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A  TEXT  BOOK  OF  PHYSIOLOGY 


•^ 


.mm 


A  TEXT  BOOK 


PHYSIOLOGY 


BY 

M.  FOSTER,  M.A.,  M.D.,  LL.D.,  F.R.S. 

PROFESSOR   OF   PHYSIOLOGY  IN   THE   UNIVERSITY   OF   C^VJIBRIDGE 
AND   FELLOW   OF  TRINITY   COLLEGE,    CAMBRIDGE 


REVISED  AND  ABIilDGED 

FROM  THE  AUTHOR- S    TEXT  BOOK   OF  PHYSIOLOGY 

IK  FIVE    VOLUMES 


/ 

Weill  gork 
MACMILLAN    AND    CO. 

AND     LONDON 

1895 

A II  rights  'reserved 


YVi?rn'ii:i'  / 


P3^ 


COPTKIGHT,   1894, 

By  MACMILLAN  AND  CO. 


NnttoDoB  ^rcss  : 

J.  S.  Gushing  &  Co.  —  Berwick  &  Smith. 

Norwood,  Mass.,  U.S.A. 


PREFACE. 

In  accordance  with  the  wishes  of  some  influential  friends, 
I  have  prepared  this  abridged  edition  of  my  text  book.  The 
abridgment  has  been  effected  by  omitting  all  the  histological 
matter,  and  all  discussions  of  a  too  theoretical  nature.  Other- 
wise, beyond  such  changes  as  the  advance  of  science  seems  to 
call  for,  the  text  which  is  left  is  the  same  as  in  the  full 
edition. 

M.   FOSTER. 
September,  189-!:. 


§§ 

1-3, 

§ 

4. 

§ 

5. 

§ 

6. 

§ 

7. 

§ 

8. 

§ 

9. 

§ 

10. 

§ 

11. 

CONTENTS. 


INTRODUCTIOK 

PAGE 

Distinctive  characters  of  living  and  dead  bodies        ....  1 

Living  substance,  food  and  waste 4 

Protoplasm  and  the  physiological  unit 5 

Histological  differentiation  and  physiological  division   of   labour. 

Tissues  and  functions 6 

The  two  chief  classes  of  tissues 6 

Muscular  and  nervous  tissues 7 

Tissues  of  digestion  and  excretion 7 

Organs.  —  Muscles  and  nerves  of  the  organs  of  nutrition  ...  8 

The  blood  and  the  vascular  system    . 8 

§    12.     The  main  problems  of  physiology 9 


BOOK   I. 

BLOOD.      THE   TISSUES   OF   MOVEMENT. 
THE   VASCULAR   MECHANISM. 

CHAPTEE   T. 
Blood. 

§    13.     The  general  work  of  the  blood 13 

SECTION  I. 

The  Clotting  of  Blood. 

§    14.     The  phenomena  of  clotting 15 

§    15.     The  characters  of  fibrin 17 

§    16.     The  features  of  serum.     Paraglobulin :  its  characters       .         .         .18 

§    17.     Serum-albumin:  its  characters 19 

§    IB.     The  circumstances  which  affect  the  rapidity  of  clotting    .         .         .20 

vii 


CONTENTS. 


§  19.  The  preparation  of  piasmine  and  fibrinogen .... 

§  20.  Fibrin-ferment :  its  action.     Nature  of  the  process  of  clotting 

§  21.  Why  blood  clots  when  shed 

§  22.  The  influence  on  clotting  exerted  by  tne  living  blood  vessels 

§  2-3.  The  nature  of  this  Influence ;  the  action  of  the  v^hite  corpuscles 


22 
23 
26 

27 
29 


SECTION  II. 

The  Corpuscles  of  the  Blood. 

The  Bed  Corpuscles. 

§  24.  The  structure  of  the  red  corpuscles ;  laky  blood ;  stroma,  and  haemo- 
globin          31 

§  25.  The  number  of  red  corpuscles  in  human  blood ;  method  of  enumer- 
ation   33 

§  26.     The  destruction  of  red  corpuscles 34 

§  27.     The  formation  of  red  corpuscles  in  the  embryo  and  in  the  adult ; 

hsematoblasts 35 

The  Wiite  Corpuscles. 

§  28.     The  structure  of  the  v^hite  corpuscles ;  characters  of  the  cell-substance    36 

§  29.     The  chemical  bodies  present  in  white  corpuscles 37 

§  30.  The  white  corpuscles  as  a  type  of  living  matter;  metabolism,  kata- 
bolic  and  anabolic  changes.     The  nature  and  relations  of  the 

'  granules  ' ;  living  substance,  food  and  waste 39 

§  31.     The  origin  of  the  white  corpuscles.     Leucocytes 42 

§  32.     The  disappearance  of  the  white  corpuscles.     Their  influence  on  the 

plasma.     Different  kinds  of  white  corpuscles 13 

Blood  Blatelets. 
§  33.     The  characters  of  blood  platelets ,        .        ,44 

SECTION  III. 

The  Chemical  Composition  of  Blood. 

§  34.  General  chemical  characters .  46 

§  35.  Chemical  composition  of  serum 46 

§  36.  Chemical  composition  of  red  corpuscles         ......  47 

§  37.  Chemical  composition  of  white  corpuscles 48 

SECTION  IV. 

The  Qoantity  of  Blood  and  its  Distribution  in  the  Body. 

§  38.     The  determination  of  the  quantity  of  blood  in  the  body,  and  the  main 

facts  of  its  distribution 49 


CONTENTS.  ix 

CHAPTER  II. 

The  Contractile  Tissues. 

PACK 

§  39.     The  movements  of  the  body  carried  out  by  means  of  various  kinds  of 

contractile  tissues 51 


SECTION  I. 

The  Phenomena  of  Muscle  and  Nekve. 

Muscular  and  Nervous  Irritability. 

§  40.     Irritability  ;  contractility  ;  stimuli 53 

§  41.     Independent  muscular  irritability ;  action  of  urari       .        .        .         .54 

§  42.    Simple  and  tetanic  contractions 55 

§  43.    The  muscle-nerve  preparation 55 

§  44.     Various  forms  of  stimuli.     Induction  Coil.     Key.     Magnetic  Inter- 

ruptor.    Electrodes.    Method  of  graphic  record      .        .        .        .56 

The  Phenomena  of  a  Simple  Muscular  Contraction. 

§  45.     The  muscle-curve.    Myographs.    Time  measurements.     Signals        .    65 

§  46.     Analysis  of  a  simple  muscle-curve 71 

§  47.     Variations  of  the  muscle-curve.    The  shortening  accompanied  by 

thickening 74 

§  48.     Simple  muscular  contractions  rare  in  the  living  body  .        .        .        .75 

Tetanic  Contractions. 

§  49.     Tetanic  contractions.    Analysis  of  the  curve  of  tetanus       .        .        .75 

§  50.     Various  degrees  of  tetanic  contractions 79 

§  51.     Diminution  and  disappearance  of  irritability  after  death     .        .        ,80 


SECTION  II. 

On  the  Changes  which  take  place  in  a  Muscle  during  a 
Contraction. 

The  Change  in  Form. 

§  52.     Gross  structure  of  muscle,  arrangement  of  muscular  fibres,  blood 

vessels  and  nerves 82 

§  53.     The  wave  of  contraction  ;  its  length,  velocity,  and  other  characters  .    83 
§  54.    The  visible  changes  which  take  place  in  a  muscular  fibre  during  a 

contraction 85 

The  Chemistry  of  Muscle. 

§  55.     Contrast  of  living  and  dead  muscle  ;  rigor  mortis         .        .        .        .80 
§  56.     Chemical  bodies  present  in  dead  muscle  ;  myosin,  syntoniu        .        .    87 


X  CONTENTS. 

PAGE 

§  57.  Chemistry  of  living  muscle  ;  muscle-plasma,  muscle-clot,  and  muscle- 
serum,  myoglobulin,  liisto-hfematin 89 

§  58.     Acid  reaction  of  rigid  muscle  ;  development  of  carbonic  acid  in  rigor 

mortis 90 

§  59.     Other  constituents  of  muscle 92 

§  60.     Chemical  changes  during  contraction  ;  development  of  carbonic  acid 

and  acid  reaction 94 

§  61.     Summary  of  the  chemistry  of  muscle 95 


Thermal  Changes. 

§  62.     Heat  given  out  during  a  contraction.    Comparison  of  muscle  v^ith  a 

steam-engine 95 


Electrical  Changes. 

§  63.     Non-polarizable  electrodes.    Muscle  currents  ;  their  distribution  and 

nature 97 

§  64.     Negative  variation  of  the  muscle  current ;  currents  of  action.    The 

rheoscopic  frog 102 


The  Changes  in  a  Nerve  during  the  passage  of  a  Nervous  Impulse. 

§  65.  The  changes  constituting  what  is  called  a  nervous  impulse  propa- 
gated along  the  nerve .     104 

§  66.     The  chemistry  of  a  nerve  ;  cholesterin,  lecithin,  cerebrin,  protagon     105 

§  67.     The  nervous  impulse ;  the  electrical  changes  accompanying  it.    These 

changes  travel  in  both  directions  along  the  nerve  ....     106 

§  68.     Summary  of  the  changes  occurring  in  a  muscle  and  nerve  as  the 

result  of  stimulation 108 


SECTION  III. 

The  Nature  of  the  Changes  through  which  an  Electric  Current 

IS    ABLE    TO    GENERATE    A    NeRVOUS    ImPULSE. 

Action  of  the  Constant  Current. 

§  69.  Action  of  the  constant  current ;  making  and  breaking  contractions.  110 
§  70.     Electrotonus.     Effect  of  the  constant  current  on  the  irritability  of 

the  nerve.     Katelectrotonus.     Anelectrotonus       .         .        .         .112 

§  71.     Electrotonic  currents 114 

§  72.     Relation  of  electrotonus  to  nervous  impulses,  and  to  the  effects  of 

the  constant  current 116 

§  7.3.     Action  of  the  constant  current  on  muscle 118 


CONTENTS.  xi 

SECTION  IV. 

The  Muscle-Nerve  preparation  as  a  Machine. 

PAGE 

§  74.  The  influence  of  the  nature  and  mode  of  application  of  the  stimulus 
on  the  magnitude  of  the  contraction.  INIaximal  and  minimal 
stimuli.  Influence  of  abruptness  and  duration  of  stimulus.  Some 
parts  of  a  nerve  more  irritable  than  others 119 

§  75.     Frequency  of  repetition  necessary  to  produce  tetanus  ;  pale  and  red 

muscles.    The  muscular  sound 121 

§  76.     The  influence  of  the  load  ;  effect  of  resistance.     The  work  done       .     123 

§  77.     The  influence  of  the  size  and  form  of  the  muscle        ....     124 


SECTION   V, 

The    Circumstances  which   determine    the    Degree    of  Irritability   of 

Muscles  and  Nerves. 

§  78.     Diminution  and  disappearance  of  irritability  after  severance  from 
the  body.     Effect  of  division  of  nerves ;  degeneration  of  nerve 

fibres 125 

§  79.    The  influence  of  temperature 127 

§  80.     The  influence  of  blood  supply 128 

§81.     The  influence  of  functional  activity.   Exercise.  Fatigue.    The  causes 

of  exhaustion 128 


SECTION  VI. 

On  some  other  Forms  of  Contractile  Tissue, 

Plain,  Synooth,  or   Unstriated  Muscular  Tissue. 

§  82.     Muscular  tissue 131 

§  83.     The  chemistiy  of  unstriated  muscle 131 

§  84.    The  characters  of  the  contraction  of  unstriated  muscle.     Peristaltic 

contractions.    '  Spontaneous '  contractions.    Tonic  contractions  .  131 

Ciliary  Movement. 

§  85.     The  action  of  cilia 134 

§  86.     Nature  of  ciliary  movement.     Circumstances  affecting  ciliary  move- 
ments        134 

AmoRboid  Movements. 

§  87.     Nature  of  an  amoeboid  movement ;  its  relation  to  a  muscular  con- 
traction      137 


xii  CONTENTS. 

CHAPTER  III. 

On  the  more  General  Features  op  Nervous  Tissues. 

PAGE 

§  88.  The  general  arrangement  of  the  nervous  system.  Cerebro-spinal 
and  splanchnic  or  sympathetic  system ;  somatic  and  splanchnic 
nerves 139 

§  89.  Grey  matter  and  white  matter  of  the  central  nervous  system.  Struc- 
ture of  a  nerve  cell  of  the  spinal  cord ;  axis-cylinder  process. 
Functions  of  nerve  cells        . 143 

§  90.  Eeiiex  actions  ;  the  machinery  required.  The  circumstances  deter- 
mining the  nature  of  a  reflex  action.  Reflex  actions  often  pur- 
poseful    144 

§    91.     Automatic  actions       . 146 

§    92.     Inhibitory  nerves 148 


CHAPTER  IV. 

The  Vascular  Mechanism. 
SECTION  I. 

The  Structure  and  Main  Features  of  the  Vascular  Apparatus. 

§    93.    The  chief  work  of  the  blood  carried  on  in  the  capillaries  and  other 

minute  vessels 149 

§    94.     The  main  features  of  the  vascular  apparatus    .....    150 

SECTION  II. 

The  Main  Facts  of  the  Circulation. 

§    95.     Behaviour  of  arteries  contrasted  with  that  of  veins  .        .        •     153 

§    96.     Blood  pressure  in  an  artery  and  in  a  vein 154 

§    97.     Methods  of  registering  blood  pressure ;    mercurial  manometer. 

Kymograph.     The  blood  pressure  curve 156 

§    98.     Characters  of  the  blood  pressure  in  various  arteries  and  veins. 
Blood  pressure  in  the  capillaries.     Fall  of  blood  pressure  in  the 

minute  vessels 159 

§    99.     The  circulation  through  the  capillaries,  and  small  vessels.    Periph- 
eral resistance 161 

Hydraulic  Principles  of  the  Circulation. 

§  100.     The  three  main  physical  facts  of  the  circulation  ;  the  central  pump, 

the  peripheral  resistance,  and  the  elastic  tubing  .        .        .     163 


CONTEJ^TS.  xiii 

PAGE 

§  101.     The  conversion  of  the  intermittent  into  a  continuous  flow  by  means 

of  the  elastic  reaction  of  tlie  arteries 164 

§  102.     Artificial  Model.    Arterial  and  venous  pressure  with  great  and  with 

little  peripheral  resistance .     160 

§  103.     Additional  aids  to  the  circulation  in  the  living  body        .        .        .     171 

Circumstances  determining  the  Bate  of  the  Flow. 

§  104.     Methods  of  determining  the  rate  of  the  flow.     HEemadromometer, 
Rheometer,  Hsematachometer.    The  plethysmographic  method. 

The  rate  of  flow  in  arteries,  veins,  and  capillaries       .        .        .  172 

§  105.     The  rate  of  flow  dependent  on  the  width  of  the  bed  .        ,        .  176 

§  106.     The  time  of  the  entire  circuit 178 

§  107.     Summary  of  the  main  physical  facts  of  the  circulation     .        .        .  179 

SECTION   III. 
The  Heart. 

The  Phenomena  of  the  Normal  Beat. 

§  108.     The  visible  movements 181 

§  109.     The  cardiac  cycle  ;  the  series  of  events  constituting  a  beat      .        .  182 

§  110.     The  change  of  form 185 

5;  111.     The  cardiac  impulse 187 

§  112.     The  sounds  of  the  heart 188 

§113.     Endocardiac  pressure.  Methods  of  determining  this.  Cardiac  sound 
and  tambour.     Piston  and  membrane  manometers.     General 

features  of  the  curve  of  endocardiac  pressure      ....  191 

§  114.     The  output  of  the  heart ;  the  methods  of  determining  this      .        .  197 

The  Mechanism  of  the  Beat. 

§  115.     The  curves  obtained  by  means  of  cardiograph  and  the  myocardio- 

graph.    The  curve  of  ventricular  pressure  compared  with  these    200 
§  116.     The  pressure  in  the  ventricle  compai-ed  with  that  in  the  aorta. 
The  differential  manometer  or  manometer  balance.    The  teach- 
ings of  this  comparison 203 

§  117.     Minimum  and  maximum  manometers.     The  negative  pressure  in 

the  cardiac  cavities 210 

§  118.     The  duration  of  the  several  phases  of  the  cardiac  cycle    .        .        .    212 

§  119.     Summary  of  the  events  constituting  a  beat 216 

§  120.     The  work  done 217 

SECTION   IV. 
The  Pulse. 

§  121.     Methods  of  recording  the  pulse.     The  sphygmograph,  sphygmo- 

scope,  and  other  instruments.     The  pulse  curve  ....    219 

§  122.     Pulse  tracing  from  an  artilicial  model ;   the  nature  of  the  pulse 

wave 223 


xiv  CONTENTS. 

PAGE 

§  123.  The  characters  of  the  pulse  curve  ;  influence  of  pressure  exerted  by 

lever 226 

§  124.  The  changes  undergone  by  the  pulse  wave  along  the  arterial  tract  227 

§  125.     The  velocity  of  the  pulse  vrave 228 

§  126.     The  length  of  the  pulse  wave 229 

§  127.  Secondary  waves.     Katacrotic  and  anacrotic  tracings       .        .         .  230 

§  128.     The  dicrotic  wave  :  its  causes 232 

§  129.  Circumstances  determining  the  prominence  of  the  dicrotic  wave    .  234 

§  130.     The  predicrotic  wave.    Anacrotic  waves 235 

§  131.     Venous  pulse       .        , 236 

SECTION  V. 

The  Regulation  and  Adaptation  op  the  Vascular  Mechanism. 

The  Begulation  of  the  Beat  of  the  Heart. 

§  132.     The  two  gi-eat  regulators  ;  changes  in  the  heart  beat  and  changes 

in  the  peripheral  mechanism 238 

The  Development  of  the  Normal  Beat. 

§  133.  Graphic  record  of  the  heart  beat.  The  beat  of  the  frog's  heart. 
The  sequence  of  events,  and  the  descending  scale  of  rhythmic 
power 239 

§  134.     Some  features  of  the  heart  beat  in  the  mammal        ....    244 

The  Government  of  the  Heart  Beat  by  the  Nervous  System. 

§  135.     Inhibition  in  the  frog  by  stimulation  of  vagus  nerves.    Features 

of  inhibition 244 

§  136.  Augmentation  of  the  heart  beat  in  the  frog.  Antagonism  of  aug- 
mentation and  inhibition.  Course  of  augmentor  fibres  in  the 
frog 246 

§  137.     Eeflex  inhibition.    Cardio-inhibitory  centre 249 

§  138.  Inhibition  in  the  mammal ;  effect  on  blood  pressure.  Eeflex  inhi- 
bition.    Course  of  augmentor  fibres  in  the  dog    ....    250 

§  139.     Action  of  atropin  and  muscarin 257 

Other  Influences  regulating  or  modifying  the  Beat  of  the  Heart. 

§  140.  Influences  of  blood,  and  substances  contained  in  the  blood.  Influ- 
ence of  the  distension  of  the  cavities.  Relation  of  heart  beat  to 
blood  pressure 258 

SECTION   VI. 

Changes  in  the  Calibre  of  the  Minute   Arteries.     Vaso-motor 

Actions. 

§  141.     Changes  of  calibre  in  arteries  as  seen  in  the  web  of  a  frog's  foot 

and  elsewhere.     Vaso-motor  nerves 262 

§  142.     The  vascular  phenomena  in  a  rabbit's  ear 263 


CO^TEIS^TS.  XV 

PAGB 

§  143.     The  effects  on  the  vessels  of  the  ear  of  dividing  and  stimulating 

the  cervical  sympathetic  nerve 264 

§  144.     Course  of  vaso-motor  fibres  of  the  ear 265 

§  145.  The  effects  on  the  vessels  of  the  submaxillary  gland  of  stimulating 
the  chorda  tympani  nerve  ;  vaso-constrictor  and  vaso-dilator 
fibres 267 

§  146.     Vaso-motor  nerves  of  other  parts  of  the  body.    Constrictor  and 

dilator  fibres  in  the  sciatic  and  brachial  nerves  ....    269 

The  Course  of  Vaso-motor  Fibres. 

§  147.     The  course  of  vaso-constrictor  fibres 273 

§  148.     The  course  of  vaso-dilator  fibres 275 

The  Effects  of  Vaso-motor  Actions. 

§  149.     Local  and  general  effects  of  the  constriction  and  dilation  of  an 

artery  or  set  of  arteries 275 

Vaso-motor  Functions  of  the  Central  Nervous  System. 

§  150.    Vaso-dilator  fibres  usually  employed  as  part  of  a  reflex  action        .  277 
§  151.     Loss  of  tone  resulting  from  the  division  of  the  spinal  chord  at  vari- 
ous levels.     Vaso-motor  centre  in  the  spinal  bulb       .        .        ,  278 

§  152.     The  Depressor  nerve 280 

§  153.     Rise  of  blood  pressure  from  stimulation  of  afferent  nerves  ;  pressor 

effects 281 

§  154.     The  limits  of  the  bulbar  vaso-motor  centre 282 

§  155.  The  relation  of  the  bulbar  vaso-motor  centre  to  other  spinal  vaso- 
motor centres.    Dilation,  tone,  and  constriction  of  blood  vessels  283 

§  156.     Summary  of  vaso-motor  actions 284 

§  157.     Instances  of  vaso-motor  actions.    Blushing.    Effect  of  warmth  on 

skin.    Vascular  changes  in  kidney  and  alimentaxy  canal   .        .  280 

§  158.     Vaso-motor  nerves  of  the  veins 288 

SECTION  VII. 

The  Capillary  Circulation. 

§  159.     The  normal  capillary  circulation.    The  axial  core  and  the  plasmatic 

layer 289 

§  160.     Changes  in  the  capillary  circulation   induced  by  irritants.    The 

phenomena  of  inflammation 290 

§  161.     The  migration  of  white  corpuscles.     Stasis 292 

§  162.     Nature  of  the  inflammatory  changes 293 

ij  163.     Changes  in  the  peripheral  resistance  due  to  changes  in  the  blood    .  294 

SECTION  VIII. 

Changes  in  the  Quantity  of  Blood. 

§  164.     Effects  of  increasing  and  of  diminishing  the  total  quantity  of  blood    296 


xvi  CONTENTS. 

SECTION  IX. 
A  Review  of  some  of  the  Features  of  the  Circulation. 

PAGE 

§  165.     The  constant  and  variable  factors 299 

§  166.     The  influence  of  the  venous  inflow  and  of  the  distension  of  the 

cavities  of  the  heart 300 

§  167.     The  heart  beat  influenced  by  tlie  quantity  and  quality  of  the  blood 

flowing  through  the  heart 300 

§  168.     The  causes  of  an  irregular  heart  beat 301 

§  169.     The  causes  of  the  sudden  cessation  of  the  heart  beat  and  of  sudden 

death 302 

§  170.     Instances  of  the  working  of  the  vaso-constrictor  mechanism   .         .  303 

§  171.     The  influence  of  bodily  exercise  on  the  vascular  mechanism    .        ,  304 

§  172.    The  influence  of  food  on  the  vascular  mechanism     ....  306 

§  173.     The  vascular  mechanism  and  the  nervous  system     ....  307 


BOOK   II. 

THE   TISSUES   OF   CHEMICAL   ACTION   WITH   THEIR    RESPECTIVE 
MECHANISMS.    NUTRITION. 

CHAPTEE  I. 

The  Tissues  and  Mechanisms  of  Digestion. 
§  174.     Food-stuffs.    The  several  stages  of  digestion 311 

SECTION  I. 
The  Characters  and  Properties  of  Saliva  and  Gastric  Juice. 

Saliva. 

§  175.     The  chemical  characters  of  saliva.     Mucin 313 

§  176.     The  properties  of  saliva.  The  action  of  saliva  on  starch.    Characters 

of  starch,  dextrins,  dextrose,  maltose 314 

§  177.     The  nature  of  the  amylolytic  action  of  saliva.    The  amylolytic 

ferment 316 

§  178.     The  characters  of  parotid,  submaxillary,  sublingual,  and  mixed 

saliva 318 

Gastric  Juice. 

§  179.     The  chemical  characters  of  gastric  juice 320 

§  180.  The  action  of  gastric  juice  on  proteids.  Artificial  gastric  juice. 
The  characters  of  the  more  important  proteid  bodies.  Forma- 
tion of  acid-albumin 321 


C0NTE:N'TS.  xvii 

PAGE 

§  181.     Peptone  and  albumose.     Tarapeptone.     Classification  of  proteids  .  325 

§  182.     Circumstances  affecting  gastric  digestion  ;  acidity,  temperature      .  327 

§  183.     Tlie  nature  of  the  action  of  gastric  juice.     Pepsin     ....  328 

§  184.     Tlie  action  of  gastric  juice  on  gelatin,  chondrin,  &c 329 

§  185.     The  action  of  gastric  juice  on  milk.    Curdling  of  milk.     Casein. 

Eennin 329 


SECTION  II. 

The  Act  of  Secretion  of  Saliva  and  Gastric  Juice  and  the  Nervous 
Mechanisms  which  regulate  it. 

§  186.     The  evidences  of  the  existence  of  a  nervous  mechanism  .        .        .  332 

§  187.     The  nerves  of  the  submaxillary  gland 333 

§  188.     The  reflex  secretion  of  saliva  by  means  of  the  chorda  tympani 

nerve 333 

§  189.     The  nature  of  the  action  of  the  chorda  tympani  nerve.     Influence 

of  atropin 335 

§  190.     The  effects  on  the  submaxillary  gland  of  stimulating  the  cervical 

sympathetic  nerve 337 

§  191.     The  nervous  mechanism  of  the  parotid  gland 338 

§  192.  The  general  features  of  the  secretion  of  gastric  juice  .  .  .  338 
§  193.     The  nervous  supply  of  the  stomach.    The  action  of  the  nerves 

obscure 339 

§  194.  ■  The  influence  of  the  absorption  of  food  in  promoting  secretion        .  339 

Tlie  Changes  in  a  Gland  constituting  the  Act  of  Secretion. 

§  195.     The  appearances  presented  by  the  pancreas  during  secretion  ;  the 

histological  changes 340 

§  196.     The  changes  in  an  albuminous  gland  during  secretion      .         .         .  342 

§  197.     The  changes  in  a  mucous  gland  during  secretion       ....  344 

§  198.     The  changes  in  the  central  cells  of  the  stomach  during  secretion     .  346 

§  199.  The  general  nature  of  secretion.  Loading  and  discharge  .  .  347 
§  200.     The  formation  of  the  ferment ;  zymogen,  trypsin,  and  trypsinogen, 

pepsin  and  pepsinogen 348 

§  201.     The  nature  of  the  act  of  secretion  itself.    The  flov?  of  fluid.    '  Secre- 
tory '  and  '  trophic '  fibres 350 

§  202.     The  functions  of  the  epithelium  of  the  ducts 351 

§  203.     The  formation  of  the  free  acid  of  gastric  juice 352 

§  204.     Why  the  .stomach  does  not  digest  itself 352 


SECTION  III. 

The  Properties  and  Characters  of  Bile,  Pancreatic  Juice,  and 
Succus  Entericus. 

Bile. 

§  205.     The  characters  of  bile 354 

§  206.     The  pigments  of  bile.     Bilirubin 356 


xviii  CONTENTS. 

PAGE 

§  207.     The  bile  salts  ;  glycocholic  and  taurocholic  acids      ....    356 
§  208.     The  action  of  bile  on  food 357 

Pancreatic  Juice. 

§  209.     The  characters  of  pancreatic  juice 358 

§  210.     The  action  of  pancreatic  juice  on  proteids ;  leucin,  tyrosin.     Its 

action  on  fats  and  on  starch 359 

Succus  Entericus. 

§  211.     Nature  and  action  of  succus  entericus 363 

§212.     Gallstones 364 


SECTION  IV. 

The  Secretion  of  Pancreatic  Juice  and  of  Bile. 

§  213.    The  secretion  of  pancreatic  juice 365 

§  214.     The  discharge  of  and  the  secretion  of  bile 366 

§  215.     The  vascular  conditions  of  the  liver  and  their  relations  to  the 
secretion  of  bile.     The    influence  of    absorbed    products    of 

digestion 367 

§  216.     The  pressure  at  which  bile  is  secreted 369 

§  217.     The  resorption  of  bile 370 

SECTION  V. 

The  Muscular  Mechanisms  of  Digestion. 

§  218.     Peristaltic  movements.     Mastication 371 

§  219.  Deglutition  ;  its  phases,  nature,  and  nervous  mechanism .        .        .  372 

§  220.     The  movements  of  the  oesophagus 375 

§  221.     The  movements  of  the  stomach 377 

§  222.     Vomiting 378 

§  223.     The  movements  of  the  small  intestine 380 

§  224.     The  movements  of  the  large  intestine 381 

§  225.     Defgecation 381 

§  226.  The  nervous  mechanisms  of  gastric  and  intestinal  movements.    The 

special  movements  of  the  rectum 383 

§  227.     Influences  bearing  on  peristaltic  movements 386 

SECTION  VI. 

The  Changes  which  the  Food  undergoes  in  the  Alimentary  Canal. 

§  228.    The  changes  in  the  mouth 387 

The  Changes  in  the  Stomach. 

§  229.     The  changes    in  the  stomach.     Chyme.     Absorption  from   the 

stomach.     Flatulence 388 


CONTENTS.  xix 


In  the  Small  Intestine. 

PAGE 

§  230.     The  changes  in  the  small  intestine.    Action  of  bile  and  pancreatic 

juice 390 

§  231.     Influence  of  bile  on  peptic  digestion 392 

§  232.     The  action  of  micro-organisms  in  the  alimentary  canal.     Indol,  &c.  393 

In  the  Large  Intestine. 
§  233.     The  changes  in  the  large  intestine.    Digestion  of  cellulose      .        .    395 

The  Fceces. 
§  234.     The  nature  and  constituents  of  the  faeces 396 

SECTION  VII. 

The  Lacteals  and  the  Lymphatic  System. 

§  235.     The  general  arrangement  of  the  lymphatics 398 

SECTION  VIII. 

The  Nature  and  Movements  of  Lymph  (including  Chyle). 

§  236.     Movements  of  lymph  continually  going  on  in  the  living  body  .        .    399 

The  Characters  of  Lyrtiph. 

§  237.  The  microscopical  characters  of  lymph 400 

§  238.  The  chemical  composition  of  lymph 401 

§  239.  The  changes  taking  place  in  lymph  during  its  passage      .        .        .  401 

§  240.  The  chemical  characters  of  serous  fluids 402 

§  241.  The  characters  of  chyle 402 

The  Movements  of  Lymph. 

§  242.     The  causes  which  maintain  the  flow  of  lymph 403 

§  243.     The  influence  of  the  nervous  system  on  the  flow  of  lymph       .        .  405 
§  244.     The  phenomena  of  transudation.    The  relation  of  transudation  to 

filtration  and  diffusion 406 

§  245.     CEdema  or  dropsy  ;  its  causes 409 

SECTION  IX. 

Absobption  from  the  Alimentary  Canal. 

§  246.     The  main  products  of  digestion,  and  the  two  paths  of  absorption 

open  for  them 412 

The  Course  taken  by  the  Several  Products  of  Digestion. 

§  247.     The  course  taken  by  the  fats 413 

§  248.     The  course  taken  by  water  and  salts 414 


XX  CONTENTS. 

FA&E 

§  249.     The  course  taken  by  sugar .....,.,.    414 
§  250.     The  course  taken  by  proteids 415 

The  Mechanism  of  Ahsot-pUon. 

§  251.     The  mechanism  of  the  absorption  of  the  fats 417 

§  252.     The  pumping  action  of  the  villi 419 

§  253.  The  mechanism  of  the  absorption  of  diffusible  substances  and  of 
water.  Eelations  of  the  process  to  diffusion.  Action  of  the 
cells.    The  two  stages  of  the  act  of  absorption ;  their  nature     .    420 


CHAPTER  II. 

§  254.     Respiration 424 

SECTION  I. 

The  Mechanics  of  Pulmonary  Eespiration. 

§  255.     The  entrance  and  exit  of  air  into  and  from  the  lungs ;  tidal  and 

stationary  air 425 

§  256.     Complemental,  reserve  or  supplemental,  and  residual  air.    Results 

of  an  openmg  into  the  pleural  chamber 426 

§  257.    The  lungs  before  birth  and  the  changes  at  birth       ....  427 

§  258.     The  pressure  exerted  in  breathing  and  the  quantity  of  air  moved    .  427 

§  259.     The  graphic  records  of  the  respiratory  movements  ;  pneumatograph  428 

§  260.     The  curve  of  respiratory  movements 432 

The  Bespiratory  Movements. 

§  261.     The  visible  movements 433 

§  262.     The  movements  of  inspiration.    The  movements  of  the  diaphragm  433 

§  263.     The  elevation  of  the  ribs 434 

§  264.     The  muscles  which  move  the  ribs 435 

§  265.     The  muscles  of  laboured  inspiration 436 

§  266.     Expiration.     The  expiratory  muscles 437 

§  267.     Facial  and  laryngeal  respiration 438 

SECTION  II. 

The  Changes  of  the  Air  in  Respiration. 

§  268.     The  changes  in  temperature 440 

§  269.     The  aqueous  vapour  in  expiration 440 

§  270.    The  gaseous  changes 440 

§271.     The  diminution  in  volume 441 

§  272.     The  organic  impurities  in  expired  air 442 

SECTION  III. 

The  Respiratory  Changes  in  the  Blood. 

§  273.     The  gases  of  arterial  and  venous  blood.     The  mercurial  gas  pump  .  443 


CONTENTS.  xxi 


The  Belations  of  Oxygen  in  the  Blood. 


§  274.  The  absorption  of  oxygen  by  blood  is  not  according  to  '  the  law  of 

pressures ' 

§  275.  The  characters  of  hsemoglobin    .... 

§  276.  The  spectroscopic  features  of  haemoglobin 

§  277.  The  spectroscopic  features  of  reduced  hsemoglobin 

§  278.  The  oxygenation  and  reduction  of  hsemoglobin 

§  279.  The  colour  of  venous  and  arterial  blood    . 

§  280.  Carbonic-oxide-hfemoglobin        .        .        ,        . 


447 
450 
451 
453 
455 
455 
457 


Products  of  the  Decomposition  of  Hcemoglobin. 

§  281.     Hsemoglobin  splits  up  into  hsematin  and  a  proteid    ....     458 
§  282.     The  features  of  hsematin.     Hsemin.     Methsemoglobin      .        .        .    460 

The  Belations  of  the  Carbonic  Acid  in  the  Blood. 
§  283.     The  carbonic  acid  of  the  blood  not  simply  absorbed         ,        .        .    461 

The  Belations  of  the  Nitrogen  in  the  Blood. 
§  284.     The  nitrogen  simply  absorbed 461 

SECTION  IV. 

The  Respiratory  Changes  in  the  Lungs. 

§  285.     The  relations  of  the  oxygen  of  the  blood  to  pressure.    Association 
of    oxygen    with,   and    dissociation    from    hsemoglobin.    The 

problem  stated 462 

§  286.     The  experimental  evidence 464 

§  287.    The  relations  of  the  oxygen  in  laboured  breathing  and  asphyxia     .    465 

The  Exit  of  Carbonic  Acid. 

§  288.     The  exit  of  carbonic  acid  from  the  blood  into  the  pulmonary  alve- 
olus the  result  of  ordinary  diffusion 466 

SECTION  V. 
The  Respiratory  Changes  in  the  Tissues. 

§  289.     The  oxidations  of  the  body  take  place  mainly  in  the  tissues  and  not 

in  the  circulating  blood.     The  respiration  of  muscle  .        .        .    467 

§  290.     The  respiration  of  other  tissues.    The  taking  in  of  oxygen  separate 

from  the  giving  out  of  carbonic  acid 469 

§  291.     A  summary  of  respiration  in  its  chemical  aspects     ....    470 

SECTION  VI. 

The  Nervous  Mechanism  of  Respiration. 

§  292.     Respiration  an  involuntaiy  act.    The  efferent  nerves,  the  respiratory 

centre 472 


xxii  CONTEl^TS. 


PAGE 


§  293.  The  complex  nature  of  the  medullary  respiratory  centre  ;  the  sub- 
sidiary spinal  mechanisms 473 

§  294.     The  action  of  the  centre  automatic 474 

§  295.     The  centre  influenced  by  afferent  impulses.    The  respiratory  action 

of  the  vagus  nerve.     Inhibitory  and  augmentor  effects        .        .    475 

§  296.  The  double  action  of  the  centre,  inspiratory  and  expiratory.  Antag- 
onism between  the  two 479 

§  297.     The  effects  of  inflation  and  suction.    Double  action  of  the  vagus 

fibres.    Self-regulating  mechanism 480 

§  298.     The  influence  on  the  respiratory  centre  of  various  afferent  impulses    483 

§  299.     The  respiratory  centre  composed  of  two  lateral  halves     ,        .        .    484 

§  300.     The  respiratory  centre  influenced  by  the  condition  of  the  blood 

brought  to  it.    Eupnoea,  hyperpncea,  and  dyspnoea    .        .        .    484 

§  301.     The  direct  character  of  these  influences.     Effects  of  heat  on  the 

respiratory  centre 486 

§  302.    The  relative  shares  taken  by  deflciency  of  oxygen  and  excess  of 

carbonic  acid  in  the  above  influences 487 

§  303.     Changes  in  the  blood  other  than  differences  in  the  gases  influence 

the  respiratory  effect.    The  effects  of  muscular  exercise     .        .    488 

§  304.    The  phenomena  and  causes  of  apnoea 489 

§  305.     Secondary  respiratory  rhythm.    The  Cheyne- Stokes  respiration     .    490 


SECTION  VII. 

The  Effects  of  Changes  in  the  Composition  and  Pressure 
OP  THE  Air  breathed. 

§  306.    The  phenomena  of  asphyxia 491 

§  307.  The  effects  on  respiration  of  breathing  various  foreign  gases  .        .  494 

§  308.  The  effects  of  a  diminution  of  pressure  in  the  air  breathed      .        .  494 

§  309.  The  effects  of  an  increase  of  atmospheric  pressure  ....  496 


SECTION  VIII. 

The  Eelations  of  the  Respiratory  System  to  the  Vascular 
AND  other  Systems. 

§  310.  The  respiratory  movements  influence  the  circulation.  The  respira- 
tory undulations  of  the  blood-pressure  curve       ....    497 

§  311.  The  effects  of  the  expansion  of  the  thorax  and  its  return  on  the 
flow  of  blood  to  and  from  the  heart  and  so  on  the  blood- 
pressure  499 

§  312.     The  effects  of  the  expansion  and  return  of  the  thorax  on  the  flow 

of  blood  through  the  lungs 502 

§  313.     The  effects  on  the  circulation  of  artificial  respiration        .         .        .     503 

§  314.     An  action  of  the  cardio-inliibitory  centre  synchronous  with  that  of 

the  respiratory  centre    . 504 

§  315.     The  effects  of  impeded  respiration  on  the  circulation  in  general. 

Traube-Hering  curves.    The  circulation  in  asphyxia  .        .        .    504 


CONTEXTS.  yyiii 

PAGE 

§  316.     The  effects  on  respiration  of  changes  in  the  circulation  through  the 

respiratory  centre  and  through  the  lungs 508 

§  317.     The  effects  of  exercise  on  respiration 509 


SECTION  IX. 

Modified  Respiratory  Movements. 

§  318.     Sighing,  yawning,  hiccough,  sobbing,  coughing,  sneezing,  laugh- 

•  ing,  and  crying 511 


CHAPTER  III. 

The  ELiMiNATiOiSr  of  Waste  Products. 
§  319.     The  chief  waste  products  and  their  channels  of  exit         .        .        .    513 

SECTION  I. 

The  Composition  and  Characters  of  Urine. 

§  320.  The  general  characters  of  urine 515 

§  321.  The  normal  organic  constituents  of  urine 516 

§  322.  The  inorganic  salts 517 

§  323.  The  non-nitrogenous  constituents  of  the  urine         ....  518 

§  324.  The  pigments  of  the  urine 519 

§  325.  Ferments  and  other  bodies  present  in  urine 519 

§  326.  The  relative  quantities  of  the  more  important  constituents  of  urine  520 

§  327.  The  acidity  of  urine 521 

§  328.  The  abnormal  constituents  of  urine 522 

SECTION  n. 

The  Secretion  of  Urine. 

§  329.     The  double  nature  of  urinary  secretion 524 

§  330.     The  vaso-motor  mechanisms  of  the  kidney.    The  renal  oncometer 

and  oncograph 525 

§  331.     The  relation  of  the  flow  of  blood  through  the  kidney  to  variations 

in  blood-pressure  and  to  vaso-motor  changes  in  general     .        .    528 

§  332.     The  vaso-constrictor  nerves  of  the  kidney 530 

§  333.     The  vaso-dilator  nerves  of  the  kidney 531 

§  334.     The  influence  on  the  renal  circulation  of  chemical  substances  in 

the  blood 531 

§  335.     The  connection  between  changes  in  the  renal  circulation  and  the 

secretion  of  urine 532 


xxiv  CONTENTS. 

Secretion  by  the  Benal  Epithelium. 

PAGE 

§  336.     The  evidence  of  the  secretory  activity  of  the  epithelium.    Experi- 
ments on  amphibia.    The  results  of  injecting  sulphindigotate  of 

sodium 533 

§  337.     The  nature  of  glomerular  secretion ;  its  relation  to  filtration  and 

diffusion.    Albuminous  urine 535 

§  338.     The  nature  of  the  work  of  the  epithelium  as  regards  the  secretion 

of  urea 538 

§  339.     The  formation  of  hippuric  acid .    539 

§  340.     The  relations  of  the  secretory  activity  of  the  kidney  to  the  secretory 

activity  of  the  skin 540 

§  341.     The  relations  of  the  secretion  of  urine  to  food  and  drink         .        .    541 

§  342.     Diuretics 542 

§  343.    Direct  action  of  the  nervous  system  on  the  kidney  ....    543 

SECTION  III. 

The  Discharge  of  Urine. 

§  344.    The  movements  of  the  ureter 544 

Micturition. 

§  345.     The  muscles  of  the  bladder,  their  action,  the  nerves  governing 

them ;  the  sphincter  vesicae 545 

§  346.    The  varying  tone  of  the  bladder 546 

§  347.     The  general  nervous  mechanism  of  micturition        ....  547 

§  348.     Involuntary  and  voluntary  micturition 548 

§  349.     Changes  of  the  urine  during  its  stay  in  the  bladder  ....  549 

SECTION  IV. 
The  Nature  and  Amount  of  Perspiration. 

§  350.     Sensible  and  insensible  perspiration.    The  characters  and  constitu- 
ents of  sweat 550 

Cutaneous  Bespiration. 

§  351.     The  nature  and  amount  of  cutaneous  respiration.    The  effects  of 

varnishing  the  skin •        •    552 

§  352.     Absorption  by  the  skin 553 

SECTION  V. 

The  Mechanism  of  the  Secretion  of  Sweat. 

§  353.    The  relation  of  sweating  to  vascular  changes.    The  nervous  mech- 
anism of  the  sweat-glands 555 

§  354.     The  sweat-nerves,  their  origin  and  course         .....  557 

§  355.     Inhibitory  sweat-nerves 558 


CONTENTS.  XXV 

CHAPTEE  IV. 
The  Metabolic  Processes  of  the  Body. 

PAGE 

§  356.    The  general  characters  of  the  metabolism  of  the  body      .        .        .    559 

SECTION  I. 

The  History  op  Glycogen. 

§  357.     The  characters  of  glycogen 561 

§  358.     The  conversion  of  glycogen  into  sugar  by  the  liver  ....  562 
§  .359.     The  iniluence  of  various  foods  in  storing  up  glycogen.    The  storage 

of  glycogen  in  the  winter  frog 563 

§  360.    The  detailed  characters  of  the  hepatic  cells  in  the  frog    .        .        .  566 
§  361.     The  histological  changes  induced  by  food  and  circumstances  in  the 

hepatic  cells  of  the  frog 567 

§  362.     The  corresponding  changes  in  the  mammal 568 

§  .363.     The  nature  and  meaning  of  these  changes 568 

§  364.     Views  as  to  the  manner  in  which  glycogen  is  stored  in  the  hepatic 

cells.     By  simple  dehydration  of  sugar 569 

§  365.     The  glycogen  formed  by  a  product  of  the  metabolism  of  the  hepatic 

cells.     Comparison  of  the  two  views 570 

§  366.    The  uses  of  glycogen.    The  formation  of  fat  as  a  store  of  carbon- 
holding  material 571 

§  367.     Glycogen  in  muscle 573 

§  368.     Glycogen  in  the  placenta  and  in  various  tissues        ....  574 

Diabetes. 

§  369.     Artificial  diabetes 575 

§  370.     The  nervous  mechanism  of  the  diabetic  puncture     ....  576 

§  371.     Temporary  diabetes  from  the  use  of  drugs.    Natural  diabetes.    The  576 

diminution  of  hepatic  glycogen  by  arsenic  and  other  agents      .  576 

SECTION  II. 

The  Spleen. 

§  372.     The  movements  of  the  spleen.    The  spleen  curve     ....  579 
§  373.     The  spleen  pulp  ;  the  white  and  red  corpuscles.    Changes  under- 
gone by  the  latter 581 

§  374.     The  chemical  constituents  of  the  spleen 582 

SECTION   III. 

The  Formation  of  the  Constituents  of  Bile. 

§  375.     The  formation  of  bilirubin  from  haemoglobin 684 

§  376.     The  nature  of  and  preparations  towards  this  formation   .        .        .    585 
§  377.     The  formation  of  bile  acids 686 


xxvi  CONTENTS. 


PAGE 

§  378.     The  relations  of  the  secretion  of  bilirubin  to  the  secretion  of  bile 

acids 587 

§  379.     The  relation  of  the  secretion  of  bile  to  the  formation  of  glycogen  .    587 


SECTION  IV. 

On  Ueea  and  on  Nitrogenous  Metabolism  in  general. 

§  380.     Urea  the  end-product  of  the  metabolism  of  proteid  matter      ,        .  589 

§  381.     Urea  probably  the  result  of  a  series  of  changes         ....  590 

§  382.     The  kreatin  of  muscle  in  its  relations  to  urea 590 

§  383.     Difficulties  presented  by  the  normal  presence  of  kreatin  in  urine    .  591 

§  384.     The  nitrogenous  metabolism  of  the  nervous  tissues  ....  592 
§  385.     The  nitrogenous  metabolism   of  the  glandular  structures.    The 

increase  of  urea  due  directly  to  food 593 

§386.     The  formation  of  urea  in  the  liver     . 594 

§  387.     The  synthesis  of  urea 595 

§  388.     Uric  acid 596 

§  389.     Other  nitrogenous  crystalline  bodies,  such  as  xanthin,  &c.       .        .  597 

§  390.     Eelations  of  urea  to  cyanogen  compounds 698 

§  391.     A  summary  of  nitrogenous  metabolism 599 

SECTION  V. 

On  some  Structures  and  Processes  op  an  obscure  Nature. 

§  392.     The  structure  and  functions  of  the  thyroid  body      ....  600 

§  393.     The  pituitary  body 601 

§  394.     The  structure,  chemical  constituents,  and  functions  of  the  supra- 
renal bodies 601 

§  395.     The  structure,  nature,  and  functions  of  the  thymus         .        .        .  602 

SECTION  VI. 
The  History  of  Eat.     Adipose  Tissue. 


§  396.  Adipose  tissue  ;  its  changes 

§  397.  The  structure  of  adipose  tissue  . 

§  398.  The  disappearance  of  fat  from  adipose  tissue 

§  399.  The  nature  of  fat  in  adipose  tissue    . 

§  400.  Fat  is  formed  in  the  body  .... 

§  401.  Formation  of  fat  from  carbohydrates  and  from  proteids 

§  402.  Limits  to  the  construction  of  fat 


604 
604 
605 
606 
606 
608 
609 


SECTION  VII. 

The  Mammary  Gland. 

§  403.     The  general  structure  of  the  mammary  gland 610 

§  404.     The  structure  of  the  alveoli ;  the  varying  appearances  of  the  epi- 
thelial cells 010 


CONTENTS.  xxvii 

PAGE 

§  405.  The  characters  of  the  dormant  mammary  gland       ....    611 

§  406.  The  connective-tissue  of  the  mammary  gland  .... 

§  407.  The  nature  of  milk ;  its  constituents  ;  their  relative  quantities  in 

different  animals 

§  408.  Colostrum,  its  chemical  and  microscopical  characters 

§  409.  The  mammary  gland  at  birth 

§  410.  The  nature  of  the  act  of  secretion  of  milk 

§  411.  The  secretion  of  milk  typical  of  metabolism  in  general     . 

§  412.  The  relations  of  the  gland  to  the  nervous  system 


612 

612 
614 
615 
615 
616 
618 


CHAPTER  V. 

NUTRITIOX. 

SECTION  I. 
The  Statistics  of  NcTRiTioif. 

§  413.    The  composition  of  the  animal  body 619 

§  414.    The  composition  of  the  starving  body  and  the  general  phenomena 

of  starvation 620 

Comparison  of  Income  and  Output  of  Material. 

§  415.     The  method  of  determining  the  income  and  output.    The  respira- 
tion chamber.     The  calculation  of  changes  in  the  body  based 

on  a  comparison  of  the  income  and  output 622 

§  416.     Nitrogenous  metabolism.    '  Tissue  '  proteids  and  '  floating '  proteids. 

Luxus  consumption 625 

§  417.     The  effects  of  fatty  and  carbohydrate  food 627 

§  418.     The  effects  of  gelatin  as  food 629 

§  419.     Peptone  as  food 629 

§  420.     The  effects  of  salts  as  food 630 

SECTION  II. 
The  Energy  of  the  Body. 

The  Income  of  Energy. 

§  421.     The  available  potential  energy  of  the  several  food-stuffs  and  of  an 

ordinary  diet 632 

The  Expenditure  of  Energy. 

§  422.     The  daily  expenditure  as  heat  and  as  work  done.     Calorimeters      .    634 
§  423.     The  energy  of  mechanical  work  does  not  arise  exclusively  from 

proteid  metabolism  ;  urea  and  muscular  exercise         .        .        .    636 

Animal  Heat. 

§  424.     The  sources  and  distribution  of  heat ;  the  muscles  the  chief  source    638 
§  425.     The  temperature   of  the  body ;   cold-blooded  and  warm-blooded 

animals 640 


xxviii  CONTENTS. 

PAGE 

§  426.     The  regulation  of  the  temperature  of  the  body  by  variation  in  the 

loss  of  heat.     The  skin  the  great  regulator 641 

§  427.     The  production  of  heat,   the  circumstances   determining  it ;   the 

effects  of  meals  and  of  labour 643 

§  428.  The  influence  of  the  nervous  mechanism  in  determining  the  pro- 
duction of  heat ;  the  effects  of  heat  and  cold  on  the  metabolism 
of  the  body  are  produced  through  the  nervous  system        .         .     645 

§  429.  The  portions  of  tlie  central  nervous  system  concerned  in  this  regu- 
lation of  heat 647 

§  430.  The  narrow  limits  within  which  the  bodily  temperature  is  main- 
tained      647 

§  431.     Abnormally  high  temperatures,  pyrexia 648 

§  432.     The  effects  of  great  heat 649 

§  433.     The  effects  of  great  cold 650 


SECTION  in. 

On  Nutrition  in  general. 

§  434.     The  general  features  of  metabolism 651 

§  435.     The  metabolism  of  muscle  as  typical  of  the  metabolism  of  tissues  ; 

the  nature  of  the  food  of  muscle 652 

§  436.     The  fate  of  the  lactic  acid  produced  by  muscle  ....     653 

§  437.     A  comparison  of  the  metabolism  of  muscular  tissue  with  that  of 

other  tissues 653 

§  438.     Proteid  substance  the  pivot  of  metabolism 654 

§  439.  The  influence  of  nerves  on  metabolism ;  katabolic  and  anabolic 
nerves.  The  influence  of  the  nervous  system  on  the  general 
nutrition  of  the  tissues.  The  phenomena  of  disease,  &c. ;  trophic 
nerves 654 


SECTION  IV. 

On  Diet. 

§  440.    The  normal  diet,  statistical  and  experimental 658 

§  441.     The  necessity  for  all  three  food-stuffs,  proteids,  fats,  and  carbo- 

hydi-ates.     The   necessity   and   importance   of   salts    including 

extractives  ;  alcohol,  &c 660 

§  442.     The  chemical  value  of  articles  of  food  to  be  corrected  by  their 

digestibility 662 

§  443.  The  physiological  value  of  a  purely  vegetable  diet  ....  664 
§  444.  The  modifications  of  a  normal  diet  needed  to  meet  variations  in  size  667 
§  445.  The  modifications  of  a  normal  diet  needed  to  meet  changes  of  climate  667 
§  446.     The  modifications  of  a  normal  diet  needed  to  promote  or  prevent 

fattening 668 

§  447.     The  modifications  of  a  normal  diet  needed  to  meet  muscular  and 

mental  labour 669 


CONTENTS,  xxix 

BOOK   III. 

THE   CENTRAL  NERVOUS   SYSTEM  AND  ITS  INSTRUMENTS. 

CHAPTEE  I. 

The  Spixal  Cord. 

SECTION  I. 
On  some  Features  of  the  Spinal  Nerves. 

PAGE 

§  448.     The  spinal  nerves 675 

§  449.     On  efferent  and  afferent  impulses 676 

§  450.     Efferent  fibres  run  in  the  anterior  root  and  afferent  fibres  in  the 

posterior  root 677 

§  451.     The  "  trophic  "  influence  of  the  ganglion  of  the  posterior  root ;  the 

degeneration  of  nerve  fibres 678 

SECTION  II. 
The  Structcre  of  the  Spinal  Cord. 

§  452.  The  general  features  of  the  cord ;  grey  and  white  matter.  The 
grouping  of  the  nerve  cells.  The  cells  of  the  anterior  and  pos- 
terior horn,  the  lateral  group,  Clarke's  column,  and  the  lateral 
horn.  The  tracts  of  white  matter.  Median  posterior  column, 
external  posterior  column.  The  evidence  of  the  differentiation 
of  the  white  matter  into  tracts.  Ascending  and  descending 
degeneration.  Descending  tracts :  crossed  and  direct  pyramidal 
tracts,  antero-lateral  descending  tract.  Ascending  tracts  :  cere- 
bellar tract,  antero-lateral  ascending  tract,  median  posterior 
tract 681 

§  453.  The  special  features  of  the  several  regions  of  the  spinal  cord.  The 
conus  medullaris,  the  lumbar,  and  cervical  swellings.  Varia- 
tions in  the  sectional  area  of  the  white  matter     ....    689 

§  454.     Variations  in  the  sectional  area  of  the  grey  matter  ....    691 

§  455.     The  relative  size,  form,  and  features  of  transverse  sections  of  the 

cord  at  different  levels 692 

§  456,     Variations  in  the  several  columns  of  white  matter  at  different  levels    693 

SECTION  III. 
The  Reflex  Actions  of  the  Cord. 

§  457.  The  difficulties  attending  the  experimental  investigation  of  the  cen- 
tral nervous  system  ;  '  shock '  and  other  effects  of  an  operation    696 

§  458.  The  differences,  as  regards  reflex  movements,  between  different 
kinds  of  animals.  The  features  of  a  reflex  act  dependent  on  the 
character  of  the  afferent  impulses 698 


XXX  CONTENTS. 


PAGE 


§  459.     The  characters  of  a  reflex  movement  dependent  on  the  strength  of 

the  stimulus 699 

§  460.    The  characters  of  a  reflex  movement  dependent  on  the  part  of  the 

body  to  which  the  stimulus  is  applied 700 

§  461.     The  complexity  of  many  reflex  movements ;  their  relation  to  intel- 
ligence      700 

§  462.     Keflex  movements  coordinated  by  afferent  impulses  other  than  the 

exciting  impulses 702 

§  463.     The  characters  of  a  reflex  movement  determined  by  the  intrinsic 

condition  of  the  cord 703 

§  464.     The  reflex  movements  carried  out  by  the  spinal  cord  in  man  .        .  704 

§  465.     Reflex  actions  resulting  in  changes  other  than  movements       .        .  706 

§  466.     The  inhibition  of  reflex  actions 707 

§  467.     The  time  required  for  reflex  actions 709 

SECTION  IV. 

The  Automatic  Actions  of  the  Spinal  Cord. 

§  468.    Automatic  actions  of  the  spinal  cord  in  the  frog  and  in  the  dog       .  711 

§  469.     Automatic  activity  dependent  on  afferent  impulses  ....  712 

§  470.     Tone  of  skeletal  muscles 713 

§  471.    Tendon  phenomena,  knee  jerk 717 

§472.     Eigidity  of  muscles  through  spinal  action  .        ,        ...        .        .  717 


CHAPTER  IL 
The  Brain. 
SECTION  I. 


On  the  Phenomena  exhibited  by  an  Animal  deprived  of  its 
Cerebral  Hemispheres. 

§  473.  The  absence  of  distinct  signs  of  volition  and  intelligence  .  .719 
§  474.  The  characters  of  the  movements  of  a  brainless  frog  .  .  .  720 
§  475.     The  phenomena  exhibited  by  birds  after  removal  of  their  cerebral 

hemispheres 723 

§  476.  The  effects  of  removing  the  cerebral  hemispheres  in  mammals  .  725 
§  477.     The  effects  of  removing  the  cerebral  hemispheres  in  the  dog    .        .    727 

SECTION  II. 

The  Machinery  of  Coordinated  Movements. 

§  478.  The  effects  of  injury  to  the  semicircular  canals.  Our  appreciation 
of  the  position  of  our  body,  the  sense  of  equilibrium.  Afferent 
impulses  and  sensations  as  factors  of  the  coordination  of  move- 
ments       729 


CONTENTS.  xxxi 

PAGE 

§  479.     The  phenomena  and  causation  of  vertigo 733 

§  480.     Forced  movements 735 

§  481.     The   parts  of  the  middle  brain  concerned  in  the  coordination  of 

movements 737 


SECTION  m. 

On  Voluntary  Movements. 

§  482.     The  real  distinction  between  voluntary  and  involuntary  movements    739 
§  483.     The  cortical  motor  areas  of  the  dog ;  the  characters  of  the  move- 
ments resulting  from  cortical  stimulation 740 

§  484.     The  cortical  motor  areas  in  the  monkey 744 

§  485.     The  cortical  motor  areas  in  the  anthropoid  ape  ....     748 

§  486.     The  movements  of  cortical  origin  carried  out  by  means  of  the 

pyramidal  tract ;  the  nature  of  the  movements  so  carried  out     .     749 
§  487.     The  results  of  the  removal  of  a  cortical  area  in  dog  and  in  the 

monkey 762 

§  488.     The  cortical  motor  areas  in  man  ;  the  area  for  speech       .         .         .     764 
§  489.     The  nature  of  the  action  of  a  motor  area  in  carrying  out  a  volun- 
tary movement ;  the  characters  of  aphasia.     The  same  as  illus- 
trated by  the   area  for  a  limb  in  the  dog ;   the  influence  of 

sensory  impulses 769 

§  490.     The  relations  of  the  motor  area  to  other  parts  of  the  central  nervous 
system  ;  the  motor  area  employed  in  movements  usually  called 

involuntary 773 

§  491.     The  passage  of  volitional  impulses  along  the  spinal  cord  in  animals    774 

§  492.     Their  passage  in  man 775 

§  493.     A  summary   of  the   chief  facts  concerning  the  carrying  out  of 

voluntary  movements 776 


SECTION  IV. 

On  the  Development  within  the  Central  Nervous  System  of 
Visual  and  op  some  other  Sensations. 

§  494.     Visual  impulses  and  sensations ;  visual  fields,  and  binocular  vision    781 
§  495.     The  decussation  of  the  optic  nerves  in  the  optic  chiasma  .        .        .     784 
§  496.     The  course  of  the  optic  tract.     The  endings  Of  the  optic  tract  in  the 
lateral  corpus  geniculatum,  the  pulvinar  and  the  anterior  corpus 
quadrigemiuum  ;  the  results  of  degeneration  and  atrophy  experi- 
ments       785 

§  497.     The  connection  of  the  three  above  bodies  with  the  cerebral  cortex  ; 
the  meaning  of  the  terms,  blindness  total  and  complete  or  par- 
tial, hemianopsia,  amblyopia.    The  difficulties  of  interpretation 
attending  experiments  on  the  vision  of  animals  ....     787 
§  498.     The  nature  of  the  movements  of  the  eyes  caused  by  stimulation  of 

the  occipital  cortex 700 

§  499.    The  effects  on  vision  of  removing  parts  of  the  occipital  cortex  in 

monkeys  and  in  dogs  ;  the  teachings  of  clinical  histories     .        .    791 


xxxii  CONTENTS. 


PAGE 


§  500.     The  probable  progressive  development  of  visual  sensations ;  lower 

and  higher  visual  centres 793 

§  501.     Sensations  of  smell.    The  cortical  area  for  smell      ....  794 

§  502.     Sensations  of  taste 795 

§  503.     Sensations  of  hearing 796 


SECTION  V. 
On  the  Development  of  Cutaneous  and  Some  Other  Sensations. 

§  504.     Sensations  of  touch,  heat,  cold,  and  pain 798 

§  505.  Theoretical  difficulties  touching  the  cortical  localisation  of  cutaneous 
sensations.  The  effects  on  cutaneous  sensations  of  removing 
regions  of  the  cortex 799 

§  506.     The  afferent  tracts  from  the  spinal  cord,  their  endings  in  the  brain  .     800 

§  507.  The  effect  of  sections  of  the  spinal  cord  on  the  transmission  of 
afferent  impulses  influencing  the  vasomotor  centre.  Other 
experiments  on  animals  as  to  the  effects  of  sections  of  the 
spinal  cord  on  the  transmission  of  sensory  impulses    .        .         .     802 

§  508.     The  teachings  of  clinical  histories ;  different  paths  for   different 

sensory  impulses 804 

§  509.  General  considerations  on  the  development  of  sensations  along  the 
spinal  cord.  The  cerebellar  tract,  the  median  posterior  tract, 
the  grey  matter  and  internuncial  tracts 805 

§  510.     The  terms  '  sensory  '  and  '  motor  '  not  an  adequate  description  of 

the  processes  in  the  central  nervous  system  ....     808 

§  511.     The  transmission  of  sensations  within  the  brain.     The  relations  of 

the  cerebellum 810 


SECTION  VI. 

On  Some  Other  Aspects  of  the  Functions  of  the  Brain. 

§  512.     Considerations  touching  the  cerebellum 812 

§  513.     Considerations  touching  the  corpora  quadrigemina  ....     814 
§  514.     The  splanchnic  functions  of  the  brain 816 

\ 

SECTION  VII. 

On  the  Time  taken  up  by  Cerebral  Operations. 

§  515.     The  reaction  period  or  reaction  time 819 

§  616.  Elementary  analysis  of  psychical  processes,  the  time  taken  up  by 
each.  The  time  required  for  discrimination,  for  the  develop- 
ment of  perception,  and  of  the  will ;  the  circumstances  influenc- 
ing them 821 


CONTENTS.  xxxiii 

SECTION   VIII. 
The  LtiMphatic  Arrangements  of  the  Brain  and  Spinal  Cord. 

I'AGR 

§  517.     The  characters  of  the  cerebro-spinal  fluid 824 

§  518.     The  renewal  of  the  cerebro-spiual  fluid.     The  purposes  served  by 

the  fluid 825 

SECTION  IX. 

The  Vascular  Arrangements  of  the  Brain  and  Spinal  Cord. 

§  519.     The  distribution  and  characters  of  the  arteries  of  the  brain      .         .     827 

§  520.     The  venous  arrangements  of  the  brain 828 

§  521.     The  supply  of  blood  to  the  brain  relatively  small.     The  methods  of 

investigating  the  circulation  of  the  brain 829 

§  522.     The  supply  of  blood  to  the  brain  modified  by  the  respiration  and  by 

changes  in  the  general  arterial  pressure.     The  vrant  of  clear 

proof  of  special  vasomotor  nerves  for  the  cerebral  arteries  .  .  831 
§  523.     The  flow  of  blood  through  the  brain  nevertheless  influenced  by 

changes  taking  place  in  the  brain  itself 833 


CHAPTEE  III. 
Sight. 

SECTION  I. 

On  the  General  Structure  of  the  Eye,  and  on  the  Formation  of 
the  Retinal  Image. 

§  524.     Dioptic  mechanisms  and  visual  impulses 834 

§  525.     The  general  structure  of  the  eye.     The  formation  of  the  retinal 

image 835 

§  526.     A  simple  optic  system  ;  its  cardinal  points.     The  refractive  surfaces 

and  media  of  the  eye 839 

§  527.     The  optic  constants  of  the  eye.    The  diagrammatic  eye    .        .        .  841 

§  528.     The  paths  of  the  rays  of  light  through  the  eye 843 

§  529.     The  retinal  image  in  relation  to  the  sensations  excited  by  it    .        .  845 

SECTION   II. 

The  Facts  of  Accommodation. 

§  530.     The  eye  can  accommodate  for  far  and  near  objects  ;  far  and  near 

limits  of  accommodation 846 

§  531.     Scheiner's  experiment 847 


xxxiv  CONTENTS. 

PAGE 

§  532.     Emmetropic,  myopic,  hypermetropic,  and  presbyopic  eyes      .        .    849 
§  533.     The  changes  observed  in  the  eye  during  accommodation  .        .        .    850 


SECTION  III. 

The  Mechanism  op  Accommodation  and  the  Movements  op 
THE  Pupil. 

§  534.     The  mechanism  for  changing  the  anterior  curvature  of  the  lens       .    854 
§  535.    The  evidence  that  such  a  mechanism  does  effect  the  result       .        .    856 

The  Changes  in  the  Pupil. 

§  5-36.     Circumstances  leading  to  constriction  and  to  dilation  of  the  pupil  .  857 

§  537.     Constriction  and  dilation 858 

§  538.     The  nerves  supplying  the  pupil 859 

§  539.     Constriction  a  reflex  act  by  means  of  the  optic  and  oculo-motor 

nerves 860 

§  540.     Changes  in  the  pupil  through  the  action  of  the  cervical  sympathetic 

nerve 862 

§  541.     The  nature  of  the  dilating  mechanism 863 

§  542.     Direct  action  of  drugs  and  other  agencies  on  the  pupil      .        '.        .  866 

§  543.     The  nervous  mechanism  of  accommodation 868 

§  544.     The  association  of  the  movements  of  accommodation  and  the  move- 
ments of  the  pupil 868 


SECTION  IV. 

Imperfections  in  the  Dioptric  Apparatus.  , 

§  545.     Imperfections  of  accommodation 870 

§  546.     Spherical  aberration 871 

§  547.     Astigmatism 871 

§  548.     Chromatic  aberration  .         .         .' 873 

§  549.     Entoptic  phenomena 874 

SECTION   V. 
On  Some  General  Features  op  Visual  Sensations. 

§  550.     The  relation  of  the  sensation   to  the  intensity  of  the  stimulus; 

Weber's  law 878 

§  551.     The  relation  of  the  sensation  to  the  duration  of  the  stimulus    .         .  881 

§  552.     Flickering  and  continuous  sensations 883 

§  553.     Sensations  produced  by  various  changes  in  the  retina  referred  to 

some  external  source  of  light 883 

§  554,     Localisation  of  visual  sensations 885 

§  565.     The  conditions  of  discrete  visual  sensations 886 

§  556.     The  region  of  distinct  vision.     The  limits  of  distinct  vision      .         .  887 

§  557.     Nature  of  the  discreteness  of  visual  sensations  ;  retinal  visual  units  889 


COI^TENTS  XXXV 

SECTION   VI. 
On  Colour  Sensations. 

PAGE 

§  558.     The  existence  of  many  kinds  of  colour  sensations      ....  891 

§  659.     The  mixing  of  colour  sensations 892 

§  5G0.     The  several  usual  colour  sensations  result   from   the  mixture  of 

simpler,  primary  sensations 893 

§  561.     The  conditions  which  determine  the  characters  of  colour  sensations  895 

§  562.     Complementary  colours 896 

§  563.     Any  colour  sensation  produced  by  the  suitable  mixture  of  three 

colour  sensations 897 

§  564.  The  Young-Helmholtz  theory  of  colour  sensations  ....  898 
§  565.     Hering's  theory  of  colour  sensations.     A  comparison  of  the  two 

theories 900 

§  566.     Variations  in  colour  vision.      Colour  blindness.      The  different 

kinds  of  colour  blindness ;  red  blind  and  green  blind ;  the 

Young-Helmholtz  explanation  of  them 906 

§  567.     The  explanation  of  colour  blindness  onHering's  theory  .         .         .  909 

§  568.     The  probable  subjective  condition  of  the  colour  blind       .        .        .  911 

§  569.     Blue  or  violet  blindness  ;  absolute  colour  blindness  ....  912 

§  570.     Colour  blindness  in  the  periphery  of  the  retina         ....  913 

§  571.     The  influence  of  the  yellow  spot 913 

v^  572.     Colour  sensations  in  relation  to  the  intensity  of  the  stimulus  .        .  913 


SECTION  VII. 

On  the  Development  of  Visual  Impulses. 

§  573.     The  blind  spot 916 

§  574.     Pui-kinje's  figures  ;  their  import 917 

§  575.     Possible  theories  as  to  the  mode  of  origin  of  visual  sensations .        .  920 
§  576.     Photochemistry  of   the   retina ;   visual   purple  ;  the   pigment   epi- 
thelium     922 

§  577.     The  functions  of  the  layer  of  rods  and  cones.    The  ophthalmoscope  926 

§  578.     Possible  differences  of  function  of  rods  and  cones     ....  927 

§  579.     Electric  currents  in  the  retina 928 


SECTION   VIII. 

On  Some  Features  of  Visual  Sensations  especially  in  Kelation 
TO  Visual  Perceptions. 

§  580.     Simultaneous  visual  sensations  ;  the  vi.sual  field        ....  929 
§  581.     The  psychological  and  physiological  methods  ;  sensations  and  per- 
ceptions ;  their  want  of  agreement 929 

§  582.     Irradiation 932 

§  583.     Simultaneous  contrast  ....  ....  932 

§  584.     After-images.     Successive  contrast    .,,....  933 


xxxvi  CONTENTS. 

PAGE 

§  585.     The  phenomena  of  '  contrast '  in  their  bearing  on  the  theories  of 

colour  Yision 935 

§  586.     Recurrent  sensations.     Ocular  phantoms  or  hallucinations      .        .    936 


SECTION  IX. 

Binocular  Vision. 

§  587.     The  movements  of  the  eye-ball ;  their  limitations.     Centre  of  rota- 
tion, visual  axis,  visual  plane 939 

§  588.     The  visual  field  and  field  of  sight  of  one  eye  and  of  both  eyes .        .  940 

§  589.     Corresponding  or  identical  points 942 

§  590.     The  movements  of  the  eye-ball ;  the  primary  position  and  secondary 

positions  ;  the  kind  of  movements  vfhich  are  possible         .        .  944 

§  591.     Listing's  Law  ;  the  experimental  proof 946 

§592.     The  muscles  of  the  eye-ball        .       • 948 

§  593.     The  action  of  the  ocular  muscles        .......  949 

§  594.     The  nervous  mechanism  of  the  movements  of  the  eye-balls ;  the 

coordination  of  the  movements 952 

§  595.     The  nervous  centres  for  the  movements  of  the  eye-balls  .        .        .  956 

§  596.     The  Horopter 957 


SECTION  X. 

On  Some  Features  of  Visual  Perceptions  and  on  Visual  Judgments. 

§  597.     On  the  differences  between  the  objective  field  of  sight  and  the  sub- 
jective field  of  vision 959 

§  598.     The  psychical  processes  belonging  to  visual  perceptions ;  illusions 

and  visual  judgments 961 

§  599.     Appreciation  of  apparent  size 962 

§  600.     Judgment  of  distance  and  of  actual  size 964 

§  601.     The  judgment  of  solidity 966 

§  602.     The  struggle  of  the  two  fields  of  vision       .        .        .        .        .        .967 


SECTION  XI. 

The  Nutrition  of  the  Eye. 

§  603.     The  arrangement  of  the  blood  vessels 969 

§  604.     The  vaso-motor  changes  in  the  eye 970 

The  Lymphatics  of  the  Eye. 

§  605.  The  lymphatic  vessels  and  lymph  spaces  of  the  eye  ....  970 
§  606.  The  aqueous  humour  ;  the  changes  taking  place  in  it ;  how  effected  972 
§  607.     The  vitreous  humour  ;  the  changes  taking  place  in  it        .         .         .     974 


CONTENTS. 


xxxvn 


SECTION  XII. 
The  Protective  Mechanisms  of  the  Eye. 


§  608.     The  eye-lids  and  their  muscles 

§  609.     The  conjunctiva  and  its  glands.    Tears.     The  secretion  of  tears 


PAGE 

976 

977 


CHAPTER  IV. 
Hearing. 


SECTION  I. 

On  the  General  Structure  of  the  Ear  and  on  the  Structure 
AND  Functions  of  the  Subsidiary  Auditory  Apparatus. 

§  610.     The  embryonic  history  of  the  ear.    The  otic  vesicle        .        .        .      980 
§611.     The  general  relations  of  the  parts  of  the  ear;   vestibule  and 
cochlea,  membranous  and  bony  labyrinth,  tympanum,  auditory 
ossicles,  membrana  tympani  and  external  meatus      .         .        .       981 
§  612.     The  general  use  of  the  several  parts 986 


§  613. 
§614. 

§615. 
§610. 
§617. 
§618. 


The  Conduction  of  Sound  through  the  Tympanum. 

The  chain  of  ossicles  as  a  lever 

Longitudinal  and  transversal  sonorous  vibrations.     The  vibrations 

of  the  tympanic  membrane 

The  conduction  of  vibrations  through  the  chain  of  ossicles 
The  conduction  of  vibrations  through  the  bones  of  the  skull 
The  action  of  the  tensor  tympani  and  stapedius  muscles 
The  Eustachian  tube 


991 
992 
993 
995 


SECTION  II. 

On  Auditory  Sensations. 

§  619.     Noises  and  musical  sounds 998 

§  G20.     The  characters  of  musical  sounds  ;  loudness,  pitch  and  quality  ; 

fundamental  and  partial  tones 998 

§  621.     The  limits  of  auditory  sensations 1000 

§  622.     Appreciation  of  differences  of  pitch 1001 

§  623.     The  number  of  vibrations  needed  to  excite  a  sensation  .        .        .  1001 

§  624.     The  characters  of  noises 1002 

§  625.     The  effects  of  exhaustion 1002 

§  626.     The  fusion  of  auditory  sensations 1003 

fj  627.     The  interference  of  vibrations.     Beats 1004 


xxxviii  CONTEI^TS. 

SECTION  III 
On  the  Development  of  Auditory  Impulses. 

PAGE 

§  628.     The  transmission  of  impulses  through  the  labyrinth  ;  the  functions 

of  the  hair  cells  of  the  cochlea 1007 

§  629.     The  analysis  of  complex  waves  of  sound ;  theories  as  to  the  mode 

of  action  of  the  organ  of  Corti 1013 

§  630.    The  appreciation  of  the  nature  of  sounds  ultimately  a  psychical 

process 1015 

§  631.     Auditory  functions  of  the  vestibular  labyrinth        ....     1016 

SECTION  IV. 

On  Auditory  Perceptions  and  Judgments. 

§  632.     Auditory  phantoms 1020 

§  633.     The  appreciation  of  outwardness  in  sounds  is  connected  with  the 

tympanum 1021 

§  634.     Hearing  binaural.    The  judgment  of  the  direction  of  sounds  .        .  1022 

§  635.     Judgment  of  the  distance  of  sounds 1023 


CHAPTER  V. 

Taste  and  Smell. 

SECTION  I. 

Olfactory  Sensations. 

§  636.     The  sensation  due  to  contact  of  particles  with  the  membrane        .     1025 

§  637.     The  chief  characters  of  olfactory  sensations 1026 

§  638.     Olfactory  judgments.    The  olfactory  nerve  the  nerve  of  smell        .     1027 

SECTION  II. 
Gustatory  Sensations. 

§  639.     Sensations  of  taste  usually  or  frequently  accompanied  by  other 

sensations 1029 

§  640.     The  different  kinds  of  taste.     Sensations  of  taste  provoked  by 

mechanical  and  electrical  stimulation 1029 

§  641.     The  conditions  under  which  taste  sensations  are  excited        .        .     1031 

§  642.     The  distribution  of  the  several  kinds  of  tastes.    Theories  as  to  the 

mode  of  origin  of  taste  sensations 1032 

§  643.    The  nerves  of  taste  ;  the  chorda  tympani 1035 


COI^TE:^fTS.  xxxix 

CHAPTER  VI. 

On  Cutaneous  and  Some  Other  Sensations. 

SECTION  I. 
The  General  Features  of  Cutaneous  Sensations. 

PAGE 

§  644.    Three  kinds  of  cutaneous  sensations,  of  pressure,  of  heat  and  cold, 

and  of  pain 1037 

Tactile  Sensations  or  Sensations  of  Pressure. 

§  645.     The  general  characters  of  tactile  sensations 1037 

§  646.     The  localisation  of  tactile  sensations 1039 

Sensations  of  Heat  and  Cold. 

§  647.     Sensations  of  heat  and  cold  due  to  sudden  changes  in  the  tem- 
perature of  the  skin 1041 

§  648.    The  general  characters  of  temperature  sensations   ....     1042 
§  649.     Tactile  and  temperature  sensations  in  parts  other  than  the  external 

skin 1043 

SECTION  II. 

On  Painful  and  Some  Other  Kinds  of  Sensation. 

§  650.  Sensations  of  pain  distinct  from  other  sensations   ....  1044 

§651.  Sensations  of  pain  are  extreme  degrees  of  common  sensibility       .  1044 

§  652.  Special  nerve  endings  not  necessary  for  sensations  of  pain     .        .  1047 

§  653.  Hunger  and  thirst 1048 

SECTION  III. 
On  the  Mode  of  Development  of  Cutaneous  Sensations. 

§  654.  The  specific  energy  of  nerves.  Special  terminal  organs  necessary 
for  the  sensations  of  touch  and  temperature  as  distinguished 
from  sensations  of  pain 1051 

§  655.     The  terminal  organs  for  sensations  of  pressure  different  from  those 

for  sensations  of  temperature 1054 

§  656.     The  terminal  organs  for  sensations  of  heat  different  from  those 

for  sensations  of  cold 1055 

§  657.     The  importance  of  contrast  in  cutaneous  sensations        .        .        .     1056 

§  658.     The  nature  of  the  terminal  organs 1057 


xl 


CONTENTS. 


SECTION  IV 

The  Muscular  Sense. 

PAeE 

§  659.     We  possess  a  sense  of  '  movement,'  of  ' position,'  and  of  ' effort' .  1059 

§  660.    The  muscular  sense  distinguished  from  the  sense  of  effort      .        .  1060 

§  661.     The  afferent  impulses  forming  the  basis  of  the  muscular  sense  are 

distinct  from  cutaneous  impulses 1061 

§  662.     They  are  derived  from  the  muscles,  ligaments,  and  tendons  .        .  1063 


SECTION  V. 

On  Tactile  Perceptions  and  Judgments. 

§  663.     The  ties  between  touch  and  the  muscular  sense      .        .        .        .  1066 

§  664.    The  ties  between  touch  and  sight 1067 

§  665.     Cutaneous  sensations  may  arise  otherwise  than  from  cutaneous 

events 1068 

§  666.     Tactile  Illusions 1069 


CHAPTER   VII. 
02sr  Some  Special  Muscular  Mechanisms. 


SECTION  I. 


The  Voice. 


667. 


§669. 
§670. 

§  671. 

§672. 
§673. 
§674. 

§675. 

§676. 
§677. 


The  laryngoscopic  view  of  the  larynx 

The  fundamental  features  of  the  voice ;    loudness,   pitch,  and 

quality.     The  main  conditions  of   the  utterance  of  voice 

adduction  and  tightening  of  the  vocal  cords  . 

The  muscles  of  the  larynx 

The  action  of  the  muscles  in  reference  to  narrowing  and  widening 

the  glottis  and  to  tightening  and  slackening  the  vocal  cords 
The  nervous  mechanisms  of  the  larynx.     The  respiratory  move 

ments  of  the  larynx 

The  nervous  mechanism  of  phonation 

The  cortical  area  for  movements  of  the  larynx 

The  different  kinds  of  voice.     Changes  in  the  glottis  other  than 

those  of  mere  adduction  and  general  tension     . 
Chest-voice  and  head- voice.    The  registers  of  the  voice.    The  com 

plexity  of  the  laryngeal  movements 

The  uses  of  the  ventricles  and  other  parts  of  the  larynx 
The  '  break '  in  the  voice  at  puberty 


1070 


1074 
1076 

1080 

1081 
1084 
1084 

1085 

1087 
1091 
1091 


CONTENTS.  xH 

SECTION  IL 
Speech. 

PAGE 

§  678.    Speech,  a  mixture  of  musical  sounds  and  of  noises         .        .        .  1092 

§  679.     Vowels  and  consonants 1092 

§  680.    The  manner  of  formation  of  the  several  vowels      ....  1093 
§  681.    The  manner  of  formation  of  the  several  groups  of  consonants      .  1096 
§  682.    The  manner  of  formation  of  the  more  important  individual  con- 
sonants           1097 

SECTION   III. 

On  some  Locomotor  Mechanisms. 

§  683.    The  general  characters  of  the  actions  of  skeletal  muscles      .        .  1101 

§  684.    The  erect  posture 1102 

§685.    Walking 1102 


BOOK   IV. 

THE  TISSUES  AND  MECHANISMS  OF  KEPRODUCTION, 
§  686.    The  general  features  of  reproduction 1109 

CHAPTER  I. 

Impregnation. 

SECTION  I. 

Menstruation. 

§  687.    The  transference  of  the  ovum  from  the  ovary  to  the  uterus.    The 

changes  in  the  uterine  mucous  membrane 1111 

SECTION  II. 

The  Male  Organs. 

§  688.    The  movements  of  the  spermatozoa 1114 

§  689.    The  chemical  constituents  of  semen.    The  vesiculae  seminales  and 

prostate 1115 


xiii  co:ntents. 


PAGE 


§  690.    Erectile  tissue ,        .        .        .    1115 

§  691.    The  nature  of  erection 1116 

§  692.     The  emission  of  semen 1117 


CHAPTEE.  II. 
Pregnancy  and  Bikth. 

SECTION    I. 

The  Placenta. 

§  693.    The  spermatozoon  enters  and  unites  with  the  ovum        .        .        .  1119 

§  694.    The  formation  of  the  decidua 1119 

§  695.    The  decidua  serotina  is  transformed  into  the  placenta.     The  shed- 
ding of  the  placenta 1120 


SECTION  II. 

The  Ndtkition  of  the  Embryo. 

§  696.    The  embryo  breathes  by  and  feeds  upon  the  maternal  blood  of  the 

placenta 1123 

§  697.    The  blood  and  blood-flow  in  the  umbilical  arteries  and  umbilical 

vein 1124 

§  698.    The  amniotic  fluid,  its  nature  and  origin,  its  relations  to  the  nutri- 
tion of  the  foetus 1126 

§  699.     The  transmission  of  food  material  from  the  mother  to  the  foetus  .  1128 

§  700.    Glycogen  in  the  foetus 1129 

§  701.    The  movements  of  the  fostus 1130 

§  702.    The  digestive  functions  of  the  foetus 1130 

§703.    The  foetal  circulation  towards  the  close  of  uterine  life    .        •        .  1132 

§  704.    The  cause  of  the  first  breath 1134 

§705.    The  changes  in  the  circulation  taking  place  at  birth       .        .        .1134 

SECTION  III. 

Parturition. 

§  706.     The  period  of  gestation 1136 

§707.     The  events  of  "  labour "  .        . 1137 

§708.     The  reflex  nature  of  parturition 1140 

§709.    The  nerves  concerned  in  the  act 1141 

§710.    The  causes  determining  the  onset  of  labour 1142 

.§  711.    The  inhibition  of  parturition 1142 


CONTENTS.  xliii 

CHAPTEE  III. 

The  Phases  of  Life. 

PAGE 

§  712.     The  composition  of  the  babe  as  compared  with  the  adult        .        .  1144 

§  713,     The  curve  of  growth  from  birth  onwards 1145 

§  714.     The  characters  of  the  nutrition  of  the  babe  and  infant  .         .        .  1146 

§715.    The  nervous  system  of  the  babe 1148 

§  716.     Dentition 1149 

§  717.     Puberty.     Differences  of  sex 1150 

§718.     Old  age 1151 

§  719.     Periodical  events 1153 

§  720.     Sleep 1153 

§721.     Other  diurnal  changes  in  the  functions 1156 


CHAPTER  IV. 

Death. 
§  722.     The  general  causation  of  death 1158 

INDEX '.        .     1161 


LIST   OF   FIGUEES. 


PIG 


PAGE 

1.  A  muscle-nerve  preparation 56 

2.  Diagram  of  du  Bois-Reymond  key 58 

3.  Diagram  illustrating  apparatus  arranged  for  experiments  with  muscle 

and  nerve 60 

4.  Diagram  of  an  induction  coil 62 

5.  The  magnetic  interrupter 63 

6.  The  magnetic  interruptor  with  Helmholtz's  arrangement  for  equaliz- 

ing the  make  and  break  shocks 64 

7.  A  muscle-curve  from  the  gastrocnemius  of  a  frog 66 

8.  The  same,  with  the  recording  surface  moving  slowly      ....  66 

9.  The  same,  with  the  recording  surface  travelling  very  rapidly        .        .  67 

10.  The  pendulum  myograph 68 

11.  Diagram  of  an  arrangement  of  a  vibrating  tuning-fork  with  a  Desprez 

signal 70 

12.  Curves  illustrating  the  measurement  of  the  velocity  of  a  nervous  impulse  73 

13.  Tracing  of  a  double  muscle-curve 76 

14.  Muscle-curve.     Single  induction-shocks  repeated  slowly       ...  76 

15.  The  same,  repeated  more  rapidly 77 

16.  The  same,  repeated  still  more  rapidly 77 

17.  Tetanus  produced  with  the  ordinary  magnetic  interruptor    ...  78 

18.  Non-polarizable  electrodes 98 

19.  Diagram  illustrating  the  electric  currents  of  nerve  and  muscle     .        .  99 

20.  Diagram  illustrative  of  the  progression  of  electric  changes   .         .        .  103 

21.  Diagram  of  ascending  and  descending  constant  current        .         .         .112 

22.  Diagram  of  the  electrotonic  changes  in  irritability        .         .        .         .114 

23.  Diagram  illustrating  electrotonic  currents 115 

24.  Scheme  of  the  nerves  of  a  segment  of  the  spinal  cord   ....  140 

25.  Apparatus  for  investigating  blood  pressure 157 

26.  Tracing  of  arterial  pressure  in  dog 158 

27.  Tracing  of  arterial  pressure  in  rabbit 159 

28.  Lud wig's  kymograph 160 

29.  Diagram  of  fall  of  blood  pressure  in  arteries,  capillaries  and  veins       .  161 

30.  Arterial  scheme 167 

31.  Tracing  from  arterial  model  with  little  peripheral  resistance         .         .  168 

32.  The  same  with  increased  peripheral  resistance 169 

33.  Ludwig's  stromuhr 173 

34.  Chauveau  and  Lortet's  hsematachometer 174 

35.  Diagram  illustrating  causes  determining  the  velocity  of  the  flow  .        .  176 

36.  Tracing  from  heart  of  cat 186 

xliv 


LIST   OF  FIGURES.  xlv 

FIG.  PAGE 

37.  Marey's  tambour,  with  cardiac  sound 192 

38.  Tracings  from  right  auricle  and  ventricle  of  horse  (Chauveauand  Marey)  193 

39.  Cui'ves  of  endocardiac  pressure  by  means  of  piston  manometer   .         .  194 

40.  The  membrane  manometer  of  Hiirthle 194 

41.  Diagram  of  the  same 195 

42.  Curve  of  ventricular  pressure  :  membrane  manometer ....  19G 

43.  Stolnikow's  apparatus  for  measuring  the  output  of  the  heart       .        .  198 

44.  Cardiometer  of  Roy  and  Adami 199 

45.  Tracing  from  the  heart  of  a  cat,  by  means  of  a  light  lever   .        .        .  200 
40.     Cardiograms 201 

47.  Myocardiogram 202 

48.  Diagram  of  application  of  aortic  and  ventricular  catheters   .        .        .  203 

49.  Simultaneous  tracings  of  ventricular  and  aortic  pressure       .        .         .  204 
60.  Diagram  of  the  differential  manometer  of  Hiirthle         ....  204 

51.  Simultaneous  curves  of  aortic  and  ventricular  pressure,  and  of  the 

differential  manometer ;  descending  systolic  plateau        .        .         .  205 

52.  The  same,  with  the  recording  surface  travelling  rapidly        .         .         .  205 

53.  Simultaneous  curves  of  aortic   and  ventricular  pressure  and  of   the 

differential  manometer  ;  ascending  sj'stolic  plateau          .        .        .  208 

54.  Diagi'am  of  ventricular  and  aortic  pressure  and  of  the  cardiac  impulse  209 

55.  Maximum  and  minimum  manometer 210 

56.  Fick's  spring  manometer 220 

57.  Diagram  of  a  sphygmogi-aph 221 

58.  Pulse  tracing  from  radial  artery 223 

59.  Diagi-am  of  artificial  pulse  tracings 224 

60.  Diagram  of  progression  of  pulse  wave 225 

61.  Pulse  tracing  with  different  pressures 226 

62.  Pulse  tracing  from  dorsalis  pedis  artery 227 

G3.     Pulse  tracing  from  carotid  artery 230 

64.  Anacrotic  pulse  tracing 231 

65.  Dicrotic  pulse  tracing 231 

66.  A  perfusion  cannula 240 

07.  Diagram  of  apparatus  for  registering  the  beat  of  a  frog's  heart    .        .  241 

68.  Inhibition  of  heart  beat  in  the  frog 245 

69.  Diagram  of  the  course  of  cardiac  augmentor  fibres  in  the  frog     .        .  247 

70.  Cardiac  inhibition  in  the  mammal 250 

71.  The  course  of  cardial  inhibitory  and  augmentor  fibres  in  the  dog.        .  254 

72.  Diagram  of  the  course  of  vaso-constrictor  fibres 266 

73.  Diagram  of  the  nerves  of  the  submaxillary  gland 267 

74.  The  depressor  nerve 281 

75.  Rise  of  pressure  due  to  stimulation  of  the  sciatic  nerve  .        .  282 

76.  Diagrammatic  representation  of  the  submaxillary  gland  of  the  dog 

with  its  nerves  and  blood  vessels 334 

77.  Alveoli  of  the  pancreas  of  a  rabbit  at  rest  and  in  activity     .        .        .  341 

78.  Changes  in  the  parotid  gland  during  secretion 343 

79.  Sections  of  the  parotid  gland  of  the  rabbit  at  rest  and  after  stimula- 

tion of  the  cervical  sympathetic  nerve 344 

80.  Mucous  cells  from  the  fresh  submaxillary  gland  of  the  dog  .        .        .  344 

81.  Alveoli  of  dog's  submaxillary  gland  in  loaded  and  in  discharged  phases  345 

82.  Gastric  gland  of  mammal  (bat)  during  activity 346 


xlvi  LIST   OF  EIGUEES. 

FIG.  PAGE 

83.  Diagram  illustrating  the  influence  of  food  on  the  secretion  of  the 

pancreatic  juice 366 

84.  Diagram  to  illustrate  the  nerves  of  the  alimentary  canal  in  the  dog  .  384 

85.  Apparatus  for  taking  tracings  of  the  movements  of  the  column  of  air 

in  respiration 429 

86.  Tracing  of  thoracic  respiratory  movements 432 

87.  Diagram  of  Ludwig's  mercurial  gas-pump 444 

88.  Diagram  of  Alvergniat's  pump 446 

89.  The  spectra  of  oxy-hsemoglobin  in  different  grades  of  concentration, 

of  (reduced)  haemoglobin,  and  of  carbonic-oxide-hgemoglobin       .  452 

90.  Spectra  of  some  derivatives  of  haemoglobin 459 

91.  Curve  of  the  effect  on  respiration  of  section  of  one  vagus    .         .         .  476 

92.  Curve  of  the  effect  on  respiration  of  section  of  both  vagus  nerves      .  477 

93.  Curve  of  the  quickening  of  respiration  by  gentle  stimulation  of  the 

central  end  of  the  vagus  trunk 477 

94.  Curve  of  respiratory  increase  due  to  stimulation  of  vagus  nerve         .  478 

95.  Curve  of  the  inhibitory  effects  of  stimulation  of  the  superior  laryngeal 

nerve 480 

96.  Curves  illustrating  the  effects  of  distension  and  collapse  of  the  lung  .  481- 

97.  Curve  shewing  the  effects  of  repeated  inflations  of  the  lungs      .        .  482 

98.  Curve  shew^ing  the  effects  of  repeated  suctions  of  the  lungs        .        .  482 

99.  Curves  of  blood-pressure  and  intra-thoracic  pressure  taken  together .  498 

100.  Curves  of  blood-pressure  during  a  suspension  of  breathing          .        .  505 

101.  Curve  shewing  Traube-Heriug  undulations 508 

102.  Eenal  oncometer 526' 

103.  Oncograph 527 

104.  Tracing  from  renal  oncometer 528 

105.  Section  of  the  liver  of  frog 564 

106.  Three  phases  of  the  hepatic  cells  of  the  frog 565 

107.  Section  of  mammalian  liver  rich  in  glycogen 568 

108.  Section  of  mammalian  liver  containing  little  or  no  glycogen       .        .  568 

109.  Normal  spleen  curve  from  dog 580 

110.  A  transverse  dorso-ventral  section  of  the  spinal  cord  (human)  at  the 

level  of  the  sixth  thoracic  nerve 682 

111.  Transverse  dorso-ventral  section  of  the  spinal  cord  (human)  at  the 

level  of  the  sixth  cervical  nerve 683- 

112.  Transverse  dorso-ventral  section  of  the  spinal  cord  (human)  at  the 

level  of  the  third  lumbar  nerve         .         .         .        .       '  .         .         .  685 

113.  Diagram  to  illustrate  the  general  arrangement  of  the  several  tracts  of 

white  matter  in  the  spinal  cord 686-687 

114.  Diagram  illustrating  some  of  the  features  of  the  spinal  cord  at  differ- 

ent levels 688 

115.  Diagram  shewing  the  united  sectional  areas  of  the  spinal  nerves  pro- 

ceeding from  below  upwards 690 

116.  Diagram  shewing  the  variations  in  the  sectional  area  of  the  grey 

matter  of  the  spinal  cord,  along  its  length 691 

117.  Diagiam  shewing  the  relative  sectional  areas  of  the  spinal  nerves  as 

they  join  the  spinal  cord 691 

118.  Diagram  shewing  the  variations  in  the  sectional  area  of  the  lateral 

columns  of  the  spinal  cord,  along  its  length 694 


LIST   OP  yiGUEES.  xlvii 

FIG.  PAGE 

119.  Diagram  shewing  the  variations  in  the  sectional  area  of  the  anterior 

columns  of  the  spinal  cord,  along  its  length 694 

120.  Diagram  shewing  the  variations  in  the  sectional  area  of  the  posterior 

columns  of  tlie  spinal  cord,  along  its  length 694 

121.  The  areas  of  the  cerebral  convolutions  of  the  dog        ....  741 

122.  Outline  of  brain  of  monlcey  to  shew  the  principal  sulci  and  gyri        .  744 

123.  Left  hemisphere  of  the  brain  of  monkey  viewed  from  the  left  side  and 

from  above 745 

124.  Mesial  aspect  of  the  left  half  of  the  brain  of  monkey  ....  747 

125.  Outline  of  horizontal  section  of  brain,  to  shew  the  internal  capsule   .  750 

126.  Outline  of  a  sagittal  section  through  tlie  hemispliere  ....  751 

127.  Outline  of  a  transverse  dorso-ventral  section  of  the  right  half  of  the 

brain 752 

128.  Transverse  dorso-ventral  section  through  the  crus  and  anterior  corpora 

quadrigemina 75S 

129.  Transverse  dorso-ventral  section  through  the  fore  part  of  the  pons    .  754 

130.  Transverse  dorso-ventral  section  through  the  pons  at  the  exit  of  the 

iifth  nerve 755 

131.  Transverse  dorsal  section  through  the  bulb  at  the  widest  part  of  the 

fourth  ventricle 756 

132.  Transverse  dorso-ventral  section  through  the  bulb  just  behind  the  pons  757 

133.  Diagram  to  illustrate  the  relative  size  of  the  pyramidal  tract  in  man, 

monkey  and  dog 761 

134.  Diagram  of  the  convolutions  and  fissures  on  the  lateral  surface  of  the 

right  cerebral  hemisphere  of  man 767 

135.  The  same  on  the  mesial  surface 767 

136.  The  right  lateral  aspect  of  the  cerebiTim  of  man  in  outline,  to  illus- 

trate the  cortical  areas 768 

137.  Mesial  surface  of  the  right  cerebral  hemisphere  of  man  in  outline,  to 

illustrate  the  cortical  areas 768 

138.  Diagram  to  illustrate  the  nervous  apparatus  of  vision  in  man     .        .  783- 

139.  Diagrammatic  outline  of  a  horizontal  section  of  the  eye,  to  illustrate 

the  relations  of  the  various  parts 836 

140.  Diagram  of  simple  optical  system 840 

141.  Diagram  of  the  schematic  or  diagrammatic  eye          ....  843 

142.  Diagram  of  the  formation  of  a  retinal  image 844 

143.  Diagram  of  Scheiner's  experiment 848 

144.  Diagram  of  images  reflected  from  the  eye          .....  852 

145.  Diagram  of  the  ciliary  muscle  as  seen  in  a  vertical  radial  section  of 

the  ciliary  region    ..........  855 

146.  Diagram  to  illustrate  accommodation 856 

147.  Diagrammatic  representation  of  the  nerves  governing  the  pupil         .  859 

148.  Diagram  illustrating  chromatic  aberration 873 

149.  Diagram  to  illustrate  entoptical  images 876 

150.  Diagram  of  three  primary  colour  sensations       .....  899 

151.  Diagram  to  illustrate  Hering's  theoiy  of  colour  vision        .         .        .  902 

152.  Diagram  illustrating  the  formation  of  Purkinjg's  figures  when  the 

illumination  is  directed  through  the  sclerotic         ....  918 

153.  Diagram  illustrating  the  formation  of  PurkinjS's  figures  when  the 

illumination  is  directed  through  the  cornea    .....  919 


xlviii  LIST   OF, FIGURES. 

FIG.  PAGE 

154.  Diagram  to  illustrate  the  principles  of  a  simple  form  of  opthalmoscope  927 

155.  rigure  to  illustrate  irradiation 932 

156.  The  visual  field  of  the  right  eye 941 

157.  The  visual  fields  (fields  of  sight)  of  the  two  eyes  w^hen  the  eyes 

converge  to  the  same  fixed  point .......  942 

158.  Diagram  illustrating  corresponding  points 943 

159.  Figure  to  illustrate  the  insertions  of  the  ocular  muscles  .         .         .  949 

160.  Diagram  to  illustrate  the  actions  of  the  ocular  muscles     .        .        .  950 

161.  Diagram  illustrating  a  simple  horopter 957 

162.  Figure  to  illustrate  the  appreciation  of  apparent  size         .        .         .  962 

163.  The  same 963 

164.  rigure  to  illustrate  an  optical  effect  produced  by  parallel  slanting 

lines 963 

165.  Figure  to  illustrate  binocular  vision 967 

166.  Diagram  to  illustrate  the  general  structure  of  the  ear      .         .         .  982 

167.  The  bony  labyrinth .  983 

168.  The  membrana  tympani    . 984 

169.  Diagram  to  illustrate  the  relations  of  auditory  passage,  tympanum 

and  Eustachian  tube 984 

170.  Frontal  section  through  the  tympanum     ......  985 

171.  Diagram  of  the  median  wall  of  the  tympanum         ....  985 

172.  The  auditory  ossicles 986 

173.  The  ossicles  in  position 987 

174.  The  ligaments  of  the  ossicles 988 

175.  The  malleus  and  incus  in  position 989 

176.  Diagram  of  the  outer  wall  of  the  tympanum  as  seen  from  the  mesial 

side 994 

177.  The  stapes  in  position 995 

178.  The  membranous  labyrinth  as  seen  from  above         ....  1008 

179.  The  membranous  labyrinth  and  the  endings  of  the  auditory  nerve    .  1009 

180.  Diagram  of  a  transverse  section  of  a  whorl  of  the  cochlea       .         .  1010 

181.  Diagram  of  the  organ  of  Corti 1011 

182.  Diagram  of  the  constituents  of  the  organ  of  Corti  ....  1012 

183.  Diagram  of  a  laryngoscopic  view  of  the  larynx        ....  1071 

184.  Diagram  of  the  superior  aperture  of  the  larynx        ....  1072 

185.  Diagram  of  the  larynx  in  vertical  section  ......  1072 

186.  Diagram  of  the  larynx  in  vertical  transverse  section         .         .         .  1073 

187.  The  larynx  as  seen  by  means  of  the  laryngoscope  in  different  con- 

ditions of  the  glottis 1075 

188.  Diagram  of  the  transverse  and  oblique  arytenoid  and  of  the  posterior 

crico-arytenoid  muscles 1076 

189.  Diagram  to  illustrate  the  thyro-aryteuoid  muscles     ....  1077 

190.  The  internal  thyro-arytenoid  muscle 1078 

191.  The  lateral  crico-arytenoid  muscle 1079 

192.  The  crico-thyroid  muscle 1079 

193.  Diagram  to  illustrate  the  contact  of  the  feet  with  the  ground  in 

walking 1104 

194.  Diagram  to  illustrate  running 1105 

195.  Diagram  to  illustrate  the  fcetal  circulation 1133 


INTRODUCTION. 


§  1.  Dissection,  aided  by  microscopical  examination,  teaches 
us  that  the  body  of  man  is  made  up  of  certain  kinds  of  material, 
so  differing  from  each  other  in  optical  and  other  physical  characters 
and  so  built  up  together  as  to  give  the  body  certain  structural 
features.  Chemical  examination  further  teaches  us  that  these 
kinds  of  material  are  composed  of  various  chemical  substances,  a 
large  number  of  which  have  this  characteristic  that  they  possess  a 
considerable  amount  of  potential  energy  capable  of  being  set  free, 
rendered  actual,  by  oxidation  or  some  other  chemical  change. 
Thus  the  body  as  a  whole  may,  from  a  chemical  point  of  view,  be 
considered  as  a  mass  of  various  chemical  substances,  representing 
altogether  a  considerable  capital  of  potential  energy. 

§  2.  This  body  may  exist  either  as  a  living  body  or  (for  a 
certain  time  at  least)  as  a  dead  body,  and  the  living  body  may  at 
any  time  become  a  dead  body.  At  what  is  generally  called  the 
moment  of  death  (but  artificially  so,  for  as  we  shall  see  the 
processes  of  death  are  numerous  and  gradual)  the  dead  body  so 
far  as  structure  and  chemical  composition  are  concerned  is  exceed- 
ingly like  the  living  body  ;  indeed  the  differences  between  the  two 
are  such  as  can  be  determined  only  by  very  careful  examination, 
and  are  still  to  a  large  extent  estimated  by  drawing  inferences 
rather  than  actually  observed.  At  any  rate  the  dead  body  at 
the  moment  of  death  resembles  the  living  body  in  so  far  as  it 
represents  a  capital  of  potential  energy.  From  that  moment 
onwards  however  the  capital  is  expended ;  by  processes  which 
are  largely  those  of  oxidation,  the  energy  is  gradually  dissipated, 
leaving  the  body  chiefly  in  the  form  of  heat.  While  these  chemi- 
cal processes  are  going  on  the  structural  features  disappear,  and 
the  body,  with  the  loss  of  nearly  all  its  energy,  is  at  last  resolved 
into  "  dust  and  ashes." 


2  THE  LIVING  AND  THE  DEAD  BODY. 

The  characteristic  of  the  dead  body  then  is  that,  being  a  mass 
of  substances  of  considerable  potential  energy,  it  is  always  more 
or  less  slowly  losing  energy  never  gaining  energy ;  the  capital  of 
energy  present  at  the  moment  of  death  is  more  or  less  slowly 
diminished,  is  never  increased  or  replaced. 

§  3.  When  on  the  other  hand  we  study  a  living  body  we  are 
struck  with  the  following  salient  facts. 

1.  The  living  body  moves  of  itself,  either  moving  one  part  of 
the  body  on  another  or  moving  the  whole  body  from  place  to  place. 
These  movements  are  active ;  the  body  is  not  simply  pulled  or 
pushed  by  external  forces,  but  the  motive  power  is  in  the  body 
itself,  the  energy  of  each  movement  is  supplied  by  the  body  itself. 

2.  These  movements  are  determined  and  influenced,  indeed 
often  seem  to  be  started,  by  changes  in  the  surroundings  of  the  body. 
Sudden  contact  between  the  surface  of  the  body  and  some  foreign 
object  will  often  call  forth  a  movement.  The  body  is  sensitive  to 
changes  in  its  surroundings,  and  this  sensitiveness  is  manifested 
not  only  by  movements  but  by  other  changes  in  the  body. 

8.  It  is  continually  generating  heat  and  giving  out  heat  to 
surrounding  things,  the  production  and  loss  of  heat,  in  the  case 
of  man  and  certain  other  animals,  being  so  adjusted  that  the 
whole  body  is  warm,  —  that  is,  of  a  temperature  higher  than  that 
of  surrounding  things. 

4.  From  time  to  time  it  eats,  —  that  is  to  say,  takes  into  itself 
supplies  of  certain  substances  known  as  food,  these  substances 
being  in  the  main  similar  to  those  which  compose  the  body  and 
being  like  them  chemical  bodies  of  considerable  potential  energy, 
capable  through  oxidation  or  other  chemical  changes  of  setting 
free  a  considerable  quantity  of  energy. 

5.  It  is  continually  breathing,  —  that  is,  taking  in  from  the 
surrounding  air  supplies  of  oxygen. 

6.  It  is  continually,  or  from  time  to  time,  discharging  from 
itself  into  its  surroundings  so-called  waste  matters,  which  waste 
matters  may  be  broadly  described  as  products  of  oxidation  of  the 
substances  taken  in  as  food,  or  of  the  substances  composing  the 
body. 

Hence  the  living  body  may  be  said  to  be  distinguished  from 
the  dead  body  by  three  main  features. 

The  living  body  like  the  dead  is  continually  losing  energy 
(and  losing  it  more  rapidly  than  the  dead  body,  the  special 
breathing  arrangements  permitting  a  more  rapid  oxidation  of  its 
substance),  but  unlike  the  dead  body  is  by  means  of  food  contin- 
ually restoring  its  substance  and  replenishing  its  store  of  energy. 

The  energy  set  free  in  the  dead  body  by  the  oxidation  and 
other  chemical  changes  of  its  substance  leaves  the  body  almost 
exclusively  in  the  form  of  heat,  whereas  a  great  deal  of  energy 
leaves  the  living  body  as  mechanical  work,  the  result  of  various 
movements  of  the  body ;  and  as  we  shall  see  a  great  deal  of  the 


INTRODUCTION.  3 

energy  which  ultimately  leaves  the  body  as  heat  exists  for  a  while 
within  the  living  body  in  other  forms  than  heat,  though  eventually 
transformed  into  heat. 

The  changes  in  the  surroundings  affect  the  dead  body  at  a 
slow  rate  and  in  a  general  way  only,  simply  lessening  or  increasing 
the  amount  or  rate  of  chemical  change  and  the  quantity  of 
heat  thereby  set  free,  but  never  diverting  the  energy  into  some 
other  form,  such  as  that  of  movement ;  whereas  changes  in  the  sur- 
roundings may  in  the  case  of  the  living  body  rapidly,  profoundly, 
and  in  special  ways  affect  not  only  the  amount  but  also  the  kind  of 
energy  set  free.  The  dead  body  left  to  itself  slowly  falls  to  pieces, 
slowly  dissipates  its  store  of  energy,  and  slowly  gives  out  heat.  A 
higher  or  lower  temperature,  more  or  less  moisture,  a  free  or  scanty 
supply  of  oxygen,  the  advent  of  many  or  few  putrefactive  organ- 
isms, —  these  may  quicken  or  slacken  the  rate  at  which  energy  is 
being  dissipated  but  do  not  divert  that  energy  from  heat  into 
motion  ;  whereas  in  the  living  body  so  slight  a  change  of  surround- 
ings as  the  mere  touch  by  a  hair  of  some  particular  surface,  may 
so  affect  the  setting  free  of  energy  as  to  lead  to  such  a  discharge 
of  energy  in  the  form  of  movement  that  the  previously  apparently 
quiescent  body  may  be  suddenly  thrown  into  the  most  violent 
convulsions. 

The  differences  therefore  between  living  substance  and  dead 
substance  though  recondite  are  very  great,  and  the  ultimate  object 
of  Physiology  is  to  ascertain  how  it  is  that  living  substance  can  do 
what  dead  substance  cannot,  —  can  renew  its  substance  and  replen- 
ish the  energy  which  it  is  continually  losing,  and  can  according  to 
the  nature  of  its  surroundings  vary  not  only  the  amount  but  also 
the  kind  of  energy  which  it  sets  free.  Thus  there  are  two  great 
divisions  of  Physiology :  one  having  to  do  with  the  renewal  of 
substance  and  the  replenishment  of  energy,  the  other  having  to 
do  with  the  setting  free  of  energy. 

§  4.  Now,  the  body  of  man  (or  one  of  the  higher  animals)  is  a 
very  complicated  structure  consisting  of  different  kinds  of  mate- 
rial which  we  call  tissues,  such  as  muscular,  nervous,  connective, 
and  the  like,  variously  arranged  in  organs,  such  as  heart,  lungs, 
muscles,  skin,  etc.,  all  built  up  to  form  the  body  according  to 
certain  morphological  laws.  But  all  this  complication,  though 
advantageous  and  indeed  necessary  for  the  fuller  life  of  man,  is 
not  essential  to  the  existence  of  life.  The  amoeba  is  a  living 
being  ;  it  renews  its  substance,  replenishes  its  store  of  energy,  and 
sets  free  energy  now  in  one  form,  now  in  another ;  and  yet  the 
amoeba  may  be  said  to  have  no  tissues  and  no  organs ;  at  all  events 
this  is  true  of  closely  allied  but  not  so  well-known  simple  beings. 
Using  the  more  familiar  amoeba  as  a  type,  and  therefore  leaving  on 
one  side  the  nucleus,  and  any  distinction  between  endosarc  and 
ectosarc,  we  may  say  that  its  body  is  homogeneous  in  the  sense 
that  if  we  divided  it  into  small  pieces,  each  piece  would  be  like  all 


4  PEOTOPLASM. 

the  others.  In  another  sense  it  is  not  homogeneous;  for  we 
know  that  the  amoeba  receives  into  its  substance  material  as  food, 
and  that  this  food  or  part  of  it  remains  lodged  in  the  body  until 
it  is  made  use  of  and  built  up  into  the  living  substance  of  the 
body ;  and  each  piece  of  the  living  substance  of  the  body  must 
have  in  or  near  it  some  of  the  material  which  it  is  about  to  build 
up  into  itself.  Further,  we  know  that  the  amoeba  gives  out  waste 
matters,  such  as  carbonic  acid  and  other  substances  ;  and  each  piece 
of  the  amoeba  must  contain  some  of  these  waste  matters  about  to 
be,  but  not  yet,  discharged  from  the  piece.  Each  piece  of  the 
amoeba  will  therefore  contain  these  three  things  :  the  actual  living 
substance,  the  food  about  to  become  living  substance,  and  the 
waste  matters  which  have  ceased  to  be  living  substance. 

Moreover,  we  have  reasons  to  think  that  the  living  substance 
does  not  break  down  into  the  waste  matters  which  leave  the  body 
at  a  single  bound,  but  that  there  are  stages  in  the  downward 
progress  between  the  one  and  the  other.  Similarly,  though  our 
knowledge  on  this  point  is  less  sure,  we  have  reason  to  think 
that  the  food  is  not  incorporated  into  the  living  substance  at  a. 
single  step,  but  that  there  are  stages  in  the  upward  progress 
from  the  dead  food  to  the  living  substance.  Each  piece  of  the^ 
body  of  the  amoeba  will  therefore  contain  substances  represent- 
ing various  stages  of  becoming  living,  and  of  ceasing  to  b& 
living,  as  well  as  the  living  substance  itself.  And  we  may 
safely  make  this  statement  though  we  are  quite  unable  to  draw 
the  line  where  the  dead  food  on  its  way  up  becomes  living,  or  the 
living  substance  on  its  way  down  becomes  dead. 

§  5.  Nor  is  it  necessary  for  our  present  purpose  to  be  able  to- 
point  out  under  the  microscope,  or  to  describe  from  a  histological 
point  of  view,  the  parts  which  are  living  and  the  parts  which  are 
dead  food  or  dead  waste.  The  body  of  the  amoeba  is  frequently 
spoken  of  as  consisting  of  '  protoplasm.'  The  name  was  originally 
given  to  the  matter  forming  the  primordial  utricle  of  the  vegetable 
cell  as  distinguished  from  the  cell  wall  on  the  one  hand,  and  from 
the  fluid  contents  of  the  cell  or  cell  sap  on  the  other,  and  also 
we  may  add  from  the  nucleus.  It  has  since  been  applied  very 
generally  to  such  parts  of  animal  bodies  as  resemble,  in  their 
general  features,  the  primordial  utricle.  Thus  the  body  of  a  white 
blood  corpuscle,  or  of  a  gland  cell,  or  of  a  nerve  cell,  is  said  tO' 
consist  of  protoplasm.  Such  parts  of  animal  bodies  as  do  not  in 
their  general  features  resemble  the  matter  of  the  primordial  utricle- 
are  not  called  protoplasm,  or,  if  they  at  some  earlier  stage  did  bear 
such  resemblance,  but  no  longer  do  so,  are  sometimes,  as  in  the  case 
of  the  substance  of  a  muscular  fibre,  called  '  differentiated  proto- 
plasm.' Protoplasm  in  this  sense  sometimes  appears,  as  in  the 
outer  part  of  most  amoebae,  as  a  mass  of  glassy-looking  material, 
either  continuous  or  interrupted  by  more  or  less  spherical  spaces 
or  vacuoles  filled  with  fluid,  sometimes  as  in  a  gland  cell  as  a  more 


INTRODUCTIOK  5 

refractive,  cloudy-looking,  or  finely  granular  material  arranged  in  a 
more  or  less  irregular  network,  or  spongework,  the  interstices  of 
which  are  occupied  either  by  fluid  or  by  some  material  different  from 
itself.  We  shall  return  however  to  the  features  of  this  '  proto- 
plasm '  when  we  come  to  treat  of  white  blood  corpuscles  and  other 
'  protoplasmic  '  structures.  Meanwhile  it  is  sufficient  for  our  pres- 
ent purpose  to  note  that  lodged  in  the  protoplasm,  discontinuous 
with  it,  and  forming  no  part  of  it,  are  in  the  first  place  collections 
of  fluid,  of  watery  solutions  of  various  substances,  occupying  the 
more  regular  vacuoles  or  the  more  irregular  spaces  of  the  network, 
and  in  the  second  place  discrete  granules  of  one  kind  or  another, 
also  forming  no  part  of  the  protoplasm  itself,  but  lodged  either  in  the 
bars  or  substance  of  the  protoplasm  or  in  the  vacuoles  or  meshes. 

Now,  there  can  be  little  doubt  that  the  fluids  and  the  discrete 
granules  are  dead  food  or  dead  waste,  but  the  present  state  of 
our  knowledge  will  not  permit  us  to  make  any  very  definite 
statement  about  the  protoplasm  itself.  We  may  probably  conclude, 
indeed  we  may  be  almost  sure,  that  protoplasm  in  the  above  sense 
is  not  all  living  substance ;  that  it  is  made  up  partly  of  the  real 
living  substance,  and  partly  of  material  which  is  becoming  living 
or  has  ceased  to  be  living ;  and  in  the  case  where  protoplasm  is 
described  as  forming  a  network,  it  is  possible  that  some  of  the 
material  occupying  the  meshes  of  the  network  may  be,  like  part  of 
the  network  itself,  really  alive.  '  Protoplasm '  in  fact,  as  in  the 
sense  in  which  we  are  now  using  it,  and  shall  continue  to  use  it, 
is  a  morphological  term ;  but  it  must  be  borne  in  mind  that  the 
same  word  '  protoplasm '  is  also  frequently  used  to  denote  what 
we  have  just  now  called 'the  real  living  substance.'  The  word 
then  embodies  a  j^hi/siological  idea ;  so  used  it  may  be  applied  to 
the  living  substance  of  all  living  structures,  whatever  the  micro- 
scopical features  of  those  structures ;  in  this  sense  it  cannot  at 
present,  and  possibly  never  will  be  recognised  by  the  microscope, 
and  our  knowledge  of  its  nature  must  be  based  on  inferences. 

Keeping  then  to  the  phrase  '  living  substance '  we  may  say 
that  each  piece  of  the  body  of  the  amcBba  consists  of  living 
substance  in  which  are  lodged,  or  with  which  are  built  up  in 
some  way  or  other,  food  and  waste  in  various  stages. 

Kow,  an  amceba  may  divide  itself  into  two,  each  half  exhibiting 
all  the  phenomena  of  the  whole ;  and  we  can  easily  imagine  the 
process  to  be  repeated  until  the  amoeba  was  divided  into  a 
multitude  of  exceedingly  minute  amoeba?,  each  having  all  the 
properties  of  the  original.  But  it  is  obvious,  as  in  the  like 
division  of  a  mass  of  a  chemical  substance,  that  the  division  could 
not  be  repeated  indefinitely.  Just  as  in  division  of  the  chemical 
inass  we  come  to  the  chemical  molecule,  further  division  of  which 
changes  the  properties  of  the  substance,  so  in  the  continued 
division  of  the  amoeba  we  should  come  to  a  stage  in  which  further 
division  interfered  with  the  physiological  actions;  we  should  come 


6  DIVISION   OF  LABOUR. 

to  a  physiological  unit,  corresponding  to  but  greatly  more  complex 
than  the  chemical  molecule.  This  unit  to  remain  a  physiological 
unit  and  to  continue  to  live  must  contain  not  only  a  portion  of 
the  living  substance  but  also  the  food  for  that  living  substance, 
in  several  at  least  of  the  stages,  from  the  initial  rav7  food  up  to  the 
final  '  living '  stages,  and  must  similarly  contain  various  stages  of 
waste. 

§  6.  Now  the  great  characteristic  of  the  typically  simple 
amoeba  (leaving  out  the  nucleus)  is  that,  so  far  as  we  can  ascer- 
tain, all  the  physiological  units  are  alike ;  they  all  do  the  same 
things.  Each  and  every  part  of  the  body  receives  food  more  or 
less  raw  and  builds  it  up  into  its  own  living  substance;  each 
and  every  part  of  the  body  may  be  at  one  time  quiescent  and 
at  another  in  motion ;  each  and  every  part  is  sensitive  and 
responds  by  movement  or  otherwise  to  various  changes  in  its  sur- 
roundings. 

The  body  of  man,  in  its  first  stage,  while  it  is  as  yet  an  ovum, 
if  we  leave  aside  the  nucleus  and  neglect  differences  caused  by  the 
unequal  distribution  of  food  material  or  yolk,  may  also  be  said  to 
be  composed  of   like  parts  or  like  physiological  units. 

By  the  act  of  segmentation  however  the  ovum  is  divided  into 
parts  or  cells  which  early  shew  differences  from  each  other ;  and 
these  differences  rapidly  increase  as  development  proceeds.  Some 
cells  put  on  certain  characters  and  others  other  characters  ;  that 
is  to  say,  the  cells  undergo  histological  differentiation.  And  this 
takes  place  in  such  a  way  that  a  number  of  cells  lying  together 
in  a  group  become  eventually  converted  into  a  tissue  ;  and  the 
whole  body  becomes  a  collection  of  such  tissues  arranged  together 
according  to  morphological  laws,  each  tissue  having  a  definite 
structure,  its  cellular  nature  being  sometimes  preserved,  sometimes 
obscured  or  even  lost. 

This  histological  differentiation  is  accompanied  by  a  physio- 
logical division  of  lahoitr.  Each  tissue  may  be  supposed  to  be 
composed  of  physiological  units,  the  units  of  the  same  tissue  being 
alike  but  differing  from  the  units  of  other  tissues ;  and  corre- 
sponding to  this  difference  of  structure,  the  units  of  different 
tissues  behave  or  act  differently.  Instead  of  all  the  units  as  in 
the  amoeba  doing  the  same  things  equally  well,  the  units  of  one 
tissue  are  told  off  as  it  were  to  do  one  thing  especially  well,  or 
especially  fully,  and  thus  the  whole  labour  of  the  body  is  divided 
among  the  several  tissues. 

§  7.  The  several  tissues  may  thus  be  classified  according  to 
the  work  which  they  have  to  do ;  and  the  first  great  distinction  is 
into  (1)  the  tissues  which  are  concerned  in  the  setting  free  of 
energy  in  special  ways,  and  (2)  the  tissues  which  are  concerned 
in  replenishing  the  substance  and  so  renewing  the  energy  of  the 
body. 

Each  physiological  unit  of  the  amoeba  while  it  is  engaged  in 


INTRODUCTION.  7 

setting  free  energy  so  as  to  move  itself,  and  by  reason  of  its 
sensitiveness  so  directing  that  energy  as  to  produce  a  movement 
suitable  to  the  conditions  of  its  surroundings,  has  at  the  same 
time  to  bear  the  labour  of  taking  in  raw  food,  of  selecting  that 
part  of  the  raw  food  which  is  useful  and  rejecting  that  which 
is  useless,  and  of  working  up  the  accepted  part  through  a  variety 
of  stages  into  its  own  living  substance  ;  that  is  to  say,  it  has  at 
the  same  time  that  it  is  feeling  and  moving  to  carry  on  the  work 
of  digesting  and  assimilating.  It  has  moreover  at  the  same  time 
to  throw  out  the  waste  matters  arising  from  the  changes  taking 
place  in  its  own  substance,  having  first  brought  these  waste 
matters  into  a  condition  suitable  for  being  thrown  out. 

§  8.  In  the  body  of  man,  movements,  as  we  shall  see,  are  broadly 
speaking  carried  out  by  means  of  muscular  tissue,  and  the  changes 
in  muscular  tissue  which  lead  to  the  setting  free  of  energy  in  the 
form  of  movement  are  directed,  governed,  and  adapted  to  the 
surroundings  of  man,  by  means  of  nervous  tissue.  Eays  of  light 
fall  on  the  nervous  substance  of  the  eye  called  the  retina,  and  set 
up  in  the  retina  changes  which  induce  in  the  optic  nerve  other 
changes,  which  in  turn  are  propagated  to  the  brain  as  nervous 
impulses,  both  the  excitation  and  the  propagation  involving  an 
expenditure  of  energy.  These  nervous  impulses  reaching  the  brain 
may  induce  other  nervous  impulses  which  travelling  down  certain 
nerves  to  certain  muscles  may  lead  to  changes  in  those  muscles 
by  which  they  suddenly  grow  short  and  pull  upon  the  bones  or 
other  structures  to  which  they  are  attached,  in  which  case  we  say 
the  man  starts ;  or  the  nervous  impulses  reaching  the  brain  may 
produce  some  other  effects.  Similarly,  sound  fallmg  on  the  ear, 
or  contact  between  the  skin  and  some  foreign  body,  or  some  change 
in  the  air  or  other  surroundings  of  the  body,  or  some  change  within 
the  body  itself  may  so  affect  the  nervous  tissue  of  the  body  that 
nervous  impulses  are  started  and  travel  to  this  point  or  to  that, 
to  the  brain  or  elsewhere,  and  eventually  may  either  reach  some 
muscular  tissue  and  so  give  rise  to  movements,  or  may  reach 
other  tissues  and  produce  some  other  effect. 

The  muscular  tissue  then  may  be  considered  as  given  up  to 
the  production  of  movement,  and  the  nervous  tissue  as  given 
up  to  the  generation,  transformation,  and  propagation  of  nervous 
impulses.  In  each  case  there  is  an  expenditure  of  energy,  which 
in  the  case  of  the  muscle,  as  we  shall  see,  leaves  the  body  partly 
as  heat,  and  partly  as  work  done,  but  in  the  case  of  nervous  tissue 
is  wholly  or  almost  wholly  transformed  into  heat  before  it  leaves 
the  body ;  and  this  expenditure  necessitates  a  replenishment  of 
energy  and  a  renewal  of  substance. 

§9.  In  order  that  these  master  tissues  —  the  nervous  and 
muscular  tissues  —  may  carry  on  their  important  works  to  the  best 
advantage,  they  are  relieved  of  much  of  the  labour  which  falls  upon 
each  physiological  unit  of  the  amoeba.     They  are  not  presented 


8  TISSUES  AND  ORGANS. 

with  raw  food ;  they  are  not  required  to  carry  out  the  necessary 
transformations  of  their  immediate  waste  matters.  The  whole  of 
the  rest  of  the  body  is  engaged  (1)  in  so  preparing  the  raw  food, 
and  so  bringing  it  to  the  nervous  and  muscular  tissues  that  these 
may  build  it  up  into  their  own  substance  with  the  least  trouble  ; 
and  (2)  in  receiving  the  waste  matters  which  arise  in  muscular 
and  nervous  tissues,  and  preparing  them  for  rapid  and  easy 
ejection  from  the  body. 

Thus  to  certain  tissues,  which  we  may  speak  of  broadly  as 
*  tissues  of  digestion,'  is  allotted  the  duty  of  acting  on  the  food  and 
preparing  it  for  the  use  of  the  muscular  and  nervous  tissues  ;  and 
to  other  tissues,  which  we  may  speak  of  as  '  tissues  of  excretion,' 
is  allotted  the  duty  of  clearing  the  body  from  the  waste  matters 
generated  by  the  muscular  and  nervous  tissues. 

§  10.  These  tissues  are  for  the  most  part  arranged  in  machines 
or  mechanisms  called  organs,  and  the  working  of  these  organs  in- 
volves movement.  The  movements  of  these  organs  are  carried  out, 
like  the  other  movements  of  the  body,  chiefly  by  means  of  muscular 
tissue  governed  by  nervous  tissue.  Hence  we  may  make  a  dis- 
tinction between  the  muscles  which  are  concerned  in  producing  an 
effect  on  the  world  outside  man's  body  —  the  muscles  by  which 
man  does  his  work  in  the  world  —  and  the  muscles  which  are  con- 
cerned in  carrying  out  the  m()vements  of  the  internal  organs  ;  and 
we  may  similarly  make  a  distinction  between  the  nervous  tissue 
concerned  in  carrying  out  the  external  work  of  the  body  and  that 
concerned  in  regulating  the  movements  and,  as  we  shall  see,  the 
general  conduct  of  the  internal  organs.  But  these  two  classes  of 
muscular  and  nervous  tissue  though  distinct  in  work  and,  as  we 
shall  see,  often  different  in  structure,  are  not  separated  or  isolated. 
On  the  contrary,  while  it  is  the  main  duty  of  the  nervous  tissue  as 
a  whole  (the  nervous  system,  as  we  may  call  it)  to  carry  out,  by 
means  of  nervous  impulses  passing  hither  and  thither,  what  may 
be  spoken  of  as  the  work  of  man,  and  in  this  sense  is  the  master 
tissue,  it  also  serves  as  a  bond  of  union  between  itself  and  the 
muscles  doing  external  work  on  the  one  hand,  and  the  organs  of 
digestion  or  excretion  on  the  other,  so  that  the  activity  and  con- 
duct of  the  latter  may  be  adequately  adapted  to  the  needs  of  the 
former. 

§  11.  Lastly,  the  food  prepared  and  elaborated  by  the  digestive 
organs  is  carried  and  presented  to  the  muscular  and  nervous 
tissues  in  the  form  of  a  complex  fluid  known  as  blood,  which 
driven  by  means  of  a  complicated  mechanism  known  as  the 
vascular  system,  circulates  all  over  the  body,  visiting  in  turn  all 
the  tissues  of  the  body,  and  by  a  special  arrangement  known  as 
the  respiratory  mechanism,  carrying  in  itself  to  the  several  tissues 
a  supply  of  oxygen  as  well  as  of  food  more  properly  so  called. 

The  motive  power  of  this  vascular  system  is  supplied  as  in  the 
case  of   the  digestive  system  by  means  of  muscular  tissue,  the 


INTRODUCTIOK  9 

activity  of  which  is  similarly  governed  by  the  nervous  system,  and 
hence  the  flow  of  blood  to  this  part  or  that  part  is  regulated 
according  to  the  needs  of  the  part. 

§  12.  The  above  slight  sketch  will  perhaps  suffice  to  shew 
not  only  how  numerous  but  how  varied  are  the  problems  with 
which  Physiology  has  to  deal. 

In  the  first  place  there  are  what  may  be  called  general  prob- 
lems, such  as,  How  the  food  after  its  preparation  and  elaboration 
into  blood  is  built  up  into  the  living  substance  of  the  several 
tissues  ?  How  the  living  substance  breaks  down  into  the  dead 
waste  ?  How  the  building  up  and  breaking  down  differ  in  the 
different  tissues  in  such  a  way  that  energy  is  set  free  in  different 
modes,  —  the  muscular  tissue  contracting,  the  nervous  tissue  thrill- 
ing with  a  nervous  impulse,  the  secreting  tissue  doing  chemical 
work,  and  the  like  ?  To  these  general  questions  the  answers  which 
we  can  at  present  give  can  hardly  be  called  answers  at  all. 

In  the  second  place  there  are  what  may  be  called  special 
problems,  such  as.  What  are  the  various  steps  by  which  the  blood 
is  kept  replenished  with  food  and  oxygen,  and  kept  free  from  an 
accumulation  of  waste,  and  how  is  the  activity  of  the  digestive, 
respiratory,  and  excretory  organs,  which  effect  this,  regulated  and 
adapted  to  the  stress  of  circumstances  ?  What  are  the  details 
of  the  working  of  the  vascular  mechanism  by  which  each  and 
every  tissue  is  forever  bathed  with  fresh  blood,  and  how  is  that 
working  delicately  adapted  to  all  the  varied  changes  of  the  body  ? 
And,  compared  with  which  all  other  special  problems  are  insignifi- 
cant and  preparatory  only.  How  do  nervous  impulses  so  flit  to  and 
fro  within  the  nervous  system  as  to  issue  in  the  movements  which 
make  up  what  we  sometimes  call  the  life  of  man  ?  It  is  to  these 
special  problems  that  we  must  chiefly  confine  our  attention,  and 
we  may  fitly  begin  with  a  study  of  the  blood. 


BOOK   I. 

BLOOD.      THE   TISSUES   OF   MOVEMENT.      THE 
VASCULAE  MECHANISM. 


CHAPTER  I. 
BLOOD. 

§  13.  The  several  tissues  are  traversed  by  minute  tubes,  —  the 
capillary  blood  vessels,  —  to  which  blood  is  brought  by  the  arteries, 
and  from  which  blood  is  carried  away  by  the  veins.  These 
capillaries  form  networks  the  meshes  of  which,  differing  in  form 
and  size  in  the  different  tissues,  are  occupied  by  the  elements  of 
the  tissue  which  consequently  lie  outside  the  capillaries. 

The  blood  flowing  along  the  capillaries  consists,  under  normal 
conditions,  of  an  almost  colourless  fluid,  the  plasma,  in  which 
are  carried  a  number  of  bodies,  the  red  and  the  white  corpuscles. 
Outside  the  capillary  walls,  filling  up  such  spaces  as  exist  between 
the  capillary  walls  and  the  cells  or  fibres  of  the  tissue,  or  between 
the  elements  of  the  tissue  themselves,  is  found  a  colourless  fluid, 
resembling  in  many  respects  the  plasma  of  blood  and  called 
lymph.  Thus  all  the  elements  of  the  tissue  and  the  outsides  of 
all  the  capillaries  are  bathed  with  lymph,  which,  as  we  shall  see 
hereafter,  is  continually  flowing  away  from  the  tissue  along 
special  channels  to  pass  into  lymphatic  vessels  and  thence  into 
the  blood. 

As  the  blood  flows  along  the  capillaries  certain  constituents 
of  the  plasma  (together  with,  at  times,  white  corpuscles,  and 
under  exceptional  circumstances  red  corpuscles)  pass  through 
the  capillary  wall  into  the  lymph,  and  certain  constituents  of  the 
lymph  pass  through  the  capillary  wall  into  the  blood  within  the 
capillary.  There  is  thus  an  interchange  of  material  between 
the  blood  within  the  capillary  and  the  lymph  outside.  A  similar 
interchange  of  material  is  at  the  same  time  going  on  between  the 
lymph  and  the  tissue  itself.  Hence,  by  means  of  the  lymph  acting 
as  middleman,  a  double  interchange  of  material  takes  place  between 
the  blood  within  the  capillary  and  the  tissue  outside  the  capillary. 
In  every  tissue,  so  long  as  life  lasts  and  the  blood  flows  through 
the  blood  vessels,  a  double  stream,  now  rapid  now  slow,  is  passing 
from  the  blood  to  the  tissue  and  from  the  tissue  to  the  blood. 
The  stream  from  the  blood  to  the  tissue  carries  to  the  tissue 
the  material  which  the  tissue  needs  for  building  itself  up  and 
for  doing    its  work,  including   the    all-important   oxygen.     The 


14  BLOOD  AN  INTERNAL   MEDIUM.  [Book  i. 

stream  from  the  tissue  to  the  blood  carries  into  the  blood  certain 
of  the  products  of  the  chemical  changes  which  have  been  taking 
place  in  the  tissue,  —  products  which  may  be  simple  waste,  to  be 
cast  out  of  the  body  as  soon  as  possible,  or  which  may  be  bodies 
capable  of  being  made  use  of  by  some  other  tissue. 

A  third  stream,  that  from  the  lymph  lying  in  the  chinks  and 
crannies  of  the  tissue  along  the  lymph  channels  to  the  larger 
lymph  vessels,  carries  away  from  the  tissue  such  parts  of  the 
material  coming  from  the  blood  as  are  not  taken  up  by  the  tissue 
itself  and  such  parts  of  the  material  coming  from  the  tissue  as  do 
not  find  their  way  into  the  blood  vessel. 

In  most  tissues,  as  in  muscle  for  instance,  the  capillary  net- 
work is  so  close  set  and  the  muscular  fibre  lies  so  near  to  the 
blood  vessel  that  the  lymph  between  the  two  exists  only  as  a  very 
thin  sheet ;  but  in  some  tissues,  as  in  cartilage,  the  blood  vessels 
lie  on  the  outside  of  a  large  mass  of  tissue,  the  interchange  be- 
tween the  central  parts  of  which  and  the  nearest  capillary 
blood  vessel  is  carried  on  through  a  long  stretch  of  lymph.  But 
in  each  case  the  principle  is  the  same ;  the  tissue,  by  the  help  of 
lymph,  lives  on  the  blood ;  and  when  in  succeeding  pages  we 
speak  of  changes  between  the  blood  and  the  tissues,  it  will  be 
understood,  whether  expressly  stated  so  or  no,  that  the  changes 
are  effected  by  means  of  the  lymph.  The  blood  may  thus  be 
regarded  as  an  internal  medium  bearing  the  same  relations  to 
the  constituent  tissues  that  the  external  medium,  the  world,  does 
to  the  whole  individual.  Just  as  the  whole  organism  lives  on  the 
things  around  it,  its  air  and  its  food,  so  the  several  tissues  live  on 
the  complex  fluid  by  which  they  are  all  bathed  and  which  is  to 
them  their  immediate  air  and  food. 

All  the  tissues  take  up  oxygen  from  the  blood  and  give  up 
carbonic  acid  to  the  blood,  but  not  always  at  the  same  rate  or  at 
the  same  time.  Moreover  the  several  tissues  take  up  from  the 
blood  and  give  up  to  the  blood  either  different  things  or  the  same 
things  at  different  rates  or  at  different  times. 

From  this  it  follows,  on  the  one  hand,  that  the  composition  and 
characters  of  the  blood  must  be  for  ever  varying  in  different  parts 
of  the  body  and  at  different  times ;  and  on  the  other  hand,  that 
the  united  action  of  all  the  tissues  must  tend  to  establish  and 
maintain  an  average  uniform  composition  of  the  whole  mass  of 
blood.  The  special  changes  which  blood  is  known  to  undergo 
while  it  passes  through  the  several  tissues  will  best  be  dealt  with 
when  the  individual  tissues  and  organs  come  under  our  considera- 
tion. At  present  it  will  be  sufficient  to  study  the  main  features 
which  are  presented  by  blood,  brought,  so  to  speak,  into  a  state  of 
equilibrium  by  the  common  action  of  all  the  tissues. 

Of  all  these  main  features  of  blood,  the  most  striking  if  not 
the  most  important  is  the  property  it  possesses  of  clotting  when 
shed. 


SEC.   1.     THE   CLOTTING   OF  BLOOD. 


§  14.  Blood,  when  shed  from  the  blood  vessels  of  a  living  body, 
is  perfectly  tiuid.  In  a  short  time  it  becomes  viscid  :  it  flows  less 
readily  from  vessel  to  vessel.  The  viscidity  increases  rapidly  until 
the  whole  mass  of  blood  under  observation  becomes  a  complete 
jelly.  The  vessel  into  which  it  has  been  shed  can  at  this  stage  be 
inverted  without  a  drop  of  the  blood  being  spilt.  The  jelly  is  of 
the  same  bulk  as  the  previously  fluid  blood,  and  if  carefully  shaken 
out  will  present  a  complete  mould  of  the  interior  of  the  vessel. 
If  the  blood  in  this  jelly  stage  be  left  untouched  in  a  glass  vessel, 
a  few  drops  of  an  almost  colourless  fluid  soon  make  their  appearance 
on  the  surface  of  the  jelly.  Increasing  in  number,  and  running 
together,  the  drops  after  a  while  form  a  superficial  layer  of  pale 
straw-coloured  fluid.  Later  on,  similar  layers  of  the  same  fluid  are 
seen  at  the  sides  and  finally  at  the  bottom  of  the  jelly,  which, 
shrunk  to  a  smaller  size  and  of  firmer  consistency,  now  forms  a 
clot  or  crassamentiwi,  floating  in  a  perfectly  fluid  seritm.  The 
shrinking  and  condensation  of  the  clot,  and  the  corresponding 
increase  of  the  serum,  continue  for  some  time.  The  upper  surface 
of  the  clot  is  generally  slightly  concave.  A  portion  of  the  clot 
examined  under  the  microscope  is  seen  to  consist  of  a  feltwork  of 
fine  granular  fibrils,  in  the  meshes  of  which  are  entangled  the  red 
and  white  corpuscles  of  the  blood.  In  the  serum  nothing  can  be 
seen  but  a  few  stray  corpuscles,  chiefly  white.  The  fibrils  are 
composed  of  a  substance  called  fibrin.  Hence  we  may  speak 
of  the  clot  as  consisting  of  fibrin  and  corpuscles  ;  and  the  act 
of  clotting  is  obviously  a  substitution  for  the  plasma  of  fibrin 
and  serum,  followed  by  a  separation  of  the  fibrin  and  corpuscles 
from  the  serum. 

In  man,  blood  when  shed  becomes  viscid  in  about  two  or 
three  minutes,  and  enters  the  jelly  stage  in  about  five  or  ten 
minutes.  After  the  lapse  of  another  few  minutes  the  first  drops 
of  serum  are  seen,  and  clotting  is  generally  complete  in  from  one 


16  PHEXOMEI^A   OF   CLOTTING.  [Book  i. 

to  several  hours.  The  times  however  will  be  found  to  vary  accord- 
ing to  circumstances.  Among  animals  the  rapidity  of  clotting 
varies  exceedingly  in  different  species.  The  blood  of  the  horse 
clots  with  remarkable  slowness ;  so  slowly  indeed  that  many  of  the 
red  and  also  some  of  the  white  corpuscles  (both  these  being  speci- 
fically heavier  than  the  plasma)  have  time  to  sink  before  viscidity 
sets  in.  In  consequence  there  appears  on  the  surface  of  the  blood 
an  upper  layer  of  colourless  plasma,  containing  in  its  deeper  por- 
tions many  colourless  corpuscles  (which  are  lighter  than  the  red). 
This  layer  clots  like  the  other  parts  of  the  blood,  forming  the  so- 
called  '  buffy  coat.'  A  similar  buffy  coat  is  sometimes  seen  in  the 
blood  of  man,  in  certain  abnormal  conditions  of  the  body. 

If  a  portion  of  horse's  blood  be  surrounded  by  a  cooling 
mixture  of  ice  and  salt,  and  thus  kept  at  about  0°C.,  clotting 
may  be  almost  indefinitely  postponed.  Under  these  circumstances 
a  more  complete  descent  of  the  corpuscles  takes  place,  and  a 
considerable  quantity  of  colourless  transparent  plasma  free  from 
blood-corpuscles  may  be  obtained.  A  portion  of  this  plasma 
removed  from  the  freezing  mixture  clots  in  the  same  manner  as 
does  the  entire  blood.  It  first  becomes  viscid  and  then  forms  a 
jelly,  which  subsequently  separates  into  a  colourless  shrunken  clot 
and  serum.  This  shews  that  the  corpuscles  are  not  an  essential 
part  of  the  clot. 

If  a  few  cubic  centimetres  of  this  colourless  plasma,  or  of  a 
similar  plasma  which  may  be  obtained  from  almost  any  blood  by 
means  which  we  will  presently  describe,  be  diluted  with  many 
times  its  bulk  of  a  0-6  p.c.  solution  of  sodium  chloride^  clotting  is 
much  retarded,  and  the  various  stages  may  be  more  easily  watched. 
As  the  fluid  is  becoming  viscid,  fine  fibrils  of  fibrin  will  be  seen  to 
be  developed  in  it,  especially  at  the  sides  of  the  containing  vessel. 
As  these  fibrils  multiply  in  number,  the  fluid  becomes  more  and 
more  of  the  consistence  of  a  jelly  and  at  the  same  time  somewhat 
opaque.  Stirred  or  pulled  about  with  a  needle,  the  fibrils  shrink 
up  into  a  small,  opaque,  stringy  mass ;  and  a  very  considerable 
bulk  of  the  jelly  may  by  agitation  be  resolved  into  a  minute 
fragment  of  shrunken  fibrin  floating  in  a  quantity  of  what  is 
really  diluted  serum.  If  a  specimen  of  such  diluted  plasma 
be  stirred  from  time  to  time,  as  soon  as  clotting  begins,  with  a 
needle  or  glass  rod,  the  fibrin  may  be  removed  piecemeal  as  it 
forms,  and  "the  jelly  stage  may  be  altogether  done  away  with. 
When  fresh  blood  which  has  not  yet  had  time  to  clot  is  stirred  or 
whipped  with  a  bundle  of  rods  (or  anything  presenting  a  large 
amount  of  rough  surface),  no  jelly-like  clotting  takes  place,  but 
the  rods  become  covered  with  a  mass  of  shrunken  fibrin.  Blood 
thus  whipped  until  fibrin  ceases  to  be  deposited,  is  found  to  have 
entirely  lost  its  power  of  clotting. 

1  A  solution  of  sodium  chloride  of  this  strength  will  hereafter  be  spoken  of  as 
'normal  saline  solution.' 


Chap.  I.]  BLOOD.  17 

Putting  these  facts  together,  it  is  very  clear  that  the  pheno- 
mena of  the  clotting  of  blood  are  caused  by  the  appearance  in  the 
plasma  of  fine  fibrils  of  fibrin.  So  long  as  these  are  scanty,  the 
blood  is  simply  viscid.  When  they  become  sufficiently  numerous, 
they  give  the  blood  the  firmness  of  a  jelly.  Soon  after  their 
formation  they  begin  to  shrink,  and  while  shrinking  enclose  in 
their  meshes  the  corpuscles  but  squeeze  out  the  fluid  parts  of  the 
blood.  Hence  the  appearance  of  the  shrunken  coloured  clot  and 
the  colovirless  serum. 

§  15.  Fibrin,  whether  obtained  by  whipping  freshly-shed  blood, 
or  by  washing  either  a  normal  clot,  or  a  clot  obtained  from  colour- 
less plasma,  exhibits  the  same  general  characters.  It  belongs  to 
that  class  of  complex  unstable  nitrogenous  bodies  called  proteids 
which  form  a  large  portion  of  all  living  bodies  and  an  essential 
part  of  all  living  structures. 

Our  knowledge  of  proteids  is  at  present  too  imperfect,  and 
probably  none  of  them  have  yet  been  prepared  in  adequate  purity, 
to  justify  us  in  attempting  to  assign  to  them  any  definite  formula ; 
but  it  is  important  to  remember  their  general  composition.  100 
parts  of  a  proteid  contain  rather  more  than  50  parts  of  carbon, 
rather  more  than  15  of  nitrogen,  about  7  of  hydrogen,  and  rather 
more  than  20  of  oxygen  ;  that  is  to  say,  they  contain  about  half 
their  weight  of  carbon,  and  only  about  ^th  their  weight  of  nitrogen ; 
and  yet  as  we  shall  see  they  are  eminently  the  nitrogenous  sub- 
stances of  the  body.  They  usually  contain  a  small  quantity 
(1  or  2  p.c.)  of  sulphur,  and  many  also  have  some  phosphorus 
attached  to  them  in  some  way  or  other.  When  burnt  they  leave 
a  variable  quantity  of  ash,  consisting  of  inorganic  salts  of  which 
the  bases  are  chiefiy  sodium  and  potassium  and  the  acids  chiefly 
hydrochloric,  sulphuric,  phosphoric,  and  carbonic. 

They  all  give  certain  reactions,  by  which  their  presence  may 
be  recognised ;  of  these  the  most  characteristic  are  the  following  : 
Boiled  with  nitric  acid  they  give  .  a  yellow  colour,  which  deepens 
into  orange  upon  the  addition  of  ammonia.  This  is  called  the 
xantJioproteic  test ;  the  colour  is  due  to  a  product  of  decomposi- 
tion. Boiled  with  the  mixture  of  mercuric  and  mercurous 
nitrates  known  as  Ilillon's  reagent  they  give  a  pink  colour. 
Mixed  with  a  strong  solution  of  sodic  hydrate  they  give  on  the 
addition  of  a  drop  or  two  of  a  very  weak  solution  of  cupric  sul- 
phate a  violet  colour  which  deepens  on  heating.  These  are  artificial 
reactions,  not  throwing  much  if  any  light  on  the  constitution  of 
proteids ;  but  they  are  useful  as  practical  tests  enabling  us  to 
detect  the  presence  of  proteids. 

The  several  members  of  the  proteid  group  are  at  present  dis- 
tinguished from  each  other  chiefiy  by  their  respective  solubilities, 
especially  in  various  saline  solutions.  Fibrin  is  one  of  the  least 
soluble  ;  it  is  insoluble  in  water,  almost  insoluble  in  dilute  neutral 
saline    solutions,   very   sparingly   soluble    in    more    concentrated 

2 


18  PROTEIDS   OF   SERUM.  [Book  i. 

neutral  saline  solutions  and  in  dilute  acids  and  alkalis,  but  is 
easily  dissolved  in  strong  acids  and  alkalis.  In  the  process  of 
solution  it  becomes  changed  into  something  which  is  no  longer 
fibrin.  In  dilute  acids  it  swells  up  and  becomes  transparent,  but 
when  the  acid  is  neutralized  returns  to  its  previous  condition. 
When  suspended  in  water  and  heated  to  100°  C.  or  even  to  75°  C, 
it  becomes  changed  ;  it  is  still  less  soluble  than  before.  It  is  said 
in  this  case  to  be  coagulated  by  the  heat ;  and  as  we  shall  see, 
nearly  all  proteids  have  the  property  of  being  changed  in  nature, 
of  undergoing  coagulation  and  so  becoming  less  soluble  than 
before,  by  being  exposed  to  a  certain  high  temperature. 

Fibrin  then  is  a  proteid  distinguished  from  other  proteids  by 
its  smaller  solubility ;  it  is  further  distinguished  by  its  peculiar 
filamentous  structure,  the  other  proteids  when  obtained  in  a  solid 
form  appearing  either  in  amorphous  granules  or  at  most  in  viscid 
masses. 

§  16.     We  may  now  return  to  the  serum. 

This  is  perfectly  fluid,  and  remains  fluid  until  it  decomposes. 
It  is  of  a  faint  straw  colour,  due  to  the  presence  of  a  special 
pigment  substance,  differing  from  the  red  matter  which  gives 
redness  to  the  red  corpuscles.  Tested  by  the  xanthoproteic  and 
other  tests  it  obviously  contains  a  large  quantity  of  proteid 
matter,  and  upon  examination  we  find  that  at  least  two  distinct 
proteid  substances  are  present  in  it. 

If  crystals  of  magnesium  sulphate  be  added  to  serum  and 
gently  stirred  until  they  dissolve,  it  will  be  seen  that  the  serum 
as  it  approaches  saturation  with  the  salt  becomes  turbid  instead 
of  remaining  clear,  and  eventually  a  white  amorphous  granular  or 
flocculent  precipitate  makes  its  appearance.  This  precipitate  may 
be  separated  by  decantation  or  filtration,  washed  with  saturated 
solutions  of  magnesium  sulphate,  in  which  it  is  insoluble,  until 
it  is  freed  from  all  other  constituents  of  the  serum,  and  thus 
obtained  fairly  pure.  It  is  then  found  to  be  a  proteid  body, 
distinguished  by  the  following  characters  among  others  :  — 

1.  It  is  (when  freed  from  any  adherent  magnesium  sulphate) 
insoluble  in  distilled  water ;  it  is  insoluble  in  concentrated 
solutions  of  neutral  saline  bodies,  such  as  magnesium  sulphate, 
sodium  chloride,  &c.,  but  readily  soluble  in  dilute  (e.g.  1  p.c) 
solutions  of  the  same  neutral  saline  bodies.  Hence  from  its 
solutions  in  the  latter  it  may  be  precipitated  either  by  adding 
more  neutral  saline  substance  or  by  removing  by  dialysis  the 
small  quantity  of  saline  substance  present.  When  obtained  in  a 
precipitated  form,  and  suspended  in  distilled  water,  it  readily 
dissolves  into  a  clear  solution  upon  the  addition  of  a  small  quan- 
tity of  some  neutral  saline  body.  By  these  various  solutions  and 
precipitations  it  is  not  really  changed  in  nature. 

2.  It  readily  dissolves  in  very  dilute  acids  (e.g.    in   hydro- 


Chap,  i.]  BLOOD.  19 

chloric  acid  even  when  dihited  to  far  less  than  1  p.c),  and  it  is 
similarly  soluble  in  dilute  alkalis ;  but  in  being  thus  dissolved  it  is 
changed  in  nature,  and  the  solutions  of  it  in  dilute  acid  and  dilute 
alkalis  give  reactions  quite  different  from  those  of  the  solution 
of  the  substance  in  dilute  neutral  saline  solutions.  By  the  acid 
it  is  converted  into  what  is  called  acid-albicmin,  by  the  alkali 
into  alkali-albumin,  both  of  which  bodies  we  shall  have  to  study 
later  on. 

3.  When  it  is  suspended  in  water  and  heated  it  becomes 
altered  in  character,  coagulated,  and  all  its  reactions  are  changed. 
It  is  no  longer  soluble  in  dilute  neutral  saline  solutions,  not  even 
in  dilute  acids  and  alkalis  ;  it  has  become  coagulated  'proteicl,  and 
is  now  even  less  soluble  than  fresh  fibrin.  When  a  solution  of  it 
in  dilute  neutral  saline  solution  is  similarly  heated,  a  similar 
change  takes  place  :  a  precipitate  falls  down  which  on  examination 
is  found  to  be  coagulated  proteid.  The  temperature  at  which 
this  change  takes  place  is  somewhere  about  75°  C,  though  shift- 
ing slightly  according  to  the  quantity  of  saline  substance  present  in 
the  solution. 

The  above  three  reactions  are  given  by  a  number  of  proteid 
bodies  forming  a  group  called  globulins,  and  the  particular  globulin 
present  in  blood-serum,  is  called  ]}araglobulin. 

One  of  the  proteids  present  in  blood-serum  is  then  para- 
globulin,  characterised  by  its  solubility  in  dilute  neutral  saline 
solutions  ;  its  insolubility  in  distilled  water  and  concentrated  saline 
solutions ;  its  ready  solubility,  and  at  the  same  time  conversion 
into  other  bodies,  in  dilute  acids  and  alkalis ;  and  in  its  becomins 
converted  into  coagulated  proteid,  and  so  being  precipitated  from 
its  solutions  at  75°  C. 

The  amount  of  it  present  in  blood-serum  varies  in  various 
animals,  and  apparently  in  the  same  animal  at  different  times.  In 
100  parts  by  weight  of  serum  there  are  generally  present  about 
8  or  9  parts  of  proteids  altogether ;  and  of  these  some  3  or  4,  more 
or  less,  may  be  taken  as  paraglobulin. 

§  17.  If  the  serum  from  which  the  paraglobulin  has  been 
precipitated  by  the  addition  of  neutral  salt,  and  removed  by  fil- 
tration, be  subjected  to  dialysis,  the  salt  added  may  be  removed, 
and  a  clear,  somewhat  diluted  serum  free  from  paraglobulin  may 
be  obtained.  This  still  gives  abundant  proteid  reactions,  so  that 
the  serum  still  contains  a  proteid,  or  some  proteids  still  more 
soluble  than  the  globulins,  since  they  will  remain  in  solution, 
and  are  not  precipitated,  even  when  dialysis  is  continued  until 
the  serum  is  practically  freed  from  both  the  neutral  salt  added 
to  it  and  the  diffusible  salts  previously  present  in  the  natural 
serum.  When  this  serum  is  heated  to  75°  C.  a  precipitate  makes 
its  appearance ;  the  proteids  still  present  are  coagulated  at  this 
temperature. 


20  PEOTEIDS   OF   SEEUM.  [Book  i. 

We  have  some  reasons  for  thinking  that  more  than  one  proteid 
is  present ;  but  they  are  all  closely  allied  to  each  other,  and  we 
may  for  the  present  speak  of  them  as  if  they  were  one,  and  call 
the  proteid  left  in  serum,  after  removal  of  the  paraglobulin,  by  the 
name  of  albumin,  or,  to  distinguish  it  from  other  albumins  found 
elsewhere,  serum-albumin.  Serum-albumin  is  distinguished  by 
being  more  soluble  than  the  globulins,  since  it  is  soluble  in  dis- 
tilled water,  even  in  the  absence  of  all  neutral  salts.  Like  the 
globulins,  though  with  much  less  ease,  it  is  converted  by  dilute 
acids  and  dilute  alkalis  into  acid-  or  into  alkali-albumin. 

The  percentage  amount  of  serum-albumin  in  serum  may  be 
put  down  as  4  or  5,  more  or  less ;  but  it  varies,  and  sometimes  is 
less  abundant  than  paraglobulin.  In  some  animals  (snakes)  it  is 
said  to  disappear  during  starvation. 

The  more  important  characters  of  the  three  proteids  which  we 
have  just  studied  may  be  stated  as  follows  :  — 
Soluble  in  water  and  in  saline  solutions  of  all 

strengths    •  .  serum-albumin. 

Insoluble  in  water,  readily  soluble  in  dilute 
saline  solutions,  insoluble  in  concentrated 

saline  solutions paraglobulin. 

Insoluble  in  water,  hardly  soluble  at  all  in 
dilute  saline  solutions,  and  very  little  solu- 
ble in  more  concentrated  saline  solutions  .  fibrin. 

Besides  paraglobulin  and  serum-albumin,  serum  contains  a 
very  large  number  of  substances,  generally  in  small  quantity, 
which,  since  they  have  to  be  extracted  by  special  methods,  are 
called  extractives ;  of  these  some  are  nitrogenous,  some  non- 
nitrogenous.  Serum  contains  in  addition  important  inorganic 
saline  substances ;  but  to  these  we  shall  return. 

§  18.  With  the  knowledge  which  we  have  gained  of  the  pro- 
teids of  clotted  blood  we  may  go  back  to  the  question  :  Clotting 
being  due  to  the  appearance  in  blood  plasma  of  a  proteid  sub- 
stance, fibrin,  which  previously  did  not  exist  in  it  as  such,  what 
are  the  causes  which  lead  to  the  appearance  of  fibrin  ? 

We  learn  something  by  studying  the  most  important  external 
circumstances  which  affect  the  rapidity  with  which  the  blood  of 
the  same  individual  clots  when  shed.     These  are  as  follows  :  — 

A  temperature  of  about  40°  C,  which  is  about  or  slightly  above 
the  temperature  of  the  blood  of  warm-blooded  animals,  is  perhaps 
the  most  favourable  to  clotting.  A  further  rise  of  a  few  degrees  is 
apparently  also  beneficial,  or  at  least  not  injurious  ;  but  upon  a  still 
further  rise  the  effect  changes,  and  when  blood  is  raj)idly  heated 
to  56°  C.  no  clotting  at  all  may  take  place.  At  this  temperature 
certain  proteids  of  the  blood  are  coagulated  and  precipitated 
before  clotting  can  take  place,  and  with  this  change  the  power  of 
the  blood  to  clot  is  wholly  lost.     If  however  the  heating  be  not 


Chap.  i.J  BLOOD.  21 

very  rapid,  the  blood  may  clot  before  this  change  has  time  to  come 
on.  When  the  temperature  instead  of  being  raised  is  lowered 
below  40°  C.  the  clotting  becomes  delayed  and  prolonged ;  and  at 
the  temperature  of  0°  or  1°  C.  the  blood  will  remain  fluid,  and  yet 
capable  of  clotting  when  withdrawn  from  the  adverse  circumstances, 
for  a  very  long,  it  might  almost  be  said  for  an  indefinite,  time. 

A  small  quantity  of  blood  shed  into  a  small  vessel  clots  sooner 
than  a  large  quantity  shed  into  a  larger  one  ;  and  in  general  the 
greater  the  amount  of  foreign  surface  with  which  the  blood  comes 
in  contact  the  more  rapid  the  clotting.  When  shed  blood  is 
stirred  or  "  whipped  "  the  fibrin  makes  its  appearance  sooner  than 
when  the  blood  is  left  to  clot  in  the  ordinary  way ;  so  that  here 
too  the  accelerating  influence  of  contact  with  foreign  bodies  makes 
itself  felt.  Similarly,  movement  of  shed  blood  hastens  clotting, 
since  it  increases  the  amount  of  contact  with  foreign  bodies.  So 
also  the  addition  of  spongy  platinum  or  of  powdered  charcoal,  or 
of  other  inert  powders,  to  tardily  clotting  blood,  will  by  influence 
of  surface,  hasten  clotting.  Conversely,  blood  brought  into  contact 
with  pure  oil  does  not  clot  so  rapidly  as  when  in  contact  with  glass 
or  metal ;  and  blood  will  continue  to  flow  for  a  longer  time  without 
clotting  through  a  tube  smeared  inside  with  oil  than  through  a 
tube  not  so  smeared.  The  influence  of  the  oil  in  such  cases  is  a 
physical  not  a  chemical  one ;  any  pure,  neutral,  inert  oil  will  do. 
As  far  as  we  know,  these  influences  affect  only  the  rapidity  with 
which  the  clotting  takes  place  ;  that  is,  the  rapidity  with  which  the 
fibrin  makes  its  appearance,  not  the  amount  of  clot,  not  the  quan- 
tity of  fibrin  formed,  though  when  clotting  is  very  much  retarded 
by  cold  changes  may  ensue  whereby  the  amount  of  clotting  which 
eventually  takes  place  is  indirectly  affected. 

Mere  exposure  to  air  exerts  apparently  little  influence  on  the 
process  of  clotting.  Blood  collected  direct  from  a  blood-vessel 
over  mercury  so  as  wholly  to  exclude  the  air,  clots,  in  a  general 
way,  as  readily  as  blood  freely  exposed  to  the  air.  It  is  only  when 
blood  is  much  laden  with  carbonic  acid,  the  presence  of  which  is 
antagonistic  to  clotting,  that  exclusion  of  air,  by  hindering  the 
escape  of  the  excess  of  carbonic  acid,  delays  clotting. 

These  facts  teach  us  that  fibrin  does  not  as  was  once  thought 
make  its  appearance  in  shed  blood  because  the  blood  when  shed 
ceases  to  share  in  the  movement  of  the  circulation,  or  because  the 
blood  is  cooled  on  leaving  the  warm  body,  or  because  the  blood  is 
then  more  freely  exposed  to  the  air ;  they  further  suggest  the  view 
that  the  fibrin  is  the  result  of  some  chemical  change,  the  conversion 
into  fibrin  of  something  which  is  not  fibrin,  the  change  like  other 
chemical  changes  being  most  active  at  an  optimum  temperature, 
and  like  so  many  other  cliemical  changes  being  assisted  by  the 
influences  exerted  by  the  presence  of  inert  bodies. 

And  we  have  direct  experimental  evidence  that  plasma  does 
contain  an  antecedent  of  fibrin  which  bv  chemical  chancfe  is 
converted  into  fibrin. 


22  PLASMA.  [Book  i. 

§  19.  If  blood  be  received  direct  from  the  blood-vessels  into 
one-third  its  bulk  of  a  saturated  solution  of  some  neutral  salt  such 
as  magnesium  sulphate,  and  the  two  gently  but  thoroughly  mixed, 
clotting,  especially  at  a  moderately  low  temperature,  will  be 
deferred  for  a  very  long  time.  If  the  mixture  be  allowed  to  stand, 
the  corpuscles  will  sink,  and  a  colourless  plasma  will  be  obtained 
similar  to  the  plasma  gained  from  horse's  blood  by  cold,  except 
that  it  contains  an  excess  of  the  neutral  salt.  The  presence  of 
the  neutral  salt  has  acted  in  the  same  direction  as  cold :  it  has 
prevented  the  occurrence  of  clotting.  It  has  not  destroyed  the 
fibrin ;  for  if  some  of  the  plasma  be  diluted  with  from  five  to  ten 
times  its  bulk  of  water,  it  will  clot  speedily  in  quite  a  normal 
fashion,  with  the  production  of  quite  normal  fibrin. 

The  separation  of  the  fluid  plasma  from  the  corpuscles  and  from 
other  bodies  heavier  than  the  plasma  is  much  facihtated  by  the  use  of 
the  centrifugal  machine.  This  consists  essentially  of  a  tireless  wheel 
with  several  spokes,  placed  in  a  horizontal  position  and  made  to  revolve 
with  great  velocity  (1000  revolutions  per  minute  for  instance)  round 
its  axis.  Tubes  of  metal  or  very  strong  glass  are  suspended  at  the  ends 
of  the  spokes  by  carefully  adjusted  joints.  As  the  wheel  rotates  with 
increasing  velocity,  each  tube  gradually  assumes  a  horizontal  position, 
bottom  outwards,  without  spilling  any  of  its  contents.  As  the  rapid 
rotation  continues  the  corpuscles  and  heavier  particles  are  driven  to  the 
bottom  of  the  tube,  and  if  a  very  rapid  movement  be  continued  for  a 
long  time  will  form  a  compact  cake  at  the  bottom  of  the  tube.  When 
the  rotation  is  stopped  the  tubes  gradually  return  to  their  upright  posi- 
tion again  without  anything  being  spilt,  and  the  clear  plasma  in  each  tube 
can  then  be  decanted  off. 

If  some  of  the  colourless,  transparent  plasma,  obtained  either 
by  the  action  of  neutral  salts  from  any  blood,  or  by  the  help  of 
cold  from  horse's  blood,  be  treated  with  some  solid  neutral  salt, 
such  as  sodium  chloride,  to  saturation,  a  white,  flaky,  somewhat 
sticky  precipitate  will  make  its  appearance.  If  this  precipitate 
be  removed,  the  fluid  no  longer  possesses  the  power  of  clotting  (or 
very  slightly  so),  even  though  the  neutral  salt  present  be  removed 
by  dialysis,  or  its  influence  lessened  by  dilution.  With  the  re- 
moval of  the  substance  precipitated,  the  plasma  has  lost  its  power 
of  clotting. 

If  the  precipitate  itself,  after  being  washed  with  a  saturated 
solution  of  the  neutral  salt  (in  which  it  is  insoluble)  so  as  to  get 
rid  of  all  serum  and  other  constituents  of  the  plasma,  be  treated 
with  a  small  quantity  of  water,  it  readily  dissolves,^  and  the 
solution  rapidly  filtered  gives  a  clear,  colourless  filtrate,  which  is 
at  first  perfectly  fluid.     Soon,  however,  the  fluidity  gives  way  to 

1  The  substance  itself  is  not  sohible  in  distilled  water,  but  a  quantity  of  the 
neutral  salts  always  clings  to  the  precipitate,  and  thus  the  addition  of  water  virtually 
gives  rise  to  a  dilute  saline  solution,  in  which  tlie  substance  is  readily  soluble. 


Chap,  i.]  BLOOD.  23 

viscidity,  and  this  in  turn  to  a  jelly  condition,  and  finally  the  jelly 
shrinks  into  a  clot  floating  in  a  clear  fluid ;  in  other  words,  the 
filtrate  clots  like  plasma.  Thus  there  is  present  in  cooled  plasma, 
and  in  plasma  kept  from  clotting  by  the  presence  of  neutral  salts, 
a  something,  precipitable  by  saturation  with  neutral  salts  ;  a  some- 
thing which,  since  it  is  soluble  in  very  dilute  saline  solutions, 
cannot  be  fibrin  itself,  but  which  in  solution  speedily  gives  rise  to 
the  appearance  of  fibrin.  To  this  substance  its  discoverer,  Denis, 
gave  the  name  of  plasjnine. 

The  substance  thus  precipitated  is  not  however  a  single  body 
but  a  mixture  of  at  least  two  bodies.  If  sodium  chloride  be 
carefully  added  to  plasma  to  an  extent  of  about  13  per  cent,  a 
white,  flaky,  viscid  precipitate  is  thrown  down  very  much  like 
plasmine.  If  after  the  removal  of  the  first  precipitate  more  sodium 
chloride  and  especially  if  magnesium  sulphate  be  added,  a  second 
precipitate  is  thrown  down,  less  viscid  and  more  granular  than  the 
first. 

The  second  precipitate  when  examined  is  found  to  be  identical 
with  the  paraglohulin,  coagulating  at  75°  C,  which  we  have 
already  seen  to  be  a  constituent  of  serum. 

The  first  precipitate  is  also  a  proteid  belonging  to  the  globulin 
group,  but  differs  from  paraglobulin  not  only  in  being  more 
readily  precipitated  by  sodium  chloride,  and  in  being  when 
precipitated  more  viscid,  but  *ilso  in  other  respects,  and  especially 
in  being  coagulated  at  a  far  lower  temperature  than  paraglobulin, 
viz.  at  56°  C.  Now,  while  isolated  paraglobulin  cannot  by  any 
means  known  to  us  be  converted  into  fibrin,  and  its  presence  in 
the  so-called  plasmine  does  not  seem  to  be  essential  to  the  for- 
mation of  fibrin  out  of  plasmine,  the  presence  in  plasmine  of  the 
body  coagulating  at  56°  C.  does  seem  essential  to  the  conversion 
of  plasmine  into  fibrin ;  and  we  have  reason  for  thinking  that  it  is 
itself  converted,  in  part  at  least,  into  fibrin.  Hence  it  has  received 
the  name  of  fibrinogen. 

§  20.     The  reasons  for  this  view  are  as  follows. 

Besides  blood,  which  clots  naturally  when  shed,  there  are 
certain  fluids  in  the  body  which  do  not  clot  naturally,  either  in 
the  body  or  when  shed,  but  which  by  certain  artificial  means  may 
be  made  to  clot,  and  in  clotting  to  yield  quite  normal  fibrin. 
Thus  the  so-called  serous  fluid  taken  some  hours  after  death  ^ 
from  the  pericardial,  pleural,  or  peritoneal  cavities,  the  fluid  found  in 
the  enlarged  serous  sac  of  the  testis,  known  as  hj'drocele  fluid,  and 
other  similar  fluids,  will  in  the  majority  of  cases,  when  obtained  free 
from  blood  or  other  admixtures,  remain  fluid  almost  indefinitely, 
shewing  no  disposition  whatever  to  clot.-     Yet  in  most  cases  at 

1  If  it  be  removed  iinmeiliatclj'  after  death  it  ,2;,.iipi-ally  elots  readily  and  finiih', 
giving  a  colourless  clot  consisting  of  fibrin  and  white  corpuscles. 

-  In  some  specimens,  however,  a  spontaneous  coagulation,  generally  slight,  but  in 
exceptional  cases  massive,  may  be  observed. 


24  FIBEIN  FEEMENT.  [Book  i. 

all  events,  these  fluids,  when  a  little  blood,  or  a  piece  of  blood  clot,. 
or  a  little  serum  is  added  to  them,  will  clot  rapidly  and  firmly,^ 
giving  rise  to  an  unmistakeable  clot  of  normal  fibrin,  differing  only 
from  the  clot  of  blood  in  that,  when  serum  is  used,  it  is  colourless,, 
being  free  from  red  corpuscles. 

Now,  blood  (or  blood  clot,  or  serum)  contains  many  things,  to 
any  one  of  which  the  clotting  power  thus  seen  might  be  attributed. 
But  it  is  found  that  in  many  cases  clotting  may  be  induced  in  the 
fluids  of  which  we  are  speaking  by  the  mere  addition  and  that 
even  in  exceedingly  small  quantity,  of  a  substance  which  can  be 
extracted  from  blood,  or  from  serum,  or  from  blood  clot,  or  even 
from  washed  fibrin,  or  indeed  from  other  sources,  —  a  substance 
whose  exact  nature  is  uncertain,  it  being  doubtful  whether  it  is  a 
proteid  at  all,  and  whose  action  is  peculiar. 

If  serum,  or  whipped  blood,  or  a  broken-up  clot  be  mixed  with 
a  large  quantity  of  alcohol  and  allowed  to  stand  some  days,  the 
proteids  present  are  in  time  so  changed  by  the  alcohol  as  to 
become  insoluble  in  water.  Hence  if  the  copious  precipitate 
caused  by  the  alcohol,  after  long  standing,  be  separated  by  filtration 
from  the  alcohol,  dried  at  a  low  temperature,  not  exceeding  40°  C, 
and  extracted  with  distilled  water,  the  aqueous  extract  contains 
very  little  proteid  matter,  —  indeed  very  little  organic  matter  at  all. 
Nevertheless  even  a  small  quantity  of  this  aqueous  extract  added 
alone  to  certain  specimens  of  hydrocele  fluid  or  other  of  the  fluids 
spoken  of  above,  will  bring  about  a  speedy  clotting.  The  same 
aqueous  extract  has  also  a  remarkable  effect  in  hastening  the 
clotting  of  fluids  which,  though  they  will  eventually  clot,  do  so 
very  slowly.  Thus,  plasma  may,  by  the  careful  addition  of  a 
certain  quantity  of  neutral  salt  and  water,  be  reduced  to  such  a 
condition  that  it  clots  very  slowly  indeed,  taking  perhaps  days  to 
complete  the  process.  The  addition  of  a  small  quantity  of  the 
aqueous  extract  we  are  describing  will,  however,  bring  about  a 
clotting  which  is  at  once  rapid  and  complete. 

The  active  substance,  whatever  it  be,  in  this  aqueous  extract 
exists  in  small  quantity  only,  and  its  clotting  virtues  are  at  once 
and  for  ever  lost  when  the  solution  is  boiled.  Further,  there  is  no 
reason  to  think  that  the  active  substance  actually  enters  into  the 
formation  of  the  fibrin  to  which  it  gives  rise.  It  appears  to  belong 
to  a  class  of  bodies  playing  an  important  part  in  physiological 
processes  and  called  ferments,  of  which  we  shall  have  more  to  say 
hereafter.  We  may  therefore  speak  of  it  as  the  fibrin  ferment,  the 
name  given  to  it  by  its  discoverer  Alexander  Schmidt. 

This  fibrin  ferment  is  present  in  and  may  be  extracted  from 
clotted  or  whipped  blood,  and  from  both  the  clot  ^  and  the  serum 
of  clotted  blood ;  and  since  in  most  if  not  all  cases  where  blood  or 

^  In  a  few  cases  no  coagulation  can  thus  be  induced. 

2  A  powerful  solution  of  fibrin  ferment  may  be  readily  prepared  by  simply 
extracting  a  washed  blood  clot  with  a  10  p.c.  solution  of  sodium  chloride. 


Chap,  t.]  BLOOD.  25 

blood  clot  or  serum  produces  clotting  in  hydrocele  or  pericardial 
fluid,  an  exactly  similar  clotting  may  be  induced  by  the  mere 
addition  of  fibrin  ferment,  we  seem  justified  in  concluding  that 
the  clotting  virtues  of  the  former  are  due  to  the  ferment  which 
they  contain. 

Now,  when  fibrinogen  is  precipitated  from  plasma  as  above 
described  by  sodium  chloride,  re-dissolved,  and  reprecipitated,  more 
than  once,  it  may  be  obtained  in  solution,  by  help  of  a  dilute 
neutral  saline  solution,  in  an  approximately  pure  condition,  at 
all  events  free  from  other  proteids.  Such  a  solution  will  not  clot 
spontaneously ;  it  may  remain  fluid  indefinitely ;  and  yet  on  the 
addition  of  a  little  fibrin  ferment  it  will  clot  readily  and  firmly, 
yielding  quite  normal  fibrin. 

This  body  fibrinogen  is  also  present  and  may  be  separated  out 
from  the  specimens  of  hydrocele,  pericardial,  and  other  fluids  which 
clot  on  the  addition  of  fibrin  ferment ;  and  when  the  fibrinogen  has 
been  wholly  removed  from  these  fluids  they  refuse  to  clot  on  the 
addition  of  fibrin  ferment. 

Paraglobulin,  on  the  other  hand,  whether  prepared  from 
plasmine  by  separation  of  the  fibrinogen,  or  from  serum,  or  from 
other  fluids  in  which  it  is  found,  cannot  be  converted  by  fibrin 
ferment  or  indeed  by  any  other  means  into  fibrin.  And  fibrinogen 
isolated  as  described  above,  or  serous  fluids  which  contain 
fibrinogen,  can  be  made,  by  means  of  flbrin  ferment,  to  yield 
quite  normal  fibrin  in  the  complete  absence  of  paraglobulin.  A 
solution  of  paraglobulin  obtained  from  serum  or  blood  clot  will,  it 
is  true,  clot  pericardial  or  hydrocele  fluids  containing  fibrinogen, 
or  indeed  a  solution  of  fibrinogen ;  but  this  is  apparently  due  to 
the  fact  that  the  paraglobulin  has  in  these  cases  some  fibrin 
ferment  mixed  with  it;  it  is  also  possible  that  under  certain 
conditions  the  presence  of  paraglobulin  may  be  favourable  to  the 
action  of  the  ferment. 

When  the  so-called  plasmine  is  precipitated  as  directed  in 
§  19,  fibrin  ferment  is  carried  down  with  the  fibrinogen  and  para- 
globulin ;  and  when  the  plasmine  is  re-dissolved  the  ferment  is 
present  in  the  solution  and  ready  to  act  on  the  fibrinogen.  Hence 
tlie  re-dissolved  plasmine  clots  spontaneously.  When  fibrinogen 
is  isolated  from  plasma  by  repeated  precipitation  and  solution,  the 
ferment  is  washed  away  from  it,  and  the  pure  ferment-free  fibrin- 
ogen, ultimately  obtained,  does  not  clot  spontaneously. 

So  far  it  seems  clear  that  there  does  exist  a  proteid  body, 
fibrinogen,  which  may  by  the  action  of  fibrin  ferment  be  directly, 
without  the  intervention  of  other  proteids,  converted  into  the 
less  soluble  fibrin.  Our  knowledge  of  the  constitution  of  proteid 
bodies  is  too  imperfect  to  enable  us  to  make  any  very  definite 
statement  as  to  the  exact  nature  of  the  change  thus  effected  ;  but 
we  may  say  this  much:  Fibrinogen  and  fibrin  have  about  the 
same    elementary    composition,    fibrin    containing    a    trifle    more 


26  FIBEINOGEN  AND   FIBEIK  [Book  i. 

nitrogen.  When  fibrinogen  is  converted  into  fibrin  by  means  of 
fibrin  ferment,  the  weight  of  the  fibrin  produced  is  always  less 
than  that  of  the  fibrinogen  which  is  consumed,  and  there  is  always 
produced  at  the  same  time  a  certain  quantity  of  another  proteid, 
belonging  to  the  globulin  family.  There  are  reasons  however 
why  we  cannot  speak  of  the  ferment  as  splitting  up  fibrinogen 
into  fibrin  and  a  globulin.  It  seems  more  probable  that  the 
ferment  converts  the  fibrinogen  first  into  a  body  which  we  might 
call  soluble  fihj'in,  and  then  turns  this  body  into  veritable  fibrin  ; 
but  further  inquiries  on  the  subject  are  needed. 

The  action  of  the  fibrin  ferment  on  fibrinogen  is  dependent  on 
other  conditions  besides  temperature ;  for  instance,  the  presence 
of  a  calcium  salt  seems  to  be  necessary.  If  blood  be  shed  into  a 
dilute  solution  of  potassium  oxalate,  the  mixture,  which  need  not 
contain  more  than  -1  p.c.  of  the  oxalate,  remains  fluid  indefinitely, 
but  clots  readily  on  the  addition  of  a  small  quantity  of  a  calcium 
salt.  Apparently  the  oxalate,  by  precipitating  the  calcium  salts 
present  in  the  blood,  prevents  the  conversion  of  the  fibrinogen 
into  fibrin.  So  also  a  solution  of  fibrinogen  which  has  been 
deprived  of  its  calcium  salts,  by  diffusion  for  instance,  will  not  clot 
on  the  addition  of  fibrin  ferment  similarly  deprived  of  its  calcium 
salts  ;  but  the  mixture  clots  readily  on  the  addition  of  a  minute 
quantity  of  calcium  sulphate.  We  shall  have  to  speak  later  on  of 
a  somewhat  analogous  part  played  by  calcium  salts  in  the  curdling 
of  milk.  It  may  be  added  that  the  presence  of  other  neutral 
salts,  such  as  sodium  chloride,  appears  to  influence  clotting. 

§  21.  We  may  conclude  then  that  the  plasma  of  blood  when 
shed,  or  at  all  events  soon  after  it  has  been  shed,  contains  fibrino- 
gen; and  it  also  seems  probable  that  the  clotting  comes  about 
because  the  fibrinogen  is  converted  into  fibrin  by  the  action  of 
fibrin  ferment ;  but  we  are  still  far  from  a  definite  answer  to  the 
question,  why  blood  remains  fluid  in  the  body  and  yet  clots  when 
shed  ? 

We  have  already  said  that  blood  or  blood  plasma,  brought  up  to 
a  temperature  of  56°  C.  as  soon  as  possible  after  its  removal  from 
the  living  blood  vessels,  gives  a  proteid  precipitate  and  loses  its 
power  of  clotting.  This  may  be  taken  to  shew  that  blood,  as  it 
circulates  in  the  living  blood  vessels,  contains  fibrinogen  as  such, 
and  that  when  the  blood  is  heated  to  56°  C,  which  is  the  coagu- 
lating point  of  fibrinogen,  the  fibrinogen  present  is  coagulated  and 
precipitated,  and  consequently  no  fibrin  can  be  formed. 

Further,  while  clotted  blood  undoubtedly  contains  an  abundance 
of  fibrin  ferment,  no  ferment,  or  a  minimal  quantity  only,  is  present 
in  blood  as  it  leaves  the  blood  vessels.  If  blood  be  received  directly 
from  the  blood  vessels  into  alcohol,  the  aqueous  extract  prepared 
as  directed  above  contains  no  ferment,  or  merely  a  trace.  Appa- 
rently the  ferment  makes  its  appearance  in  the  blood  as  the  result 
of  changes  taking  place  in  the  blood  after  it  has  been  shed. 


Chap,  i.]  BLOOD.  27 

We  might  from  this  be  inclined  to  conclude  that  blood  clots 
when  shed  but  not  before,  because,  fibrinogen  being  always  present, 
the  shedding  brings  about  changes  which  produce  fibrin  ferment, 
not  previously  existing,  and  this  acting  on  the  fibrinogen  gives  rise 
to  fibrin.  But  we  meet  with  the  following  difficulty.  A  very 
considerable  quantity  of  very  active  ferment  may  be  injected  into 
the  blood  current  of  a  living  animal  without  necessarily  producing 
any  clotting  at  all.  Obviously,  either  blood  within  the  blood 
vessels  does  not  contain  fibrinogen  as  such,  and  the  fibrinogen 
detected  by  heating  the  blood  to  56°  C.  is  the  result  of  changes 
which  have  already  ensued  before  that  temperature  is  reached ; 
or  in  the  living  circulation  there  are  agencies  at  work  which 
prevent  any  ferment  which  may  be  introduced  into  the  circula- 
tion from  producing  its  usual  effects  on  fibrinogen  ;  or  there  are 
agencies  at  work  which  destroy  or  do  away  with  the  fibrin,  little 
by  little,  as  it  is  formed. 

§  22.  And  indeed  when  we  reflect  how  complex  blood  is,  and 
of  what  many  and  great  changes  it  is  susceptible,  we  shall  not 
wonder  that  the  question  we  are  putting  cannot  be  answered  off 
hand. 

The  corpuscles  with  which  blood  is  crowded  are  living  structures, 
and  consequently  are  continually  acting  upon  and  being  acted 
upon  by  the  plasma.  The  red  corpuscles  it  is  true  are,  as  we  shall 
see,  peculiar  bodies,  with  a  restricted  life  and  a  very  specialized 
work,  and  possibly  their  influence  on  the  plasma  is  not  very  great ; 
but  we  have  reason  to  think  that  the  relations  between  the  white 
corpuscles  and  the  plasma  are  close  and  important. 

Then  again  the  blood  is  not  only  acting  upon  and  being  acted 
upon  by  the  several  tissues  as  it  flows  through  the  various 
capillaries,  but  along  the  whole  of  its  course,  through  the  heart, 
arteries,  capillaries,  and  veins,  is  acting  upon  and  being  acted  upon 
by  the  vascular  walls,  which  like  the  rest  of  the  body  are  alive, 
and  being  alive  are  continually  undergoing  and  promoting  change. 

That  relations  of  some  kind,  having  a  direct  influence  on  the 
clotting  of  blood,  do  exist  between  the  blood  and  the  vascular 
walls  is  shewn  by  the  following  facts. 

After  death,  when  all  motion  of  the  blood  has  ceased,  the 
blood  remains  for  a  long  time  fluid.  It  is  not  till  some  time 
afterwards,  at  an  epoch  when  post-mortem  changes  in  the  blood 
and  in  the  blood  vessels  have  had  time  to  develope  themselves, 
that  clotting  begins.  Thus,  some  hours  after  death  the  blood  in 
the  great  veins  may  be  found  still  perfectly  fluid.  Yet  such  blood 
has  not  lost  its  power  of  clotting ;  it  still  clots  when  removed 
from  the  body,  and  clots  too  when  received  over  mercury  without 
exposure  to  air,  shewing  that,  though  the  blood,  being  highly 
venous,  is  rich  in  carbonic  acid  and  contains  little  or  no  oxygen,  its 
fluidity  is  not  due  to  any  excess  of  carbonic  acid  or  absence  of  oxy- 
gen.    Eventually  it  does  clot  even  witliin  tlie  vessels,  but  perhaps 


28  INFLUENCE   OF   BLOOD   VESSELS.  [Book  i. 

never  so  firmly  and  completely  as  when  shed.  It  clots  first  in 
the  larger  vessels,  but  remains  fluid  in  the  smaller  vessels  for  a 
very  long  time,  for  many  hours  in  fact,  since  in  these  the  same 
bulk  of  blood  is  exposed  to  the  influence  of,  and  reciprocally 
exerts  an  influence  on,  a  larger  surface  of  the  vascular  walls, 
than  in  the  larger  vessels.  And  if  it  be  urged  that  the  result  is 
here  due  to  influences  exerted  by  the  body  at  large,  by  the  tissues 
as  well  as  by  the  vascular  walls,  this  objection  will  not  hold  good 
against  the  following  experiment. 

If  the  jugular  vein  of  a  large  animal,  such  as  an  ox  or  horse, 
be  carefully  ligatured  when  full  of  blood,  and  the  ligatured  por- 
tion excised,  the  blood  in  many  cases  remains  perfectly  fluid, 
along  the  greater  part  of  the  length  of  the  piece,  for  twenty-four 
or  even  forty-eight  hours.  The  piece  so  ligatured  may  be  sus- 
pended in  a  framework  and  opened  at  the  top  so  as  to  imitate  a 
living  test-tube,  and  yet  the  blood  will  often  remain  long  fluid, 
though  a  portion  removed  at  any  time  into  a  glass  or  other  vessel 
will  clot  in  a  few  minutes.  If  two  such  living  test-tubes  be  pre- 
pared, the  blood  may  be  poured  from  one  to  the  other  without 
clotting  taking  place. 

A  similar  relation  of  the  fluid  to  its  containing  living  wall 
is  seen  in  the  case  of  those  serous  fluids  which  clot  spontane- 
ously. If,  so  soon  after  death  as  the  body  is  cold  and  the  fat 
is  solidified,  the  pericardium  be  carefully  removed  from  a  sheep 
by  an  incision  round  the  base  of  the  heart,  the  pericardial 
fluid  (which,  as  we  have  already  seen,  during  life,  and  some 
little  time  after  death,  possesses  the  power  of  clotting)  may  be 
kept  in  the  pericardial  bag  as  in  a  living  cup  for  many  hours 
without  clotting,  and  yet  a  small  portion  removed  with  a  pipette 
clots  at  once. 

This  relation  between  the  blood  and  the  vascular  wall  may 
be  disturbed  or  overridden :  clotting  may  take  place  or  may  be 
induced  within  the  living  blood  vessel.  "When  the  lining  mem- 
brane is  injured,  as  when  an  artery  or  vein  is  sharply  ligatured, 
or  when  it  is  diseased,  as  for  instance  in  aneurism,  a  clot  is  apt  to 
be  formed  at  the  injured  or  diseased  spot ;  and  in  certain  morbid 
conditions  of  the  body  clots  are  formed  in  various  vascular  tracts. 
Absence  of  motion,  which  in  shed  blood,  as  we  have  seen,  is  im- 
favourable  to  clotting,  is  apt  within  the  body  to  lead  to  clotting. 
Thus  when  an  artery  is  ligatured,  the  blood  in  the  tract  of  artery 
on  the  cardiac  side  of  the  ligature,  between  the  ligature  and  the 
branch  last  given  off  by  tbe  artery,  ceasing  to  share  in  the  circula- 
tion, remains  motionless  or  nearly  so,  and  along  this  tract  a  clot 
forms,  firmest  next  to  the  ligature  and  ending  near  where  the 
branch  is  given  off;  this  perhaps  may  be  explained  by  the  fact 
that  the  walls  of  the  tract  suffer  in  their  nutrition  by  the  stagna- 
tion of  the  blood,  and  that  consequently  the  normal  relation  be- 
tween them  and  the  contained  blood  is  disturbed. 


Chap,  i.]  BLOOD.  29 

That  the  blood  within  the  living  blood  vessels,  though  not 
actually  clotting  under  normal  circumstances,  may  easily  be  made 
to  clot,  that  the  blood  is  in  fact  so  to  speak  always  on  the  point 
of  clotting,  is  shewn  by  the  fact  that  a  foreign  body,  such  as  a 
needle  thrust  into  the  iaterior  of  a  blood  vessel  or  a  thread  dra\vn 
through  and  left  in  a  blood  vessel,  is  apt  to  become  covered  with 
fibrin.  Some  influence  exerted  by  the  needle  or  thread,  whatever 
may  be  the  character  of  that  influence,  is  sufficient  to  determine  a 
clotting,  which  otherwise  would  not  have  taken  place. 

The  same  instability  of  the  blood  as  regards  clotting  is  strikingly 
shewn,  in  the  case  of  the  rabbit  at  least,  by  the  result  of  injecting 
into  the  blood  vessels  a  small  quantity  of  a  solution  of  a  peculiar 
proteid  prepared  from  certain  structures  such  as  the  thymus  body. 
Massive  clotting  of  the  blood  in  almost  all  the  blood  vessels,  small 
and  large,  takes  place  with  great  rapidity,  leading  to  the  sudden 
death  of  the  animal.  In  contrast  to  this  effect  may  be  mentioned 
the  result  of  injecting  into  the  blood  vessels  of  a  dog  a  quantity 
of  a  solution  of  a  body  called  alhumose,  of  which  we  shall  hereafter 
have  to  treat  as  a  product  of  the  digestion  of  proteid  substances, 
to  the  extent  of  -3  grm.  per  kilo  of  body  weight.  So  far  from 
producing  clotting,  the  injected  alhumose  has  such  an  effect  on 
the  blood  that  for  several  hours  after  the  injection  shed  blood  will 
refuse  to  clot  of  itself  and  remain  quite  fluid,  though  it  can  be 
made  to  clot  by  special  treatment. 

§  23.  All  the  foregoing  facts  tend  to  shew  that  the  blood 
as  it  is  flowing  through  the  healthy  blood  vessels  is,  so  far  as 
clotting  is  concerned,  in  a  state  of  unstable  equilibrium,  which 
may  at  any  moment  be  upset,  even  within  the  blood  vessels, 
and  which  is  upset  directly  the  blood  is  shed,  with  clotting 
as  a  result.  Our  present  knowledge  does  not  permit  us  to 
make  an  authoritative  statement  as  to  the  exact  nature  of  this 
equilibrium.  There  are  reasons  however  for  thinking  that  the 
wliite  corpuscles  play  an  important  part  in  the  matter.  Where- 
ever  clotting  occurs  naturally,  white  corpuscles  are  present ;  and 
this  is  true  not  only  of  blood  but  also  of  such  specimens  of  i^eri- 
cardial  or  other  .serous  fluids  as  clot  naturally.  And  nianv  argu- 
ments, which  we  cannot  enter  upon  here,  may  be  adduced  all 
pointing  to  the  same  conclusion,  that  the  wiiite  corpuscle^  play 
an  important  part  in  the  process  of  clotting  But  it  would  lead 
us  too  far  into  controversial  matters  to  attempt  to  define  what 
that  part  is,  or  to  explain  the  exact  nature  of  the  equilibrium  of 
which  we  have  spoken.  What  we  do  know  is  that  in  blood  soon 
after  it  has  been  shed,  the  body  which  we  have  called  fibrinogen 
is  present  as  also  the  body  which  we  have  called  fibrin  ferment, 
that  the  latter  acting  on  the  former  will  produce  fibrin,  and  that 
the  appearance  of  fibrin  is  undoubtedly  the  cause  of  what  is  called 
clotting.  We  seem  justified  in  concluding  that  the  clotting  of 
shed  blood  is  due  to  the  conversion  by  ferment  of  fibrinogen  into 


30  CLOTTING   OF   BLOOD.  [Book  i. 

fibrin.  The  further  inference  that  clotting  within  the  body  is  the 
same  thing  as  clotting  outside  the  body  and  similarly  due  to  the 
transformation  of  fibrinogen  by  ferment  into  fibrin,  though  prob- 
able, is  not  proved.  We  do  not  yet  know  the  exact  nature  and 
condition  of  the  blood  within  the  living  blood  vessels,  and  until 
we  know  that  we  cannot  satisfactorily  explain  why  blood  in  the 
living  blood  vessels  is  usually  fluid  but  can  at  times  clot. 


SEC.  2.  THE  CORPUSCLES  OF  THE  BLOOD. 


The  Red  Corpuscles. 

§  24.  The  redness  of  blood  is  due  exclusively  fo  the  red 
corpuscles.  The  plasma  as  seen  in  thin  layers  within  the  living 
blood  vessels  appears  colourless,  as  does  also  a  thin  layer  of  serum  ; 
but  a  thick  layer  of  serum  (and  probably  of  plasma)  has  a  faint 
yellowish  tinge  due  as  we  have  said  to  the  presence  of  a  small 
quantity  of  a  special  pigment. 

A  single  red  corpuscle  seen  by  itself  under  the  microscope  is 
a  fairly  homogeneous,  imperfectly  translucent  biconcave  disc  of  a 
very  faint  colour,  looking  yellow  rather  than  red ;  but  when  several 
corpuscles  lie  one  upon  the  top  of  the  other  the  mass  appears 
distinctly  red ;  and  though  a  single  corpuscle  is  somewhat  trans- 
lucent, a  comparatively  thin  layer  of  blood  is  opaque ;  type  for 
instance  cannot  be  read  through  even  a  thin  layer  of  blood. 

When  a  quantity  of  whipped  blood  (or  blood  otherwise  de- 
prived of  fibrin)  is  frozen  and  thawed  several  times  it  changes 
colour,  becoming  of  a  darker  hue,  and  is  then  found  to  be  much 
more  transparent,  so  that  type  can  now  be  easily  read  through  a 
moderately  thin  layer.  It  is  then  spoken  of  as  laky  blood.  The 
same  change  may  be  effected  by  shaking  the  blood  with  ether,  or 
by  adding  a  small  quantity  of  bile  salts,  and  in  other  ways.  Upon 
examination  of  laky  blood  it  is  found  that  the  red  corpuscles  are 
*  broken  up '  or  at  least  altered,  and  that  the  redness  which  pre- 
viously was  confined  to  them  is  now  diffused  through  the  serum. 
Normal  blood  is  opaque  because  each  corpuscle  while  permitting 
some  rays  of  light  (chiefly  red)  to  pass  througli,  reflects  many 
others,  and  the  brightness  of  the  hue  of  normal  blood  is  due  to 
this  reflection  of  light  from  the  surfaces  of  the  several  corpuscles. 
Laky  blood  is  transparent  because  there  are  no  longer  intact 
corpuscles  to  present  surfaces  for  the  reflection  of  light,  and  the 
darker  hue  of  laky  blood  is  similarly  due  to  the  absence  of 
reflection  from  the  several  corpuscles. 


32  STRUCTURE   OF   RED    CORPUSCLE.         [Book  i. 

When  laky  blood  is  allowed  to  stand  a  sediment  is  formed  (and 
may  be  separated  by  the  centrifugal  machine)  which  on  exami- 
nation is  found  to  consist  of  discs,  or  fragments  of  discs,  of  a 
colourless  substance  exhibiting  under  high  powers  an  obscurely 
spongy  or  reticular  structure.  These  colourless  thin  discs  seen 
flat-wise  often  appear  as  mere  rings.  The  substance  composing 
them  stains  with  various  reagents  and  may  thus  be  made  more 
evident. 

The  red  corpuscle  then  consists  obviously  of  a  colourless  frame- 
w^ork,  with  which  in  normal  conditions  a  red  colouring  matter  is 
associated ;  but  by  various  means  the  colouring  matter  may  be 
driven  from  the  framework  and  dissolved  in  the  serum. 

The  framework  is  spoken  of  as  stroma ;  it  is  a  modified  or 
differentiated  protoplasm,  and  upon  chemical  analysis  yields  a  pro- 
teid  substance  belonging  to  the  globnlin  group,  and  other  matters, 
among  which  is  the  peculiar  complex  fat  called  lecithin,  of  which 
we  shall  h^ve  to  speak  in  treating  of  nervous  tissue. 

The  red  colouring  matter  which  in  normal  conditions  is  asso- 
ciated with  this  stroma  may  by  appropriate  means  be  isolated,  and, 
in  the  case  of  the  blood  of  many  animals,  obtained  in  a  crystalline 
form.  It  is  called  Hcemoglolin,  and  may  by  proper  methods  be 
split  up  into  a  proteid  belonging  to  the  globulin  group,  and  into  a 
coloured  pigment,  containing  iron,  called  Hcematin.  Haemoglobin 
is  therefore  a  very  complex  body.  It  is  found  to  have  remarkable 
relations  to  oxygen,  and  indeed  as  we  shall  see  the  red  corpuscles 
by  virtue  of  their  haemoglobin  have  a  special  work  in  respiration ; 
they  carry  oxygen  from  the  lungs  to  the  several  tissues.  We 
shall  therefore  defer  the  further  study  of  haemoglobin  until  we 
have  to  deal  with  respiration. 

Though  the  haemoglobin,  as  is  seen  in  laky  blood,  is  readily 
soluble  in  serum  (and  it  is  also  soluble  in  plasma),  in  the  intact 
normal  blood  it  remains  confined  to  the  corpuscle ;  obviously 
there  is  some  special  connection  between  the  stroma  and  the  hae- 
moglobin ;  it  is  not  until  the  stroma  is  altered,  we  may  perhaps 
say  killed  (as  by  repeated  freezing  and  thawing),  that  it  loses  its 
hold  on  the  haemoglobin,  which  thus  set  free  passes  into  solution 
in  the  serum.  The  disc  of  stroma  when  separated  from  the  haemo- 
globin has  as  we  have  just  said  an  obscurely  spongy  texture  ;  but 
we  do  not  know  accurately  the  exact  condition  of  the  stroma  in 
the  intact  corpuscle  or  how  it  holds  the  haemoglobin.  There  is 
certainly  no  definite  membrane  or  envelope  to  the  corpuscle,  for 
by  exposing  blood  to  a  high  temperature,  60°  C,  the  corpuscle 
will  break  up  into  more  or  less  spherical  pieces,  each  still  consisting 
of  stroma  and  haemoglobin. 

The  quantity  of  stroma  necessary  to  hold  a  quantity  of  haemo- 
globin is  exceedingly  small.  Of  the  total  solid  matter  of  a 
corpuscle  more  than  90  p.  c.  is  haemoglobin.  A  red  corpuscle  in 
fact  is  a  quantity  of  htemoglobin  held  together  in  the  form  of  a 


Chap,  i.]  BLOOD.  3? 

disc  by  a  minimal  amount  of  stroma.  Hence  whatever  effect  the 
stroma  per  se  may  have  upon  the  plasma,  this,  in  the  case  of 
mammals  at  all  events,  must  be  insignificant :  the  red  corpuscle 
is  practically  simply  a  carrier  of  haemoglobin. 

§  25.  The  average  number  of  red  corpuscles  in  human  blood 
may  be  probal)ly  put  down  at  about  5  millions  in  a  cubic  milli- 
meter (the  range  in  different  mammals  is  said  to  be  from  3  to  18 
millions),  but  the  relation  of  corpuscle  to  plasma  varies  a  good 
deal  even  in  health,  and  very  much  in  disease.  Obviously  the 
relation  may  be  affected  (1)  by  an  increase  or  decrease  of  the 
plasma,  (2)  by  an  actual  decrease  or  increase  of  red  corpuscles. 
Now  the  former  must  frequently  take  place.  The  blood  as  we 
have  already  urged  is  always  being  acted  upon  by  changes  in  the 
tissues  and  indeed  is  an  index  of  those  changes ;  hence  the  plasma 
must  be  continually  changing,  though  always  strivmg  to  return  to 
the  normal  condition.  Thus  when  a  large  quantity  of  water  is 
discharged  by  the  kidney,  the  skin  or  the  bowels,  that  water  comes 
really  from  the  blood,  and  the  drain  of  water  must  tend  to  dimin- 
ish the  bulk  of  the  plasma,  and  so  to  increase  the  relative  number 
of  red  corpuscles,  though  the  effect  is  probably  soon  remedied  by 
the  passage  of  water  from  the  tissues  into  the  blood.  So  again 
when  a  large  quantity  of  water  is  drunk,  this  passes  into  the 
blood  and  tends  temporarily  to  dilute  the  plasma  (and  so  to  dimin- 
ish the  relative  number  of  red  corpuscles),  though  this  condition 
is  in  turn  soon  remedied  by  the  passage  of  the  superfluous  fluid  to 
the  tissues  and  excretory  organs.  The  greater  or  less  number 
of  red  corpuscles  then  in  a  given  bulk  of  blood  may  be  simply 
due  to  less  or  more  plasma,  but  we  have  reason  to  think  that  the 
actual  number  of  the  corpuscles  in  the  blood  does  vary  from  time 
to  time.  This  is  especially  seen  in  certain  forms  of  disease,  which 
may  be  spoken  of  under  the  general  terra  of  anaemia  (there  being 
several  kinds  of  anaemia),  in  which  the  number  of  red  corpuscles 
is  distinctly  diminished. 

The  redness  of  blood  may  however  be  influenced  not  only  by 
the  number  of  red  corpuscles  in  each  cubic  millimeter  of  blood 
but  also  by  the  amount  of  hcBmoglobin  in  each  corpuscle,  and  to 
a  less  degree  by  the  size  of  the  corpuscles.  If  we  compare,  with 
a  common  stand  ird,  the  redness  of  two  specimens  of  blood  un- 
equally red,  and  then  determine  the  relative  number  of  corpuscles 
in  each,  we  may  find  that  the  less  red  specimen  has  as  many 
corpuscles  as  the  redder  one,  or  at  least  the  deficiency  in  redness 
is  greater  than  can  be  accounted  for  by  the  paucity  of  red  cor- 
puscles. Obviously  in  such  a  case  the  red  corpuscles  have  too 
little  haemoglobin.  In  some  cases  of  aniemia  the  deficiency  of 
haemoglobin  in  each  corpuscle  is  more  striking  than  the  scantiness 
of  red  cin-puscles. 

Tlie  number  of  corpuscles  in  a  specimen  of  blood  is  determined  by 
mixing  a  small   but  carofullv  measured  quantity  of  the  blood  with  a 
F.  ^  3 


34  NUMBER  OF   RED   CORPUSCLES.  [Book  i. 

large  quantity  of  some  indifferent  fluid,  e.g.  a  5  p.c.  solution  of  sodium 
sulphate,  and  then  actually  counting  the  corpuscles  in  a  known  minimal 
bulk  of  the  mixture. 

This  perhaps  may  be  most  conveniently  done  by  the  method  gener- 
ally known  as  that  of  Gowers  (Hsemacytometer)  improved  by  Malassez. 
A  glass  slide,  in  a  metal  frame,  is  ruled  into  minute  rectangles,  e.g. 
;imm.  by  ^mm.,  so  as  to  give  a  convenient  area  of  J^th  of  a  square  mm. 
Three  small  screws  in  the  frame  permit  a  coverslip  to  be  brought  to  a 
fixed  distance,  e.g.  ^mm.,  from  the  surface  of  the  slide.  The  blood 
having  been  diluted,  e.g.  to  100  times  its  volume,  a  small  quantity  of 
the  diluted  (and  thoroughly  mixed)  blood,  sufficient  to  occupy  fully 
the  space  between  the  coverslip  and  the  glass  slide  when  the  former  is 
brought  to  its  proper  position,  is  placed  on  the  slide,  and  the  coverslip 
brought  down.  The  volume  of  diluted  blood  now  lying  over  each  of 
the  rectangles  will  be  tJo^^^  (s^'^s)  of  a  cubic  mm. ;  and  if,  when  the 
corpuscles  have  subsided,  the  number  of  corpuscles  lying  within  a 
rectangle  be  counted,  the  result  will  give  the  number  of  corpuscles 
previously  distributed  through  ^^o^^^  ^^  ^  cubic  mm,  of  the  diluted 
blood.  This  multiplied  by  100  will  give  the  number  of  corpuscles  in 
1  cubic  mm.  of  the  diluted  blood,  and  again  multiplied  by  100  the 
number  in  1  cubic  mm.  of  the  entire  blood.  It  is  advisable  to  count 
the  number  of  corpuscles  in  several  of  the  rectangles,  and  to  take  the 
average.  For  the  convenience  of  counting,  each  rectangle  is  subdi- 
vided into  a  number  of  very  small  squares,  e.g.  into  20,  each  with  a 
side  of  2'otli  mm.,  and  so  an  area  of  4^0^^  '^^  ^  square  mm. 

Since  the  actual  number  of  red  corpuscles  in  a  specimen  of 
blood  (which  may  be  taken  as  a  sample  of  the  whole  blood)  is 
sometimes  more,  sometimes  less,  it  is  obvious  that  either  red 
corpuscles  may  be  temporarily  withdrawn  from  and  returned  to 
the  general  blood  current,  or  that  certain  red  corpuscles  are  after 
a  while  made  away  with,  and  that  new  ones  take  their  place. 
We  have  no  satisfactory  evidence  of  the  former  being  the  case  in 
normal  conditions,  whereas  we  have  evidence  that  old  corpuscles 
do  die  and  that  new  ones  are  born. 

§  26.  The  red  corpuscles,  we  have  already  said,  are  continually 
engaged  in  carrying  oxygen,  by  means  of  their  haemoglobin,  from 
the  lungs  to  the  tissues ;  they  load  themselves  with  oxygen  at  the 
lungs  and  unload  at  the  tissues.  It  is  extremely  unlikely  that 
this  act  should  be  repeated  indefinitely  without  leading  to  changes 
which  may  be  familiarly  described  as  wear  and  tear,  and  which 
would  ultimately  lead  to  the  death  of  the  corpuscles. 

We  shall  have  to  state  later  on  that  the  liver  discharges  into 
the  alimentary  canal,  as  a  constituent  of  bile,  a  considerable 
quantity  of  a  pigment  known  as  bilirubin,  and  that  this  substance 
has  remarkable  relations  with,  and  indeed  may  be  regarded  as  a 
derivative  of  hcematin,  which  as  we  have  seen  (§  24)  is  a  product 
of  the  decomposition  of  hsemoglobin.  It  appears  probable  in  fact 
that  the  bilirubin  of  bile  (and  this  as  we  shall  see  is  the  chief 
biliary  pigment  and  the  source  of  the  other  biliary  pigments)  is 


Chap.   i.J  BLOOD.  35 

not  formed  wholly  anew  in  the  body  but  is  manufactured  in  some 
way  or  other  out  of  hiematiu  derived  from  haemoglobin.  This 
must  entail  a  daily  consumption  of  a  considerable  quantity  of 
haemoglobin,  and,  since  we  know  no  other  source  of  htemoglobin 
besides  the  red  corpuscles,  and  have  no  evidence  of  red  corpuscles 
continuing  to  exist  after  having  lost  their  haemoglobin,  must 
therefore  entail  a  daily  destruction  of  many  red  corpuscles. 

Even  in  health  then  a  number  of  red  corpuscles  must  be 
continually  disappearing ;  and  in  disease  the  rapid  and  great 
diminution  which  may  take  place  in  the  number  of  red  corpuscles 
shews  that  large  destruction  may  occur. 

We  cannot  at  present  accurately  trace  out  the  steps  of  this 
disappearance  of  red  corpuscles.  In  the  spleen  pulp,  red  corpuscles 
have  been  seen  in  various  stages  of  disorganisation,  some  of  them 
lying  within  the  substance  of  large  colourless  corpuscles,  and  as  it 
were  being  eaten  by  them.  There  is  also  evidence  that  destruction 
takes  place  in  the  liver  itself,  and  indeed  elsewhere. 

§  27.  This  destruction  of  red  corpuscles  necessitates  the  birth 
of  new  corpuscles,  to  keep  up  the  normal  supply  of  haemoglobin ; 
and  indeed  the  cases  in  which  after  even  great  loss  of  blood  by 
haemorrhage  a  healthy  ruddiness  returns,  and  that  often  rapidly, 
shewing  that  the  lost  corpuscles  have  been  replaced,  as  well  as 
the  cases  of  recovery  from  the  disease  anaemia,  prove  that  red 
corpuscles  are,  even  in  adult  life,  born  somewhere  in  the  body. 

In  the  developing  embryo  of  the  mammal  the  red  corpuscles 
of  the  blood  are  not  haemoglobin-holding  non-nucleated  discs  of 
stroma,  but  coloured  nucleated  cells  which  have  arisen  by  the 
development  of  haemoglobin  and  stroma  in  the  '  undifferentiated 
protoplasmic '  cell  substance  of  certain  cells. 

Still  later  on  in  the  life  of  the  embryo  the  nucleated  red  cor- 
puscles are  replaced  by  ordinary  red  corpuscles,  by  non-nucleated 
discs  composed  almost  exclusively  of  htemoglobin-holding  stroma. 
How  the  transformation  takes  place,  and  especially  how  the 
nucleus  comes  to  be  absent  is  at  present  a  matter  of  considerable 
dispute. 

In  the  adult  as  in  the  embryo  the  red  corpuscles  appear  to  be 
formed  out  of  preceding  coloured  nucleated  cells.  In  the  interior 
of  bones  is  a  peculiar  tissue  called  marrow,  which  in  most  parts, 
being  very  full  of  blood  vessels,  is  called  red  marrow.  In  this 
rel  marrow  the  capillaries  and  minute  veins  form  an  intricate 
labyrinth  of  relatively  wide  passages  with  very  thin  walls,  and 
through  this  labyrinth  the  flow  of  blood  is  comparatively  slow. 
In  the  passages  of  this  labyrinth  are  found  coloured  nucleated 
cells,  that  is  to  say,  cells  the  cell-substance  of  which  has  under- 
gone more  or  less  differentiation  into  haemoglobin  and  stroma. 
And  there  seems  to  be  going  on  in  red  marrow  a  multiplication  of 
such  coloured  nucleated  cells,  some  of  which  transformed,  in  some 
way  or  other,  into  red  non-nucleated  discs,  that  is  into  ordinary 


36  FORMATION   OF   RED   CORPUSCLES.       [Book  i. 

red  corpuscles,  pass  away  into  the  general  blood  current.  In  other 
words,  a  formation  of  red  corpuscles,  not  wholly  unlike  that  which 
takes  place  in  the  embryo,  is  in  the  adult  continually  going  on  in 
the  red  marrow  of  the  bones. 

A  similar  formation  of  red  corpuscles  has  also  been  described, 
though  with  less  evidence,  as  taking  place  in  the  spleen,  especially 
under  particular  circumstances,  such  as  after  great  loss  of  blood. 

The  formation  of  red  corpuscles  is  therefore  a  special  process 
taking  place  in  special  regions ;  we  have  no  satisfactory  evidence 
that  the  ordinary  wliite  corpuscles  of  the  blood  are,  as  they  travel 
in  the  current  of  the  circulation,  transformed  into  red  corpuscles. 

The  red  corpuscles  then,  to  sum  up,  are  useful  to  the  body  on 
account  of  the  haemoglobin  which  constitutes  so  nearly  the  whole 
of  their  solid  matter.  What  functions  the  stroma  may  have  besides 
the  mere  so  to  speak  mechanical  one  of  holding  the  haemoglobin  in 
the  form  of  a  corpuscle,  we  do  not  know.  The  primary  use  of  the 
hsemoglobin  is  to  carry  oxygen  from  the  lungs  to  the  tissues,  and 
it  would  appear  that  it  is  advantageous  to  the  economy  that  the 
haemoglobin  should  be  as  it  were  bottled  up  in  corpuscles  rather 
than  simply  diffused  through  the  plasma.  How  long  a  corpuscle 
may  live,  fetching  and  carrying  oxygen,  we  do  not  exactly  know ; 
the  red  corpuscles  of  one  animal,  e.  g.  a  bird,  injected  into  the 
vessels  of  another,  e.  g.  a  mammal,  disappear  within  a  few  days ; 
but  this  affords  no  measure  of  the  life  of  a  corpuscle  in  its  own 
home.  Eventually  however  the  red  corpuscle  dies,  its  place  being 
supplied  by  a  new  one.  The  haemoglobin  set  free  from  the  dead 
corpuscles  appears  to  have  a  secondary  use  in  forming  the  pigment 
of  the  bile  and  possibly  other  pigments. 


The   JMiite  or  Colourless   Corpuscles. 

§  28.  The  white  corpuscles  are  far  less  numerous  than  the  red; 
a  specimen  of  ordinary  healthy  blood  will  contain  several  hundred 
red  corpuscles  to  each  white  corpuscle,  though  the  proportion,  even 
in  health,  varies  considerably  under  different  circumstances,  rang- 
ing from  1  in  300  to  1  in  700.  But  though  less  numerous,  the 
white  corpuscles  are  probably  of  greater  importance  to  the  blood 
itself  than  are  the  red  corpuscles ;  the  latter  are  chiefly  limited  to 
the  special  work  of  carrying  oxygen  from  the  lungs  to  the  tissues, 
while  the  former  probably  exert  a  considerable  influence  on  the 
blood  plasma  itself,  and  help  to  maintain  it  in  a  proper  condition. 

The  white  corpuscle,  which  is  often  taken  as  the  type  of  '  a 
cell,'  consists  of  a  cell-body  or  cell-substance  and  a  nucleus. 
Several  varieties  or  kinds  of  white  corpuscle  are  found  in  the 
blood,  differing  from  each  other  as  to  size,  as  to  the  characters  of 
the  nucleus,  as  to  the  characters  of  the  cell-substance,  as  to  the 
extent  to  which   they  exhibit  '  amoeboid '  movements,  whereby 


Chap,  i.]  BLOOD.  37 

what  may  be  called  the  normal  or  resting  spherical  shape  is 
variously  changed  (we  shall  study  these  amceboid  movements 
later  on),  and  in  other  respects ;  but  at  present  we  will  deal  with 
those  features  only  which  they  have  in  common,  and  speak  of 
the  white  corpuscle  as  if  all  were  alike. 

The  cell  body  of  the  white  corpuscle  may  be  taken  as  a  good 
example  of  what  we  have  called  undifferentiated  protoplasm. 
It  may  perhaps  be  best  considered  as  consisting  of  a  uniformly 
transparent  but  somewhat  refractive  material  forming  the  ground 
substance  or  basis,  in  which  occur  vacuoles  of  varying  size  1jut 
all  for  the  most  part  minute,  and  in  which  are  imbedded  particles 
also  of  varying  size  but  also  for  the  most  part  minute.  Some 
maintain  that  the  ground  substance  exists  in  the  form  of  a  net- 
work, the  interstices  of  which  are  filled  up  either  with  fluid  or 
with  some  material  different  in  nature  from  that  of  which  the 
bars  of  the  network  are  composed  ;  but  without  entering  into  the 
discussion  of  a  debated  question,  we  may  say  that  the  evidence 
for  the  natural  existence  of  sudh  a  network  is  not  convincing. 
The  imbedded  particles  are  in  some  cases  extremely  small,  and 
for  the  most  part  distributed  uniformly  over  the  cell  body,  giving 
it  the  finely  granular  or  even  hyaline  aspect  spoken  of  above ;  in 
other  cases  however  the  partialis  are  relatively  large  and  obvi- 
ously discrete,  making  the  corpuscle  coarsely  granular,  the  coarse 
granules  being  sometimes  confined  to  one  or  another  part  of  the 
cell  body.  These  particles  or  granules,  whether  coarse  or  fine,  vary 
in  nature :  they  behave  differently  towards  various  staining  and 
other  reagents.  Some  of  them,  as  shewn  by  their  greater  refrac- 
tive power,  their  staining  with  osmic  acid,  and  their  solution  by 
solvents  of  fat,  are  fatty  in  nature  ;  others  may  similarly  be  shewn 
by  their  reactions  to  be  proteid  in  nature ;  and  in  certain  cases 
some  of  the  granules  are  carbohydrate  in  nature. 

The  material  in  which  these  granules  are  imbedded,  and  which 
forms  the  greater  part  of  the  cell  body,  has  no  special  optical 
features ;  so  far  as  can  be  ascertained  it  appears  under  the  micro- 
scope to  be  homogeneous,  no  definite  structure  can  be  detected  in 
it.  It  must  be  borne  in  mind  that  the  whole  corpuscle  consists 
largely  of  water,  the  total  solid  matter  amounting  to  not  much 
more  than  10  per  cent.  The  transparent  material  of  the  cell  body 
must  therefore  be  in  a  condition  which  we  may  call  semifluid,  or 
semisolid,  without  being  called  upon  to  define  what  we  exactly 
mean  by  these  terms.  This  ai^proach  to  fiuidity  appears  to  be 
connected  with  the  great  mobility  of  the  cell  body  as  shewn  in  its 
amoeboid  movements. 

§  29.  When  we  submit  to  chemical  examination  a  sufficient 
mass  of  white  corpuscles  separated  out  from  the  blood  by  special 
means  and  obtained  tolerably  free  from  red  corpuscles  and  plasma 
(or  apply  to  the  white  blood  corpuscles  the  chemical  results 
obtained  from  the  more  easily  procured  lymph  corpuscles,  which 


38  COMPOSITION   OF    WHITE   CORPUSCLES.    [Book  i. 

as  we  shall  see  are  very  similar  to  and  indeed  in  many  ways 
closely  related  to  the  white  corpuscles  of  the  blood),  we  find  that 
this  small  solid  matter  of  the  corpuscle  consists  largely  of  certain 
proteids,  or  of  substances  more  or  less  allied  to  proteids.  Our 
knowledge  of  these  proteids  and  other  substances  is  as  yet  im- 
perfect, but  we  are  probably  justified  in  making  the  following 
statement. 

There  is  present,  in  somewhat  considerable  quantity,  a  sub- 
stance of  a  peculiar  nature,  which  since  it  is  confined  to  the 
nuclei  of  the  corpuscles  and  further  seems  to  be  present  in  all 
nuclei,  has  been  called  nuclein.  This  nuclein,  which  though  a 
complex  nitrogenous  body  is  different  in  composition  and  nature 
from  proteids,  is  remarkable  on  the  one  hand  for  being  a  very 
stable  inert  body,  and  on  the  other  for  containing  a  large  quantity 
(acording  to  some  observers  nearly  10  p.c.)  of  phosphorus,  which 
appears  to  enter  in  a  certain  way  into  the  structure  of  the  mole- 
cule, whereas  in  the  case  of  proteids  the  phosphorus,  which  is  not 
always  present,  is,  as  it  were,  attached  to  the  molecule. 

The  substance  however  which  is  present  in  the  greatest  quan- 
tity is  one  also  at  present  not  thoroughly  understood,  which 
though  it  appears  to  exist  in  the  cell  body  apart  from  the  nucleus, 
and  indeed  to  form  a  large  part  of  the  solid  matter  of  the  cell 
body,  has  since  it  seems  to  be  a  compound  of  nuclein  and  albumin 
(or  some  other  proteid)  been  called  nucleo-albumin.  It,  like 
nuclein,  contains  a  considerable  quantity  of  phosphorus,  by  which 
as  well  as  by  other  features  it  is  distinguished  from  the  globulins, 
though  in  some  respects  it  seems  allied  to  that  class  of  proteids, 
and  to  a  somewhat  similar  proteid,  myosin,  of  which  we  shall  have 
to  speak  later  on  as  a  constituent  of  muscle. 

Besides  these  two  bodies,  the  white  corpuscles  also  contain  a 
globulin  which,  under  the  name  of  cell  globulin,  has  been  distin- 
guished from  the  globulin  or  paraglobulin  of  blood,  as  well  as  a 
body  or  bodies  like  to  or  identical  with  serum  albumin. 

Next  in  importance  to  the  proteids,  as  constant  constituents  of 
the  white  corpuscles,  come  certain  fats.  Among  these  the  most 
conspicuous  is  the  complex  fatty  body  lecithin. 

In  the  case  of  many  corpuscles  at  all  events  we  have  evidence 
of  the  presence  of  a  member  of  the  large  group  of  carbohydrates, 
comprising  starches  and  sugar,  viz.  the  starch -like  body  glycogen, 
which  we  shall  have  to  study  more  fully  hereafter.  This  glycogen 
may  exist  in  the  living  corpuscle  as  glycogen,  but  it  is  very  apt 
after  the  death  of  the  corpuscle  to  become  changed  by  hydration 
into  some  form  of  sugar,  such  as  maltose  or  dextrose. 

Lastly,  the  ash  of  the  white  corpuscles  is  characterised  by 
containing  a  relatively  large  quantity  of  potassium  and  of  phos- 
phates and  by  being  relatively  poor  in  chlorides  and  in  sodium. 
But  in  this  respect  the  corpuscle  is  merely  an  example  of  what 
seems  to  be  a  general   rule  (to  which   however   there    may  be 


Chap,  i-l  BLOOD.  39 

exceptions),  that  while  the  elements  of  the  tissues  themselves  are 
rich  in  potassium  and  phosphates,  the  blood  plasma  or  lymph  on 
which  they  live  abounds  in  chlorides  and  sodium  salts. 

§  30.  In  the  broad  features  above  mentioned,  tlie  white  blood 
corpuscle  may  be  taken  as  a  picture  and  example  of  all  living 
tissues.  If  we  examine  the  histological  elements  of  any  tissue, 
whether  we  take  an  epithelium  cell,  or  a  nerve  cell,  or  a  cartilage 
cell,  or  a  muscular  fibre,  we  meet  with  very  similar  features. 
Studying  the  element  morphologically,  we  find  a  nucleus  ^  and  a 
cell  body,  the  nucleus  having  the  general  characters  described 
above  with  frequently  other  characters  introduced,  and  the  cell 
body  consisting  of  at  least  more  than  one  kind  of  material,  the 
materials  being  sometimes  so  disposed  as  to  produce  the  optical 
effect  simply  of  a  transparent  mass  in  which  granules  are  imbedded, 
in  which  case  we  speak  of  the  cell  body  as  protoplasmic,  but  at 
other  times  so  arranged  that  the  cell  body  possesses  differentiated 
structure.  Studying  the  element  from  a  chemical  point  of  view 
we  find  proteids  always  present,  and  among  these  bodies  identical 
with  or  more  or  less  closely  allied  to  such  proteids  as  globulin  and 
myosin,  we  generally  have  evidence  of  the  presence  also  of  fat  of 
some  kind  and  of  some  member  or  members  of  the  carbohydrate 
group,  and  the  ash  always  contains  potassium  and  phosphates, 
with  sulphates,  chlorides,  sodium  and  calcium,  to  which  may  be 
added  magnesium  and  iron. 

We  stated  in  the  Introduction  that  living  matter  is  always 
undergoing  chemical  change ;  this  continued  chemical  change  we 
may  denote  by  the  term  metabolism.  We  further  urged  that  so 
lonij  as  living  matter  is  alive,  the  chemical  change  or  metabolism 
is  of  a  double  kind.  On  the  one  hand,  the  living  substance  is 
continually  breaking  down  into  simpler  bodies,  with  a  setting  free 
of  energy ;  this  part  of  the  metabolism  we  may  speak  of  as  made 
up  of  hataboUc  changes.  On  the  other  hand,  the  living  substance 
is  continually  building  itself  up,  embodying  energy  into  itself  and 
so  replenishing  its  store  of  energy ;  this  part  of  the  metabolism 
we  may  speak  of  as  made  up  of  anabolic  changes.  We  also  urged 
that  in  every  piece  of  living  tissue  there  might  be  (1)  the  actual 
living  substance  itself,  (2)  material  which  is  present  for  the  pur- 
pose of  becoming,  and  is  on  the  way  to  become,  living  substance, 
that  is  to  say,  food  undergoing  or  about  to  undergo  anabolic 
changes,  and  (3)  material  which  has  resulted  from,  or  is  resulting 
from,  the  breaking  down  of  the  living  sul)stance,  that  is  to  say, 
material  which  has  undergone  or  is  undergoing  katabolic  changes, 
and  which  we  speak  of  under  the  general  term  '  waste.'  In  using 
the  word  "  living  substance,"  however,  though  we  may  for  con- 
venience sake  speak  of  the  really  living  part  as  a  substance,  we 
may  repeat  that  in  reality  it  is  not  a  substance  in  the  chemical 
sense  of  the  word,  but  material  undergoing  a  series  of  changes. 

^  The  existence  of  multinuclear  structures  does  not  affect  the  prcseut  argument. 


40  METABOLISM.  [Book  i. 

If,  now,  we  ask  the  question,  which  part  of  the  body  of  the 
white  corpuscle  (or  of  a  similar  element  of  another  tissue)  is  the 
real  living  substance,  and  which  part  is  food  or  waste,  we  ask  a 
question  which  we  cannot  as  yet  definitely  answer.  We  have  at 
present  no  adequate  morphological  criteria  to  enable  us  to  judge, 
by  optical  characters,  what  is  really  living  and  what  is  not. 

One  thing  we  may  perhaps  say ;  the  material  which  appears 
in  the  cell  body  in  the  form  of  distinct  granules,  merely  lodged 
in  the  more  transparent  material,  cannot  be  part  of  the  real  living 
substance  ;  it  must  be  either  food  or  waste.  Some  of  these  granules 
are  fat,  and  we  have  at  times  an  opportunity  of  observing  that 
they  have  been  introduced  into  the  corpuscle  from  the  surrounding 
plasma.  The  white  corpuscle  as  we  have  said  has  the  power  of 
executing  amoeboid  movements ;  it  can  creep  round  objects, 
envelope  them  with  its  own  substance,  and  so  put  them  inside 
itself.  The  granules  of  fat  thus  introduced  may  be  subsequently 
extruded  or  may  disappear  within  the  corpuscle ;  in  the  latter 
case  they  are  obviously  changed,  and  apparently  made  use  of 
by  the  corpuscle.  In  other  words,  these  fatty  granules  are  ap- 
parently food  material,  on  their  way  to  be  worked  up  into  the 
living  substance  of  the  corpuscle. 

But  we  have  also  evidence  that  similar  granules  of  fat  may 
make  their  appearance  wholly  within  the  corpuscle ;  they  are  pro- 
ducts of  the  activity  of  the  corpuscle.  We  have  further  reason 
to  think  that  in  some  cases,  at  all  events,  they  arise  from  the 
breaking  down  of  the  living  substance  of  the  corpuscle,  that  they 
are  what  we  have  called  waste  products. 

But  all  the  granules  visible  in  a  corpuscle  are  not  necessarily 
fatty  in  nature ;  some  of  them  may  undoubtedly  be  granules  of 
protend  or  allied  matter,  and  it  is  possible  that  some  of  them  may 
at  times  be  of  carbohydrate  or  other  nature.  In  all  cases  however 
they  are  either  food  material  or  waste  products.  And  what  is 
true  of  the  easily  distinguished  granules  is  also  true  of  other 
substances,  in  solution  or  in  a  solid  form,  but  so  disposed  as  not  to 
be  optically  recognised. 

Hence  a  part,  and  it  may  be  no  inconsiderable  part,  of  the 
body  of  a  white  corpuscle  may  be  not  living  substance  at  all,  but 
either  food  or  waste.  Further,  it  does  not  necessarily  follow  that 
the  whole  of  any  quantity  of  material,  fatty  or  otherwise,  intro- 
duced into  the  corpuscle  from  without,  should  actually  be  built  up 
into  and  so  become  part  of  the  living  substance  ;  the  changes  frcm 
raw  food  to  living  substance  are  as  we  have  already  said  probably 
many,  and  it  may  be  that  after  a  certain  number  of  changes,  few 
or  many,  part  only  of  the  material  is  accepted  as  worthy  of  being 
made  alive,  and  the  rest,  being  rejected,  becomes  at  once  waste 
matter ;  or  the  material  may,  even  after  it  has  undergone  this  or 
that  change,  never  actually  enter  into  the  living  substance  but  all 
become  waste  matter.     We  say  waste  matter,  but  this  does  not 


Chap,  i.]  BLOOD.  41 

mean  useless  matter.  The  matter  so  formed  may  without  entering 
into  the  living  substance  be  of  some  subsidiary  use  to  the  corpuscle, 
or  as  probably  more  often  happens,  being  discharged  from  the  cor- 
puscle, may  be  of  use  to  some  other  part  of  the  l)ody.  We  do  not 
know  how  the  living  substance  builds  itself  up,  but  we  seem  com- 
pelled to  admit  that,  in  certain  cases  at  all  events,  it  is  able  in 
some  way  or  other  to  produce  changes  on  material  wdiile  that 
material  is  still  outside  the  living  substance  as  it  were,  before  it 
enters  into  and  indeed  without  its  ever  actually  entering  into  the 
composition  of  the  living  substance.  On  the  other  hand,  we  must 
equally  admit  that  some  of  the  waste  substances  are  the  direct 
products  of  the  katabolic  changes  of  the  living  substance  itself,  and 
were  actually  once  part  of  the  living  substance.  Hence  w^e  ought 
perhaps  to  distinguish  the  products  of  the  activity  of  living  matter 
into  waste  products  proper,  the  direct  results  of  katabolic  changes, 
and  into  by-products  which  are  the  results  of  changes  effected  by 
the  living  matter  outside  itself  and  which  cannot  therefore  be  con- 
sidered as  necessarily  either  anabolic  or  katabolic. 

Concerning  the  chemical  characters  of  the  livinsj  matter  itself 
we  cannot  at  present  make  any  very  definite  statement.  We  may 
say  that  proteid  substance  enters  in  some  way  into  its  structure 
and  indeed  forms  a  large  part  of  it,  but  we  are  not  justified  in 
saying  that  the  living  substance  consists  only  of  proteid  matter  in 
a  peculiar  condition.  And  indeed  the  persistency  with  which 
some  representative  of  fatty  bodies  and  some  representative  of 
carbohydrates  always  appear  in  living  tissue,  would  perhaps  rather 
lead  us  to  suppose  that  these  equally  with  proteid  material  were 
essential  to  its  structure.  Again,  though  the  behaviour  of  the 
nucleus  as  contrasted  with  that  of  the  cell  body  leads  us  to 
suppose  that  the  living  substance  of  the  former  is  a  different  kind 
of  living  substance  from  that  of  the  latter,  we  do  not  know  exactly 
in  what  the  difference  consists.  The  nucleus  as  we  have  seen 
contains  mcclein,  which  perhaps  we  may  regard  as  a  largely  modi- 
fied pi'oteid ;  but  a  body  which  is  remarkable  for  its  stability,  for 
the  difficulty  with  which  it  is  changed  by  chemical  reagents, 
cannot  be  regarded  as  an  integral  part  of  the  essentially  mobile 
living  substance  of  the  nucleus. 

In  this  connection  it  may  be  worth  while  again  to  call  attention 
to  the  fact  that  the  corpuscle  contains  a  very  large  quantity  indeed 
of  water,  viz.  about  90  p.c.  Part  of  this,  we  do  not  know  how  much, 
probably  exists  in  a  more  or  less  definite  combination  wdth  the 
protoplasm,  somewhat  after  the  manner  of,  to  use  what  is  a  mere 
illustration,  the  water  of  crystallization  of  salts.  If  we  imagine  a 
whole  group  of  different  complex  salts  continually  occupied  in  turn 
in  being  crystallized,  and  being  decrystallized,  the  water  thus  en- 
gaged by  the  salts  w^ill  give  us  a  rough  image  of  the  water  which 
passes  in  and  out  of  the  substance  of  the  corpuscle  as  the  result  of 
its  metabolic  activitv.     We  mit^lit  call  this  "  water  of  metabolism." 


42  ORIGIN   OF   WHITE  CORPUSCLES.  [Book  i. 

Another  part  of  the  water,  carrying  in  this  case  substances  in 
sohition,  probably  exists  in  spaces  or  interstices  too  small  to  be  seen 
with  even  the  highest  powers  of  the  microscope.  Still  another 
part  of  the  water  similarly  holding  substances  in  solution  exists  at 
times  in  definite  spaces  visible  under  the  microscope,  more  or  less 
regularly  spherical,  and  called  vacuoles. 

We  have  dwelt  thus  at  length  on  the  white  corpuscle  in  the 
first  place  because  as  we  have  already  said  what  takes  place  in  it 
is  in  a  sense  a  picture  of  what  takes  place  in  all  living  structures, 
and  in  the  second  place  because  the  facts  which  we  have  mentioned 
help  us  to  understand  how  the  white  corpuscle  may  carry  on  in 
the  blood  a  work  of  no  unimportant  kind ;  for  from  what  has  been 
said  it  is  obvious  that  the  white  corpuscle  is  continually  acting 
upon  and  being  acted  upon  by  the  plasma. 

§  31.  To  understand  however  the  work  of  these  white  cor- 
puscles we  must  learn  what  is  known  of  their  history. 

In  successive  drops  of  blood  taken  at  different  times  from  the 
same  individual,  the  number  of  colourless  corpuscles  will  be  found 
to  vary  very  much,  not  only  relatively  to  the  red  corpuscles,  but 
also  absolutely.     They  must  therefore  '  come  and  go.' 

In  treating  of  the  lymphatic  system  we  shall  have  to  point  out 
that  a  very  large  quantity  of  fluid  called  lymph,  containing  a  very 
considerable  number  of  bodies  very  similar  in  their  general  cha- 
racters to  the  white  corpuscles  of  the  blood,  is  being  continually 
poured  into  the  vascular  system  at  the  point  where  the  thoracic 
duct  joins  the  great  veins  on  the  left  side  of  the  neck,  and  to 
a  less  extent  where  the  other  large  lymphatics  join  the  venous 
system  on  the  right  side  of  the  neck.  These  corpuscles  of  lymph, 
which,  as  we  have  just  said,  closely  resemble,  and  indeed  are  with 
difficulty  distinguished  from  the  white  corpuscles  of  the  blood, 
but  of  which,  when  they  exist  outside  the  vascular  system,  it 
will  be  convenient  to  speak  of  as  leucocytes,  are  found  along  the 
whole  length  of  the  lymphatic  system,  but  are  more  numerous 
in  the  lymphatic  vessels  after  these  have  passed  through  the 
lymphatic  glands.  These  lymphatic  glands  are  partly  composed  of 
what  is  known  as  adenoid  tissue,  a  special  kind  of  connective 
tissue  arranged  as  a  delicate  network.  The  meshes  of  this  are 
crowded  with  colourless  nucleated  cells,  which  though  varying  in 
size  are  for  the  most  part  small,  the  nucleus  being  surrounded 
by  a  relatively  small  quantity  of  cell-substance.  Many  of  these 
cells  show  signs  that  they  are  undergoing  cell  division,  and  we  have 
reason  to  think  that  cells  so  formed,  acquiring  a  larger  amount  of 
cell-substance,  become  ordinary  leucocytes.  In  other  words,  leuco- 
cytes multiply  in  the  lymphatic  glands,  and  leaving  the  glands  by 
the  lympathic  vessels,  make  their  way  to  the  blood.  Patches  and 
tracts  of  similar  adenoid  tissue,  not  arranged  however  as  distinct 
glands  but  similarly  occupied  by  developing  leucocytes  and  simi- 
larly connected  with  lymphatic  vessels,  are  found  in  various  parts 


Chap,  i.]  BLOOD.  43 

of  the  body,  especially  in  the  mucous  membranes.  Moreover,  the 
leucocytes  appear  to  multiply  by  division  during  their  abode  in 
the  various  lymph  passages.  Hence  we  may  conclude  that  from 
various  parts  of  the  body,  the  lymphatics  are  continually  bringing 
to  the  blood  an  abundant  supply  of  leucocytes,  and  that  these 
become  the  ordinary  white  corpuscles  of  the  blood.  This  is 
probably  the  chief  source  of  the  white  corpuscles,  for  though  the 
white  corpuscles  have  been  seen  dividing  in  the  blood  itself,  no 
large  increase,  so  far  as  we  know,  takes  plac3  in  that  way. 

§  3.2.  It  follows  that  since  white  corpuscles  are  thus  continu- 
ally being  added  to  the  blood,  white  corpuscles  must  as  continually 
either  be  destroyed,  or  ba  transformed,  or  escape  from  the  interior 
of  the  blood  vessels ;  otherwise  the  blood  would  soon  be  blocked 
with  white  corpuscles. 

Some  do  leave  the  blood  vessels.  In  treating  of  the  circulation 
we  shall  have  to  point  out  that  white  corpuscles  are  able  to  pierce 
the  walls  of  the  capillaries  and  minute  veins  and  thus  to  make 
their  way  from  the  interior  of  the  blood  vessels  into  spaces  filled 
with  lymph,  the  "  lymph  spaces,"  as  they  are  called,  of  the  tissue 
lying  outside  the  blood  vessels.  This  is  spoken  of  as  the  "  migra- 
tion of  the  white  corpuscles."  In  an  "  inflamed  area "  large 
numbers  of  white  corpuscles  are  thus  drained  away  from  the 
blood  into  the  lymph  spaces  of  the  tissue ;  and  it  is  probable  that 
a  similar  loss  takes  place,  more  or  less,  under  normal  conditions. 
These  migrating  corpuscles  may,  by  following  the  devious  tracts 
of  the  lymph,  find  their  way  back  into  the  blood ;  some  of  them 
however  may  remain,  and  undergo  various  changes.  Thus,  in 
inflamed  areas,  when  suppuration  follows  inflammation,  the  white 
corpuscles  which  have  migrated  may  become  '  pus  corpuscles,'  or, 
where  thickening  and  growth  follow  upon  inflammation,  may, 
according  to  many  authorities,  become  transformed  into  temporary 
or  permanent  tissue,  especially  connective  tissue  ;  but  this  trans- 
formation into  tissue  is  disputed.  When  an  inflammation  subsides 
without  leaving  any  effect  a  few  corpuscles  only  will  be  found  in 
the  tissue  ;  those  which  had  previously  migrated  must  therefore 
have  been  disposed  of  in  some  way  or  other. 

In  speaking  of  the  formation  of  red  corpuscles  (§  27)  we  saw 
that  not  only  it  is  not  proved  that  the  nucleated  corpuscL'S  which 
give  rise  to  red  corpuscles  are  ordinary  white  corpuscles,  but  that 
in  all  probability  the  real  hsematoblasts,  the  parents  of  red  cor- 
puscles, are  special  corpuscles  developed  in  the  situations  where  the 
manufacture  of  red  corpuscles  takes  place.  So  far  therefore  from 
assuming,  as  is  sometimes  done,  that  the  white  corpuscles  of  the 
blood  are  all  of  them  on  their  way  to  become  red  corpuscles,  it 
may  be  doubted  whether  any  of  them  are.  In  any  case  however, 
even  making  allowance  for  those  wliich  migrate,  a  very  consider- 
able number  of  the  white  corpuscles  must  'disappear'  in  some  way 
or  other  from  the  blood  stream,  and  we  uiay  perhaps  speak  of 


44  BLOOD   PLATELETS.  [Book  i. 

their  disappearance  as  being  a  '  destruction '  or  '  dissolution.'  We 
have  as  yet  no  exact  knowledge  to  guide  us  in  this  matter,  but 
we  can  readily  imagine  that,  upon  the  death  of  the  corpuscle,  the 
substances  composing  it,  after  undergoing  changes,  are  dissolved 
by  and  become  part  of  the  plasma.  If  so,  the  corpuscles  as  they 
die  must  repeatedly  influence  the  composition  and  nature  of  the 
plasma. 

But  if  they  thus  afl'ect  the  plasma  in  their  death,  it  is  even 
more  probable  that  they  influence  it  during  their  life.  Being 
alive  they  must  be  continually  taking  in  and  giving  out.  As  we 
have  already  said  they  are  known  to  ingest,  after  the  fashion  of 
an  amoeba,  solid  particles  of  various  kinds  such  as  fat  or  carmine, 
present  in  the  plasma,  and  probably  digest  such  of  these  particles 
as  are  nutritious.  But  if  they  ingest  these  solid  matters  they 
probably  also  carry  out  the  easier  task  of  ingesting  dissolved 
matters.  If  however  they  thus  take  in,  they  must  also  give  out ; 
and  thus  by  the  removal  on  the  one  hand  of  various  substances 
from  the  plasma,  and  by  the  addition  on  the  other  hand  of  other 
substances  to  the  plasma,  they  must  be  continually  influencing  the 
plasma.  We  have  already  said  that  the  white  corpuscles  in  shed 
blood  as  they  die  are  supposed  to  play  an  important  part  in  the 
clotting  of  blood  ;  similarly  they  may  during  their  whole  life  be 
engaged  in  carrying  out  changes  in  the  proteids  of  the  plasma 
which  do  not  lead  to  clotting,  but  which  prepare  the  proteids  for 
their  various  uses  in  the  body. 

Pathological  facts  afford  support  to  this  view.  The  disease 
called  leucocytheemia  (or  leuksemia)  is  characterised  by  an  increase 
of  the  white  corpuscles,  both  absolute  and  relative  to  the  red 
corpuscles,  the  increase,  due  to  an  augmented  production  or 
possibly  to  a  retarded  destruction,  being  at  times  so  great  as  to 
give  the  blood  a  pinkish  grey  appearance,  like  that  of  blood  mixed 
with  pus.  We  accordingly  find  that  in  this  disease  the  plasma  is 
in  many  ways  profoundly  affected  and  fails  to  nourish  the  tissues. 
As  a  further  illustration  of  the  possible  actions  of  the  white 
corpuscles  we  may  state  that,  in  certain  diseases  in  which  minute 
organisms,  such  as  bacteria,  make  their  appearance  in  the  blood 
and  tissues,  white  corpuscles  may  attack  and  devour  these  bacteria, 
thus  acting  as  "  phagocytes,"  and  in  this  way,  or  otherwise,  by 
exerting  some  influence  on  the  bacteria  or  the  products  of  their 
activity,  modify  the  course  of  the  disease  of  which  the  bacteria  are 
the  essential  cause. 

Blood  Platelets. 

§  33.  In  a  drop  of  blood  examined  with  care  immediately 
after  removal,  may  be  seen  a  number  of  exceedingly  small  bodies 
(2  /A  to  3  /u-  in  diameter)  frequently  disc-shaped  but  sometimes  of  a 
rounded  or  irregular  form,  homogeneous  in  appearance  when  quite 


Chap,  i.]  BLOOD.  45 

fresh  but  apt  to  assume  a  faintly  granular  aspect.  They  are 
called  blood  platelets.  They  have  been  supposed  by  some  to  become 
developed  into  and  indeed  to  be  early  stages  of  the  red  corpuscles, 
and  hence  have  been  called  hsematoblasts ;  but  this  view  has  not 
been  confirmed,  indeed,  as  we  have  seen  (§  27),  the  real  ha?mato- 
blasts  or  developing  red  corpuscles  are  of  quite  a  different  nature. 

They  speedily  undergo  change  after  removal  from  the  body, 
apparently  dissolving  in  the  plasma ;  they  break  up,  part  of  their 
substance  disappearing,  while  the  rest  becomes  granular.  Their 
granular  remains  are  apt  to  run  together,  forming  in  the  plasma  the 
shapeless  masses  which  have  long  been  known  and  described  as 
"  lumps  of  protoplasm."  By  appropriate  reagents,  however,  these 
platelets  may  be  fixed  and  stained  in  the  condition  in  which  they 
appear  after  leaving  the  body. 

The  substance  composing  them  is  peculiar,  and  though  we 
may  perhaps  speak  of  them  as  consisting  of  living  material,  their 
nature  is  at  present  obscure.  They  may  be  seen  within  the  living 
blood  vessels,  and  therefore  must  be  regarded  as  real  parts  of  the 
blood  and  not  as  products  of  the  changes  taking  place  in  blood 
after  it  has  been  shed. 

When  a  needle  or  thread  or  other  foreign  body  is  introduced 
into  the  interior  of  a  blood  vessel,  they  are  apt  to  collect  upon,  and 
indeed  are  the  precursors  of  the  clot  which  in  most  cases  forms 
around  the  needle  or  thread.  They  are  also  found  in  the  thromhi 
or  plugs  which  sometimes  form  in  the  blood  vessels  as  the  result  of 
disease  or  injury.  Indeed  it  has  been  maintained  that  what  are 
called  lohite  thromhi  (to  distinguish  them  from  red  thrombi,  which 
are  plugs  of  corpuscles  and  fibrin)  are  in  reality  aggregations  of 
blood  platelets ;  and  for  various  reasons  blood  platelets  have  been 
supposed  to  play  an  important  part  in  the  clotting  of  blood,  carrying 
out  the  work  which  in  this  respect  is  by  others  attributed  to  the 
white  corpuscles.  But  no  very  definite  statement  can  at  present 
be  made  about  this ;  and  indeed  the  origin  and  whole  nature 
of  these  blood  platelets  is  at  present  obscure. 


SEC.   3.     THE   CHEMICAL  COMPOSITION  OF   BLOOD. 


§  34.  We  may  now  pass  briefly  in  review  the  chief  chemical 
characters  of  blood,  remembering  always  that,  as  we  have  already 
urged,  the  chief  chemical  interests  of  blood  are  attached  to  the 
changes  which  it  undergoes  in  the  several  tissues;  these  will  be 
considered  in  connection  with  each  tissue  at  the  appropriate  place. 

The  average  specific  gravity  of  human  blood  is  1055,  varying 
from  1045  to  1075  within  the  limits  of  health. 

The  reaction  of  blood  as  it  flows  from  the  blood  vessels  is 
found  to  be  distinctly  though  feebly  alkaline.  If  a  drop  be  placed 
on  a  piece  of  faintly-red  highly-glazed  litmus  paper,  and  then 
wiped  off,  a  blue  stain  will  be  left. 

The  whole  blood  contains  a  certain  quantity  of  the  gases, 
oxygen,  carbonic  acid,  and  nitrogen,  which  are  held  in  the  blood  in 
a  peculiar  way,  and  which  are  given  off  from  blood  when  exposed 
to  a  vacuum  or  to  an  atmosphere  of  suitable  composition ;  the 
relative  amounts  differ  in  different  kinds  of  blood,  and  so  serve 
especially  to  distinguish  arterial  from  venous  blood.  These  gases 
of  blood  we  shall  study  in  connection  with  respiration. 

The  normal  blood  consists  of  corpuscles  and  plasyna. 

If  the  corpuscles  be  supposed  to  retain  the  amount  of  water 
proper  to  them,  blood  may,  in  general  terms,  be  considered  as 
consisting  by  weight  of  from  about  one-third  to  somewhat  less 
than  one-half  of  corpuscles,  the  rest  being  plasma.  As  we  have 
already  seen,  the  number  of  corpuscles  in  a  specimen  of  blood  is 
found  to  vary  considerably,  not  only  in  different  animals  and  in 
different  individuals,  but  in  the  same  individual  at  different  times. 

The  plasma  is  resolved  by  the  clotting  of  the  blood  into  serum 
and  fibrin. 

§  36.     The  serum  contains  in  100  parts 
Proteid  substances  about  8  or  9  parts. 

Fats,  various  extractives,  and  saline  matters  „     2  or  1      „ 

Water  „       90 


Chap,  i.]  BLOOD.  47 

The  proteids  are  ijaraglobulin  and  serum  albumin  (there  being 
probably  more  than  one  kind  of  serum  albumin)  in  varying  pro- 
portion. We  may  perhaps,  roughly  speaking,  say  that  they  occur 
in  about  equal  quantities. 

Conspicuous  and  striking  as  are  the  results  of  clotting,  mas- 
sive as  appears  to  be  the  clot  which  is  formed,  it  must  be  remem- 
bered that  by  far  the  greater  part  of  the  clot  consists  of  corpuscles. 
The  amount  by  weight  of  fibrin  required  to  bind  together  a  number 
of  corpuscles  in  order  to  form  even  a  large  firm  clot  is  exceedingly 
small.  Thus  the  average  quantity  by  weight  of  fibrin  in  human 
blood  is  said  to  be  -2  p.c. ;  the  amount  however  which  can  be 
obtained  from  a  given  quantity  of  plasma  varies  extremely,  the 
variation  being  due  not  only  to  circumstances  affecting  the  blood, 
but  also  to  the  method  employed. 

The  fats,  which  are  scanty,  except  after  a  meal  or  in  certain 
pathological  conditions,  consist  of  the  neutral  fats,  stearin,  palmitin, 
and  olein,  with  a  certain  quantity  of  their  respective  alkaline  soaps. 
The  peculiar  complex  fat  lecithin  occurs  in  very  small  quantities 
only  ;  the  amount  present  of  the  peculiar  alcohol  cholesterin  which 
has  so  fatty  an  appearance  is  also  small.  Among  the  extractives 
present  in  serum  may  be  put  down  nearly  all  the  nitrogenous 
and  other  substances  which  form  the  extractives  of  the  body  and 
of  food,  such  as  urea,  kreatin,  sugar,  lactic  acid,  &c.  A  very 
large  number  of  these  have  been  discovered  in  the  blood  under 
various  circumstances,  the  consideration  of  which  must  be  left  for 
the  present.  The  peculiar  odour  of  blood  or  of  serum  is  probably 
due  to  the  presence  of  volatile  bodies  of  the  fatty  acid  series.  The 
faint  yellow  colour  of  serum  is  due  to  a  special  yellow  pigment. 
The  most  characteristic  and  important  chemical  feature  of  the 
saline  constitution  of  the  serum  is  the  preponderance,  at  least  in 
man  and  most  animals,  of  sodium  salts  over  those  of  potassium. 
In  this  respect  the  serum  offers  a  marked  contrast  to  the  corpuscles. 
Less  marked,  but  still  striking,  is  the  abundance  of  chlorides  and 
the  poverty  of  phosphates  in  the  serum  as  compared  with  the 
corpuscles.  The  salts  may  in  fact  briefly  be  described  as  consisting 
chiefly  of  sodium  chloride,  with  some  amount  of  sodium  carbonate, 
or  more  correctly  sodium  bicarbonate,  and  potassium  chloride,  with 
small  quantities  of  sodium  sulphate,  sodium  phosphate,  calcium 
phosphate,  and  magnesium  phosphate.  And  of  even  the  small 
quantity  of  phosphates  found  in  the  ash,  part  of  the  phosphorus 
exists  in  the  serum  itself,  not  as  a  phosphate  but  as  phosphorus  in 
some  organic  body. 

§  36.  The  red  corpuscles  contain  less  water  than  the  serum, 
the  amount  of  solid  matter  being  variously  estimated  at  from  30  to 
40  or  more  p.c.  The  solids  are  almost  entirely  organic  matter,  the 
inorganic  salts  amounting  to  less  than  1  p.c.  Of  the  organic  matter 
again  by  far  the  larger  part  consists  of  haemoglobin.  In  100  parts 
of  the  dried  organic  matter  of  the  corpuscles  of  human  blood,  about 


48  COMPOSITION   OF  BLOOD.  [Book  i. 

90  parts  are  licemoglobin,  about  8  parts  are  proteid  substances, 
and  about  2  parts  are  other  substances.  Of  these  other  substances 
one  of  the  most  important,  forming  about  a  quarter  of  them  and 
apparently  being  always  present,  is  lecithin.  Cholesterin  appears 
also  to  be  normally  present.  The  proteid  substances  which  form 
the  stroma  of  the  red  corpuscles  appear  to  belong  chiefly  to  the 
globulin  family.  As  regards  the  inorganic  constituents,  the  cor- 
puscles are  distinguished  by  the  relative  abundance  of  the  salts 
of  potassium  and  of  phosphates.  This  at  least  is  the  case  in  man  ; 
the  relative  quantities  of  sodium  and  potassium  in  the  corpuscles 
and  serum  respectively  appear  however  to  vary  in  different 
animals  ;  in  some  the  sodium  salts  are  in  excess  even  in  the 
corpuscles. 

§  37.  The  proteid  matrix  of  the  loliite  corjmscles,  we  have 
stated  to  be  composed  of  nucleo-albumin,  globulin,  and  possibly 
other  proteids.  The  nuclei  contain  nuclein.  The  white  cor- 
puscles are  found  to  contain,  in  addition  to  proteid  material, 
lecithin  and  other  fats,  glycogen,  extractives  and  inorganic  salts, 
there  being  in  the  ash  as  in  that  of  the  red  corpuscles  a  pre- 
ponderance of  potassium  salts  and  of  phosphates. 

The  main  facts  of  interest,  then,  in  the  chemical  composition  of 
the  blood  are  as  follows :  The  red  corpuscles  consist  chiefly  of 
hcemoglobin.  The  organic  solids  of  serum  consist  partly  of  serum- 
albumin,  and  partly  of  paraglobulin.  The  serum  or  plasma 
contrasts,  in  man  at  least,  with  the  corpuscles,  inasmuch  as  the 
former  contains  chiefly  chlorides  and  sodium  salts  while  the  latter 
are  richer  in  phosphates  and  potassium  salts.  The  extractives  of 
the  blood  are  remarkable  rather  for  their  number  and  variability 
than  for  their  abundance,  the  most  constant  and  important  being 
perhaps  urea,  kreatin,  sugar,  and  lactic  acid. 


SEC.    4.     THE   QUANTITY   OF   BLOOD,    AND    ITS 
DISTRIBUTION   IN  THE  BODY. 


§  38.  The  quantity  of  blood  contained  in  the  whole  vascular 
system  is  a  balance  struck  between  the  tissues  which  give  to  and 
those  which  take  away  from  the  blood.  Thus  the  tissues  of  the 
alimentary  canal  largely  add  to  the  blood  water  and  the  material 
derived  from  food,  while  the  excretory  organs  largely  take  away 
water  and  the  other  substances  constituting  the  excretions.  Other 
tissues  both  give  and  take ;  and  the  considerable  drain  from  the 
blood  to  the  lymph  spaces  which  takes  place  in  the  capillaries  is 
met  by  the  flow  of  lymph  into  the  great  veins. 

From  the  result  of  a  few  observations  on  executed  criminals  it 
has  been  concluded  that  the  total  quantity  of  blood  in  the  human 
body  is  about  Jgth  of  the  body  weight.  But  in  various  animals, 
the  proportion  of  the  weight  of  the  blood  to  that  of  the  body  has 
been  found  to  vary  very  considerably  in  different  individuals  ,  and 
probably  this  holds  good  for  man  also,  —  at  all  events  within  cer- 
tain limits. 

In  the  same  individual  the  quantity  probably  does  not  vary 
largely.  A  sudden  drain  upon  the  water  of  the  blood  by  great 
activity  of  the  excretory  organs,  as  by  profuse  sweating,  or  a 
sudden  addition  to  the  water  of  the  blood,  as  by  drinking  large 
quantities  of  water  or  by  injecting  fluid  into  the  blood  vessels,  is 
rapidly  compensated  by  the  passage  of  water  from  the  tissues  to 
the  blood  or  from  the  Ijlood  to  the  tissues.  As  we  have  already 
said,  the  tissues  are  continually  striving  to  keep  up  an  average 
composition  of  the  blood,  and  in  so  doing  keep  up  an  average 
quantity.  In  starvation  the  quantity  (and  quality)  of  the  blood 
is  maintained  for  a  long  time  at  the  expense  of  the  tissues,  so 
that  after  some  days  deprivation  of  food  and  drink,  while  the  fat, 
the  muscles,  and  other  tissues  have  been  largely  diminished,  the 
quantity  of  blood  remains  nearly  the  same. 

4 


50  QUANTITY  OF  BLOOD.  [Book  i. 

The  total  quantity  of  blood  present  in  an  animal  body  is  estimated 
in  the  following  way  :  As  much  blood  as  possible  is  allowed  to  escape 
from  the  vessels ;  this  is  measured  directly.  The  vessels  are  then 
washed  out  with  water  or  normal  saline  solution,  and  the  washings 
carefully  collected,  mixed,  and  measured.  A  known  quantity  of  blood 
is  diluted  with  water  or  normal  saline  solution  until  it  possesses  the 
same  tint  as  a  measured  specimen  of  the  washings.  This  gives  the 
amount  of  blood  (or  rather  of  haemoglobin)  in  the  measured  specimen, 
from  which  the  total  quantity  in  the  whole  washings  is  calculated. 
Lastly,  the  whole  body  is  carefully  minced  and  washed  free  from  blood. 
The  washings  are  collected  and  filtered,  and  the  amount  of  blood  in 
them  is  estimated  as  before  by  comparison  with  a  specimen  of  diluted 
blood.  The  quantity  of  blood,  as  calculated  from  the  two  washings, 
together  with  the  escaped  and  directly  measured  blood,  gives  the  total 
quantity  of  blood  in  the  body. 

The  method  is  not  free  from  objections,  but  other  methods  are  open 
to  still  graver  objections. 

The  blood  is  in  round  numbers  distributed  as  follows  :  — 
About  one-fourth  in  the  heart,  lungs,  large  arteries,  and  veins, 
„     „     liver, 
„  „  „     „     skeletal  muscles, 

„  „  „     „     other  organs. 

Since  in  the  heart  and  great  blood  vessels  the  blood  is  simply 
in  transit,  without  undergoing  any  great  changes  (and  in  the 
lungs,  so  far  as  we  know,  the  changes  are  limited  to  respiratory 
changes),  it  follows  that  the  changes  which  take  place  in  the  blood 
passing  through  the  liver  and  skeletal  muscles  far  exceed  those 
which  take  place  in  the  rest  of  the  body. 


CHAPTER   11. 
THE  CONTEACTILE  TISSUES. 


§  39.  In  order  that  the  blood  may  nourish  the  several  tissues 
it  is  carried  to  and  from  them  by  the  vascular  mechanism  ;  and 
this  carriage  entails  active  movements.  In  order  that  the  blood 
may  adequately  nourish  the  tissues,  it  must  be  replenished  by  food 
from  the  alimentary  canal,  and  purified  from  waste  by  the  excretory 
organs  ;  and  both  these  processes  entail  movements.  Hence  before 
we  proceed  further  we  must  study  some  of  the  general  characters 
of  the  movements  of  the  body. 

Most  of  the  movements  of  the  body  are  carried  out  by  means 
of  the  muscles  of  the  trunk  and  limbs,  which  being  connected  with 
the  skeleton  are  frequently  called  skeletal  muscles.  A  skeletal 
muscle  when  subjected  to  certain  influences  suddenly  shortens, 
bringing  its  two  ends  nearer  together;  and  it  is  the  shortening 
which,  by  acting  upon  various  bony  levers  or  by  help  of  other 
mechanical  arrangements,  produces  the  movement.  Such  a  tem- 
porary shortening,  called  forth  by  certain  influences  and  due  as  we 
shall  see  to  changes  taking  place  in  the  muscular  tissue  forming 
the  chief  part  of  the  muscle,  is  technically  called  a  contraction  of 
the  muscle  ;  and  the  muscular  tissue  is  spoken  of  as  a  contractile 
tissue.  The  heart  is  chiefly  composed  of  muscular  tissue,  differing 
in  certain  minor  features  from  the  muscular  tissue  of  the  skeletal 
muscles ;  and  the  beat  of  the  heart  is  essentially  a  contraction  of 
the  musclar  tissue  composing  it,  a  shortening  of  the  peculiar 
muscular  fibres  of  which  the  heart  is  chiefly  made  up.  The 
movements  of  the  alimentary  canal  and  of  many  other  organs  are 
similarly  the  results  of  the  contraction  of  the  muscular  tissue 
entering  into  the  composition  of  those  organs,  of  the  shortening  of 
certain  muscular  fibres  built  up  into  those  organs.  In  fact  almost 
all  the  movements  of  the  body  are  the  results  of  the  contraction  of 
muscular  fibres,  of  various  nature  and  variously  disposed. 


52  THE  CONTEACTILE   TISSUES.  [Book  i. 

Some  few  movements  however  are  carried  out  by  structures 
which  cannot  be  called  muscular.  Thus  in  the  pulmonary  passages 
and  elsewhere  movement  is  effected  by  means  of  cilia  attached  to 
epithelium  cells ;  and  elsewhere,  as  in  the  case  of  the  migrating 
white  corpuscles  of  the  blood,  transference  from  place  to  place  in 
the  body  is  brought  about  by  amoeboid  movements.  But,  as  we 
shall  see,  the  changes  in  the  epithelium  cell  or  white  corpuscle 
which  are  at  the  bottom  of  ciliary  or  amoeboid  movements  are  in 
all  probability  fundamentally  the  same  as  those  which  take  place 
in  a  muscular  fibre  when  it  contracts.  They  are  of  the  nature  of 
a  contraction,  and  hence  we  may  speak  of  all  these  as  different 
forms  of  contractile  tissue. 

Of  all  these  various  forms  of  contractile  tissue  the  skeletal 
muscles,  on  account  of  the  more  complete  development  of  their 
functions,  will  be  better  studied  first ;  the  others,  on  account 
of  their  very  simplicity,  are  in  many  respects  less  satisfactorily 
understood. 

All  the  ordinary  skeletal  muscles  are  connected  with  nerves. 
We  have  no  reason  for  thinking  that  they  are  thrown  into  con- 
traction, under  normal  conditions,  otherwise  than  by  the  agency  of 
nerves.  Muscles  and  nerves  being  thus  so  closely  allied,  and 
having  besides  so  many  properties  in  common,  it  will  conduce 
to  clearness  and  brevity  if  we  treat  them  together. 


SEC.   1.     THE  PHENOMENA  OF  MUSCLE  AND  NERVE. 


Muscular  and  Nervous  h^ritahility. 

§  40.  The  skeletal  muscles  of  a  frog,  the  brain  and  spinal 
cord  of  which  have  been  destroyed,  do  not  exhibit  any  spontaneous 
movements  or  contractions,  even  though  the  nerves  be  otherwise 
quite  intact.  Left  undisturbed  the  whole  body  may  decompose 
without  any  contraction  of  any  of  the  skeletal  muscles  having 
been  witnessed.  Neither  the  skeletal  muscles  nor  the  nerves 
distributed  to  them  possess  any  power  of  automatic  action. 

If  however  a  muscle  be  laid  bare  and  be  more  or  less  violently 
disturbed,  —  if  for  instance  it  be  pinched,  or  touched  with  a  hot 
wire,  or  brought  into  contact  with  certain  chemical  substances, 
or  subjected  to  the  action  of  galvanic  currents,  —  it  will  move,  that 
is  contract,  whenever  it  is  thus  disturbed.  Though  not  exhibiting 
any  spontaneous  activity,  the  muscle  is  (and  continues  for  some 
time  after  the  general  death  of  the  animal  to  be)  irritahle. 
Though  it  remains  quite  quiescent  when  left  untouched,  its 
powers  are  then  dormant  only,  not  absent.  These  require  to  be 
roused  or  '  stimulated '  by  some  change  or  disturbance  in  order 
that  they  may  manifest  themselves.  The  substances  or  agents 
which  are  thus  able  to  evoke  the  activity  of  an  irritable  muscle 
are  spoken  of  as  stimuli. 

But  to  produce  a  contraction  in  a  muscle  the  stimulus  need 
not  be  applied  directly  to  the  muscle  ;  it  may  be  applied  indirectly 
by  means  of  the  nerve.  Thus,  if  the  trunk  of  a  nerve  be  pinched, 
or  .subjected  to  sudden  heat,  or  dipped  in  certain  chemical  sub- 
stances, or  acted  upon  by  various  galvanic  currents,  contractions 
are  seen  in  the  muscles  to  which  branches  of  the  nerve  are 
distributed. 

The  nerve  like  the  muscle  is  irritable  ;  it  is  thrown  into  a  state 
of  activity  by  a  stimulus  ;  but  unlike  tlie  muscle  it  does  not  itself 
contract.  The  stimulus  does  not  give  rise  in  the  nerve  to  any 
visible  change  of  form ;  but  that  changes  of  some  kind  or  other 


54  MUSCULAE   IKEITABILITY.  [Book  i. 

are  set  up  and  propagated  along  the  nerve  down  to  the  muscle  is 
shewn  by  the  fact  that  the  muscle  contracts  when  a  part  of  the 
nerve  at  some  distance  from  itself  is  stimulated.  Both  nerve  and 
muscle  are  irritable,  but  only  the  muscle  is  contractile, —  i.  e.,  mani- 
fests its  irritability  by  a  contraction.  The  nerve  manifests  its 
irritability  by  transmitting  along  itself,  without  any  visible  altera- 
tion of  form,  certain  molecular  changes  set  up  by  the  stimulus. 
We  shall  call  these  changes  thus  propagated  along  a  nerve, 
'nervous  impulses.' 

§  41.  We  have  stated  above  that  the  muscle  may  be  thrown 
into  contractions  by  stimuli  applied  directly  to  itself.  But  it 
might  fairly  be  urged  that  the  contractions  so  produced  are  in 
reality  due  to  the  fact  that  the  stimulus,  although  apparently 
applied  directly  to  the  muscle,  is,  after  all,  brought  to  bear  on  some 
or  other  of  the  many  fine  nerve-branches,  which  as  we  shall  see  are 
abundant  in  the  muscle,  passing  among  and  between  the  muscular 
fibres  in  which  they  finally  end.  The  following  facts  however  go 
far  to  prove  that  the  muscular  fibres  themselves  are  capable  of 
being  directly  stimulated  without  the  intervention  of  any  nerves. 
When  a  frog  (or  other  animal)  is  poisoned  with  urari,  the  nerves 
may  be  subjected  to  the  strongest  stimuli  without  causing  any 
contractions  in  the  muscles  to  which  they  are  distributed ;  yet 
even  ordinary  stimuli  applied  directly  to  the  muscle  readily  cause 
contractions.  If  before  introducing  the  urari  into  the  system, 
a  ligature  be  passed  underneath  the  sciatic  nerve  in  one  leg,  —  for 
instance  the  right,  —  and  drawn  tightly  round  the  whole  leg  to  the 
exclusion  of  the  nerve,  it  is  evident  that  the  urari  when  injected 
into  the  back  of  the  animal,  will  gain  access  to  the  right  sciatic 
nerve  above  the  ligature,  but  not  below,  while  it  will  have  free 
access  to  the  rest  of  the  body,  including  the  whole  left  sciatic.  If, 
as  soon  as  the  urari  has  taken  effect,  the  two  sciatic  nerves  be 
stimulated,  no  movement  of  the  left  leg  will  be  produced  by  stimu- 
lating the  left  sciatic,  whereas  strong  contractions  of  the  muscles  of 
the  right  leg  below  the  ligature  will  follow  stimulation  of  the  right 
sciatic,  whether  the  nerve  be  stimulated  above  or  below  the  ligature. 
Now,  since  the  upper  parts  of  both  sciatics  are  equally  exposed  to 
the  action  of  the  poison,  it  is  clear  that  the  failure  of  the  left  nerve 
to  cause  contraction  is  not  attributable  to  any  change  having  taken 
place  in  the  upper  portion  of  the  nerve,  else  why  should  not  the 
right,  which  has  in  its  upper  portion  been  equally  exposed  to  the 
action  of  the  poison,  also  fail  ?  Evidently  the  poison  acts  on  some 
parts  of  the  nerve  lower  down.  If  a  single  muscle  be  removed  from 
the  circulation  (by  ligaturing  its  blood  vessels),  previous  to  the 
poisoning  with  urari,  that  muscle  will  contract  when  any  part  of  the 
nerve  going  to  it  is  stimulated,  though  no  other  muscle  in  the  body 
will  contract  when  its  nerve  is  stimulated.  Here  the  whole  nerve 
right  down  to  the  muscle  has  been  exposed  to  the  action  of  the 
poison;  and  yet  it  has  lost  none  of  its  power  over  the  muscle.    On 


Chap,  ii.]  THE   CONTRACTILE   TISSUES.  55 

the  other  hand,  if  the  muscle  be  allowed  to  remain  in  the  body, 
and  so  be  exposed  to  the  action  of  the  poison,  but  the  nerve  be 
divided  high  up  and  the  part  connected  with  the  muscle  gently 
lifted  up  before  the  urari  is  introduced  into  the  system,  so  that  no 
blood  flows  to  it  and  so  that  it  is  protected  from  the  influence  of 
the  poison,  stimulation  of  the  nerve  will  be  found  to  produce  no 
contractions  in  the  muscle,  though  stimuli  applied  directly  to  the 
muscle  at  once  cause  it  to  contract.  From  these  facts  it  is  clear 
that  urari  poisons  the  ends  of  the  nerve  within  the  muscle  long 
before  it  affects  the  trunk ;  and  it  is  exceedingly  probable  that  it 
is  the  very  extreme  ends  of  the  nerves  (possibly  the  end-plates,  or 
peculiar  structures  in  which  the  nerve  fibres  end  in  the  muscular 
fibres,  —  for  urari  poisoning,  at  least  when  profound,  causes  a  slight 
but  yet  distinctly  recognisable  effect  in  the  microscopic  appearance 
of  these  structures)  which  ^re  affected.  The  phenomena  of  urari 
poisoning  therefore  go  far  to  prove  that  muscles  are  capable  of 
being  made  to  contract  by  stimuli  applied  directly  to  the  muscular 
fibres  themselves ;  and  there  are  other  facts  which  support  this 
view. 

§  42.  When  in  a  recently  killed  frog  we  stimulate  by  various 
means  and  in  various  ways  the  muscles  and  nerves,  it  will  be 
observed  that  the  movements  thus  produced,  though  very  various, 
may  be  distniguished  to  be  of  two  kinds.  On  the  one  hand,  the 
result  may  be  a  mere  twitch,  as  it  were,  of  this  or  that  muscle  ; 
on  the  other  hand,  one  or  more  muscles  may  remain  shortened, 
contracted  for  a  considerable  time,  —  a  limb  for  instance  being 
raised  up  or  stretched  out,  and  kept  raised  up  or  stretched  out  for 
many  seconds.  And  we  find  upon  examination  that  a  stimulus 
may  be  applied  either  in  such  a  way  as  to  produce  a  mere  twitch, 
—  a  passing,  rapid  contraction  which  is  over  and  gone  in  a  fraction 
of  a  second,  —  or  in  such  a  way  as  to  keep  the  muscle  shortened  or 
contracted  for  so  long  time  as,  up  to  certain  limits,  we  may  choose. 
The  mere  twitch  is  called  a  single  or  simijlc  muscular  contraction  ; 
the  sustained  contraction,  which  as  we  shall  see  is  really  the  result 
of  rapidly  repeated  simple  contractions,  is  called  a  tetanic  con- 
traction. 

§  43.  In  order  to  study  these  contractions  adequately,  we  must 
have  recourse  to  the  '  graphic  method '  as  it  is  called,  and  obtain  a 
tracing  or  other  record  of  the  change  of  form  of  the  muscle.  To 
do  this  conveniently,  it  is  best  to  operate  with  a  muscle  isolated 
from  the  rest  of  the  body  of  a  recently  killed  animal,  and  carefully 
prepared  in  such  a  way  as  to  remain  irritable  for  some  time.  The 
muscles  of  cold  blooded  animals  remain  irritable  after  removal 
from  the  body  far  longer  than  those  of  warm  blooded  animals,  and 
hence  those  of  the  frog  are  generally  made  use  of.  We  shall  study 
presently  the  conditions  which  determine  this  maintenance  of  the 
irritability  of  muscles  and  nerves  after  removal  from  the  body. 

A  muscle  thus  isolated,  with  its  nerve  left  attached  to  it,  is 


56 


ELECTEICAL   STIMULI. 


[Book  i. 


called  a  muscle-nerve  preparation.  The  most  convenient  muscle 
for  this  purpose  in  the  frog  is  perhaps  the  gastrocnemius,  which 
should  be  dissected  out  so  as  to  leave  carefully  preserved  the 
attachment  to  the  femur  above,  some  portion  of  the  tendon  (tendo 
achillis)  below,  and  a  considerable  length  of  the  sciatic  nerve  with 
its  branches  going  to  the  muscle.     Fig.  1. 


Fig.  1.    A  Muscle-nerve  Preparation. 

VI,  the  muscle,  gastrocnemius  of  frog ;  n,  the  sciatic  nerve,  all  the  branches 
being  cut  away  except  that  supplying  the  muscle  ;  /,  femur ;  d.  clamp  ;  t.  a.  tenda 
achillis  ;  sp.  c.  end  of  spinal  canal. 

§  44.  We  may  apply  to  such  a  muscle-nerve  preparation  the 
various  kinds  of  stimuli  spoken  of  above,  —  mechanical,  such  as 
pricking  or  pinching  ;  thermal,  such  as  sudden  heating ;  chemical, 
such  as  acids  or  other  active  chemical  substances,  or  electrical ; 
and  these  we  may  apply  either  to  the  muscle  directly,  or  to  the 
nerve,  thus  affecting  the  muscle  indirectly.  Of  all  these  stimuli 
by  far  the  most  convenient  for  general  purposes  are  electrical 
stimuli  of  various  kinds ;  and  these,  except  for  special  purposes, 
are  best  applied  to  the  nerve,  and  not  directly  to  the  muscle. 

Of  electrical  stimuli  again,  the  currents,  as  they  are  called, 
generated  by  a  voltaic  cell  are  most  convenient,  though  the 
electricity  generated  by  a  rotating  magnet,  or  that  produced  by 
friction,  may  be  employed.  Making  use  of  a  cell  or  battery  of  cells, 
Daniell's,  Grove's,  Leclanchd,  or  any  other,  we  must  distinguish 
between  the  current  produced   by  the  cell   itself    (the  constant 


Chap,  ii.]  THE   CONTEACTILE   TISSUES.  57 

current  as  we  shall  call  it)  and  the  induced  current  obtained  from 
the  constant  current  by  means  of  an  induction  coil,  as  it  is  called ; 
for  the  physiological  effects  of  the  two  kinds  of  current  are  in 
many  ways  different. 

It  may  perhaps  be  worth  while  to  remind  the  reader  of  the  following 
facts :  — 

In  a  galvanic  battery,  the  substance  (plate  of  zinc  for  instance) 
which  is  acted  upon  and  used  up  by  the  liquid  is  called  the  positive 
element,  and  the  substance  which  is  not  so  acted  upon  and  used  up 
(plate  &c.  of  copper,  platinum,  or  carbon,  &c.)  is  called  the  negative 
element.  A  galvanic  action  is  set  up  when  the  positive  (zinc)  and  the 
negative  (copper)  elements  are  connected  outside  the  battery  by  some 
conducting  material,  such  as  a  wire,  and  the  current  is  said  to  flow  in  a 
circuit  or  circle  from  the  zinc  or  positive  element  to  the  copper  or 
negative  element  inside  the  battery,  and  then  from  the  copper  or  negative 
element  back  to  the  zinc  or  positive  element  through  the  wire  outside 
the  battery.  If  the  conducting  wire  be  cut  through,  the  current  ceases 
to  flow ;  but  if  the  cut  ends  be  brought  into  contact,  the  current  is  re- 
established and  continues  to  flow  so  long  as  the  contact  is  good.  The 
ends  of  the  wires  are  called  '  poles,'  or  when  used  for  physiological 
purposes,  in  which  case  they  may  be  fashioned  in  various  ways,  are 
spoken  of  as  electrodes.  When  the  poles  are  brought  into  contact  or 
are  connected  by  some  conducting  material,  galvanic  action  is  set  up, 
and  the  current  flows  through  the  battery  and  wires  ;  this  is  spoken  of 
as  "  making  the  current "  or  "  completing  or  closing  the  circuit."  When 
the  poles  are  drawn  apart  from  each  other,  or  when  some  non-conducting 
material  is  interposed  between  them,  the  galvanic  action  is  arrested  ; 
this  is  spoken  of  as  "  breaking  the  current "  or  "  opening  the  circuit." 
The  current  passes  from  the  wire  connected  with  the  negative  (copper) 
element  in  the  battery  to  the  wire  connected  with  the  positive  (zinc) 
element  in  the  battery ;  hence  the  pole  connected  with  the  copper 
(negative)  element  is  called  the  positive  pole,  and  that  connected  with 
the  zinc  (positive)  element  is  called  the  negative  pole.  When  used  for 
physiological  purposes  the  positive  pole  becomes  the  positive  electrode, 
and  the  negative  pole  the  negative  electrode.  The  positive  electrode  is 
often  spoken  of  as  the  anode  (ana,  up),  and  the  negative  electrode  as 
the  kathode  (kata,  down). 

A  piece  of  nerve  of  ordinary  length,  though  not  a  good  conductor, 
is  still  a  conductor,  and  when  placed  on  the  electrodes,  completes  the 
circuit,  permitting  the  current  to  pass  through  it ;  in  order  to  remove 
the  nerve  from  the  influence  of  the  current  it  must  be  lilted  off"  from 
the  electrodes.  This  is  obviously  inconvenient ;  and  hence  it  is  usual 
to  arrange  a  means  of  opening  or  closing  the  circuit  at  some  point  along 
one  of  the  two  wires.  This  maybe  done  in  various  ways,  — by  fastening 
one  part  of  the  wire  into  a  cup  of  mercury  and  so  by  dipping  the  other 
part  of  tlie  wire  into  the  cup  to  close  the  circuit  and  make  the  current, 
and  by  lifting  it  out  of  the  mercury  to  open  the  circuit  and  break  the 
current ;  or  by  arranging,  between  the  two  parts  of  the  wires,  a 
moveable  bridge  of  good  conducting  material  such  as  brass,  which  can 
be  put  down  to  close   the  circuit  or  raised   up   to  open  the  circuit ;   or  in 


58 


IXDUCTIOX   COIL. 


[Book 


other  -ways.     Such  a  means  of  closing  and  opening  a   circuit  and  so  of 
making  or  breaking  a  current  is  called  a  hey. 

A  key  which  is  frequently  used  by  physiologists  goes  by  the  name  of 
du  Bois-Eeymond's  key ;  though  undesirable  in  many  respects  it  has 
the  advantage  that  it  can  be  used  in  two  different  ways.  It  may  be 
arranged  as  in  A,  Fig.  2.  In  this  case,  when  the  brass  bridge  of  K, 
the  key  is  put  down  (dotted  outline  in  the  figure),  so  as  to  form  a 
means  of  good  conduction  between  the  brass  plates  to  which  the  wires 
are  screwed,  the  circuit  is  closed  and  the  current  passes  from  the  posi- 
tive pole  (end  of  the  negative  —  copper —  element)  to  the  positive  electrode 
or  anode.  An.  through  the  nerve,  to  the  negative  electrode  or  kathode 
Kat.  and  thence  back  to  the  negative  pole  (end  of  the  positive  —  zinc  — 


Fig.  2.    Diagram  of  Dd  Bois-Eetmojtd  Key  used,  A,  for  Making  and  Breaking, 
B,  FOR  Short  Circuiting. 

element)  in  the  battery  ;  on  raising  the  brass  bridge  (continuous  outline 
in  the  figure)  the  circuit  is  opened,  the  current  broken,  and  no  current 
passes  through  the  electrodes.  Or  it  may  be  arranged  as  in  B.  In 
this  case  if  the  brass  bridge  be  '  down,'  the  resistance  offered  by  it  is  so 
small  compared  with  the  resistance  offered  by  the  nerve  between  the 
electrodes,  that  the  whole  current  from  the  battery  passes  through  the 
bridge,  back  to  the  battery,  and  none,  or  only  an  infinitesimal  portion, 
passes  into  the  nerve.  When  on  the  other  hand  the  bridge  is  raised, 
and  so  the  conduction  between  the  two  sides  suspended,  the  current  is 
not  able  to  pass  directly  from  one  side  to  the  other,  but  can  and  does 
pass  along  the  wire  through  the  nerve  back  to  the  battery.  Hence  in 
arrangement  A,  '  putting  down  the  key'  as  it  is  called  makes  a  current 
in  the  nerve,  and  '  raising  '  or  '  opening  the  key '  breaks  the  current.  In 
arrangement  B,  however,  putting  down  the  key  diverts  the  current  from 
the  nerve  by  sending  it  through  the  bridge,  and  so  back  to  tlie  battery ; 
the  current  instead  of  making  the  longer  circuit  through  the  electrodes 
makes  the  shorter  circuit  through  the  key ;  hence  this  is  called  '  short 
circuiting.'     When  the   bridge  is  raised   the  current  passes  through    the 


Chap,  ii.]  THE   CONTKACTILE   TISSUES.  59 

nerve  on  the  electrodes.  Thus  '  putting  down  '  and  '  raising  or  *  opening ' 
the  key  have  contrary  efl'ects  in  A  and  B.  In  B,  it  will  be  observed, 
the  battery  is  always  at  work,  the  current  is  always  flowing  either 
through  the  electrodes  (key  up)  or  through  the  key  (key  down);  in 
A,  the  battery  is  not  at  work  until  the  circuit  is  made  by  putting 
down  the  key.  And  in  many  cases  it  is  desirable  to  take  so  to  speak 
a  sample  of  the  current  while  the  battery  is  iu  full  swing  rather  than 
just  as  it  begins  to  Avork.  Moreover  in  B  the  electrodes  are,  when  the 
key  is  down,  wholly  shut  off  from  tlie  current ;  whereas  in  A,  when 
the  key  is  up,  one  electrode  is  still  in  direct  connection  with  the  battery  ; 
and  this  connection,  leading  to  what  is  known  as  unipolar  action,  may 
give  rise  to  stimulation  of  the  nerve.  Hence  the  use  of  the  key  iu 
the  form  B. 

Other  forms  of  key  may  be  used.  Thus  in  the  Morse  key  {F,  Fig. 
3)  contact  is  made  by  pressing  down  a  lever  handle  (Jia);  when  the 
pressure  is  removed,  the  handle,  driven  up  by  a  spring,  breaks  contact. 
In  the  arrangement  shewn  in  the  figure  one  wire  from  the  battery 
being  brought  to  the  binding  screw  b,  while  the  binding  screw  a  is 
connected  with  the  other  wire,  putting  down  the  handle  makes  connec- 
tion between  a  and  h,  and  thus  makes  a  current.  By  arranging  the  wires 
iu  the  several  binding  screws  in  a  different  way,  the  making  contact  by 
depressing  the  handle  may  be  used  to  short  circuit. 

In  an  "  induction  coil,"  Figs.  3  and  4,  the  wire  connecting  the  two 
elements  of  a  battery  is  twisted  at  some  part  of  its  course  irto  a  close 
spiral,  called  the  xirimary  coil.  Thus  in  Fig.  3  the  wire  a;"'  connected 
with  the  copper  or  negative  plate  cp.  of  the  battery,  E^  joins  the 
primary  coil  pr.  c,  and  then  passes  on  as  y'",  through  the  •'  key  "  F, 
to  the  positive  (zinc)  plate  z.p.  of  the  battery.  Over  this  primary  coil, 
but  quite  unconnected  with  it,  slides  another  coil,  — the  secondary  coil,  s.c.  ; 
the  ends  of  the  wire  forming  this  coil,  y"  and  x",  are  continued  on  in 
the  arrangement  illustrated  in  the  figure  as  y'  and  y,  and  as  x'  and  x, 
and  terminate  in  electrodes.  If  these  electrodes  are  in  contact  or  con- 
nected with  conducting  material,  the  circuit  of  the  secondary  coil  is  said 
to  be  closed  ;  otherwise  it  is  open. 

In  such  an  arrangement  it  is  found  that  at  the  moment  when 
the  primary  circuit  is  closed, —  /.  e.  when  the  primary  current  is  "  made," 
a  secondary  "  induced  "  current  is,  for  an  exceedingly  brief  period  of 
time,  set  up  in  the  secondary  coil.  Thus  in  F^ig.  2  when,  hj  moving 
the  "  key  "  F,  y'"  and  x'"  (previously  not  in  connection  with'  each  other) 
are  put  into  connection  and  the  primary  current  tlius  made,  at  that 
instant  a  current  appears  in  the  wires  y"  x"  etc.,  but  almost  immediately 
disappears.  A  similar  almost  instantaneous  current  is  also  developed 
when  the  primary  current  is  "  broken,"  but  not  till  then.  So  long  as 
the  primary  current  flows  with  uniform  intensity,  no  current  is  induced 
in  tlie  secondary  coil.  It  is  only  when  the  primarj^  current  is  either 
made  or  broken,  or  suddenly  varies  in  intensity,  that  a  current  appears 
in  the  secondary  coil.  In  each  case  the  current  is  of  very  brief 
duration,  gone  in  an  instant  almost,  and  may  therefore  be  spoken  of  as 
"a  shock,"  an  induction  shock, — being  called  a  ''making  shock  "  when 
it  is  caused  by  the  making,  and  a  "breaking  shock"  when  it  is  caused 
by  the  breaking,  of  the  primary  circuit.     The  direction  of  the  current 


60 


INDUCTIOi^  COIL. 


[Book  i. 


^^nrrrr^^^. 


Chap,  ii.]  THE   CONTRACTILE   TISSUES.  61 

Fig.  3.    Diagram  illustrating  Apparatus  arranged  for  Experiments 
WITH  Muscle  and  Nerve. 

A.  The  moist  chamber  containing  the  muscle-nerve  preparation.  The  muscle 
m,  supported  by  the  chimp  cL,  which  firmly  grasps  the  end  of  the  femur  /',  is 
connected  by  means  of  the  S  hook  s  and  a  thread  with  the  lever  /,  placed  below 
the  moist  chamber.  The  nerve  n,  with  the  portion  of  the  spinal  column  n'  still 
attached  to  it,  is  placed  on  tho  electrode-holder  el,  in  contact  with  the  wires 
X,  II.  The  whole  of  the  interior  of  the  glass  case  gl.  is  kept  saturated  with 
moisture,  and  the  electrode-liolder  is  so  constructed  that  a  piece  of  moistened 
blotting-paper  may  be  placed  on  it  without  coming  into  contact  with  the 
nerve. 

B.  The  revolving  cylinder  bearing  the  smoked  paper  on  which  the  lever  writes. 

C.  Du  Bois-Reymond's  key  arranged  for  short-circuiting.  The  wires  x  and  y  of 
the  electrode-holder  are  connected  through  binding  screws  in  the  floor  of  the 
moist  chamber  with  the  wires  x',  if ,  and  these  are  secured  in  the  key,  one  on 
either  side.  To  the  same  key  are  attached  the  wires  x"  y"  coming  from  the 
secondary  coils  s.  c.  of  the  induction-coil  D.  This  secondary  coil  can  be  made  to 
slide  up  and  down  over  the  primary  coil  jtr.  c,  with  which  are  connected  the  two 
wires  x'"  and  y'".  x'"  is  connected  directly  witli  one  pole,  for  instance  tlie  copper 
pole  c. ;).  of  the  battery  E.  y'"  is  carried  to  a  binding  screw  a  of  the  Morse  key 
F,  and  is  continued  as  y'^  from  another  binding  screw  h  of  the  key  to  the  zinc 
pole  z.  p.  of  the  battery. 

Supposing  everything  to  be  arranged,  and  the  battery  charged,  on  depressing  the 
handle  ha,  of  the  Morse  key  F,  a  current  will  be  made  in  the  primary  coil  pr.  c, 
passing  from  c.  p.  through  x"'  to  pr.  c,  and  thence  through  //'"  to  a,  thence  to  h, 
and  so  through  z/'^  to  s.  p.  On  removing  the  finger  from  the  handle  of  F,  a  spring 
thrusts  up  the  handle,  and  the  primary  circuit  is  in  consequence  immediately 
broken. 

At  the  instant  that  tlie  primary  current  is  either  made  or  broken,  an  induced 
current  is  for  the  instant  developed  in  the  secondary  coil  s.  c.  If  the  cross  bar  li  in 
the  du  Bois-Keymoud's  key  be  raised  (as  shewn  in  the  thick  line  in  the  figure),  the 
wires  x"  x'  x,  the  nerve  between  the  electrodes  and  the  wires  y,  y',  y"  form  the 
complete  secondary  circuit,  and  the  nerve  consequently  experiences  a  making  or 
breaking  induction-shock  whenever  the  primary  current  is  made  or  broken.  If  the 
cross  bar  of  the  du  Bois-Reymond's  key  be  shut  down,  as  in  the  dotted  line  h'  in  the 
figure,  the  resistance  of  the  cross  bar  is  so  slight  compared  with  tliat  of  the  nerve 
and  of  the  wires  going  from  the  key  to  the  nerve,  that  the  whole  secondary  (induced) 
current  passes  from  x"  to  //"  (or  from  y"  to  x"),  along  the  cross  bar,  and  practicallv 
none  passes  into  the  nerve.  The  nerve  being  thus  "  short-circuited."  is  not  affected 
by  any  changes  in  the  current. 

The  figure  is  intended  merely  to  illustrate  the  general  method  of  studying  muscular 
contraction ;  it  is  not  to  be  supposed  that  the  details  here  given  are  universally 
adopted  or  indeed  the  best  for  all  purposes. 

in  the  making  shock  is  opposed  to  that  of  the  primary  current ;  thus  in 
the  figure  while  the  primary  current  flows  from  a;'"  to  y'",  the  induced 
making  shock  flows  from  y  to  x.  The  current  of  the  breaking  shock 
on  the  other  hand  flows  in  the  same  direction  as  the  primary  current 
from  X  to  //,  and  is  therefore  in  direction  the  reverse  of  tlie  making 
sliock.  Compare  Fig.  3,  where  arrangement  is  shewn  in  a  diagrammatic 
manner. 

The  current  from  the  battery,  upon  its  first  entrance  into  the 
primary  coil,  as  it  passes  along  each  twist  of  that  coil,  gives  rise  in  the 
neighbouring  twists  of  the  same  coil  to  a  momentary  induced  current 
having  a  direction  opposite  to  its  own,  and  therefore  tending  to  weaken 
itself.     It   is   not    until    this    'sclf-intluction '    has    passed    off  that   the 


62 


II^DUCTION   COIL. 


[Book 


current  in  the  primary  coil  is  established  in  its  full  strength.  Owing 
to  thiis  delay  in  the  full  establishment  of  the  current  in  the  primary 
coil,  the  induced  current  in  the  secondary  coil  is  developed  more  slowly 


Fig.  4.    Diagram  of  an  Induction  Coil. 

+  positive  pole,  end  of  negative  element;  —  negative  pole,  end  of  positive 
element  of  battery  ;  K,  du  Bois-Reymond's  key ;  pr.  c.  primary  coil,  current  shewn  by 
feathered  arrow ;  sc.  c.  secondary  coil,  current  shewn  by  unfeathered  arrow. 

than  it  would  be  were  no  such  '  self-induction '  present.  On  the  other 
hand,  when  the  current  from  the  battery  is  '  broken/  or  '  shut  off'  from 
the  primary  coil,  no  such  delay  is  offered  to  its  disappearance,  and 
consequently  the  induced  current  in  the  secondary  coil  is  developed 
with  unimpeded  rapidity.  We  shall  see  later  on  that  a  rapidly  de- 
veloped current  is  more  effective  as  a  stimulus  than  is  a  more  slowly 
developed  current.  Hence  the  making  shock,  where  rapidity  of  pro- 
duction is  interfered  with  by  the  self-induction  of  the  primary  coil,  is 
less  effective  as  a  stimulus  than  the  breaking  shock,  whose  development 
is  not  thus  interfered  with. 

The  strength  of  the  induced  current  depends,  on  the  one  hand,  on 
the  strength  of  the  current  passing  through  the  primary  coil,  —  that  is,, 
on  the  strength  of  the  battery.  It  also  depends  on  the  relative  position 
of  the  two  coils.  Thus,  if  a  secondary  coil  is  brought  nearer  and  nearer 
to  the  primary  coil  and  made  to  overlap  it  more  and  more,  the 
induced  current  becomes  stronger  and  stronger,  though  the  current 
from  the  battery  remains  the  same.  With  an  ordinary  battery,  the 
secondary  coil  may  be  pushed  to  some  distance  away  from  the  primary 
coil,  and  yet  shocks  sufficient  to  stimulate  a  muscle  will  be  obtained. 
For  this  purpose  however  the  two  coils  should  be  in  the  same  line ;. 
when  the  secondary  coil  is  placed  cross-wise,  at  right  angles  to  the 
primary,  no  induced  current  is  developed,  and  at  intermediate  angles 
the  induced  current  has  intermediate  strengths. 

When  the  primary  current  is  repeatedly  and  rapidly  made  and 
broken,  the  secondary  current  being  developed  with  each  make  and 
with  each  break,  a  rapidly  recurring  series  of  alternating  currents  is 
developed  in  the  secondary  coil  and  passes  through  its  electrodes.  We 
shall  fref4uently  speak  of  this  as  the  interrujjted  induction  current,  or 
more  briefly  the  interrupted   current ;  it  is  sometimes  spoken  of  as  the- 


Chap,  ii.] 


THE    CONTKACTILE   TISSUES. 


G3 


faradaic  current,  and  the  application  of  it  to  any  tissue  is  spoken  of  as- 
faradization. 

Such  a  repeated  breaking  and  making  of  the  primary  current  may 
be  effected  in  many  various  ways.  In  the  instrument  commonly  used 
for  the  purpose,  the  primary  current  is  made  and  broken  by  means  of  a 
vibrating  steel  slip  working  against  a  magnet ;  hence  the  instrument  i& 
called  a  magnetic  interruptor.     See  Fig.  5. 


Fig.  5.     The  Magnetic  Interruptor. 

The  two  wires  x  and  y  from  the  battery  are  connected  with  the  two 
brass  pillars  a  and  d  by  means  of  screws.  Directly  contact  is  thus- 
made,  the  current,  indicated  in  the  figure  by  the  thich  interrupted  line, 
passes  in  the  direction  of  the  arrows,  up  the  pillar  a,  along  the  steel 
spring  b,  as  far  as  the  screw  c,  the  point  of  wliich,  armed  with  platinum, 
is  in  contact  with  a  small  platinum  plate  on  b.  The  current  passes 
from  b  through  c  and  a  connecting  wire  into  the  primary  coil  p.  Upon 
its  entering  into  the  primary  coil,  an  induced  (making)  current  is  for 
the  instant  developed  in  the  secondary  coil  (not  shewn  in  the  figure). 
From  the  primary  coil  p  the  current  passes,  by  a  connecting  wire, 
through  the  double  spiral  m,  and,  did  nothing  happen,  would  continue 
to  pass  from  m  by  a  connecting  wire  to  the  pillar  d,  and  so  by  the  wire 
y  to  the  battery.  The  whole  of  this  course  is  indicated  by  tlie  thick 
interrupted  line  with  its  arrows. 

As  the  current  however  passes  through  tlie  spirals  in,  the  iron  cores 
of  these  are  made  magnetic.  They  in  consequence  draw  down  the  iron 
bar  e,  fixed  at  the  end  of  the  spring  b,  the  flexibility  of  the  spring 
allowing  this.  But  when  e  is  drawn  down,  the  platinum  plate  on  the 
upper  surface  of  b  is  also  drawn  away  from  the  screw  c,  and  thus  the 
current  is  "  broken  "  at  b.  (Sometimes  the  screw/  is  so  arranged  that 
when  e  is  drawn  down  a  platinum  plate  on  the  under  surface  of  b  is 
brought   into  contact  with   the   platinum-armed   point    of  the   screw  /. 


64 


INDUCTION   COIL. 


[Book  t. 


The  current  then  passes  from  h  not  to  c  but  to  /,  and  so  down  the 
pillar  d,  in  the  direction  indicated  by  the  thin  interrupted  line,  and  out 
to  the  battery  by  the  wire  y,  and  is  thus  cut  oil'  from  the  primary  coil. 
But  this  arrangement  is  unnecessary.)  At  the  instant  that  the  cur- 
rent is  thus  broken  and  so  cut  off  from  the  primary  coil,  an  induced 
(breaking)  current  is  for  the  moment  developed  in  the  secondary  coil. 
But  the  current  is  cut  off  not  only  from  the  primary  coil,  but  also 
from  the  spirals  m  ;  in  consequence  their  cores  cease  to  be  magnetised, 
the  bar  e  ceases  to  be  attracted  by  them,  and  the  spring  h,  by  virtue  of 
its  elasticity,  resumes  its  former  position  in  contact  with  the  screw  c. 
This  return  of  the  spring  however  re-establishes  the  current  in  the 
primary  coil  and  in  the  spirals,  and  the  spring  is  drawn  down,  to  be 
released  once  more  in  the  same  manner  as  before.  Thus  as  long  as 
the  current  is  passing  along  x,  the  contact  of  h  with  c  is  alternately 
being  made  and  broken,  and  the  current  is  constantly  passing  into  and 
being  shut  off  from  p,  the  periods  of  alternation  being  determined  by 
the  periods  of  vibration  of  the  spring  6.  With  each  passage  of  the 
current  into,  or  withdrawal  from  the  primary  coil,  an  induced  (making 
and,  respectively,  breaking)  current  is  developed  in  a  secondary  coil. 

As  thus  used,  each  'making  shock,'  as  explained  above,  is  less 
powerful  than  the  corresponding  ■  breaking  shock ; '  and  indeed  it 
sometimes  happens  that  instead  of  each  make  as  well  as  each  break 
acting  as  a  stimulus,  giving  rise  to  a  contraction,  the  '  breaks '  only  are 
effective,  the  several  '  makes '  giving  rise  to  no  contractions. 

By  what  is  known  as  Helmholtz's  arrangement.   Fig.   6,  however. 


Fig.  6.  The  Magnetic  Interruptor  with  Helmholtz  arrangement  for  equal- 
izing THE  Make  and  Break  Shocks. 


the  making  and  breaking  shocks  may  be  equalized.     For  this  purpose 
the  screw  c  is  raised  out  of  reach  of  the  excursions  of  the  spring  h,  and 


Chap,  ii.]  THE   CONTRACTILE   TISSUES.  65 

a  moderately  thick  wire  w,  offering  a  certain  amount  only  of  resistance, 
is  interposed  between  the  upper  binding  screw  a'  on  the  pillar  a,  and 
the  binding  screw  c'  leading  to  the  primary  coil.  Under  these  arrange- 
ments the  current  from  tlie  battery  passes  through  a',  along  the  inter- 
posed wire  to  c',  through  the  primary  coil  and  thus  as  before  to  m. 
As  before,  by  the  magnetization  of  w,  e  is  drawn  down  and  b  brought 
in  contact  with/.  As  the  result  of  tliis  contact,  the  current  from  the 
battery  can  now  pass  by  a,f,  and  d  (shewn  by  the  tliin  interrupted  line) 
back  to  the  battery  ;  but  not  the  whole  of  the  current,  some  of  it  can 
still  pass  along  the  wire  w  to  the  primary  coil,  the  relative  amount 
being  determined  by  the  relative  resistances  offered  by  the  two  courses. 
Hence  at  each  successive  magnetization  of  m,  tlie  current  in  the 
primary  coil  does  not  entirely  disappear  when  b  is  brought  in  contact 
with//  it  is  only  so  far  diminished  that  vi  ceases  to  attract  e,  and 
hence  by  the  release  of  6  from  /  the  whole  current  once  more  passes 
along  w.  Since  at  what  corresponds  to  the  '  break '  the  current  in 
the  primarj''  coil  is  diminished  only,  not  absolutely  done  away  with, 
self-induction  makes  its  appearance  at  the  '  break '  as  well  as  at  the 
'make;'  thus  the  'breaking'  and  '  making '  induced  currents  or  shocks 
in  the  secondary  coil  are  equalized.  They  are  both  reduced  to  the 
lower  efficiency  of  the  '  making '  shock  in  the  old  arrangement ; 
hence  to  produce  the  same  strength  of  stimulus  with  this  arrange- 
ment a  stronger  current  must  be  applied  or  the  secondary  coil  pushed 
over  the  primary  coil  to  a  greater  extent  than  with  the  other  arrange- 
ment. 


The  Phenomena  of  a  Simple  Muscular  Contraction. 


§  45.  If  the  far  end  of  the  nerve  of  a  muscle-nerve  preparation 
(Figs.  1  and  3)  be  laid  on  electrodes  connected  with  the  secondary 
coil  of  an  induction-machine,  the  passage  of  a  single  induction- 
shock,  which  may  be  taken  as  a  convenient  form  of  an  almost  mo- 
mentary stimulus,  will  produce  no  visible  change  in  the  nerve,  but 
the  muscle  will  give  a  twitch,  a  short,  sharp  contraction,  —  i.  c,  will 
for  an  instant  shorten  itself,  becoming  thicker  the  while,  and  then 
return  to  its  previous  condition.  If  one  end  of  the  muscle  be  attached 
to  a  lever,  while  the  other  is  fixed,  the  lever  will  by  its  movements 
indicate  the  extent  and  duration  of  the  shortening.  If  the  point 
of  the  lever  be  brought  to  bear  on  some  rapidly  travelling  surface, 
on  which  it  leaves  a  mark  (being  for  this  purpose  armed  with  a 
pen  and  ink  if  the  surface  be  plain  paper,  or  with  a  bristle  or 
finely  pointed  piece  of  platinum  foil  if  the  surface  be  smoked  glass 
or  paper),  so  long  as  the  muscle  remains  at  rest  the  lever  will 
describe  an  even  line,  which  we  may  call  the  base  line.  If  how- 
ever the  muscle  shortens,  the  lever  will  rise  above  the  base  line 
and  thus  describe  some  sort  of  curve  above  the  base  line.     Now, 

5 


66  A   SIMPLE   MUSCULAR   COXTRACTIOK    [Book  i. 

it  is  found  that  when  a  single  induction-shock  is  sent  tlirough  the 
nerve  the  twitch  which  the  muscle  gives  causes  the  lever  to  de- 
scribe some  such  curve  as  that  shewn  in  Fig.  7 ;  the  lever  (after  a 
brief  interval  immediately  succeeding  the  opening  or  shutting 'the 
key,  of  which  we  shall  speak  presently)  rises  at  first  rapidly  but 
afterwards  more  slowly,  shewing  that  the  muscle  is  correspondingly 
shortening,  then  ceases  to  rise,  shewing  that  the  muscle  is  ceasing 


Fig.  7.    A  Muscle-curve  from  the  Gastrocnemius  of  the  Frog, 

This  curve,  like  all  succeeding  ones,  unless  otherwise  indicated,  is  to  be  read 
from  left  to  right,  —  that  is  to  say,  while  the  lever  and  tuning-fork  were  stationary 
the  recording  surface  was  travelling  from  right  to  left. 

a  indicates  the  moment  at  which  the  induction-shock  is  sent  into  the  nerve ;  b  the 
commencement,  c  tlie  maximum,  and  d  the  close  of  the  contraction. 

Below  the  muscle-curve  is  the  curve  drawn  by  a  tuning-fork  making  100  double 
vibrations  a  second,  each  complete  curve  representing  therefore  one-hundredth  of 
a  second. 

to  grow  shorter ;  then  descends,  shewing  that  the  muscle  is  length- 
ening again  ;  and  finally,  sooner  or  later,  reaches  and  joins  the  base 
line,  shewing  that  the  muscle  after  the  shortening  has  regained 
its  previous  natural  length.  Such  a  curve  described  by  a  muscle 
during  a  twitch  or  simple  muscular  contraction,  caused  by  a  single 
induction-shock  or  by  any  other  stimulus  producing  the  same  effect, 
is  called  a  curve  of  a  simple  muscular  contraction  or,  more  shortly, 
a  "  muscle-curve."  It  is  obvious  that  the  exact  form  of  the  curve 
described  by  identical  contractions  of  a  muscle  will  depend  on  the 
rapidity  with  which  the  recording  surface  is  travelling.  Thus  if 
the  surface  be  travelling  slowly  the  up-stroke  corresponding  to 
the  shortening  will  be  very  abrupt  and  the  down-stroke  also  very 
steep,  as  in  Fig.  8,  which  is  a  curve  from  a 
gastrocnemius  muscle  of  a  frog,  taken  with  a 
slowly  moving  drum,  the  tuning-fork  being 
the  same  as  that  used  in  Fig.  7  ;  indeed  with 
a  very  slow  movement,  the  two  may  be  hardly 
separable  from  each  other.  On  the  other 
hand,  if  the  surface  travel  very  rapidly  the  ^j 

curve  may  be  immensely  long  drawn  out,  as       mmumfMijmj^mmm 
in  Fig.   9,   which  is  a  curve  from  a  gastro-  Fig.  8. 

cnemius  muscle  of  a  frog,  taken  with  a  very 

rapidly  moving    pendulum    myograph,  the   tuning-fork    marking 
about  500  vibrations  a  second.     On  examination,  however,  it  will 


Chap,  ii.] 


THE   CONTRACTILE   TISSUES. 


67 


be  found  that  both  these  extreme  curves  are  funda- 
mentally the  same  as  the  medium  one,  when 
account  is  taken  of  the  different  rapidities  of  the 
travelling  surface  in  the  several  cases. 

In  order  to  make  the  '  muscle-curve  '  complete, 
it  is  necessary  to  mark  on  the  recording  surface  the 
exact  time  at  which  the  induction-shock  is  sent  into 
tlie  nerve,  and  also  to  note  the  speed  at  which  the 
recording  surface  is  travelling. 

In  the  pendulum  myograph  the  rate  of  move- 
ment can  be  calculated  from  the  length  of  the 
pendulum ;  but  even  in  this  it  is  convenient,  and 
in  the  case  of  the  spring  myograph  and  revolving 
cylinder  is  necessary,  to  measure  the  rate  of  move- 
ment directly  by  means  of  a  vibrating  tuning-fork 
or  of  some  body  vibrating  regularly.  Indeed  it  is 
best  to  make  such  a  direct  measurement  with  each 
curve  that  is  taken. 

A  tuning-fork,  as  is  known,  vibrates  so  many 
times  a  second  according  to  its  pitch.  If  a  tuning- 
fork,  armed  with  a  light  marker  on  one  of  its  prongs 
and  vibrating  say  100  a  second,  —  i.e.,  executing  a 
double  vibration,  moving  forwards  and  backwards, 
100  times  a  second,  —  be  brought  while  vibrating  to 
make  a  tracing  on  the  recording  surface  immedi- 
ately below  the  lever  belonging  to  the  muscle,  we 
can  use  the  curve  or  rather  curves  described  by  the 
tuning-fork  to  measure  the  duration  of  any  part  or 
of  the  whole  of  the  muscle-curve.  It  is  essential 
that  at  starting  the  point  of  the  marker  of  the 
tuning-fork  should  be  exactly  underneath  the  marker 
of  the  lever,  or  rather,  since  the  point  of  the  lever 
as  it  moves  up  and  down  describes  not  a  straight 
line  but  an  arc  of  a  circle  of  which  its  fulcrum  is 
the  centre  and  itself  (from  the  fulcrum  to  the  tip 
of  the  marker)  the  radius,  that  the  point  of  the 
marker  of  the  tuning-fork  should  be  exactly  on 
the  arc  described  by  the  marker  of  the  lever,  either 
above  or  below  it,  as  may  prove  most  convenient. 
If  then  at  starting  the  tuning-fork  marker  be  thus 
on  the  arc  of  the  lever  marker,  and  we  note  on  the 
curve  of  the  tuning-fork  the  place  where  the  arc 
of  the  lever  cuts  it  at  the  beginning  and  at  the  end 
of  the  muscle-curve,  as  at  Fig.  7,  we  can  count  the 
number  of  vibrations  of  the  tuning-fork  which  have 
taken  place  between  the  two  marks,  and  so  ascer- 
tain the  whole  time  of  the  muscle-curve ;  if  for 
instance  there  have  been  10  doul)le  vibrations,  each 


¥iG  9. 


68 


PENDULUM   MYOGEAPH. 


[Book  i. 


Fig  10.    The  Pendulum  Myograph. 
The  figure  is  diagrammatic,  the  essentials  onlv  of  the  instrument  being  shewn 
The  smoked  glass  plate  A  swings  with  the  pendulum  B  on   carefully  adjusted 


Chap.  ii.J  THE   CONTEACTILE   TISSUES  6^ 

bearings  at  C.  The  contrivances  by  which  the  glass  plate  can  be  removed  and 
replaced  at  pleasure  are  not  shewn.  A  second  glass  plate  so  arranged  that  the 
first  glass  plate  may  be  moved  up  and  down  without  altering  the  swing  of  the 
pendulum  is  also  omitted.  Before  commencing  an  experiment  the  pendulum  is 
raised  up  (in  the  figure  to  the  right),  and  is  kept  in  that  position  by  the  tooth  a 
catching  on  the  spring-catch  b.  On  depressing  the  catch  b  the  glass  plate  is  set 
free,  swings  into  the  new  position  indicated  by  the  dotted  lines,  and  is  held  in  that 
position  by  the  tooth  a'  catching  on  the  catch  b'.  In  the  course  of  its  swing  the 
tooth  a'  coming  into  contact  with  the  projecting  steel  rod  c,  knocks  it  on  one  side 
into  the  position  indicated  by  the  dotted  line  c'.  The  rod  c  is  in  electric  continuity 
with  the  wire  x  of  the  primary  coil  of  an  induction-machine.  The  screw  d  is 
similarly  in  electric  continuity  with  the  wire  y  of  the  same  primary  coil.  The 
screw  d  and  the  rod  c  are  armed  with  platinum  at  the  points  in  which  they  are  in 
contact,  and  both  are  insulated  by  means  of  the  ebonite  block  e.  As  long  as  c  and  d 
are  in  contact  the  circuit  of  the  primary  coil  to  which  x  and  y  belong  is  closed. 
When  in  its  swing  the  tooth  a'  knocks  c  away  from  d,  at  that  instant  the  circuit  is 
broken,  and  a  '  breaking '  shock  is  sent  through  the  electrodes  connected  with  the 
secondary  coil  of  the  machine,  and  so  through  the  nerve.  The  lever  /,  the  end  only 
of  which  is  shewn  in  the  figure,  is  brought  to  bear  on  the  glass  plate,  and  when  at 
rest  describes  a  straight  line,  or  more  exactly  an  arc  of  a  circle  of  large  radius.  The 
tuning-fork  /",  the  ends  only  of  the  two  limbs  of  which  are  shewn  in  the  figure 
placed  immediately  below  the  lever,  serves  to  mark  the  time. 

occupying  y J^-  sec,  the  whole  curve  has  taken  Jq  sec.  to  make. 
In  the  same  way  we  can  measure  the  duration  of  the  rise  of  the 
curve  or  of  the  fall,  or  of  any  part  of  it. 

Though  the  tuning-fork  may,  by  simply  striking  it,  be  set 
going  long  enough  for  the  purposes  of  an  observation,  it  is 
convenient  to  keep  it  going  by  means  of  an  electric  current  and 
a  magnet,  very  much  as  the  spring  in  the  '  magnetic  interruptor ' 
(Fig.  5)  is  kept  going. 

It  is  not  necessary  to  use  an  actual  tuning-fork;  any  rod, 
armed  with  a  marker,  which  can  be  made  to  vibrate  regularly, 
and  whose  time  of  vibration  is  known,  may  be  used  for  the  pur- 
pose ;  thus  a  reed,  made  to  vibrate  by  a  blast  of  air,  is  sometimes 
employed. 

The  exact  moment  at  which  the  induction-shock  is  thrown 
into  the  nerve  may  be  recorded  on  the  muscle-curve  by  means  of 
a  '  signal,'  which  may  be  applied  in  various  ways, 

A  liglit  steel  lever  armed  with  a  marker  is  arranged  over  a  small 
coil  by  means  of  a  light  spring  in  such  a  way  that  when  the  coil  by 
the  pa.ssage  of  a  current  through  it  becomes  a  magnet  it  pulls  the 
lever  down  to  itself;  on  the  current  being  broken,  and  the  magneti- 
zation of  the  coil  ceasing,  the  lever  by  help  of  the  spring  flies  up.  The 
marker  of  such  a  lever  is  placed  immediately  under  {i.e.,  at  some  point 
on  the  arc  described  by)  the  marker  of  the  muscle  (or  other)  lever. 
Hence  by  making  a  current  in  the  coil  and  putting  the  signal  lever 
down,  or  by  breaking  an  already  existing  current,  and  letting  the 
signal  lever  fly  up,  we  can  make  at  pleasure  a  mark  corresponding  to 
any  part  we  i)lease  of  the  nuiscle  (or  other)  curve. 

If  in  onler  to  magnetize  the  coil  of  the  signal,  we  use,  as  we  may 
do,  the  primary  current  which  generates  the  induction-shock,  the  break- 
ing or  making  of  the  primary  current,  whichever  we  use  to  produce  the 


70  GRAPHIC   RECOED   OF  A  CONTRACTION.     [Book  i. 

induction-shock,  will  make  the  signal  lever  fly  up  or  come  down. 
Hence  we  shall  have  on  the  recording  surface,  under  the  muscle,  a 
mark  indicating  the  exact  moment  at  which  the  primary  current  Avas 
broken  or  made.  Now,  the  time  taken  up  by  the  generation  of  the 
induced  current  and  its  passage  into  the  nerve  between  the  electrodes 
is  so  infinitesimally  small,  that  we  may,  without  appreciable  erroi',  take 
the  moment  of  the  breaking  or  making  of  the  primary  current  as 
the  moment  of  the  entrance  of  the  induction-shock  into  the  nerve. 
Thus  we  can  mark  below  the  muscle-curve,  or,  by  describing  the  arc  of 
the  muscle  lever,  on  the  muscle-curve  itself,  the  exact  moment  at  which 
the  induction-shock  falls  into  the  nerve  between  the  electrodes,  as  is 
done  at  a  in  Figs.  7,  8,  9. 

In  the  pendulum  myograph  a  separate  signal  is  not  needed.  If, 
having  placed  the  muscle  lever  in  the  position  in  which  we  intend  to 
make  it  record,  we  allow  the  glass  plate  to  descend  until  the  tooth  a' 
just  touches  the  rod  c  (so  that  the  rod  is  just  about  to  be  knocked 
down,  and  so  break  the  primary  circuit)  and  make  on  the  base  line, 
which  is  meanwhile  being  described  by  the  lever  marker,  a  mark  to 
indicate  where  tlie  point  of  the  marker  is  under  these  circumstances, 
and  then  bring  back  the  plate  to  its  proper  position,  the  mark  which 
we  have  made  will  mark  the  moment  of  the  breaking  of  the  primary 
circuit  and  so  of  the  entrance  of  the  induction-shock  into  the  nerve. 
For  it  is  just  when,  as  the  glass  pLite  swings  down,  the  marker  of  the 
lever  comes  to  the  mark  which  we  have  made  that  the  rod  c  is  knocked, 
back  and  the  primary  current  is  broken. 


Pig.  11.    Diagram  op  an  Arrangement  of  a  Vibrating  Tuning-fork 
WITH  A  Desprez  Signal. 

The  current  flows  along  the  wire  /connected  with  the  positive  (4-)  pole  or  end 
of  the  negative  plate  {N)  of  the  battery,  through  the  tuning-fork,  down  the  pin 
connected  with  the  end  of  the  lower  prong,  to  the  mercury  in  the  cup  Hg,  and  so  by 
a  wire  (shewn  in  the  figure  as  a  black  line  bent  at  right  angles)  to  the  binding 
screw  e.  From  this  binding  screw  part  of  the  current  flows  through  the  coil  a 
between  the  prongs  of  the  tuning-fork,  and  thence  by  the  wire  c  to  the  binding 
screw  a,  while  another  part  flows  through  the  wire  ry,  through  the  coil  of  the 
Desprez  signal  back  by  the  wire  h,  to  the  binding  screw  a.  From  the  binding 
screw  a  the  current  passes  back  to  the  negative  (— )  pole  or  end  of  the  positive 
element  (P)  of  the  battery.  As  the  current  flows  through  the  coil  of  the  Desprez 
signal  from  g  to  h,  the  core  of  coil  becoming  magnetized  draws  to  it  the  marker  of 
the  signal.  As  the  current  flows  through  the  coil  d,  the  core  of  that  coil,  also 
becoming  magnetized,  draws  up  the  lower  prong  of  the  fork.  But  the  pin  is  so 
adjusted  that  the  drawing  up  of  the  prong  lifts  the  point  of  the  pin  out  of  the 
mercury.  In  consequence,  the  current,  being  thus  broken  at  Hg,  flows  neither 
through  d  nor  through  the  Desprez  signal.  In  consequence,  the  core  of  the  Desprez 
thus  ceasing  to  be  magnetized,  the  marker  flies  back,  being  usually  assisted  by  a 


Chap,  ii.]  THE   CONTRACTILE   TISSUES.  71 

spring  (not  shewn  in  the  figure).  But,  in  consequence  of  the  current  ceasing  to  flow 
through  d,  tlie  core  of  d  ceases  to  lift  up  tiie  prong,  and  the  pin,  in  the  descent  of 
the  prong,  makes  contact  once  more  with  the  mercury.  The  re-establisiuneut  of  the 
current,  however,  once  more  acting  on  the  two  coils,  again  pulls  ujtou  tiie  marker  of 
the  signal,  and  again,  by  magnetizing  the  core  of  d,  pulls  u]j  tiie  prong  and  once 
more  breaks  tlie  current.  I'hus  the  current  is  continually  made  and  broken,  the 
rapidity  of  tlie  interruptions  being  determined  by  the  vibration  periods  of  tiie 
tuning-fork,  and  the  lever  of  the  signal  rising  and  falling  synchronously  with  the 
movements  of  the  tuning-fork. 

A  '  signal '  like  the  above,  in  an  improved  form  known  as  Desprez's, 
may  be  used  also  to  record  time,  and  thus  the  awkwardness  of  bringing 
a  large  tuning-furk  up  to  the  recording  surface  obviated.  For  this  pur- 
pose the  signal  is  introduced  into  a  circuit,  the  current  of  whicli  is 
continually  being  made  and  broken  by  a  tuning-fork  (Fig.  10).  The 
tuning-fork,  once  set  vibrating,  continues  to  make  and  break  the  current 
at  each  of  its  vibrations,  and,  as  stated  above,  is  kept  vibrating  by  the 
current.  But  each  make  or  break  caused  by  the  tuning-fork  alfects 
also  the  small  coil  of  the  signal,  causing  the  lever  of  tlie  signal  to  fall 
down  or  fly  up.  Thus  tlie  signal  describes  vibration  curves  synchronous 
with  those  of  the  tuning-fork  driving  it.  The  signal  may  similarly  be 
worked  by  means  of  vibrating  agents  other  than  a  tuning-fork. 

Various  recording  surfaces  may  be  used.  The  form  most  generally 
ttseful  is  a  cylinder  covered  with  smoked  paper,  and  made  to  revolve  by 
clockwork  or  otherwise  ;  such  a  cylinder  driven  by  clockwork  is  shewn 
in  Fig.  3,  B.  By  using  a  cylinder  of  large  radius  with  adequate  gear, 
a  high  speed,  some  inches  for  instance  in  a  second,  can  be  obtained.  In 
the  spring  myograph  a  smoked  glass  plate  is  thrust  rapidly  forward 
along  a  groove,  by  means  of  a  spring  suddenly  thrown  into  action.  In 
the  pendulum,  myograph,  Fig.  9,  a  smoked  glass  plate  attached  to  the 
lower  end  of  a  long  frame,  swinging  like  a  pendulum,  is  suchlenly  let  go 
at  a  certain  height,  and  so  swings  rapidly  through  an  arc  of  a  circle. 
The  disadvantage  of  the  last  two  methods  is  that  the  surface  travels  at 
a  continually  changing  rate,  whereas,  in  the  revolving  cylinder,  careful 
construction  and  adjustment  will  secure  a  very  uniform  rate. 


§  46.  Having  thus  obtained  a  time  record,  and  an  indication 
of  the  exact  moment  at  which  the  induction-shock  falls  into  the 
nerve,  we  may  for  present  purposes  consider  the  muscle-curve 
complete.  The  study  of  such  a  curve,  as  for  instance  that  sliewn 
in  Fig.  7,  taken  from  the  gastrocnemius  of  a  frog,  teaches  ns  the 
following  facts  :  — 

1.  That  although  the  passage  of  the  induced  current  from 
electrode  to  electrode  is  practically  instantaneous,  its  effect,  meas- 
ured from  the  entrance  of  the  shock  into  the  nerve  to  the  return 
of  the  muscle  to  its  natural  length  after  the  shortening,  takes 
an  appreciable  time.  In  tlie  figure,  the  whole  curve  from  a  to  d 
takes  up  about  the  same  time  as  eleven  double  vibrations  of  the 
tuning-fork.  Since  each  double  vibration  here  represents  100th  of 
a  second,  the  duration  of  the  whole  curve  is  rather  more  than 
iV  sec. 


72  MUSCLE-CUEVE.  [Book  i. 

2.  In  the  first  portion  of  this  period,  from  a  to  h,  there  is  no 
visible  change,  no  raising  of  the  lever,  no  shortening  of  the  muscle. 

3.  It  is  not  until  h  —  that  is  to  saj^  after  the  lapse  of  about 
•j-^  sec.  —  that  the  shortening  begins.  The  shortening  as  shewn 
by  the  curve  is  at  first  slow,  but  soon  becomes  more  rapid,  and 
then  slackens  again  until  it  reaches  a  maximum  at  c  ;  the  whole 
shortening  occupying  rather  more  than  ^^-q  sec. 

4.  Arrived  at  the  maximum  of  shortening,  the  muscle  at  once 
begins  to  relax,  the  lever  descending  at  first  slowly,  then  more 
rapidly,  and  at  last  more  slowly  again,  until  at  d  the  muscle  has 
regained  its  natural  length ;  the  whole  return  from  the  maximum 
of  contraction  to  the  natural  length  occupying  rather  more  than 

10  0  ^®'^- 

Thus  a  simple  muscular  contraction,  a  simple  spasm  or  twitch, 

produced  by  a  momentary  stimulus,  such  as  a  single  induction- 
shock,  consists  of  three  main  phases :  — 

1.  A  phase  antecedent  to  any  visible  alteration  in  the  muscle. 
This  phase,  during  which  invisible  preparatory  changes  are  taking 
place  in  the  nerve  and  muscle,  is  called  the  '  latent  2Jeriod.' 

2.  A  phase  of  shortening,  or,  in  the  more  strict  meaning  of 
the  word,  contraction. 

3.  A  phase  of  relaxation  or  return  to  the  original  length. 

In  the  case  we  are  considering,  the  electrodes  are  supposed 
to  be  applied  to  the  nerve  at  some  distance  from  the  muscle. 
Consequently  the  latent  period  of  the  curve  comprises  not  only 
the  preparatory  actions  which  may  be  going  on  in  the  muscle 
itself,  but  also  the  changes  necessary  to  conduct  the  immediate 
effect  of  the  induction-shock,  from  the  part  of  the  nerve  between 
the  electrodes  along  a  considerable  length  of  nerve  down  to  the 
muscle.  It  is  obvious  that  these  latter  changes  might  be  elimi- 
nated by  placing  the  electrodes  on  the  muscle  itself,  or  on  the 
nerve  close  to  the  muscle.  If  this  were  done,  the  muscle  and 
lever  being  exactly  as  before,  and  care  were  taken  that  the 
induction-shock  entered  into  the  nerve  at  the  new  spot,  at  the 
moment  when  the  point  of  the  lever  had  reached  exactly  the  same 
point  of  the  travelling  surface  as  before,  two  curves  would  be 
gained  having  the  relations  shewn  in  Fig.  12.  The  two  curves 
resemble  each  other  in  almost  all  points,  except  that  in  the  curve 
taken  with  the  shorter  piece  of  nerve,  the  latent  period,  the 
distance  a  to  h  as  compared  with  the  distance  a  to  h'  is  shortened : 
the  contraction  begins  rather  earlier.  A  study  of  the  two  curves 
teaches  us  the  following  two  facts  :  — 

1.  Shifting  the  electrodes  from  a  point  of  the  nerve  at  some 
distance  from  the  muscle  to  a  point  of  the  nerve  close  to  the 
muscle,  has  only  shortened  the  latent  period  a  very  little.  Even 
when  a  very  long  piece  of  nerve  is  taken,  the  difference  in  the  two 
curves  is  very  small,  and,  indeed,  in  order  that  it  may  be  clearly 
recognized  or  measured,  the  travelling  surface  must  be  made  to 


Chap,  ii.]  THE   CONTRACTILE   TISSUES. 


73 


travel  very  rapidly.     It  is  obvious,  therefore,  that  by  far  the  greater 
part  of  the  latent  period  is  taken  up  by  changes  in  the  muscle 


Fig.  12.     Curves   illustrating   the   Measurement   of    the   Velocity   of   a 

Nervous  Impulse. 

The  same  muscle-nerve  preparation  is  stimulated  (1)  as  far  as  possible  from  the 
muscle,  (2)  as  near  as  possible  to  the  muscle ;  both  contractions  are  registered  in 
exactly  the  same  way. 

In  (1),  the  stimulus  enters  the  nerve  at  the  time  indicated  by  the  line  a,  the  con- 
traction begins  at  b' ;  the  whole  latent  period  therefore  is  indicated  by  the  distance 
from  a  to  b'. 

In  (2),  the  stimulus  enters  the  nerve  at  exactly  the  .same  time  a  ;  the  contraction 
begins  at  b:  the  latent  period  therefore  is  indicated  by  the  distance  between  d  and  b. 

The  time  taken  up  by  the  nervous  impulse  in  passing  along  the  length  of  nerve 
between  1  and  2  is  therefore  indicated  by  tiie  distance  between  6  and  b',  wliich  may 
be  measured  by  the  tuning-fork  curve  below  •  each  double  vibration  of  the  tuning- 
fork  corresponds  to  yl^  or  0083  sec. 

itself,  changes  antecedent  to  the  shortening  becoming  actvially 
visible.  Of  course,  even  when  the  electrodes  are  placed  close  to 
the  muscle,  the  latent  period  includes  the  changes  going  on  in  the 
short  piece  of  nerve  still  lying  between  the  electrodes  and  the 
muscular  fibres.  To  eliminate  this  with  a  view  of  determining 
the  latent  period  in  the  muscle  itself,  the  electrodes  might  be 
placed  directly  on  the  muscle  poisoned  with  urari.  If  this  were 
done,  it  would  be  found  that  the  latent  period  remained  about  the 
same,  —  that  is  to  say,  that  in  all  cases  the  latent  period  is  chiefly 
taken  up  by  changes  in  the  muscular  as  distinguished  from  the 
nervous  elements. 

2.  Such  difference  as  does  exist  between  the  two  curves  in 
the  figure,  indicates  the  time  taken  up  by  the  propagation,  along 
the  piece  of  nerve,  of  the  changes  set  up  at  the  far  end  of  the  nerve 
by  the  induction-shock.  These  changes  we  have  already  spoken 
of  as  constituting  a  nervous  impulse ;  and  the  above  experiment 
shews  that  it  takes  a  small  but  yet  distinctly  appreciable  time 
for  a  nervous  impulse  to  travel  along  a  nerve.  In  the  figure  the 
difference  between  the  two  latent  periods,  the  distance  between  b 
and  b',  seems  almost  too  small  to  measure  accurately  ;  but  if  a 
long  piece  of  nerve  be  used  for  the  experiment,  and  the  recording 
surface  be  made  to  travel  very  fast,  the  difference  between  the 
duration  of  the  latent  period  when  the  induction-shock  is  sent  in 
at  a  point  close  to  the  muscle,  and  that  when  it  is  sent  in  at  a 
point  as  far  away  as  possible  from  the  muscle,  may  be  satisfactorily 
measured  in  fractions  of  a  second.     If  the  lencfth  of  nerve  between 


74  VELOCITY   OF  NEEVOUS   IMPULSE.       [Book  i. 

the  two  points  be  accurately  measured,  the  rate  at  which  a  nervous 
impulse  travels  along  the  nerve  to  a  muscle  can  thus  be  easily 
calculated.  This  has  been  found  to  be  in  the  frog  about  28,  and 
in  man  about  33  metres  per  second,  but  varies  considerably, 
especially  in  warm-blooded  animals. 

Thus  when  a  momentary  stimulus,  such  as  a  single  induction- 
shock,  is  sent  into  a  nerve  connected  with  a  muscle,  the  following 
events  take  place :  a  nervous  impulse  is  started  in  the  nerve,  and 
this  travelling  down  to  the  muscle  produces  in  the  muscle  first  the 
invisible  changes  which  occupy  the  latent  period,  secondly  the 
changes  which  bring  about  the  visible  shortening  or  contraction 
proper,  and  thirdly  the  changes  which  bring  about  the  relaxation 
and  return  to  the  original  length.  The  changes  taking  place  in 
these  several  phases  are  changes  of  living  matter :  they  vary  with 
the  condition  of  the  living  substance  of  the  muscle,  and  only  take 
place  so  long  as  the  muscle  is  alive.  Though  the  relaxation  which 
brings  back  the  muscle  to  its  original  length  is  assisted  by  the 
muscle  being  loaded  with  a  weight,  or  otherwise  stretched,  tliis  is 
not  essential  to  the  actual  relaxation,  and  with  the  same  load  the 
return  will  vary  according  to  the  condition  of  the  muscle  ;  the 
relaxation  must  be  considered  as  an  essential  part  of  the  whole 
contraction,  no  less  than  the  shortening  itself. 

§  47.  Not  only,  as  we  shall  see  later  on,  does  the  whole  con- 
traction vary  in  extent  and  character  according  to  the  condition  of 
the  muscle,  the  strength  of  the  induction-shock,  the  load  which  the 
muscle  is  bearing,  and  various  attendant  circumstances,  but  the 
three  phases  may  vary  independently.  The  latent  period  may  be 
longer  or  shorter,  the  shortening  may  take  a  longer  or  shorter 
time  to  reach  the  same  height,  and  especially  the  relaxation  may 
be  slow  or  rapid,  complete  or  imperfect.  Even  when  the  same 
strength  of  induction-shock  is  used,  the  contraction  may  be  short 
and  sharp,  or  very  long  drawn  out,  so  that  the  curves  described  on 
a  recording  surface,  travelling  at  the  same  rate  in  the  two  cases, 
appear  very  different ;  and,  under  certain  circumstances,  as  when  a 
muscle  is  fatigued,  the  relaxation,  more  particularly  the  last  part 
of  it,  may  be  so  slow,  that  it  may  be  several  seconds  before  the 
muscle  really  regains  its  original  length.  We  may  add  that  the 
latent  period,  which  in  an  ordinary  experiment  on  a  frog's  gastro- 
cnemius is  so  conspicuous,  may,  under  certain  circumstances,  be  so 
shortened  as  almost,  if  not  wholly,  to  disappear.  Indeed,  it  is 
maintained  by  some  that  the  occurrence  of  the  latent  period  is 
not  an  essential  feature  of  the  whole  act. 

Hence,  if  we  say  that  the  duration  of  a  simple  muscular  con- 
traction of  the  gastrocnemius  of  a  frog  under  ordinary  circumstances 
is  about  yV  ^®^-'  ^^^  which  ^^  ^  is  taken  up  by  the  latent  period,  ^os 
by  the  contraction,  and  yf-g  by  the  relaxation,  these  must  be  taken 
as  '  round  numbers,'  stated  so  as  to  be  easily  remembered.  The 
duration  of  each  phase  as  well  as  of  the  whole  contraction  varies  in 


Chap,  ii.]  THE   CONTRACTILE   TISSUES.  75 

different  animals,  in  different  muscles  of  the  same  animal,  and  in 
the  same  muscle  under  different  conditions. 

The  muscle-curve  which  we  have  been  discussing  is  a  curve  of 
changes  in  the  length  only  of  the  muscle  ;  but  if  the  muscle,  instead 
of  being  suspended,  were  laid  flat  on  a  glass  plate,  and  a  lever  laid 
over  its  belly,  we  should  find,  upon  sending  an  induction-shock 
into  the  nerve,  that  the  lever  was  raised,  shewing  that  the  muscle 
during  the  contraction  became  thicker.  And  if  we  took  a  graphic 
record  of  the  movements  of  the  lever,  we  should  obtain  a  curve 
very  similar  to  the  one  just  discussed ;  after  a  latent  period  the 
lever  would  rise,  shewing  that  the  muscle  was  getting  thicker,  and 
afterwards  would  fall,  shewing  that  the  muscle  was  becoming  thin 
again.  In  other  words,  in  contraction  the  lessening  of  the  muscle 
lengthwise  is  accompanied  by  an  increase  crosswise ;  indeed,  as  we 
shall  see  later  on,  the  muscle  in  contracting  is  not  diminished  in 
bulk  at  all  (or  only  to  an  exceedingly  small  extent,  about  ^  o  olio"  °^ 
its  total  bulk),  but  makes  up  for  its  diminution  in  length  by 
increasing  in  its  other  diameters. 

§  48.  A  single  induction-shock  is,  as  we  have  said,  the  most 
convenient  form  of  stimulus  for  producing  a  simple  muscular  con- 
traction, but  this  may  also  be  obtained  by  other  stimuli,  provided 
that  these  are  sufficiently  sudden  and  short  in  their  action,  as,  for 
instance,  by  a  prick  of,  or  sharp  blow  on,  the  nerve  or  muscle.  For 
the  production  of  a  single,  simple  muscular  contraction,  the  changes 
in  the  nerve  leading  to  the  muscle  must  be  of  such  a  kind  as  to 
constitute  what  may  be  called  a  single  nervous  impulse,  and  any 
stimulus  which  will  evoke  a  single  nervous  impulse  only  may  be 
used  to  produce  a  simple  muscular  contraction. 

As  a  rule,  however,  most  stimuli  other  than  single  induction- 
shocks  tend  to  produce  in  a  nerve  several  nervous  impulses,  and, 
as  we  shall  see,  the  nervous  impulses  which  issue  from  the  central 
nervous  system,  and  so  pass  along  nerves  to  muscles,  are,  as  a  rule, 
not  single  and  simple,  but  complex.  Hence,  as  a  matter  of  fact, 
a  simple  mviscular  contraction  is  within  the  living  body  a  com- 
paratively rare  event  (at  least  as  far  as  the  skeletal  muscles  are 
concerned,)  and  cannot  easily  be  produced  outside  the  body  other- 
wise than  by  a  single  induction-shock.  The  ordinary  form  of 
muscular  contraction  is  not  a  simple  muscular  contraction,  but  the 
more  complex  form  known  as  a  tetanic  contraction,  to  the  study 
of  which  we  must  now  turn. 


Tetanic  Contractions. 

§  49.  If  a  single  induction-shock  he  followed  at  a  certain 
interval  by  a  second  shock  of  the  same  strength,  the  first  simple 
contraction  will  be  followed  by  a  second  simple  contraction,  both 


76 


TETANUS. 


[Book  i. 


contractions  being  separate  and  distinct ;  and,  if  the  shocks  be 
repeated,  a  series  of  rhythmically-recurring,  separate,  simple  con- 
tractions may  be  obtained.  If,  however,  the  interval  between  two 
shocks  be  made  short,  —  if,  for  instance,  it  be  made  only  just  long 
enough  to  allow  the  first  contraction  to  have  passed  its  maximum 
before  the  latent  period  of  the  second  is  over,  —  the  curves  of  the 
two  contractions  will  bear  some  such  relation  to  each  other  as 
that  shewn  in  Fig.  13.  It  will  be  observed  that  the  second  curve 
is  almost  in  all  respects  like  the  first,  except  that  it  starts,  so  to 
speak,  from  the  first  curve  instead  of  from  the  base-line.  The 
second  nervous  impulse  has  acted  on  the  already  contracted 
muscle,  and  made  it  contract  again  just  as  it  would  have  done  if 
there  had  been  no  first  impulse,  and  the  muscle  had  been  at  rest. 
The  two  contractions  are  added  together,  and  the  lever  is  raised 
nearly  double  the  height  it  would  have  been  by  either  alone.  If 
in  the  same  way  a  third  shock  follows  the  second  at  a  sufficiently 


v/VWV^w/^/-, 


Fig.  13.    Tracing  of  a  Double  Muscle-ctjrve. 

While  the  muscle  (gastrocnemius  of  frog)  was  engaged  in  the  first  contraction 
(whose  complete  course,  had  nothing  interveued,  is  indicated  by  the  dotted  line),  a 
second  induction-shock  was  thrown  in,  at  such  a  time  that  the  second  contraction 
began  just  as  the  first  was  beginning  to  decline.  The  second  curve  is  seen  to  start 
from  the  first,  as  does  the  first  from  the  base-line. 

short  interval,  a  third  curve  is  piled  on  the  top  of  the  second  ;  the 
same  with  a  fourth,  and  so  on.  A  more  or  less  similar  result 
would  occur  if  the  second  contraction  began  at  another  phase 
of  the  first.  The  combined  effect  is,  of  course,  greatest  when 
the  second  contraction  begins  at  the  maximum  of  the  first,  being 
less  both  before  and  afterwards. 

Hence,  the  result  of  a  repetition  of  shocks  will  depend  largely 
on  the  rate  of  repetition.  If,  as  in  Fig.  14,  the  shocks  follow  each 
other  so  slowly  that  one  contraction  is  over,  or  almost  over,  before 
the  next  begins,  each  contraction  will  be  distinct,  or  nearly  distinct, 
and  there  will  be  little  or  no  combined  effect. 


Fig.  14.    Muscle-curve.    Single  Induction-shock  repeated  slowly. 


Chap,  ii.] 


THE   CONTPvACTILE   TISSUES. 


77 


If,  however,  the  shocks  be  repeated  more  rapidly,  as  in  Fig.  15, 
each  succeeding  contraction  will  start  from  some  part  of  the 
preceding  one,  and  the  lever  will  be  raised  to  a  greater  height  at 
each  contraction. 


Fig.  15.  Muscle-curve.    Single  Induction-shock  repeated  more  r.\^pidlt. 

If  the  frequency  of  the  shocks  be  still  further  increased,  as  in 
Fig.  16,  the  rise  due  to  the  combination  of  contraction  will  be  still 
more  rapid,  and  a  smaller  part  of  each  contraction  will  be  visible 
on  the  curve. 


Fig.  16.  Muscle-curve.  Single  Induction-shock  repeated  still  more  rapidly. 

In  each  of  these  three  curves  it  will  be  noticed  that  the 
character  of  the  curve  changes  somewhat  during  its  development. 
The  change  is  the  result  of  commencing  fatigue,  caused  by  the 
repetition  of  the  contractions,  the  fatigue  manifesting  itself  by  an 
increasing  prolongation  of  each  contraction,  shew^n  especially  in  a 
delay  of  relaxation,  and  by  an  increasing  diminution  in  the  height 
of  the  contraction.  Thus  in  Fig.  14  the  contractions,  quite  distinct 
at  first,  become  fused  later  ;  the  fifth  contraction,  for  instance,  is 
prolonged  so  that  the  sixth  begins  before  the  lever  has  reached 
the  base  line  ;  yet  the  summit  of  the  sixth  is  hardly  higlier  than 
the  summit  of  the  fifth,  since  the  sixth,  though  starting  at  a  higher 
level,  is  a  somewhat  weaker  contraction.  So,  also,  in  Fig.  15,  the 
lever  rises  rapidly  at  first,  but  more  slowly  afterwards,  owing  to  an 
increasing  diminution  in  the  height  of  the  single  contractions.  In 
Fig.  16  the  increment  of  rise  of  the  curve  due  to  each  contraction 
diminishes  very  rapidly,  and  though  the  lever  does  continue  to 


78  TETAKUS.  [Book  r. 

rise  during  the  whole  series,  the  ascent,  after  about  the  sixth 
contraction,  is  very  gradual  indeed,  and  the  indications  of  the 
individual  contractions  are  much  less  marked  than  at  first. 

Hence,  when  shocks  are  repeated  with  sufficient  rapidity,  it 
results  that,  after  a  certain  number  of  shocks,  the  succeeding 
impulses  do  not  cause  any  further  shortening  of  the  muscle,  any 
further  raising  of  the  lever,  but  merely  keep  up  the  contraction 
already  existing.  The  curve  thus  reaches  a  maximum,  which  it 
maintains,  subject  to  the  depressing  effects  of  exhaustion,  so  long 
as  the  shocks  are  repeated.  When  these  cease  to  be  given,  the 
muscle  returns  to  its  natural  length. 

When  the  shocks  succeed  each  other  still  more  rapidly  than 
in  Fig.  16,  the  individual  contractions,  visible  at  first,  may  become 
fused  together  and  wholly  lost  to  view  in  the  latter  part  of  the 
curve.  When  the  shocks  succeed  each  other  still  more  rapidly 
(the  second  contraction  beginning  in  the  ascending  portion  of 
the  first),  it  becomes  difficult  or  impossible  to  trace  out  any  of 
the  single  contractions.^  The  curve  then  described  by  the  lever 
is  of  the  kind  shewn  in  Fig.  17,  where  the  primary  current  of  an 


Fig.  17.    Tetanus  produced  with  the  ordinary  Magnetic  Interruptok  of  an 
Induction-machine.     (Recording  surface  travelling  slowly.) 
The  interrupted  current  is  thrown  in  at  a. 

induction-machine  was  rapidly  made  and  broken  by  the  magnetic 
interruptor.  Fig.  4.  The  lever,  it  will  be  observed,  rises  at  a  (the 
recording  surface  is  travelling  too  slowly  to  allow  the  latent  period 
to  be  distinguished),  at  first  very  rapidly,  —  in  fact,  in  an  unbroken 
and  almost  a  vertical  line,  —  and  so  very  speedily  reaches  the  maxi- 
mum, which  is  maintained  so  long  as  the  shocks  continue  to  be 
given ;  when  these  cease  to  be  given,  the  curve  descends,  at  first 
very  rapidly,  and  then  more  and  more  gradually  towards  the  base 
line,  which  it  reaches  just  at  the  end  of  the  figure. 

This  condition  of  muscle,  brought  about  by  rapidly  repeated 
shocks,  this    fusion   of   a   number   of   simple    twitches    into    an 

1  The  ease  with  which  the  individual  contractions  can  be  made  out  depends  in 
part,  it  need  hardly  be  said,  on  the  rapidity  with  which  the  recording  surface  travels. 


Chap,  ii.]  THE   CO^^TRACTILE   TISSUES.  79 

apparently  smooth,  continuous  effort,  is  known  as  tetanus,  or 
tetanic  contraction.  The  above  facts  are  most  clearly  shewn 
when  induction-shocks,  or  at  least  galvanic  currents  in  some 
form  or  other,  are  employed.  They  are  seen,  however,  what- 
ever be  the  form  of  stimulus  employed.  Thus,  in  the  case  of 
mechanical  stimuli,  while  a  single  quick  blow  may  cause  a  single 
twitch,  a  pronounced  tetanus  may  be  obtained  by  rapidly  striking 
successively  fresh  portions  of  a  nerve.  With  chemical  stimulation, 
as  when  a  nerve  is  dipped  in  acid,  it  is  impossible  to  secure  a 
momentary  application ;  hence  tetanus,  generally  irregular  in 
character,  is  the  normal  result  of  this  mode  of  stimulation.  In 
the  living  body,  the  contractions  of  the  skeletal  muscles,  brought 
about  either  by  the  will  or  otherwise,  are  generally  tetanic  in 
character.  Even  very  short,  sharp  movements,  sucli  as  a  sudden 
jerk  of  a  limb,  or  a  wink  of  the  eyelid,  are,  in  reality,  examples  of 
tetanus  of  short  duration. 

If  the  lever,  instead  of  being  fastened  to  the  tendon  of  a  muscle 
hung  vertically,  be  laid  across  the  belly  of  a  muscle  placed  in  a 
horizontal  position,  and  the  muscle  be  thrown  into  tetanus  by  a 
repetition  of  induction-shocks,  it  will  be  seen  that  each  shortening 
of  the  muscle  is  accompanied  by  a  corresponding  thickening,  and 
that  the  total  shortening  of  the  tetanus  is  accompanied  by  a  cor- 
responding total  thickening.  And,  indeed,  in  tetanus  we  can 
observe  more  easily  than  in  a  single  contraction  that  the  muscle  in 
contracting  changes  in  form  only,  not  in  bulk.  If  a  living  muscle, 
or  group  of  muscles,  be  placed  in  a  glass  jar,  or  chamber,  the  closed 
top  of  which  is  prolonged  into  a  narrow  glass  tube,  and  the 
chamber  be  so  filled  with  water  (or,  preferably,  with  a  solution  of 
sodium  chloride,  "6  p.  c.  in  strength,  the  "normal  saline  solution," 
which  is  less  injurious  to  the  tissue  than  simple  water)  that 
the  fluid  rises  up  into  the  narrow  tube,  it  is  obvious  that  any 
change  in  the  bulk  of  the  muscle  will  be  easily  shewn  by  a  rising 
or  falling  of  the  column  of  fluid  in  the  narrow  tube.  It  is  found 
that  when  the  muscle  is  made  to  contract,  even  in  the  most 
forcible  manner,  the  change  of  level  in  the  heioht  of  the  column 
which  can  be  observed  is  practically  insignificant :  there  appears 
to  be  a  fall  indicating  a  diminution  of  bulk  to  the  extent  of  about 
one  ten-thousandth  of  the  total  bulk  of  the  muscle.  So  that  we 
may  fairly  say  that  in  a  tetanus,  and  hence  in  a  simple  contraction, 
the  lessening  of  the  length  of  the  muscle  causes  a  corresponding 
increase  in  tlie  other  directions :  the  substance  of  the  muscle  is 
displaced  not  diminished. 

§  50.  So  far  we  have  spoken  simply  of  an  induction-shock,  or 
of  induction-shocks,  without  any  reference  to  their  strength,  and 
of  a  living  or  irritable  muscle,  without  any  reference  to  the  degree 
or  extent  of  its  irritability;  but  induction-shocks  may  vary  in 
strength,  and  the  irritability  of  the  muscle  may  vary. 

If  we  slide  the  secondary  coil  a  long  way  from  the  primary 


80  VAEIATIONS   OF  lERITABILITY.  [Book  i. 

coil,  and  thus  make  use  of  extremely  feeble  induction-shocks,  we 
shall  probably  find  that  these  shocks,  applied  even  to  a  quite  fresh 
muscle-nerve  preparation,  produce  no  contraction.  If  we  then 
gradually  slide  the  secondary  coil  nearer  and  nearer  the  primary 
coil,  and  keep  on  trying  the  effects  of  the  shocks,  we  shall  find 
that,  after  a  while,  in  a  certain  position  of  the  coils,  a  very  feeble 
contraction  makes  its  appearance.  As  the  secondary  coil  comes 
still  nearer  to  the  primary  coil,  the  contractions  grow  greater  and 
greater.  After  a  while,  however,  and  that,  indeed,  in  ordinary 
circumstances  very  speedily,  increasing  the  strength  of  the  shock 
no  longer  increases  the  height  of  the  contraction ;  the  maximum 
contraction  of  which  the  muscle  is  capable  with  such  shocks 
however  strong  has  been  reached. 

If  we  use  a  tetanizing  or  interrupted  current,  we  shall  obtain 
the  same  general  results  ;  we  may,  according  to  the  strength  of  the 
current,  get  no  contraction  at  all,  or  contractions  of  various  extent 
up  to  a  maximum,  which  cannot  be  exceeded.  Under  favourable 
conditions  the  maximum  contraction  may  be  very  considerable : 
the  shortening  in  tetanus  may  amount  to  three-fifths  of  the  total 
length  of  the  muscle. 

The  amount  of  contraction  then  depends  on  the  strength  of 
the  stimulus,  whatever  be  the  stimulus ;  but  this  holds  good 
within  certain  limits  only ;  to  this  point  however  we  shall  return 
later  on. 

§  51.  If,  having  ascertained  in  a  perfectly  fresh  muscle-nerve 
preparation  the  amount  of  contraction  produced  by  this  and  that 
strength  of  stimulus,  we  leave  the  preparation  by  itself  for  some 
time,  say  for  a  few  hours,  and  then  repeat  the  observations,  we 
shall  find  that  stronger  stimuli,  stronger  shocks,  for  instance,  are 
required  to  produce  the  same  amount  of  contraction  as  before  ;  that 
is  to  say,  the  irritability  of  the  preparation,  the  power  to  respond 
to  stimuli,  has  in  the  meanwhile  diminished.  After  a  further 
interval,  we  should  find  the  irritability  still  further  diminished : 
even  very  strong  shocks  would  be  unable  to  evoke  contractions 
as  large  as  those  previously  caused  by  weak  shocks.  At  last  we 
should  find  that  no  shocks,  no  stimuli,  however  strong,  were  able 
to  produce  any  visible  contraction  whatever.  The  amount  of 
contraction,  in  fact,  evoked  by  a  stimulus  depends  not  only  on  the 
strength  of  the  stimulus  but  also  on  the  degree  of  irritability  of 
the  muscle-nerve  preparation. 

Immediately  upon  removal  from  the  body,  the  preparation 
possesses  a  certain  amount  of  irritability,  not  differing  very 
materially  from  that  which  the  muscle  and  nerve  possess  while 
within,  and  forming  an  integral  part  of  the  body  ;  but  after  re- 
moval from  the  body  the  preparation  loses  irritability,  the  rate  of 
loss  being  dependent  on  a  variety  of  circumstances  ;  and  this  goes 
on  until,  since  no  stimulus  which  we  can  apply  will  give  rise  to 
a  contraction,  we  say  the  irritability  has  wholly  disappeared. 


Chap,  it.]  TPIE   CONTRACTILE   TISSUES.  81 

We  might  take  this  disappearance  of  irritability  as  marking 
the  death  of  the  preparation,  but  it  is  followed  sooner  or  later  by 
a  curious  change  in  the  muscle,  which  is  called  rigor  mortis,  and 
which  we  shall  study  presently  ;  and  it  is  convenient  to  regard 
this  rigor  mortis  as  marking  the  death  of  the  muscle. 

The  irritable  muscle,  then,  when  stimulated  either  directly,  the 
stimulus  being  applied  to  itself,  or  indirectly,  the  stimulus  being 
applied  to  its  nerve,  responds  to  the  stimulus  by  a  change  of 
form  which  is  essentially  a  shortening  and  thickening.  By  the 
shortening  (and  thickening),  the  muscle  in  contracting  is  able  to 
do  work,  to  move  the  parts  to  which  it  is  attached  ;  it  thus  sets 
free  energy.  We  have  now  to  study  more  in  detail  how  this 
energy  is  set  free  and  the  laws  which  regulate  its  expenditure. 


SEC.  2.     ON   THE   CHANGES   WHICH   TAKE  PLACE  IN 
A  MUSCLE  DURING  A  CONTRACTION. 


The  Change  in  Form. 

§  52.  An  ordinary  skeletal  muscle  consists  of  elementary 
muscle  fibres,  bound  together  in  variously  arranged  bundles  by 
connective  tissue  which  carries  blood  vessels,  nerves  and  lym- 
phatics. 

The  contraction  of  a  muscle  is  the  contraction  of  all  or  some  of 
its  elementary  fibres,  the  connective  tissue  being  passive  ;  hence 
while  those  fibres  of  the  muscle  which  end  directly  in  the  tendon, 
in  contracting  pull  directly  on  the  tendon,  those  which  do  not  so 
end  pull  indirectly  on  the  tendon  by  means  of  the  connective 
tissue  between  the  bundles,  which  connective  tissue  is  continuous 
with  the  tendon. 

Each  muscle  is  supplied  by  one  or  more  branches  of  nerves 
which,  running  in  the  connective  tissue,  divide  into  smaller 
branches  and  twigs  between  the  bundles  and  between  the  fibres, 
and  eventually  end  in  such  a  way  that  every  muscular  fibre  is  sup- 
plied with  at  least  one  nerve  fibre,  which  joins  the  muscular  fibre 
somewhere  about  the  middle  between  its  two  ends  or  sometimes 
nearer  one  end,  in  a  special  nerve  ending  called  an  end-plate.  It 
follows  that  when  a  muscular  fibre  is  stimulated  by  means  of  a 
nerve  fibre,  the  nervous  impulse  travelling  down  the  nerve  fibre 
falls  into  the  muscular  fibre  not  at  one  end  but  at  about  its  mid- 
dle ;  it  is  the  middle  of  the  fibre  which  is  affected  first  by  the 
nervous  impulse,  and  the  changes  in  the  muscular  substance 
started  in  the  middle  of  the  muscular  fibre  travel  thence  to  the 
two  ends  of  the  fibre.  In  an  ordinary  skeletal  muscle  however,  the 
fibres  and  bundles  of  fibres  begin  and  end  at  different  distances 
from  the  ends  of  the  muscle,  and  the  nerve  or  nerves  going  to 
the  muscle  divide  and  spread  out  in  the  muscle  in  such  a  way 
that  the  end-plates,  in  which  the  individual  fibres  of  the  nerve 
end,  are  distributed  widely  over  the   muscle   at  very  different 


Chap,  ir.]  THE   CONTRACTILE   TISSUES.  83 

distances  from  the  ends  of  the  muscle.  Hence,  if  we  suppose 
a  single  nervous  impulse,  such  as  that  generated  by  a  single 
induction-shock,  or  a  series  of  such  impulses  to  be  started  at 
the  same  time  at  some  part  of  the  trunk  of  the  nerve  in  each  of 
the  fibres  of  the  nerve  going  to  the  muscle,  these  impulses  will 
reach  very  different  parts  of  the  muscle  at  about  the  same  time 
and  the  contractions  which  they  set  going  will  Ijegin,  so  to  speak, 
nearly  all  over  the  whole  muscle  at  the  same  time,  and  will  not  all 
start  in  any  particular  zone  or  area  of  the  muscle. 

§  53.  The  wave  of  contraction.  We  have  seen,  however,  that 
under  the  influence  of  urari  the  nerve  fibre  is  unable  to  excite 
contractions  in  a  muscular  fibre,  although  the  irritability  of  the 
muscular  fibre  itself  is  retained.  Hence,  in  a  muscle  poisoned  by 
urari  the  contraction  begins  at  that  part  of  the  muscular  substance 
which  is  first  affected  by  the  stimulus,  and  we  may  start  a  con- 
traction in  what  part  of  the  muscle  we  please  by  properly  placing 
the  electrodes. 

Some  muscles,  such  for  instance  as  the  sartorius  of  the  frog, 
though  of  some  length  are  composed  of  fibres  which  run  parallel 
to  each  other  from  one  end  of  the  muscle  to  the  other.  If  such  a 
muscle  be  poisoned  with  urari  so  as  to  eliminate  the  action  of  the 
nerves  and  stimulated  at  one  end  (an  induction-shock  sent  through 
a  pair  of  electrodes  placed  at  some  little  distance  apart  from  each 
other  at  the  end  of  the  muscle  may  be  employed,  but  better 
results  are  obtained  if  a  mode  of  stimulation,  of  which  we  shall 
have  to  speak  presently,  viz.  the  application  of  the  "  constant  cur- 
rent," be  adopted),  the  contraction  which  ensues  starts  from  the 
end  stimulated,  and  travels  thence  along  the  muscle.  If  two 
levers  be  made  to  rest  on,  or  be  suspended  from,  two  parts  of  such 
a  muscle  placed  horizontally,  the  parts  being  at  a  known  distance 
from  each  other  and  from  the  part  stimulated,  the  progress  of  the 
contraction  may  be  studied. 

The  movements  of  the  levers  indicate  in  this  case  the  thicken- 
ing of  the  fibres  which  is  taking  place  at  the  parts  on  which 
the  levers  rest  or  to  which  they  are  attached ;  and  if  we  take 
a  graphic  record  of  these  movements,  bringing  the  two  levers  to 
mark,  one  immediately  below  the  other,  we  shall  find  that  the 
lever  nearer  the  part  stimulated  begins  to  move  earlier,  reaches  its 
maximum  earlier,  and  returns  to  rest  earlier  than  does  the  farther 
lever.  The  contraction,  started  by  the  stimulus,  in  travelling  along 
the  muscle  from  the  part  stimulated  reaches  the  nearer  lever  some 
little  time  before  it  reaches  the  farther  lever,  and  has  passed  by 
the  nearer  lever  some  little  time  before  it  has  passed  by  the 
farther  lever ;  and  the  farther  apart  the  two  levers  are  the  greater 
will  be  the  difference  in  time  between  their  movements.  In  other 
words  the  contraction  travels  along  the  muscle  in  the  form  of  a 
wave,   each    part    of   the    muscle    in    succession    from    the    end 


84  THE   WAVE   OF   CONTRACTION.  [Book  i. 

stimulated  swelling  out  and  shortening  as  the  contraction  reaches 
it,  and  then  returning  to  its  original  state.  And  what  is  true  of 
the  collection  of  parallel  fibres  which  we  call  the  muscle  is  also 
true  of  each  fibre,  for  the  swelling  at  any  part  of  the  muscle  is 
only  the  sum  of  the  swelling  of  the  individual  fibres ;  if  we  were 
able  to  take  a  single  long  fibre  and  stimulate  it  at  one  end,  we 
should  be  able  under  the  microscope  to  see  a  swelling  or  bulging 
accompanied  by  a  corresponding  shortening,  i.e.  to  see  a  con- 
traction sweep  along  the  fibre  from  end  to  end. 

If  in  the  graphic  record  of  the  two  levers  just  mentioned 
we  count  the  number  of  vibrations  of  the  tuning-fork  which 
intervene  between  the  mark  on  the  record  which  indicates  the 
beginning  of  the  rise  of  the  near  lever  (that  is,  the  arrival  of  the 
contraction  wave  at  this  lever)  and  the  mark  which  indicates  the 
beginning  of  the  rise  of  the  far  lever,  this  will  give  us  the  time 
which  it  has  taken  the  contraction  wave  to  travel  from  the  near  to 
the  far  lever.  Let  us  suppose  this  to  be  "005  sec.  Let  us  suppose 
the  distance  between  the  two  levers  to  be  15  mm.  The  con- 
traction wave  then  has  taken  "005  sec.  to  travel  15  mm.,  that  is 
to  say  it  has  travelled  at  the  rate  of  3  meters  per  sec.  And  indeed 
we  find  by  this,  or  by  other  methods,  that  in  the  frog's  muscles  the 
contraction  wave  does  travel  at  a  rate  which  may  be  put  down  as 
from  3  to  4  meters  a  second,  though  it  varies  under  different  con- 
ditions. In  the  warm  blooded  mammal  the  rate  is  somewhat 
greater,  and  may  probably  be  put  down  at  5  meters  a  second 
in  the  excised  muscle,  rising  possibly  to  10  meters  in  a  muscle 
within  the  living  body. 

If  again  in  the  graphic  record  of  the  two  levers  we  count,  in 
the  case  of  either  lever,  the  number  of  vibrations  of  the  tuning- 
fork  which  intervene  between  the  mark  where  the  lever  begins  to 
rise  and  the  mark  where  it  has  finished  its  fall  and  returned  to  the 
base  line,  we  can  measure  the  time  intervening  between  the 
contraction  wave  reaching  the  lever,  and  leaving  the  lever  on  its 
way  onward,  that  is  to  say,  we  can  measure  the  time  which  it  has 
taken  the  contraction  wave  to  pass  over  the  part  of  the  muscle  on 
which  the  lever  is  resting.  Let  us  suppose  this  time  to  be  say 
•1  sec.  But  a  wave  which  is  travelling  at  the  rate  of  3  m.  a 
second  and  takes  1  sec.  to  pass  over  any  point  must  be  300  mm. 
long.  And  indeed  we  find  that  in  the  frog  the  length  of  the 
contraction  wave  may  be  put  down  as  varying  from  200  to 
400  mm.;  and  in  the  mammal  it  is  not  very  different. 

Now  the  very  longest  muscular  fibre  is  stated  to  be  at  most 
only  about  40  mm.  in  length  ;  hence,  in  an  ordinary  contraction, 
during  the  greater  part  of  the  duration  of  the  contraction  the 
whole  length  of  the  fibre  will  be  occupied  by  the  contraction 
wave.  Just  at  the  beginning  of  the  contraction  there  will  be 
a  time  when  the  front  of  the  contraction  wave  has  reached  for 


Chap,  ii.]  THE   CONTRACTILE   TISSUES.  85 

instance  only  lialf-way  down  the  fibre  (supposing  the  stimulus 
to  be  applied,  as  in  the  case  we  have  been  discussing,  at  one  end 
only),  and  just  at  the  end  of  the  contraction  there  will  be  a  time 
for  instance  when  the  contraction  has  left  the  half  of  the  fibre 
next  to  the  stimulus,  but  has  not  yet  cleared  away  from  the  other 
half.  But  nearly  all  the  rest  of  the  time  every  part  of  the  fibre 
will  be  in  some  phase  or  other  of  contraction,  though  the  parts 
nearer  the  stimulus  will  be  in  more  advanced  phases  than  the 
parts  farther  from  the  stimulus. 

This  is  true  when  a  muscle  of  parallel  fibres  is  stimulated 
artificially  at  one  end  of  the  muscles,  and  when  therefore  each 
fibre  is  stimulated  at  one  end.  It  is  of  course  all  the  more  true 
when  a  muscle  of  ordinary  construction  is  stimulated  by  means  of 
its  nerve.  The  stimulus  of  the  nervous  impulse  impinges,  m  this 
case,  on  the  muscle  fibre  at  the  end-plate  which,  as  we  have  said, 
is  placed  towards  the  middle  of  the  fibre,  and  the  contraction 
wave  travels  from  the  end-plate  in  opposite  directions  toward 
each  end,  and  has  accordingly  only  about  half  the  length  of  the 
fibre  to  run  in.  All  the  more  therefore  must  the  whole  fibre  be 
in  a  state  of  contraction  at  the  same  time. 

§  54.  We  may  now  turn  to  the  question.  What  takes  place  in 
a  muscular  fibre  when  a  contraction  wave  sweeps  over  it  ? 

Optical  Changes.  Although  undoubtedly  the  optical  features 
of  a  muscular  fibre  change  while  it  is  contracting,  it  is  very  diffi- 
cult to  make  an  authoritative  statement  as  to  what  those  changes 
are.  In  the  first  place  a  contraction  wave,  even  when  it  is  travel- 
ling with  relative  slowness,  travels  so  rapidly  that  the  individual 
features  cannot  be  seized  by  the  eye.  We  are  confined  to  con- 
clusions drawn  from  the  study  of  short  local  contractions,  local 
thickenings  and  shortenings  which  may  be  obtained  in  the  living 
fibre  and  fixed  by  the  action  of  osmic  acid  vapour  or  by  other 
means ;  and  it  has  to  be  assumed  that  these  local  bulgings  give  a 
true  picture  of  a  normal  contraction  wave  by  which,  as  we  have 
just  seen,  the  whole  length  of  a  fibre  is  occupied  at  the  same  time. 
In  the  second  place  the  minute  structure  of  a  muscular  fibre  has 
been  and  still  is  the  subject  of  fierce  dispute. 

If  we  adopt  the  view  that  the  fibre  is  made  up  of  dim 
bands  or  discs  of  dim  substance  alternating  with  bright  bands 
or  discs  of  bright  substance,  with  transverse  markings  in  the 
middle  of  each  bi'ight  band  forming  a  line  "  intermediate "  be- 
tween the  two  adjacent  dim  bands,  we  may,  according  to  some 
observers,  say  that  during  a  contraction  there  seems  to  be  an 
interchange  between  the  dim  and  bright  bands  so  that,  in  ordinary 
light,  at  the  height  of  the  contraction,  in  the  broadest  part  of  one 
of  the  bulgings  just  spoken  of,  the  previously  obscure  "  interme- 
diate line  "  becomes  a  conspicuous  dark  band,  the  interval  between 
two  such  changed  intermediate  lines  becoming  relatively  and  uui- 


86  CHEMISTRY   OF   MUSCLE.  [Book  i. 

formly  bright ;  in  other  words  there  is  a  sort  of  reversal  of  the 
situation,  what  was  bright  becoming,  in  its  middle  at  least,  dark, 
and  what  was  dim  becoming  relatively  bright.  When  the  fibre 
is  examined  under  polarized  light,  by  which  the  dim  bands  are 
shown  to  be  largely  composed  of  doubly  refractive,  anisotropic 
material  and  the  bright  bands  chiefly  of  singly  refractive,  isotropic 
material,  it  is  found  that  the  above  apparent  reversal  is  not  based 
on  any  reversal  of  the  refractive  material,  the  anisotropic  (dim) 
band  remains  anisotropic,  and  the  isotropic  (bright)  band  remains 
isotropic.  But  wMle  both  bands  become  broader  (across  the  fibre) 
and  thinner  (shorter  along  the  length  of  the  fibre),  the  anisotropic 
band  does  not  become  so  much  thinner  as  does  the  isotropic  band, 
in  other  words  the  dim  doubly  refractive  band  or  disc  increases  in 
bulk  at  the  expense  of  the  bright  singly  refractive  band.  And 
this  accords  with  another  feature  of  the  fibre  during  contraction  ; 
namely,  that  the  sarcolemma,  which  in  the  fibre  at  rest  presents 
a  quite  even  line,  is  then  indented  at  the  middle  of  the  bright 
band  at  about  the  position  of  the  intermediate  line,  and  bulges 
out  opposite  the  dim  band,  that  is  opposite  the  enlarged  aniso- 
tropic disc. 

It  is  useless,  however,  to  dwell  on  these  matters  until  the  minute 
structure  of  the  fibre  has  been  more  clearly  and  satisfactorily  made 
out  than  it  is  at  present.  A  contraction  is  obviously  a  transloca- 
tion of  molecules  of  the  muscle  substance  and  may,  very  roughly, 
be  compared  to  the  movement  by  which  a  company,  say  of  one  hun- 
dred soldiers  ten  ranks  deep,  with  ten  men  in  each  rank,  extends 
out  into  a  double  line  of  two  ranks  with  fifty  men  in  each  rank. 
The  movement  of  translocation  is  obviously,  in  striated  muscle,  a 
very  complicated  one,  but  how  the  striation  helps  the  movement 
we  do  not  at  present  really  know.  All  we  can  say  is  that  when 
swift  and  rapid  contraction  is  needed,  the  contractile  tissue  em- 
ployed puts  on  in  nearly  all  cases  the  striated  structure. 


The  Chemistry  of  Muscle. 

§  55.  We  said,  in  the  Introduction,  that  it  was  difficult  to 
make  out  with  certainty  the  exact  chemical  differences  between 
dead  and  living  substance.  Muscle  however  in  dying  undergoes 
a  remarkable  chemical  change,  which  may  be  studied  with  com- 
parative ease.  We  have  already  said  that  all  muscles,  within  a 
certain  time  after  removal  from  the  body,  or,  if  still  remaining  part 
of  the  body,  within  a  certain  time  after  '  general '  death  of  the 
body,  lose  their  irritability,  and  that  the  loss  of  irritability,  which 
even  when  rapid,  is  gradual,  is  succeeded  by  an  event  which  is 
somewhat  more  sudden,  viz.  the  entrance  into  the  condition  known 
as  rigor  mortis.     The  occurrence  of  rigor  mortis,  or  cadaveric  rigid- 


Chap,  ir.]  THE   CONTRACTILE   TISSUES.  87 

ity,  as  it  is  sometimes  called,  which  may  be  considered  as  the  token 
of  the  death  of  the  muscle,  is  marked  by  the  following  features. 
The  living  muscle  possesses  a  certain  translucency,  the  rigid  muscle 
is  distinctly  more  opaque.  The  living  muscle  is  very  extensible 
and  elastic,  it  stretches  readily  and  to  a  considerable  extent  when 
a  weight  is  hung  upon  it  or  when  any  traction  is  applied  to  it, 
but  speedily  and,  under  normal  circumstances,  completely  returns 
to  its  original  length  when  the  weight  or  traction  is  removed ;  as 
we  shall  see  however  the  rapidity  and  completeness  of  the  return 
depends  on  the  condition  of  the  muscle,  a  well-nourished,  active 
muscle  regaining  its  normal  length  much  more  rapidly  and  com- 
pletely than  a  tired  and  exhausted  muscle.  A  dead,  rigid  muscle 
is  much  less  extensible  and  at  the  same  time  much  less  elastic ; 
the  muscle  now  requires  considerable  force  to  stretch  it,  and  when 
the  force  is  removed,  does  not,  as  before,  return  to  its  former 
length.  To  the  touch  the  rigid  muscle  has  lost  much  of  its 
former  softness,  and  has  become  firmer  and  more  resistant.  The 
entrance  into  rigor  mortis  is  moreover  accompanied  by  a  shorten- 
ing or  contraction,  which  may,  under  certain  circumstances,  be 
considerable.  The  energy  of  this  contraction  is  not  great,  so  that 
any  actual  shortening  is  easily  prevented  by  the  presence  of  even 
a  slight  opposing  force. 

Now  the  chemical  features  of  the  dead,  rigid  muscle  are  also 
strikingly  different  from  those  of  the  living  muscle. 

§  56.  If  a  dead  muscle,  from  which  all  fat,  tendon,  fascia,  and 
connective  tissue  have  been  as  much  as  possible  removed,  and 
which  has  been  freed  from  blood  by  the  injection  of  'normal'  saline 
solution,  be  minced  and  repeatedly  washed  with  water,  the  washings 
will  contain  certain  forms  of  albumin  and  certain  extractive  bodies, 
of  which  we  shall  speak  directly.  When  the  washing  has  been 
continued  until  the  wash-water  gives  no  proteid  reaction,  a  large 
portion  of  muscle  will  still  remain  undissolved.  If  this  be  treated 
with  a  10  p.  c.  solution  of  a  neutral  salt,  ammonium  chloride  being 
the  best,  a  large  portion  of  it  will  become  dissolved ;  the  solution 
however  is  more  or  less  imperfect  and  filters  with  difficulty.  If  the 
filtrate  be  allowed  to  fall  drop  by  drop  into  a  large  quantity  of 
distilled  water,  a  white  flocculent  matter  will  be  precipitated. 
This  flocculent  precipitate  is  myosin.  Myosin  is  a  proteid,  giving 
the  ordinary  proteid  reactions,  and  having  the  same  general 
elementary  composition  as  other  proteids.  It  is  soluble  in  dilute 
saline  solutions,  especially  those  of  ammonium  chloride,  and  may 
be  classed  in  the  globulin  family,  though  it  is  not  so  soluble  as 
paraglobulin,  requiring  a  stronger  solution  of  a  neutral  snlt  to 
dissolve  it ;  thus  while  soluble  in  a  5  or  10  p.  c.  solution  of  such  a 
salt,  it  is  far  less  soluble  in  a  1  p.  c.  solution,  which  as  we  have 
seen  readily  dissolves  paraglobulin.  From  its  solutions  in  neutral 
saline  solution  it   is   precipitated   by  saturation  with  a  neutral 


88  CHEMISTRY   OF  MUSCLE.  [Book  i. 

salt,  preferably  sodium  chloride,  and  may  be  purified  by  being 
washed  with  a  saturated  solution,  dissolved  again  in  a  weaker 
solution,  and  reprecipitated  by  saturation.  Dissolved  in  saline 
solutions  it  readily  coagulates  when  heated,  i.e.  is  converted  into 
coagulated  proteid,  and  it  is  worthy  of  notice  that  it  coagulates 
at  a  comparatively  low  temperature,  viz.  about  56°  C. ;  this  it  will 
be  remembered  is  the  temperature  at  which  fibrinogen  is  coagu- 
lated, whereas  paraglobulin,  serum  albumin,  and  many  other  pro- 
teids  do  not  coagulate  until  a  higher  temperature,  75°  C,  is  reached. 
Solutions  of  myosin  are  precipitated  by  alcohol,  and  the  precipitate, 
as  in  the  case  of  other  proteids,  becomes  by  continued  action  of  the 
alcohol  altered  into  coagulated  insoluble  proteid. 

We  have  seen  that  paraglobulin,  and  indeed  any  member  of 
the  globulin  group,  is  very  readily  changed  by  the  action  of  dilute 
acids  into  a  body  called  acid  albumin,  characterised  by  not  being 
soluble  either  in  water  or  in  dilute  saline  solutions  but  readily 
soluble  in  dilute  acids  and  alkalis,  from  its  solutions  in  either  of 
which  it  is  precipitated  by  neutralisation,  and  by  the  fact  that  the 
solutions  in  dilute  acids  and  alkalis  are  not  coagulated  by  heat. 
When  therefore  a  globulin  is  dissolved  in  dilute  acid,  what  takes 
place  is  not  a  mere  solution  but  a  chemical  change ;  the  globulin 
cannot  be  got  back  from  the  solution,  it  has  been  changed  into 
acid-albumin.  Similarly  when  globulin  is  dissolved  in  dilute 
alkalis  it  is  changed  into  alkali  alhumin  ;  and  broadly  speaking 
alkali  albumin  precipitated  by  neutralisation  can  be  changed  by 
solution  with  dilute  acids  into  acid  albumin,  and  acid  albumin  by 
dilute  alkalis  into  alkali  albumin. 

Now  myosin  is  similarly,  and  even  more  readily  than  is 
globulin,  converted  into  acid  albumin,  and  by  treating  a  muscle 
either  washed  or  not,  directly  with  dilute  hydrochloric  acid,  the 
myosin  may  be  converted  into  acid  albumin  and  dissolved  out. 
Acid  albumin  obtained  by  dissolving  muscle  in  dilute  acid  used  to 
be  called  syntonin,  and  it  used  to  be  said  that  a  muscle  contained 
syntonin  ;  the  muscle  however  contains  myosin,  not  syntonin,  but 
it  may  be  useful  to  retain  the  word  syntonin  to  denote  acid  albumin 
obtained  by  the  action  of  dilute  acid  on  myosin.  By  the  action 
of  dilute  alkalis,  myosin  may  similarly  be  converted  into  alkali 
albumin. 

From  what  has  been  stated  above  it  is  obvious  that  myosin  has 
many  analogies  with  fibrin,  and  we  have  yet  to  mention  other 
striking  analogies ;  it  is  however  much  more  soluble  than  fibrin, 
and  speaking  generally  it  may  be  said  to  be  intermediate  in  its 
character  between  fibrin  and  globulin.  On  keeping,  and  especially 
on  drying,  its  solubility  is  much  diminished. 

Of  the  substances  which  are  left  in  washed  muscle,  from  which 
all  the  myosin  has  been  extracted  by  ammonium  chloride  solution, 
little  is  known.     If  washed  muscle  be  treated  directly  with  dilute 


Chap,  ii.]  THE   CONTRACTILE   TISSUES.  89 

hydrochloric  acid,  a  large  part  of  the  material  of  the  muscle  passes, 
as  we  have  said,  at  once  into  syntonin.  The  quantity  of  syntonin 
thus  obtained  may  be  taken  as  roughly  representing  the  quantity 
of  myosin  previously  existing  in  the  muscle.  A  more  prolonged 
action  of  the  acid  may  dissolve  out  other  proteids,  besides  myosin, 
left  after  the  washing.  The  portion  insoluble  in  dilute  hydro- 
chloric acid  consists  in  part  of  the  gelatine  yielding  and  other 
substances  of  the  sarcolemma  and  of  the  connective  and  other 
tissues  between  the  bundles,  of  the  nuclei  of  these  tissues  and  of 
the  fibres  themselves,  and  in  part,  possibly,  of  some  portions  of 
the  muscle  substance  itself.  We  are  not  however  at  present  in  a 
position  to  make  any  very  definite  statement  as  to  the  relation  of 
the  myosin  to  the  structural  features  of  muscle.  Since  the  dim 
bands  are  rendered  very  indistinct  by  the  action  of  10  p.c.  sodium 
chloride  solution,  we  may  perhaps  infer  that  myosin  enters  largely 
into  the  composition  of  the  dim  bands,  and  therefore  of  the  fibrilhie ; 
but  it  would  be  hazardous  to  say  much  more  than  this. 

§  57.  Living  muscle  may  be  frozen,  and  yet,  after  certain 
precautions  will,  on  being  thawed,  regain  its  irritability,  or  at  all 
events  will  for  a  time  be  found  to  be  still  living  in  the  sense  that 
it  has  not  yet  passed  into  rigor  mortis.  We  may  therefore  take 
living  muscle  which  has  been  frozen  as  still  living. 

If  living  contractile  onuscle,  freed  as  much  as  possible  from 
blood,  be  frozen,  and  while  frozen,  minced,  and  rubbed  up  in  a 
mortar  with  four  times  its  weight  of  snow  containing  1  p.c.  of 
sodium  chloride,  a  mixture  is  obtained  which  at  a  temperature 
just  below  0°  C.  is  sufficiently  fluid  to  be  filtered,  though  with 
difficulty.  The  slightly  opalescent  filtrate,  or  muscle  i^lcisma  as  it 
is  called,  is  at  first  quite  fluid,  but  will  when  exposed  to  the 
ordinary  temperature  become  a  solid  jelly,  and  afterwards  separate 
into  a  clot  and  scrum.  It  will  in  fact  clot  like  blood  plasma,  with 
this  difference,  that  the  clot  is  not  firm  and  fibrillar,  but  loose, 
granular,  and  flocculent.  During  the  clotting  the  fluid,  which 
before  was  neutral  or  slightly  alkaline,  becomes  distinctly  acid. 

The  clot  is  myosin.  It  gives  all  the  reactions  of  myosin  obtained 
from  dead  muscle. 

The  serum  contains  an  albumin  very  similar  to,  if  not  identical 
with,  serum  albumin,  a  globulin  differing  somewhat  from,  and 
coagulating  at  a  lower  temperature  than  paraglobulin,  and  which 
to  distinguish  it  from  the  globulin  of  blood  has  been  called  inyo- 
glohulin,  some  other  proteids  which  need  not  be  described  here, 
and  various  '  extractives '  of  which  we  shall  speak  directly.  Such 
muscles  as  are  red  also  contain  a  small  quantity  of  haemoglobin 
and  possibly,  another  allied  red  pigment. 

Thus  while  dead  muscle  contains  myosin,  albumin,  and  other 
proteids,  extractives,  and  certain  insoluble  matters,  together  with 
gelatinous   and   other   substances   not   referable   to   the    muscle 


^0  RIGOR  MORTIS.  [Book  i. 

substance  itself,  living  muscle  contains  no  myosin,  but  some 
substance  or  substances  which  bear  somewhat  the  same  relation  to 
myosin  that  the  antecedents  of  fibrin  do  to  fibrin,  and  which  give 
rise  to  myosin  upon  the  death  of  the  muscle.  There  are  indeed 
reasons  for  thinking  that  the  myosin  arises  from  the  conversion  of 
a  previously  existing  body,  which  may  be  called  unyosinogen,  and 
that  the  conversion  takes  place,  or  may  take  place,  by  the  action 
of  a  special  ferment,  the  conversion  of  myosinogen  into  myosin 
being  very  analogous  to  the  conversion  of  fibrinogen  into  fibrin. 

We  may  in  fact  speak  of  rigor  mortis  as  characterised  by  a 
clotting  of  the  muscle  plasma,  comparable  to  the  clotting  of  blood 
plasma,  but  differing  from  it  inasmuch  as  the  product  is  not  fibrin 
but  myosin.  The  rigidity,  the  loss  of  suppleness,  and  the  dimin- 
ished translucency  appear  to  be  at  all  events  largely,  though 
probably  not  wholly,  due  to  the  change  from  the  fluid  plasma  to  the 
solid  myosin.  We  might  compare  a  living  muscle  to  a  number  of 
fine  transparent  membranous  tubes  containing  blood  plasma.  When 
this  blood  plasma  entered  into  the  'jelly'  stage  of  clotting,  the 
system  of  tubes  would  present  many  of  the  phenomena  of  rigor 
mortis.  They  would  lose  much  of  their  suppleness  and  translucency, 
and  acquire  a  certain  amount  of  rigidity. 

§  58.  There  is  however  one  very  marked  and  important 
difference  between  the  rigor  mortis  of  muscle  and  the  clotting 
of  blood.  Blood  during  its  clotting  undergoes  a  slight  change 
only  in  its  reaction ;  but  muscle  during  the  onset  of  rigor  mortis 
becomes  distinctly  acid. 

A  living  muscle  at  rest  is  in  reaction  neutral,  or,  possibly  from 
some  remains  of  lymph  adhering  to  it,  faintly  alkaline.  If  on  the 
other  hand  the  reaction  of  a  thoroughly  rigid  muscle  be  tested,  it 
will  be  found  to  be  most  distinctly  acid.  This  development  of  an 
acid  reaction  is  witnessed  not  only  in  the  solid  untouched  fibre  but 
also  in  expressed  muscle  plasma ;  it  seems  to  be  associated  in  some 
way  with  the  appearance  of  the  myosin. 

The  exact  causation  of  this  acid  reaction  has  not  at  present 
been  clearly  worked  out.  Since  the  coloration  of  the  litmus  pro- 
duced is  permanent,  carbonic  acid,  which  as  we  shall  immediately 
state,  is  set  free  at  the  same  time,  cannot  be  regarded  as  the  active 
acid,  for  the  reddening  of  litmus  produced  by  carbonic  acid  speedily 
disappears  on  exposure.  On  the  other  hand,  it  is  possible  to  ex- 
tract from  rigid  muscle  a  certain  quantity  of  lactic  acid,  or  rather 
of  a  variety  of  lactic  acid  known  as  sarcolactic  acid  ^ ;  and  we  may 
probably  regard  the  acid  reaction  of  rigid  muscle  as  due  to  a  new 
formation  or  to  an  increased  formation  of  this  sarcolactic  acid. 
There  is  reason  however  to  think  that  the  establishment  of  ttie 

1  There  are  many  varieties  of  lactic  acid,  which  are  isomeric,  having  the  same 
composition  CsHgOg,  but  differ  in  their  reactions  and  especially  in  the  solubility  of 
their  zinc  salts.     The  variety  present  in  muscle  is  distinguished  as  sarcolactic  acid. 


Chap,  ir.]  THE   CONTRACTILE   TISSUES.  91 

acid  reaction  is  not  a  perfectly  simple  process  but  a  more  or  less 
complex  one,  other  substances  besides  sarcolactic  acid  intervening. 

Coincident  with  the  appearance  of  this  acid  reaction,  though 
as  we  have  said,  not  the  direct  cause  of  it,  a  large  development  of 
carbonic  acid  takes  place  when  muscle  becomes  rigid.  Irritable 
living  muscular  substance  like  all  living  substance  is  continually 
respiring,  that  is  to  say,  is  continually  consuming  oxygen  and 
giving  out  carbonic  acid.  In  the  body,  the  arterial  blood  going  to 
the  muscle  gives  up  some  of  its  oxygen,  and  gains  a  quantity  of 
carbonic  acid,  thus  becoming  venous  as  it  passes  through  the 
muscle  capillaries.  Even  after  removal  from  the  body,  the  living 
muscle  continues  to  take  up  from  the  surrounding  atmosphere  a 
certain  quantity  of  oxygen  and  to  give  out  a  certain  quantity  of 
carbonic  acid. 

At  the  onset  of  rigor  mortis  there  is  a  very  large  and  sudden 
increase  in  this  production  of  carbonic  acid,  in  fact  an  outburst  as  it 
were  of  that  gas.  This  is  a  phenomenon  deserving  special  attention. 
Knowing  that  the  carbonic  acid  which  is  the  outcome  of  the 
respiration  of  the  whole  body  is  the  result  of  the  oxidation  of  car- 
bon-holding substances,  we  might  very  naturally  suppose  that  the 
increased  production  of  carbonic  acid  attendant  on  the  development 
of  rigor  mortis  is  due  to  the  fact  that  during  that  event  a  certain 
quantity  of  the  carbon-holding  constituents  of  the  muscle  are 
suddenly  Oxidized.  But  such  a  view  is  negatived  by  the  following 
facts.  In  the  first  place,  the  increased  production  of  carbonic  acid 
during  rigor  mortis  is  not  accompanied  by  a  corresponding  in- 
crease in  the  consumption  of  oxygen.  In  the  second  place,  a 
muscle  (of  a  frog  for  instance)  contains  in  itself  no  free  or  loosely 
attached  oxygen ;  when  subjected  to  the  action  of  a  mercurial  air- 
pump  it  gives  off  no  oxygen  to  a  vacuum,  offering  in  this  respect 
a  marked  contrast  to  blood  ;  and  yet,  when  placed  in  an  atmosphere 
free  from  oxygen,  it  will  not  only  continue  to  give  off  carbonic 
acid  while  it  remains  alive,  but  will  also  exhibit  at  the  onset  of 
rigor  mortis  the  same  increased  production  of  carbonic  acid  that 
is  shewn  by  a  muscle  placed  in  an  atmosphere  containing  ox}'gen. 
It  is  obvious  that  in  such  a  case  the  carbonic  acid  does  not  arise 
from  the  direct  oxidation  of  the  muscle  substance,  for  there  is  no 
oxygen  present  at  the  time  to  carry  on  that  oxidation.  We  are 
driven  to  suppose  that  during  rigor  mortis,  some  complex  body, 
containing  in  itself  ready  formed  carbonic  acid  so  to  speak,  is  split 
up,  and  thus  carbonic  acid  is  set  free,  the  process  of  oxidation  by 
which  that  carbonic  acid  was  formed  out  of  the  carbon-holding  con- 
stituents of  the  muscle  having  taken  place  at  some  anterior  date. 

Living  resting  muscle,  then,  is  alkaline  or  neutral  in  reaction, 
and  the  substance  of  its  fibres  contains  a  plasma  capable  of  clotting. 
Dead  rigid  muscle  on  the  other  hand  is  acid  in  reaction,  and  no 
longer  contains  a  plasma  capable  of  clottmg,  but  is  laden  with  the 


92  RIGOR  MORTIS.  [Book  i. 

solid  myosin.  Further,  the  change  from  the  living  irritable  con- 
dition to  that  of  rigor  mortis  is  accompanied  by  a  large  and  sudden 
development  of  carbonic  acid. 

It  is  found  moreover  that  there  is  a  certain  amount  of  parallel- 
ism between  the  intensity  of  the  rigor  mortis,  the  degree  of  acid 
reaction  and  the  quantity  of  carbonic  acid  given  out.  If  we 
suppose,  as  we  fairly  may  do,  that  the  intensity  of  the  rigidity  is 
dependent  on  the  quantity  of  myosin  deposited  in  the  fibres,  and 
the  acid  reaction  to  the  development  if  not  of  lactic  acid,  at  least 
of  some  other  substance,  the  parallelism  between  the  three  products, 
myosin,  acid-producing  substance,  and  carbonic  acid,  would  suggest 
the  idea  that  all  three  are  the  results  of  the  splitting-up  of  the 
same  highly  complex  substance.  No  one  has  at  present  however 
succeeded  in  isolating  or  in  otherwise  definitely  proving  the  exist- 
ence of  such  a  body,  and  though  the  idea  seems  tempting,  it  may 
in  the  end  prove  erroneous. 

§  59.  As  to  the  other  proteids  of  muscle,  such  as  the  albumin 
and  the  globulin,  we  know  as  yet  nothing  definite  concerning  the 
parts  which  they  play  and  the  changes  which  they  undergo  in  the 
living  muscle  or  in  rigor  mortis. 

Besides  the  fat  which  is  found,  and  that  not  unfrequently  in 
abundance,  in  the  connective  tissue  between  the  fibres,  there  is 
also  present  in  the  muscular  substance  within  the  sarcolemma, 
always  some,  and  at  times  a  great  deal,  of  fat,  chiefiy  ordinary  fat, 
viz.  stearin,  palmitin,  and  olein  in  variable  proportion,  but  with  a 
small  quantity  of  the  more  complex  fat  lecithin  ;  the  latter  probably 
is  derived  from  the  nerve  fibres.  As  to  the  function  of  these  several 
tats  in  the  life  of  the  muscle  we  know  little  or  nothing. 

Carhohydrcttes,  the  third  of  the  three  great  classes  in  which  we 
may  group  the  energy-holding  substances  of  which  the  animal 
body  and  its  food  are  alike  composed,  viz.  proteids,  fat  and  carbo- 
hydrates, are  represented  in  muscle  by  a  peculiar  body,  glycogen, 
which  we  shall  have  to  study  in  detail  later  on.  We  must  here 
merely  say  that  glycogen  is  a  body  closely  allied  to  starch,  having 
a  formula,  which  may  be  included  under  the  general  formula  for 
starches  n  (CgHioOs),  and  may  like  it  be  converted  by  the  action  of 
acids,  or  by  the  action  of  particular  ferments  known  as  amylolytic 
ferments,  into  some  form  of  sugar,  dextrose  (CeHiaOe)  or  some 
allied  sugar.  Many,  if  not  all,  living  muscles  contain  a  certain 
amount,  and  some,  under  certain  circumstances,  a  considerable 
amount  of  glycogen.  During  or  after  rigor  mortis  this  glycogen  is 
very  apt  to  be  converted  into  dextrose,  or  an  allied  sugar.  The 
muscles  of  the  emliryo  at  an  early  stage  contain  a  relatively 
enormous  quantity  of  glycogen,  a  fact  which  suggests  that  the 
glycogen  of  muscle  is  carbohydrate  food  of  the  muscle  about  to  be 
wrought  up  into  the  living  muscular  substance. 

The  bodies  which  we  have  called  extractives  are  numerous  and 


Chap,  ii.]  THE  CONTRACTILE   TISSUES.  93 

varied.  They  are  especially  interesting  since  it  seems  probable 
that  they  are  waste  products  of  the  metabolism  of  the  muscular 
substance,  and  the  study  of  them  may  be  expected  to  throw  light 
on  the  chemical  change  which  muscular  substance  undergoes  during 
life.  Since,  as  we  shall  see,  muscular  substance  forms  by  far  the 
greater  part  of  the  nitrogenous  —  that  is,  proteid  —  portion  of  the 
body,  the  nitrogenous  extractives  of  muscle  demand  peculiar  atten- 
tion. Now,  the  body  urea,  which  we  shall  have  to  study  in  detail 
later  on,  far  exceeds  in  importance  all  the  other  nitrogenous  extrac- 
tives of  the  body  as  a  whole,  since  it  is  practically  the  one  forin  in 
which  nitrogenous  waste  leaves  the  body;  if  we  include  with  urea, 
the  closely  allied  uric  acid  (which  for  present  purposes  may  simply 
be  regarded  as  a  variety  of  urea),  we  may  say  broadly  that  all  the 
nitrogen  taken  in  as  food  sooner  or  later  leaves  the  body  as  urea ; 
compared  with  this  all  other  nitrogenous  waste  thrown  out  from 
the  body  is  insignificant.  Of  the  urea  which  thus  leaves  the  body, 
a  considerable  portion  must  at  some  time  or  other  have  existed,  or, 
to  speak  more  exactly,  its  nitrogen  must  have  existed  as  the  nitrogen 
of  the  proteids  of  muscular  substance.  Nevertheless,  no  urea  at  all 
or  an  absolutely  minimal  quantity  only  is,  in  normal  conditions, 
present  in  muscular  substance  either  living  and  irritable,  or  dead 
and  rigid ;  urea  does  not  arise  in  muscular  substance  itself  as  one 
of  the  immediate  waste  products  of  muscular  substance. 

There  is,  however,  always  present,  in  relatively  considerable 
amount,  on  an  average  about  "25  p.c.  of  wet  muscle,  a  remarkable 
body,  kreatin.  This  is  in  one  sense  a  compound  of  urea :  it  may 
be  split  up  into  urea  and  sarcosin.  This  latter  body  is  a  methyl 
glycin,  that  is  to  say,  a  glycin  in  which  methyl  has  been  sub- 
stituted for  hydrogen,  and  glycin  itself  is  amido-acetic  acid,  a 
compound  of  amidogen,  that  is  a  representative  of  ammonia,  and 
acetic  acid.  Hence  kreatin  contains  urea,  which  has  close  relations 
with  ammonia,  together  with  another  representative  of  ammonia, 
and  a  surplus  of  carbon  and  hydrogen  arranged  as  a  body  belonging 
to  the  fatty  acid  series.  We  shall  have  to  return  to  this  kreatin, 
and  to  consider  its  relations  to  urea  and  to  muscle,  when  we  come 
to  deal  with  urine. 

The  other  nitrogenous  extractives,  such  as  karnin,  hypoxanthin 
(or  sarkin),  xanthin,  taurin,  &c.,  occur  in  small  quantity,  and  need 
not  be  dwelt  on  here. 

Among  non-nitrogenous  extractives,  the  most  important  is  the 
sarcolactic  acid,  of  which  we  have  already  spoken ;  to  this  may 
be  added  sugar  in  some  form  or  other,  either  coming  from  glycogen 
or  from  some  other  source. 

The  ash  of  muscle,  like  the  ash  of  the  blood  corpuscles,  and, 
indeed,  the  ash  of  the  tissues  in  general,  as  distinguished  from  the 
blood,  or  plasma,  or  lymph  on  which  the  tissues  live,  is  character- 
ised by  the  preponderance  of  potassium  salts  and  of  phosphates ; 
these  form  in  fact  nearly  80  p.c.  of  the  whole  ash. 


94  CHEMICAL   CHANGES.  [Book  i. 

§  60.  We  may  now  pass  on  to  the  question,  What  are  the 
chemical  changes  which  take  place  when  a  living,  resting  muscle 
enters  into  a  contraction  ?  These  changes  are  most  evident  after 
the  muscle  has  been  subjected  to  a  prolonged  tetanus  ;  but  there 
can  be  no  doubt  that  the  chemical  events  of  a  tetanus  are,  like 
the  physical  events,  simply  the  sum  of  the  results  of  the  consti- 
tuent single  contractions. 

In  the  first  place,  the  muscle  becomes  acid,  not  so  acid  as  in 
rigor  mortis,  but  still  sufficiently  so,  after  a  vigorous  tetanus,  to 
turn  blue  litmus  distinctly  red.  The  cause  of  the  acid  reaction, 
like  that  of  rigor  mortis,  is  not  quite  clear,  but  is  in  all  probability 
the  same  in  both  cases. 

In  the  second  place,  a  considerable  quantity  of  carbonic  acid  is 
set  free ;  and  the  production  of  carbonic  acid  in  muscular  contrac- 
tion resembles  the  production  of  carbonic  acid  during  rigor  mortis 
in  that  it  is  not  accompanied  by  a  corresponding  increase  in 
the  consumption  of  oxygen.  This  is  evident  even  in  a  muscle 
through  which  the  circulation  of  blood  is  still  going  on  ;  for  though 
the  blood  passing  through  a  contracting  muscle  gives  up  more 
oxygen  than  the  blood  passing  through  a  resting  muscle,  the  increase 
in  the  amount  of  oxygen  taken  up  falls  below  the  increase  in  the 
carbonic  acid  given  out.  But  it  is  still  more  markedly  shewn  in  a 
muscle  removed  from  the  body ;  for  in  such  a  muscle  both  the 
contraction  and  the  increase  in  the  production  of  carbonic  acid  will 
go  on  in  the  absence  of  oxygen.  A  frog's  muscle,  suspended  in  an 
atmosphere  of  nitrogen,  will  remain  irritable  for  some  considerable 
time,  and  at  each  vigorous  tetanus  an  increase  in  the  production 
of  carbonic  acid  may  be  readily  ascertained. 

Moreover,  there  seems  to  be  a  correspondence  between  the 
energy  of  the  contraction  and  the  amount  of  carbonic  acid  and 
the  degree  of  acid  reaction  produced,  so  that,  though  we  are  now 
treading  on  somewhat  uncertain  ground,  we  are  naturally  led  to  the 
view  that  the  essential  chemical  process,  lying  at  the  bottom  of  a 
muscular  contraction  as  of  rigor  mortis,  is  the  splitting-up  of  some 
highly  complex  substance.  But  here  the  resemblance  between  rigor 
mortis  and  contraction  ends.  We  have  no  satisfactory  evidence  of 
the  formation  during  a  contraction  of  any  body  like  myosin.  And 
this  difference  in  chemical  results  tallies  with  an  important  physical 
difference  between  rigid  muscle  and  contracting  muscle.  The 
rigid  muscle,  as  we  have  seen,  becomes  less  extensible,  less  elastic, 
less  translucent ;  the  contracting  muscle  remains  no  less  trans- 
lucent, elastic,  and  extensible  than  the  resting  muscle,  —  indeed, 
there  are  reasons  for  thinking  that  the  muscle  in  contracting 
becomes  actually  more  extensible  for  the  time  being. 

But  if,  during  a  contraction,  myosin  is  not  formed,  what  changes 
of  proteid  or  nitrogenous  matter  do  take  place  ?  We  do  not  know. 
We  have  no  evidence  that  kreatin,  or  any  other  nitrogenous 
extractive,  is  increased  by  the  contraction  of  muscle  ;  we  have  no 


Chap.  h.J  THE   CONTRACTILE   TISSUES.  95- 

satisfactory  evidence  of  any  nitrogen  waste  at  all  as  the  result  of  a 
contraction  ;  and,  indeed,  as  we  shall  see  later  on,  the  study  of  the 
waste  products  of  the  body  as  a  whole  leads  us  to  believe  that  the: 
energy  of  the  work  done  by  the  muscles  of  the  body  comes  from 
the  potential  energy  of  carbon  compounds,  and  not  of  nitrogen 
compounds  at  all.     But  to  this  point  we  shall  have  to  return. 

§  61.  We  may  sum  up  the  chemistry  of  muscle  somewhat  as 
follows :  — 

During  life  the  muscular  substance  is  continually  taking  up 
from  the  blood,  that  is  from  the  lymph,  proteid,  fatty  and  carbo- 
hydrate material,  saline  matters  and  oxygen  ;  these  it  builds  up 
into  itself,  how,  we  do  not  know,  and  so  forms  the  peculiar  complex 
living  muscular  substance.  The  exact  nature  of  this  living  sub- 
stance is  unknown  to  us.  What  we  do  know  is  that  it  is  largely 
composed  of  proteid  material,  and  that  such  bodies  as  myosinogen, 
myoglobulin,  and  albumin,  being  always  present  in  it,  have 
probably  something  to  do  with  the  building  of  it  up. 

During  rest  this  muscular  substance,  while  taking  in  and  build- 
ing itself  up  out  of,  or  by  means  of,  the  above-mentioned  materials, 
is  continually  giving  off  carbonic  acid,  and  continually  forming 
nitrogenous  waste,  such  as  kreatin.  It  also  probably  gives  off  some 
amount  of  sarcolactic  acid,  and  possibly  other  non-nitrogenous 
waste  matters. 

During  a  contraction  there  is  a  great  increase  in  the  amount 
of  carbonic  acid  given  off,  an  increased  formation  of  lactic  acid, 
and  possibly  other  changes  giving  rise  to  an  acid  reaction,  a  greater 
consumption  of  oxygen,  though  the  increase  is  not  equal  to  the 
increase  of  carbonic  acid,  but,  as  far  as  we  can  learn,  no  increase 
of  nitrogenous  waste. 

During  rigor  mortis,  there  is  a  similar  increased  production  of 
carbonic  acid  and  of  some  other  acid-producing  substance,  ac- 
companied by  a  remarkable  conversion  of  myosinogen  into  myosin,, 
by  which  the  rigidity  of  the  dead  fibre  is  brought  about. 


Tlicrmal    Changes. 

§  62.  The  chemical  changes  during  a  contraction  set  free  a 
quantity  of  energy,  but  only  a  portion  of  this  energy  appears  in 
the  '  work  done  ; '  a  considerable  portion  takes  on  the  form  of  heat. 
Though  we  shall  have  hereafter  to  treat  this  subject  more  fully, 
the  leading  facts  may  be  given  here. 

Whenever  a  muscle  contracts,  its  temperature  rises,  indicating 
that  heat  is  given  out.  When  a  mercury  thermometer  is  plunged 
into  a  mass  of  muscles,  such  as  tliose  of  the  thigh  of  the  dog,  a  rise 
of  the  mercury  is  observed  upon  the  muscles  being  tlirown  into  a 
prolonged  contraction.  More  exact  results  however  are  obtained 
by  means  of  a  thermopile,  by  the  help  of  which  the  rise  of  tempera- 


96  THERMAL   CHANGES.  [Book  i. 

ture  caused  by  a  few  repeated  single  contractions,  or,  indeed,  by  a 
single  contraction,  may  be  observed,  and  the  amount  of  heat  given 
out  approximatively  measured. 

The  thermopile  may  consist  either  of  a  single  junction,  in  the  form  uf 
a  needle  plunged  into  the  substance  of  the  muscle  ;  or  of  several  junctions 
either  in  the  shape  of  a  flat  surface  carefully  opposed  to  the  surface  of 
muscle  (the  pile  being  balanced  so  as  to  move  with  the  contracting 
muscle,  and  thus  to  keep  the  contact  exact),  or  in  the  shape  of  a  thin 
wedge,  the  edge  of  which,  comprising  the  actual  junctions,  is  thrust  into 
a  mass  of  muscles  and  held  in  position  by  them.  In  all  cases  the  fellow 
junction  or  junctions  must  be  kept  at  a  constant  temperature. 

Another  delicate  method  of  determining  the  changes  of  temperature 
of  a  tissue  is  based  upon  the  measurement  of  alterations  in  electric 
resistance  which  a  fine  wire,  in  contact  with  or  plunged  into  the  tissue, 
undergoes  as  the  temperature  of  the  tissue  changes. 

It  has  been  calculated  that  the  heat  given  out  by  the  muscles  of 
the  thigh  of  a  frog  in  a  single  contraction  amounts  to  3T  micro-units 
of  heat  ^  for  each  gramme  of  muscle,  the  result  being  obtained  by 
dividing  by  five  the  total  amount  of  heat  given  out  in  five  succes- 
sive single  contractions.  It  will,  however,  be  safer  to  regard  these 
figures  as  illustrative  of  the  fact  that  the  heat  given  out  is  consider- 
able rather  than  as  data  for  elaborate  calculations.  Moreover,  we 
have  no  satisfactory  quantitative  determinations  of  the  heat  given 
out  by  the  muscles  of  warm  blooded  animals,  though  there  can  be 
no  doubt  that  it  is  much  greater  than  that  given  out  by  the  muscles 
of  the  frog. 

There  can  hardly  be  any  doubt  that  the  heat  thus  set  free  is 
the  product  of  chemical  changes  within  the  muscle,  changes,  which, 
though  they  cannot,  for  the  reasons  given  above  (§  60),  be  regarded 
as  simple  and  direct  oxidations,  yet,  since  they  are  processes 
dependent  on  the  antecedent  entrance  of  oxygen  into  the  muscle, 
may  be  spoken  of  in  general  terms  as  a  combustion.  So  that  the 
muscle  may  be  likened  to  a  steam-engine,  in  which  the  combus- 
tion of  a  certain  amount  of  material  gives  rise  to  the  development 
of  energy  in  two  forms,  as  heat  and  as  movement,  there  being 
certain  quantitative  relations  between  the  amount  of  energy  set 
free  as  heat  and  that  giving  rise  to  movement.  We  must,  however, 
carefully  guard  ourselves  against  pressing  this  analogy  too  closely. 
In  the  steam-engine,  we  can  distinguish  clearly  between  the  fuel 
which,  through  its  combustion,  is  the  sole  source  of  energy,  and  the 
machinery,  which  is  not  consumed  to  provide  energy,  and  only 
suffers  wear  and  tear.  In  the  muscle  we  cannot  with  certainty  at 
present  make  such  a  distinction.  It  may  be  that  the  chemical 
clianges  at  the  bottom  of  a  contraction  do  not  involve  the  real 
living  material  of  the  fibre,  but  only  some  substance,  manufactured 
by  the  living  material  and  lodged  in  some  way,  we  do  not  know 

^  The  micro-unit  being  a  milligramme  of  water  raised  one  degree  centigrade. 


Chap,  h.]  THE    CONTRACTILE   TISSUES.  97 

how,  in  the  living  material ;  it  may  be  that  when  a  fibre  contracts 
it  is  this  substance  within  the  fibre  which  explodes,  and  not  the  fibre 
itself..  If  we  further  suppose  that  this  substance  is  some  complex 
compound  of  carbon  and  hydrogen,  into  which  no  nitrogen  enters,  we 
shall  have  an  explanation  of  the  difficulty  referred  to  above  (§  60), 
namely,  that  nitrogenous  waste  is  not  increased  by  a  contraction. 
The  special  contractile,  carbon-hydrogen  substance  may  then  be 
compared  to  tlie  charge  of  a  gun,  the  products  of  its  explosion 
being  carbonic  and  sarcolactic  acids,  while  tlie  real,  living  material 
of  the  fibre  may  be  compared  to  the  gun  itself  ;  but  to  a  gun  which 
itself  is  continually  undergoing  change,  far  beyond  mere  wear  and 
tear,  among  the  products  of  which  change  nitrogenous  bodies  like 
kreatin  are  conspicuous.  This  view  will  certainly  explain  why 
kreatin  is  not  increased  during  the  contraction  while  the  carbonic 
and  lactic  acids  are.  But  it  must  be  remembered  that  such  a  view 
is  not  yet  proved ;  it  may  be  the  living  material  of  the  fibre  as  a 
whole  which  is  continually  breaking  down  in  an  explosive  decom- 
position, and  as  continually  building  itself  up  again  out  of  the 
material  supplied  by  the  blood. 

In  a  steam-engine  only  a  certain  amount  of  the  total  potential 
energy  of  the  fuel  issues  as  work,  the  rest  being  lost  as  heat,  the 
proportion  varying,  but  the  work  rarely,  if  ever,  exceeding  one- 
tenth  of  the  total  energy,  and  generally  being  less.  In  the  case  of 
the  muscle  we  are  not  at  present  in  a  position  to  draw  up  an  exact 
equation  between  the  latent  energy  on  the  one  hand  and  the  two 
forms  of  actual  energy  on  the  other.  We  have  reason  to  think 
that  the  proportion  between  heat  and  work  varies  considerably 
under  different  circumstances,  the  work  sometimes  rising  as  high 
as  one-fifth,  or,  according  to  some,  as  high  even  as  one-half,  some- 
times possibly  sinking  as  low  as  one  twenty-fourth  of  the  total 
energy  ;  and  observations  seem  to  shew*  that  the  greater  the  re- 
sistance which  the  muscle  has  to  overcome,  the  larger  the  proportion 
of  the  total  energy  expended,  which  goes  out  as  work  done.  The 
muscle,  in  fact,  seems  to  be  so  far  self-regulating,  that  the  more 
work  it  has  to  do,  the  greater,  within  certain  limits,  is  the  economy 
with  which  it  works. 

Lastly,  it  must  be  remembered  that  the  giving  out  of  heat  by 
the  muscle  is  not  confined  to  the  occasions  when  it  is  actually  con- 
tracting. When,  at  a  later  period,  we  treat  of  the  heat  of  the  body 
generally,  evidence  will  be  brought  forward  that  the  muscles,  even 
when  at  rest,  are  giving  rise  to  heat,  so  that  the  heat  given  out  at 
a  contraction  is  not  some  wholly  new  phenomenon,  but  a  temporary 
exaggeration  of  what  is  continually  going  on  at  a  more  feeble 
rate. 

Electrical  Changes. 

§  63.  Besides  chemical  and  thermal  changes  a  remarkable 
electric  change  takes  place  whenever  a  muscle  contracts. 


98 


THERMAL   CHANGES. 


[Book  i. 


Muscle-currents.  If  a  muscle  be  removed  in  an  ordinary 
manner  from  the  body,  and  two  non-polarisable  electrodes,^  con- 
nected with  a  delicate  galvanometer  of  many  convolutions  and 


Fig.  18.     Non-polarisable  Electrodes. 

a,  the  glass  tube ;  s,  the  amalgamated  zinc  slips  connected  with  their  respective 
wires;  z.  s.,  the  zinc  sulphate  solution,  ch.  c,  the  plug  of  china  clay;  c,  the  portion 
of  the  china-clay  plug  projecting  from  the  end  of  the  tube  tliis  can  be  moulded  into 
any  required  form. 

high  resistance,  be  placed  on  two  points  of  the  surface  of  the 
muscle,  a  deflection  of  the  galvanometer  will  take  place,  indicating 
the  existence  of  a  current  passing  through  the  galvanometer  from 
the  one  point  of  the  muscle  to  the  other,  the  direction  and 
amount  of  the  deflection  varying  according  to  the  position  of  the 
points.  The  '  muscle-currents  '  thus  revealed  are  seen  to  the  best 
advantage  when  the  muscle  chosen  is  a  cylindrical  or  prismatic 
one  with  parallel  fibres,  and  when  the  two  tendinous  ends  are  cut 
off  by  clean  incisions  at  right  angles  to  the  long  axis  of  the  muscle. 
The  muscle  then  presents  a  transverse  section  (artificial)  at  each 
end,  and  a  longitudinal  surface.  We  may  speak  of  the  latter  as 
being  divided  into  two  equal  parts  by  an  imaginary  transverse  line 
on  its  surface  called  the  '  equator,'  containing  all  the  points  of  the 
surface  midway  between  the  two  ends.  Fig.  19  is  a  diagrammatic 
representation  of  such  a  muscle,  the  line  ab  being  the  equator.  In 
such  a  muscle  the  development  of  the  muscle-currents  is  found  to 
be  as  follows. 


'  These  (Fig.  18)  consist  essentially  of  a  slip  of  thoroughly  amalgamated  zinc 
dipping  into  a  saturated  solution  of  zinc  .sulphate,  which,  in  turn,  is  brought  into 
connection  with  the  nerve  or  muscle  by  means  of  a  plug  or  bridge  of  cliiiia-clay, 
moistened  with  normal  sodium  chloride  solution  ,  it  is  important  that  the  zinc  should 
lie  thoroughly  amalgamated.  This  form  of  electrodes  gives  rise  to  less  polari.sation 
than  do  simple  platinum  or  copper  electrodes.  The  clay  affords  a  connection  be- 
tween the  zinc  and  the  tissue  which  neither  acts  on  the  tissue  nor  is  acted  on  by  the 
tissue.  Contact  of  any  tissue  with  copper  or  platinum  is  in  itself  sufiBcient  to 
develope  a  current. 


Chap,  ii.] 


THE   CONTRACTILE   TISSUES. 


99 


The  greatest  deflection  is  observed  when  one  electrode  is  placed 
at  the  mid-point  or  equator  of  the  muscle,  and  the  other  at  either 
cut  end ;  and  the  deflection  is  of  such  a  kind  as  to  shew  that  posi- 
tive currents  are  continually  passing  from  the  equator  through  the 
galvanometer  to  the  cut  end  :  that  is  to  say,  the  cut  end  is  negative 
relatively  to  the  equator.  The  currents  outside  the  muscle  may  be 
considered  as  completed  by  currents  in  the  muscle  from  the  cut  end 
to  the  equator.     In  the  diagram  Fig.  19,  the  arrows  indicate  the 


/ 


Fig.  19.  Diagram  illustrating  the  Electric  Ccrrents  of  Nerve  and  Muscle. 

Being  purely  diagrammatic,  it  may  serve  for  a  piece  either  of  nerve  or  of  muscle, 
except  that  the  currents  at  the  transverse  section  cannot  be  shewn  in  a  nerve.  The 
arrows  shew  the  direction  of  the  current  through  the  galvanometer. 

ab  the  e(|uator.  The  strongest  currents  are  those  shewn  by  the  dark  lines,  as 
from  «,  at  equator,  to  x  or  to  (/  at  the  cut  ends.  The  current  from  «  to  c  is  weaker 
than  from  a  to  y,  though  both,  as  shewn  by  the  arrows,  have  the  same  direction.  A 
current  is  shewn  from  e,  which  is  near  the  equator,  to  f,  which  is  farther  from  tlie 
equator.  The  current  (in  muscle)  from  a  point  in  the  circumference  to  a  point 
nearer  tlie  centre  of  the  transverse  section  is  shewn  at  gh.  From  a  to  i  or  from 
X  to  ?/  there  is  no  current,  as  indicated  by  the  dotted  lines. 

direction  of  the  currents.  If  the  one  electrode  be  placed  at  the 
equator  ah,  the  effect  is  the  same  at  whichever  of  the  two  cut  ends  x 
or  y  the  other  is  placed.  If,  one  electrode  remaining  at  the  equator, 
the  other  be  shifted  from  the  cut  end  to  a  spot  c  nearer  to  the 
equator,  the  current  continues  to  have  the  same  direction,  but  is  of 
less  intensity  in  proportion  to  the  nearness  of  the  electrodes  to  each 
other.  If  the  two  electrodes  be  placed  at  unequal  distances  e  and  /, 
one  on  either  side  of  the  equator,  there  will  be  a  feeble  current  from 
the  one  nearer  the  equator  to  the  one  farther  off,  and  the  current 
will  be  the  feebler,  the  more  nearly  they  are  equidistant  from  the 
equator.  If  they  are  quite  equidistant,  as,  for  instance,  when  one  is 
placed  on  one  cut  end  r,  and  the  other  on  the  other  cut  end  y,  there 
will  be  no  current  at  all. 

If  one  electrode  be  placed  at  the  circumference  of  the  transverse 
section  and  the  other  at  the  centre  of  the  transverse  section,  there 


100  MUSCLE   CURRENTS.  [Book  i. 

will  be  a  current  through  the  galvanometer  from  the  former  to 
the  latter;  there  will  be  a  current  of  similar  direction  but  of  less 
intensity  when  one  electrode  is  at  the  circumference  g  of  the  trans- 
verse section,  and  the  other  at  some  point  h  nearer  the  centre  of  the 
transverse  section.  In  fact,  the  points  which  are  relatively  most 
positive  and  most  negative  to  each  other  are  points  on  the  equator 
and  the  two  centres  of  the  transverse  sections  ;  and  the  intensity  of 
the  current  between  any  two  points  will  depend  on  the  respective 
distances  of  those  points  from  the  equator  and  from  the  centre  of 
the  transverse  section. 

Similar  currents  may  be  observed  when  the  longitudinal  surface 
is  not  the  natural  but  an  artificial  one ;  indeed  they  may  be  wit- 
nessed in  even  a  piece  of  muscle  provided  it  be  of  cylindrical  shape 
and  composed  of  parallel  fibres. 

These  '  muscle-currents '  are  not  mere  transitory  currents  dis- 
appearing as  soon  as  the  circuit  is  closed  ;  on  the  contrary,  they 
last  a  very  considerable  time.  They  must,  therefore,  be  maintained 
by  some  changes  going  on  in  the  muscle,  by  continued  chemical 
action  in  fact.  They  disappear  as  the  irritability  of  the  muscle 
vanishes,  and  are  connected  with  those  nutritive,  so-called  vital 
changes  which  maintain  the  irritability  of  the  muscle. 

Muscle-currents,  such  as  have  just  been  described,  may,  we  re- 
peat, be  observed  in  any  cylindrical  muscle  suitably  prepared,  and 
similar  currents,  with  variations  which  need  not  be  discussed  here, 
may  be  seen  in  muscles  of  irregular  shape  with  obliquely  or  other- 
wise arranged  fibres.  And  Du  Bois-Eeymond,  to  whom  chiefly  we 
are  indebted  for  our  knowledge  of  these  currents,  has  been  led  to 
regard  them  as  essential  and  important  properties  of  living  muscle. 
He  has  moreover  advanced  the  theory  that  muscle  may  be  con- 
sidered as  composed  of  electro-motive  particles  or  molecules,  each 
of  which,  like  the  muscle  at  large,  has  a  positive  equator  and  nega- 
tive ends,  the  whole  muscle  being  made  up  of  these  molecules  in 
somewhat  the  same  way  (to  use  an  illustration  which  must  not, 
however,  be  strained  or  considered  as  an  exact  one)  as  a  magnet 
may  be  supposed  to  be  made  up  of  magnetic  particles,  each  with 
its  north  and  south  pole. 

There  are  reasons,  however,  for  thinking  that  these  muscle- 
currents  have  no  such  fundamental  origin,  that  they  are  in  fact  of 
surface  and  indeed  of  artificial  origin.  Without  entering  into  the 
controversy  on  this  question,  the  following  important  facts  may  be 
mentioned  :  — 

1.  When  a  muscle  is  examined  while  it  still  retains  uninjured 
its  natural  tendinous  terminations,  the  currents  are  much  weaker 
than  when  artificial  transverse  sections  have  been  made ;  the 
natural  tendinous  end  is  less  negative  than  the  cut  surface.  But 
the  tendinous  end  becomes  at  once  negative  when  it  is  dipped 
in  water  or  acid,  indeed,  when  it  is  in  any  way  injured.  The 
less  roughly,  in  fact,  a  muscle  is  treated  the  less  evident  are  the 


Chap,  ii.]  THE   CONTRACTILE   TISSUES.  101 

muscle-currents ;  and  it  is  maintained  that  if  adequate  care  be 
taken  to  maintain  a  muscle  in  an  absolutely  natural  condition,  no 
such  currents  as  those  we  have  been  describing  exist  at  all,  that 
natural  living  muscle  is  isoelectric,  as  it  is  called. 

2.  The  surface  of  the  uninjured  inactive  '  ventricle  of  the  frog's 
heart,  which  is  practically  a  mass  of  muscle,  is  isoelectric,  no  current 
is  obtained  when  the  electrodes  are  placed  on  any  two  points  of  the 
surface.  If,  however,  any  part  of  the  surface  be  injured,  or  if  the 
ventricle  be  cut  across  so  as  to  expose  a  cut  surface,  the  injured  spot 
or  the  cut  surface  becomes  at  once  powerfully  negative  towards 
the  uninjured  surface,  a  strong  current  being  developed  which  passes 
through  the  galvanometer  from  the  uninjured  surface  to  the  cut 
surface  or  to  the  injured  spot.  The  negativity  thus  developed  in 
a  cut  surface  passes  oil'  in  the  course  of  some  hours,  but  may  be 
restored  by  making  a  fresh  cut  and  exposing  a  fresh  surface. 

The  temporary  duration  of  the  negativity  after  injury,  and  its 
renewal  upon  fresh  injury,  in  the  case  of  the  ventricle,  in  contrast 
to  the  more  permanent  negativity  of  injured  skeletal  muscle,  is 
explained  by  the  difi'erent  structure  of  the  two  kinds  of  muscle. 
The  cardiac  muscle,  as  we  shall  hereafter  see,  is  composed  of  short 
fibre-cells ;  when  a  cut  is  made  a  certain  number  of  these  fibre- 
cells  are  injured,  giving  rise  to  negativity,  but  the  injury  done  to 
them  stops  with  them,  and  is  not  propagated  to  the  cells  with 
which  they  are  in  contact ;  hence,  upon  their  death  the  negativity 
and  the  current  disappear.  A  fresh  cut  involving  new  cells,  pro- 
duces fresh  negativity  and  a  new  current.  In  the  long  fibres  of 
the  skeletal  muscle,  on  the  other  hand,  the  effects  of  the  injury 
are  slowly  propagated  along  the  fibre  from  the  spot  injured. 

Now,  when  a  muscle  is  cut  or  injured,  the  substance  of  the 
fibres  dies  at  the  cut  or  injured  surface.  And  many  physiologists, 
among  whom  the  most  prominent  is  Hermann,  have  been  led,  by 
the  above  and  other  facts,  to  the  conclusion  that  muscle-currents 
do  not  exist  naturally  in  untouched,  uninjured  muscles,  that  the 
muscular  substance  is  naturally,  when  living,  isoelectric,  but  that 
whenever  a  portion  of  the  muscular  substance  dies,  it  becomes 
while  dying  negative  to  the  living  substance,  and  thus  gives  rise 
to  currents.  They  explain  the  typical  currents  (as  they  might  be 
called)  manifested  by  a  muscle  with  a  natural  longitudinal  surface 
and  artificial  transverse  sections,  by  the  fact  that  the  dying  cut 
ends  are  negative  relatively  to  the  rest  of  the  muscle. 

Du  Bois-Reymond  and  those  with  him  offer  special  explanations 
of  the  above  facts  and  of  other  objections  which  have  been  urged 
against  the  tlieory  of  naturally  existing  electro-motive  molecules. 
Into  these  we  cannot  enter  here.  We  must  rest  content  with  the 
statement  that  in  an  ordinary  muscle  currents,  such  as  have  Iteeu 
described,  may  be  witnessed,  but  that  strong  arguments  may  be 

'  The  necessity  of  its  being  inactive  will  be  seen  subsequently. 


102  MUSCLE  CURRENTS.  [Book  i. 

adduced  in  favour  of  the  view  that  these  currents  are  not  '  natural ' 
phenomena,  but  essentially  of  artificial  origin.  It  will  therefore  be 
best  to  speak  of  them  as  currents  of  rest. 

§  64.  Currents  of  action.  Negative  variation  of  the  Muscle- 
current.  The  controversy  whether  the  '  currents  of  rest '  observable 
in  a  muscle  be  of  natural  origin  or  not,  does  not  affect  the  truth 
or  the  importance  of  the  fact  that  an  electrical  change  takes  place, 
and  a  current  is  developed  in  a  muscle  whenever  it  enters  into  a 
contraction.  When  currents  of  rest  are  observable  in  a  muscle, 
these  are  found  to  undergo  a  diminution  upon  the  occurrence  of  a 
contraction,  and  this  diminution  is  spoken  of  as  '  the  negative 
variation '  of  the  currents  of  rest.  The  negative  variation  may  be 
seen  when  a  muscle  is  thrown  into  a  single  contraction,  but  is  most 
readily  shewn  when  the  muscle  is  tetauized.  Thus,  if  a  pair  of 
electrodes  be  placed  on  a  muscle,  one  at  the  equator,  and  the 
other  at  or  near  the  transverse  section,  so  that  a  considerable 
deflection  of  the  galvanometer  needle,  indicating  a  considerable 
current  of  rest,  be  gained,  the  needle  of  the  galvanometer  will, 
when  the  muscle  is  tetanized  by  an  interrupted  current  sent 
through  its  nerve  (at  a  point  too  far  from  the  muscle  to  allow  of 
any  escape  of  the  current  into  the  electrodes  connected  with  the 
galvanometer),  swing  back  towards  zero  ;  it  returns  to  its  original 
deflection  when  the  tetanizing  current  is  shut  off. 

Not  only  may  this  negative  variation  be  shewn  by  the  galvano- 
meter, but  it,  as  well  as  the  current  of  rest,  may  be  used  as  a 
galvanic  shock,  and  so  employed  to  stimulate  a  muscle,  as  in  the 
experiment  known  as  '  the  rheoscopic  frog.'  For  this  purpose  the 
muscles  and  nerves  need  to  be  in  thoroughly  good  condition,  and 
very  irritable.  Two  muscle-nerve  preparations,  A  and  B,  having 
been  made,  and  each  placed  on  a  glass  plate  for  the  sake  of  insula- 
tion, the  nerve  of  the  one,  B,  is  allowed  to  fall  on  the  muscle  of  the 
othei'.  A,  in  such  a  way  that  one  point  of  the  nerve  comes  in 
contact  with  the  equator  of  the  muscle,  and  another  point  with 
one  end  of  the  muscle  or  with  a  point  at  some  distance  from  the 
equator.  At  the  moment  the  nerve  is  let  fall  and  contact  made,  a 
current,  viz.  the  '  current  of  rest '  of  the  muscle  A,  passes  through 
the  nerve ;  this  acts  as  a  stimulus  to  the  nerve,  and  so  causes 
a  contraction  in  the  muscle  connected  with  a  nerve.  Thus  the 
muscle  A  acts  as  a  battery,  the  completion  of  the  circuit  of  which 
by  means  of  the  nerve  of  B  serves  as  a  stimulus,  causing  the 
muscle  B  to  contract. 

If,  while  the  nerve  of  B  is  still  in  contact  with  the  muscle  of  A, 
the  nerve  of  the  latter  is  tetanized  with  an  interrupted  current, 
not  only  is  the  muscle  of  A  thrown  into  tetanus,  but  also  that  of 
B  ;  the  reason  being  as  follows.  At  each  spasm  of  which  the 
tetanus  of  A  is  made  up,  there  is  a  negative  variation  of  the 
muscle  current  of  A.  Each  negative  variation  of  the  muscle 
current  of  A  serves  as  a  stimulus  to  the  nerve  of  B,  and  is  hence 


CUAP.   11.] 


THE   CONTRACTILE   TISSUES. 


103 


the  cause  of  a  spasm  in  the  muscle  of  B;  and  the  stimuli  following 
each  other  rapidly,  as  being  produced  by  the  tetanus  of  A,  they 
must  do,  the  spasms  in  B  to  which  they  give  rise  are  also  fused  into 
a  tetanus  in  B.  B,  in  fact,  contracts  in  harmony  with  A.  This 
experiment  shews  that  the  negative  variation  accompanying  the 
tetanus  of  a  muscle,  though  it  causes  only  a  single  swing  of  the 
galvanometer,  is  really  made  up  of  a  series  of  negative  variations, 
each  single  negative  variation  corresponding  to  the  single  spasms 
of  which  the  tetanus  is  made  up. 

But  an  electrical  change  may  be  manifested  even  in  cases  when 
no  currents  of  rest  exist.  We  have  stated  (§  63)  that  the  surface 
of  the  uninjured  inactive  ventricle  of  the  frog's  heart  is  isoelectric, 
no  currents  being  observed  when  the  electrodes  of  a  galvanometer 
are  placed  on  two  points  of  the  surface.  Xevertheless,  a  most 
distinct  current  is  developed  whenever  the  ventricle  contracts. 
This  may  be  shewn  either  by  the  galvanometer  or  by  the  rheo- 
scopic  frog.  If  the  nerve  of  an  irritable  muscle-nerve  preparation 
be  laid  over  a  pulsating  ventricle,  each  beat  is  responded  to  by  a 
twitch  of  the  muscle  of  the  preparation.  In  the  case  of  ordinary 
muscles,  too,  instances  occur  in  which  it  seems  impossible  to  regard 
the  electrical  change  manifested  during  the  contraction  as  the 
mere  diminution  of  a  preexisting  current. 

Accordingly  those  who  deny  the  existence  of  '  natural '  muscle- 
currents  speak  of  a  muscle  as  developing  during  a  contraction  a 
'  current  of  action,'  occasioned  as  they  believe  by  the  muscular  sub- 
stance as  it  is  entering  into  the  state  of  contraction,  becoming 
negative  towards  the  muscular  substance  which  is  still  at  rest,  or 
has  returned  to  a  state  of  rest.  In  fact,  they  regard  the  negativity 
of  muscular  substance  as  characteristic  alike  of  beginning  death 
and  of  a  beginning  contraction.  So  that  in  a  muscular  contraction 
a  wave  of  negativity,  starting  from  the  end-plate  when  indirect,  or 
from  the  point  stimulated  when  direct  stimulation  is  used,  passes 
along  the  muscular  substance  to  the  ends  or  end  of  the  fibre. 

If,  for  instance,  we  suppose  two  electrodes  placed  on  two  points 

(Fig.  20),  A  and  B,  of  a  fibre  about 
to  be  stimulated  by  a  single  induc- 
tion-shock at  one  end.  ]5efore  the 
stimulation  the  fibre  is  isoelectric, 
and  the  needle  of  the  galvanometer 
stands  at  zero.  At  a  certain  time 
after  the  shock  has  been  sent 
through  the  stimulating  electrodes 
{x),  as  the  wave  of  contraction  is 
travelling  down  the  fibre,  the  sec- 
tion of  the  fibre  beneath  A  will 
become  negative  towards  the  rest 
of  the  fibre,  and  so  negative  towards 
Fig.  20.  the  portion  of  the  fibre  under  B, 


104  MUSCLE   CURRENTS.  [Book  i. 

i.e.  A  will  be  negative  relatively  to  B,  and  this  will  be  shewn  by 
a  deflection  of  the  needle.  A  little  later,  B  will  be  entering  into 
contraction,  and  will  be  becoming  negative  towards  the  rest  of  the 
iibre,  including  the  part  under  A,  whose  negativity  by  this  time 
is  passing  off;  that  is  to  say,  B  will  now  be  negative  towards  A, 
and  this  will  be  shewn  by  a  deflection  of  the  needle  in  a  direction 
opposite  to  that  of  the  deflection  which  has  just  previously  taken 
place.  Hence,  between  two  electrodes  placed  along  a  fibre,  a  single 
wave  of  contraction  will  give  rise  to  two  currents  of  different 
phases,  to  a  diphasic  change ;  and  this,  indeed,  is  found  to  be 
the  case. 

This  being  so,  it  is  obvious  that  the  electrical  result  of  tetanizing 
a  muscle  when  wave  after  wave  follows  along  each  fibre,  is  a  com- 
plex matter ;  but  it  is  maintained  that  the  apparent  negative 
variation  of  tetanus  can  be  explained  as  the  net  result  of  a  series  of 
currents  of  action,  due  to  the  individual  contractions,  the  second 
phase  of  the  current  in  each  contraction  being  less  marked  than 
the  first  phase.  We  cannot,  however,  enter  more  fully  here  into  a 
discussion  of  this  difficult  subject. 

When  we  study,  as  we  may  do  with  the  help  of  appropriate 
apparatus,  the  rapidity  with  which  the  electrical  change  accompany- 
ing a  muscular  contraction  travels,  we  find  it  to  be  the  same  as 
that  of  the  contraction  wave  itself.  The  older  observations  seemed 
to  shew  that  the  electrical  change  fell  entirely  within  the  latent 
period,  and  might,  therefore,  be  regarded  as  an  outward  token  of 
invisible  molecular  processes,  occupying  the  latent  period,  and 
sweeping  along  the  muscular  fibre  ahead  of  and  preparing  for  the 
visible  change  of  form.  And,  indeed,  since  we  are  led  to  regard 
the  change  of  form  as  the  result  of  chemical  processes  taking  place 
in  the  muscular  substance,  we  must  suppose  that  the  change  of 
form  is  preceded  by  molecular  chemical  changes.  But,  as  we  have 
said,  a  latent  period  of  measurable  length  does  not  appear  to  be 
an  essential  feature  of  a  muscular  contraction ;  we  may,  under 
certain  circumstances,  fail  to  detect  a  latent  period.  And  some 
recent  observations  seem  to  shew  that  the  electrical  change  and 
the  change  of  form  may  begin  at  the  same  time.  Indeed,  some 
have  maintained  that  the  former  is  the  result  of  the  latter,  and 
not,  as  suggested  above,  of  the  forerunning  molecular  events.  The 
question  however  is  one  which  cannot  at  present  be  regarded  as 
settled. 

The   Changes  in  a  Nerve  during  the  passage  of  a  Nervous 

Impulse. 

§  65.  The  change  in  the  form  of  a  muscle  during  its  contrac- 
tion is  a  thing  which  can  be  seen  and  felt ;  but  the  changes  in  a 
nerve  during  its  activity  are  invisible  and  impalpable.  We  stimu- 
late one  end  of  a  nerve  going  to  a  muscle,  and  we  see  this  followed 


Chap,  ii.]  THE   CONTRACTILE   TISSUES.  105 

by  a  contraction  of  the  muscle  attached  to  the  other  end ;  or  we 
stimulate  a  nerve  still  connecte.l  with  the  central  nervous  system, 
and  we  see  this  followed  by  certain  movements,  or  by  other  tokens 
which  shew  that  disturbances  have  been  set  up  in  the  central 
nervous  system.  We  know  therefore  that  some  changes  or  other, 
constituting  what  we  have  called  a  nervous  impulse,  have  been 
propagated  along  the  nerve ;  but  the  changes  are  such  as  we 
cannot  see.  It  is  possible,  however,  to  learn  something  about 
them. 

§  66.  The  cliemistnj  of  a  nerve.  The  medulla  of  a  medullated 
nerve  fibre  is  usually  spoken  of  as  fatty,  and  yet  is  in  reality  very 
largely  composed  of  a  substancs  which  is  not  (in  the  strict  sense 
of  the  word)  a  fat.  Wlien  we  examine  chemically  a  quantity  of 
nerve  (or  what  is  practically  the  same  thing  a  quantity  of  that 
part  of  the  central  nervous  system  which  is  called  white  matter, 
and  which  is  chieliy  compose  1,  like  a  nsrve,  of  medullated  nerves, 
and  is  to  be  preferred  for  chemical  examination  because  it  contains 
a  relatively  sm  dl  quantity  of  connective  tissue),  we  find  that  a 
very  large  proportion,  according  to  some  observers  about  half,  of 
the  dried  matter  consists  of  the  peculiar  body  cholesterin.  Now 
cholesterin  is  not  a  fat  but  an  alcohol ;  like  glycerine,  however, 
which  is  also  an  alcohol,  it  forms  compound?  with  fatty  acids ; 
and  though  we  do  not  know  definitely  the  chemical  condition 
in  which  cholesterin  exists  during  life  in  the  medulla,  it  is  more 
than  probable  that  it  exists  in  some  combination  with  some  of 
the  really  fatty  bodies  also  present  in  the  medulla,  and  not  in  a 
free  isolated  state.  It  is  singular  that  besides  being  present  in 
such  large  quantities  in  nervous  tissue,  and  to  a  small  extent 
in  other  tissues  and  in  blood,  cholesterin  is  a  normal  constituent 
of  bile,  and  forms  the  greater  part  of  gall  stones  when  these  are 
present;  in  gall  stones  it  is  undoubtedly  present  in  a  free  state. 
Besides  cholesterin  '  white '  nervous  matter  contains  a  less  but 
still  considerable  quantity  of  a  complex  fat,  wdiose  nature  is 
disputed.  According  to  some  authorities  rather  less  than  half 
this  complex  fat  consists  of  the  peculiar  body  lecithin,  which  we 
have  already  seen  to  be  present  also  in  blood  corpuscles  and  else- 
where. Lecithin  contains  the  radicle  of  stearic  acid  (or  of  oleic, 
or  of  palmitic  acid)  associated  not,  as  in  ordinary  fats,  with  simple 
glycerine,  but  with  the  more  complex  glycerin-phosphoric  acid, 
and  further  combined  with  a  nitrogenous  body,  neurin,  an  am- 
monia compound  of  some  considerable  complexity ;  it  is  therefore 
of  remarkable  nature  since,  though  a  fat,  it  contains  both  nitrogen 
and  phosphorus.  According  to  the  same  authorities  the  remainder 
of  the  complex  fat  consists  of  another  fatty  body,  also  apparently 
containing  nitrogen  but  no  phosphorus,  called  cerehrin.  Other 
authorities  regard  both  these  bodies,  lecithin  and  cerebrin,  as 
products  of  decomposition  of  a  still  more  complex  fat,  called 
protagon.     Obviously  the  fat  of  the  white  matter  of  the  central 


106  THE   CHEMISTRY  OF  NERVES.  [Book  i. 

nervous  system  and  of  spinal  nerves  (of  which  fat  by  far  the 
greater  part  must  exist  in  the  medulla,  and  form  nearly  the  whole 
of  the  medulla)  is  a  very  complex  body  indeed,  especially  so  if  the 
cholesterin  exists  in  combination  with  the  lecithin,  or  cerebrin  (or 
protagon).  Being  so  complex  it  is  naturally  very  unstable,  and  in- 
deed, in  its  instability  resembles  proteid  matter.  Hence  probably 
the  reason  why  the  medulla  changes  so  rapidly  and  so  profoundly 
after  the  death  of  the  nerve.  It  seems  moreover  that  a  certain 
though  small  quantity  of  proteid  matter  forms  part  of  the  medulla, 
and  it  is  possible  that  this  exists  in  some  kind  of  combination  with 
the  complex  fat ;  but  our  knowledge  on  this  point  is  imperfect. 

The  presence  in  such  large  quantity  of  this  complex  fatty 
medulla  renders  the  chemical  examination  of  the  other  consti- 
tuents of  a  nerve  very  difficult,  and  our  knowledge  of  the  chemical 
nature  of,  and  of  the  chemical  changes  going  on  in,  the  axis-cylinder, 
is  as  yet  limited.  Examined  under  the  microscope  the  axis-cylinder 
gives  the  xanthoproteic  reaction  and  other  indications  that  it  is 
largely  proteid  in  nature.  From  nervous  matter,  and  especially 
from  the  grey  matter  of  the  brain  and  spinal  cord,  there  may  by 
appropriate  methods  be  extracted  certain  proteids  similar  to  those 
found  in  leucocytes  and  other  cells  (§  29 j  namely,  a  nucleo-albu- 
min  and  one  or  more  globulins  ;  these  aie  probably  constituents 
both  of  the  nerve  cells  and  of  the  axis-cylinders  which  are  pro- 
cesses of  cells.  Since  kreatin  and  a  lactic  acid  are  present  among 
the  '  extractives  '  of  nervous  tissue,  we  may  infer  that  in  a  broad 
way  the  chemical  changes  in  nerves  are  similar  to  those  in 
muscles.     Beyond  this  we  can  say  very  little. 

After  the  fats  of  the  medulla  (and  the  much  smaller  quantity 
of  fat  present  in  the  axis-cylinder),  the  proteids  of  the  axis-cylinder, 
and  the  other  soluble  substances  present  in  one  or  the  other,  or 
gathered  round  the  nuclei  of  the  neurilemma,  have  by  various 
means  been  dissolved  out  of  a  nerve  fibre  certain  substances  still 
remain.  One  of  these  in  small  quantity  is  the  nuclein  of  the 
nuclei :  another  in  larger  quantity  is  the  substance  nciirol^eratin 
which  forms  a  supporting  framework  for  the  medulla,  and  whose 
most  marked  characteristic  is  perhaps  its  resistance  to  solution. 

In  the  ash  of  nerves  there  is  a  preponderance  of  potassium 
salts  and  phosphates  but  not  so  marked  as  in  the  case  of  muscle. 

§  67.  The  nervous  impulse.  The  chemical  analogy  between 
the  substance  of  the  muscle  and  that  of  the  axis-cylinder  would 
naturally  lead  us  to  suppose  that  the  progress  of  a  nervous  im- 
pulse along  a  nerve  fibre  was  accompanied  by  chemical  changes 
similar  to  those  taking  place  in  a  muscle  fibre.  Whatever  changes 
however  do  or  may  take  place  are  too  slight  to  be  recognized  by 
the  means  at  our  disposal.  We  have  no  satisfactory  evidence 
that  in  a  nerve  even  repeated  nervous  impulses  can  give  rise  to 
an  acid  reaction  or  that  the  death  of  a  nerve  fibre  leads  to  such  a 
reaction.   The  grey  matter  of  the  central  nervous  system  it  is  true 


Chap,  ii.]  THE   CONTRACTILE   TISSUES.  107 

is  said  to  be  faintly  alkaline  during  life  and  to  become  acid  aftc^r 
death  ;  but  in  this  grey  matter  nerve  cells  are  relatively  aljundaut ; 
the  white  matter,  composed  chiefly  of  nerve  fibres,  is  and  remains, 
during  action  as  well  as  rest,  and  even  after  death,  neutral  or 
slightly  alkaline. 

Nor  have  we  satisfactory  evidence  that  the  progress  of  a 
nervous  impulse  is  accompanied  by  any  setting  free  of  energy  in 
the  form  of  heat. 

In  fact,  beyond  the  terminal  results,  such  as  a  muscular  con- 
traction in  the  case  of  a  nerve  going  to  a  muscle,  or  some  affection 
of  the  central  nervous  sy^em  in  the  case  of  a  nerve  still  in  con- 
nection with  its  nervous  centre,  there  is  one  event  and  one  event 
only  which  we  are  able  to  recognize  as  the  objective  token  of  a 
nervous  impulse,  and  that  is  an  electric  change.  For  a  piece  of 
nerve  removed  from  the  body  exhibits  nearly  the  same  electric 
phenomena  as  a  piece  of  muscle  It  has  an  equator  which  is 
electrically  positive  relatively  to  the  two  cut  ends.  In  fact  the  dia- 
gram Fig.  ly,  and  the  description  which  was  given  in  §  63  of  the 
electric  changes  in  muscle  may  be  applied  almost  as  well  to  a 
nerve,  except  that  the  currents  are  in  all  cases  much  more  feeble 
in  the  case  of  nerves  than  of  muscles,  and  the  special  currents 
from  the  circumference  to  the  centre  of  the  transverse  sections 
cannot  well  be  shewn  in  a  slender  nerve ;  indeed  it  is  doubtful  if 
they  exist  at  all. 

During  the  passage  of  a  nervous  impulse  the  '  natural  nerve 
current'  undergoes  a  negative  variation,  just  as  the  'natural 
muscle  current'  undergoes  a  negative  variation  during  a  con- 
traction. There  are  moreover  reasons  in  the  case  of  the  nerve,  as 
in  the  case  of  the  muscle,  which  lead  us  to  doubt  the  pre-exist- 
ence  of  any  such  '  natural '  currents.  A  nerve  in  an  absolutely 
natural  condition  appears  to  be,  like  a  muscle,  isoelectric ;  hence 
we  may  say  that  in  a  nerve  during  the  passage  of  a  nervous 
impulse,  as  in  a  muscle  during  a  muscular  contraction,  a  '  current 
of  action '  is  developed. 

This  'current  of  action'  or  'negative  variation'  may  be  shewn 
either  by  the  galvanometer  or  by  the  rheoscopic  frog.  If  the 
nerve  of  the  'muscle  nerve  preparation'  B  (see  §  64)  be  placed 
in  an  appropriate  manner  on  a  thoroughly  irritable  nerve  vl  (to 
which  of  course  no  muscle  need  be  attached),  touching  for 
instance  the  equator  and  one  end  of  the  nerve,  tlien  single  induc- 
tion-shocks sent  into  the  far  end  of  A  will  cause  single  spasms 
in  the  muscle  of  B,  while  tetanization  of  A,  i.  e.  rapidly  repeated 
shocks  sent  into  A,  will  cause  tetanus  of  the  muscle  of  B. 

That  this  current,  whether  it  be  regarded  as  an  independent 
'  current  of  action '  or  as  a  negative  variation  of  a  '  pre-existing ' 
current,  is  an  essential  feature  of  a  nervous  impulse  is  shewn  by 
the  fact  that  the  degree  or  intensity  of  the  one  varies  with  that 
of  the  other.     Thev  both  travel  too  at  the  same  rate.     In  describ- 


108  ELECTRIC   CURRENTS   IN   NERVES.        [Book  i. 

ing  the  muscle-curve,  and  the  method  of  measuring  the  muscular 
latent  period,  we  have  incidentally  shewn  (§  46)  how  at  the  same 
time  the  velocity  of  the  nervous  impulse  may  be  measured,  and 
stated  that  the  rate  in  the  nerves  of  a  frog  is  about  28  meters  a 
second.  By  means  of  a  special  and  somewhat  complicated 
apparatus  it  is  ascertained  that  the  current  of  action  travels  along 
an  isolated  piece  of  nerve  at  the  same  rate.  It  also,  like  the 
contraction,  travels  in  the  form  of  a  wave,  rising  rapidly  to  a 
maximum  at  each  point  of  the  nerve  and  then  more  gradually 
declining  again.  The  length  of  the  wave  may  by  special  means 
be  measured,  and  is  found  to  be  about  |8  mm. 

When  an  isolated  piece  of  nerve  is  stimulated  in  the  middle, 
the  current  of  action  is  propagated  equally  well  in  both  direc- 
tions, and  that  whether  the  nerve  be  a  chiefly  sensory  or  a  chiefly 
motor  nerve,  or  indeed  if  it  be  a  nerve-root  composed  exclusively 
of  motor  or  of  sensory  fibres.  Taking  the  current  of  action  as 
the  token  of  a  nervous  impulse,  we  infer  from  this  that  when  a 
nerve  fibre  is  stimulated  artificially  at  any  part  of  its  course,  the 
nervous  impulse  set  going  travels  in  both  directions. 

"We  used  just  now  the  phrase  '  tetanization  of  a  nerve,'  mean- 
ing the  application  to  a  nerve  of  rapidly  repeated  shocks  such  as 
would  produce  tetanus  in  the  muscle  to  which  the  nerve  was 
attached,  and  we  shall  have  frequent  occasion  to  employ  the 
phrase.  It  must  however  be  understood  that  there  is  in  the 
nerve,  in  an  ordinary  way,  no  summation  of  nervous  impulses  com- 
parable to  the  summation  of  muscular  contractions.  Putting  aside 
certain  cases  which  we  cannot  discuss  here  we  may  say  that  the 
series  of  shocks  sent  in  at  the  far  end  of  the  nerve  start  a  series 
of  impulses ;  these  travel  down  the  nerve  and  reach  the  muscle 
as  a  series  of  distinct  impulses ;  and  the  first  changes  in  the 
muscle,  the  molecular  changes  which,  sweeping  along  the  fibre, 
initiate  the  change  of  form,  and  which  we  may  perhaps  speak  of 
as  constituting  a  muscle  impulse,  also  probably  form  a  series  the 
members  of  which  are  distinct.  It  is  not  until  these  molecular 
changes  become  transformed  into  visible  changes  of  form  that 
any  fusion  or  summation  takes  place. 

§  68.  Putting  together  the  facts  contained  in  this  and  the  pre- 
ceding sections,  the  following  may  be  taken  as  a  brief  approximate 
history  of  what  takes  place  in  a  muscle  and  nerve  when  the  latter 
is  subjected  to  a  single  induction-shock.  At  the  instant  that  the 
induced  current  passes  into  the  nerve,  changes  occur,  of  whose 
nature  we  know  nothing  certain  except  that  they  cause  a  '  current 
of  action '  or  '  negative  variation  '  of  the  '  natural '  nerve  current. 
These  changes  propagate  themselves  along  the  nerve  in  both 
directions  as  a  nervous  impulse  in  the  form  of  a  wave,  having 
a  wave-length  of  about  18  mm.,  and  a  velocity  (in  frog's  nerve)  of 
about  28  m.  per  sec.  Passing  down  the  nerve  fibres  to  the  muscle, 
flowing  along  the  branching  and  narrowing  tracts,  the  wave  at  last 


Chap,  ir.]  THE   CONTRACTILE   TISSUES.  109 

breaks  on  the  end-plates  of  the  fibres  of  the  muscle.  Here  it  is 
transmuted  into  what  we  have  called  a  muscle  impure,  which 
with  a  greatly  diminished  velocity  (about  3  m.  per  sec),  travels 
from  each  end-plate  in  both  directions  to  the  end  of  the  fibre, 
where  it  appears  to  be  lost,  at  all  events  we  do  not  know  what 
becomes  of  it.  As  this  impulse  wave  sweeps  along  the  fibre  it 
initiates  an  explosive  decomposition  of  material,  leading  to  a 
discharge  of  carbonic  acid,  to  the  appearance  of  some  substance  or 
substances  with  an  acid  reaction,  and  probably  of  other  unknown 
things,  with  a  considerable  development  of  heat.  This  explosive 
decomposition  gives  rise  to  the  visible  contraction  wave ;  the  fibre, 
as  the  wave  passes  over  it,  swells  and  shortens  and  thus  brings  its 
two  ends  nearer  together. 

When  repeated  shocks  are  given,  wave  follows  wave  of  nervous 
impulse,  muscle  impulse,  and  visible  contraction  ;  but  the  last  do 
not  keep  distinct,  they  are  fused  into  the  continued  shortening 
which  we  call  tetanus. 


SEC.  3.  THE  NATUEE  OF  THE  CHANGES  THEOUGH 
WHICH  AN  ELECTEIC  CUEEENT  IS  ABLE  TO  GENE- 
EATE  A  NEEVOUS  IMPULSE. 


Action  of  the  Constant  Current. 

§  69.  In  the  preceding  account,  the  stimulus  applied  in  order 
to  give  rise  to  a  nervous  impulse  has  always  been  supposed  to  be 
an  induction-shock,  single  or  repeated.  This  choice  of  stimulus  has 
been  made  on  account  of  the  almost  momentary  duration  of  the 
induced  current.  Had  we  used  a  current  lasting  for  some  consider- 
able time,  the  problems  before  us  would  have  become  more  com- 
plex, in  consequence  of  our  having  to  distinguish  between  the 
events  taking  place  while  the  current  was  passing  through  the 
nerve,  from  those  which  occurred  at  the  moment  when  the  current 
was  thrown  into  the  nerve,  or  at  the  moment  when  it  was  shut 
off  from  the  nerve.  These  complications  do  arise  when,  instead  of 
employing  the  induced  current  as  a  stimulus,  we  use  a  constant 
current,  i.e.  when  we  pass  through  the  nerve  (or  muscle)  a  current 
direct  from  the  battery,  without  the  intervention  of  any  induc- 
tion-coiL 

Before  making  the  actual  experiment,  we  might,  perhaps, 
naturally  suppose  that  the  constant  current  would  act  as  a  stimu- 
lus throughout  the  whole  time  during  which  it  was  applied ;  that,  so 
long  as  the  current  passed  along  the  nerve,  nervous  impulses  would 
be  generated,  and  that  these  would  throw  the  muscle  into  some- 
thing at  all  events  like  tetanus.  And  under  certain  conditions  this 
does  take  place ;  occasionally  it  does  happen  that  at  the  moment 
the  current  is  thrown  into  the  nerve  the  muscle  of  the  muscle- 
nerve  preparation  falls  into  a  tetanus,  which  is  continued  until  the 
current  is  shut  off ;  but  such  a  result  is  exceptional.  In  the  vast 
majority  of  cases  what  happens  is  as  follows.  At  the  moment  that 
the  circuit  is  made,  the  moment  that  the  current  is  thrown 
into  the  nerve,  a  single  twitch,  a  simple  contraction,  the  so-called 
making  contraction,  is  witnessed  ;  but  after  this  has  passed  away 


Chap,  ii.]  THE   CONTRACTILE   TISSUES.  Ill 

the  muscle  remains  absolutely  quiescent  in  sjiite  of  the  current 
continuing  to  pass  through  the  nerve,  and  this  quiescence  is 
maintained  until  the  circuit  is  broken,  until  the  current  is  shut 
off  from  the  nerve,  when  another  simple  contraction,  the  so- 
called  breaking  contraction,  is  observed.  The  mere  passage  of  a 
constant  current  of  uniform  intensity  through  a  nerve  does  not, 
under  ordinary  circumstances,  act  as  a  stimulus  generating  a 
nervous  impulse;  such  an  impulse  is  only  set  up  when  the 
current  either  falls  into  or  is  shut  off  from  the  nerve.  It  is 
the  entrance  or  the  exit  of  the  current,  and  not  the  continuance  of 
the  current,  which  is  the  stimulus.  The  quiescence  of  the  nerve 
and  muscle  during  the  passage  of  the  current  is,  however,  dependent 
on  the  current  remaining  uniform  in  intensity  or  at  least  not  being 
suddenly  increased  or  diminished.  Any  sufficiently  sudden  and 
large  increase  or  diminution  of  the  intensity  of  the  current  will 
act  like  the  entrance  or  exit  of  a  current,  and,  by  generating  a 
nervous  impulse,  give  rise  to  a  contraction.  If  the  intensity  of  the 
current,  however,  be  very  slowly  and  gradually  increased  or  di- 
minished, a  very  wide  range  of  intensity  may  be  passed  through 
without  any  contraction  being  seen.  It  is  the  sudden  change  from 
one  condition  to  another,  and  not  the  condition  itself,  which  causes 
the  nervous  impulse. 

In  many  cases,  both  a  '  making '  and  a  '  breaking '  contraction, 
each  a  simple  twitch,  are  observed,  and  this  is  perhaps  the 
commonest  event ;  but  when  the  current  is  very  weak,  and  again 
when  the  current  is  very  strong,  either  the  breaking  or  the  making 
contraction  may  be  absent,  i.e.  there  may  be  a  contraction  only 
when  the  current  is  thrown  into  the  nerve,  or  only  when  it  is 
shut  off  from  the  nerve. 

Under  ordinary  circumstances  the  contractions  witnessed  with 
the  constant  current,  either  at  the  make  or  at  the  break,  are  of  the 
nature  of  a  '  simple'  contraction,  but,  as  has  already  been  said,  the 
application  of  the  current  may  give  rise  to  a  very  pronounced 
tetanus.  Such  a  tetanus  is  seen  sometimes  when  the  current 
is  made,  lasting  during  the  application  of  the  current,  sometimes 
when  the  current  is  broken,  lasting  some  time  after  the  current  has 
been  wholly  removed  from  the  nerve.  The  former  is  spoken  of  as 
a  '  making,'  the  latter  as  a  '  breaking  '  tetanus.  But  these  excep- 
tional results  of  the  application  of  the  constant  current  need  not 
detain  us  now. 

The  great  interest  attached  to  the  action  of  the  constant 
current  lies  in  the  fact  that  during  the  passage  of  the  current, 
in  spite  of  the  absence  of  all  nervous  impulses,  and,  therefore, 
of  all  muscular  contractions,  the  nerve  is  for  the  time  both  between 
and  on  each  side  of  the  electrodes  profoundly  modified  in  a  most 
peculiar  manner.  This  modification,  important  both  for  the  light 
it  tlu'ows  on  the  generation  of  nervous  impulses  and  for  its  practical 
applications,  is  known  under  the  name  of  clcctrotonus. 


112  ELECTROTOXUS.  [Book  i. 

§  70.  Electrotonus.  The  marked  feature  of  the  electrotonic 
condition  is  that  the  nerve,  though  apparently  quiescent,  is  changed 
in  respect  to  its  irritability ;  and  that  in  a  different  way  in  the 
neighbourhood  of  the  two  electrodes  respectively. 

Suppose  that  on  the  nerve  of  a  muscle-nerve  preparation  are 
placed  two  (non-polarizable)  electrodes  (Fig.  21,  a,  k),  connected 
with  a  battery  and  arranged  with  a  key  so  that  a  constant  current 
can  at  pleasure  be  thrown  into  or  shut  off  from  the  nerve. 
This  constant  current,  whose  effects  we  are  about  to  study,  may  be 
called  the  '  polarizing  current.'  Let  a  be  the  positive  electrode  or 
anode,  and  k  the  negative  electrode  or  kathode,  both  placed  at 
some  distance  from  the  muscle,  and  also  with  a  certain  interval 
between  each  other.  At  the  point  x  let  there  be  applied  a  pair  of 
electrodes  connected  with  an  induction-coil.  Let  the  muscle 
further  be  connected  with  a  lever,  so  that  its  contractions  can 
be  recorded,  and  their  amount  measured.  Before  the  polarizing 
current  is  thrown  into  the  nerve,  let  a  single  induction-shock 
of  known  intensity  (a  weak  one  being  chosen,  or  at  least  not 
one  which  would  cause  in  the  muscle  a  maximum  contraction)  be 
thrown  in  at  x.     A  contraction  of  a  certain  amount  will  follow. 


«  CliSBC 


a 


Fig.  21.     Muscle-nerve  Preparations,  with  the  nerve  exposed  in  ^  to  a  rfesce^cf/fl^r 
and  in  B  to  an  ascending  constant  current. 

In  each  n  is  the  anode,  k  the  kathode  of  the  constant  current,     x  represents  the 
spot  where  the  induction-shocks  used  to  test  the  irritability  of  the  nerve  are  sent  in. 

That  contraction  may  be  taken  as  a  measure  of  the  irritability  of 
the  nerve  at  the  point  x.  Now,  let  the  polarizing  current  be 
thrown  in,  and  let  the  kathode  or  negative  pole  be  nearest  the 
muscle,  as  in  Fig.  21  A,  so  that  the  current  passes  along  the 
nerve  in  a  direction  from  the  central  nervous  system  towards  the 
muscle ;  such  a  current  is  spoken  of  as  a  descending  one.  The 
entrance  of  the  polarizing  current  into  the   nerve    will  produce 


Chap,  ii.]  THE   CONTEACTILE    TISSUES.  113 

a  '  making '  contraction ;  this  we  may  neglect.  If  while  the 
current  is  passing,  the  same  induction-shock  as  before  be  sent 
through  X,  the  contraction  which  results  will  be  found  to  be 
greater  than  on  the  former  occasion.  If  the  polarizing  current  be 
now  shut  off',  a  '  breaking '  contraction  will  probably  be  produced  ; 
this  also  we  may  neglect.  If  now  the  point  x,  after  a  short 
interval,  be  again  tested  with  the  same  induction-shock  as  before, 
the  contraction  will  be  no  longer  greater,  but  of  the  same  amount, 
or  perhaps  not  so  great,  as  at  first.  During  the  passage  of 
the  polarizing  current,  therefore,  the  irritability  of  the  nerve  at 
the  point  x  has  been  temporarily  increased,  since  the  same  shock 
applied  to  it  causes  a  greater  contraction  during  the  presence  than 
in  the  absence  of  the  current.  But  this  is  only  true  so  long  as  the 
polarizing  current  is  a  descending  one,  so  long  as  the  point  x  lies 
on  the  side  of  the  kathode.  On  the  other  hand,  if  the  polarizing 
current  had  been  an  ascending  one,  with  the  anode  or  positive  pole 
nearest  the  muscle,  as  in  Fig.  21  B,  the  irritability  of  the  nerve  at 
X  would  have  been  found  to  be  diminished  instead  of  increased  by 
the  polarizing  current ;  the  contraction  obtained  during  the  passage 
of  the  constant  current  would  be  less  than  before  the  passage  of 
the  current,  or  might  be  absent  altogether,  and  the  contraction 
after  the  current  had  been  shut  off  would  be  as  great  or  perhaps 
greater  than  before.  That  is  to  say,  when  a  constant  current  is 
applied  to  a  nerve,  the  irritability  of  the  nerve  between  the  polar- 
izing electrodes  and  the  muscle  is,  during  the  passage  of  the 
current,  increased  when  the  kathode  is  nearest  the  muscle  (and 
the  polarizing  current  descending),  and  diminished  when  the  anode 
is  nearest  the  muscle  (and  the  polarizing  current  ascending).  The 
same  result,  mutatis  mutandis,  and  with  some  qualifications  which 
we  need  not  discuss,  would  be  gained  if  x  were  placed  not  between 
the  muscle  and  the  polarizing  current,  but  on  the  far  side  of  the 
latter.  Hence,  it  may  be  stated  generally  that  during  the  passage 
of  a  constant  current  through  a  nerve,  the  irritability  of  the  nerve 
is  increased  in  the  region  of  the  kathode,  and  diminished  in 
the  region  of  the  anode.  The  changes  in  the  nerve  which  give 
rise  to  this  increase  of  irritability  in  the  region  of  the  kathode 
are  spoken  of  as  katclcctrotonus,  and  the  nerve  is  said  to  be 
in  a  katelectrotonic  condition.  Similarly  the  changes  in  the 
region  of  the  anode  are  spoken  of  as  anelcctrotonus,  and  the  nerve 
is  said  to  be  in  an  anelectrotonic  condition.  It  is  also  often  usual 
to  speak  of  the  katelectrotonic  increase,  and  anelectrotonic  decrease 
of  irritability. 

This  law  remains  true  whatever  be  the  mode  adopted  for 
determining  the  irritability.  The  result  holds  good  not  only 
with  a  single  induction-shock,  but  also  with  a  tetanizing  inter-, 
rupted  current,  with  chemical  and  with  mechanical  stimuli.  It 
further  appears  to  hold  good  not  only  in  a  dissected  nerve-muscle 
preparation,  but  also  in  the  intact  nerves  of  the  living  body.     The 


114 


ELECTKOTONUS. 


[Book 


increase  and  decrease  of  irritability  are  most  marked  in  the 
immediate  neighbourhood  of  the  electrodes,  but  spread  for  a 
considerable  distance  in  each  direction  in  the  extrapolar  regions. 
The  same  modification  is  not  confined  to  the  extrapolar  region, 
but  exists  also  in  the  intrapolar  region.  In  the  intrapolar  region 
there  must  be  of  course  a  neutral  or  indifferent  point,  where  the 
katelectrotonic  increase  merges  into  the  anelectrotonic  decrease, 
and  where,  therefore,  the  irritability  is  unchanged.  When  the 
polarizing  current  is  a  weak  one,  this  indifferent  point  is  nearer  the 
anode  than  the  kathode,  but  as  the  polarizing  current  increases  in 
intensity,  draws  nearer  and  nearer  the  kathode  (see  Fig.  22). 

The  amount  of  increase  and  decrease  is  dependent:  (1)  On  the 
strength  of  the  current,  the  stronger  current  up  to  a  certain  limit 
producing  the  greater  effect.  (2)  On  the  irritability  of  the  nerve, 
the  more  irritable,  better  conditioned  nerve  being  the  more  affected 
by  a  current  of  the  same  intensity. 

In  the  experiments  just  described  the  increase  or  decrease  of 
irritability  is  taken  to  mean  that  the  same  stimulus  starts  in  the  one 
case  a  larger  or  more  powerful,  and  in  the  other  case  a  smaller  or 
less  energetic  impulse  ;  but  we  have  reason  to  think  that  the  mere 
propagation  or  conduction  of  impulses  started  elsewhere  is  also 
affected  by  the  electrotonic  condition.  At  all  events  anelectrotonus 
appears  to  offer  an  obstacle  to  the  passage  of  a  nervous  impulse. 


<?  ,B 


Fig.  22.  Diagram  illustrating  the  Variations  of  Irritability  during  Electro- 
TONUS,  WITH  Polarizing  Currents  of  Increasing  Intensity  (from  Pfliiger). 

The  anode  is  supposed  to  be  placed  at  A,  the  kathode  at  B  ;  AB  is  consequently 
the  intrapolar  district.  In  each  of  the  three  curves,  the  portion  of  the  curve  below 
the  base  line  represents  diminished  irritability,  that  above,  increased  irritability. 
yi  represents  the  effect  of  a  weak  current ;  the  indifferent  point  x^  is  near  the 
anode  A.  In  yg.  ^  stronger  current,  the  indifferent  point  x^  is  nearer  the  kathode 
B,  the  diminution  of  irritability  in  anelectrotonus  and  the  increase  in  katelectro- 
tonus  being  greater  than  in  y^ ;'  the  effect  also  spreads  for  a  greater  distance  along 
the  extrapolar  regions  in  both  directions.  In  y^  the  same  events  are  seeu  to  be  still 
more  marked. 


§  71.  Electrotonic  Currents.  Daring  the  passage  of  a  constant 
current  through  a  nerve,  variations  in  the  electric  currents  belonging 
to  the  nerve  itself  may  be  observed ;  and  these  variations  have  certain 
relations  to  the  variations  of  the  irritability  of  the  nerve.  Thus,  if 
a  constant  current,   supplied  by  the  battery  F  (Fig.   23),   be  applied 


Chap,  ii.] 


THE   CONTRACTILE   TISSUES. 


115 


to  a  piece  of  nerve  by  means  of  two  non-polarizable  electrodes  p,  p\ 
the  "  currents  of  rest "  obtainable  from  various  points  of  the  nerve 
will  be  different  during  tlie  passage  of  the  polarizing  current  from 
those  which  were  manifest  before  or  after  the  current  was  applied  ;  and, 
moreover,  the  changes  in  the  nerve-currents  produced  by  the  polarizing 
current  will  not  be  the  same  in  the  neighbourhood  of  the  anode  (^?) 
as  those  in  the  neighbourhood  of  the  kathode  (/:<').  Thus  let  G  and  H  be 
two  galvanometers  so  connected  with  the  two  ends  of  the  nerve  as  to 
afford  good  and  clear  evidence  of  the  "currents  of  rest."  Before 
the  polarizing  current  is  thrown  into  the  nerve,  the  needle  of  U  will 
occupy  a  position  indicating  the  passage  of  a  current  of  a  certain 
intensity  from  h  to  h'  through  the  galvanometer  (from  the  positive 
longitudinal  surface  to  the  negative  cut  end  of  the  nerve),  the  circuit 
being  completed  by  a  current  in  the  nerve  from  li  to  A,  i.e.  the  current 


4- 


'f-i 


iih 


9    m>      >    \(/ 


■mmmM.. , 

/   ^ — est     " 


Fig.  23.    Diagram  illustrating  Electrotonic  Currents. 

P  the  polarizing  battery,  with  k  a  key,/?  the  anode,  and  p'  the  kathode.  At  the  left 
end  of  tlie  piece  of  nerve  the  natural  current  flows  through  the  galvanometer  (7 
from  g  to  7',  in  the  direction  of  the  arrows  ;  its  direction,  thereifore,  is  the  same 
as  that  of  the  polarizing  current ;  consequently  it  a])pears  increased,  as  indicated 
by  the  sign  +.  The  current  at  the  other  end  of  the  piece  of  nerve,  from  /(  to  h\ 
through  the  galvanometer  //,  Hows  in  a  contrary  direction  to  the  polarizing 
current ;  it  consequently  appears  to  be  diminished,  as  indicated  by  the  sign  — . 

_  N.  B.  For  simplicity's  sake,  the  polarizing  current  is  here  supposed  to  be  thrown 
m  at  the  middle  of  a  i)iece  of  nerve,  and  the  galvanometer  jilaced  at  the  two  ends. 
Of  course  it  will  be  understood  that  the  former  mav  be  thrown  in  anywhere,  and  the 
latter  connected  with  any  two  pairs  of  points  which' will  give  currents. 


116  ELECTROTONUS.  [Book  i. 

will  flow  in  the  direction  of  the  arrow.  Similarly  the  needle  of  G  will 
by  its  deflection  indicate  the  existence  of  a  current  flowing  from  g  to  g^ 
through  the  galvanometer,  and  from  ^'  to  g  through  the  nerve,  in  the 
direction  of  the  arrow. 

At  the  instant  that  the  polarizing  current  is  thrown  into  the  nerve 
B.t pp',  the  currents  at  gg',  hli'  will  undergo  a  "  negative  variation  ; "  that  is, 
the  nerve  at  each  point  will  exhibit  a  "  current  of  action  "  correspond- 
ing to  the  nervous  impulse,  which,  at  the  making  of  the  polarizing 
current,  passes  in  both  directions  along  the  nerve,  and  may  cause  a 
contraction  in  the  attached  muscle.  The  current  of  action  is,  as  we 
have  seen,  of  extremely  short  duration  :  it  is  over  and  gone  in  a  small 
fraction  of  a  second.  It  therefore  must  not  be  confounded  with  a 
permanent  eflect,  which,  in  the  case  we  are  dealing  with,  is  observed  in 
both  galvanometers.  This  effect,  which  is  dependent  on  the  direction 
of  the  polarizing  current,  is  as  follows  :  Supposing  that  the  polarizing 
current  is  flowing  in  the  direction  of  the  arrow  in  the  figure,  that  is, 
passes  in  the  nerve  from  the  positive  electrode  or  anode  j9  to  the  negative 
electrode  or  kathode  p',  it  is  found  that  the  current  through  the 
galvanometer  G  is  increased,  while  that  through  ^is  diminished.  The 
polarizing  current  has  caused  the  appearance  in  the  nerve  outside  the 
electrodes  of  a  current,  having  the  same  direction  as  itself,  called  the 
' electrotonic '  current;  and  this  electrotonic  current  adds  to,  or  takes 
away  from,  the  natural  nerve-current  or  "  current  of  rest,"  according  as 
it  is  flowing  in  the  same  direction  as  that,  or  in  an  opposite  direction. 

The  strength  of  the  electrotonic  current  is  dependent  on  the  strength 
of  the  polarizing  current,  and  on  the  length  of  the  intrapolar  region, 
which  is  exposed  to  the  polarizing  current.  When  a  strong  polarizing 
current  is  used,  the  electromotive  force  of  the  electrotonic  current  may 
be  much  greater  than   that  of  the  natural  nerve-current. 

The  strength  of  the  electrotonic  current  varies  with  the  irritability, 
or  vital  condition  of  the  nerve,  being  greater  with  the  more  irritable 
nerve ;  and  a  dead  nerve  will  not  manifest  electrotonic  currents.  More- 
over, the  propagation  of  the  current  is  stopped  by  a  ligature,  or  by 
crushing  the  nerve. 

We  may  speak  of  the  conditions  which  give  rise  to  this  electrotonic 
current  as  a  joAysj'cai  electrotonus  analogous  to  that  phynological  electro- 
tonns,  which  is  made  known  by  variations  in  irritability.  The  physical 
electrotonic  current  is  probably  due  to  the  escape  of  the  polarizing 
current  along  the  nerve  under  the  peculiar  conditions  of  the  living 
nerve  j  but  we  must  not  attempt  to  enter  here  into  this  difficult  subject, 
or  into  the  allied  question  as  to  the  exact  connection  between  tlie 
physical  and  the  physiological  electrotonus,  though  there  can  be  little 
doubt  that  the  latter  is  dependent  on  the  former. 

§  72.  These  variations  of  irritability  at  the  kathode  and  anode 
respectively,  thus  brought  about  by  the  action  of  the  constant 
current,  are  interesting  theoretically,  because  we  may  trace  a  con- 
nection between  them  and  the  nervous  impulse  which  is  the  result 
of  the  making  or  breaking  of  a  constant  current. 

For  we  have  evidence  that  a  nervous  impulse  is  generated 
when    a  portion  of   the    nerve  passes    suddenly  from  a  normal 


Chap.  ii.J  THE   CONTRACTILE   TISSUES.  117 

condition  to  a  state  of  katelectrotonus,  or  from  a  state  of  anelec- 
trotonus  back  to  a  normal  condition ;  but  that  the  passage  from 
a  normal  condition  to  anelectrotonus  or  from  katelectrotonus 
back  to  a  normal  condition  is  unable  to  generate  an  impulse. 
Hence,  when  a  constant  current  is  '  made,'  the  impulse  is  gen- 
erated only  at  the  kathode  where  the  nerve  passes  suddenly  into 
katelectrotonus  ;  when  the  current,  on  the  other  hand,  is  'broken,' 
the  impulse  is  generated  only  at  the  anode  where  the  nerve  passes 
suddenly  back  from  anelectrotonus  into  a  normal  condition.  We 
have  an  indirect  proof  of  this  in  the  facts  to  which  we  drew 
attention  a  little  while  back,  viz.  that  a  contraction  sometimes 
occurs  at  the  '  breaking  '  only,  sometimes  at  the  '  making '  only 
of  the  constant  current,  sometimes  at  both.  For  it  is  found  that 
this  depends  partly  on  the  strength  of  the  current  in  relation  to 
the  irritability  of  the  nerve,  partly  on  the  direction  of  the  current, 
whether  ascending  or  descending  ;  and  the  results  obtained  with 
strong,  medium  and  weak  descending  and  ascending  currents  have 
been  stated  in  the  form  of  a  '  law  of  contraction.'  We  need  not 
enter  into  the  details  of  this  '  law,'  but  will  merely  say  that  the 
results  which  it  formulates  are  best  explained  by  the  hypothesis 
just  stated.  We  may  add  that  when  the  constant  current  is 
applied  to  certain  structures  composed  of  plain  muscular  fibres, 
whose  rate  of  contraction  we  have  seen  to  be  slow,  the  making 
contraction  may  be  actually  seen  to  begin  at  the  kathode  and 
travel  towards  the  anode,  and  the  breaking  contraction  to  begin 
at  the  anode  and  travel  thence  towards  the  kathode. 

Since  in  katelectrotonus  the  irritability  is  increased,  and  in 
anelectrotonus  decreased,  both  the  entrance  from  the  normal 
condition  into  katelectrotonus,  and  the  return  from  anelectrotonus 
to  the  normal  condition,  are  instances  of  a  passage  from  a  lower 
stage  of  irritability  to  a  higher  stage  of  irritability.  Hence,  the 
phenomena  of  electrotonus  would  lead  us  to  the  conception  that  a 
stimulus  in  provoking  a  nervous  impulse  produces  its  effect  by,  in 
some  way  or  other,  suddenly  raising  the  irritability  to  a  higher 
pitch.  But  what  we  are  exactly  to  understand  by  raising  the 
irritability,  what  molecular  change  is  the  cause  of  the  rise,  and 
how  either  electric  or  other  stimuli  can  produce  this  change,  are 
matters  which  we  cannot  discuss  here. 

Besides  their  theoretical  importance,  the  phenomena  of  electro- 
tonus have  also  a  practical  interest.  When  an  ascending  current 
is  passed  along  a  nerve  going  to  a  muscle  or  group  of  muscles,  the 
region  between  the  electrodes  and  the  muscle  is  thrown  into 
anelectrotonus,  and  its  irritability  is  diminished.  If  the  current 
be  of  adequate  strength,  the  irritability  may  be  so  much  lessened 
that  nervous  impulses  cannot  be  generated  in  that  part  of  the 
nerve,  or  cannot  pass  along  it.  Hence,  by  this  means  the  irregular 
contractions  of  muscles  known  as  '  cramp '  may  be  abolished. 
Similarly,  by  bringing  into  a  condition  of  anelectrotonus  a  portion 


118  EFFECTS   OF   CONSTANT   CURRENT.       [Book  i. 

of  a  sensory  nerve  in  which  violent  impulses  are  being  generated, 
giving  rise  in  the  central  nervous  system  to  sensations  of  pain,  the 
impulses  are  toned  down  or  wholly  abolished,  and  the  pain  ceases. 
So  on  the  other  hand  we  may  at  pleasure  heighten  the  irritability 
of  a  part  by  throwing  it  into  katelectrotonus.  In  this  way  the 
constant  current,  properly  applied,  becomes  a  powerful  remedial 
means. 

Lastly,  though  we  are  dealing  now  with  nerves  going  to  muscles, 
that  is  to  say,  with  motor  nerves  only,  we  may  add  that  what  we 
have  said  about  electrotonus  and  the  development  of  nervous 
impulses  by  it  appears  to  apply  equally  well  to  sensory  nerves. 

§  73.  In  a  general  way  muscular  fibres  behave  towards  an 
electric  current  very  much  as  do  nerve  fibres  ;  but  there  are 
certain  important  differences. 

In  the  first  place,  muscular  fibres,  devoid  of  nerve  fibres,  are 
much  more  readily  thrown  into  contractions  by  the  breaking  and 
making  of  a  constant  current  than  by  the  more  transient 
induction-shock ;  the  muscular  substance  seems  to  be  more 
sluggish  than  the  nervous  substance  and  requires  to  be  acted  upon 
for  a  longer  time.  This  fact  may  be  made  use  of,  and  indeed  is  in 
medical  practice  made  use  of,  to  determine  the  condition  of  the 
nerves  supplying  a  muscle.  If  the  intramuscular  nerves  be  still  in 
good  condition,  the  muscle  as  a  whole  responds  readily  to  single 
induction-shocks  because  these  can  act  upon  the  intramuscular 
nerves.  If  these  nerves  on  the  other  hand  have  lost  their  irrita- 
bility, the  muscle  does  not  respond  readily  to  single  induction- 
shocks,  or  to  the  interrupted  current,  but  can  still  easily  be  thrown 
into  contractions  by  the  constant  current. 

In  the  second  place,  while  in  a  nerve  no  impulses  are  as  a  rule 
generated  during  the  passage  of  a  constant  current,  between  the 
break  and  the  make,  provided  that  it  is  not  too  strong,  and  that  it 
remains  uniform  in  strength,  in  an  urarized  muscle  on  the  other 
hand,  even  with  moderate  and  perfectly  uniform  currents,  a  kind  of 
tetanus  or  apparently  a  series  of  rhythmically  repeated  contractions 
is  very  frequently  witnessed  during  the  passage  of  the  current. 
The  exact  nature  and  cause  of  these  phenomena  in  muscle,  we 
must  not  however  discuss  here. 


SEC.  4  THE   MUSCLE-NERVE   PREPAEATION   AS  A 

MACHINE. 


§  74.  The  facts  described  in  the  foregoing  sections  shew  that  a 
muscle  with  its  nerve  may  be  justly  regarded  as  a  machine  which, 
when  stimulated,  will  do  a  certain  amount  of  work.  But  the 
actual  amount  of  work  which  a  muscle-nerve  preparation  will  do  is 
found  to  depend  on  a  large  number  of  circumstances,  and  conse- 
quently to  vary  within  very  wide  limits.  These  variations  will  be 
largely  determined  by  the  condition  of  the  muscle  and  nerve  in 
repect  to  their  nutrition ;  in  other  words,  by  the  degree  of  irrita- 
bility manifested  by  the  miiscle  or  by  the  nerve  or  by  both.  But 
quite  apart  from  the  general  influences  affecting  its  nutrition  and 
thus  its  irritability,  a  muscle-nerve  preparation  is  affected,  as 
regards  the  amount  of  its  work,  by  a  variety  of  other  circumstances, 
which  we  may  briefly  consider  here,  reserving  to  a  succeeding 
section  the  study  of  variations  in  irritability. 

We  may  here  remark  that  a  muscle  may  be  thrown  into 
contraction  under  two  different  conditions.  In  the  one  case  it  may 
be  free  to  shorten :  by  the  lifting  of  the  weight  or  otherwise,  the 
one  end  of  the  muscle  may  approach  the  other ;  and  this  is  the 
kind  of  contraction  which  we  have  taken,  and  may  take  as  the 
ordinary  one.  But  the  muscle  may  be  placed  under  such  circum- 
stances that,  when  it  contracts,  the  one  end  is  not  brought  nearer 
to  the  other,  the  muscle  remains  of  the  same  length,  and  the 
effect  of  the  contraction  is  manifested  only  as  an  increased  strain. 
In  this  latter  case,  the  contraction  is  spoken  of  as  an  "isometric," 
in  the  former  case  as  an  "  isotonic  "  contraction. 

The  influence  of  the  nature  and  mode  of  application  of  the 
stimulus.  When  we  apply  a  weak  stimulus,  a  weak  induction- 
shock,  to  a  nerve,  we  get  a  small  contraction,  a  slight  shortening  of 
the  muscle ;  when  we  apply  a  stronger  stimulus,  a  stronger  in- 
duction-shock, we  get  a  larger  contraction,  a  greater  shortening  of 
the  muscle.  We  take,  other  things  being  equal,  the  amount  of 
contraction  of  the  muscle  as  a  measure  of  the  nervous  impulse, 
and  sny  that  in  the  former  case  a  weak  or  slight,  in  the  latter  case 
a  stronger  or  larger  nervous  impulse  has  been  generated.  Now 
the  muscle  of  the  muscle-nerve  preparation  consists  of  many 
muscular  fibres  and  the  nerve  of  many  nerve  fibres  ;  and  we  may 


120  CHARACTERS   OF   STIMULI.  [Book  i. 

fairly  suppose  that  in  two  experiments  we  may  in  the  one  experi- 
ment bring  the  iuduction-shock  or  other  stimulus  to  bear  on  a  few 
nerve  fibres  only,  and  in  the  other  experiment  on  many  or  even  all 
the  fibres  of  the  nerve.  In  the  former  case  only  those  muscular 
fibres  in  which  the  few  nerve  fibres  stimulated  end  will  be  thrown 
into  contraction,  the  others  remaining  quiet,  and  the  shortening 
of  the  muscle  as  a  whole,  since  only  a  few  fibres  take  part  in  it, 
will  necessarily  be  less  than  when  all  the  fibres  of  the  nerve  are 
stimulated  and  all  the  fibres  of  the  muscle  contract.  That  is  to 
say,  the  amount  of  contraction  will  depend  on  the  number  of 
fibres  stimulated.  For  simplicity's  sake  however  we  will  in  what 
follows,  e.ccept  when  otherwise  indicated,  suppose  that  when  a 
nerve  is  stimulated,  all  the  fibres  are  stimulated  and  all  the 
muscular  fibres  contract. 

This  baing  premised,  we  may  say  that,  other  things  being  equal, 
the  magnitude  of  a  nervous  impulse,  and  so  the  magnitude  of  the 
ensuing  contraction,  is  directly  dependent  on  what  we  may  call 
the  strength  of  the  stimulus.  Thus  taking  a  single  induction- 
shock  as  the  most  manageable  stimulus,  we  find  that  if,  before  we 
begin,  we  place  the  secondary  coil  (Fig.  4,  sc.)  a  long  way  off  the 
primary  coil  2^'^-  c.,  no  visible  effect  at  all  follows  upon  the 
discharge  of  the  induction-shock.  The  passage  of  the  momentary 
weak  current  is  either  unable  to  produce  any  nervous  impulse  at 
all,  or  the  weak  nervous  impulse  to  which  it  gives  rise  is  unable 
to  stir  the  sluggish  muscular  substance  to  a  visible  contraction. 
As  we  slide  the  secondary  coil  towards  the  primary,  sending  in  an 
induction-shock  at  each  new  position,  we  find  that  at  a  certain 
distance  between  the  secondary  and  primary  coils,  the  muscle 
responds  to  each  induction-shock  ^  with  a  contraction  which  makes 
itself  visible  by  the  slightest  possible  rise  of  the  attached  lever. 
This  position  of  the  coils,  the  battery  remaining  the  same  and 
other  things  being  equal,  marks  the  minimal  stimulus  giving  rise 
to  the  minimal  contraction.  As  the  secondary  coil  is  brought 
nearer  to  the  primary,  the  contractions  increase  in  height  corre- 
sponding to  the  increase  in  the  intensity  of  the  stimulus.  Very 
soon  however  an  increase  in  the  stimulus  caused  by  further  sliding 
the  secondary  coil  over  the  primary  fails  to  cause  any  increase 
in  the  contraction.  This  indicates  that  the  maximal  stimulus 
giving  rise  to  the  maximal  contraction  has  been  reached ;  though 
the  shocks  increase  in  intensity  as  the  secondary  coil  is  pushed 
further  and  further  over  the  primary,  the  contractions  remain  of 
the  same  height,  until  fatigue  lowers  them. 

With  single  induction-shocks  then  the  muscular  contraction, 
and  by  inference  the  nervous  impulse,  increases  with  an  increase  in 
the  intensity  of  the  stimulus,  between  the  limits  of  the  minimal 

1  Tn  these  experiments  either  the  breaking  or  making  shock  must  be  tised,  not 
sometimes  one  anrl  sometimes  the  otlier,  for,  as  we  liave  stated,  the  two  kinds  of 
shock  differ  in  efficiency,  the  breaking  being  the  most  potent. 


Chap,  ii.]  THE   CONTRACTILE   TISSUES.  121 

and  maximal  stimuli ;  and  this  dependence  of  the  nervous  impulse, 
and  so  of  the  contraction,  on  the  strength  of  the  stimulus  may  be 
observed  not  only  in  electric  but  in  all  kinds  of  stimuli. 

It  may  here  be  remarked  that  in  order  for  a  stimulus  to  be 
effective,  a  certain  abruptness  in  its  action  is  necessary.  Thus 
as  we  have  seen  the  constant  current  when  it  is  passing  through 
a  nerve  with  uniform  intensity  does  not  give  rise  to  a  nervous 
impulse,  and  indeed  it  may  be  increased  or  diminished  to  almost 
any  extent  without  generating  nervous  impulses,  provided  that  the 
change  be  made  gradually  enough  ;  it  is  only  when  there  is  a 
sudden  change  that  the  current  becomes  effective  as  a  stimulus. 
And  the  reason  why  the  breaking  induction-shock  is  more  potent 
as  a  stimulus  than  the  making  shock  is  because  as  we  have  seen 
(§  44)  the  current  which  is  induced  in  the  secondary  coil  of  an 
induction-machine  at  the  breaking  of  the  primary  circuit,  is  more 
rapidly  developed,  and  has  a  sharper  rise  than  the  current  which 
appears  when  the  primary  circuit  is  made.  Similarly  a  sharp  tap 
on  a  nerve  will  produce  a  conti action,  when  a  gradually  increasing 
pressure  will  fail  to  do  so ,  and  in  general  the  efficiency  of  a 
stimulus  of  any  kind  will  depend  in  part  on  the  suddenness  or 
abruptness  of  its  action. 

A  stimulus,  in  order  that  it  may  be  eff'ective,  must  have  an 
action  of  a  certain  duration,  the  time  necessary  to  produce  an  effect 
varying  according  to  the  strength  of  the  stimulus  and  being  differ- 
ent in  the  case  of  a  nerve  from  what  it  is  in  the  case  of  a  muscle. 
It  would  appear  that  an  electric  current  applied  to  a  nerve  must 
have  a  duration  of  at  least  about  '0015  sec.  to  cause  any  contrac- 
tion at  all,  and  needs  a  longer  time  than  this  to  produce  its  full 
effect.  A  muscle  fibre  apart  from  its  nerve  fibre  requires  a  still 
longer  duration  of  the  stimulus,  and  hence,  as  we  have  already 
stated,  a  muscle  poisoned  by  urari,  or  which  has  otherwise  lost 
the  action  of  its  nerves,  will  not  respond  as  readily  to  induction- 
shocks  as  to  the  more  slowly  acting,  breaking  and  making  of  a 
constant  current. 

In  the  ca.'-e  of  electric  stimuli,  the  same  current  will  produce 
a  stronger  contraction  when  it  is  sent  along  the  nerve  than  when 
it  is  sent  across  the  nerve ;  indeed  it  is  maintained  that  a  current 
which  passes  through  a  nerve  in  an  absolutely  transverse  direction 
is  powerless  to  generate  impulses. 

§  75.  We  have  seen  that  when  single  stimuli  are  repeated 
with  sufficient  fre([uency,  the  indivitlual  contractions  are  fused 
into  tetanus ;  as  the  frequency  of  the  repetition  is  increased,  the 
individual  contractions  are  less  obvious  on  the  curve,  until  at 
last  we  get  a  curve  on  which  they  seem  to  be  entirely  lost  and 
which  we  may  speak  of  as  a  complete  tetanus.  By  such  a  tetanus 
a  much  greater  contraction,  a  much  greater  shortening  of  the 
muscle  is  of  course  obtained  than  by  single  contractions. 

The  exact  frequency  of  repetition  required  to   produce  com- 


122  REPETITION  OF  CONTRACTIONS  IN  TETANUS.  [Book  i. 

plete  tetanus  will  depend  chiefly  on  the  length  of  the  individual 
contractions,  and  tMs  varies  in  different  animals,  in  different 
muscles  of  the  same  animal,  and  in  the  same  muscle  under  differ- 
ent conditions.  In  a  cold  blooded  animal  a  single  contraction  is 
as  a  rule  more  prolonged  than  in  a  warm  blooded  animal,  and 
tetanus  is  consequently  produced  in  the  former  by  a  less  frequent 
repetition  of  the  stimulus.  A  tired  muscle  has  a  longer  contrac- 
tion than  a  fresh  muscle,  and  hence  in  many  tetanus  curves  the 
individual  contractions,  easily  recognised  at  first,  disappear  later 
on,  owing  to  the  individual  contractions  being  lengthened  out  by 
the  exhaustion  caused  by  the  tetanus  itself.  In  many  animals, 
e.  g.  the  rabbit,  some  muscles  (such  as  the  adductor  magnus 
femoris)  are  pale,  while  others  (such  as  the  semitendinosus)  are 
red.  The  red  muscles  are  not  only  more  richly  supplied  with 
blood  vessels,  but  the  muscle  substance  of  the  fibres  contains 
more  haemoglobin  than  the  pale,  and  there  are  other  structural 
differences.  iSTow  the  single  contraction  of  one  of  these  red  muscles 
is  more  prolonged  than  the  single  contraction  of  one  of  the  pale 
muscles  produced  by  the  same  stimulus.  Hence  the  red  muscles 
are  thrown  into  complete  tetanus  with  a  repetition  of  much  less 
frequency  than  that  required  for  the  pale  muscles.  Thus,  ten 
stimuli  in  a  second  are  quite  sufficient  to  throw  the  red  muscles 
of  the  rabbit  into  complete  tetanus,  while  the  pale  muscles 
require  at  least  twenty  stimuli  in  a  second. 

So  long  as  signs  of  the  individual  contractions  are  visible  on 
the  curve  of  tetanus  it  is  easy  to  recognise  that  each  stimulation 
produces  one  of  the  constituent  single  contractions,  and  that  the 
number  so  to  speak  of  the  vibrations  of  the  muscle  making  up 
the  tetanus  corresponds  to  the  number  of  stimulations ;  but  the 
question  whether,  when  we  increase  the  number  of  stimulations 
beyond  that  necessary  to  produce  a  complete  tetanus,  we  still 
increase  the  number  of  constituent  single  contractions  is  one  not 
so  easy  to  answer.  And  connected  with  this  question  is  another 
difficult  one.  What  is  the  rate  of  repetition  of  single  contrac- 
tions making  up  those  tetanic  contractions  which  as  we  have  said 
are  the  kind  of  contractions  by  which  the  voluntary,  and  indeed 
other  natural,  movements  of  the  body  are  carried  out  ?  What  is 
the  evidence  that  these  are  really  tetanic  in  character  ? 

When  a  muscle  is  thrown  into  tetanus,  a  more  or  less  musical 
sound  is  produced.  This  may  be  heard  by  applying  a  stethoscope 
directly  over  a  contracting  muscle,  and  a  similar  sound  but  of  a 
more  mixed  origin  and  less  trustworthy  may  be  heard  when  the 
masseter  muscles  are  forcibly  contracted  or  when  a  finger  is  placed 
in  the  ear,  and  the  muscles  of  the  same  arm  are  contracted. 

When  the  stethoscope  is  placed  over  a  muscle,  the  nerve  of 
which  is  stimulated  by  induction-shocks  repeated  with  varying 
frequency,  the  note  heard  will  vary  with  the  frequency  of  the 
shocks,  being  of    higher  pitch  with   the   more  frequent   shocks. 


Chap,  ii.]  THE   CONTRACTILE   TISSUES.  123 

Now  it  has  been  thought  that  the  vibrations  of  the  muscle  giving 
rise  to  the  "  muscle  sound "  are  identical  with  the  single  con- 
tractions making  up  the  tetanus  ot  the  muscle.  And  since,  in 
the  human  body,  when  a  muscle  is  thrown  into  contraction  in  a 
voluntary  efi'ort,  or  indeed  in  any  of  the  ordinary  natural  move- 
ments of  the  body,  the  fundamental  tone  of  the  sound  corresponds 
to  about  19  or  20  vibrations  a  second,  it  has  been  concluded  that 
the  contraction  taking  place  in  such  cases  is  a  tetanus  of  which 
the  individual  contractions  follow  each  other  about  19  or  20  times 
a  second.  But  investigations  seem  to  shew  that  the  vibrations 
giving  rise  to  the  muscle  sound  do  not  really  correspond  to  the 
shortenings  and  relaxations  of  the  individual  contractions,  and 
that  the  pitch  of  the  note  cannot  therefore  be  taken  as  an  indica- 
tion of  the  number  of  single  contractions  making  up  the  tetanus  ; 
indeed,  as  we  shall  see  in  speaking  of  the  sounds  of  the  heart,  a 
single  muscular  contraction  may  produce  a  sound  which  though 
differing  from  the  sound  given  out  during  tetanus  has  to  a  certain 
extent  musical  characters.  Nevertheless  the  special  characters 
of  the  muscle  sound  given  out  by  muscles  in  the  natural  move- 
ments of  the  body  may  be  taken  as  shewing  at  least  that  the 
contractions  of  the  muscle  in  these  movements  are  tetanic  in 
nature,  and  the  similarity  of  the  note  in  all  the  voluntary  efforts  of 
the  body  and  indeed  in  all  movements  carried  out  by  the  central 
nervous  system  is  at  least  consonant  with  the  view  that  the 
repetition  of  single  contractions  is  of  about  the  same  frequency 
in  all  these  movements.  What  that  frequency  is,  and  whether 
it  is  exactly  identical  in  all  these  movements,  has  not  at  present 
been  clearly  determined ;  though  certain  markings  on  the  myro- 
graphic  tracings  of  these  movements  and  other  facts  seem  to 
indicate  that  it  is  about  12  a  second. 

§  76.  Tlie  Influence  of  the  Load.  It  might  be  imagined  that 
a  muscle,  which,  when  loaded  with  a  given  weight,  and  stimulated 
by  a  current  of  a  given  intensity,  had  contracted  to  a  certain 
extent,  would  only  contract  to  half  that  extent  when  loaded  with 
twice  the  weight  and  stimulated  with  the  same  stimulus.  Such 
however  is  not  necessarily  the  case ;  the  height  to  which  the 
weight  is  raised  may  be  in  the  second  instance  as  great,  or  even 
greater,  than  in  tlie  first.  That  is  to  say,  the  resistance  offered 
to  the  contraction  actually  augments  the  contraction,  the  ten- 
sion of  the  muscular  tibre  increases  the  facility  with  which  the 
explosive  changes  resulting  in  a  contraction  take  place.  And  we 
have  other  evidence  that  anything  which  tends  to  stretch  the 
muscular  fibres,  that  any  tension  of  tlie  mu.'^cular  fibres,  whether 
during  rest  or  during  contraction,  increases  the  metabolism  of  the 
muscle.  There  is,  of  course,  a  limit  to  this  favourable  action  of 
the  resistance.  As  the  load  continues  to  be  increased,  the  height 
of  the  contraction  is  dimini.shed,  and  at  last  a  point  is  reached  at 
which  the  muscle  is  unable  (even  when  the  stimulus  chosen  is 
the  strongest  possible)  to  lift  the  load  at  all. 


100 

150 

200 

250 

7 

5 

2 

0 

700 

750 

400 

0 

124  THE   WORK   DONE.  [Book  i. 

In  a  muscle  viewed  as  a  machine  we  have  to  deal  not  merely 
with  the  height  of  the  contraction,  that  is  with  the  amount  of 
shortening,  but  with  the  work  done.  And  this  is  measured  by 
multiplying  the  number  of  units  of  height  to  which  the  load  is 
raised  into  the  number  of  units  of  weight  of  the  load.  Hence  it 
is  obvious  from  the  foregoing  obfervations  that  the  work  done 
must  be  largely  dependent  on  the  weight  itself.  Thus  there  is  a 
certain  weight  of  load  with  which  in  any  given  muscle,  stimu- 
lated by  a  given  stimulus,  the  most  work  will  be  done ;  as  may 
be  seen  from  the  following  example : 

Load,  in  grammes    .  -. 0    50 

Height  of  contractions  in  millimeters  14      9 
Work  done,  in  gram-millimeters  ...     0  450 

§  77.  The  In/licence  of  the  Size  and  Form  of  the  Muscle.  Since 
all  known  muscular  fibres  are  much  shorter  than  the  wave-length 
of  a  contraction,  it  is  obvious  that  the  longer  the  fibre,  the  greater 
will  be  the  shortening  caused  by  the  same  contraction  wave,  the 
greater  will  be  the  height  of  the  contraction  with  the  same 
stimulus.  Hence  in  a  muscle  of  parallel  fibres,  the  height  to 
which  the  load  is  raised  as  the  result  of  a  given  stimulus  applied 
to  its  nerve,  will  depend  on  the  length  of  the  fibres,  while  the 
maximum  weight  of  load  capable  of  being  lifted  will  depend  on 
the  number  of  the  fibres,  since  the  load  is  distributed  among 
them.  Of  two  muscles  therefore  of  equal  length  (and  of  the 
same  quality)  the  most  work  will  be  done  by  that  which  has  the 
larger  number  of  fibres,  that  is  to  say,  the  fibres  being  of  equal 
width,  which  has  the  greater  sectional  area ;  and  of  tw^o  muscles 
with  equal  sectional  areas,  the  most  work  will  be  done  by  that 
which  is  the  longer.  If  the  two  muscles  are  unequal  both  in 
Isngth  and  sectional  area,  the  work  done  will  be  the  greater  in  the 
one  wiiich  has  the  larger  bulk,  which  contains  the  greater  number 
of  cubic  units.  In  speaking  therefore  of  the  work  which  can  be 
done  by  a  muscle,  we  may  use  as  a  standard  a  cubic  unit  of  bulk, 
or,  the  specific  gravity  of  the  muscle  being  the  same,  a  unit  of 
weight. 

We  learn  then  from  the  foregoing  paragraphs  that  the  work 
done,  by  a  muscle-nerve  preparation,  will  depend,  not  only  on  the 
acti\dty  of  the  nerve  and  muscle  as  determined  by  their  own 
irritability,  but  also  on  the  character  and  mode  of  application  of 
the  stimulus,  on  the  kind  of  contraction  (whether  a  single  spasm, 
or  a  slowly  repeated  tetanus  or  a  rapidly  repeated  tetanus)  on  the 
load  itself,  and  on  the  size  and  form  of  the  muscle.  Taking 
the  most  favourable  circumstance i,  viz.  a  well-nourished,  lively 
preparation,  a  maximum  stimulus  causing  a  rapid  tetanus  and  an 
appropriate  load,  we  may  determine  the  maximum  work  done  by 
a  given  weight  of  muscle,  say  one  gramme.  This  in  the  case  of 
the  muscles  of  the  frog  has  been  estimated  at  about  four  gram- 
meters  for  one  gramme  of  muscle. 


SEC.  5.  THE  CIECUMSTANCES  WHICH  DETEEMINE 
THE  DEGREE  OF  IRRITABILITY  OF  MUSCLES  AND 
NERVES. 


§  78.  A  muscle-nerve  preparation,  at  the  time  that  it  is  re- 
moved from  the  body,  possesses  a  certain  degree  of  irritability,  it 
responds  by  a  contraction  of  a  certain  amount  to  a  stimulus  of  a 
certain  strength,  applied  to  the  nerve  or  to  the  muscle.  After  a 
while,  the  exact  period  depending  on  a  variety  of  circumstances, 
the  same  stimulus  produces  a  smaller  contraction,  i.e.  the  irritability 
of  the  preparation  has  diminished.  In  other  words,  the  muscle, 
or  nerve,  or  both,  have  become  partially  '  exhausted  ; '  and  the 
exhaustion  svibsequently  increases,  the  same  stimulus  producing 
smaller  contractions,  until  at  last  all  irritability  is  lost,  no  stimulus 
however  strong  producing  any  contraction,  whether  applied  to  the 
nerve  or  directly  to  the  muscle  ;  and  eventually  the  muscle,  as  we 
have  seen,  becomes  rigid.  The  progress  of  this  exhaustion  is  more 
rapid  in  the  nerves  than  in  the  muscles ;  for  some  time  after  the 
nerve  trunk  has  ceased  to  respond  to  even  the  strongest  stimulus, 
contractions  may  be  obtained  by  applying  the  stimulus  directly  to 
the  muscle.  It  is  much  more  rapid  in  the  warm  blooded  than  in 
the  cold  blooded  animals.  The  muscles  and  nerves  of  the  former 
lose  their  irritability,  when  removed  from  the  body,  after  a  period 
varying  according  to  circumstances  from  a  few  minutes  to  two  or 
three  hours  ;  those  of  cold  blooded  animals  (or  at  least  of  an 
amphibian  or  a  reptile)  may,  under  favourable  conditions,  remain 
irritable  for  two,  three,  or  even  more  days.  The  duration  of 
irritability  in  warm  blooded  animals  may,  however,  be  considerably 
prolonged  by  reducing  the  temperature  of  the  body  before  death. 

If  with  some  thin  body  a  sharp  blow  be  struck  across  a  innscle  which 
has  entered  into  the  later  stages  of  exhaustion,  a  wheal  lasting  for 
several  seconds  is  developed.  This  wheal  appears  to  be  a  contraction 
wave  limited  to  tlie  part  struck,  and  disappearing  very  slowly,  without 
extending  to  the  neighbouring  muscular  substance.     It  has  been  called 


126  DEGENERATION   OF  NERVES.  [Book  i. 

an  '  iclio-muscular  ^  contraction,  because  it  may  be  brought  out  even 
when  ordinary  stimuli  have  ceased  to  produce  any  effect.  It  may  how- 
ever be  accompanied  at  its  beginning  by  an  ordinary  contraction.  It 
is  readily  produced  in  the  living  body  on  the  pectoral  and  other  muscles 
of  persons  suffering  from  phthisis  and  other  exhausting  diseases. 

This  natural  exhaustion  and  diminution  of  irritability  in 
muscles  and  nerves  removed  from  the  body  may  be  modified  both 
in  the  case  of  the  muscle  and  of  the  nerve,  by  a  variety  of  circum- 
stances. Similarly,  while  the  nerve  and  muscle  still  remain  in  the 
body,  the  irritability  of  the  one  or  of  the  other  may  be  modified 
either  in  the  v^ay  of  increase  or  of  decrease  by  certain  general 
influences,  of  v^hich  the  most  important  are,  severance  from  the 
central  nervous  system,  and  variations  in  temperature,  in  blood 
supply,  and  in  functional  activity. 

The  Effects  of  Severance  from  the  Central  Nervous  System. 
"When  a  nerve,  such  for  instance  as  the  sciatic,  is  divided  in 
situ,  in  the  living  body,  there  is  first  of  all  observed  a  slight 
increase  of  irritability,  noticeable  especially  near  the  cut  end  ;  but 
after  a  while  the  irritability  diminishes,  and  gradually  disappears. 
Both  the  slight  initial  increase  and  the  subsequent  decrease  begin 
at  the  cut  end  and  advance  centrifugally  towards  the  peripheral 
terminations.  This  centrifugal  feature  of  the  loss  of  irritability  is 
often  spoken  of  as  the  Ritter-Valli  law.  In  a  mammal  it  may  be 
two  or  three  days,  in  a  frog,  as  many,  or  even  more  weeks,  before 
irritability  has  disappeared  from  the  nerve  trunk.  It  is  maintained 
in  the  small  (and  especially  in  the  intramuscular)  branches  for 
still  longer  periods. 

This  centrifugal  loss  of  irritability  is  the  forerunner  in  the 
peripheral  portion  of  the  divided  nerve  of  structural  changes 
which  proceed  in  a  similar  centrifugal  manner.  In  the  central 
portion  of  the  divided  nerve  these  structural  changes  may  be 
traced  as  far  only  as  the  next  node  of  Eanvier.  Beyond  this 
the  nerve  usually  remains  in  a  normal  condition.  Such  a  degen- 
eration may  be  observed  to  extend  down  to  the  very  endings  of 
the  nerve  in  the  muscle,  including  the  end-plates,  but  does  not 
at  first  affect  the  muscular  substance  itself.  The  muscle,  though 
it  has  lost  all  its  nervous  elements,  still  remains  irritable  towards 
stimuli  applied  directly  to  itself :  an  additional  proof  of  the 
existence  of  an  independent  muscular  irritability. 

If  the  muscle  thus  deprived  of  its  nervous  elements  be  left  to 
itself  its  irritability,  however  tested,  sooner  or  later  diminishes ;  but 
if  the  muscle  be  periodically  thrown  into  contractions  by  artificial 
stimulation  with  the  constant  current,  the  decline  of  irritability 
and  attendant  loss  of  nutritive  power  may  be  postponed  for  some 
considerable  time.  But  so  far  as  our  experience  goes  at  present 
the  artificial  stimulation  cannot  fully  replace  the  natural  one,  and 
sooner  or  later  the  muscle  like  the  nerve  suffers  degeneration,  loses 


Chap,  ii.]  THE   CONTRACTILE   TISSUES.  127 

all  irritability  and  ultimately  its  place  is  taken  by  connective 
tissue. 

For  some  time  the  irritability  of  the  muscle,  as  well  towards 
stimuli  applied  directly  to  itself  as  towards  those  applied  through 
the  impaired  nerve,  seems  to  be  diminished ;  but  after  a  while  a 
peculiar  condition  (to  which  we  have  already  alluded,  §  73)  sets 
in,  in  which  the  muscle  is  found  to  be  not  easily  stimulated  by 
single  induction-shocks  but  to  respond  readily  to  the  make  or 
break  of  a  constant  current.  In  fact  it  is  said  to  become  even 
more  sensitive  to  the  latter  mode  of  stimulation  than  it  was 
when  its  nerve  was  intact  and  functionally  active.  At  the  same 
time  it  also  becomes  more  irritable  towards  direct  mechanical 
stimuli,  and  very  frequently  fibrillar  contractions,  more  or  less 
rhythmic  and  apparently  of  spontaneous  origin,  though  their 
causation  is  obscure,  make  their  appearance.  This  phase  of 
heightened  sensitiveness  of  a  muscle,  especially  to  the  constant 
current,  appears  to  reach  its  maximum,  in  man  at  about  the 
seventh  week  after  nervous  impulses  have  ceased,  owing  to  injury 
to  the  nerves  or  nervous  centre,  to  reach  the  muscle. 

§  79.  The  infiucnce  of  temperature.  AVe  have  already  seen 
that  sudden  heat  (and  the  same  might  be  said  of  cold  when 
sufficiently  intense),  applied  to  a  limited  part  of  a  nerve  or  muscle, 
as  when  the  nerve  or  muscle  is  touched  with  a  hot  wire,  will ' 
act  as  a  stimulus.  It  is  however  much  more  difficult  to  gene- 
rate nervous  or  muscular  impulses  by  exposing  a  whole  motor 
nerve  ^  or  muscle  to  a  gradual  rise  of  temperature. 

A  muscle  may  be  gradually  cooled  to  0°  C.  or  below  without 
any  contraction  being  caused ;  but  when  it  is  heated  to  a  limit, 
which  in  the  case  of  frog's  muscles  is  about  45",  of  mammalian 
muscles  about  50°,  a  sudden  change  takes  place :  the  muscle  falls, 
at  the  limiting  temperature,  into  a  rigor  mortis,  which  is  initiated 
by  a  forciljle  contraction  or  at  least  shortening. 

Moderate  warmth,  e.  g.  in  the  frog  an  increase  of  temperature 
up  to  somewhat  below  45°  C,  favours  both  muscular  and  nervous 
irritability.  All  the  molecular  processes  are  hastened  and  facili- 
tated :  the  contraction  is  for  a  given  stimulus  greater  and  more 
rapid,  i.  e.  of  shorter  duration,  and  nervous  impulses  are  generated 
more  readily  by  slight  stimuli.  Owing  to  the  quickening  of  the 
chemical  changes,  the  supply  of  new  material  may  prove  insuffi- 
cient; hence  muscles  and  nerves  removed  from  the  body  lose  their 
irrital;)ility  more  rapidly  at  a  high  than  at  a  low  temperature. 

The  gradual  application  of  cold  to  a  nerve  produces  effects 
which  differ  according  to  the  kind  of  stimulus  employed  in  testing 
the  condition  of  the  nerve  ;  but  it  may  be  stated  in  general  that  a 
low  temperature,  especially  one  near  to  0°,  slackens  all  the  mole- 
cular processes,  so  that  the  wave  of  nervous  impulse  is  lessened 
and  prolonged,  the  velocity  of  its  passage  being  much  diminished, 

*  The  aftion  of  cold  and  licat  on  sensory  nerves  will  be  considered  in  the  later 
portion  of  tlie  work. 


128  INFLUENCE   OF  ACTIVITY.  [Book  i. 

e.  g.  from  28  metres  to  1  metre  per  sec.     At  about  0°,  the  irrita- 
bility of  the  nerve  disappears  altogether. 

When  a  muscle  is  exposed  to  similar  cold,  e.  g.  to  a  tempera- 
ture very  little  above  zero,  the  contractions  are  remarkably  pro- 
longed ;  they  are  diminished  in  height  at  the  same  time,  but  not 
in  proportion  to  the  increase  of  their  duration.  Exposed  to  a 
temperature  of  zero  or  below,  muscles  soon  lose  their  irritability, 
without  however  uudergoing  rigor  mortis. 

§  80.  The  influence  of  hlood  supply.  When  a  muscle  still 
within  the  body  is  deprived  by  any  means  of  its  proper  blood 
supply,  as  when  the  blood  vessels  going  to  it  are  ligatured,  the 
same  gradual  loss  of  irritability  and  final  appearance  of  rigor 
mortis  are  observed  as  in  muscles  removed  from  the  body.  Thus 
if  the  abdominal  aorta  be  ligatured,  the  muscles  of  the  lower 
limbs  lose  their  irritability  and  finally  become  rigid.  So  also  in 
systemic  death,  when  the  blood  supply  to  the  muscles  is  cut  off  by 
the  cessation  of  the  circulation,  loss  of  irritability  ensues,  and  rigor 
mortis  eventually  follows.  In  a  human  corpse  the  muscles  of  the 
body  enter  into  rigor  mortis  in  a  fixed  order :  first  those  of  the  jaw 
and  neck,  then  those  of  the  trunk,  next  those  of  the  arms,  and 
lastly  those  of  the  legs.  The  rapidity  with  which  rigor  mortis 
comes  on  after  death  varies  considerably,  being  determined  both  by 
external  circumstances  and  by  the  internal  conditions  of  the  body. 
Thus  external  warmth  hastens  and  cold  retards  the  onset.  After 
great  muscular  exertion,  as  in  hunted  animals,  and  when  death 
closes  wasting  diseases,  rigor  mortis  in  most  cases  comes  on  rapidly. 
As  a  general  rule  it  may  be  said  that  the  later  it  is  in  making  its 
appearance,  the  more  pronounced  it  is,  and  the  longer  it  lasts  ;  but 
there  are  many  exceptions,  and  when  the  state  is  recognised  as 
being  fundamentally  due  to  a  clotting  of  the  muscle  substance,  it 
is  easy  to  understand  that  the  amount  of  rigidity,  i.e.  the  amount 
of  the  clot,  and  the  rapidity  of  the  onset,  i.  e.  the  quickness  with 
which  clotting  takes  place,  may  vary  independently.  When 
rigor  mortis  has  once  become  thoroughly  established  in  a  muscle 
through  deprivation  of  blood,  it  cannot  be  removed  by  any  sub- 
sequent supply  of  blood.  Mere  loss  of  irritability,  even  though 
complete,  if  stopping  short  of  the  actual  clotting  of  the  muscle 
substance,  may  be  with  care  removed. 

The  influence  of  blood  supply  cannot  be  so  satisfactorily  studied 
in  the  case  of  nerves  as  in  the  case  of  muscles ;  there  can  however 
be  little  doubt  that  the  effects  are  analogous. 

§  81.  The  influence  of  functional  activity.  This  too  is  more 
easily  studied  in  the  case  of  muscles  than  of  nerves. 

When  a  muscle  within  the  body  is  unused,  it  wastes ;  when 
used,  it  (within  certain  limits)  grows.  Both  these  facts  shew  that 
the  nutrition  of  a  muscle  is  favourably  affected  by  its  functional 
activity.  Part  of  this  may  be  an  indirect  effect  of  the  increased 
blood  supply  which  occurs  when  a  muscle  contracts.     When  a 


Chap,  ii.]  THE   CONTRACTILE   TISSUES.  129 

nerve  going  to  a  muscle  is  stimulated,  the  blood  vessels  of  the 
muscle  dilate.  Hence  at  the  time  of  the  contraction  more  blood 
flows  through  the  muscle,  and  this  increased  flow  continues  for 
some  little  while  after  the  contraction  of  the  muscle  has  ceased. 
But,  apart  from  the  blood  supply,  it  is  probable  that  the  ex- 
haustion caused  by  a  contraction  is  immediately  followed  by  a 
reaction  favourable  to  the  nutrition  of  the  muscle ;  and  this  is  a 
reason,  possibly  the  chief  reason,  why  a  muscle  is  increased  by  use, 
that  is  to  say,  the  loss  of  substance  and  energy  caused  by  the  con- 
traction is  subsequently  more  than  made  up  for  by  increased  met- 
abolism during  the  following  period  of  rest. 

A  muscle,  even  within  the  body,  after  prolonged  action  is 
fatigued,  i.  e.  a  stronger  stimulus  is  required  to  produce  the  same 
contraction ;  in  other  words,  its  irritability  may  be  lessened  by 
functional  activity.  Whether  functional  activity  therefore  is  in- 
jurious or  beneficial  depends  on  its  amount  in  relation  to  the 
condition  of  the  muscle. 

It  is  worthy  of  notice  that  a  motor  nerve  is  far  less  susceptible 
of  being  fatigued  by  artificial  stimulation  than  is  a  muscle ;  in 
fact  it  seems  extremely  difficult  to  tire  a  nerve  by  mere  stimula- 
tion. In  an  animal  poisoned  by  urari  the  sciatic  nerve  may  be 
stimulated  continuously  with  powerful  currents  for  even  several 
hours  and  yet  remain  irritable.  So  long  as  the  urari  is  produc- 
ing its  usual  effect,  the  muscles  sheltered  by  it  are  not  thrown 
into  contraction  by  the  stimulation  of  the  nerve  and  so  are  not 
fatigued ;  as  the  effect  of  the  urari  passes  off,  contractions  make 
their  appearance  in  response  to  the  stimulation  of  the  sciatic 
nerve,  shewing  that  this,  in  spite  of  its  having  been  stimulated 
for  so  long  a  time,  has  not  been  exhausted.  And  other  experi- 
ments point  to  a  similar  conclusion.  It  would  seem  that  the 
molecular  processes  constituting  a  nervous  impulse  unlike  those 
constituting  a  muscular  contraction,  are  of  such  a  nature  or  take 
place  in  such  a  way,  that  after  the  development  of  one  impulse 
the  substance  of  the  nerve  fibre  is  at  once  ready  for  the  develop- 
ment of  a  second  impulse. 

The  sense  of  fatigue  of  which,  after  prolonged  or  unusual  exer- 
tion, we  are  conscious  in  our  own  bodies,  is  probably  of  complex 
origin,  and  its  nature,  like  that  of  the  normal  muscular  sense  of 
which  we  shall  have  to  speak  hereafter,  is  at  present  not  thoroughly 
understood.  It  seems  to  be  in  the  first  place  the  result  of  changes 
in  the  muscles  themselves,  but  is  possibly  also  caused  by  changes  in 
the  nervous  apparatus  concerned  in  muscular  action,  and  especially 
in  those  parts  of  the  central  nervous  system  which  are  concerned 
in  the  production  of  voluntary  impulses.  In  any  case  it  cannot  be 
taken  as  an  adequate  measure  of  the  actual  fatigue  of  the  muscles  ; 
for  a  man  who  says  he  is  absolutely  exhausted  may  under  excite- 
ment perform  a  very  large  amount  of  work  with  his  already  weary 
muscles.  The  will  in  fact  rarely  if  ever  calls  forth  the  greatest 
contractions  of  which  the  muscles  are  capable. 


130  CAUSES   OF  EXHAUSTION.  [Book  ii. 

Absolute  (temporary)  exhaustion  of  the  muscles,  so  that  the 
strongest  stimuli  produce  no  contraction,  may  be  produced  even 
within  the  body  by  artificial  stimulation :  recovery  takes  place 
on  rest.  Out  of  the  body  absolute  exhaustion  takes  place  readily. 
Here  also  recovery  may  take  place.  Whether  in  any  given  case  it 
does  occur  or  not,  is  determined  by  the  amount  of  contraction 
causing  the  exhaustion,  and  by  the  previous  condition  of  the 
muscle.  In  all  cases  recovery  is  hastened  by  renewal  (natural  or 
artificial)  of  the  blood  stream. 

The  more  rapidly  the  contractions  follow  each  other,  the  less 
the  interval  between  any  two  contractions,  the  more  rapid  the 
exhaustion.  A  certain  number  of  single  induction-shocks  repeated 
rapidly,  say  every  second  or  oftener,  bring  about  exhaustive  loss 
of  irritability  more  rapidly  than  the  same  number  of  shocks 
repeated  less  rapidly,  for  instance  every  5  or  10  seconds.  Hence 
tetanus  is  a  ready  means  of  producing  exhaustion. 

In  exhausted  muscles  the  elasticity  is  much  diminished ;  the 
tired  muscle  returns  less  readily  to  its  natural  length  than  does 
the  fresh  one. 

The  exhaustion  due  to  contraction  may  be  the  result :  —  Either 
of  the  consumption  of  the  store  of  really  contractile  material 
present  in  the  muscle.  Or  of  the  accumulation  in  the  tissue 
of  the  products  of  the  act  of  contraction.  Or  of  both  of  these 
causes. 

The  restorative  influence  of  rest,  in  the  case  of  a  muscle 
removed  from  the  circulation,  may  be  explained  by  supposing  that 
during  the  repose,  either  the  internal  changes  of  the  tissue 
manufacture  new  explosive  material  out  of  the  comparatively  raw 
material  already  present  in  the  fibres,  or  the  directly  hurtful  pro- 
ducts of  the  act  of  contraction  undergo  changes  by  which  they  are 
converted  into  comparatively  inert  bodies.  A  stream  of  fresh 
blood  may  exert  its  restorative  influence  not  only  by  quickening 
the  above  two  events,  but  also  by  carrying  off  the  immediate  waste 
products  while  at  the  same  time  it  brings  new  raw  material.  It  is 
not  known  to  what  extent  each  of  these  parts  is  played.  That  the 
products  of  contraction  are  exhausting  in  their  effects,  is  shewn  by 
the  facts  that  the  injection  of  a  solution  of  the  muscle-extractives 
into  the  vessels  of  a  muscle  produces  exhaustion,  and  that  exhausted 
muscles  are  recovered  by  the  simple  injection  of  inert  saline  solu- 
tions into  their  blood  vessels.  But  the  matter  has  not  yet  been 
fully  worked  out. 

One  important  element  brought  by  fresh  blood  is  oxygen.  This, 
as  we  have  seen,  is  not  necessary  for  the  carrying  out  of  the  actual 
contraction,  and  yet  is  essential  to  the  maintenance  of  irritability. 
The  oxygen  absorbed  by  the  muscle  apparently  enters  in  some 
peculiar  way  into  the  formation  of  that  complex  explosive  material 
the  decomposition  of  which  in  the  act  of  contraction,  though  it 
gives  rise  to  carbonic  acid  and  other  products  of  oxidation,  is  not 
in  itself  a  process  of  direct  oxidation. 


SEC.  6.     ON  SOME  OTHER  FORMS   OF   CONTRACTILE 

TISSUE. 


Plain,  Smooth   or    Unstriated   Muscular    Tissue. 

§  82.  This,  in  vertebrates  at  all  events,  rarely  occurs  in  isolated 
masses  or  muscles,  as  does  striated  muscular  tissue,  but  is  usually- 
found  taking  part  in  the  structure  of  complex  organs,  such  for 
instance  as  the  intestines  ;  hence  the  investigation  of  its  properties 
is  beset  with  many  difficulties. 

§  83.  So  far  as  we  know  plain  muscular  tissue  in  its  chemical 
features  resembles  striated  muscular  tissue.  It  contains  albumin, 
some  forms  of  globulin,  and  antecedents  of  myosin  which  upon  the 
death  of  the  fibres  become  myosin ;  for  plain  muscular  tissue  after 
death  becomes  rigid,  losing  its  extensibility  and  probably  becoming 
acid,  though  the  acidity  is  not  so  marked  as  in  striated  muscle. 
Kreatin  has  also  been  found,  as  well  as  glycogen,  and  indeed  it 
seems  probable  that  the  whole  metabolism  of  plain  muscular 
tissue  is  fundamentally  the  same  as  that  of  the  striated  muscles. 

§  84.  In  their  general  physical  features  plain  muscular  fibres 
also  resemble  striated  fibres,  and  like  them  they  are  irritable  and 
contractile  ;  when  stimulated  they  contract.  The  fibres  vary  in 
natural  length  in  different  situations,  those  of  the  blood  vessels  for 
instance  being  shorter  and  stouter  than  those  of  the  intestine ;  but 
in  the  same  situation  the  fibres  may  also  be  found  in  one  of  two 
different  conditions.  In  the  one  case  the  fibres  are  long  and  thin, 
in  the  other  case  they  are  reduced  in  length,  it  may  be  to  one  half 
or  even  to  one  third,  and  are  correspondingly  thicker,  broader 
and  less  pointed  at  the  ends,  their  total  bulk  remaining  unaltered. 
In  the  former  case  they  are  relaxed  or  elongated,  in  the  latter  case 
they  are  contracted. 

The  facts  of  the  contraction  of  plain  muscular  tissue  may  be 
studied  in  the  intestine,  the  muscular  coat  of  which  consists  of  an 
outer  thin  sheet  composed  of  fibres  and  bundles  of  fibres  disposed 
longitudinally  and  of  an  inner  much  thicker  sheet  of  fibres  disposed 
circularly  ;  in  the  ureter  a  similar  arrangement  of  two  coats  obtain.^. 

If  a  mechanical  or  electrical  (or  indeed  any  other)  stinmlus  be 


132  CONTRACTION   OF   PLAIN   MUSCLES.      [Book  i. 

brought  to  bear  on  a  part  of  a  fresh  living  still  warm  intestine  (the 
small  intestine  is  the  best  to  work  with)  a  circular  contraction  is 
seen  to  take  place  at  the  spot  stimulated ;  the  intestine  seems 
nipped  in  ringwise,  as  if  tied  round  with  an  invisible  cord ;  and  the 
part  so  constricted,  previously  vascular  and  red,  becomes  pale  and 
bloodless.  The  individual  fibres  of  the  circular  coat  m  the  region 
stimulated  have  each  become  shorter,  and  the  total  effect  of  the 
shortening  of  the  multitude  of  fibres  all  having  the  same  circular 
disposition  is  to  constrict  or  narrow  the  lumen  or  tube  of  the  in- 
testine. The  longitudinally  disposed  fibres  of  the  outer  longitudinal 
coat  in  a  similar  manner  contract  or  shorten  in  a  longitudinal 
direction,  but  this  coat  being  relatively  much  thinner  than  the 
circular  coat,  the  longitudinal  contraction  is  altogether  over- 
shadowed by  the  circular  contraction.  A  similar  mode  of  contrac- 
tion is  also  seen  when  the  ureter  is  similarly  stimulated. 

The  contraction  thus  induced  is  preceded  by  a  very  long  latent 
period  and  lasts  a  very  considerable  time,  in  fact  several  seconds, 
after  which  relaxation  slowly  takes  place.  We  may  say  then  that 
over  the  circularly  dispersed  fibres  of  the  intestine  (or  ureter)  at 
the  spot  in  question  there  has  passed  a  contraction-wave  remarkable 
for  its  long  latent  period  and  for  the  slowness  of  its  development, 
the  wave  being  propagated  from  fibre  to  fibre.  From  the  spot  so 
directly  stimulated,  the  contraction  may  pass  also  as  a  wave  (with 
a  length  of  1  cm.  and  a  velocity  of  from  20  to  30  millimetres  a 
second  in  the  ureter),  along  the  circular  coat  both  upwards  and 
downwards.  The  longitudinal  fibres  at  the  spot  stimulated  are  as 
we  have  said  also  thrown  into  contractions  of  altogether  similar 
character,  and  a  wave  of  contraction  may  thus  also  travel  longitudi- 
nally along  the  longitudinal  coat  both  upwards  and  downwards. 
It  is  evident  however  that  the  wave  of  contraction  of  which  we  are 
now  speaking  is  in  one  respect  different  from  the  wave  of  contrac- 
tion treated  of  in  dealing  with  striated  muscle.  Tn  the  latter  case 
the  contraction-wave  is  a  simple  wave  propagated  along  the  in- 
dividual fibre  and  starting  from  the  end-plate  or,  in  the  case  of 
direct  stimulation,  from  the  part  of  the  fibre  first  affected  by  the 
stimulus ;  we  have  no  evidence  that  the  contraction  of  one  fibre 
can  communicate  contraction  to  neighbouring  fibres  or  indeed  in 
any  way  influence  neighbouring  fibres.  In  the  case  of  the  intestine 
or  ureter,  the  wave  is  complex,  being  the  sum  of  the  contraction- 
waves  of  several  fibres  engaged  in  different  phases  and  is  propagated 
from  fibre  to  fibre,  both  in  the  direction  of  the  fibres,  as  when  the 
whole  circumference  of  the  intestine  is  engaged  in  the  contraction, 
or  when  the  wave  travels  longitudinally  along  the  longitudinal  coat, 
and  also  in  a  direction  at  right  angles  to  the  axes  of  the  fibres,  as 
when  the  contraction-wave  travels  lengthways  along  the  circular 
coat  of  the  intestine,  or  when  it  passes  across  a  breadth  of  the 
longitudinal  coat ;  that  is  to  say,  the  changes  leading  to  contraction 
are  communicated  not  only  in  a  direct  manner  across  the  cement 


Chap,  ii.]  THE   CONTRACTILE   TISSUES.  133 

substance  uniting  the  fibres  of  a  bundle  but  also  in  an  indirect 
manner,  probably  by  means  of  nerve  fibres,  from  bundle  to  bundle 
across  the  connective  tissue  between  them.  Moreover,  it  is  obvious 
that  even  the  contraction-wave  which  passes  along  a  single  un- 
striated  fibre  differs  from  that  passing  along  a  striated  fibre,  in 
the  very  great  length  both  of  its  latent  period  and  of  the  duration 
of  its  contraction.  Hence,  much  more  even  than  in  the  case  of  a 
striated  muscle,  the  whole  of  each  fibre  must  be  occupied  by  the 
contraction-wave,  and  indeed  be  in  nearly  the  same  phase  of  the 
contraction  at  the  same  time. 

Waves  of  contraction  thus  passing  along  the  circular  and  longi- 
tudinal coats  of  the  intestine  constitute  what  is  called  peristaltic 
action. 

Like  the  contractions  of  striated  muscle  the  contractions  of 
plain  muscles  may  be  started  by  stimulation  of  nerves  going 
to  the  part,  the  nerves  supplying  plain  muscular  tissue,  running 
for  the  most  part  as  we  have  said  in  the  so-called  sympathetic 
system,  but  being  as  we  shall  see  ultimately  connected  with 
the  spinal  cord  or  brain.  Here  however  we  come  upon  an  im- 
portant distinction  between  the  striated  skeletal  muscles,  and 
the  plain  muscles  of  the  viscera.  As  a  general  rule  the  skeletal 
muscles  are  thrown  into  contraction  only  by  nervous  impulses 
reaching  them  along  their  nerves ;  spontaneous  movements  of 
the  skeletal  muscles,  that  is  contractions  arising  out  of  changes 
in  the  muscles  themselves  are  extremely  rare,  and  when  they 
occur  are  abnormal ;  so-called  '  cramps '  for  instance,  which  are 
prolonged  tetanic  contractions  of  skeletal  muscles  independent  of 
the  will,  though  their  occurrence  is  largely  due  to  the  condition  of 
the  muscle  itself,  generally  the  result  of  overwork,  are  probably 
actually  started  by  nervous  impulses  reaching  them  from  without. 
On  the  other  hand  the  plain  muscles  of  the  viscera,  of  the  intestine, 
uterus  and  ureter,  for  instance,  and  of  the  blood  vessels  very  fre- 
quently fall  into  contractions  and  so  carry  out  movements  of  the 
organs  to  which  they  belong  quite  independently  of  the  central 
nervous  system.  These  organs  exhibit  '  spontaneous  '  movements 
quite  apart  from  the  will,  quite  apart  from  the  central  nervous 
system,  and  under  favourable  circumstances  continue  to  do  this  for 
some  time  after  they  have  been  entirely  isolated  and  removed  from 
the  body.  So  slight  indeed  is  the  connection  between  the  move- 
ments of  organs  and  parts  supplied  with  plain  muscular  fibres,  and 
the  will,  that  these  muscular  fibres  have  sometimes  been  called 
involuntary  muscles  ;  but  this  name  is  undesirable  since  some 
muscles  consisting  entirely  of  plain  muscular  fibres  (e.  g.  the  ciliary 
muscles  by  which  the  eye  is  accommodated  for  viewing  objects  at 
different  distances)  are  directly  under  the  infiuence  of  the  will, 
and  some  mxiscles  composed  of  striated  fibres  {c.  g.  those  of  the 
heart)  are  wholly  removed  from  the  influence  of  the  will. 

We  shall  best  studv  however  the  facts  relatinij  to  the  move- 


134  CILIARY  MOVEMENT.  [Book  i. 

ments  of  parts  provided  with  plain  muscular  fibres  when  we  come 
to  consider  the  parts  themselves. 

Like  the  skeletal  muscles,  whose  nervous  elements  have  been 
rendered  functionally  incapable  (§  73),  plain  muscles  are  much 
more  sensitive  to  the  making  and  breaking  of  a  constant  current 
than  to  induction  shocks ;  a  current,  when  very  brief,  like  that  of 
an  induction-shock,  produces  little  or  no  effect. 

The  contraction  of  plain  muscular  fibres  is  as  we  said  very  slow 
in  its  development  and  very  long  in  its  duration,  even  when  started 
by  a  momentary  stimulus,  such  as  a  single  induction-shock.  The 
contraction  after  a  stimulation  often  lasts  so  long  as  to  raise  the 
question,  whether  what  has  been  produced  is  not  a  single  contrac- 
tion but  a  tetanus.  Tetanus,  however,  that  is  the  fusion  of  a  series 
of  contractions,  seems  to  be  of  rare  occurrence,  though  probably  it 
may  be  induced,  in  plain  muscular  tissue ;  but  some  of  the  ends 
of  tetanus  are  gained  by  a  kind  of  contraction  which,  not  prom- 
inent in  skeletal  muscle,  becomes  of  great  importance  in  plain 
muscular  tissue,  by  a  kind  of  contraction  called  a  tonic  contraction. 
The  subject  is  one  not  without  difficulties,  but  it  would  appear  that 
a  plain  muscular  fibre  may  remain  for  a  very  considerable  time  in 
a  state  of  contraction,  the  amount  of  shortening  thus  maintained 
being  either  small  or  great :  it  is  then  said  to  be  in  a  state  of 
tonic  contraction.  This  is  especially  seen  in  the  case  of  the  plain 
muscular  tissue  of  the  arteries,  and  we  shall  have  to  return  to  this 
matter  in  dealing  with  the  circulation. 

The  muscular  tissue  which  enters  into  the  construction  of  the 
heart  is  of  a  peculiar  nature,  being  on  the  one  hand  striated,  and 
on  the  other  in  some  respects  similar  to  plain  muscular  tissue,  but 
this  we  shall  consider  in  dealing  with  the  heart  itself. 


Ciliary   Movement. 

§  85.  Nearly  all  the  movements  of  the  body  which  are  not  due 
to  physical  causes,  such  as  gravity,  the  diffusion  of  liquids  &c.,  are 
carried  out  by  muscles,  either  striated  or  plain ;  but  some  small 
and  yet  important  effects  in  the  way  of  movement  are  produced 
by  the  action  of  cilia,  and  by  those  changes  of  form  which  are 
called  amoeboid.  Cilia  are  generally  appendages  of  epithelial 
cells. 

§  86.  Ciliary  action,  in  the  form  in  which  it  is  most  common 
in  mammals  and  indeed  vertebrates,  consists  in  the  cilium  {i.  e.  the 
tapering  filament  spoken  of  above)  being  at  one  moment  straight 
or  vertical,  at  the  next  moment  being  bent  down  suddenly  into  a 
hook  or  sickle  form,  and  then  more  slowly  returning  to  the  straight 
erect  position.  When  the  cilia  are  vigorous,  this  double  move- 
ment is  repeated  with  very  great  rapidity,  so  rapidly  that  the 
individual  movements  cannot  be  seen ;  it  is  only  when,  by  reason 


Chap,  ii.]  THE   CONTRACTILE   TISSUES.  135 

of  fatigue,  the  action  becomes  slow  that  the  movement  itself  can 
be  seen ;  what  is  seen  otherwise  is  simply  the  effect  of  the 
movement.  The  movements  when  slow  have  been  counted  at 
about  eight  (double  movements)  in  a  second;  probably  when 
vigorous  they  are  repeated  from  twelve  to  twenty  times  a  second. 

The  flexion  takes  place  in  one  direction  only,  and  all  the  cilia 
of  each  cell,  and  indeed  of  all  the  cells  of  the  same  epithelium 
move  in  the  same  direction.  Moreover  the  same  direction  is 
maintained  during  the  whole  life  of  the  epithelium ;  thus  the  cilia 
of  the  epithelium  of  the  trachea  and  bronchial  passages  move 
during  the  whole  of  life  in  such  a  way  as  to  drive  the  fluid  lying 
upon  them  upwards  towards  the  mouth  ;  so  far  as  we  know  in 
vertebrates,  or  at  least  in  mammals,  the  direction  is  not  and  cannot 
by  any  means  be  reversed. 

The  flexion  is  very  rapid  but  the  return  to  the  erect  position 
is  much  slower ;  hence  the  total  effect  of  the  blow,  supposing  the 
cilium  and  the  cell  to  be  fixed,  is  to  drive  the  thin  layer  of  fluid  in 
which  the  cilium  is  working,  and  which  always  exists  over  the 
epithelium,  and  any  particles  which  may  be  floating  in  that  fluid, 
in  the  same  direction  as  that  in  which  the  blow  is  given.  If  the 
cell  be  not  attached  but  floating  free  the  effect  of  the  blow  may 
be  to  drive  the  cell  itself  backward ;  and  when  perfectly  fresh 
ciliated  epithelium  is  teased  out  and  examined  in  an  inert  fluid 
such  as  normal  saline  solution,  isolated  cells  or  small  groups  of 
cells  may  be  seen  rowing  themselves  about  as  it  were  by  the 
action  of  their  cilia. 

All  the  cilia  of  a  cell  move,  as  we  have  just  said,  in  the  same 
direction,  but  not  quite  at  the  same  time.  If  we  call  the  side  of 
the  cell  towards  which  the  cilia  bend  the  front  of  the  cell  and  the 
opposite  side  the  back,  the  cilia  at  the  back  move  a  trifle  before 
those  at  the  front  so  that  the  movement  runs  over  the  cell  in  the 
direction  of  the  movement  itself.  Similarly,  taking  any  one  cell, 
the  cilia  of  the  cells  behind  it  move  slightly  before,  and  the  cilia 
of  the  cells  in  front  of  it  slightly  after,  its  own  cilia  move.  Hence 
in  this  way  along  a  whole  stretch  of  epithelium  the  movement  or 
bending  of  the  cilia  sweeps  over  the  surface  in  ripples  or  waves, 
very  much  as,  when  the  wind  blows,  similar  waves  of  bending 
sweep  over  a  field  of  corn  or  tall  grass.  By  this  arrangement  the 
efficacy  of  the  movement  is  secured,  and  a  steady  stream  of  fluid 
carrying  particles  is  driven  over  the  surface  in  a  uniform  continued 
direction ;  if  the  cilia  of  separate  cells,  and  still  more  if  the 
separate  cilia  of  each  cell,  moved  independently  of  the  others,  all 
that  would  be  produced  would  be  a  series  of  minute  '  wobbles,'  of 
as  little  use  for  driving  the  fluid  definitely  onwards  as  the  efforts 
of  a  boat's  crew  all  rowing  out  of  time  are  for  propelling  the  boat. 

Swift  bending  and  slower  straightening  is  the  form  of  ciliary 
movement  generally  met  with  in  the  ciliated  epithelium  of  mam- 
mals and  indeed  of  vertebrates ;  but  among  the  invertebrates  we 


136  CILIARY  MOVEMENT.  [Book  i. 

find  other  kinds  of  movement,  suchi  as  a  to  and  fro  movement, 
equally  rapid  in  both  directions,  a  cork-screw  movement,  a  simple 
undulatory  movement,  and  many  others.  In  each  case  the  kind  of 
movement  seems  adapted  to  secure  a  special  end.  Thus  even  in 
the  mammal  while  the  one-sided  blow  of  the  cilia  of  the  epithelial 
cells  secures  a  flow  of  fluid  over  the  epithelium,  the  tail  of  the 
spermatozoon,  which  is  practically  a  single  cilium,  by  moving  to 
and  fro  m  an  undulatory  fashion  drives  the  head  of  the  sperma- 
tozoon onwards  in  a  straight  line,  like  a  boat  driven  by  a  single 
oar  worked  at  the  stern. 

Why  and  exactly  how  the  cilium  of  the  epithelial  cells  bends 
swiftly  and  straightens  slowly,  always  acting  in  the  same  direction, 
is  a  problem  difficult  at  present  to  answer  fully.  Some  have  thought 
that  the  body  of  the  cell  is  contractile,  or  contains  contractile 
mechanisms  pulling  upon  the  cilia,  which  are  thus  simple  passive 
puppets  in  the  hands  of  the  cells.  But  there  is  no  satisfactory 
evidence  for  such  a  view.  On  the  whole  the  evidence  is  in  favour 
of  the  view  that  the  action  is  carried  out  by  the  cilium  itself,  that 
the  bending  is  a  contraction  of  the  cilium,  and  that  the  straight- 
ening corresponds  to  the  relaxation  of  a  muscular  fibre.  But  even 
then  the  exact  manner  in  which  the  contraction  bends  and  the 
relaxation  straightens  the  filament  is  not  fully  explained.  We 
have  no  positive  evidence  that  a  longitudinal  half,  the  inside  we 
might  say,  of  the  filament  is  contractile,  and  the  other  half,  the 
outside,  elastic,  a  supposition  which  has  been  made  to  explain  the 
bending  and  straightening.  In  fact  no  adequate  explanation  of 
the  matter  has  as  yet  been  given,  and  it  is  really  only  on  general 
grounds  we  conclude  that  the  action  is  an  effect  of  contractility. 

In  the  vertebrate  animal,  cilia  are,  so  far  as  we  know,  wholly 
independent  of  the  nervous  system,  and  their  movement  is  prob- 
ably ceaseless.  In  such  animals  however  as  Infusoria,  Hydrozoa, 
&c.  the  movements  in  a  ciliary  tract  may  often  be  seen  to  stop  and 
to  go  on  again,  to  be  now  fast  now  slow,  according  to  the  needs 
of  the  economy,  and,  as  it  almost  seems,  according  to  the  will  of 
the  creature ;  indeed  in  some  of  these  animals  the  ciliary  move- 
ments are  clearly  under  the  influence  of  the  nervous  system. 

Observations  with  galvanic  currents,  constant  and  interrupted, 
have  not  led  to  any  satisfactory  results,  and,  so  far  as  we  know  at 
present,  ciliary  action  is  most  affected  by  changes  of  temperature 
and  chemical  media.  Moderate  heat  quickens  the  movements,  but 
a  rise  of  temperature  beyond  a  certain  limit  (about  40°C.  in  the  case 
of  the  pharyngeal  membrane  of  the  frog)  becomes  injurious ;  cold 
retards.  Very  dilute  alkalis  are  favourable,  acids  are  injurious. 
An  excess  of  carbonic  acid  or  an  absence  of  oxygen  diminishes  or 
arrests  the  movements,  either  temporarily  or  permanently,  accord- 
ing to  the  length  of  the  exposure.  Chloroform  or  ether  in  slight 
doses  diminishes  or  suspends  the  action  temporarily,  in  excess 
kills  and  disorganises  the  cells. 


Chap.  ii.J  THE   CONTRACTILE   TISSUES.  137 


Amcehoid  Movements. 

§  87.  The  white  blood  corpuscles,  as  we  have  said  (§  28),  are 
able  of  themselves  to  change  their  form  and  by  repeated  changes 
of  form  to  move  from  place  to  place.  Such  movements  of  the 
substance  of  the  corpuscles  are  called  amoeboid,  since  they  closely 
resemble  and  appear  to  be  identical  in  nature  with  the  movements 
executed  by  the  amoeba  and  similar  organisms.  The  movement 
of  the  endoplasm  of  the  vegetable  cell  seems  also  to  be  of  the 
same  kind. 

The  amoeba  changes  its  form  (and  shifts  its  place)  by  throw- 
ing out  projections  of  its  substance,  called  pseudopodia,  which 
may  be  blunt  and  short,  broad  bulgings  as  it  were,  or  may  be  so 
long  and  thin  as  to  be  mere  filaments,  or  may  be  of  an  intermedi- 
ate character.  As  we  watch  the  outline  of  the  hyaline  ectosarc 
we  may  see  a  pseudopodium  beginning  by  a  slight  bulging  of  the 
outline ;  the  bulging  increases  by  the  neighbouring  portions  of 
the  ectosarc  moving  into  it,  the  movement  under  the  microscope 
reminding  one  of  the  flowing  of  melted  glass.  As  the  pseudo- 
podium grows  larger  and  engages  the  whole  thickness  of  the 
ectosarc  at  the  spot,  the  granules  of  the  endosarc  may  be  seen 
streaming  into  it  forming  a  core  of  endosarc  in  the  middle  of  the 
bulging  of  ectosarc.  The  pseudopodium  may  continue  to  grow 
larger  and  larger  at  the  expense  of  the  rest  of  the  body,  and 
eventually  the  whole  of  the  amoeba  including  the  nucleus  may  as 
it  were  have  passed  into  the  pseudopodium  ;  the  body  of  the 
amoeba  will  now  occupy  the  place  of  the  pseudopodium  instead  of 
its  old  place  ;  in  other  words  it  will  in  changing  its  form  have  also 
changed  its  place. 

During  all  these  movements,  and  during  all  similar  amoeboid 
movements,  the  bulk  of  the  organism  will,  so  far  as  can  be 
ascertained,  have  remained  unchanged ;  the  throwing  out  a  pseu- 
dopodium in  one  direction  is  accompanied  by  a  corresponding 
retraction  of  the  body  in  other  directions  ;  if  as  sometimes  happens 
the  organism  throws  out  pseudopodia  in  various  directions  at  the 
same  time,  the  main  body  from  which  the  pseudopodia  project  is 
reduced  in  thickness ;  from  being  a  spherical  lump  for  instance  it 
becomes  a  branched  film.  The  movement  is  brought  about  not 
by  increase  or  decrease  of  substance  but  by  mere  translocation  of 
particles ;  a  particle  which  at  one  moment  was  in  one  position 
moves  into  a  new  position,  several  particles  thus  moving  towards 
the  same  point  cause  a  bulging  at  that  point,  and  several  particles 
moving  away  from  the  same  point  cause  a  retraction  at  that 
point;  but  no  two  particles  get  nearer  to  each  other  so  as  to 
occupy  together  less  space  and  thus  lead  to  condensation  of  sub- 
stance, or  get  farther  from  each  other  so  as  to  occupy  more  space 
and  thus  lead  to  increase  of  bulk. 


138  AMCEBOID  MOVEMENTS.  [Book  t. 

In  this  respect,  in  that  there  is  no  change  of  bulk,  but  only  a 
shifting  of  particles  in  their  relative  position  to  each  other,  the 
amoeboid  movement  resembles  a  muscular  contraction ;  but  in 
other  respects  the  two  kinds  of  movement  seem  different,  and 
the  question  arises,  have  we  the  right  to  speak  of  the  substance 
which  can  only  execute  amoeboid  movements  as  being  contractile  ? 
We  may,  if  we  admit  that  contractility  is  at  bottom  simply  the 
power  of  shifting  the  relative  position  of  particles,  and  that 
lauscular  contraction  is  a  specialized  form  of  contraction,  the 
shifting  of  particles  is  specialized  in  the  sense  that  it  has  always 
a  definite  relation  to  the  long  axis  of  the  fibre. 

The  protoplasm  of  the  amoeba  or  of  a  white  corpuscle  is,  as  we 
iave  said,  of  a  consistency  which  we  for  want  of  better  terms  call 
semi-solid  or  semi-fluid.  Consequently  when  no  internal  changes 
are  prompting  its  particles  to  move  in  this  or  that  direction,  the 
influences  of  the  surroundings  will  tend  to  give  the  body,  as  they 
will  other  fluid  or  semi-fluid  drops,  a  spherical  form.  Hence  the 
natural  form  of  the  white  corpuscle  is  more  or  less  spherical.  If 
under  the  influence  of  some  stimulus  internal  or  external,  some 
of  the  particles  are  stirred  to  shift  their  place,  amoeboid  move- 
ments follow,  and  the  spherical  form  is  lost.  If  however  all  the 
particles  were  stirred  to  move  with  equal  energy,  they  would 
neutralize  each  other's  action,  no  protrusion  or  retraction  would 
take  place  at  any  point  of  the  surface  and  the  body  would  remain 
a  sphere.  Hence  in  extreme  stimulation,  in  what  in  the  muscle 
corresponds  to  complete  tetanus,  the  form  of  the  body  is  the  same 
as  in  rest ;  and  the  tetanized  sphere  would  not  be  appreciably 
smaller  than  the  sphere  at  rest,  for  that  would  imply  change  of 
bulk,  but  this  as  we  have  seen  does  not  take  place.  This  result 
shews  strikingly  the  difference  between  the  general  contractility 
of  the  amoeba,  and  the  special  contractility  of  the  muscle. 


CHAPTER  III. 


ON   THE   MORE   GENERAL  FEATURES   OF  NERVOUS 

TISSUES. 


§  88.  In  the  preceding  chapter  we  have  dealt  with  the  pro- 
perties of  nerves  going  to  muscles,  the  nerves  which  we  called 
mo^or,  and  have  incidentally  spoken  of  other  nerves  which  we  called 
sensory.  Both  these  kinds  of  nerves  are  connected  with  the  brain 
and  spinal  cord  and  form  part  of  the  General  Nervous  System. 
We  shall  have  to  study  hereafter  in  detail  the  brain  and  spinal  cord  ; 
but  the  nervous  system  intervenes  so  repeatedly  in  the  processes 
carried  out  by  other  tissues  that  it  will  be  desirable,  before  pro- 
ceeding further,  to  discuss  some  of  its  more  general  features. 

The  Nervous  System  consists  (1)  of  the  Brain  and  Spinal  Cord 
forming  together  the  cerebrospinal  axis,  or  central  nervous  system  ; 
(2)  of  the  nerves  passing  from  that  axis  to  nearly  all  parts  of  the 
body,  those  which  are  connected  with  the  spinal  cord  being  called 
spinal,  and  those  which  are  connected  with  the  brain,  within  the 
cranium,  being  called  cranial ;  and  (3)  of  ganglia  distributed  along 
the  nerves  in  various  parts  of  the  body. 

The  spinal  cord  obviously  consists  of  a  number  of  segments  or 
metameres,  following  in  succession  along  its  axis,  each  metamere 
giving  off  on  each  side  a  pair  of  spinal  nerves  ;  and  a  similar 
division  into  metameres  may  be  traced  in  the  brain,  though  less 
distinctly,  since  the  cranial  nerves  are  arranged  in  manner  some- 
what different  from  that  of  the  spinal  nerves.  We  may  take  a 
single  spinal  metamere,  represented  diagrammatically  in  Fig.  24, 
as  illustrating  the  general  features  of  the  nervous  system  ;  and 
since  the  half  on  one  side  of  the  median  line  resembles  the  half 
on  the  other  side,  we  may  deal  with  one  lateral  half  only. 

Each  spinal  nerve  arises  by  two  roots.  The  metamere  of  the 
central  nervous  system  C  consists,  as  we  shall  hereafter  see,  of  grey 


140 


A  NEURAL   METAMEEE. 


[Book  i. 


Tig.  24.    Scheme  of  the  Nerves  of  a  Segment  of  the  Spinal  Cord. 

Gr  grey,  FT  white  matter  of  spinal  cord.  A  anterior,  P  posterior  root.  G  ganglion 
on  the  posterior  root.  N  whole  nerve,  N'  spinal  nerve  proper,  ending  in  M  skeletal 
or  somatic  muscle,  5  somatic  sensory  cell  or  surface,  X  in  other  ways.  V  visceral 
nerve  (white  ramus  communicans)  passing  to  a  ganglion  of  the  sympathetic  chain 
2,  and  passing  on  as  V  to  supply  the  more  distant  ganglion  a,  then  as  V"  to  the 
peripheral  ganglion  o-'  and  ending  in  m  splanchnic  muscle,  s  splanchnic  sensory 
cell  or  surface,  x  other  possible  splanchnic  endings. 

From  2  is  given  off  the  revehent  nerve  r.  v  (grey  ramus  communicans),  which 
partly  passes  backward  towards  the  spinal  cord,  and  partly  runs  as  v.  m,  in  connection 


Chap.  III.]     FEATUEES    OF   NERVOUS    TISSUES.  141 

with  the  spinal  nerve,  to  supply  vasomotor  (constrictor)  fibres  to  the  muscles  (7//)  of 
blood  vessels  in  certain  parts,  for  example,  in  the  limbs. 

Sy,  the  sympathetic  chain  uniting  the  ganglia  of  the  series  2.  The  terminations 
of  the  other  nerves  arising  from  2,  a,  a  are  not  shewn. 

The  figure  is  necessarily  schematic,  and  must  not  be  taken  to  shew  that  the 
visceral  branch  joins  only  the  ganglion  belonging  to  the  same  segment  as  the  spinal 
nerve ;  the  visceral  branch  joins  the  sympathetic  chain,  passing  to  other  ganglia 
besides  the  one  of  the  same  segment,  indeed  in  some  cases  does  not  join  this  ac  all. 

matter  Gr  in  the  interior,  and  white  matter  W  on  the  outside. 
From  the  anterior  part  of  grey  matter  is  given  off  the  anterior 
nerve  root  A,  and  from  the  posterior  part  the  posterior  nerve 
root  P.  The  latter  passes  into  a  swelling  or  ganglion  G,  "  the 
ganglion  of  the  posterior  root,"  or  more  shortly  "  the  spinal  gan- 
glion ;  "  the  anterior  root  does  not  pass  into  this  ganglion.  Beyond 
the  ganglion  the  roots  join  to  form  the  nerve  trunk  N.  We  shall 
later  on  give  the  evidence  that  the  nerve  fibres  composing  the 
posterior  root  F  are  exclusively  (or  nearly  so),  occupied  in  carry- 
ing nervous  impulses  from  the  tissues  of  the  body  to  the  central 
nervous  system,  and  that  the  fibres  composing  the  anterior  root  A 
are  similarly  occupied  in  carrying  impulses  from  the  central  ner- 
vous system  to  the  several  tissues ;  that  is  to  say  the  former  is 
made  up  of  sensory  fibres,  or,  (since  the  impulses  passing  along 
them  to  the  central  system  may  give  rise  to  effects  other  than 
sensations)  afferent  fibres,  while  the  latter  is  made  up  of  motor, 
or,  (since  the  impulses  passing  along  them  from  the  central  ner- 
vous system  may  produce  effects  other  than  movements)  efferent 
fibres.  The  nerve  trunk  N  is  consequently  a  mixed  nerve  com- 
posed of  afferent  and  efferent  fibres. 

By  far  the  greater  part  of  this  mixed  nerve,  dividing  into 
various  branches,  is  distributed  {N')  to  the  skin  and  the  skeletal 
muscles,  some  of  the  fibres  (motor)  ending  in  muscular  fibres  (J/), 
others  (sensory)  ending  in  epithelial  cells  {S)  connected  with  the 
skin,  which  we  shall  consider  hereafter  under  the  name  of  sen- 
sory epithelial  cells,  while  others,  X,  after  dividing  into  minute 
branches  and  forming  plexuses  end,  in  ways  not  yet  definitely 
determined,  in  tissues  associated  with  the  skin  or  skeletal  muscles. 
Morphologists  distinguish  the  parts  which  go  to  form  the  skin, 
skeletal  muscles,  &c.  as  somatic,  from  the  splanchnic  parts  which 
go  to  form  the  viscera.  We  may  accordingly  call  this  main  part 
of  the  spinal  nerve  the  somatic  division  of  the  nerve. 

Soon  after  the  mixed  nerve  A"  leaves  the  spinal  canal  it 
gives  off  a  branch  V,  which  under  the  name  of  (white)  ramus 
communicans,  runs  into  the  longitudinal  series  of  ganglia  (H) 
conspicuous  in  the  thorax  as  the  main  sympathetic  chain.  This 
branch  is  destined  to  supply  the  viscera,  and  might,  therefore,  be 
called  the  splanchnic  division  of  the  spinal  nerve.  We  may  say 
at  once,  without  entering  into  details,  that  the  whole  of  the 
sympathetic  system  with  its  ganglia,  plexuses  and  nerves  is  to 
be  regarded  as  a  development  or  expansion  of  the  visceral  or 
splanchnic  divisions  of  certain  spinal  nerves.     By  means  of  this 


142  SOMATIC  AND   SPLANCHNIC   NERVES.     [Book  i. 

system  splanchnic  fibres  from  the  central  nervous  system  are 
distributed  to  the  tissues  of  the  viscera,  some  of  them  on  their 
way  passing  through  secondary  ganglia  a,  and,  it  may  be,  tertiary 
ganglia.  The  majority  of  these  splanchnic  fibres  seem  to  be  effer- 
ent in  nature,  carrying  impulses  from  the  central  nervous  system 
to  the  tissues,  some  ending  in  plain  muscular  fibres  (m)  others  in 
other  ways  (^x)  ;  but  some  of  the  fibres  are  afferent  (s)  and  con- 
vey impulses  from  the  viscera  to  the  central  nervous  system,  and 
it  is  possible  that  some  of  these  begin  or  end  in  epithelial  cells  of 
the  viscera. 

All  the  fibres  issuing  from  the  main  sympathetic  chain  do  not 
however  pass  to  the  tissues  of  the  viscera ;  a  certain  number  of 
fibres  turn  back  (r.  v.)  from  the  ganglion  to  join  the  spinal  nerve 
and  run  for  the  most  part  peripherally  in  the  somatic  nerve,  though 
some  of  them  pass  backwards  to  the  spinal  cord,  ending  probably 
in  the  membranes  of  the  cord.  In  the  case  of  many  of  the  spinal 
nerves  the  communicating  branch  from  the  spiual  nerve  consists 
distinctly  of  two  parts,  a  '  white  ramus '  consisting  chiefiy  of 
medullated  and  a  '  grey  ramus '  consisting  chiefly  of  non-medul- 
lated  fibres ;  in  these  cases  these  backward  turning  '  revehent ' 
fibres  run  in  the  grey  ramus ;  but  in  the  case  of  some  of  the  spinal 
nerves  it  is  not  possible  to  distinguish  a  grey  ramus  separate  from 
a  white  ramus. 

We  shall  have  occasion  in  the  next  chapter  to  speak  of 
nerves  or  rather  nerve  fibres  which  by  influencing  the  mus- 
cles of  the  blood  vessels  govern  the  calibre  of  those  vessels  and 
are  spoken  of  as  vaso-motor  nerves.  Some  of  these,  in  their 
action,  constrict  or  narrow  the  blood  vessels,  and  are  hence  called 
vaso-constrictor  nerve  fibres.  All  these  vaso-constrictor  nerve 
fibres  issuing  from  the  central  nervous  system  pass  to  the  splanch- 
nic system,  to  the  sympathetic  chain;  but  while  some  of  them, 
continuing  in  the  splanchnic  system,  are  distributed  to  the  blood 
vessels  of  the  viscera,  others  turning  back  by  revehent  branches 
and  running  with  ordinary  somatic  fibres  in  the  (somatic)  spinal 
nerves  are  distributed  to  the  blood  vessels  certainly  of  the  skin 
and  possibly  of  other  somatic  tissues.  These  are  represented 
in  the  figure  by  m'.  As  we  shall  see,  other  nerve  fibres,  having 
other  functions',  take  a  similar  course. 

A  nerve  fibre  is  fundamentally  a  prolongation  of  a  nerve  cell ; 
the  axis  cylinder  which  is  the  essential  part  of  a  medullated  fibre, 
and  constitutes  practically  the  whole  of  a  non-medullated  fibre,  is 
the  prolongation  of  a  process,  the  so-called  axis  cylinder  process  of 
a  nerve  cell.  When  we  examine  a  nerve  we  find  that  along  its 
course  it  consists  exclusively  of  nerve  fibres  bound  together  by 
connective  tissue  carrying  blood  vessels  and  lymphatics,  the  somatic 
(spinal  and  cranial)  nerves  being  composed  chiefly  of  medullated 
fibres,  mixed  with  which  are  some  non-medullated  fibres,  and 
the  sympathetic  nerves  being  composed  chiefly  of  non-medullated 
fibres,  some  of  them   containing  hardly   any  medullated  fibres. 


Chap,  hi.]  GENERAL  FEATURES  OF  NERVOUS  TISSUES.  143 

Nerve  cells  are  not  normal  constituents  of  nerves ;  they  are  found 
only  in  the  central  nervous  system,  from  which  the  nerves  issue, 
or  in  the  ganglia,  spinal  and  sympathetic  ganglia,  through  which 
they  pass  or  with  which  they  make  connections ;  (we  may  omit 
for  the  present  the  nerve  cells  in  which,  or  in  connection  with 
which  certain  nerve  fibres  end  at  the  periphery).  Hence  all  the 
nerve  fibres  of  the  body  are  processes  of  nerve  cells  situated  in 
the  central  nervous  system,  or  in  the  spinal  ganglia  and  corre- 
sponding ganglia  on  certain  cranial  nerves,  or  in  sympathetic 
ganglia. 

§  89.  In  the  central  nervous  system,  the  nerve  cells  are 
found  in  the  so  called  '  grey  matter.'  The  '  white  matter,'  putting 
aside  certain  exceptions,  consists  so  far  as  its  nervous  elements 
are  concerned  exclusively  of  nerve  fibres.  Confining  ourselves 
for  the  present  to  the  spinal  cord,  we  find  that  the  fibres  of  the 
anterior  root,  efferent  fibres,  are  processes  of  cells,  prolongations  of 
the  axis  cylinder  processes  of  cells,  lying  in  the  grey  matter  of  the 
spinal  cord,  but  that  the  fibres  of  the  posterior  root  are  (putting  aside 
certain  minor  exceptions)  processes  of  cells  lying  in  the  ganglion 
of  that  root.  The  cell  whose  axis  cylinder  process  becomes  an 
efferent  fibre  of  an  anterior  root  has  other  processes,  which  do  not 
become  axis  cylinders  of  nerve  fibres  but  end  by  a  division  more 
or  less  distinctly  arborescent  in  the  grey  matter.  The  fibre  of  a 
posterior  root  whose  axis  cylinder  is  a  process  of  a  cell  in  the  spi- 
nal ganglion  running  and  dividing  in  the  spinal  cord,  in  a  manner 
of  which  we  shall  have  to  speak  later  on,  finally  ends  in  the  grey 
matter  also  in  a  more  or  less  distinctly  arborescent  fashion. 
The  grey  matter  also  contains  nerve  cells  whose  axis  cylinder  and 
other  processes  both  begin  and  end  in  the  grey  matter ;  that  is  to 
say  the  body  of  the  nerve  cell  lies  in  the  grey  matter,  and  all  its 
processes  finally  end,  also  in  a  more  or  less  distinctly  arborescent 
fashion,  in  the  grey  matter,  without  leaving  the  spinal  cord  or  at 
least  the  central  nervous  system,  though  the  fibre  which  its  axis 
cylinder  becomes  (and  there  may  be  more  than  one  such)  may 
traverse  for  a  while  the  white  matter. 

The  grey  matter  of  the  spinal  cord  (and  the  same  is  true 
though  in  a  much  more  complex  way  of  the  grey  matter  of  tlie 
brain)  may  therefore  be  considered  as  a  centre  or  a  number  of 
centres,  nervous  centres,  connected  on  the  one  hand  with  afferent 
and  on  the  other  hand  with  efferent  fibres.  In  some  cases  the 
connection  between  the  afferent  and  the  efferent  fibres  may  be 
simple  and  direct ;  the  terminations  of  the  afferent  fibre  may  come 
into  direct  connection  with  the  nerve  cell,  a  process  of  which  is 
the  efferent  fibre.  In  other  cases  the  connection  may  be  an  indirect 
one  ;  between  the  two  intervenes  a  cell,  with  certain  processes  of 
which  the  afferent  fibre  is  connected,  and  another  process  (or  other 
processes)  of  which  is  connected  with  the  cell  whose  process  is  the 
efferent  fibre ;  or  more  than  one  such  cell  may  intervene.  And  recent 
inquiry  shews  that  the  usual  mode  of  connection  of  one  cell  with 


144  REFLEX  ACTIONS.  [Book  i. 

another  is  that  the  arborescent  terminations  of  the  axis  cylinder 
process  of  the  one  cell  are  applied  to  the  arborescent  processes  (not 
of  the  nature  of  axis  cylinder  processes)  of  the  other  cell,  in  such  a 
way  that  the  substances  of  the  two  cells  are  in  abundant  contact, 
but  not  in  actual  continuity. 

The  various  nervous  centres  thus  supplied  by  the  grey  matter 
of  the  spinal  cord  and  brain  have  two  important  classes  of  functions 
called  respectively  reflex  actions  and  automatic  actions. 

§  90,  Reflex  actions.  In  a  reflex  action  afferent  impulses 
reaching  the  nervous  centre  give  rise  to  the  discharge  of  efferent 
impulses,  the  discharge  following  so  rapidly  and  in  such  a  way  as  to 
leave  no  doubt  that  it  is  caused  by  the  advent  at  the  centre  of  the 
afferent  impulses.  Thus  a  frog  from  which  the  brain  has  been 
removed  while  the  rest  of  the  body  has  been  left  intact  will 
frequently  remain  quite  motionless  (as  far  at  least  as  the  skeletal 
muscles  are  concerned)  for  an  almost  indefinite  time  ;  but  if  its 
skin  be  pricked,  or  if  in  other  ways  afferent  impulses  be  generated 
in  afferent  fibres  by  adequate  stimulation,  movements  of  the  limbs 
or  body  will  immediately  follow.  Obviously  in  this  instance  the 
stimulation  of  afferent  fibres  has  been  the  cause  of  the  discharge 
of  impulses  along  efferent  fibres. 

The  machinery  involved  in  such  a  reflex  act  consists  of  three 
parts :  (1)  the  afferent  fibres,  (2)  the  nerve  centre,  in  this  case  the 
spinal  cord,  and  (3)  the  efferent  fibres.  If  any  one  of  these  three 
parts  be  missing  the  reflex  act  cannot  take  place ;  if  for  instance 
the  afferent  nerves  or  the  efferent  nerves  be  cut  across  in  their 
course,  or  if  the  centre,  the  spinal  cord,  be  destroyed,  the  reflex 
action  cannot  take  place. 

Keflex  actions  can  be  carried  out  by  means  of  the  brain,  as  we 
shall  see  while  studying  that  organ  in  detail,  but  the  best  and 
clearest  examples  of  reflex  action  are  manifested  by  the  spinal 
cord;  in  fact,  reflex  action  is  one  of  the  most  important  func- 
tions of  the  spinal  cord.  We  shall  have  to  study  the  various 
reflex  actions  of  the  spinal  cord  in  detail  hereafter,  but  it  will  be 
desirable  to  point  out  here  some  of  their  general  features. 

When  we  stimulate  the  nerve  of  a  muscle-nerve  preparation 
the  result,  though  modified  in  part  by  the  condition  of  the  muscle 
and  nerve,  whether  fresh  and  irritable  or  exhausted  for  instance, 
is  directly  dependent  on  the  nature  and  strength  of  the  stimulus. 
If  we  use  a  single  induction-shock  we  get  a  simple  contraction,  if 
the  interrupted  current  we  get  a  tetanus,  if  we  use  a  weak  shock 
we  get  a  slight  contraction,  if  a  strong  shock  a  large  contraction, 
and  so  on  ;  and  throughout  our  study  of  muscular  contractions  we 
assumed  that  the  amount  of  contraction  might  be  taken  as  a 
measure  of  the  magnitude  of  the  nervous  impulses  generated  by 
the  stimulus.  And  it  need  hardly  be  said  that  when  we  stimulate 
certain  fibres  only  of  a  motor  nerve,  it  is  only  the  muscular  fibres 
in  which  those  nerve  fibres  end,  which  are  thrown  into  con- 
traction. 


CnAi'.  III.]  GENERAL  FEATURES  OF  NERVOUS  TISSUES.   145 

In  a  reflex  action  on  the  other  hand  the  movements  called  forth 
by  the  same  stimulus  may  be  in  one  case  insigniticant,  and  in 
another  violent  and  excessive,  the  result  depending  on  the  arrange- 
ments and  condition  of  the  central  portion  of  the  reflex  mechanism. 
Thus  the  mere  contact  of  a  hair  with  the  mucous  membrane  lining 
the  larynx,  a  contact  which  can  originate  only  the  very  slightest 
afferent  impulses,  may  call  forth  a  convulsive  fit  of  coughing,  in 
which  a  very  large  number  of  muscles  are  thrown  into  violent  con- 
tractions ;  whereas  the  same  contact  of  the  hair  with  other  surfaces 
of  the  body  may  produce  no  obvious  effect  at  all.  Similarly,  while 
in  the  brainless  but  otlierwise  normal  frog,  a  slight  touch  on  the  skin 
of  the  flank  will  produce  nothing  but  a  faint  flicker  of  the  under- 
lying muscles,  the  same  touch  on  the  same  part  of  a  frog  poisoned 
with  strychnia  will  produce  violent  lasting  tetanic  contractions  of 
nearly  all  the  muscles  of  the  body.  Motor  impulses  as  we  have 
seen  travel  along  motor  nerves  without  any  great  expenditure  of 
energy  and  probably  without  increasing  that  expenditure  as  they 
proceed ;  and  the  same  is  apparently  the  case  with  afferent  impulses 
passing  along  afferent  nerves.  When  however  in  a  reflex  action 
aflerent  impulses  reach  the  nerve  centre,  a  change  in  the  nature  and 
magnitude  of  the  impulses  takes  place.  It  is  not  that  in  the  nerve 
centre  the  afferent  impulses  are  simply  turned  aside  or  reflected  into 
efferent  impulses  ;  and  hence  the  term  "  reflex  "  action  is  a  bad  one. 
It  is  rather  that  the  afferent  impulses  act  afresh  as  it  were  as  a 
stimulus  to  the  nerve  centre,  producing  according  to  circumstances 
and  conditions  either  a  few  weak  efferent  impulses  or  a  multitude 
of  strong  ones.  The  nerve  centre  may  be  regarded  as  a  collection 
of  explosive  charges  ready  to  be  discharged  and  so  to  start  efferent 
impulses  along  certain  efferent  nerves,  and  these  charges  are 
so  arranged  and  so  related  to  certain  afferent  nerves,  that  aflerent 
impulses  reaching  the  centre  along  those  nerves  may  in  one  case 
discharge  a  few  only  of  the  charges  and  so  give  rise  to  feeble 
movements,  and  in  another  case  discharge  a  very  large  number  and 
so  give  rise  to  large  and  violent  movements.  In  a  reflex  action 
then  the  number,  intensity,  character  and  distribution  of  the  efferent 
impulses,  and  so  the  kind  and  amount  of  movement,  will  depend 
chiefly  on  what  takes  place  in  the  centre,  and  this  will  in  turn 
depend  on  the  one  hand  on  the  condition  of  tlie  centre  and,  on 
the  other,  on  the  special  relations  of  the  centre  to  the  afferent 
impulses.  At  the  same  time  we  are  able  to  recognize  in  most 
reflex  actions  a  certain  relation  between  the  strength  of  the 
stimulus,  that  is  to  say  the  magnitude  of  the  afferent  impulses, 
and  the  extent  of  the  movement,  that  is  to  say  the  magnitude 
of  the  efferent  impulses. 

We  may  add,  without  going  more  fully  into  the  subject  here, 
that  in  most  reflex  actions  a  special  relation  may  be  observed 
between  the  part  stimulated  and  the  resulting  movement.  In  the 
simplest  cases  of  reflex  action  tliis  relation  is  merely  of  such  a 
kind  that  the  muscles  thrown  into  action  are  those  governed  by  a 

10 


146  AUTOMATIC   ACTIONS.  [Book  i. 

motor  nerve  which  is  the  fellow  of  the  sensory  nerve,  the  stimula- 
tion of  which  calls  forth  the  movement.  In  the  more  complex 
reflex  actions  of  the  brainless  frog,  and  in  other  cases,  the  relation 
is  of  such  a  kind  that  the  resulting  movement  bears  an  adaptation 
to  the  stimulus  :  the  foot  is  withdrawn  from  the  stimulus,  or 
the  movement  is  calculated  to  push  or  wipe  away  the  stimulus. 
In  other  words,  a  certain  purpose  is  evident  in  the  reflex  action. 

Thus  in  all  cases,  except  perhaps  the  very  simplest,  the  move- 
ments called  forth  by  a  reflex  action  are  exceedingly  complex 
compared  with  those  which  result  from  the  direct  stimulation  of  a 
motor  trunk. 

§  91.  Automatic  actions.  Efferent  impulses  frequently  issue 
from  the  brain  and  spinal  cord  and  so  give  rise  to  movements 
without  being  obviously  preceded  by  any  stimulation.  Such  move- 
ments are  spoken  of  as  automatic  or  spontaneous.  The  efferent 
impulses  in  such  cases  are  started  by  changes  in  the  nerve  centre 
which  are  not  the  immediate  result  of  the  arrival  at  the  nerve 
centre  of  afferent  impulses  from  without,  but  which  appear  to 
arise  in  the  nerve  centre  itself.  Changes  of  this  kind  may  recur 
rhythmically ;  thus,  as  we  shall  see,  we  have  reason  to  think  that 
in  a  certain  part  of  the  central  nervous  system  called  the  spinal 
bulb,  or  medulla  oblongata,  changes  of  the  nervous  material,  re- 
curring  rhythmically,  lead  to  the  rhythmic  discharge  along  certain 
nerves  of  efferent  impulses  whereby  muscles  connected  with  the 
chest  are  rhythmically  thrown  into  action  and  a  rhythmically 
repeated  breathing  is  brought  about.  And  other  similar  rhythmic 
automatic  movements  may  be  carried  out  by  various  parts  of  the 
spinal  cord. 

From  the  brain  itself  a  much  more  varied  and  apparently 
irregular  discharge  of  efferent  impulses,  not  the  obvious  result  of 
any  immediately  foregoing  afferent  impulses,  and  therefore  not 
forming  part  of  reflex  actions,  is  very  common,  constituting  what 
we  speak  of  as  volition,  efferent  impulses  thus  arising  being  called 
volitional  or  voluntary  impulses.  The  spinal  cord  apart  from  the 
brain  does  not  appear  capable  of  executing  these  voluntary  move- 
ments ;  but  to  this  subject  we  shall  return  when  we  come  to  speak 
of  the  central  nervous  system  in  detail. 

While  reflex  and  automatic  actions  are  thus  frequently  carried 
out  by  the  grey  matter  of  tlie  central  nervous  system,  of  which  grey 
matter  nerve  cells  are  conspicuous  constituents,  it  is  at  least  not 
absolutely  proved  that  either  kind  of  action  is  carried  out  by  the 
other  portions  of  the  nervous  system  in  which  nerve  cells  are 
found. 

As  regards  the  ganglia  on  the  posterior  roots  of  spinal 
nerves  it  can  be  definitely  affirmed  that  these  act  neither  as  auto- 
matic centres  nor  as  centres  of  reflex  action.  The  nerve  cell  of 
such  a  ganglion  serves  to  govern  the  nutrition  of  the  afferent 
nerve  fibre  to  which  it  is  attached  by  a  T  shaped  junction ;  and  a 
portion  of  the  fibre  which  is  cut  away  from  its  nerve  cell  in  the 


Chap,  hi.]  GENERAL  FEATURES  OF  NERVOUS  TISSUES.  147 

ganglion,  whether  it  be  the  portion  which  is  passing  from 
the  skin  or  other  tissue  to  the  ganglion,  or  the  portion  which  is 
passing  from  the  ganglion  to  the  spinal  cord  as  part  of  the 
posterior  root,  degenerates  and  dies  on  the  side  of  the  cut  away 
from  the  ganglion.  This,  however,  is  not  a  feature  confined  to 
these  spinal  ganglia;  an  efferent  fibre  of  the  anterior  root 
similarly  degenerates  when  it  is  cut  away  from  the  nerve  cell  in 
the  grey  matter,  of  the  axis  cylinder  process  of  which  it  is  as  we 
have  said  a  prolongation.  Speaking  generally,  a  nerve  cell  governs 
the  nutrition  of,  acts  as  a  trophic  centre  as  it  is  called  to,  the 
nerve  fibre  into  which  its  axis  cylinder  process  is  continued. 

As  regards  the  sympathetic  ganglia,  though  there  are  some 
results  which  appear  to  indicate  that  certain  of  these  ganglia  may 
act  in  a  simple  and  rudimentary  way  as  centres  of  reflex  action, 
these  cases  are  by  no  means  clear;  and  it  may  be  distinctly 
affirmed  that  these  ganglia  do  not  generally  act  as  centres  of 
reflex  action  in  the  same  way  as  does  the  grey  matter  of  the 
central  nervous  system. 

Of  the  fibres  running  in  a  ramus  from  a  spinal  nerve  to  the 
ganglion  of  the  same  nietamere,  some  may  end  in  connection 
with  the  nerve  cells  of  that  ganglion.  In  that  case  the  nerve 
fibre,  which  is  a  medullated  one,  appears  to  end  in  an  arborescence 
in  contact  with  a  nerve  cell ,  and  that  nerve  cell  gives  off  one  or 
more  processes  which  become  nerve  fibres,  but  non-medullated 
nerve  fibres.  Other  fibres  of  the  ramus  may  simply  pass  through 
that  ganglion,  passing  by  its  nerve  cells,  and  end  in  connection  with 
the  nerve  cells  of  some  other  ganglion,  which  nerve  cells  similarly 
give  rise  to  non-medullated  nerve  fibres.  The  nerve  fibres  leaving 
a  ganglion  are  more  numerous  than  those  reaching  it  from  the 
central  nervous  system  ;  and  while  the  latter  are  medullated, 
the  former  are  increasingly  non-medullated.  Hence  in  the  gan- 
glion there  is  a  spreading  and  distribution  of  nervous  impulses ; 
as  to  what  changes  in  the  nature  of  the  impulses  may  be  effected 
as  they  pass  through  the  nerve  cells  of  a  ganglion  is  not  at 
present  clear. 

There  seems  at  first  sight  evidence  of  some  strength  that  these 
sympathetic  ganglia,  unlike  the  ganglia  on  the  posterior  roots,  may 
serve  as  centres  of  rhythmic  automatic  action.  Several  organs  of 
the  body  containing  muscular  tissue,  the  most  notable  lieing  the 
heart,  are  during  life  engaged  in  rhythmic  automatic  movements, 
and  in  many  cases  continue  these  movements  after  removal  from 
the  body.  In  nearly  all  these  cases  ganglia  are  present  in  con- 
nection with  the  muscular  tissue  ;  and  the  presence  and  intact 
condition  of  these  ganglia  seem  at  all  events  in  many  cases  in 
some  way  essential  to  the  due  performance  of  tlie  rhythmic 
automatic  movements.  Indeed  it  has  been  thought  that  the 
movements  in  question  are  really  due  to  the  rhythmic  automatic 
generation  in  the  cells  of  these  ganglia  of  efferent  impulses 
which  passing  down  to  the  appropriate  muscular  fibres  call  forth 


148  AUTOMATIC  ACTIONS.  [Book  i. 

the  rhythmic  movement.  When  we  come  to  study  these  move- 
ments in  detail,  we  shall  find  reasons  for  coming  to  the  conclusion 
that  this  view  is  not  supported  by  adequate  evidence. 

§  92.  InMhitory  nerves.  We  have  said  that  the  fibres  of  the 
anterior  root  should  be  called  efferent  rather  than  motor  because 
though  they  all  carry  impulses  outward  from  the  central  nervous 
system  to  the  tissues,  the  impulses  which  they  carry  do  not 
in  all  cases  lead  to  the  contraction  of  muscular  fibres.  Some  of 
these  efferent  fibres  are  distributed  to  glandular  structures,  for 
instance,  to  the  salivary  glands,  and  impulses  passing  along  these 
lead  to  changes  in  epithelial  cells  and  their  surroundings  whereby, 
without  any  muscular  contraction  necessarily  intervening,  secre- 
tion is  brought  about :  the  action  of  these  fibres  of  secretion  we 
shall  study  in  connection  with  digestion. 

Besides  this  there  are  efferent  fibres  going  to  muscular  tissue 
or  at  all  events  to  muscular  organs,  the  impulses  passing  along 
which,  so  far  from  bringing  about  muscular  contraction,  diminish, 
hinder  or  stop  movements  already  in  progress.  Thus  if  when  the 
heart  is  beating  regularly,  tliat  is  to  say,  when  the  muscular  fibres 
which  make  up  the  greater  part  of  the  heart  are  rhythmically 
contracting,  the  branches  of  the  pneumogastric  nerve  going  to  the 
heart  be  adequately  stimulated,  for  instance  with  the  interrupted 
current,  the  heart  will  stop  beating ;  and  that  not  because  the 
muscles  of  the  heart  are  thrown  into  a  continued  tetanus,  the 
rhythmic  alternation  of  contraction  and  relaxation  being  replaced 
by  sustained  contraction,  but  because  contraction  disappears  alto- 
gether, all  the  muscular  fibres  of  the  heart  remaining  for  a 
considerable  time  in  complete  relaxation  and  the  whole  heart 
being  quite  flaccid.  If  a  weaker  stimulus  be  employed  the  beat 
may  not  be  actually  stopped  but  slowed  or  weakened.  And,  as  we 
shall  see,  there  are  many  other  cases  where  the  stimulation  of 
efferent  fibres  hinders,  weakens,  or  altogether  stops  a  movement 
already  in  progress.  Such  an  effect  is  called  an  inhibition,  and 
the  fibres,  stimulation  of  which  produces  the  effect,  are  called 
'  inhibitory '  fibres. 

The  phenomena  of  inhibition  are  not,  however,  confined  to 
such  cases  as  the  heart,  where  the  efferent  nerves  are  connected 
with  muscular  tissues.  In  fact  it  is  probable,  though  not  actually 
proved  in  every  case,  that  wherever  in  any  tissue,  energy  is  being 
set  free,  nervous  impulses  brought  to  bear  on  the  tissue  may  affect 
the  rate  or  amount  of  the  energy  set  free  in  two  different  ways  ;  on 
the  one  hand,  they  may  increase  or  quicken  the  setting  free  of 
energy,  and  on  the  other  hand  they  may  slacken  or  hinder  the 
setting  free  of  energy.  And  in  at  all  events  a  large  number  of 
cases  it  is  possible  to  produce  the  one  effect  by  means  of  one  set 
of  nerve  fibres,  and  the  other  effect  by  another  set  of  nerve  fibres. 
We  shall  have  occasion  however  to  study  the  several  instances  of 
this  double  action  in  the  appropriate  places. 


CHAPTER  IV. 


THE   VASCULAR  MECHANISM. 


SEC.    1.     THE   STRUCTURE  AND  MAIN   FEATURES  OF 
THE   VASCULAR   APPARATUS. 


§  93.  The  blood,  as  we  have  said,  is  the  internal  medium  on 
which  the  tissues  live ;  from  it  these  draw  their  food  and  oxygen,  to 
it  they  give  up  the  products  or  waste  matters  which  they  form.  The 
tissues,  with  some  few  exceptions,  are  traversed  by,  and  thus  the 
elements  of  the  tissues  surrounded  by,  networks  of  minute,  thin- 
walled  tubes,  the  capillary  Hood  vessels.  The  elementary  striated 
muscle  fibre,  for  instance,  is  surrounded  by  capillaries,  running  in 
the  connective  tissue  outside  but  close  to  the  sarcolemma,  arranged 
in  a  network  with  more  or  less  rectangular  meshes.  These  capil- 
laries are  closed  tubes  with  continuous  walls,  and  the  blood,  which, 
as  we  shall  see,  is  continually  streaming  through  them,  is  as  a 
whole  confined  to  their  channels,  and  does  not  escape  from  them. 
The  elements  of  the  tissues  lie  outside  the  capillaries,  and  form 
extra-vascular  islets  of  different  form  and  size  in  the  different 
tissues,  surrounded  by  capillary  networks.  But  the  walls  of  the 
capillaries  are  so  thin  and  of  such  a  nature  that  certain  of  the 
constituents  of  the  blood  pass  from  the  interior  of  the  capillary 
through  the  capillary  wall  to  the  elements  of  the  tissue  outside 
the  capillary,  and,  similarly,  certain  of  the  constituents  of  the 
tissue,  to  wit,  certain  substances,  the  result  of  the  metabolism 
continually  going  on  in  the  tissue,  pass  from  the  tissue  outside 
the  capillary  through  the  capillary  wall  into  the  blood  flowing 
through  the  capillary.     Thus,  as  we  have  already  said,  §  13,  there 


150         MAIN   FEATURES   OF   THE   APPARATUS.      [Book  i. 

is  a  continual  interchange  of  material  between  the  blood  in  the 
capillary,  and  the  elements  of  the  tissue  outside  the  capillary,  the 
lymph  acting  as  middle  man.  By  this  interchange  the  tissue 
lives  on  the  blood  and  the  blood  is  affected  by  its  passage  through 
the  tissue.  In  the  small  arteries  which  end  in,  and  in  the  small 
veins  which  begin  in  the  capillaries,  a  similar  interchange  takes 
place ;  but  the  amount  of  interchange  diminishes  as,  passing  in 
each  direction  from  the  capillaries,  the  walls  of  the  arteries  and 
veins  become  thicker ;  and  indeed,  in  all  but  the  minute  veins 
and  arteries,  the  interchange  is  so  small  that  it  may  practically 
be  neglected.  It  is  in  the  capillaries  (and  minute  arteries  and 
veins)  that  the  business  of  the  blood  is  done ;  it  is  in  these  that 
the  interchange  takes  place ;  and  the  object  of  the  vascular 
mechanism  is  to  cause  the  blood  to  flow  through  these  in  a 
manner  best  adapted  for  carrying  on  this  interchange  under 
varying  circumstances.  The  use  of  the  arteries  is  in  the  main 
simply  to  carry  the  blood  in  a  suitable  manner  from  the  heart 
to  the  capillaries,  the  use  of  the  veins  is  in  the  main  simply  to 
carry  the  blood  from  the  capillaries  back  to  the  heart,  and  the  use 
of  the  heart  is  in  the  main  simply  to  drive  the  blood  in  a  suitable 
manner  through  the  arteries  into  the  capillaries  and  from  the 
capillaries  back  along  the  veins  to  itself  again.  The  structure  of 
these  several  parts  is  adapted  to  these  several  uses. 


Main   Features   of  the   Apparatus. 

§  94.  We  may  pass  briefly  in  review  some  of  the  main 
features  of  the  several  parts  of  the  vascular  apparatus,  heart, 
arteries,  veins  and  capillaries. 

The  heart  is  a  muscular  pump,  that  is  a  pump  the  force  of 
whose  strokes  is  supplied  by  the  contraction  of  muscular  fibres, 
working  intermittently,  the  strokes  being  repeated  so  many  times 
(in  man  about  72  times)  a  minute.  It  is  so  constructed  and 
furnished  with  valves  in  such  a  way  that  at  each  stroke  it  drives 
a  certain  quantity  of  blood  with  a  certain  force  and  a  certain 
rapidity  from  the  left  ventricle  into  the  aorta  and  so  into  the 
arteries,  receiving  during  the  stroke  and  the  interval  between  that 
stroke  and  the  next,  the  same  quantity  of  blood  from  the  veins 
into  the  right  auricle.  We  omit  for  simplicity's  sake  the  pul- 
monary circulation  by  which  the  same  quantity  of  blood  is  driven 
at  the  stroke  from  the  right  ventricle  into  the  lungs  and  received 
into  the  left  auricle.  The  rhythm  of  the  beat,  that  is  the  fre- 
quency of  repetition  of  the  strokes,  and  the  characters  of  each 
beat  or  stroke,  are  determined  by  changes  taking  place  in  the 
tissues  of  the  heart  itself,  though  they  are  also  influenced  by 
causes  working  from  without. 

The  arteries  are  tubes,  with  relatively  stout  walls,  branching 


Chap,  iv.]  THE   VASCULAR  MECHANISM.  151 

from  the  aorta  all  over  the  body.  The  constitution  of  their  walls, 
especially  of  the  middle  coat,  gives  the  arteries  two  salient  proper- 
ties. In  the  first  place  they  are  vei-y  elastic,  in  the  sense  that 
they  will  stretch  readily,  both  lengthways  and  crosswise,  when 
pulled,  and  return  readily  to  their  former  size  and  shape  when 
the  pull  is  taken  off.  If  Huid  be  driven  into  one  end  of  a  piece 
of  artery,  the  other  end  of  which  is  tied,  the  artery  will  swell  out 
to  a  very  great  extent,  but  return  immediately  to  its  former 
calibre  when  the  fluid  is  let  out.  This  elasticity  is  chiefly  due  to 
the  elastic  elements  in  the  coats,  elastic  membranes  and  feltwurks, 
but  the  muscular  fibres  being  themselves  also  elastic  contribute  to 
the  result.  By  reason  of  their  possessing  such  stout  elastic  walls 
the  arteries  when  empty  do  not  collapse  but  remain  as  open  tubes. 
In  the  second  place  the  arteries  by  virtue  of  their  muscular  ele- 
ments are  contractile;  when  stimulated  either  directly  as  by 
applying  an  electric  or  mechanical  stimulus  to  the  arterial  walls 
or  indirectly  by  means  of  the  so-called  vaso-motor  nerves,  which 
we  shall  have  to  study  presently,  the  arteries  shrink  in  calibre, 
the  circularly  disposed  muscular  fibres  contracting  and  so,  in  pro- 
portion to  the  amount  of  their  contraction,  narrowing  the  lumen 
or  bore  of  the  vessel.  The  contraction  of  these  arterial  muscular 
fibres,  like  that  of  all  plain  non-striated  muscular  fibres,  is  slow 
and  long  continued,  with  a  long  latent  period,  as  compared  with 
the  contraction  of  skeletal  striated  muscular  fibres.  Owing  to 
this  muscular  element  in  the  arterial  walls,  the  calibre  of  an 
artery  may  be  very  narrow,  or  very  wide,  or  in  an  intermediate 
condition  between  the  two,  neither  very  narrow  nor  very  wide, 
according  as  the  muscular  fibres  are  very  much  contracted,  or  not 
contracted  at  all,  or  only  moderately  contracted.  Further,  while 
the  relative  proportion  of  elastic  and  muscular  elements  differs  in 
different  arteries,  as  a  general  rule  the  elastic  elements  predomi- 
nate in  the  larger  arteries  and  the  muscular  elements  in  the 
smaller  arteries,  so  that  the  larger  arteries  may  be  spoken  of  as 
eminently  elastic,  or  as  especially  useful  on  account  of  their 
elastic  properties,  and  the  smaller  arteries  as  eminently  muscular, 
or  as  especially  useful  on  account  of  their  muscular  properties. 
Thus  in  the  minute  arteries  which  are  just  passing  into  capillaries 
the  muscular  coat,  though  composed  often  of  a  single  layer,  and 
that  sometimes  an  imperfect  one,  of  muscular  fibres,  is  a  much 
more  conspicuous  and  important  part  of  the  arterial  wall  than  that 
furnished  by  the  elastic  elements. 

The  arteries  branching  out  from  a  single  aorta  down  to  multi- 
tudinous capillaries  in  nearly  every  part  of  the  body,  diminish  in 
bore  as  they  divide.  Where  an  artery  divides  into  two  or  gives  off 
a  branch,  though  the  bore  of  each  division  is  less  than  that  of  the 
artery  before  the  division  or  branching,  the  two  together  are 
greater ;  that  is  to  say,  the  united  sectional  area  of  the  branches 
is  greater  than  the   sectional    area    of   the  trunk.      Hence  the 


152  MAIN   FEATURES   OF   THE   APPARATUS.      [Book  i. 

sectional  area  of  the  arterial  bed  through  which  the  blood  flows 
goes  on  increasing  from  the  aorta  to  the  capillaries.  If  all  the 
arterial  branches  were  thrown  together  into  one  channel,  this 
would  form  a  hollow  cone  with  its  apex  at  the  aorta  and  its  base 
at  the  capillaries.  The  united  sectional  area  of  the  capillaries 
may  be  taken  as  several  hundred  times  that  of  the  sectional  area 
of  the  aorta,  so  greatly  does  the  arterial  bed  widen  out. 

The  capillaries  are  channels  of  variable  but  exceedingly  small 
size.  The  thin  sheet  of  cemented  epithelioid  plates  which  forms 
the  only  wall  of  a  capillary  is  elastic,  permitting  the  channel  offered 
by  the  same  capillary  to  differ  much  in  width  at  different  times, 
to  widen  when  blood  plasma  and  blood  corpuscles  are  being  pressed 
through  it  and  to  narrow  again  when  the  pressure  is  lessened  or 
cut  off.  The  same  thin  sheet  permits  water  and  substances, 
including  gases,  in  solution  to  pass  through  itself  from  the  blood 
to  the  tissue  outside  the  capillary  and  from  the  tissue  to  the 
blood,  and  thus  carries  on  the  interchange  of  material  between  the 
blood  and  the  tissue.  In  certain  circumstances  at  all  events  white 
and  even  red  corpuscles  may  also  pass  through  the  wall  to  the 
tissue  outside. 

The  minute  arteries  and  veins  with  which  the  capillaries  are 
continuous  allow  of  a  similar  interchange  of  material,  the  more  so 
the  smaller  they  are. 

The  walls  of  the  veins  are  thinner,  weaker  and  less  elastic 
than  those  of  the  arteries,  and  possess  a  very  variable  amount  of 
muscular  tissue  ;  they  collapse  when  the  veins  are  empty.  Though 
all  veins  are  more  or  less  elastic  and  some  veins  are  distinctly 
muscular,  the  veins  as  a  whole  cannot,  like  the  arteries,  be 
characterized  as  eminently  elastic  and  contractile  tubes ;  they 
are  rather  to  be  regarded  as  simple  channels  for  conveying  the 
blood  from  the  capillaries  to  the  heart,  having  just  so  much 
elasticity  as  will  enable  them  to  accommodate  themselves  to  the 
quantity  of  blood  passing  through  them,  the  same  vein  being  at 
one  time  full  and  distended  and  at  another  time  empty  and 
shrunk,  and  only  gifted  with  any  great  amount  of  muscular 
contractility  in  special  cases  for  special  reasons.  The  united 
sectional  area  of  the  veins,  like  that  of  the  arteries,  diminishes 
from  the  capillaries  to  the  heart;  but  the  united  sectional  area 
of  the  vense  cavae  at  their  junction  with  the  right  auricle  is 
greater  than,  nearly  twice  as  great  as,  that  of  the  aorta  at  its 
origin.  The  total  capacity  also  of  the  veins  is  much  greater  than 
that  of  the  arteries.  The  veins  alone  can  hold  the  total  mass  of 
blood  which  in  life  is  distributed  over  both  arteries  and  veins. 
Indeed  nearly  the  whole  blood  is  capable  of  being  received  by 
what  is  merely  a  part  of  the  venous  system,  viz.  the  vena  portae 
and  its  branches. 


SEC.   2.     THE   MAIN   FACTS   OF   THE   CIECULATION. 


§  95.  Before  we  attempt  to  study  in  detail  the  working  of 
these  several  parts  of  the  mechanism,  it  will  be  well,  even  at  the 
risk  of  some  future  repetition,  to  take  a  brief  survey  of  some 
of  the  salient  features. 

At  each  beat  of  the  heart,  which  in  man  is  repeated  about  72 
times  a  minute,  the  contraction  or  systole  of  the  ventricles  drives 
a  quantity  of  blood  with  very  great  force  into  the  aorta  (and  the 
same  quantity  of  blood  with  less  force  into  the  pulmonary  artery) ; 
the  actual  amount  varies  from  time  to  time,  but  180  c.c.  (4  to  6  oz.) 
may  be  taken  as  a  rather  high  estimate.  The  discharge  of  blood 
from  the  ventricle  into  the  aorta  is  very  rapid,  and  the  time 
taken  up  by  it  is,  as  we  shall  see,  less  than  the  time  which  inter- 
venes between  it  and  the  next  discharge  of  the  next  beat.  So 
that  the  flow  from  the  heart  into  the  arteries  is  most  distinctly 
intermittent,  sudden,  rapid  discharges  alternating  with  relatively 
longer  intervals,  during  which  the  arteries  receive  no  blood  from 
the  heart. 

At  each  beat  of  the  heart  just  as  much  blood  flows,  as  we  shall 
see,  from  the  veins  into  the  right  auricle  as  escapes  from  the  left 
ventricle  into  the  aorta ;  but,  as  we  shall  also  see,  this  inflow  is 
much  slower,  takes  a  longer  time,  than  the  discharge  from  the 
ventricle. 

When  the  finger  is  placed  on  an  artery  in  the  living  body,  a 
sense  of  resistance  is  felt,  and  this  resistance  seems  to  be  increased 
at  intervals,  corresponding  to  the  heart  beats,  the  artery  at  each 
heart  beat  being  felt  to  rise  up  or  expand  under  the  finger, 
constituting  what  we  shall  study  hereafter  as  the  jmlse.  In  certain 
arteries  this  pulse  may  be  seen  by  the  eye.  When  the  finger  is 
similarly  placed  on  a  corresponding  vein,  very  little  resistance  is 
felt,  and  under  ordinary  circumstances  no  pulse  can  be  perceived 
by  the  touch  or  by  the  eye. 

When  an  artery  is  severed,  the  flow  of  blood  from  the  proximal 
cut  end,  that  on  the  heart  side,  is  not  equable,  but  comes  in  jets, 


154  BLOOD  PKESSUEE.  [Book  i. 

corresponding  to  the  heart  beats,  though  the  flow  does  not  cease 
between  the  jets.  The  blood  is  ejected  with  considerable  force, 
and  may,  in  a  large  artery  of  a  large  animal,  be  spurted  out  to  the 
distance  of  some  feet.  The  larger  the  artery  and  the  nearer  to  the 
heart,  the  greater  the  force  with  which  the  blood  issues,  and  the 
more  marked  the  intermittence  of  the  flow.  The  flow  from  the 
distal  cut  end,  that  away  from  the  heart,  may  be  very  slight,  or 
may  take  place  with  considerable  force  and  marked  intermittence, 
according  to  the  amount  of  collateral  communication. 

When  a  corresponding  vein  is  severed,  the  flow  of  blood,  which 
is  chiefly  from  the  distal  cut  end,  that  in  connection  with  the 
capillaries,  is  not  jerked  but  continuous  ;  the  blood  comes  out  with 
comparatively  little  force,  and  '  wells  up '  rather  than  '  spurts  out.' 
The  flow  from  the  proximal  cut  end,  that  on  the  heart  side,  may 
amount  to  nothing  at  all,  or  may  be  slight,  or  may  be  considerable, 
depending  on  the  presence  or  absence  of  valves  and  the  amount 
of  collateral  communication. 

When  an  artery  is  ligatured,  the  vessel  swells  on  the  proximal 
side,  towards  the  heart,  and  the  throbbing  of  the  pulse  may  be 
felt  right  up  to  the  ligature.  On  the  distal  side,  the  vessel  is 
empty  and  shrunk,  and  no  pulse  can  be  felt  in  it  unless  there 
be  free  collateral  communication. 

When  a  vein  is  ligatured,  the  vessel  swells  on  the  distal  side, 
away  from  the  heart,  but  no  pulse  is  felt ;  while  on  the  proximal 
side,  towards  the  heart,  it  is  empty  and  collapsed  unless  there  be 
too  free  collateral  communication. 

§  96.  When  the  interior  of  an  artery,  for  instance  the  carotid, 
is  placed  in  communication  with  a  long  glass  tube  of  not  too  great 
a  bore,  held  vertically,  the  blood,  immediately  upon  the  communi- 
cation being  effected,  may  be  seen  to  rush  into  and  to  fill  the  tube 
for  a  certain  distance,  forming  in  it  a  column  of  blood  of  a  certain 
height.  The  column  rises  not  steadily  but  by  leaps,  each  leap 
corresponding  to  a  heart  beat,  and  each  leap  being  less  than  its 
predecessor;  and  this  goes  on,  the  increase  in  the  height  of  the 
column  at  each  heart  beat  each  time  diminishing,  until  at  last 
the  column  ceases  to  rise,  and  remains  for  a  while  at  a  mean  level, 
above  and  below  which  it  oscillates  with  slight  excursions  at  each 
heart  beat. 

To  introduce  such  a  tube,  an  artery,  say  the  carotid  of  a  rabbit, 
is  laid  bare,  ligatured  at  a  convenient  spot,  V  Fig.  25,  and  further 
temporarily  closed  a  little  distance  lower  down  nearer  the  heart  by  a 
small  pair  of  '  bull-dog '  forceps,  hd,  or  by  a  ligature  -which  can  be 
easily  slipped.  A  V-shaped  cut  is  now  made  in  the  artery  between 
the  forceps,  bd,  and  the  ligature  V  (only  the  drop  or  two  of  blood 
which  happens  to  remain  enclosed  between  the  two  being  lost)  :  the 
end  of  the  tube,  represented  by  c  in  the  figure,  is  introduced  into  the 
artery  and  .secured  by  the  ligature  /.  The  interior  of  the  tube  is  now 
in   free  communication   with  the  interior  of  the   artery,   but  the   latter 


Chap,  iv.]  THE   VASCULAR   MECHANISM.  155 

is,  by  means  of  the  forceps,  at  present  shut  off  from  the  heart.  On 
removing  the  forceps  a  direct  communication  is  at  once  established 
between  the  tube  and  the  artery  below;  in  consequence  the  blood  from 
the  heart  flows  through  the  artery  into  the  tube. 

This  experiment  shews  that  the  blood  as  it  is  flowing  into  the 
carotid  is  exerting  a  considerable  pressure  on  the  walls  of  the 
artery.  At  the  moment  when  the  forceps  is  removed,  there  is 
nothing  but  the  ordinary  pressiire  of  the  atmosphere  to  counter- 
balance this  pressure  within  the  artery,  and  consequently  a 
quantity  of  blood  is  pressed  out  into  the  tube  ;  and  this  goes  on 
until  the  column  of  blood  in  the  tube  reaches  such  a  height  that 
its  weight  is  equal  to  the  pressure  within  the  artery,  whereupon 
no  more  blood  escapes.  The  whole  column  continues  to  be  raised 
a  little  at  each  heart  beat,  but  sinks  as  much  during  the  interval 
between  each  two  beats,  and  thus  oscillates,  as  we  have  said, 
above  and  below  a  mean  level.  In  a  rabbit  this  column  of  blood 
will  generally  have  the  height  of  about  90  cm.  (.3  feet) ;  that  is  to 
say,  the  pressure  which  the  blood  exerts  on  the  walls  of  the  carotid 
of  a  rabbit  is  equal  to  the  pressure  exerted  by  a  column  of  rabbit's 
blood  90  cm.  high.  This  is  equal  to  the  pressure  of  a  column 
of  water  about  95  cm.  high,  and  to  the  pressure  of  a  column  of 
mercury  about  70  mm.  high. 

If  a  like  tube  be  similarly  introduced  into  a  corresponding 
vein,  say  the  jugular  vein,  it  will  be  found  that  the  column  of 
blood,  similarly  formed  in  the  tube,  will  be  a  very  low  one,  not 
more  than  a  very  few  centimeters  high  ;  and  that  while  the  level 
of  the  column  may  vary  a  good  deal,  owing  as  we  shall  see  later 
to  the  influence  of  the  respiratory  movements,  there  will  not,  as 
in  the  artery,  be  oscillations  corresponding  to  the  heart  beats. 

We  learn,  then,  from  this  simple  experiment,  that  in  the  carotid 
of  the  rabbit  the  blood,  while  it  flows  through  that  vessel,  is 
exerting  a  considerable  mean  pressure  on  the  arterial  walls,  equi- 
valent to  that  of  a  column  of  mercury  about  70  mm.  high,  but  that 
in  the  jugular  vein  the  blood  exerts  on  the  venous  walls  a  very 
slight  mean  pressure,  equivalent  to  that  of  a  column  of  blood  a  few 
centimeters  high,  or  of  a  column  of  mercury  three  or  four  milli- 
meters high.  We  speak  of  this  mean  pressure  exerted  by  the 
blood  on  the  walls  of  the  blood  vessels  as  blood  pressure,  and  we 
say  that  the  blood  pressure  in  the  carotid  of  the  rabbit  is  very 
high  (70  mm.  Hg.),  while  that  in  the  jugular  vein  is  very  low  (only 
3  or  4  mm.  Hg.). 

In  the  normal  state  of  things,  the  blood  flows  through  the 
carotid  to  the  arterial  branches  beyond,  and  through  the  jugular 
vein  towards  the  heart ;  the  pressure  exerted  by  the  blood  on  the 
artery,  or  on  the  vein  is  a  lateral  pressure  on  the  walls  of  the 
artery  and  vein  respectively.  In  the  above  experiment  the  pres- 
sure measured  is  not  exactly  this,  but  the  pressure  exerted  at  the 
end  of  the  artery  (or  of  the  vein)  where  the  tube  is  attached.    We 


156  BLOOD  PRESSURE.  [Book  i. 

might  directly  measure  the  lateral  pressure  in  the  carotid  by  some- 
what modifying  the  procedure  described  above.  We  might  connect 
the  carotid  with  a  tube,  the  end  of  which  was  not  straight  but 
made  in  the  form  of  a  [-  piece,  and  might  introduce  the  |-  piece 
in  such  a  way  that  the  blood  should  flow  along  one  limb  (the 
vertical  limb)  of  the  |-  piece  from  the  proximal  to  the  distal  part 
of  the  carotid,  and  at  the  same  time  by  the  other  (horizontal)  limb 
of  the  I-  piece  into  the  main,  upright  part  of  the  glass  tube.  The 
column  of  blood  in  the  tube  would  then  be  a  measure  of  the 
pressure  which  the  blood,  as  it  is  flowing  along  the  carotid,  is 
exerting  on  a  portion  of  its  walls  corresponding  to  the  mouth  of 
the  horizontal  limb  of  the  |-  piece.  If  we  were  to  introduce 
into  the  aorta,  at  the  place  of  origin  of  the  carotid,  a  similar 
(larger)  h-  piece,  and  to  connect  the  glass  tube  with  the  horizontal 
limb  of  the  |-  piece  by  a  piece  of  elastic  tubing  of  the  same  length 
and  bore  as  the  carotid,  the  column  of  blood  rising  up  in  the  tube 
would  be  the  measure  of  the  lateral  pressure  exerted  by  the  blood 
on  the  walls  of  the  aorta  at  the  origin  of  the  carotid  artery,  and 
transmitted  to  the  rigid  glass  tube  through  a  certain  length  of 
elastic  tubing.  And,  indeed,  what  is  measured  in  the  experiment 
previously  described  is  not  the  lateral  pressure  in  the  carotid  itself 
at  the  spot  where  the  glass  tube  is  introduced,  but  the  lateral 
pressure  of  the  aorta  at  the  origin  of  the  carotid,  modified  by  the 
influences  exerted  by  the  length  of  the  carotid  between  its  origin 
and  the  spot  where  the  tube  is  introduced. 

§  97.  Such  an  experiment  as  the  one  described  has  the  dis- 
advantages that  the  animal  is  weakened  by  the  loss  of  the  blood, 
which  goes  to  form  the  column  in  the  tube,  and  that  the  blood 
in  the  tube  soon  clots,  and  so  brings  the  experiment  to  an  end. 
Blood  pressure  may  be  more  conveniently  studied  by  connecting 
the  interior  of  the  artery  (or  vein)  with  a  mercury  gauge  or 
manometer,  Fig.  25,  the  proximal,  descending  limb  of  which,  m, 
is  filled  above  the  mercury  with  some  innocuous  fluid,  as  is  also 
the  tube  connecting  the  manometer  with  the  artery.  Using  such 
an  instrument  we  should  observe  very  much  the  same  facts  as  in 
the  more  simple  experiment. 

Immediately  that  communication  is  established  between  the 
interior  of  the  artery  and  the  manometer,  blood  rushes  from  the 
former  into  the  latter,  driving  some  of  the  mercury  from  the  de- 
scending limb,  m,  into  the  ascending  limb,  m',  and  thus  causing 
the  level  of  the  mercury  in  the  ascending  limb  to  rise  rapidly. 
This  rise  is  marked  by  jerks  corresponding  with  the  heart  beats. 
Having  reached  a  certain  level,  the  mercury  ceases  to  rise  any 
more.  It  does  not,  however,  remain  absolutely  at  rest,  but  under- 
goes oscillations  ;  it  keeps  rising  and  falling.  Each  rise,  which  is 
very  slight  compared  with  the  total  height  to  which  the  mercury 
has  risen,  has  the  same  rhythm  as  the  systole  of  the  ventricle. 
Similarly,  each  fall  corresponds  with  the  diastole. 


Chap.  iv.1  THE   VASCULAR  MECHANISM. 


15: 


Fig.  25 


158  BLOOD  PRESSUEE.  [Book  j. 

Fig.  25.    Apparatus  for  investigating  Blood  Pressure. 

At  the  upper  right-hand  corner  is  seen,  on  an  enlarged  scale,  the  carotid  artery, 
clamped  by  the  forceps  hd,  with  the  vagus  nerve  v  lying  by  its  side.  The  artery 
has  been  ligatured  at  /',  and  the  glass  cannula  c  has  been  introduced  into  the  artery 
between  the  ligature  /'  and  the  forceps  bd,  and  secured  in  position  by  the  ligature  /. 
The  shrunken  artery  on  the  distal  side  of  the  cannula  is  seen  at  ca'. 

p.b.  is  a  box  containing  a  bottle  holding  a  saturated  solution  of  sodium  car- 
bonate, or  of  sodium  bicarbonate,  or  a  mixture  of  the  two,  and  capable  of  being 
raised  or  lowered  at  pleasure.  The  solution  flows  by  the  tube  p.t.  regulated  by  the 
clamp  c"  into  the  tube  t.  A  syringe,  with  a  stopcock,  may  be  substituted  for  the 
bottle,  and  attached  at  c".  This,  indeed,  is  in  many  respects  a  more  convenient  plan. 
The  tube  t  is  connected  with  the  leaden  tube  t,  and  the  stopcock  c  with  the  mano- 
meter, of  which  711  is  the  descending  aud  m'  the  ascending  limb,  and  s  the  support. 
The  mercury  in  the  ascending  limb  bears  on  its  surface  the  float  _//,  a  long  rod 
attached  to  which  is  fitted  with  the  pen  p,  writing  on  the  recording  surface  r.  The 
clamp  cl.  at  the  end  of  the  tube  t  has  an  arrangement  shewn  on  a  larger  scale  at 
the  right-hand  upper  corner. 

The  descending  tube  m  of  the  manometer  and  the  tube  t  being  completely  filled 
along  its  whole  length  with  fluid  to  the  exclusion  of  aU  air,  the  cannula  c  is  filled 
with  fluid,  slipped  into  the  open  end  of  the  thick-walled  india  rubber  tube  i,  until  it 
meets  the  tube  t  (whose  position  within  the  india  rubber  tube  is  shewn  by  the  dotted 
lines),  and  is  then  securely  fixed  in  this  position  by  the  clamp  cl. 

The  stopcocks  c  and  c"  are  now  opened,  and  the  pressure-bottle  raised  or  fluid 
driven  in  by  the  syringe  until  the  mercury  in  the  manometer  is  raised  to  the 
required  height.  The  clamp  c"  is  then  closed  and  the  forceps  bd  removed  from  the 
artery.  The  pressure  of  the  blood  in  the  carotid  ca.  is  in  consequence  brought  to 
bear  through  t  upon  the  mercury  in  the  manometer. 

If  a  float,  swimming  on  the  top  of  the  mercury  in  the  ascending 
limb  of  the  manometer,  and  bearing  a  brush  or  other  marker,  be 
brought  to  bear  on  a  travelling  surface,  some  such  tracing  as  that 
represented  in  Fig.  26  will  be  described.     Each  of  the  smaller 


Fig.  26.   Tracing  of  Arterial  Pressure  with  a  Mercury  Manometer 

The  smaller  curves  p  p  are  the  pulse-curves.  The  space  from  r  to  r  embraces 
a  respiratory  undulation.  The  tracing  is  taken  from  a  dog,  and  the  irregularities 
visible  in  it  are  those  frequently  met  with  in  this  animal. 

curves  (p,  p)  corresponds  to  a  heart  beat,  the  rise  corresponding  to 
the  systole,  and  the  fall  to  the  diastole  of  the  ventricle.  The  larger 
undulations  (r,  r)  in  the  tracing,  which  are  respiratory  in  origin, 
will  be  discussed  hereafter.  In  Fig.  27  are  given  two  tracings 
taken  from  the  carotid  of  a  rabbit ;  in  the  lower  curvp,  the  record- 
ing surface  is  travelling  more  rapidly  than  in  the  upper  curve  ; 
otherwise  the  curves  are  alike  and  repeat  the  general  features  of 
the  curve  from  the  dog. 


Chap,  xv.]  THE   VASCULAR   MECHANISM.  159 


Fig.  27.    Blood  Pressure  Curves  from  the  Carotid  of  Rabbit,  the  Time 
Marker  in  each  case  marking  Seconds. 

Desmption  of  Experivient.  Into  a  carotid,  or  other  blood  vessel, 
prepared  as  explained,  a  small  glass  tube,  of  suitable  bore,  called  a 
cannula,  is  introduced  by  the  method  described  above,  and  is  subse- 
quently connected  by  means  of  a  short  piece  of  india  rubber  tubing  (Fig. 
25  i),  and  a  leaden  or  other  tube  t,  which  is  at  once  flexible  and  yet  not 
extensible,  with  the  descending  limb,  m,  of  the  manometer  or  mercury 
gauge.  The  cannula,  tube,  and  descending  limb  of  the  manometer  are 
all  tilled  with  some  fluid  which  tends  to  prevent  clotting  of  the 
blood,  the  one  chosen  being  generally  a  strong  .solution  of  sodium 
bicarbonate,  but  other  fluids  may  be  chosen.  In  order  to  avoid  loss 
of  blood,  a  quantity  of  fluid  is  injected  into  the  flexible  tube  suf- 
ficient to  raise  the  mercury  in  the  ascending  limb  of  the  manometer 
to  a  level  a  very  little  below  what  may  be  beforehand  guessed  at 
as  the  probable  mean  pressure.  When  the  forceps  bd  is  removed, 
the  pressure  of  the  blood  in  the  carotid  is  transmitted  through  the 
flexible  tube  to  the  manometer,  the  level  of  the  mercury  in  the  ascend- 
ing limb  of  which  rises  a  little,  or  sinks  a  little  at  first,  or  may  do 
neither,  according  to  the  success  with  which  the  probable  mean  pres- 
sure has  Vjeen  guessed,  and  continues  to  exhibit  the  characteristic 
oscilhitions  until  the  experiment  is  brought  to  an  end  by  the  blood 
clotting  or  otherwise. 

Tracings  of  the  movements  of  the  column  of  mercury  in  the  mano- 
meter may  be  taken  either  on  a  smoked  surface  of  a  levolving  cylinder 
(Fig.  1),  or  by  means  of  ink  on  a  continuous  roll  of  paper,  as  in  the 
more  complex  kymograph   (Fig.   28). 

§  98.  By  the  help  of  the  manometer  applied  to  various 
arteries  and  veins  we  learn  the  following  facts : 

(1)  The  mean  blood  pressure  is  high  in  all  the  arteries,  but 
is  greater  in  the  larger  arteries  nearer  the  heart  than  in  the 
smaller  arteries  farther  from  the  heart ;  it  diminishes,  in  fact, 
along  the  arterial  tract  from  the  heart  towards  the  capillaries. 

(2)  The  mean  blood  pressure  is  low  in  the  veins,  but  is  greater 
in  the  smaller  veins  nearer  the  capillaries  than  in  the  larger  veins 
nearer  the  heart,  diminishing,  in  fact,  from  the  capillaries  towards 
the  heart.     In  the  large  veins  near  the  heart  it  may  be  negative, 


160 


BLOOD  PEESSURE. 


[Book  i. 


that  is  to  say,  the  pressure  of  blood  in  the  vein  bearing  on  the 
proximal  descending  limb  of    the  manometer  may  be  less  than 


Fig.  28.    Ludwig's  Kymograph  for  recording  on  a  continuous  roll,  of  paper. 

the  pressure  of  the  atmosphere  on  the  ascending  distal  limb,  so 
that  when  communication  is  made  between  the  interior  of  the  vein 
and  the  manometer,  the  mercury  sinks  in  the  distal  and  rises  in 
the  proximal  limb,  being  sucked  up  towards  the  vein. 

The  manometer  cannot  well  be  applied  to  the  capillaries,  but  we 
may  measure  the  blood  pressure  in  the  capillaries  in  an  indirect  way. 
It  is  well  known  that  when  any  portion  of  the  skin  is  pressed  upon, 
it  becomes  pale  and  bloodless  ;  this  is  due  to  the  pressure  driving 
the  blood  out  of  the  capillaries  and  minute  vessels,  and  preventing 
any  fresh  blood  entering  into  them.  By  carefully  investigating 
the  amount  of  pressure  necessary  to  prevent  the  blood  entering 
the  capillaries  and  minute  arteries  of  the  web  of  the  frog's  foot,  or 
of  the  skin  beneath  the  nail  or  elsewhere  in  man,  the  internal 
pressure  which  the  blood  is  exercising  on  the  walls  of  the  capil- 
laries and  minute  arteries  and  veins  may  be  approximately  deter- 
mined. In  the  frog's  web  this  has  been  found  to  be  equal  to 
about  7  or  11  mm.  mercury.  In  the  mammal,  the  capillary  blood 
pressure  is  naturally  higher  than  this,  and  may  be  put  down  at 


Chap,  iv.]  THE   VASCULAR   MECHANISM. 


161 


from  15  to  20  mm.     It  is,  therefore,  considerable,  being  greater 
than  that  in  the  veins,  though  less  than  that  in  the  arteries. 

(3)  There  is  thus  a  continued  decline  of  blood  pressure  from 
the  root  of  the  aorta,  through  the  arteries,  capillaries  and  veins  to 
the  right  auricle.  We  find,  however,  on  examination,  that  the  most 
marked  fall  of  pressure  takes  place  between  the  small  arteries  on 
the  one  side  of  the  capillaries,  and  the  small  veins  on  the  other, 
the  curve  of  pressure  being  somewhat  of  the  form  given  in 
Fig.  29,  which  is  simply  intended  to  shew  this  fact  graphically, 
and  has  not  been  constructed  by  exact  measurements. 


Fig.  29.     Diagram  of  Blood  Pressttre. 

A,  Arteries.      P,  Peripheral  Kegion   (minute  arteries,  capillaries  and  veins). 

V,  Veins. 

(4)  In  the  arteries  this  mean  pressure  is  marked  by  oscillations 
corresponding  to  the  heart  beats,  each  oscillation  consisting  of  a 
rise  (increase  of  pressure  above  the  mean)  corresponding  to  the 
systole  of  the  ventricle,  followed  by  a  fall  (decrease  of  pressure 
below  the  mean)  corresponding  to  the  diastole  of  the  ventricle. 

(5)  These  oscillations,  which  we  may  speak  of  as  the  pulse, 
are  largest  and  most  conspicuous  in  the  large  arteries  near  the 
heart,  diminish  from  the  heart  towards  the  capillaries,  and  are, 
under  ordinary  circumstances,  wholly  absent  from  the  veins  along 
their  whole  extent  from  the  capillaries  to  the  heart. 

Obviously  a  great  change  takes  place  in  that  portion  of  the 
circulation  which  comprises  the  capillaries,  the  minute  arteries 
leading  to  and  the  minute  veins  leading  away  from  the  capillaries, 
and  which  we  may  speak  of  as  the  "  peripheral  region."  It  is  here 
that  a  great  drop  of  pressure  takes  place  ;  it  is  here,  also,  that  the 
pulse  disappears. 

§  99.  if  the  web  of  a  frog's  foot  be  examined  with  a  micro- 
scope, the  blood,  as  judged  of  by  the  movements  of  the  corpuscles, 
is  seen  to  be  passing  in  a  continuous  stream  from  the  small 
arteries  through  the  capillaries  to  the  veins.  The  velocity  is 
greater  in  the  arteries  than  in  the  veins,  and  greater  in  both  than 
in  the  capillaries.     In  the  arteries  faint  pulsations,  synchronous 

11 


162  CAPILLARY   CIRCULATION.  [Book  i. 

witli  the  heart's  beat,  are  frequently  visible ;  but  these  disappear 
in  the  capillaries,  in  which  the  flow  is  even ;  that  is,  not  broken  by 
pulsations,  and  this  evenness  of  flow  is  continued  on  along  the 
veins  so  far  as  we  can  trace  them.  Not  infrequently  variations  in 
velocity  and  in  the  distribution  of  the  blood,  due  to  causes  which 
will  be  hereafter  discussed,  are  witnessed  from  time  to  time. 

The  character  of  the  flow  through  the  smaller  capillaries  is 
very  variable.  Sometimes  the  corpuscles  are  seen  passing  through 
the  channel  in  single  file  with  great  regularity ;  at  other  times 
they  may  be  few  and  far  between.  Some  of  the  capillaries,  as 
we  have  said,  are  wide  enough  to  permit  two  or  more  corpuscles 
abreast.  In  all  cases  the  blood,  as  it  passes  through  the  capillary, 
stretches  the  walls  and  expands  the  tube.  Sometimes  a  corpuscle 
may  remain  stationary  at  the  entrance  into  a  capillary,  the  channel 
itself  being  for  some  little  distance  entirely  free  from  corpuscles. 
Sometimes  many  corpuscles  will  appear  to  remain  stationary  in  one 
or  more  capillaries  for  a  brief  period,  and  then  move  on  again.  Any 
one  of  these  conditions  readily  passes  into  another ;  and,  especially 
with  a  somewhat  feeble  circulation,  instances  of  all  of  them  may 
be  seen  in  the  same  field  of  the  microscope.  It  is  only  when  the 
vessels  of  the  web  are  unusually  full  of  blood  that  all  the  capil- 
laries can  be  seen  equally  filled  with  corpuscles.  The  long,  oval^ 
red  corpuscle  moves  with  its  long  axis  parallel  to  the  stream, 
occasionally  rotating  on  its  long  axis,  and  sometimes,  in  the  larger 
channels,  on  its  short  axis.  The  flexibility  and  elasticity  of  a 
corpuscle  are  well  seen  when  it  is  being  driven  into  a  capillary 
narrower  than  itself,  or  when  it  becomes  temporarily  lodged  at 
the  angle  between  two  diverging  channels. 

These,  and  other  phenomena  on  which  we  shall  dwell  later  on, 
may  be  readily  seen  in  the '  web  of  the  frog's  foot  or  in  the 
stretched-out  tongue  or  in  the  mesentery  of  the  frog ;  and  essen- 
tially similar  phenomena  may  be  observed  in  the  mesentery  or 
other  transparent  tissue  of  a  mammal.  All  over  the  body, 
wherever  capillaries  are  present,  the  corpuscles  and  the  plasma 
are  being  driven  in  a  continuous,  and  though  somewhat  irre- 
gular, yet,  on  the  whole,  steady  flow  through  channels  so  minute 
that  the  passage  is  manifestly  attended  with  considerable  diffi- 
culties. 

It  is  obvious  that  the  peculiar  characters  of  the  flow  through 
the  minute  arteries,  capillaries,  and  veins,  afford  an  explanation 
of  the  great  change,  taking  place  in  the  peripheral  region,  between 
the  arterial  flow  and  the  venous  flow.  The  united  sectional  area 
of  the  capillaries  is,  as  we  have  seen,  some  hundreds  of  times 
greater  than  the  sectional  area  of  the  aorta ;  but  this  united 
sectional  area  is  made  up  of  thousands  of  minute  passages,  vary- 
ing in  man  from  5  to  20  ^i,  some  of  them,  therefore,  being  in 
an  undistended  condition,  smaller  than  the  diameter  of  a  red 
corpuscle.     Even  were  the  blood  a  simple  liquid  free  from  all 


Chap,  iv.]  THE   VASCULAE   MECHANISM.  163 

corpuscles,  these  extremely  minute  passages  would  occasion  a 
very  great  amount  of  friction,  and  thus  present  a  considerable 
obstacle  or  resistance  to  the  flow  of  blood  through  them.  Still 
greater  must  be  the  friction  and  resistance  occasioned  by  the 
actual  blood  with  its  red  and  white  corpuscles.  The  blood,  in  fact, 
meets  with  great  difficulties  in  its  passage  through  the  peripheral 
region,  and  sometimes,  as  we  shall  see,  the  friction  and  resistance 
are  so  great  in  the  peripheral  vessels  of  this  or  that  area  that  no 
blood  at  all  passes  through  them,  and  an  arrest  of  the  flow  takes 
place  in  the  area. 

The  resistance  to  the  flow  of  blood  thus  caused  by  the  friction 
generated  in  so  many  minute  passages  is  one  of  the  most  important 
physical  facts  in  the  circulation.  In  the  large  arteries  the  friction 
is  small ;  it  increases  gradually  as  they  divide,  but  receives  its 
chief  and  most  important  addition  in  the  minute  arteries  and 
capillaries  :  it  is  relatively  greater  in  the  minute  arteries  than  in 
the  capillaries  on  account  of  the  flow  being  more  rapid  in  the 
former,  for  friction  diminishes  rapidly  with  a  diminution  in  the 
rate  of  flow.  We  may  speak  of  it  as  the  '  peripheral  friction,' 
and  the  resistance  which  it  offers  as  the  'peripheral  resistance.' 
It  need,  perhaps,  hardly  be  said  that  this  peripheral  resistance 
not  only  opposes  the  flow  of  blood  through  the  capillaries  and 
minute  arteries  themselves  where  it  is  generated,  but,  working 
backwards  along  the  whole  arterial  system,  has  to  be  overcome 
by  the  heart  at  each  systole  of  the  ventricle. 


Hydraulic  Principles  of  the  Circulation. 

§  100.  In  the  circulation,  then,  the  following  three  facts  of 
fundamental  importance  are  met  with  : 

1.  The  systole  of  the  ventricle,  driving  at  intervals  a  certain 
quantity  of  blood,  with  a  certain  force,  into  the  aorta. 

2.  The  peripheral  resistance  just  described. 

3.  A  long  stretch  of  elastic  tubing  (the  arteries),  reacliing 
from  the  ventricle  to  the  region  of  peripheral  resistance. 

From  these  facts  we  may  explain  the  main  phenomena  of  the 
circulation,  which  we  have  previously  sketched,  on  purely  physical 
principles,  without  any  appeal  to  the  special  properties  of  living 
tissues,  beyond  the  provision  that  the  ventricle  remains  capable 
of  good  rhythmical  contractions,  that  the  arterial  walls  retain 
their  elasticity,  and  that  the  friction  between  the  blood  and  the 
lining  of  the  peripheral  vessels  remains  the  same  ;  we  may  thus 
explain^  the  high  pressure  and  pulsatile  flow  in  the  arteries,  the 
steady  stream  through  the  capillaries,  the  low  pressure  and  the 
uniform  pulseless  flow  in  the  veins,  and,  finally,  the  continued  flow 
of  the  blood  from  the  aorta  to  the  mouths  of  the  venre  cava3. 

All  the  above  phenomena  in  fact  are  the  simple  results  of  an 


164  HYDRAULIC  PRINCIPLES.  [Book  i. 

intermittent  force  (like  that  of  the  systole  of  the  ventricle)  working 
in  a  closed  circuit  of  branching  tubes  so  arranged  that,  while  the 
individual  tubes  first  diminish  in  calibre  (from  the  heart  to  the 
capillaries)  and  then  increase  (from  the  capillaries  to  the  heart), 
the  area  of  the  bed  first  increases  and  then  diminishes,  the  tubes 
together  thus  forming  two  cones  placed  base  to  base  at  the  capil- 
laries, with  their  apices  converging  to  the  heart,  and  presenting 
at  their  conjoined  bases  a  conspicuous  peripheral  resistance,  the 
tubing  on  one  side,  the  arterial,  being  eminently  elastic,  and  on 
the  other,  the  venous,  affording  a  free  and  easy  passage  for  the 
blood.  It  is  the  peripheral  resistance  (for  the  resistance  offered 
by  the  friction  in  the  larger  vessels  may,  when  compared  with 
this,  be  practically  neglected),  reacting  through  the  elastic  walls 
of  the  arteries  upon  the  intermittent  force  of  the  heart,  which 
gives  the  circulation  of  the  blood  its  peculiar  features. 

§  101.  Circumstances  deterTnining  the  character  of  the  flow. 
When  fluid  is  driven  by  an  intermittent  force,  as  by  a  pump, 
through  a  perfectly  rigid  tube,  such  as  a  glass  one  (or  a  system  of 
such  tubes),  there  escapes  at  each  stroke  of  the  pump  from  the 
distal  end  of  the  tube  (or  system  of  tubes)  just  as  much  fluid  as 
enters  it  at  the  proximal  end.  What  happens  is  very  like  what 
would  happen  if,  with  a  wide  glass  tube  completely  filled  with 
billiard  balls  lying  in  a  row,  an  additional  ball  were  pushed  in  at 
one  end ;  each  ball  would  be  pushed  on  in  turn  a  stage  further, 
and  the  last  ball  at  the  further  end  would  tumble  out.  The 
escape,  moreover,  takes  place  at  the  same  time  as  the  entrance. 

This  result  remains  the  same  when  any  resistance  to  the  flow  is 
introduced  into  the  tube,  as,  for  instance,  when  the  end  of  the  tube 
is  narrowed.  The  force  of  the  pump  remaining  the  same,  the 
introduction  of  the  resistance  undoubtedly  lessens  the  quantity 
of  fluid  issuing  at  the  distal  end  at  each  stroke,  but  it  at  the 
same  time  lessens  the  quantity  entering  at  the  proximal  end  ; 
the  inflow  and  outflow  remain  equal  to  each  other,  and  still  occur 
at  the  same  time. 

In  an  elastic  tube,  such  as  an  india  rubber  one  (or  in  a  system 
of  such  tubes),  whose  sectional  area  is  sufficiently  great  to  offer 
but  little  resistance  to  the  progress  of  the  fluid,  the  flow  caused 
by  an  intermittent  force  is  also  intermittent.  The  outflow  being 
nearly  as  easy  as  the  inflow,  the  elasticity  of  the  walls  of 
the  tube  is  scarcely  at  all  called  into  play.  The  tube  behaves 
practically  like  a  rigid  tube.  When,  however,  sufficient  resistance 
is  introduced  into  any  part  of  the  course,  the  fluid,  being  unable 
to  pass  by  the  resistance  as  rapidly  as  it  enters  the  tube  from 
the  pump,  tends  to  accumulate  on  the  proximal  side  of  the  re- 
sistance. This  it  is  able  to  do  by  expanding  the  elastic  walls  of 
the  tube.  At  each  stroke  of  the  pump  a  certain  quantity  of  fluid 
enters  the  tube  at  the  proximal  end.  Of  this  only  a  fraction  can 
pass  through  the  resistance  during  the  stroke.     At  the  moment  when 


Chap.  IV.]  THE   VASCULAR   MECHANISM.  165 

the  stroke  ceases,  the  rest  still  remains  on  the  proximal  side  of  the 
resistance,  the  elastic  tube  having  expanded  to  receive  it.  During 
the  interval  between  this  and  the  next  stroke,  the  distended 
elastic  tube,  striving  to  return  to  its  natural  undistended  con- 
dition, presses  on  this  extra  quantity  of  fluid  which  it  contains 
and  tends  to  drive  it  past  the  resistance. 

Thus  in  the  rigid  tube  (and  in  the  elastic  tube  without  the 
resistance)  there  issues,  from  the  distal  end  of  the  tube,  at  each 
stroke,  just  as  much  tluid  as  enters  it  at  the  proximal  end,  while 
between  the  strokes  there  is  perfect  quiet.  In  the  elastic  tube 
with  resistance,  on  the  contrary,  the  quantity  which  passes  the 
resistance  is  only  a  fraction  of  that  which  enters  the  tube  from 
the  pump  at  any  one  stroke,  the  remainder  or  a  portion  of  the 
remainder  continuing  to  pass  during  the  interval  between  the 
strokes.  In  the  former  case,  the  tube  is  no  fuller  at  the  end  of  the 
stroke  than  at  the  beginning  ;  in  the  latter  case  there  is  an  accu- 
mulation of  fluid  between  the  pump  and  the  resistance,  and  a 
corresponding  distension  of  that  part  of  the  tube,  at  the  close  of 
each  stroke,  —  an  accumulation  and  distension,  however,  wdiich  go 
on  diminishing  during  the  interval  between  that  stroke  and  the 
next.  The  amount  of  fluid  thus  remaining  after  the  stroke  will 
depend  on  the  amount  of  resistance  in  relation  to  the  force  of  the 
stroke,  and  on  the  distensibility  of  the  tube  ;  and  the  amount  which 
passes  the  resistance  before  the  next  stroke  will  depend  on  the 
degree  of  elastic  reaction  of  which  the  tube  is  capable.  Thus,  if  the 
resistance  be  very  considerable  in  relation  to  the  force  of  the  stroke, 
and  the  tube  very  distensible,  only  a  small  portion  of  the  fluid  will 
pass  the  resistance,  the  greater  part  remaining  lodged  between  the 
pump  and  the  resistance.  If  the  elastic  reaction  be  great,  a  large 
portion  of  this  will  be  passed  on  through  the  resistance  before  the 
next  stroke  comes.  In  other  words,  the  greater  the  resistance  (in 
relation  to  the  force  of  the  stroke),  and  the  more  the  elastic  force 
is  brought  into  play,  the  less  intermittent,  the  more  nearly  conti- 
nuous, will  be  the  flow  on  the  far  side  of  the  resistance. 

If  the  first  stroke  be  succeeded  by  a  second  stroke  before  its 
quantity  of  fluid  has  all  passed  by  the  resistance,  there  will  be  an 
additional  accumulation  of  fluid  on  the  near  side  of  the  resistance, 
an  additional  distension  of  the  tube,  an  additional  strain  on  its 
elastic  powers,  and,  in  consequence,  the  flow  between  this  second 
stroke  and  the  third  will  be  even  more  marked  than  that  between 
the  first  and  the  second,  though  all  three  strokes  were  of  the  same 
force,  the  addition  being  due  to  the  extra  amount  of  elastic  force 
called  into  play.  In  fact,  it  is  evident  that,  if  there  be  a  sufficient 
store  of  elastic  power  to  fall  back  upon,  by  continually  repeating 
the  strokes  a  state  of  things  will  be  at  last  arrived  at,  in  which  the 
elastic  force,  called  into  play  by  the  continually  increasing  dis- 
tension of  the  tube  on  the  near  side  of  the  resistance,  will  be 
sufficient  to  drive  through  the  resistance,  between  each  two  strokes. 


166  ARTIFICIAL   MODEL.  [Book  i. 

just  as  much  fluid  as  enters  the  near  end  of  the  system  at  each 
stroke.  In  other  words,  the  elastic  reaction  of  the  walls  of  the 
tube  will  have  converted  the  intermittent  into  a  continuous  flow. 
The  flow  on  the  far  side  of  the  resistance  is  in  this  case  not  the 
direct  result  of  the  strokes  of  the  pump.  The  force  of  the  pump 
is  spent,  first  in  getting  up,  and  afterwards  in  keeping  up  the 
distension  of  the  tube  on  the  near  side  of  the  resistance;  the 
immediate  cause  of  the  continuous  flow  lies  in  the  distension  of 
the  tube,  which  leads  it  to  empty  itself  into  the  far  side  of  the 
resistance  at  such  a  rate  that  it  discharges  through  the  resistance 
during  a  stroke  and  in  the  succeeding  interval  just  as  much  as  it 
receives  from  the  pump  by  the  stroke  itself. 

This  is  exactly  what  takes  place  in  the  vascular  system.  The 
friction  in  the  minute  arteries  and  capillaries  presents  a  consider- 
able resistance  to  the  flow  of  blood  through  them  into  the  small 
veins.  In  consequence  of  this  resistance,  the  force  of  the  heart's 
beat  is  spent  in  maintaining  the  whole  of  the  arterial  system  in  a 
state  of  great  distension ;  the  arterial  walls  are  put  greatly  on  the 
stretch  by  the  pressure  of  the  blood  thrust  into  them  by  the  re- 
peated strokes  of  the  heart;  this  is  the  pressure  which  we  spoke  of 
above  as  blood  pressure.  The  greatly  distended  arterial  system  is, 
by  the  elastic  reaction  of  its  elastic  walls,  continually  tending  to 
empty  itself  by  overflowing  through  the  capillaries  into  the  venous 
system  ;  and  it  overflows  at  such  a  rate,  that  just  as  much  blood 
passes  from  the  arteries  to  the  veins  during  each  systole  and  its 
succeeding  diastole  as  enters  the  aorta  at  each  systole. 

§  102.  Indeed,  the  important  facts  of  the  circulation  which 
we  have  as  yet  studied  may  be  roughly  but  successfully  imitated 
on  an  artificial  model.  Fig.  30,  in  which  an  elastic  syringe  repre- 
sents the  heart,  a  long  piece  of  elastic  India  rubber  tubing  the 
arteries,  another  piece  of  tubing  the  veins,  and  a  number  of 
smaller  connecting  pieces  the  minute  arteries  and  capillaries.  If 
these  connecting  pieces  be  made  at  first  somewhat  wide,  so  as  to 
offer  no  great  resistance  to  the  flow  from  the  artificial  arteries 
to  the  artificial  veins,  but  be  so  arranged  that  they  may  be  made 
narrow,  by  the  screwing-up  of  clamps  or  otherwise,  it  is  possible  to 
illustrate  the  behaviour  of  the  vascular  mechanism  when  the  peri- 
pheral resistance  is  less  than  usual  (and  as  we  shall  see  later  on,  it 
is  possible  in  the  living  organism  either  to  reduce  or  to  increase 
what  may  be  considered  as  the  normal  peripheral  resistance),  and 
to  compare  that  behaviour  with  the  behaviour  of  the  mechanism 
when  the  peripheral  resistance  is  increased. 

The  whole  apparatus  being  placed  flat  on  a  table,  so  as  to 
avoid  differences  in  level  in  different  parts  of  it,  and  filled  with 
water,  but  so  as  not  to  distend  the  tubing,  the  two  manometers 
attached,  one.  A,  to  the  arterial  side  of  the  tubing,  and  the  other, 
V,  to  the  venous  side,  ought  to  shew  the  mercury  standing  at 
equal  heights  in  both  limbs  of  both  instruments,  since  nothing 


Chap,  iv.]  THE   VASCULAR   MECHANISM. 


167 


but  the  pressure  of  the  atmosphere  is  bearing  uu  the  fluid  in  the 
tubes,  and  that  equally  all  over. 


Fig.  30.    Arterial  Scheme. 

P,  unshaded,  is  an  elastic  tube  to  represent  the  arterial  sj-stem  branching  at 
Xand  Y,  and  ending  in  the  region  of  peripheral  resistance,  including  the  capillaries, 
which  are  imitated  by  filling  loosely  witli  small  pieces  of  sponge  the  parts  shewn  as 
dilated  in  tlie  figure.  Tlie  capillaries  are  gathered  up  into  the  venous  system,  shaded, 
which  terminates  at  0.  ^Yater  is  driven  into  tlie  arterial  system  at  P  by  means  of 
an  elastic  bag-syringe,  or  any  other  form  of  pump.  Clamps  are  placed  ou  the 
undilated  tubes  c,  c',  c".  When  these  clamps  are  tightened,  the  only  access  for  the 
water  from  tlie  arterial  to  the  venous  side  is  through  the  dilated  parts  filled  with 
sponge,  which  offer  a  considerable  resistance  to  the  flow  of  fluid  tlirough  them. 
When  the  clamps  are  unloosed  the  fluid  passes,  witli  much  less  resistance,  through 
the  undilated  tulies.  Thus  by  tightening  or  loosening  the  clamps  the  "  peripheral  " 
resistance  may  be  increased  or  diminished  at  pleasure. 

At  .1,  on  the  arterial  side,  and  at  V,  on  the  venous  side,  manometers  can  be 
attached.  At  a  and  v  (and  also  at  x  and  y)  by  means  of  clamps,  the  flow  of  fluid 
from  an  artery  and  from  a  vein,  under  various  conditions,  may  be  observed.     At  Sa, 

S'a,  and  Sr,  s])hyginographs  may  be  applied. 

If  now,  the  connecting  pieces  being  freely  open,  that  is  to  say, 
the  peripheral  resistance  being  very  little,  we  imitate  a  ventricular 
beat  by  the  stroke  of  the  pump,  we  shall  observe  the  following. 
Almost  immeiliately  after  the  stroke  the  mercury  in  the  arterial 
manometer  will  rise,  but  will  at  once  fall  again,  and  very  shortly 
afterwards  the  mercury  in  the  venous  tube  will  in  a  similar  manner 
rise  and  fall.  If  we  repeat  the  strokes  with  a  not  too  rapid  rhythm, 
each  stroke  having  the  same  force,  and  make,  as  may  by  a  simple 
contrivance  be  effected,  the  two  manometers  write  on  the  same 
recording  surface,  we  shall  obtain  curves  like  those  of  Fig.  31, 
A  and  V.  At  each  stroke  of  the  pump  the  mercury  in  the 
arterial  manometer  rises,  but  forthwith  falls  again  to  or  nearly  to 


168  ARTIFICIAL   MODEL.  [Book  i. 

the  base  line ;  no  mean  arterial  pressure,  or  very  little,  is  estab- 
lished.    The  contents  of  the  ventricle  (syringe)  thrown  into  the 


Fig.  31.    Tracings  taken  fkom  an  artificial  scheme  with  the  peripheral 

resistance  slight. 

A,  Arterial.     V,  Venous  Manometer.     This  figure,  to  save  space,  is  on  a  smaller 
scale  than  the  corresponding  Fig.  32. 

arterial  system  distend  it,  but  the  passage  through  the  peri- 
pheral region  is  so  free  that  an  equal  quantity  of  fluid  passes 
through  to  the  veins  immediately,  and  hence  the  mercury  at 
once  falls.  But  the  fluid  thus  passing  easily  into  the  veins 
distends  these  too,  and  the  mercury  in  their  manometer  rises 
too,  but  only  to  fall  again,  as  a  corresponding  quantity  issues 
from  the  ends  of  the  veins  into  the  basin,  which  serves  as  an 
artiticial  auricle.  Now  introduce  '  peripheral  resistance '  by  screw- 
ing up  the  clamps  on  the  connecting  tubes,  and  set  the  pump  to 
work  again  as  before.  With  the  first  stroke  the  mercury  in  the 
arterial  manometer.  Fig.  32,  A',  rises  as  before,  but  instead  of 
falling  rapidly,  it  falls  slowly,  because  it  now  takes  a  longer  time 
for  a  quantity  of  fluid  equal  to  that  which  has  been  thrust  into 
the  arterial  system  by  the  ventricular  stroke  to  pass  through  the 
narrowed  peripheral  region.  Before  the  curve  has  fallen  to  the 
base  line,  before  the  arterial  system  has  had  time  to  discharge 
through  the  narrowed  peripheral  region  as  much  fluid  as  it 
received  from  the  ventricle,  a  second  stroke  drives  more  fluid  into 
the  arteries,  distending  them  this  time  more  than  it  did  before, 
and  raising  the  mercury  to  a  still  higher  level.  A  third,  a  fourth, 
and  succeeding  strokes  produce  the  same  effect,  except  that  the 
additional  height  to  which  the  mercury  is  raised  at  each  stroke 
becomes  at  each  stroke  less  and  less,  until  a  state  of  things  is 
reached  in  which  the  mercury,  being  on  the  fall  when  the  stroke 
takes  place,  is  by  the  stroke  raised  just  as  high  as  it  was  before,  and 
then  beginning  to  fall  again,  is  again  raised  just  as  high,  and  so  on. 
With  each  succeeding  stroke  the  arterial  system  has  become  more 
and  more  distended ;  but  the  more  distended  it  is  the  greater  is 
the  elastic  reaction  brought  into  play.  This  greater  elastic  reaction 
more  and  more  overcomes  the  obstacle  presented  by  the  peripheral 


Chap,  iv.]  THE   VASCULAR   MECHANISM. 


169 


resistance,  and  drives  the  fluid  more  and  more  rapidly  through 
the  peripheral  region.    At  last  the  arterial  system  is  so  distended, 


VI 


Fig   32. 


Tracings  taken  from  an  artificial  scheme  with  the  i'Ekipheral 
resistance  considerable. 


A',  Arterial,  V,  Venous  Manometer. 

and  the  force  of  the  elastic  reaction  so  great,  that  during  the  stroke 
and  the  succeeding  interval  just  as  much  fluid  passes  through  the 
peripheral  region  as  enters  the  arteries  at  the  stroke.  In  other 
words,  the  repeated  strokes  have  established  a  mean  arterial  pres- 
sure which  at  the  point  where  the  manometer  is  affixed  is  raised 
slightly  at  each  ventricular  stroke,  and  falls  equally  between  the 
strokes. 

Turning  now  to  the  venous  manometer,  Fig.  32  V,  we  ob- 
serve that  each  stroke  of  the  pump  produces  on  this  much  less 
effect  than  it  did  before  the  introduction  of  the  increased  peri- 
pheral resistance.  The  mercury,  instead  of  distinctly  rising  and 
falling  at  each  stroke,  now  shews  nothing  more  than  very  gentle 
undulations  ;  it  feels  to  a  very  slight  degree  only  the  dii'ect  effect 
of  the  ventricular  stroke ;  it  is  simply  raised  slightly  above  tlie 
base  line,  and  remains  fairly  steady  at  this  level.  The  slight  rise 
marks  the  mean  pressure  exerted  by  the  fluid  at  the  place  of 
attachment  of  the  manometer.  This  mean  '  venous  '  pressure  is  a 
continuation  of  the  mean  arterial  pressure  so  obvious  in  the  arterial 
manometer,  but  is  much  less  than  that  because  a  large  part  of  the 
arterial  mean  pressure  has  been  expended  in  driving  the  fluid  past 


170  ARTIFICIAL   MODEL.  [Book  i. 

the  peripheral  resistance.  What  remains  is,  however,  sufficient 
to  drive  the  fluid  along  the  wide  venous  tubing  right  to  the 
open  end. 

Thus  this  artificial  model  may  be  made  to  illustrate  how  it 
comes  about  that  the  blood  flows  in  the  arteries  at  a  relatively 
high  pressure,  which  at  each  ventricular  systole  is  raised  slightly 
above,  and  at  each  diastole  falls  slightly  below  a  certain  mean 
level,  and  flows  in  the  veins  at  a  much  lower  pressure,  which  does 
not  shew  the  immediate  efl'ect  of  each  heart  beat. 

If  two  manometers,  instead  of  one,  were  attached  to  the 
arterial  system,  one  near  the  pump  and  the  other  farther  off,  close 
to  the  peripheral  resistance,  the  pressure  shewn  by  the  near 
manometer  would  be  found  to  be  greater  than  that  shewn  by 
the  far  one.  The  pressure  at  the  far  point  is  less  because  some  of 
the  pressure  exerted  at  the  near  point  has  been  used  to  drive  the 
fluid  from  the  near  point  to  the  far  one.  Similarly  on  the  venous 
side,  a  manometer  placed  closed  to  the  peripheral  region  would  shew 
a  higher  pressure  than  that  shewn  by  one  farther  off,  because  it  is 
the  pressure  still  remaining  in  the  veins  near  the  capillaries  which, 
assisted  as  we  shall  see  by  other  events,  drives  the  blood  onward 
to  the  larger  veins.  The  blood  pressure  is  at  its  highest  at  the 
root  of  the  aorta,  and  at  its  lowest  at  the  mouths  of  the  venae  cavse, 
and  is  falling  all  the  way  from  one  point  to  the  other,  because  all 
the  way  it  is  being  used  up  to  move  the  blood  from  one  point  to 
the  other.  The  great  drop  of  pressure  is,  as  we  have  said,  in  the 
peripheral  region,  because  more  work  has  to  be  done  in  driving 
the  blood  through  this  region  than  in  driving  the  blood  from  the 
heart  to  this  region,  or  from  this  region  to  the  heart. 

The  manometer  on  the  arterial  side  of  the  model  shews,  as  we 
have  seen,  an  oscillation  of  pressure,  a  pulse  due  to  each  heart 
beat ;  and  the  same  pulse  may  be  felt  by  placing  a  finger  or  rendered 
visible  by  placing  a  light  lever  on  the  arterial  tube.  It  may 
further  be  seen  that  this  pulse  is  most  marked  nearest  the  pump 
and  becomes  fainter  as  we  pass  to  the  periphery ;  but  we  must 
reserve  the  features  of  the  pulse  for  a  special  study.  On  the 
venous  side  of  the  model  no  pulse  can  be  detected  by  the  mano- 
meter or  by  the  finger,  provided  that  the  peripheral  resistance  be 
adequate.  If  the  peripheral  resistance  be  diminished,  as  by 
unscrewing  the  clamps,  then,  as  necessarily  follows  from  what  has 
gone  before,  the  pulse  passes  over  on  to  the  venous  side ;  and, 
as  we  shall  have  occasion  to  point  out  later  on,  in  the  living 
organism  the  peripheral  resistance  in  particular  areas  may  be  at 
times  so  much  lessened  that  a  distinct  pulsation  appears  in  the 
veins. 

If  in  the  model,  when  the  pump  is  in  full  swing,  and  arterial 
pressure  well  established,  the  arterial  tube  be  pricked  or  cut,  or 
the  small  side  tube  a  be  opened,  the  water  will  gush  out  in  jets,  as 
does  blood   from  a  cut  artery  in  the  living  body,  whereas  if  the 


Chap,  iv.]  THE   VASCULAR   MECHAXISM.  171 

venous  tube  be  similarly  pricked  or  cut,  or  the  small  tube  v  be 
opened,  the  water  will  simply  ooze  out  or  well  up.  as  does  blood 
from  a  vein  in  the  living  body.  If  the  arterial  tube  be  ligatured,  it 
will  swell  on  the  pump  side,  and  shrink  on  the  peripheral  side  ;  if 
the  venous  tube  be  ligatured,  it  will  swell  on  the  side  nearest  the 
capillaries  and  shrink  on  the  other  side.  In  short,  the  dead  model 
will  shew  all  the  main  facts  of  the  circulation  which  we  have  as 
yet  described. 

§  103.  In  the  living  body,  however,  there  are  certain  helps  to 
the  circulation  which  cannot  be  imitated  by  such  a  model  without 
introducing  great  and  undesirable  complications  ;  but  these  chietiy 
affect  the  flow  along  the  veins. 

The  veins  are  in  many  places  provided  with  valves  so  con- 
structed as  to  offer  little  or  no  resistance  to  the  flow  from  the 
capillaries  to  the  heart,  but  effectually  to  block  a  return  towards 
the  capillaries.  Hence  any  external  pressure  brought  to  bear 
i;pon  a  vein  tends  to  help  the  blood  to  move  forward  towards  the 
heart.  In  the  various  movements  carried  out  by  the  skeletal 
muscles,  such  an  external  pressure  is  brought  to  bear  on  many  of 
the  veins,  and  hence  these  movements  assist  the  circulation. 
Even  passive  movements  of  the  limbs  have  a  similar  effect. 

The  flow  along  the  large  veins  of  the  abdomen  is  assisted  by 
the  pressure  rhythmically  brought  to  bear  on  them  through  the 
movements  of  the  diaphragm  in  breathing,  as  well  as,  at  times,  by 
the  forcible  contractions  of  the  abdominal  muscles.  Again,  the 
movements  of  the  alimentary  canal,  carried  out  by  means  of  plain, 
muscular  tissue,  promote  the  flow  along  the  veins  coming  from 
that  canal,  and  when  we  come  to  study  the  spleen  we  shall  see 
that  the  plain,  muscular  fibres,  which  are  so  abundant  in  that 
organ  in  some  animals,  serve  by  rhythmical  contractions  to 
pump  the  blood  regularly  away  from  the  spleen  along  the  splenic 
veins. 

When  we  come  to  deal  with  respiration,  we  shall  see  that  each 
enlargement  of  the  chest  constituting  an  inspiration  tends  to  draw 
the  blood  towards  the  chest,  and  each  return  or  retraction  of  the 
chest  walls  in  expiration  has  an  opposite  effect,  and,  if  powerful 
enough,  may  drive  the  blood  away  from  the  chest.  The  arrange- 
ment of  the  valves  of  the  heart  causes  this  action  of  the  respiratory 
pump  to  promote  the  flow  of  blood  in  the  direction  of  the  normal 
circulation  ;  and,  indeed,  were  the  heart  perfectly  motionless  the 
working  of  this  respiratory  pump  alone  would  tend  to  drive  the 
blood  from  the  venre  cavffi  through  the  heart  into  the  aorta,  and  so 
to  keep  up  the  circulation  ;  the  force  so  exerted,  however,  would, 
without  the  aid  of  the  heart,  be  able  to  overcome  a  very  small 
part  only  of  the  resistance  in  the  capillaries  and  small  vessels  of 
the  lungs,  and  so  would  prove  actually  ineffectual. 

There  are,  then,  several  helps  to  the  flow  along  the  veins,  but 
it  must  be  remembered  that  however  useful,  they  are  helps  only 


172  THE   RATE    OF   FLOW.  [Book  i. 

and  not  the  real  cause  of  the  circulation.  The  real  cause  of  the 
flow  is  the  ventricular  stroke,  and  this  is  sufficient  to  drive  the 
blood  from  the  left  ventricle  to  the  right  auricle,  even  when  every 
muscle  of  the  body  is  at  rest,  and  breathing  is  for  a  while  stopped, 
• —  when,  therefore,  all  the  helps  we  are  speaking  of  are  wanting. 


Circumstances  determining  the  Rate  of  the  Flow. 

§  104.  We  may  now  pass  on  to  consider  briefly  the  rate  at 
which  the  blood  flows  through  the  vessels,  and  first  the  rate  of 
flow  in  the  arteries. 

When  even  a  small  artery  is  severed,  a  considerable  quantity 
of  blood  escapes  from  the  proximal  cut  end  in  a  very  short  space  of 
time.  That  is  to  say,  the  blood  moves  in  the  arteries  from  the  heart 
to  the  capillaries  with  a  very  considerable  velocity.  By  various 
methods,  this  velocity  of  the  blood  current  has  been  measured  at 
different  parts  of  the  arterial  system  ;  the  results,  owing  to  imper- 
fections in  the  methods  employed,  cannot  be  regarded  as  satis- 
factorily exact,  but  may  be  accepted  as  approximately  true.  They 
shew  that  the  velocity  of  the  arterial  stream  is  greatest  in  the 
largest  arteries  near  the  heart,  and  diminishes  from  the  heart 
towards  the  capillaries.  Thus  in  a  large  artery  of  a  large  animal, 
such  as  the  carotid  of  a  dog  or  horse,  and  probably  in  the  carotid  of 
a  man,  the  blood  flows  at  the  rate  of  300  or  500  mm.  a  second. 
In  the  very  small  arteries  the  rate  is  probably  only  a  few  mm.  a 
second. 

Methods.  The  Hsemadromometer  of  Volkmann.  An  arteiy,  e.g.  a 
carotid,  is  clamped  in  two  places,  and  divided  between  the  clamps.  Two 
cannulse,  of  a  bore  as  nearly  equal  as  possible  to  that  of  the  artery,  or  of 
a  known  bore,  are  inserted  in  the  two  ends.  The  two  cannulae  are  con- 
nected by  means  of  two  stopcocks,  which  work  together,  w^ith  the  two 
ends  of  a  long  glass  tube,  bent  in  the  shape  of  a  JJ,  ^^^'^  tilled  with 
normal  saline  solution,  or  with  a  coloured,  innocuous  fluid.  The  clamps 
on  the  artery  being  released,  a  turn  of  the  stopcocks  permits  the  blood 
to  enter  the  proximal  end  of  the  long  U  ^"be,  along  which  it  courses, 
driving  the  fluid  out  into  the  artery  through  the  distal  end.  Attached 
to  the  tube  is  a  graduated  scale,  by  means  of  which  the  velocity  with 
which  the  blood  flows  along  the  tube  may  be  read  off. 

The  Rheometer  (Stromuhr)  of  Ludwig.  The  principle  of  this 
consists  in  measuring  the  time  which  it  takes  the  flow  through  an 
artery  to  fill  and  refill  a  vessel  of  known  capacity  a  certain  number 
of  times.  The  instrument  (Fig.  33),  which  consists  of  two  glass  bulbs, 
one  being  of  known  capacity,  is  connected,  like  the  foregoing  in- 
strument, with  two  cannulse  fixed  in  the  two  ends  of  a  severed 
artery,   and   is  so   arranged   that   the  bulb    of  known    capacity   can    be 


Chap,  iv.]  THE   VASCULAR   MECHANISM. 


173 


repeatedly  filled  and  refilled  in  succession.  From  the  length  of  time 
it  takes  to  fill  tlie  bulb  a  certain  number  of  times  the  flow  tlirough  the 
artery  is  calculated. 


Fig.  33.    Ludwig's  Stuomuhr  and  a  Diagrammatic  kepkesentation  of  the  same. 

G  and  H  fit  into  the  cannulte  placed  respectively  into  the  proximal  and  distal 
cut  ends  of  the  artery  under  examination.  D  is  a  metal  disc  revolving  on  a  lower 
similar  disc  E.  A  and  B  are  glass  bulbs  (which  can  l>e  tilled  through  C)  tixetl  u])ou 
Jj ,  the  capacity  of  ^1  up  to  the  mark  x  is  known.  Holes  are  bored  through  D  and 
E  in  such  a  way  that  in  the  position  shewn  in  the  figure  fluid  passes  from  G 
through  a'  and  a  into  ^4,  and  so  by  B,  h  aud  b'  to  //.  If  the  disc  D  be  turned 
througli  two  right  angles,  fluid  passes  from  a'  to  b  and  so  by  B,  A,  and  a  to  b' .  If 
it  be  turned  through  one  right  angle  only  the  fluid  passes  directly  from  G  to  H 
without  entering  the  bulbs  at  all.  -^^1  is  filled  with  pure  oil  up  to  the  mark  .r,  B 
with  detibriuated  blood.  The  blood  is  allowed  to  flow  from  G  into  ^-1  until  the 
whole  of  the  oil  is  driven  into  B,  the  defibrinated  blood  occupying  which  is  driven 
into  //.  Tlien,  by  a  rapid  turn,  tlie  position  of  A  and  B  is  reversed,  aud  the  oil 
driven  back  into  .^1  ,■  then  again  by  another  turn  back  from  ^4  into  B,  and  so  on 
until  clotting  stops  the  observation.  The  time  which  it  takes  the  flow  through  (t 
to  fill  .4  (uj)  to  the  nuirk  ./)  alternately  witii  blood  and  oil,  being  thus  determined, 
the  sectional  area  of  G  and  the  ca])acity  of  A  being  known,  the  velocity  of  the  flow 
through  G  may  be  calculated. 


The  Haemataithometer  of  Vierordt  is  constructed  on  the  princii)le  of 
measuring  the  velocity  of  the  current  by  observing  the  amount  of  devia- 
tion undeigone  by  a  pendulum,  the  free  end  of  which  hangs  loosely  in 
the  stream. 

An  instrument  based  on  the  same  principle  has  been  invented  by 
Chauveau  and  improved  by  Lortet,  Fig.  34.  A  somewliat  wide  tube, 
the  wall  of  which  is  at  one  point  composeci  of  an  india  rubber  membrane, 
is  introduced  between  the  two  cut  ends  of  an  artery.  A  long,  liglit 
lever  pierces  the   india  rubber  membrane.     The  short,  e.\panded  arm  of 


174 


MEASUEE]\lEisT   OF  KATE   OF  FLOW.      [Book  i. 


this  lever  projecting  Avithin  the  tube  (and  corresponding  to  the  pendulum 
of  Vierordt's  instrument)  is  moved  on  its  fulcrum  in  the  india  rubber 
ring  by  the  current  of  blood  passing  through  the  tube,  the  greater  the 
velocity  of   the   current,  the   larger  being  the  excursion   of  the  lever.    ' 


Fig.  34.    H.i:matacho3ietek  of  Chauveau  and  Lortet. 


The  movements  of  the  short  arm  give  rise  to  corresponding  movements 
in  the  opposite  direction  of  the  long  arm  outside  the  tube,  and  these, 
by  means  of  a  marker  attached  to  the  end  of  the  long  arm,  may  be 
directly  inscribed  on  a  recording  surface.  This  instrument  is  best 
adapted  for  observing  changes  in  the  velocity  of  the  flow.  For  deter- 
mining actual  velocities  it  has  to  be  experimentally  graduated. 

The  rapidity  of  tlie  flow,  and  especially  variations  in  the  rapidity,  may 
also  be  studied  in  a  more  indirect  manner  by  means  of  the  following 
method,  called  the  '  plethysmographic  method.' 

The  principle  of  the  plethymiogra'ph  is  that  changes  in  the  volume 
of  a  part  or  of  an  organ  of  the  body,  are  measured  by  the  displacement 
of  fluid  in  a  chamber  with  rigid  walls  surrounding  the  part  or  organ. 
A  part  of  the  body,  the  arm,  for  instance,  is  introduced  into  a  cham- 
ber with  rigid  walls,  such  as  a  large  glass  cylinder,  which  is  filled 
with  fluid,  the  opening  by  which  the  arm  is  introduced  being  closed 
Avith  an  india  rubber  ring  or  Avith  plaster  of  Paris.  The  cavity  of  the 
chamber  is  connected,  at  one  spot,  Avitli  a  narroAv  glass  tube,  open  at 
tlie  end,  in  which  the  fluid,  after  the  introduction  of  the  arm,  stands  at 
a  certain  level.  Any  change  in  the  volume  of  the  arm  manifests  itself 
by  a  change  in  the  level  of  the  fluid  in  the  tube  ;  wdien  the  arm  shrinks 
the  level  falls,  AA'hen  the  arm  swells,  the  level  rises.  And  by  means  of 
a  piston  Avorking  in  the  tube,  or  by  a  float  bearing  a  marker  and 
swimming  on  the  top  of  the  fluid,  or  by  other  contrivances,  a  graphic 
record  of  the  changes  in  the  level  of  the  fluid  in  the  tube  and  so  of  the 
changes  in  the  volume  of  the  arm  may  be  obtained.  Such  an  instru- 
ment is  called  a  plethysmograph ;  and,  as  Ave  shall  see  it  may  be  applied 
in  various  AA'ays  to  various  parts  and  organs  of  the  body. 


Chap,  iv.]  THE   VASCULAR   MECHANISM.  175 

Now,  changes  in  the  volume  of  the  arui  are  mainly  caused  (we  may 
for  the  present  neglect  other  causes)  by  changes  in  the  quantity  of 
blood  present  in  that  portion  of  the  arm  which  lies  within  the  cylinder. 
Upon  examination  it  is  found  that  besides  certain  slower  changes  of 
volume  which  take  place  from  time  to  time,  there  are  changes  of  volume 
corresponding  to  each  heart  beat.  At  each  heart  beat  the  volume  first 
increases  and  then  decreases  again,  reaching  before  the  next  heart  beat 
the  same  measure  which  it  had  just  preceding  the  beat ;  there  is,  we 
may  say,  a  pulsation  of  volume  like  the  actual  pulse  ;  and  we  may,  by 
the  graphic  method,  obtain  a  curve  of  the  changes  in  volume,  a  "  volume 
curve."  An  increase  of  volume,  a  rise  of  the  curve,  means  that  the 
blood  is  flowing  into  the  arm,  within  the  cylinder,  by  the  (axillary) 
artery  at  the  level  of  the  rim  of  the  cylinder,  more  swiftly  than  it  is 
flowing  out  by  the  (axillary)  vein  or  veins  at  the  same  level ;  a  decrease 
of  volume,  a  fall  of  the  curve,  means  that  the  blood  is  flowing  in  less 
swiftly  than  it  is  flowing  out ;  and  a  stationary  volume,  the  curve 
neither  rising  nor  falling,  means  that  the  blood  is  flowing  in  just  as  fast 
as  it  is  flowing  out.  The  steeper  the  ascent  of  the  volume  curve,  the 
greater  is  the  rapidity  of  the  arterial  inflow,  and  any  lessening  of  the 
steepness  of  the  ascent  means  a  diminution  of  that  rapidity ;  when 
the  steepness  is  lessened  so  much  that  the  curve  runs  parallel  to  the 
base  line,  then,  whatever  the  actual  height  of  the  curve,  the  inflow  by 
the  artery  is  only  just  as  rapid  as  the  outflow  by  the  vein.  Hence,  the 
dimensions  of  the  parts  of  the  apparatus  being  known,  we  may  calculate 
how  many  more  or  how  many  less  cubic  cm.  of  blood  are  flowing  per 
second,  or  per  fraction  of  a  second,  in  by  the  artery,  than  ai'e  flowing 
out  by  the  vein.  But,  as  we  have  seen,  the  flow  in  the  veins  is  constant 
so  far  as  each  individual  heart  beat  is  concerned  :  it  is  not  directly 
influenced  by  each  heart  beat.  Hence,  having  obtained  by  means  of 
the  instrument  a  curve  of  the  change  of  volume  of  the  arm,  we  may 
from  that  calculate  out  a  curve  of  the  changes  in  rapidit}^  of  the  flow 
in  the  artery  at  the  level  of  the  mouth  of  the  cylinder.  In  this 
way  it  is  ascertained  that  with  each  heart  beat  the  rapidity  of  the  flow 
at  first  rises  very  quickly,  then  more  slowly,  then  ceases  to  rise,  after 
which  it  sinks,  and,  indeed,  sinks  to  such  a  degree  as  to  shew  that 
the  blood  at  this  moment  is  flowing  less  rapidly  in  the  artery  than  in 
the  vein,  but  subsequently  rises  again  to  flxll  once  more,  just  before  the 
next  heart  beat,  to  the  same  rate  as  at  the  beginning  of  the  beat  which 
is  being  studied.  Moreover,  it  is  possible  by  help  of  certain  assump- 
tions to  calculate  the  amount  of  the  whole  flow  through  the  artery 
(and  through  the  vein)  in  a  given  time,  that  is  to  say,  the  actual 
rapidity  of  the  flow. 

In  the  capillaries,  the  rate  is  slowest  of  all.  In  the  web  of  the 
frog  the  flow  as  judged  by  the  movement  of  the  red  corpuscles  may 
be  directly  measured  under  the  microscope  by  means  of  a  micro- 
meter, and  is  found  to  be  about  half  a  millimeter  in  a  second  ; 
but  this  is  probably  a  low  estimate,  since  it  is  only  when  the 
circulation  is  somewhat  slow,  slower,  perhaps,  than  what  ought  to 
be  considered  the  normal  rate,  that  the  red  corpuscles  can  be 
distinctly  seen.     In  the  mammal   the   rate  has  been  estimated 


176 


THE   EATE   OF  FLOW. 


[Book  i. 


at  about   '75  millimeters  a  second,  but  is  probably  quicker  even 
than  this. 

As  regards  the  veins,  the  flow  is  very  slow  in  the  small  veins 
emerging  from  the  capillaries  but  increases  as  these  join  into  larger 
trunks,  until  in  a  large  vein,  such  as  the  jugular  of  the  dog,  the 
rate  is  about  200  mm.  a  second. 

§  105.  It  will  be  seen,  then,  that  the  velocity  of  the  flow  is  in 
inverse  proportion  to  the  width  of  the  bed,  to  the  united  sectional 
areas  of  the  vessels.  It  is  greatest  at  the  aorta,  it  diminishes 
along  the  arterial  system  to  the  capillaries,  to  the  united  bases 
of  the  cones  spoken  of  in  §  94,  where  it  is  least,  and  from  thence 
increases  again  along  the  venous  system. 

And,  indeed,  it  is  this  width  of  the  bed  and  this  alone  which 
determines  the  general  velocity  of  the  flow  at  various  parts  of  the 
system.  The  slowness  of  the  flow  in  the  capillaries  is  not  due  to 
there  being  so  much  more  friction  in  their  narrow  channels  than  in 
the  wider  canals  of  the  larger  arteries ;  for  the  peripheral  resist- 
ance caused  by  the  friction  in  the  capillaries  and  small  arteries  is 
an  obstacle  not  only  to  the  flow  of  blood  through  these  small 
vessels,  where  the  resistance  is  actually  generated,  but  also  to  the 
escape  of  the  blood  from  the  large  into  the  small  arteries,  and, 
indeed,  from  the  heart  into  the  large  arteries.  It  exerts  its 
influence  along  the  whole  arterial  tract.  And  it  is  obvious  that  if 
it  were  this  peripheral  resistance  which  checked  the  flow  in  the 
capillaries,  there  could  be  no  recovery  of  velocity  along  the  venous 
tract. 

The  blood  is  flowing  through  a  closed  system  of  tubes,  the 
blood  vessels,  under  the  influence  of  one  propelling  force,  the  systole 
of  the  ventricle  ;  for  this  is  the  force  which  drives  the  blood  from 
ventricle  to  auricle,  though,  as  we  have  seen,  its  action  is  modified 
in  the  several  parts  of  the  system.  In  such  a  system  the  same 
quantity  of  fluid  must  pass  each  section  of  the  system  at  the  same 
time,  otherwise   there  would   be   a   block   at   one   place,  and  a 

deficiency  at  another.  If,  for  instance, 
a  fluid  is  made  to  flow  by  some  one 
force,  pressure  or  gravity,  through  a 
tube  A  (Fig.  35)  with  an  enlargement 
B,  it  is  obvious  that  the  same  quantity 
of  fluid  must  pass  through  the  section 
h  as  passes  through  the  section  a  in 
the  same  time,  —  for  instance,  in  a 
second.  Otherwise,  if  less  passes  through  h  than  a,  the  fluid  would 
accumulate  in  B,  or  if  more,  B  would  be  emptied.  In  the  same 
way  just  as  much  must  pass  in  the  same  time  through  the  section 
c  as  passes  through  a  or  &.  But  if  just  as  many  particles  of  water 
have  to  get  through  the  narrow  section  a  in  the  same  time  as 
they  have  to  get  through  the  broader  section  c,  they  must  move 
more  quickly  through  a  than  through  c,  or  more  slowly  through  c 


Fig.  35. 


Chap,  iv.]  THE   VASCULAR   MECHANISM.  177 

than  through  a.  For  the  same  reason,  water  flowing  along  a  river 
impelled  by  one  force,  viz.  that  of  gravity,  rushes  rapidly  through 
a  '  narrow,'  and  flows  sluggishly  when  the  river  widens  out  into 
a  '  broad.'  The  flow  through  B  will  be  similarly  slackened  if  B, 
instead  of  being  simply  a  single  enlargement  of  the  tube  A,  consists 
of  a  number  of  small  tubes  branching  out  from  A,  with  a  united 
sectional  area  greater  than  the  sectional  area  of  ^.  In  each  of 
such  small  tubes,  at  the  line  c,  for  instance,  the  flow  will  be  slower 
than  at  a,  where  the  small  tubes  branch  out  from  A,  or  at  &,  where 
they  join  again  to  form  a  single  tube.  Hence  it  is  that  the  blood 
rushes  swiftly  through  the  arteries,  flows  slowly  through  the 
capillaries,  but  quickens  its  pace  again  in  the  veins. 

An  apparent  contradiction  to  this  principle  that  the  rate  of 
flow  is  dependent  on  the  width  of  the  bed  is  seen  in  the  case 
where,  the  fluid  having  alternative  routes,  one  of  the  routes  is 
temporarily  widened.  Suppose  that  a  tube  A  divides  into  two 
branches  of  equal  length  x  and  y,  which  unite  again  to  form  the 
tube  V.  Suppose,  to  start  with,  that  x  and  y  are  of  equal 
diameter:  then  the  resistance  offered  by  each  being  equal,  the 
flow  will  be  equally  rapid  through  the  two,  being  just  so  rapid 
that  as  much  fluid  passes  in  a  given  time  through  x  and  y  together 
as  passes  through  A  or  through  V.  But  now  suppose  y  to  be 
widened :  the  widening  will  diminish  the  resistance  offered  by  y, 
and,  in  consequence,  supposing  that  no  material  change  takes 
place  in  the  pressure  or  force  which  is  driving  the  fluid  along,  more 
fluid  will  now  pass  along  ?/  in  a  given  time  than  did  before  ,  that  is 
to  say,  the  rapidity  of  the  flow  in  y  will  be  increased.  It  will  be 
increased  at  the  expense  of  the  flow  through  x,  since  it  will  still 
hold  good  that  tlie  flow  through  x  and  y  together  is  equal  to  the 
flow  through  A  and  through  V.  We  shall  have  occasion  later  on 
to  point  out  that  a  small  artery,  or  a  set  of  small  arteries,  may 
be  more  or  less  suddenly  widened,  without  materially  affecting  the 
general  blood  pressure  which  is  driving  the  blood  through  the 
artery  or  set  of  arteries.  In  such  cases  the  flow  of  blood  through 
the  widened  artery  or  arteries  is,  for  the  time,  being  increased  in 
rapidity,  not  only  in  spite  of,  but  actually  in  consequence  of  the 
artery  being  widened. 

It  must  be  understood,  in  fact,  that  this  dependence  of  the 
rapidity  of  the  flow  on  the  width  of  the  bed  applies  to  the  general 
rate  of  flow  of  the  whole  circulation  ;  and  that  while,  on  account  of 
the  width  of  the  bed,  the  flow  through  the  capillaries  is  slower 
than  through  the  small  arteries  and  veins,  that  through  the  small 
arteries  slower  than  through  the  larger  arteries,  and  that  through 
the  small  veins  slower  than  through  the  larger  veins,  the  actual 
rapidity  in  any  individual  capillary,  small  artery  or  small  vein,  or 
in  any  individual  sets  of  these,  varies  largely  from  time  to  time, 
owing  to  changes  of  circumstances,  prominent  among  which  are 
changes  in  the  resistance  to  tlie  flow,  —  changes  which,  as  we  shall 

12 


178  TIME   OF  THE   ENTIRE   CIRCUIT.  [Book  i. 

see,  may  be  brought  about  in  various  ways.  Hence,  any  numerical 
statement  as  to  the  rate  of  flow  in  these  vessels  must  be  regarded 
as  a  general  statement  only. 

Moreover,  it  must  be  remembered  that  though  we  speak  of  the 
flow  past  a  point  of  a  large  artery  as  being  of  a  certain  rapidity, 
say  300  mm.  a  second,  that  rapidity  is  continually  varying.  The 
cause  of  the  flow  through  the  whole  system  is  the  pressure  of  the 
ventricular  systole  manifested  as  what  we  have  called  blood 
pressure.  At  each  point  along  the  system  nearer  the  left  ventricle, 
and  therefore  further  from  the  right  auricle,  the  pressure  is  greater 
than  at  a  point  further  from  the  left  ventricle,  and  so  nearer  the 
right  auricle ;  it  is  this  difference  of  pressure  which  is  the  real 
cause  of  the  flow  from  the  one  point  to  the  other;  and  other 
things  being  equal  the  rapidity  of  the  flow  will  depend  on  the 
amount  of  the  difference  of  pressure.  But  the  pressure  exerted 
by  the  ventricle  is  not  constant ;  it  is  intermittent,  rhythmically 
rising  and  falling.  Hence  at  every  point  along  the  arterial  system 
the  flow  is  increased  in  rapidity  during  the  temporary  increase  of 
pressure  due  to  the  ventricular  systole,  and  diminished  during  the 
subsequent  temporary  decrease,  the  increase  and  decrease  being 
the  more  marked  the  nearer  the  point  to  the  heart ;  this  is  shewn 
in  observations  made  by  means  of  Chauveau  and  Lortet's  instru- 
ment or  by  the  plethysmographic  method  (§  104). 

§  106.  Time  of  the  entire  circuit.  It  is  obvious  from  the  fore- 
going that  a  red  corpuscle  in  performing  the  whole  circuit,  in 
travelling  from  the  left  ventricle  back  to  the  left  ventricle,  would 
spend  a  large  portion  of  its  time  in  the  capillaries,  minute  arteries,, 
and  veins.  The  entire  time  taken  up  in  the  whole  circuit  has 
been  approximately  estimated  by  measuring  the  time  it  takes 
for  an  easily  recognized  chemical  substance,  after  injection  into 
the  jugular  vein  of  one  side,  to  appear  in  the  blood  of  the  jugular 
vein  of  the  other  side. 

While  small  quantities  of  blood  are  being  drawn  at  frequently 
repeated  intervals  from  the  jugular  vein  of  one  side,  or  "wliile  the  blood 
from  the  vein  is  being  allowed  to  fall  in  a  minute  stream  on  an  absorb- 
ent paper  covering  some  travelling  surface,  an  iron  salt  such  as  potas- 
sium ferrocyanide  (or  preferably  sodium  ferrocyanide  as  being  less 
injurious)  is  injected  into  the  jugular  vein  of  the  other  side.  If  the- 
time  of  the  injection  be  noted,  and  the  time  after  the  injection  into  one- 
side  at  which  evidence  of  the  presence  of  the  iron  salt  can  be  detected 
in  the  sample  of  blood  from  the  vein  of  the  other  side  be  noted,  this 
gives  the  time  it  has  taken  the  salt  to  perform  the  circuit ;  and  on  the 
supposition  that  mere  diffusion  does  not  materially  affect  the  result,  the 
time  which  it  takes  the  blood  to  perform  the  same  circuit  is  thereby 
given. 

A  modification  of  this  method,  doing  away  with  the  necessity  of 
withdrawing  blood,  is  based  on  the  fact  that  the  electrical  conductivity 
of  the  blood  may  be  changed  by  altering  the  saline  constituents.     Two- 


Chap,  iv.]  THE   VASCULAK   MECHANISM.  179 

(non-polarisable)  electrodes  are  placed  one  on  each  side  of  some  part  of 
a  blood  vessel,  artery  or  vein,  say  the  right  jugular  or  femoral  vein 
(previously  laid  bare  and  insulated),  and  are  connected  with  a  Wheat- 
stone  bridge  and  galvanometer,  as  in  the  usual  way  of  observing 
clianges  in  electrical  resistance.  If  a  solution  of  salt  be  now  injected 
into  some  other  vessel,  say  the  left  jugular,  the  blood  laden  with  the 
extra  quantity  of  salt,  when  it  reaches  the  seat  of  the  electrodes  will 
give  rise  to  a  change  in  the  electrical  resistance  through  the  blood 
vessel  with  its  contained  blood  between  the  electrodes,  and  this  will  be 
indicated  by  a  movement  of  the  galvanometer.  If  the  times  of  the 
injection,  and  of  the  movement  of  the  galvanometer  be  noted,  the 
interval  between  the  two  will  give  the  time  it  takes  the  blood  con- 
taining the  salt  to  pass  from  the  seat  of  injection  to  the  seat  of  the 
electrodes. 

In  the  horse  this  time  has  been  experimentally  determined  at 
about  30  sees,  and  in  the  dog  at  about  15  sees.  In  man  it 
is  probably  from  20  to  25  sees. 

We  may  arrive  at  a  similar  result  indirectly  by  means  of  a 
calculation.  Taking  the  quantity  of  blood  as  •j'^  of  the  body 
weight,  the  blood  of  a  man  weighing  75  kilos  would  be  about 
5,760  grm.  If  180  grms.  left  the  ventricle  at  each  beat,  a 
quantity  equivalent  to  the  whole  blood  would  pass  through  the 
heart  in  32  beats,  i.e.  in  less  than  half  a  minute. 

Taking  the  rate  of  flow  through  the  capillaries  at  about  1mm. 
a  sec,  it  would  take  a  corpuscle  as  long  a  time  to  get  through 
about  20  mm.  of  capillaries  as  to  perform  the  whole  circuit. 
Hence,  if  any  corpuscle  had  in  its  circuit  to  pass  through  10  mm. 
of  capillaries,  half  the  whole  time  of  its  journey  would  be  spent  in 
the  narrow  channels  of  the  capillaries.  Inasmuch  as  the  purposes 
served  by  the  blood  are  chiefly  carried  out  in  the  capillaries,  it  is 
obviously  of  advantage  that  its  stay  in  them  should  be  prolonged. 
Since,  however,  the  average  length  of  a  capillary  is  about  -5  mm., 
about  half  a  second  is  spent  in  the  capillaries  of  the  tissues  and 
another  half  second  in  the  capillaries  of  the  lungs. 

§  107.  We  may  now  briefly  summarise  the  broad  features  of 
the  circvilation,  which  we  have  seen  may  be  explained  on  purely 
physical  principles,  it  being  assumed  that  the  ventricle  delivers 
a  certain  quantity  of  blood  with  a  certain  force  into  the  aorta 
at  regular  intervals,  and  that  the  physical  properties  of  the  blood 
vessels  remain  the  same. 

We  have  seen  that,  owing  to  the  peripheral  resistance  offered 
by  the  capillaries  and  small  vessels,  the  direct  effect  of  the 
ventricular  sti'oke  is  to  establisli  in  the  arteries  a  mean  arterial 
pressure,  which  is  greatest  at  the  root  of  the  aorta  and  diminishes 
towards  the  small  arteries  ,  some  of  it  being  used  up  to  drive  the 
blood  from  the  aorta  to  the  small  arteries,  but  which  retains  at 
the  region  of  the  small  arteries  sufficient  power  to  drive  through  the 
small  arteries,  capillaries  and  veins  just  as  much  blood  as  is  being 


180  MAIN  FEATURES   OE   CIRCULATIOK      [Book  i. 

thrown  into  the  aorta  by  the  ventricular  stroke.  We  have  seen 
further  that  in  the  large  arteries  at  each  stroke  the  pressure 
rises  and  falls  a  little  above  and  below  the  mean,  thus  constituting 
the  pulse,  but  that  this  extra  distension  with  its  subsequent  recoil 
diminishes  along  the  arterial  tract  and  finally  vanishes ;  it  dimin- 
ishes and  vanishes  because  it,  too,  like  the  whole  force  of  the 
ventricular  stroke,  of  a  fraction  of  which  it  is  the  expression,  is  used 
up  in  establishing  the  mean  pressure  ;  we  shall,  however,  consider 
again  later  on  the  special  features  of  this  pulse.  We  have  seen 
further  that  the  task  of  driving  the  blood  through  the  peripheral 
resistance  of  the  small  arteries  and  capillaries  consumes  much  of 
this  mean  pressure,  which  consequently  is  much  less  in  the  small 
veins  than  in  the  corresponding  small  arteries,  but  that  sufficient 
remains  to  drive  the  blood,  even  without  the  help  of  the  auxiliary 
agents  which  are  generally  in  action,  from  the  small  veins  right 
back  to  the  auricle.  Lastly,  we  have  seen  that  while  the  above 
is  the  cause  of  the  flow  from  ventricle  to  auricle,  the  changing 
rate  of  the  flow,  the  diminishing  swiftness  in  the  arteries,  the 
sluggish  crawl  through  the  capillaries,  the  increasing  quickness 
through  the  veins  are  determined  by  the  changing  width  of  the 
vascular  '  bed.' 

Before  we  proceed  to  consider  any  further  details  as  to  the 
phenomena  of  the  flow  through  the  vessels,  we  must  turn  aside  to 
study  the  heart. 


SEC.  3.     THE  HEAET. 


§  108.  The  heart  is  a  valvular  pump  which  works  on  me- 
chanical principles,  but  the  motive  power  of  which  is  supplied 
by  the  contraction  of  its  muscular  fibres.  Its  action  consequently 
presents  problems  which  are  partly  mechanical,  and  partly  vital. 
Eegarded  as  a  pump,  its  effects  are  determined  by  the  frequency  of 
the  beats,  by  the  force  of  each  beat,  by  the  character  of  each  beat, 
—  whether,  for  instance,  slow  and  lingering,  or  sudden  and  sharp,  — 
and  by  the  quantity  of  fluid  ejected  at  each  beat.  Hence,  with  a 
given  frequency,  force,  and  character  of  beat,  and  a  given  quantity 
ejected  at  each  beat,  the  problems  which  have  to  be  dealt  with  are 
for  the  most  part  mechanical.  The  vital  problems  are  chiefly  con- 
nected with  the  causes  which  determine  the  frequency,  force,  and 
character  of  the  beat.  The  quantity  ejected  at  each  beat  is 
governed  not  only  by  the  action  of  the  heart  itself,  but  also  and 
indeed  more  so  by  what  is  going  on  in  the  rest  of  the  body. 

The  Phenomena  of  the  Normal  Beat. 

The  visible  movements.  When  the  chest  of  a  mammal  is 
opened,  and  artificial  respiration  kept  up,  the  heart  may  be 
watched  beating.  Owing  to  the  removal  of  the  chest-wall,  what 
is  seen  is  not  absolutely  identical  with  what  takes  place  within 
the  intact  chest,  but  the  main  events  are  the  same  in  both  cases. 
A  complete  beat  of  the  whole  heart,  or  cardiac  cycle,  may  be 
observed  to  take  place  as  follows. 

The  great  veins,  inferior  and  superior  vente  cavee  and  pulmonary 
veins,  are  seen,  while  full  of  blood,  to  contract  in  the  neighbourhood 
of  the  heart :  the  contraction  runs  in  a  peristaltic  wave  towards 
the  auricles,  increasing  in  intensity  as  it  goes.  Arrived  at  the 
auricles,  which  are  then  full  of  blood,  the  wave  suddenly  spreads, 
at  a  rate  too  rapid  to  be  fairly  judged  by  the  eye,  over  the  whole 
of  those  organs,  which  accordingly  contract  with  a  sudden  sharp 


182  THE   CAEDIAC   CYCLE.  [Book  i. 

systole.  In  the  systole,  the  walls  of  the  auricles  press  towards  the 
auriculo-ventricular  orifices,  and  the  auricular  appendages  are 
drawn  inwards,  becoming  smaller  and  paler.  During  the  auricular 
systole,  the  ventricles  may  be  seen  to  become  turgid.  Then 
follows,  as  it  were  immediately,  the  ventricular  systole,  during 
which  the  ventricles  become  more  conical.  Held  between  the 
fingers  they  are  felt  to  become  tense  and  hard.  As  the  systole 
progresses,  the  aorta  and  pulmonary  arteries  expand  and  elongate, 
the  apex  is  tilted  slightly  upwards,  and  the  heart  twists  somewhat 
on  its  long  axis,  moving  from  the  left  and  behind  towards  the 
front  and  right,  so  that  more  of  the  left  ventricle  becomes  dis- 
played. As  the  systole  gives  way  to  the  succeeding  diastole,  the 
ventricles  resume  their  previous  form  and  position,  the  aorta  and 
pulmonary  artery  shrink  and  shorten,  the  heart  turns  back 
towards  the  left,  and  thus  the  cycle  is  completed. 

In  the  normal  beat,  the  two  ventricles  are  perfectly  synchronous 
in  action ;  they  contract  at  the  same  time  and  relax  at  the  same 
time,  and  the  two  auricles  are  similarly  synchronous  in  action. 
It  has  been  maintained,  however,  that  the  synchronism  may  at 
times  not  be  perfect. 

Before  we  attempt  to  study  in  detail  the  several  parts  of  this 
complicated  series  of  events,  it  will  be  convenient  to  take  a  rapid 
survey  of  what  is  taking  place  within  the  heart  during  such  a  cycle. 

§  109.  The  cardiac  cycle.  We  may  take  as  the  end  of  the 
cycle  the  moment  at  which  the  ventricles  having  emptied  their 
contents  have  relaxed  and  returned  to  the  diastolic  or  resting 
position  and  form.  At  this  moment  the  blood  is  flowing  freely 
with  a  fair  rapidity,  but,  as  we  have  seen,  at  a  very  low  pressure, 
through  the  vense  cavse  into  the  right  auricle  (we  may  confine 
ourselves  at  first  to  the  right  side),  and  since  there  is  now  nothing 
to  keep  the  tricuspid  valve  shut,  some  of  this  blood  probably  finds 
its  way  into  the  ventricle  also.  This  goes  on  for  some  little  time, 
and  then  comes  the  sharp,  short  systole  of  the  auricle,  which, 
since  it  begins,  as  we  have  seen,  as  a  wave  of  contraction  running 
forwards  along  the  ends  of  the  vense  cavse,  drives  the  blood  not  back- 
wards into  the  veins,  but  forwards  into  the  ventricle ;  this  result 
is  further  secured  by  the  fact  that  the  systole  has  behind  it  on  the 
venous  side  the  pressure  of  the  blood  in  the  veins,  increasing  as 
we  have  seen  backwards  towards  the  capillaries,  and  before  it  the 
relatively  empty  cavity  of  the  ventricle  in  which  the  pressure 
is  at  first  very  low.  By  the  complete  contraction  of  the  auricular 
walls  the  complete  or  nearly  complete  emptying  of  the  cavity 
is  ensured.  No  valves  are  present  in  the  mouth  of  the  superior 
vena  cava,  for  they  are  not  needed  ;  and  the  imperfect  Eustachian 
valve  at  the  mouth  of  the  inferior  vena  cava  cannot  be  of  any 
great  use  in  the  adult,  though  in  its  more  developed  state  in 
the  foetus  it  had  an  important  function  in  directing  the  blood  of 
the  inferior  vena  cava  through  the  foramen  ovale  into  the  left 


Chap.  iv.J  THE   VASCULAR   MECHANISM.  183 

auricle.  The  valves  in  the  coronary  vein  are,  however,  probably 
of  some  use  in  preventing  a  retlux  into  that  vessel. 

As  the  blood  is  being  driven  by  the  auricular  systole  into  the 
ventricle,  a  reflex  current  is  probably  set  up,  by  which  the  blood, 
passing  along  the  sides  of  the  ventricle,  gets  between  them  and 
the  flaps  of  the  tricuspid  valve  and  so  tends  to  float  these  up. 
It  is  further  probable  that  the  same  reflux  current,  continuing 
somewhat  later  than  the  flow  into  the  ventricle,  is  sufficient 
to  bring  the  flaps  into  apposition,  without  any  regurgitation  into 
the  auricle,  at  the  close  of  the  auricular  systole,  before  the  ventri- 
cular systole  has  begun. 

The  auricular  systole  is,  as  we  have  said,  immediately  followed 
by  that  of  the  ventricle.  Whether  the  contraction  of  the  ven- 
tricular walls  (which  as  we  shall  see  is  a  simple  though  prolonged 
contraction  and  not  a  tetanus)  begins  at  one  point,  and  swiftly 
travels  over  the  rest  of  the  fibres,  or  begins  all  over  the  ventricle 
at  once,  is  a  question  not  at  present  definitely  settled ;  but  in  any 
case  the  walls  exert  on  the  contents  a  pressure  which  is  soon 
brought  to  bear  on  the  whole  contents  and  very  rapidly  rises  to  a 
maximum.  The  effect  of  this  increasing  intra-ventricular  pressure 
upon  the  valve  is  undoubtedly  to  render  the  valve  more  firmly 
and  securely  closed  ;  but  the  exact  behaviour  of  the  valve  in 
thus  firmly  closing  is  a  matter  on  which  observers  are  not  agreed. 
From  the  disposition  of  the  flaps  of  the  valve,  and  their  relations 
to  the  papillary  muscles,  the  chordie  tendinea^  of  a  papillary 
muscle  being  attached  to  the  edges  of  and  spreading  over  the 
surfaces  of  two  adjacent  flaps,  we  may  infer  that  when  the 
papillary  muscles  contract,  taking  their  share  in  the  whole  ventri- 
cular systole,  they  on  the  one  hand  bring  at  least  the  edges,  if  not 
part  of  the  surfaces  of  adjacent  flaps,  into  opposition,  and,  on  the 
other  hand,  tend  to  pull  down  the  whole  of  the  valve,  more  or  less 
in  the  form  of  a  narrow  funnel,  into  the  cavity  of  the  ventricle.  If 
we  assume,  as  some  observers  do,  that  the  papillary  muscles  begin 
their  contraction  at  the  same  time  as  the  rest  of  the  ventricular 
wall,  we  may  conclude  that  the  valve  is  in  this  manner  firmly 
closed  by  their  action  at  the  very  beginning  of  the  systole.  Other 
observers  find  that  a  tracing,  obtained  by  attaching  a  hook  to  the 
apex  of  one  of  the  flaps  of  the  valve,  and  connecting  it  with  a 
thread  passing  through  the  auriculo-ventricular  orifice,  and  the 
auricle  to  a  lever,  indicates  that  the  apex  of  the  flap  does  not 
begin  to  move  downwards  until  some  appreciable  time  after  the 
beginning  of  the  systole.  This  they  interpret  as  meaning  that  the 
papillary  muscles  do  not  begin  to  contract  until  some  time  after 
the  ventricular  wall  has  begun  its  contraction ;  (and  the  tracing 
in  question  similarly  indicates  that  the  papillary  muscle  ceases  its 
contraction  before  the  ventricular  wall  does).  If  we  assume  this 
interpretation  of  the  tracing  to  be  correct,  we  must  conclude  that, 
at  the  first,  the  pressure  exerted  by  the  commencing  systole  would 


184  •       THE   CARDIAC    CYCLE.  [Book  i. 

tend,  while  bringing  the  edges  of  the  flaps  together,  to  bulge  the 
whole  valve  upwards  towards  the  auricle,  but  that,  later,  when  the 
papillary  muscles  contract,  these  pull  the  valve  in  a  funnel  shape 
down  into  the  ventricle  with  the  edges  of  the  flaps  in  complete 
apposition.  On  the  one  view,  the  papillary  muscles  serve  merely  to 
secure  the  adequate  closure  of  the  valve ;  on  the  other  view,  they 
add  to  the  pressure  exerted  by  the  ventricular  wall,  by  pulling 
the  already  closed  valve  down  on  the  ventricular  contents,  or, 
according  to  an  old  opinion,  obviate,  by  their  shortening,  the 
slackening  of  the  chordse  which  might  result  from  the  shortening 
of  ventricle  during  the  systole.  Whichever  view  be  taken,  it  may 
be  worth  while  to  remark  that  the  borders  of  the  valves  are 
excessively  thin,  so  that  when  the  valve  is  closed,  these  thin 
portions  are  pressed  flat  together  back  to  back  ;  hence,  while  the 
tougher  central  parts  of  the  valves  bear  the  force  of  the  ventricular 
systole,  the  opposed  thin,  membranous  edges,  pressed  together  by 
the  blood,  more  completely  secure  the  closure  of  the  orifice. 

At  the  commencement  of  the  ventricular  systole,  the  semilunar 
valves  of  the  pulmonary  artery  are  closed,  and  are  kept  closed  by 
the  high  pressure  of  the  blood  in  the  artery.  As,  however,  the 
ventricle  continues  to  press  with  greater  and  greater  force  on  its 
contents,  making  the  ventricle  hard  and  tense  to  the  touch,  the 
pressure  within  the  ventricle  becomes  at  length  greater  than  that 
in  the  pulmonary  artery,  and  this  greater  pressure  forces  open  the 
semilunar  valves,  and  allows  the  escape  of  the  contents  into  the 
artery.  The  ventricular  systole  may  be  seen  and  felt  in  the 
exposed  heart  to  be  of  some  duration ;  it  is  strong  enough  and  long 
enough  to  empty  the  ventricle  more  or  less  completely,  —  indeed,  in 
some  cases,  it  may  last  longer  than  the  discharge  of  blood,  so  that 
there  is  then  a  brief  period  during  which  the  ventricle  is  empty 
but  yet  contracted. 

During  the  ventricular  systole  the  semilunar  valves  are  pressed 
outwards  towards  but  not  close  to  the  arterial  walls,  reflux  currents 
probably  keeping  them  in  an  intermediate  position,  so  that  their 
orifice  forms  an  equilateral  triangle  with  curved  sides ;  they 
offer  little  obstacle  to  the  escape  of  blood  from  the  cavity  of  the 
ventricle.  The  exact  mode  and  time  of  closure  of  the  semilunar 
valves  is  a  matter  which  has  been  and,  indeed,  is  still  disputed, 
and  which  we  shall  have  to  discuss  in  some  detail  later  on. 
Meanwhile  it  will  be  sufficient  to  say,  after  the  blood  has  ceased 
to  flow  from  the  ventricle  into  the  aorta,  whether  this  be  due  to 
the  cessation  of  the  ventricular  systole,  or  to  the  whole  of  the 
ventricular  contents  having  been  already  discharged,  a  reflux  of 
blood  in  the  aorta  towards  the  ventricle  at  once  completely  fills 
and  renders  tense  the  pockets,  causing  their  free  margins  to  come 
into  close  and  firm  contact,  and  thus  entirely  blocks  the  way. 
The  corpora  Arantii  meet  in  the  centre,  and  the  thin,  membranous 
festoons  or  lunulse  are  brought  into  exact  apposition.     As  in  the 


Chap,  iv.l  THE   VASCULAR   MECHANISM.  185 

tricuspid  valves,  so  here,  while  tlie  pressure  of  the  blood  is  borne 
by  the  tougher  bodies  of  the  several  valves,  each  two  thin,  adjacent 
lunulse,  pressed  together  by  the  blood  acting  on  both  sides  of 
them,  are  kept  in  complete  contact,  without  any  strain  being 
put  upon  them  ;  in  this  way  the  orifice  is  closed  in  a  most  efficient 
manner. 

As  the  ventricular  systole  passes  off,  the  muscular  walls  relax- 
ing, the  ventricle  returns  to  its  previous  form  and  position,  and 
the  cycle  is  once  more  ended. 

What  thus  takes  place  in  the  right  side  takes  place  in  the  left 
side  also.  There  is  the  same  sudden,  sharp,  auricular  systole 
beginning  at  the  roots  of  the  pulmonary  veins,  the  same  systole  of 
the  ventricle,  but,  as  we  shall  see,  one  much  more  powerful  and 
exerting  much  more  force ;  the  mitral  valve  with  its  two  flaps 
acts  in  the  main  like  the  tricuspid  valve,  and  the  action  of  the 
semilunar  valves  of  the  aorta  simply  repeats  that  of  the  valves  of 
the  pulmonary  artery. 

We  may  now  proceed  to  study  some  of  the  cardiac  events  in 
detail. 

§  110.  Tlie  change  of  form.  The  exact  determination  of  the 
changes  in  form  and  position  of  the  heart,  especially  of  the  ven- 
tricles, during  a  cardiac  cycle  is  attended  with  difficulties. 

The  ventricles,  for  instance,  are  continually  changing  their  form ; 
they  change  while  their  cavities  are  being  filled  from  the  auricles, 
they  change  while  the  contraction  of  their  walls  is  getting  up 
the  pressure  on  their  contents,  they  change  while  under  the 
influence  of  that  pressure  their  contents  are  being  discharged  into 
the  arteries,  and  they  change  when,  their  cavities  having  been 
emptied,  their  muscular  walls  relax. 

With  regard  to  changes  in  external  form,  there  seems  no  doubt 
that  the  side-to-side  diameter  is  much  lessened  during  the  systole. 
There  is  also  evidence  that  the  front-to-back  diameter  is  greater 
during  the  systole  than  during  the  diastole,  the  increase  taking 
place  during  the  first  part  of  the  systole.  If  a  light  lever 
be  placed  so  as  to  press  very  gently  on  the  surface  of  the  heart  of 
a  mammal,  the  chest  having  been  opened  and  artificial  respiration 
being  kept  up,  some  such  curve  as  that  represented  in  Fig.  36 
may  be  obtained.  The  rise  of  the  lever  in  describing  such  a  curve 
is  due  to  the  elevation  of  the  part  of  the  front  surface  of  the  heart 
on  which  the  lever  is  resting  Such  an  elevation  might  be  caused, 
especially  if  the  lever  were  placed  near  the  apex,  by  the  heart 
being  "  tilted "  upwards  during  the  systole,  but  only  a  small 
portion  at  most  of  the  rise  can  be  attributed  to  this  cause  ;  the 
rise  is  perhaps  best  seen  when  the  lever  is  placed  in  the  middle 
portion  of  the  ventricle,  and  must  be  chiefly  due  to  an  increase  in 
the  front-to-back  diameter  of  the  ventricle  during  the  beat.  We 
shall  discuss  this  curve  later  on  in  connection  with  other  curves, 
and  may  here  simply  say  that  the  part  of  the  curve  from  h'  to  d 


186 


THE   CHANGE   OF  FOEM. 


[Book  i. 


probably  corresponds  to  the  actual  systole  of  the  ventricle,  that  is, 
to  the  time  during  which  the  fibres  of  the  ventricle  are  under- 
going contraction,  the  sudden  fall  from  d  onwards  representing 
the  relaxation  which  forms  the  first  part  of  the  diastole.     If  this 


a\  6\6<  c\   c'\  d,\         \ 
Fig.  36.     Tracing  from  Heart  of  Cat,  obtained  by  placing  a  light  lever 

ON    THE    ventricle,    THE    CHEST     HAVING    BEEN    OPENED.^       ThE    TUNING-FORK 
CURVE    MARKS    50    VIBRATIONS    PER    SEC. 

interpretation  of  the  curve  be  correct,  it  is  obvious  that  the 
front-to-back  diameter  is  greater  during  the  whole  of  the  systole 
than  it  is  during  diastole,  since  the  lever  is  raised  up  all  this  time. 
It  may,  however,  be  argued  that  the  heart  thus  exposed  is  subject 
to  abnormal  conditions  and  is,  in  diastole,  somewhat  flattened  by 
the  weight  of  its  contents,  that  this  flattening  is  increased  by  even 
slight  pressure,  and  that  therefore  the  above  conclusion  is  not 

1  The  vertical  or  rather  curved  lines  (segments  of  circles)  introduced  into  this 
and  many  other  curves  are  of  use  for  the  purpose  of  measuring  parts  of  the  curve. 
A  complete  curve  should  exhibit  an  'abscissa'  line.  This  may  be  drawn  by- 
allowing  the  lever,  arranged  for  the  experiment  but  remaining  at  rest,  to  mark  with 
its  point  on  the  recording  surface  set  in  motion  ;  a  straight  line,  the  abscissa  line, 
is  thus  described,  and  may  be  drawn  before  or  after  the  curve  itself  is  made, 
and  mav  be  placed  above  or,  preferably,  below  the  curve.  When  a  tuning-fork 
or  other  time  marker  is  used,  the  line  of  the  time  marker  or  a  line  drawn  through 
the  curves  of  the  tuning-fork  will  serve  as  an  abscissa  line.  After  a  tracing  has 
been  made,  the  recording  surface  should  be  brought  back  to  such  a  position  that 
the  point  of  the  lever  coincides  with  some  point  of  the  curve  which  it  is  desired  to 
mark ;  if  the  lever  be  then  gently  moved  up  and  down,  the  point  of  the  lever 
will  describe  a  segment  of  a  circle  (the  centre  of  which  lies  at  the  axis  of  the 
lever),  which  segment  should  be  made  long  enough  to  cut  both  the  curve  and 
the  abscissa  line  (the  tuning-fork  curves  or  other  time-marking  line)  where  this  is 
drawn.  By  moving  the  recording  surface  backwards  and  forwards,  similar  seg- 
ments of  circles  may  be  drawn  'through  other  points  of  the  curve.  The  lines 
n,  b,  c  in  Fig,  36  were"  thus  drawn.  The  distance  between  any  two  of  these  points 
may  thus  be  measured  on  the  tuning-fork  curve  or  other  time  curve,  or  on  the  abscissa 
line.  Similar  lines  may  be  drawn  on  the  tracing  after  its  removal  from  the  recording 
instrument  in  the  following  wav.  Take  a  pair  of  compasses,  the  two  points  of  which 
are  fixed  just  as  far  apart  as  the  length  of  the  lever  used  in  the  experiment,  measured 
from  its  axis  to  its  writing  point.  By  means  of  the  compasses  find  the  position  on 
the  tracing  of  the  centre  of  the  circle  of  which  any  one  of  the  previously  drawn 
curved  lines  forms  a  segment.  Through  this  centre  draw  a  line  parallel  to  the 
abscissa.  Bv  keeping  one  point  of  the  compass  on  this  line  but  moving  it  along 
the  line  backwards  or  ff)rwards,  a  segment  of  a  circle  may  be  drawn  so  as  to  cut 
any  point  of  the  curve  that  may  be  desired,  and  also  the  abscissa  line  or  the 
tirne  line.  Such  a  segment  of  a  circle  may  be  used  for  the  same  purposes  as 
the  original  one  and  any  number  of  such  segments  may  be  drawn 


Chap.  IV.]  THE   VASCULAR   MECHANISM.  187 

valid.     And,  indeed,  it  is  maintained  by  some  that  the  front-to- 
back  diameter  does  actually  diminish  during  systole. 

But  it  is  at  least  clear  that  the  front-to-back  diameter,  even  if 
it  does  not  increase,  diminishes  far  less  than  does  the  side-to-side 
diameter;  and  hence  during  the  systole  there  is  a  change  in  tlie 
form  of  the  section  of  the  base  of  the  ventricles.  During  the 
diastole  this  has  somewhat  the  form  of  an  ellipse  with  the  long 
axis  from  side  to  side,  but  with  the  front  part  of  the  ellipse  much 
more  convex  than  the  back,  since  the  back  surface  of  the  ventricles 
is  somewhat  flattened.  During  the  systole  this  ellipse  is  converted 
into  a  figure  much  more  nearly  resembling  a  circle.  It  is  urged, 
moreover,  that  the  whole  of  the  base  is  constricted,  and  that  the 
greater  efficiency  of  the  auriculo-ventricular  valves  is  thereby 
secured. 

As  to  the  behaviour  of  the  long  diameter  from  base  to  apex, 
observers  are  not  agreed  ;  some  maintain  that  it  is  shortened,  and 
others  that  it  is  practically  unchanged.  And,  in  any  case,  a  change 
in  this  diameter  plays  little  or  no  part  in  the  expulsion  of  the 
contents  of  the  ventricle  ;  this  expulsion  is  effected  by  the  contrac- 
tion of  the  more  transversely  disposed  fibres,  whereby  the  cavity  is 
reduced  to  an  elongated  slit.  Moreover,  if  any  shortening  does  take 
place  it  must  be  compensated  by  the  elongation  of  the  great  vessels, 
which,  as  stated  above,  may  be  seen  in  an  inspection  of  the  beating 
heart.  For  there  is  evidence  that  the  apex,  though,  as  we  have 
seen,  it  is  somewhat  twisted  round  during  the  systole,  and  at  the 
same  time  brought  closer  to  the  chest-wall,  does  not  change  its 
position  up  or  down,  i.e.  in  the  long  axis  of  the  body.  If  in  a 
rabbit  or  dog  a  needle  be  thrust  through  the  chest-wall  so  that  its 
point  plunges  into  the  apex  of  the  heart,  though  the  needle 
quivers,  its  head  moves  neither  up  nor  down,  as  it  would  do  if  its 
point  in  the  apex  moved  down  or  up. 

During  systole,  broadly  speaking,  the  ventricles  undergo  a 
diminution  of  total  volume,  equal  to  the  volume  of  contents 
discharged  into  the  great  vessels  (for  the  walls  themselves  like  all 
muscular  structures  retain  their  volume  during  contraction  save 
for  changes  which  may  take  place  in  the  quantity  of  blood 
contained  in  their  blood  vessels,  or  of  lymph  in  the  intermuscular 
spaces),  while  they  undergo  a  change  of  form  which  may  be 
described  as  that  from  a  roughly  hemispherical  figure  with  an 
irregularly  elliptical  section  to  a  more  regular  cone  with  a  more 
nearly  circular  base. 

§  111.  Cardiac  Impuhe.  If  the  hand  be  placed  on  the  chest, 
a  shock  or  impulse  will  be  felt  at  each  beat,  and  on  examination 
this  impulse,  '  cardiac  impulse,'  will  be  found  to  be  synchronous 
with  the  systole  of  the  ventricle.  In  man,  the  cardiac  impulse  may 
be  most  distinctly  felt  in  the  fifth  costal  interspace,  about  an  inch 
below  and  a  little  to  the  median  side  of  the  left  nipple.  In  an 
animal  the  same  impulse  may  also  be  felt  in  another  way,  viz. 


188  THE   CARDIAC    IMPULSE.  [Book  i, 

by  making  an  incision  through  the  diaphragm  from  the  abdo- 
men, and  placing  the  finger  between  the  chest-wall  and  the 
apex.  It  then  can  be  distinctly  recognized  as  the  result  of  the 
hardening  of  the  ventricle  during  the  systole.  And  the  impulse 
which  is  felt  on  the  outside  of  the  chest  is  chiefly  the  effect  of 
the  same  hardening  of  the  stationary  portion  of  the  ventricle 
in  contact  with  the  chest-wall,  transmitted  through  the  chest- 
wall  to  the  finger.  In  its  flaccid  state,  during  diastole,  the 
apex  is  (in  a  standing  position  at  least)  at  this  point  in  contact 
with  the  chest-wall,  lying,  somewhat  flattened,  between  it  and  the 
tolerably  resistant  diaphragm.  During  the  systole,  while  being 
brought  even  closer  to  the  chest-wall,  by  the  tilting  of  the  ventricle 
and  by  the  movement  to  the  front  and  to  the  right  of  which  we 
have  already  spoken,  it  suddenly  grows  tense  and  hard,  and  becomes 
rounder.  The  ventricles,  in  executing  their  systole,  have  to  contract 
against  resistance.  They  have  to  produce  within  their  cavities, 
pressures  greater  than  those  in  the  aorta  and  pulmonary  arteries, 
respectively.  This  is,  in  fact,  the  object  of  the  systole.  Hence, 
during  the  swift  systole,  the  ventricular  portion  of  the  heart 
becomes  suddenly  tense,  somewhat  in  the  same  way  as  a  bladder 
full  of  fluid  would  become  tense  and  hard  when  forcibly  squeezed. 
The  sudden  pressure  exerted  by  the  ventricle  thus  rendered  sud- 
denly tense  and  hard,  aided  by  the  closer  contact  of  the  apex  with 
the  chest-wall  (which,  however,  by  itself,  without  the  hardening  of 
contraction,  would  be  insufficient  to  produce  the  effect),  gives  an 
impulse  or  shock  both  to  the  chest-wall  and  to  the  diaphragm.  If 
the  modification  of  the  sphygmograph  (an  instrument  of  which  we 
shall  speak  later  on,  in  dealing  with  the  pulse),  called  the  cardio- 
graph, be  placed  on  the  spot  where  the  impulse  is  felt  most 
strongly,  the  lever  is  seen  to  be  raised  during  the  systole  of  the 
ventricles,  and  to  fall  again  as  the  systole  passes  away,  very  much 
as  if  it  -were  placed  on  the  heart  directly,  A  tracing  may  thus  be 
obtained,  see  Fig.  46,  of  which  we  shall  have  to  speak  more  fully 
later  on,  see  §  115.  If  the  button  of  the  lever  be  placed, 
not  on  the  exact  spot  of  the  impulse,  but  at  a  little  distance 
from  it,  the  lever  will  be  depressed  during  the  systole.  While 
at  the  spot  of  impulse  itself  the  contact  of  the  ventricle  is 
increased  during  systole,  away  from  the  spot  the  ventricle  (owing 
to  its  change  of  form  and  subsequently  to  its  diminution  in 
volume)  retires  from  the  chest- wall,  and  hence,  by  the  mediastinal 
attachments  of  the  pericardium,  draws  the  chest-wall  after  it. 

§  112.  The  Sounds  of  the  Heart.  When  the  ear  is  applied  to 
the  chest,  either  directly  or  by  means  of  a  stethoscope,  two  sounds 
are  heard,  —  the  first  a  comparatively  long,  dull,  booming  sound, 
the  second  a  short,  sharp,  sudden  one.  Between  the  first  and 
second  sounds  the  interval  of  time  is  very  short,  too  short  to  be 
easily  measured,  but  between  the  second  and  the  succeeding  first 
sound  there  is  a  distinct  pause.     The  sounds   have  been  likened 


Chap.  iv.  j  THE    VASCULAR   MECHANISM.  189 

to  the  pronunciation  of  the  syllables  lubb  diip,  so  that  the  cardiac 
cycle,  as  far  as  the  sounds  are  concerned,  might  be  represented 
by  :  —  lubb,  dup,  pause. 

The  second  sound,  which  is  short  and  sharp,  presents  no  diffi- 
culties. It  is  coincident  in  point  of  time  with  the  closure  of  the 
semilunar  valves,  and  is  heard  to  the  best  advantage  over  the 
second  right  costal  cartilage,  close  to  its  junction  with  the  sternum, 
i.  e.  at  the  point  where  the  aortic  arch  comes  nearest  to  the  surface, 
and  to  which  sounds  generated  at  the  aortic  orifice  would  be  best 
conducted.  Its  characters  are  such  as  would  belong  to  a  sound 
generated  by  membranes  like  the  semilunar  valves  being  suddenly 
made  tense,  and  so  thrown  into  vibrations.  It  is  obscured  and 
altered,  or  replaced  by  '  a  murmur,'  when  the  semilunar  valves 
are  affected  by  disease,  and  may  be  artificially  obliterated,  a 
murmur  taking  its  place,  by  passing  a  wire  down  the  arteries,  and 
hooking  up  the  aortic  valves.  There  can  be  no  doubt,  in  fact, 
that  the  second  sound  is  due  to  the  semilunar  valves  being  thrown 
into  vibrations  at  their  sudden  closure.  The  sound  heard  at  the 
second  right  costal  cartilage  is  chiefly  that  generated  by  the  aortic 
valves,  and  murmurs  or  other  alterations  in  the  sound  caused  by 
changes  in  the  aortic  valves  are  heard  most  clearly  at  this  spot. 
But  even  here  the  sound  is  not  exclusively  of  aortic  origin,  for 
in  certain  cases,  in  which  the  semilunar  valves  on  the  two  sides 
of  the  heart  are  not  wholly  synchronous  in  action,  the  sound 
heard  here  is  double  ("  reduplicated  second  sound "  ),  one  being 
due  to  the  aorta,  and  one  to  the  pulmonary  artery.  When  the 
sound  is  listened  to  on  the  left  side  of  the  sternum  at  the  same 
level,  the  pulmonary  artery  is  supposed  to  have  the  chief  share  in 
producing  what  is  heard,  and  changes  in  the  sound  heard  more 
clearly  here  than  on  the  right  side  are  taken  as  indications  of 
mischief  in  the  pulmonary  valves. 

The  first  sound,  longer,  duller,  and  of  a  more  '  booming ' 
character  than  the  second,  heard  with  greatest  distinctness  at  the 
spot  where  the  cardiac  impulse  is  felt,  presents  many  difficulties 
in  the  way  of  a  complete  explanation.  It  is  heard  distinctly  when 
the  chest-walls  are  removed.  The  cardiac  impulse,  therefore,  can 
have  little  or  nothing  to  do  with  it  In  point  of  time,  it  is 
coincident  with  the  systole  of  the  ventricles,  and  may  be  heard  to 
the  greatest  advantage  at  the  spot  of  the  cardiac  impulse  •,  that  is 
to  say,  at  the  place  where  the  ventricles  come  nearest  to  the 
surface,  and  to  which  sounds  generated  in  the  ventricles  would  be 
best  conducted. 

It  is  more  closely  coincident  with  the  closure  and  consequent 
vibrations  of  the  auriculo-ventricular  valves  than  with  the  entire 
systole ;  for  on  the  one  hand  it  dies  away  before  the  second 
sound  begins,  whereas,  as  we  shall  see,  the  actual  systole  lasts 
at  least  up  to  the  closure  of  the  semilunar  valves,  and  on 
the  other  hand  the  auriculo-ventricular  valves  cease  to  be  tense 


190  THE    SOUNDS    OF   THE    HEART.  [Book  r. 

and  to  vibrate  so  soon  as  the  contents  of  the  ventricle  are  driven 
out.  This  suggests  that  the  sound  is  caused  by  the  sudden 
tension  of  the  auriculo-ventricular  valves,  and  this  vievi^  is  sup- 
ported by  the  facts  that  the  sound  is  obscured,  altered  or 
replaced  by  murmurs  when  the  tricuspid  or  mitral  valves  are 
diseased,  and  that  the  sound  is  also  altered  or,  according  to 
some  observers,  wholly  done  away  with  when  blood  is  prevented 
from  entering  the  ventricles  by  ligature  of  the  vense  cavse.  On 
the  other  hand,  the  sound  has  not  that  sharp  character  which 
one  would  expect  in  a  sound  generated  by  the  vibration  of 
membranes  such  as  the  valves  in  question,  but  in  its  booming 
qualities  rather  suggests  a  muscular  sound.  Further,  according 
to  some  observers,  the  sound,  though  somewhat  modified,  may 
still  be  heard  when  the  large  veins  are  clamped  so  that  no  blood 
enters  the  veutricle,  and,  indeed,  may  be  recognized  in  the  few 
beats  given  by  a  mammalian  ventricle  rapidly  cut  out  of  the 
living  body  by  an  incision  carried  below  the  auriculo-ventricular 
ring.  Hence  the  view  has  been  adopted  that  this  first  sound 
is  a  muscular  sound.  In  discussing  the  muscular  sound  of  skeletal 
muscle  (see  §  75),  we  saw  reasons  to  distrust  the  view  that  this 
sound  is  generated  by  the  repeated,  individual,  simple  contrac- 
tions which  make  up  the  tetanus,  and  hence  corresponds  in  tone 
to  the  number  of  those  simple  contractions  repeated  in  a  second, 
and  to  adopt  the  view  that  the  sound  is  really  due  to  a  repetition . 
of  unequal  tensions  occurring  in  a  muscle  during  the  contraction. 
Now,  the  ventricular  systole  is  undoubtedly  a  simple  contraction,  a 
prolonged  simple  contraction,  not  a  tetanus,  and,  therefore,  under 
the  old  view  of  the  nature  of  a  muscular  sound,  could  not  produce 
such  a  sound ;  but  accepting  the  other  view,  and  reflecting  how 
complex  must  be  the  course  of  the  systolic  wave  of  contraction 
over  the  twisted  fibres  of  the  ventricle,  we  shall  not  find  great 
difficulty  in  supposing  that  that  wave  is  capable  in  its  progress  of 
producing  such  repetitions  of  unequal  tensions  as  might  give  rise 
to  a  '  muscular  sound,'  and,  consequently,  in  regarding  the  first 
sound  as  mainly  so  caused.  Accepting  such  a  view  of  the  origin  of 
the  sound  we  should  expect  to  find  the  tension  of  the  muscular 
fibres,  and  so  the  nature  of  sound,  dependent  on  the  quantity  of 
fluid  present  in  the  ventricular  cavities  and  hence  modified  by  liga- 
ture of  the  great  veins,  and  still  more  by  the  total  removal  of  the 
auricles  with  the  auriculo-ventricular  valves.  We  may  add  that 
we  should  expect  to  find  it  modified  by  the  escape  of  blood  from 
the  ventricles  into  the  arteries  during  the  systole  itself,  and  might 
regard  this  as  explaining  why  it  dies  away  before  the  ventricle  has 
ceased  to  contract. 

Moreover,  seeing  that  the  auriculo-ventricular  valves  must  be 
thrown  into  sudden  tension  at  the  onset  of  the  ventricular  systole, 
which,  as  we  have  seen,  is  developed  with  considerable  rapidity, 
not  far  removed  at  all  events  from  the  rapidity  with  which  the 


Chap.  iv.J  THE    VASCULAR   MECHANISM.  191 

semilunar  valves  are  closed,  a  rapidity,  therefore,  capable  of  giving 
rise  to  vibrations  of  the  valves  adequate  to  produce  a  sound,  it  is 
difficult  to  escape  the  conclusion  that  the  closure  of  these  valves 
must  also  generate  a  sound,  which  in  a  normally  beating  heart  is 
mingled  with  the  sound  of  muscular  origin. 

If  we  accept  this  view  that  the  sound  is  of  double  origin, 
partly  '  muscular,'  partly  '  valvular,'  both  causes  being  dependent 
on  the  tension  of  the  ventricular  cavities,  we  can  perhaps  more 
easily  understand  how  it  is  that  the  normal  first  sound  is  at  times 
so  largely,  indeed,  we  may  say  so  completely  altered  and  obscured 
in  diseases  of  the  auriculo-ventricular  valves,  and  how  it  may  also 
be  modified  in  character  by  changes,  such  as  hypertrophy,  of  the 
muscular  walls. 

Since  the  left  ventricle  forms  the  entire  left  apex  of  the 
heart,  the  murmurs  or  other  changes  of  the  first  sound  heard  most 
distinctly  at  the  spot  of  cardiac  impulse  belong  to  the  mitral  valve 
of  the  left  ventricle.  Murmurs  generated  in  the  tricuspid  valve 
of  the  right  ventricle  are  heard  more  distinctly  in  the  median  line 
below  the  end  of  the  sternum. 

§  113.  Endocardiac  Pressure.  Since  it  is  the  pressure  exerted 
upon  the  contents  of  the  ventricle  by  the  contraction  of  the 
ventricular  walls  which  drives  the  blood  from  the  heart  into  the 
aorta,  and  so  maintains  the  circulation,  the  study  of  this  pressure, 
endocardiac  pressure,  is  of  great  importance.  The  mercurial 
manometer,  so  useful  in  a  general  way  in  the  study  of  arterial 
pressure,  is  unsuited  for  the  study  of  endocardiac  pressure,  since 
the  great  inertia  of  the  mercury  prevents  the  instrument  respond- 
ing properly  to  the  exceedingly  rapid  changes  of  pressure  which 
take  place  in  the  heart.  We  are  obliged  to  have  recourse  to  other 
instruments. 

One  method,  having  been  used  by  Chauveau  and  Marey  in 
researches  which  have  become  '  classic,'  deserves  to  be  noticed, 
though  it  is  not  now  employed.  It  consists  in  introducing,  in  a 
large  animal,  such  as  a  horse,  through  a  blood  vessel  into  a  cavity 
of  the  heart,  a  tube  ending  in  an  elastic  bag,  Fig.  37  A,  both  tube 
and  bag  being  filled  with  air,  and  the  tube  being  connected  with 
a  recordin;^  '  tambour.' 


A  tube  of  appropriate  curvature,  A.  h.  Fig.  37,  is  furnished  at  its 
end  with  an  elastic  bag  or  '  ampulla  '  a.  Such  an  instrument  is 
spoken  of  as  a  'cardiac  sound.'  When  it  is  desired  to  explore  simul- 
taneously both  auricle  and  ventricle,  the  sound  is  furnished  with  two 
ampulLne,  one  at  the  extreme  end  and  the  other  at  such  a  distance  that 
when  the  former  is  within  the  cavity  of  the  ventricle  the  latter  is 
within  the  cavity  of  the  auricle.  Each  '  ampulla '  communicates 
by  a  separate  air-tight  tube  with  an  air-tight  tambour  (Fig.  37  B) 
on  which  a  lever  rests,  so  that  any  pressure  on  the  ampulla  is 
communicated  to  the  cavity  of  its  respective  tambour,   the  lever  of 


192 


ENDOCAEDIAC   PEESSUEE. 


[Book  i. 


which  is  raised  in  proportion.  When  two  ampullae  are  used,  the 
writing  points  of  both  levers  are  brought  to  bear  on  the  same  re- 
cording surface  exactly  underneath  each  other.  The  tube  is  carefully 
introduced  through  the  right  jugular  vein  into  the  right  side  of  the 
heart  until  the  lower  (ventricular)  ampulla  is  fairly  in  the  cavity  of 
the  right  ventricle,  and,  consequently,  the  upper  (auricular)  ampulla 
in  the  cavity  of  the  right  auricle.  Changes  of  pressure  on  either 
ampulla,  then,  cause  movements  of  the  corresponding  lever.  When  the 
pressure,  for  instance,  on  the  ampulla  in  the  auricle  is  increased,  the 
auricular    lever    is    raised    and    describes    on    the  recording    surface  an 


Fig.  37.     Marey's  Tambour,  with  Cardiac  Sound. 

A.  A  simple  cardiac  sound  such  as  may  be  used  for  exploration  of  the  left 
ventricle.  The  portion  a  of  the  ampulla  at  the  end  is  of  thin  india  rubber,  stretched 
over  an  open  framework  with  metallic  supports  above  and  below.  The  long  tube  b 
serves  to  introduce  it  into  the  cavity  which  it  is  desired  to  explore. 

B.  The  Tambour.  The  metal  chamber  m  is  covered  in  an  air-tight  manner 
with  the  india  rubber  c,  bearing  a  thin,  metal  plate  m',  to  which  is  attached  the  lever/, 
moving  on  the  hinge  h.  The  whole  tambour  can  be  placed  by  means  of  the  clamp 
cl  at  any  height  on  the  upright  .•?'.  The  india  rubber  tube  t  serves  to  connect  the 
interior  of  the  tambour  either  with  the  cavity  of  the  ampulla  of  A  or  with  any  other 
cavity.  Supposing  that  the  tube  t  were  connected  with  b,  any  pressure  exerted  on 
a  would  cause  the  roof  of  the  tambour  to  rise  and  the  point  of  the  lever  would  be 
proportionately  raised. 


ascending    curve ;    when  the  pressure  is  taken  off,  the  curve  descends, 
—  and  so  also  with  the  ventricle. 

The  'sound'  may  in  a  similar  manner  be  introduced  through  the 
carotid  artery  into  the  left  ventricle,  being  slipped  past  the  aortic 
valves,  and  thus  the  changes  taking  place  in  that  chamber  also  may  be 
explored. 


Chap.  iv.J  THE    VASCULAR    MECHANISM. 


193 


When  this  instrument  is  applied  to  the  right  auricle  and 
ventricle  some  such  record  is  obtained  as  that  shewn  in  Fig.  38, 
where  the  upper  curve  is  a  tracing  taken  from  the  right  auricle, 
and  the  lower  curve  from  the  right  ventricle  of  the  horse, 
both  curves  being  taken  simultaneously  on  the  same  recording 
surface.  In  these  curves  the  rise  of  the  lever  indicates  pressure 
exerted  upon  the  corresponding  ampulla,  and  the  upper  curve, 
from  the  right  auricle,  shews  the  sudden,  brief  pressure  b  exerted 
by  the  sudden  and  brief  auricular  systole.  The  lower  curve,  from 
the  right  ventricle,  shews  that  the  pressure  exerted  by  the  ventric- 
ular systole  begins  almost  immediately  after  the  auricular  systole, 
increases  very  rapidly  indeed,  so  that  the  lever  rises  in  almost  a 
straight  line  up  to  c',  is  continued  for 
some  considerable  time,  and  then  falls 
very  rapidly  to  reach  the  base  line. 
The  figure,  it  must  be  understood,  does 
not,  by  itself,  give  any  information  as 
to  the  relative  amounts  of  pressure 
exerted  by  the  auricle  and  ventricle 
respectively  ;  indeed,  the  movements  of 
the  auricular  lever  are  much  too  great 
compared  with  those  of  the  ventricular 
lever.  The  figure  is  chiefiy  useful  for 
giving  a  graphic  general  view  of  the 
series  of  events  within  the  cardiac  cavi- 
ties during  a  cardiac  cycle,  the  short 
auricular  pressure,  the  long-continued 
ventricular  pressure,  lasting  nearly  half 
the  whole  period,  and  the  subsequent 
pause  when  both  parts  are  at  rest  or  in 
diastole. 

Among  the  more  trustworthy  methods  of  recording  the 
changes  of  endocardiac  pressure,  we  may  first  mention  that  of 
Roy  and  Rolleston. 


Fig.  38.  Simultaneous  tracings 
FROM  THE  Right  .A.uricle,  am» 
Ventricle,  of  the  Horse. 
(After  Chauveau  and  Maeey.) 


By  means  of  a  short  cannula  introduced  through  a  large  vessel,  or 
directly,  as  a  trocar,  through  the  walls  of  the  ventricle  (or  auricle),  the 
blood  in  the  cavity  is  brought  to  bear  on  an  easily  moving  piston. 
The  movements  of  the  piston  are  recorded  by  a  lever,  and  the  evils 
of  inertia  are  met  by  making  the  piston  and  lever  work  against  the 
torsion  of  a  steel  ribbon,  the  length  of  which,  and  consequently  the 
resistance  offered  by  which,  and  hence  the  excursions  of  the  piston, 
can  be  varied  at  pleasure. 


We  give  as  examples  of  curves  obtained  by  this  method 
two  curves  from  the  left  ventricle,  one  (Fig.  39  A)  of  a 
rapidly  beating,  and  the  other  (Fig.  39  B)  of  a  slowly  beating 
heart. 

13 


194 


ENDOCARDIAC   PRESSUEE. 


[Book  i. 


Fig.  39.    Cueves  of  Endocardiac  Pressure.  From  Left  Ventricle  of  Dog. 
(Roy  and   RoUeston.) 

A.  a  quickly  beating,  B.  a  more  slowly  beating  heart. 

An  instrument  which  has  been  much  used  of  late,  and  the  use 
of  which  has  given  very  valuable  results  is  the  "  membrane-mano- 
meter" of  Hiirthle. 


Fig.  40.    The  Membrane-manometer  of  Hurthle.^ 


1  For  this  figure  I  am  indebted  to  Mr.  Albrecht,  the  University  Instrument- 
maker  at  Tubingen. 


Chap,  iv.]  THE   VASCULAR   MECHANISM. 


195 


Fig.  41.  Diagram  to  il- 
llstrate  the  essen- 
tial   parts    of   hur- 

THLe's    membrane    MA- 
NOMETER. 


This  consists  essentially  of  a  very  small  metal  drum  or  tambour 
(Fig.  41  a)  somewhat  like  that  of  IMarey,  but 
hemispherical  and  not  more  than  15  mm.  in 
diameter,  ending  below  in  a  tube  b.  In  Fig. 
40  the  instrument,  with  its  holder,  is  seen  from 
above.  The  second  lever,  which  is  motionless, 
is  for  the  purpose  of  describing  the  base  line. 
The  screw-tap  on  the  tube  leading,  in  the  figure, 
up  to  the  tambour,  is  for  the  purpose  of  diminish- 
ing the  calibre  of  the  tube  and  so  of  'damping  ' 
the  instrument.  On  the  right  of  the  tambour  in 
the  figure  are  seen  the  arrangements  for  adjust- 
ing the  levers.  In  Fig.  41  the  tube  b  by  which 
the  catheter  is  connected  with  the  tambour, 
is,  for  convenience  of  illustration,  'shewn  as 
directed  parallel  to  the  lever,  instead  of,  as 
in  the  instrument  itself,  at  right  angles  to  it. 
The  roof  of  the  tambour  is  supplied  bj^  a  care- 
fully chosen  delicate  elastic  membrane  c  which 

bears  at  its  centre  a  thin  metal  disc  d,  connected  by  a  short  upright 
e  with  a  lever  I. 

A  catheter,  opened  at  the  end  or  with  a  lateral  'eye  '  and  filled  with 
a  solution  of  magnesium  sulphate  or  with  some  fluid  tending  to  check 
the  clotting  of  blood,  is  introduced  into  the  cavity  of  the  heart  which 
it  is  desired  to  explore.  It  mnj  be  introduced  by  the  jugular  vein  into 
the  right  auricle,  and  past  the  auricle  into  the  right  ventricle,  or  through 
the  carotid  artery  into  the  aorta,  and  so,  between  the  semilunar  valves, 
or  piercing  one  of  the  flaps  (the  perforation  seems  to  introduce  no  error) 
into  the  cavity  of  the  left  ventricle ;  or  the  end  of  the  catheter  may  be 
left  in  the  aorta  above  the  semilunar  valves  when  it  is  desired  to 
investigate  the  pressure  at  the  root  of  the  aorta.  The  cavity  of  the 
tambour  also  is  filled,  not  with  air,  as  in  Alarey's  tambour,  but  with  the 
same  fluid  as  is  the  catheter,  or  with  water  j  and  the  tube  of  the  tambour 
is  connected  with  the  catheter. 

Variations  of  pressure  within  the  cavity  of  the  heart  are  transmitted 
through  tlie  fluid  of  the  catheter  to  the  fluid  in  the  tambour,  and  thus  put 
into  movement  the  elastic  roof  of  the  tambour  ;  the  movements  of  the 
elastic  roof  are,  in  turn,  transmitted  to  the  lever,  which  records,  in  the 
usual  manner,  on  some  recording  surface.  For  measuring  the  amount 
of  the  changes  of  pressure,  the  instrument  must  be  graduated  experi- 
mentally. There  are  many  details  in  the  instrument  which  need  not  be 
described  here;  but  we  may  state  that  the  instrument  may  be  'damped,' 
rendered  less  sensitive,  and  thus  the  features  of  the  curves  due  to 
inertia  lessened,  by  narrowing,  through  a  screw-tap,  the  communication 
between  the  catheter  and  the  cavity  of  the  tambour. 

The  memljrane  of  the  tambour  may,  by  means  of  an  ivory  button, 
be  brought  to  bear  on  one  end  of  a  slip  of  steel,  placed  horizontally 
and  fastened  at  the  other  end,  so  as  to  act  as  a  spring.  The  instrument 
then  becomes  a  "  spring-manometer."  The  small  movements  of  the 
spring  caused  by  the  movements  of  the  membrane  of  the  tambour  are 
magnified  by  a  recording  lever. 


196  ENDOCARDIAC   PRESSURE.  [Book  i. 

Fig.  42  gives  a  curve  of  endocardiac  pressure  of  the  left 
ventricle  of  the  dog  obtained  by  this 
method.  The  recording  surface  is 
travelling  quickly,  and  the  movements 
of  the  lever  are  not  great. 

The    manometer    of     Gad    differs 
Fig.  42.    Cukte  of  Pressure     from  that  of  Hiirthle  in  the  membrane 
IN  THE  Left  vsentricle  of     being  replaced  by  a  thin,  elastic  disc 
THE  Dog,  Hurthle's  Mem-         e   p,p<-„] 

BRANE-MANOMETER.  r  ^  •  p       ti 

in  tiie  instrument  of  Irey  and 
Krehl,  which  is  a  modification  of  one  by  Tick,  the  transmission 
is  effected  partly  by  fluid  and  partly  iDy  an  air  tambour,  the 
button  of  which  presses  against  a  horizontal  steel  spring. 

A  catheter,  filled  with  fluid  to  prevent  clotting  and  introduced  into 
a  cavity  of  the  heart,  is  connected  with  a  glass  cylinder,  maintained 
carefully  in  a  vertical  position,  the  lower  half  of  which  is  filled  with 
the  same  fluid  as  is  the  catheter.  The  upper  half  of  the  cylinder,  con- 
taining air  only,  is  connected  by  a  very  narrow,  in  fact  a  capillary  tube, 
with  a  small  tambour.  The  changes  of  pressure  within  the  heart  are 
transmitted  through  the  fluid  of  the  catheter  to  the  air  in  the  cylinder, 
and  so  to  the  air  in  the  tambour,  the  membrane  of  which  moves 
accordingly  in  and  out.  A  button  on  the  membrane  presses  on  a  hori- 
zontal steel  spring,  and  the  small  movements  of  the  membrane  thus 
transmitted  to  the  spring  are  recorded  by  means  of  a  magnifying 
lever. 

Other  instruments  have  been  employed  by  other  observers. 

When  we  examine  the  curves  which  we  have  given  (Figs.  38, 
39,  42),  obtained  by  three  several  methods,  we  find  that  they  agree 
in  the  following  main  features.  The  curve  of  pressure  in  the 
ventricle,  whether  right  or  left,  rises  at  the  very  beginning  of  the 
systole  with  very  great  rapidity,  very  soon  reaches  its  maximum  or 
nearly  its  maximum,  maintains  nearly  the  same  height  for  some 
time,  and  then  very  rapidly  descends  to  the  base  line  (which  in 
these  figures  indicates  the  pressure  of  the  atmosphere)  or  even 
falls,  for  a  brief  space,  slightly  below  it,  and  remains  at  or  near  the 
base  line,  until,  at  the  next  beat,  it  repeats  the  same  changes. 
This  means  that  the  contraction  of  the  ventricular  walls  in  the 
systole  acts  in  such  a  manner  as  very  suddenly  to  raise  up  to  a 
certain  height  the  pressure  within  the  ventricle,  which  during  the 
diastole  was  at,  or  not  far  removed  from  that  of  the  atmosphere, 
that  the  pressure  is  maintained  without  any  very  great  change  for 
a  considerable  time,  and  that  it  then  falls  back  to  its  original  level 
with  great  suddenness,  almost,  if  not  quite,  as  suddenly  as  it  was 
raised.  These  are  the  important  features  of  the  pressure  within 
the  ventricle ;  in  these  features  all  the  three  curves  agree.  We 
may  add  that  the  same  features  are  shewn  also  in  curves  of  pres- 


Chap,  iv.j  THE    VASCULAR    MECHANISM.  197 

sure  taken  by  other  methods  ;  and,  indeed,  as  shewn  in  Fig.  36  and 
in  others  which  we  shall  give,  corresponding  features  occur  in 
curves  of  other  changes  in  the  heart.  All  these  curves  shew  a 
flattening  maintained,  with  smaller  variations,  during  the  con- 
tinuance of  the  systole ;  this  is  so  characteristic  that  it  has  been 
called  the  '  systolic  plateau.'  It  is  true  that  curves  of  ventri- 
cular pressure  taken  by  certain  methods,  that  of  Frey  and  Krehl's 
for  instance,  do  not  shew  this  '  plateau,'  the  curve  in  such  cases 
rising  gradually  to  a  maximum  and  immediately  beginning  to  fall, 
so  that  the  summit  is  a  simple  peak.  And  it  is  argued  that  such 
a  curve  is  the  true  curve  of  ventricular  pressure  always  obtained 
so  long  as  the  blood  in  the  ventricle  has  free  access  to  the  interior 
of  the  catheter,  and  that  the  plateau  is  only  seen  when  the  end  of 
the  catheter  is  too  near  the  apex,  and  its  opening  closed,  at  the 
height  of  the  systole,  by  the  ventricular  walls  coming  together ;  the 
top  of  the  true  curve  is  thus,  as  it  were,  cut  off.  But  the  evidence 
is,  on  the  whole,  opposed  to  this  view,  and  we  shall  accept  the 
plateau  as  being  a  true  representation. 

Though  the  curves  given  above  agree  in  these  main  features, 
they  differ  in  many  minor  features,  and  other  features  also  of  minor 
value  appear  in  curves  of  endocardiac  pressure  according  to  the 
various  circumstances  in  which  the  heart  finds  itself.  Some  of 
these  minor  features  we  shall  presently  find  useful  in  discussing 
the  mechanism  of  the  beat. 

§  114.  The  output.  Since  the  use  of  the  pressure  exerted  by 
the  ventricle  is  to  drive  a  quantity  of  blood  out  of  the  ventricle 
into  the  aorta  (or  pulmonary  artery)  it  is  important  to  study  the 
'  output '  or  quantity  of  blood  so  driven  out ;  and  since,  vmder 
normal  circumstances,  the  quantity  ejected  by  the  right  ventricle 
is  the  same  as  that  ejected  by  the  left  ventricle,  we  may  confine 
our  attention  to  the  latter. 

The  normal  or  average  output  has  been  calculated  in  various 
ways,  by  help  of  certain  assumptions ;  but  these  we  may  put  on 
one  side  since  the  matter  has  now  been  made  the  subject  of  direct 
experimental  determination. 

Methods.  Metliod  of  Stolnikow.  This  consists  in  allowing  the 
blood  to  flow  from  the  carotid  into  a  vessel  until  a  certain  measured 
quantity  has  escaped,  and  then  returning  tliis  blood  to  the  right 
auricle  while  the  blood  from  the  carotid  is  flowing  into  a  second 
similar  vessel  to  be  similarly  returned,  and  in  repeating  tliis  mananivre 
a  certain  number  of  times.  One  carotid  is  tied  (tlie  animal  being  a 
dog),  and  tlie  arch  of  the  aorta  plugged  beyond  (Fig.  43  j))-  ^'^^ 
circulation  is  thus  contiued  to  the  lungs  and  the  coronary  system. 
Into  the  other  carotid  is  tied  a  tube  connected  by  a  forked  branching 
la  and  2a  witli  two  vessels  I.  and  II.,  which  also  comnumicate  by  a 
similar  forked  branching  \v  and  2v  witli  the  right  auricle.  The  blood 
is  allowed  to  flow  througli  la  into  I.  until  a  certain  quantity  has 
escaped.     Then  la  is  closed,  while  2a  and  \v  are  opened.     The  blood 


198 


THE  OUTPUT  OF  THE  HEART. 


[Book  i. 


from  I.  flows  back  by  Iw  to  the  right  auricle,  while  the  blood  from  the 
carotid  flows  into  H.  by  2a.  When  a  certain  quantity  has  escaped 
into  II.,  the  action  is  reversed,  and  I.  is   once  more  tilled ;  and  so  on. 


Fig.  43.    Diagram  of  Stolnikow's  Apparatus. 

In  this  way  the  quantity  of  blood  which  the  heart  delivers,  its  *  output ' 
during  a  given  time  can  be  measured  ;  the  quantity  discharged  at  a 
single  beat  can  similarly  be  determined.  By  means  of  recording  floats 
in  I.  and  II.,  a  graphic  record  of  the  output  may  also  be  obtained. 

The  other  methods  are  pletliysmographic  (§  104)  in  nature.  The 
volume  of  the  heart  changes  only  with  the  volume  of  its  contents, 
for  we  may  neglect,  in  the  first  instance  at  least,  as  insignificant  the 
changes  of  volume  due  to  changes  in  the  amount  of  blood  held  by  the 
coronary  system,  and  we  may  wholly  neglect  the  changes  of  volume  due 
to  changes  in  the  quantity  of  lymph  present  in  the  cardiac  tissues. 
An  increase  in  the  volume  of  the  heart  means  that  more  blood  is  flowing 
into  it  than  is  leaving  it,  a  decrease  that  more  is  leaving  it  than  is 
flowing  into  it.  Hence,  if  we  measure  the  diminution  of  volume  which 
takes  place  during  the  systole,  this  gives  us  the  volume  of  blood  dis- 
charged by  the  two  ventricles  during  that  systole,  the  effect  of  clianges 
in  the  auricles  being  neglected  ;  and  since  the  two  ventricles  discharge 
equal  quantities,  half  this  will  give  ns  the  quantity  of  blood  discharged 
by  the  loft  ventricle  during  the  systole. 

In    the  method  of   Tigerstedt  and   others  the    pericardial  cavity  is 


Chap,  iv.]  THE    VASCULAR   MECHANISM. 


199 


employed  as  the  plethysmographic  chamber,  the  changes  of  volume  in 
it  being  transmitted  by  air  to  the  recording  apparatus.  A  cannula  is 
introduced  into  the  pericardium,  a  little  air  entering  at  the  same  time, 
and  is  connected  by  an  air  tube  with  a  delicate  piston,  tlie  movements 
of  which  are  recorded  in  the  usual  way. 


girn±: 


Fig.  44.    Cakdiometer  of  Rot  and  Adami. 

In  the  method  of  Roy  and  Adami  the  heart  is  placed  in  a  rigid 
metal  box,  Fig.  44  h,  the  cavity  of  whicli,  filled  with  Avarmed  oil,  is 
connected  witli  a  light  piston  c  and  so  with  a  recording  lever.  The 
pericardium  being  laid  open,  the  two  halves  of  the  box  are  placed 
round  tlio  heart,  are  securely  fixed  by  means  of  an  india  rubber  rin"  a, 
to  the  parietal  pericardium  round  the  roots  of  the  great  vessels,  and  are 
brought  together.  The  cavity  is  then  filled  with  oil,  and  the  piston, 
also  filled  with  oil,  is   brought   into  connection  with   the  box,  tlie  lever 


200 


THE  OUTPUT  OF  THE  HEART. 


[Book  i. 


aud  rod  of  the  piston  being  placed  by  means  of  the  india  rubber  spring  d, 
in  such  a  position  that  the  pressure  within  the  box  is  some  few  mm.  Hg 
below  that  of  the  atmosphere. 

By  these  methods  it  has  been  determined  that  the  diminution 
of  the  volume  of  the  heart  at  a  systole,  the  "  contraction  volume" 
as  it  has  inconveniently  been  called,  that  is  to  say,  the  quantity 
of  blood  discharged  at  a  systole,  the  output  of  a  systole,  or  the 
"  pulse-volume  "  as  we  may  call  it,  for  it  is  this  which  causes  the 
pulse,  varies  very  much  under  various  circumstances.  We  shall 
have  to  discuss  later  on  some  of  the  influences  bearing  on  its 
amount.  Meanwhile  we  merely  call  attention  to  the  fact  that  it  does 
vary  largely,  and  that  any  numerical  statement  as  to  a  normal 
pulse-volume  has  relatively  little  value. 

Another  fact  of  considerable  importance  brought  to  light  by 
these  methods  is  that  under  certain  circumstances,  at  all  events,  the 
output  by  the  left  ventricle  during  a  number  of  beats  may  be  less 
than  the  intake  through  the  right  auricle.  This  means  that  under 
these  circumstances  the  ventricle  does  not  at  the  systole  discharge 
the  whole  of  its  contents ;  some  of  the  blood  remains  behind  in 
the  cavity  of  the  ventricle  at  the  close  of  the  systole.  Hence  the 
assumption  that  the  ventricle,  in  its  systole,  always  discharges 
the  whole  of  its  contents,  so  as  to  be  quite  empty  at  the  onset  of 
diastole,  is  not  true ;  the  ventricle  may  completely  empty  itself 
but  it  by  no  means  always  does  so. 


The  Mechanism  of  the  Beat. 

§  115.  We  may  now  attempt  to  consider  in  rather  more 
detail  what  we  may  call  the  mechanism  of  the  beat,  that  is  to  say, 
the  exact  manner  in  which  the  heart  receives  and  ejects  the  blood. 
For  this  purpose  we  shall  need  certain  data  in  addition  to  those 
on  which  we  have  already  dwelt. 

In  addition  to  the  curve  obtained  by  placing  a  light  lever  on 
the  exposed  heart  (Fig.  45),  a  method  which  though  useful  is  open 


Fig.  45.     (Repeated  from  Fig.  36. > 


Chap,  iv.]  THE    VASCULAR   MECHANISM. 


201 


to  objection,  we  may  obtain  what  is  very  nearly  the  same  thing, 
viz.  a  cardiographic  tracing  (Fig.  46)  or  cardiogram,  that  is  to  say, 
a  tracing  of  the  cardiac  impulse,  a  curve  of  the  changes  iu  the 
pressure  exerted  by  the  apex  of  the  heart  on  the  chest-wall. 

Various  forms  of  cardiograph  have  been  used  to  record  tlie  cardiac 
impulse.  In  some  the  pressure  of  the  impulse  is  transmitted  directly 
to  a  lever  which  writes  upon  a  travelling  surface.  In  others;  the 
nnpulse  is,  by  means  of  an  ivory  button,  brought  to  bear  on  an  air- 
chamber,  connected  by  a  tube  with  a  tambour  like  that  in  Fig.  37 ;  the 
pressure  of  the  cardiac  impulse  compresses  the  air  in  the  air-chamber, 
and  through  this  the  air  in  the  chamber  of  tlie  tambour,  whereupon  the 
lever  is  raised.  In  others  the  impulse,  being  received  by  a  small, 
elastic  bag  filled  with  fluid  and  introduced  through  an  opening  matle 
in  the  chest-wall,  the  pleura  being  left  intact,  is  transmitted  through 
fluid  along  a  tube  to  a  membrane-manometer.  Or,  to  avoid  opening 
tlie  chest-wall,  the  tube  may  be  made  to  begin  in  a  small,  trumpet- 
shaped  opening  or  "  receiver  "  covered  with  an  elastic  membrane,  bearing 
a  central  button  of  cork  or  other  material ;  the  button  being  lightly 
pressed  on  the  spot  where  the  impulse  is  felt,  the  impulse  is  transmitted 
along  the  fluid  of  the  tube  from  tlie  elastic  membrane  of  the  receiver 
to  that  of  the  manomciter. 

In  Fig.  46  we  give  two  such  cardiograms  obtained  by  different 
methods,  in  Fig.  54  a  more  diagrammatic  curve. 


Fig.  46.    Cardiograms. 
The  left-hand  figure  is  from  rjoy  and  Adami. 

Since  it  is  the  contraction  of  the  ventricular  fibres  which  is  the 
actual  propelling  force,  an  exact  record  of  this  contraction,  after 
the  manner  of  a  muscle-curve,  would  serve,  could  it  be  obtained, 
as  the  basis  of  discussion.  Owing  to  the  intricate  arrangement  of 
the  cardiac  muscular  fibres,  such  a  simple  record  cannot  be 
obtained ;  the  nearest  approacli  to  it  is  the  record  of  the  changes 
in  the  distance  between  two  points  on  the  surface  of  the  heart 
brought  about  durim^  a  beat. 


202  THE   MECHANISM  OF  THE   BEAT.         [Book  i. 

In  the  instrument  of  Roy  and  Adami,  by  an  ingenious  arrangement 
into  the  details  of  which  we  need  not  go,  a  delicate  rod  placed  horizon- 
tally in  connection  with  two  points  of  the  surface  of  the  heart,  of  the 
ventricles,  for  instance,  as  it  glides  to  and  fro,  according  as  the  two 
points  approach  or  recede  from  each  other,  records  its  movements  by 
means  of  a  light  lever. 

We  give  in  Fig.  47  such  a  myocardiographic  tracing,  as  it 
is  called  ;  the  rise  of  the  lever  indicates  an 
approach,  the  fall  a  receding  of  two  points 
taken  transversely  across  the  ventricle  of  a 
dog. 

What  conclusions  can  we  draw  from  the 
features  of  the  various  curves  which  we  have 
given  ?  We  have  reproduced  in  some  cases 
more  than  one  curve  representing  the  same 
event,  for  the  important  reason  that  certain 
Fig.  47.  Myocardio-  of  the  features  of  almost  every  curve  are 
Roy^'aJd  AdTmi.^^^'  ^^e,  to  some  extent  at  least,  to  the  instru- 
ment itself,  and  must  not  be  taken  as  exact 
records  of  what  is  actually  taking  place  in  the  heart ;  the  inertia 
of  one  or  other  part  of  this  or  that  instrument  used  plays  a  more 
or  less  important  part  in  determining  the  form  of  the  curve.  It 
will  therefore  be  readily  understood  that  the  interpretation  of 
various  heart  curves  is  attended  with  great  difficulties,  and  has 
led  to  much  discussion.  We  must  content  ourselves  here  with 
confining  our  attention  to  the  more  important  points,  leaving  many 
details,  however  interesting,  on  one  side. 

Let  us  begin  with  the  beginning  of  the  ventricular  systole. 
All  the  curves,  curve  of  endocardiac  pressure,  cardiogram,  myocar- 
diogram,  and  others,  shew  the  important  fact  that  the  systole  begins 
suddenly  and  increases  swiftly  until  it  reaches  the  beginning  of 
what  we  have  called  the  "  systolic  plateau,"  c  in  Figs.  38,  39,  45, 
3  in  Fig.  46,  d  in  Fig.  47. 

In  some  curves,  as  in  Figs.  38,  39  B,  42,  the  rise  is  unbroken ; 
in  others,  as  in  Figs.  39  A,  45,  the  rise  is  marked  with  a  shoulder. 
In  Fig.  47,  this  shoulder  h  has  been  interpreted,  by  those  who 
maintain  that  papillary  muscles  begin  their  contraction  later  than 
the  main  ventricular  wall,  as  indicating  that  event.  We  will  not 
discuss  the  question  here. 

In  some  of  the  pressure  curves, as  in  Fig.  38,  the  rise  of  pressure 
in  the  ventricle  due  to  the  actual  systole  is  preceded  by  a  slight 
temporary  rise.  This  has  been  interpreted  as  indicating  a  slight 
rise  of  pressure  in  the  ventricle  due  to  the  auricular  systole  just 
preceding  the  ventricular  systole  ;  but  this  interpretation  has  been 
debated,  and  indeed  the  slight  rise  in  question  is  not  always  seen. 
Similarly,  some  curves  shew  a  gradual  but  very  slight  increase  of 
pressure  in  the  ventricle  during  the  preceding  diastole  ;  this  has 
been  interpreted  as  indicating  a  rise  of  pressure  due  to  the  gradual 


Chap,  iv.]  THE    VASCULAR   MECHANISM. 


203 


inflow  of  blood  from  the  auricle  and  veins ,  but  it,  too,  is  not 
always  present.  Both  the  steady 
though  slight  rise  of  the  lever 
throughout  the  diastole,  with  a 
sudden  increase  at  the  end,  coin- 
cident with  the  auricular  systole, 
are  often  seen  in  cardiograms ;  see 
the  diagrammatic  curve  in  Fig.  54. 
The  ventricle  as  a  whole  enlarges 
under  the  venous  inflow,  and  is  more 
suddenly  enlarged  by  the  auricular 
systole. 

The  feature  on  which  we  wish  to 
insist  is  the  rapid  rise  of  the  intra- 
ventricular pressure,  and  the  sudden 
change  at  the  commencement  of  the 
systolic  plateau.  "What  does  this 
sudden  change  mean  ?  To  answer 
this  question  we  must  ascertain  what 
is  taking  place  at  the  same  time  in 
the  aorta. 

§  116.  If  two  catheters  be  in- 
troduced at  the  same  time  into  the 
left  side  of  the  heart  of  a  dog,  being 
so  arranged  that  while  the  end  of 
one  catheter  lies  in  the  left  ventricle. 
Fig.  48,  V,  that  of  the  other  lies  in 
the  aorta  A*^  above  the  semilunar 
valves,  and  if  each  catheter  be  con- 
nected with  a  membrane-manometer, 
the  two  manometers  recording  on 
the  same  surface,  one  below  the 
other,  we  obtain  some  such  result 
as  that  shewn  in  Fig.  49. 

An  examination  of  the  two  curves  thus  obtained  shews  us  the 
following.  At  0,  the  beginning  of  the  ventricular  systole,  or  rather 
the  time  when  the  contraction  of  the  ventricular  fibres  is  beginning 
to  raise  the  pressure  within  the  ventricle,  no  effect  is  being  produced 
in  the  aorta ;  the  blood  in  the  aorta  is  completely  sheltered  by 
the  closed  aortic  valves.  A  little  later,  however,  at  1,  the  pressure 
in  the  aorta  begins  to  rise.  This  means  that  the  semilunar  valves 
are  now  opened,  so  that  the  force  of  the  ventricular  systole  can 
make  itself  felt  in  the  aorta.  Up  to  1,  the  pressure  in  the 
ventricle,  though  increasing,  is  still  less  than  that  remaining  in  the 
aorta  after  the  last  beat,  but  at  1  the  pressure  in  tlie  ventricle 
becomes  equal  to  or  rather  slightly  greater  than  that  in  the  aorta, 
and  the  valves  are  thrown  open. 

This  is  also  shewn  by  comparing,  as  may  be  done  by  means 


Fig.  48.  Diagram  illustrating 
the  method  of  recording  si- 
MULTANEOUSLY THE  Pressure  ix 
THE  Left  Ventricle  and  at  the 
ROOT  OF  the  Aorta.     Hurthle. 


204 


THE   MECHANISM   OF   THE   BEAT.        [Book  i. 


of  the  "  differential  mauometer,"  the  changes  of  pressure  in  the 
ventricle  and  in  the  aorta  at  the  same  time. 


0    12 


34  5 


Fig.  49.    Simultaneous  Tracings  of  Venteiculae  and  Aoetic  Pressure. 

HiJRTHLE. 

On  the  left  side  the  recording  surface  is  travelling  slowly,  on  the  right  more 
swiftly,  the  tuning-fork  vibrations,  t,  being  100  a  second. 

J",  aortic.  V.  ventricular  curve,  x — x  base  line  to  each.  The  vertical  lines 
1,  2,  3,  4,  5,  cut  each  curve  at  exactly  the  same  time. 

In  the  differential  manometer,  Fig.  60,  the  two  tamhours  of  two 
membrane  manometers  T  and  Tj  (the  mouths  of  the  tubes  opening  into 
each  are  seen   in  section)  are  arranged  so  that  the  central  discs  of  both, 


T  T, 

Fig.  50.    Diagram  of  the  Differential  Manometer  of  Hurthle. 

d  and  d-^,  work  on  a  balance  above  them.  When  the  pressure  in  the 
two  tambours  is  equal,  the  balance  is  horizontal ;  any  difference  of 
pressure  between  the  two  leads  to  an  upward  or  downward  movement 
of  one  or  other  arm,  and  this  working  against  the  light  steel  spring  s,  by 
means  of  e  and  e'  moves  the  lever  I. 

In  Figs.  51,  52  we  give  simultaneous  tracings  of  the  pressure 
in  the  left  ventricle  F,  and  in  the  aorta  A^,  and  of  the  movements 
of  the  lever  of  the  balance  indicating  differences  of  pressure  D 
between  the  ventricle  and  the  aorta.  At  the  base  line  x — x  of  D  the 
two  pressures  are  equal.  The  course  of  the  curve  below  this  base 
line  indicates  that  the  pressure  in  the  ventricle  is  below  that  of  the 
aorta;  as  the  curve  approaches  towards  the  base  line  the  pressure 
in  the  ventricle  becomes  more  and  more  nearly  equal  to  that  in 
the  aorta ;  and  such  part  of  the  curve  as  lies  above  the  base  line 
indicates  (except  in  so  far  as  it  may  be  due  to  the  inertia  of  the 


Chap,  iv.]  THE    VASCULAR   MECHANISM. 


^05 


instrument)  that    the  pressure  in    the  ventricle  is  for  the  time 
being  above  that  iu  the  aorta. 


t 


Fig,  51.    Simultaneous  Curves  of  Aortic  and  Ventricular  Pressure  and 
OF  THE  Differential  Manometer.     Hurthle. 

.1".  aorta.  V.  ventricle.  D.  differential  manometer,  x — x,  the  base  Hue  in  each 
respectively.  The  recordmg  surface  is  travelling  slowly,  the  time  marker  t,  t  mark- 
in"  seconds. 


0   1     2 


3    4 


0     1 


3     4 


0      1     2 


3   4 


0    1 


Fig.  .52.    The  same. 


N, 


3    4 


The  recording  surface  is  travelling  quickly  ;  the  vibrations  of  the  tuning-fork  t, 
t,  are  100  (double  vibrations)  a  second. 

An  examination  of  the  figures  shews  that  the  pressures  in  the 
ventricle  and  the  aorta  become  equal  at  the  mark  (1).  Before 
this  though  the  pressure  in  the  ventricle  is  rising  rapidly  that  in 
the  aorta  is  not  rising,  indeed  is  continuing  to  sink ;  the  closed 


206  THE   MECHANISM   OF   THE   BEAT.         [Book  i. 

semilunar  valves  shelter  the  blood  in  the  aorta  from  the  ventricu- 
lar pressure.  But  immediately  after  (1)  the  pressure  in  the  aorta 
also  begins  to  rise  ;  this  shews  that  the  semilunar  valves  are  now 
open,  the  blood  in  the  ventricle  and  that  in  the  aorta  now  forming 
a  continuous  column,  and  allowing  the  pressure  of  the  ventricle  to 
be  felt  in  the  aorta.  A  very  slight  excess  of  pressure  on  the 
ventricular  side  of  the  valves  is  sufficient  to  push  aside  the  flaps 
of  the  valve ;  so  that  we  may  fairly  say  that  the  valves  open 
immediately  after  (1),  which  marks  the  point  at  which  the  curve 
of  difference  of  pressure  between  the  ventricle  and  the  aorta  has 
reached  the  base  line  x — x ;  that  is  to  say,  at  which  the  difference 
between  the  two  has  become  nil. 

It  will  be  observed,  however,  that  the  mark  (1)  cuts  the  ventri- 
cular curve  not  at  the  summit  of  its  rise  but  short  of  this  ;  the 
pressure  in  the  ventricle  continues  to  rise  after  the  valves  are 
open,  the  curve  continues  after  this  to  ascend  rapidly  up  to  (2), 
which  marks  the  beginning  of  the  systolic  plateau.  During  the 
interval  between  (1)  and  (2)  the  pressure  is  rising  in  the  aorta  also. 
During  this  interval  the  pressure  in  the  ventricle,  continuing  to 
rise,  becomes  greater  than  that  in  the  aorta,  the  curve  of  difference 
rises  above  the  base  line  ;  but  the  excess  of  pressure  in  the  ventricle 
does  not  become  very  great,  the  curve  of  difference  does  not  rise  to 
any  great  height,  because  that  very  excess  of  pressure  is  used  up 
in  driving  the  contents  of  the  ventricle  into  the  aorta  through  the 
open  semilunar  valves. 

During  this  interval  the  pressure  in  the  aorta  continues  to 
rise  because,  until  the  height  of  pressure  at  (2)  is  reached,  the 
pressure  is  not  yet  sufficient  to  drive  the  blood  on  along  the 
arterial  system  with  adequate  rapidity. 

With  the  point  (2)  the  systolic  plateau  begins.  During  this 
plateau  the  exact  course  taken  by  the  curve  of  ventricular  pressure 
differs  in  different  cases.  We  will  take  first  the  perhaps  more 
ordinary  case  in  which  the  curve  with  intermediate  variations 
which  we  may  at  present  pass  over  gradually  declines  until  the 
point  (3)  is  reached,  when  the  plateau  comes  to  an  end  by  reason 
of  the  sudden  fall  of  the  ventricular  pressure. 

There  can  be  no  doubt  that  the  sudden  fall  after  (3)  is  due  to 
the  sudden  cessation  of  the  contraction  of  the  ventricular  walls,  to 
their  sudden  relaxation.  But  what  is  taking  place  during  the 
systolic  plateau  before  this  point  is  reached  ? 

It  used  to  be  argued,  taking  coimt  of  the  distension  only  of 
the  aorta  as  indicated  by  the  sphygmograph,  an  instrument  of 
which  we  shall  speak  later  on,  that  the  ventricular  contents 
escape  into  the  aorta  during  the  period  of  the  distension  of  the 
aorta  and  during  this  only,  having  ceased  to  flow  by  the  time  that 
this  distension  passes  away  giving  place  to  a  sequent  shrinking 
of  the  aorta.  Now  when  this  period  of  distension  is  carefully 
measured  it  is  found  to  be  much  shorter  than  the  systole  of  the 


Chap,  iv.]  THE  VASCULAE  MECHANISM.  207 

ventricle,  as  measured  by  the  length  of  the  systolic  plateau. 
Hence,  it  being  further  assumed  that  the  whole  of  the  contents 
of  the  ventricle  were  ejected  at  each  systole,  it  was  inferred 
that  the  ventricle  remained  empty  and  yet  contracted  for 
an  appreciable  period  after  the  discharge  of  its  contents.  And 
this  led,  in  turn,  to  a  great  divergence  of  opinion  as  to  the  exact 
time  at  which  the  semilunar  valves  were  closed. 

But  when  we  carefully  explore  the  pressure  in  the  aorta  and 
in  the  ventricle  at  the  same  time,  making  use  of  the  differential 
manometer,  we  come  upon  facts  which  seem  to  disprove  this  view. 
Examining  Fig.  52  we  find  that,  while  during  the  systolic  plateau 
the  pressure  is  falling  in  both  aorta  and  ventricle,  the  curve 
of  difference  of  pressure  D  remains  above  the  base  line,  though 
not  far  above  it  and  continually  approaching  it,  up  to  the  mark  (3) 
at  the  very  end  of  the  plateau.  At  this  point,  however,  at  the  end 
of  the  plateau,  at  the  beginning  of  relaxation,  a  very  great  difference 
of  pressure  is  established ;  while  the  ventricular  pressure  falls 
suddenly  and  soon  reaches  or  even  passes  the  base  line  (becoming 
in  the  latter  case  negative,  i.e.  below  that  of  the  atmosphere),  the 
pressure  in  the  aorta  undergoes  relatively  little  change,  —  indeed, 
immediately  afterwards  receives  an  increase  of  which  we  shall 
have  to  speak  later  on  as  the  dicrotic  crest  of  the  pulse  wave; 
and  the  curve  of  difference  D  falls  with  very  great  abruptness. 

The  interpretation  of  this  seems  to  be  as  follows.  During 
the  whole  of  the  systolic  plateau  up  to  the  mark  (3)  the  semi- 
lunar valves  are  open,  the  cavity  of  the  ventricle  and  the  root 
of  the  aorta  form  a  common  cavity  which  is  occupied  by  a 
continuous  column  of  blood.  Hence  the  curves  of  ventricular 
and  aortic  pressure,  of  the  pressure  at  the  one  end  and  at  the 
other  end  of  this  column,  follow  the  same  general  course,  and, 
indeed,  shew  the  same  secondary  variations ;  this  general  course 
is,  in  the  case  which  we  are  studying,  a  descending  one  by 
reason,  as  we  have  said,  of  the  relatively  free  escape  of  blood  from 
the  arterial  system  through  the  peripheral  resistance.  But  the 
column  of  blood  in  question  is  a  column  in  motion,  the  ventricular 
pressure  is  driving  the  blood  from  the  ventricle  into  the  aorta ;  to 
effect  this  the  pressure  in  the  ventricle  must  continue  to  be  higher 
than  that  which  it  is  itself  generfiting  in  the  aorta,  the  curve  of 
difference  must  remain  above  the  base  line.  And,  since  the  curve 
of  difference  does  remain  above  the  base  line  right  up  to  the  mark 
(3),  we  may  infer  that  up  to  this  point  blood  dties  pass  from  the 
ventricle  into  the  aorta.  At  (3),  however,  there  is  a  sudden  change. 
The  systole  suddenly  ceases,  and  with  that  the  curve  of  difference 
suddenly  sinks  below  the  base  line ;  the  flow  from  ventricle  ceases 
not  because  there  is  no  more  blood  to  come,  but  because  the  pressure 
in  the  ventricle  now  becomes  lower  than  that  in  the  aorta ;  and, 
indeed,  the  blood  would  How  back  from  the  aorta  to  the  region  of 
lower  pressure,  to  the  ventricle,  were  it  not  that  the  very  first  effect 


208 


THE   MECHANISM  OF  THE   BEAT.         [Book  i. 


of  the  reflux  is  to  close  the  semilunar  valves.  So  soon  as  these 
are  closed,  the  pressures  in  the  ventricle  and  the  aorta,  which  were 
previously  following  similar  courses,  now  take  separate  courses ;  the 
latter  falls  suddenly,  the  former  decreases  gradually,  and  continues 
to  decrease  until  the  next  systole  once  more  opens  the  semilunar 
valves.  We  may  add  that  this  view  is  consistent  with  the  conclu- 
sion mentioned  in  §  114,  that  not  only  the  pulse-volume  may  vary, 
but  also,  at  times  at  least,  the  whole  contents  are  not  driven  out 
at  the  systole,  some  blood  remaining  behind. 

Moreover,  the  pressure  does  not  always  gradually  decline 
during  the  systolic  plateau ;  sometimes  it  gradually  rises  during 
the  whole  of  the  period  of  the  plateau,  reaching  its  highest  point 
just  before  the  final  sudden  fall.     This  is  shewn  in  Fig.  53. 


Fig.  53.    Curve  of  Aortic  and  Ventricular  Pressure,  with  an 

ASCENDING  SySTOLIC  PlATEAU.       HuRTHLE. 

In  this  figure  the  general  features  are  the  same  as  in  Fig.  52, 
save  that  the  curve  of  ventricular  pressure  rises  during  the  whole 
of  the  systolic  plateau.  But  the  curve  of  aortic  pressure  also  rises 
in  a  corresponding  manner,  and  the  curve  of  difference,  if  shewn, 
would  be  the  same  as  in  Fig.  52.  The  explanation  of  the  difference 
between  the  two  cases  is  that  in  Fig.  52  the  peripheral  resistance 
in  the  arterial  flow  (§  99)  is  not  very  great,  and  the  ventricular 
systole  soon  overcomes  it  to  such  an  extent  as  to  lead  at  once  to 
some  fall  of  pressure  in  the  aorta  (and  in  the  ventricle).  In  Fig. 
53  the  peripheral  resistance  is  very  great ;  it  is  not  overcome  at 
first,  the  ventricle  does  its  best  working  against  it,  and  produces 
the  most  effect,  raising  the  pressure  to  the  highest  point,  just 
before  its  systole  *comes  to  an  end.  We  may  add  that  a  similar 
course  of  the  curve  may  be  seen  even  when  the  pressure  in  the 
aorta  is  not  very  high,  provided  that  the  pulse-volume,  the  quantity 
discharged  at  the  systole  is  very  great;  the  form  of  the  curve 
depends  on  the  relative  amounts  which  are  entering  the  arterial 
system  from  the  heart,  and  leaving  it  by  the  peripheral  vessels. 

It  is  possible  that  under  some  circumstances  the  whole  of  the 


Chap,  iv.]  THE  VASCULxVR   MECHANISM. 


209 


contents  may  be  discharged  before  the  actual  systole  ends ;  but 
the  observations  and  arguments  which  we  have  just  related, 
shew  that  such  an  event  must  be  regarded  as  of  exceptional,  and 
not,  as  has  been  contended,  of  normal  occurrence. 

Of  the  smaller  secondary  variations  visible  on  the  systolic 
plateau,  conspicuous  in  some  curves  (4,  5,  6,  7  in  Fig.  46),  various 
explanations  have  been  given.  Into  the  discussion  of  these  we 
cannot  enter  here  ;  we  may  however  say  that  in  many  observations, 
which  we  may  probably  regard  as  correct,  these  secondary  markings 
are  identical  in  the  curves  of  ventricular  pressure,  of  aortic  pressure 
and  of  the  cardiac  impulse,  or  of  the  cliange  in  the  outward  form 
of  the  heart ;  the  events  which  cause  them  tell  in  the  same  way 
on  all  three. 


8ystole 


Diastole 


Fig. 


54.    DiAORAM  OF  Ventricular  and  Aortic  Pressure  and  of  the 
Cardiac  Impulse.     Hurtiile. 


We  give  in  Fig.  54  a  diagram  of  the  cardiac  events  according 

to  the  exposition  which  we  have  just  made.     The  curves  previously 

given  were  copies  of  actual  curves  obtained  by  experiment;  this 

is  a  constructed  diagram.     The  upper  curve  is  the  curve  of  the 

cardiac  impulse.     The  middle  curve  is  the  curve  of  pressure  in  the 

14 


210 


NEGATIVE  PEESSURE. 


[Book  i. 


left  ventricle  ;  the  unbroken  line  represents  the  course  of  the  curve 
when,  the  peripheral  resistance  being  small,  the  pressure  needed 
to  drive  onward  the  blood  is  not  very  high,  in  the  figure  less  than 
150  mm.  Hg.  The  dotted  line  represents  the  course  of  the  curve 
when,  the  peripheral  resistance  being  great,  the  pressure  is  high, 
in  the  figure  nearly  200  mm.  Hg.  The  lower  curve  is  the  curve  of 
pressure  at  the  root  of  the  aorta,  the  unbroken  and  the  dotted 
lines  having  the  same  significance  as  in  the  ventricular  curve. 
The  line  0  marks  the  commencement  of  the  ventricular  systole, 
the  line  1  the  opening  of  the  semilunar  valves,  and  3  the  end 
of  the  systole.  The  line  4  marks  the  beginning  of  what  in  dealing 
with  the  pulse,  we  shall  speak  of  as  the  dicrotic  wave.  The  semi- 
lunar valves  are  closed  between  3  and  4 ;  the  closure  is  the  result 
at  3  of  the  cessation  of  the  systole  and  as  we  shall  see  the  cause 
at  4  of  the  dicrotic  wave  of  the  pulse.  The  time  is  given  in  tenths 
of  a  second. 

§  117.  In  many  curves,  as  in  some  of  those  given  above,  the 
pressure  in  the  ventricle  at  the  beginning  of  diastole  falls  not  only 
to  the  base  line,  which  is  the  line  of  atmospheric  pressure,  but  even 
below  it ;  that  is  to  say,  becomes  negative.  Such  a  negative  pressure 
may  be  shewn  by  means  of  a  minimum  manometer,  that  is,  a  mano- 
meter arranged  so  as  to  shew  the  lowest  pressure  which  has  been 
reached  in  a  series  of  events.     The  mercury  manometer,  which  as  we 


T"iG.  55.    The  Maximum  Manometer  of  Goltz  and  Gaule. 

At  e  a  connection  is  made  with  the  tube  leading  to  the  heart.  When  the  screw 
clamp  k  is  closed,  the  valve  v  comes  into  action,  and  the  instrument,  in  the  position 
of  the  valve  shewn  in  the  figure,  is  a  maximum  manometer.  By  reversing  the 
direction  of  ?■  it  is  converted  into  a  minimum  manometer.  When  ^  is  opened,  the 
variations  of  pressure  are  conveyed  along  a,  and  the  instrument  then  acts  like  an 
ordinary  manometer. 


Chap,  iv.]  THE   VASCULAE   MECHANISM.  211 

have  said,  is  unsuitable  for  following  the  rapid  changes  constituting  a 
single  beat,  may  be  used  as  a  maximum  or  minimum  instrument 
for  determining  the  highest  or  lowest  pi'essure  reached  in  one  or 
other  of  the  heart's  cavities  during  a  series  of  beats. 

The  principle  of  one  form  of  maximum  manometer,  Fig.  55,  consists 
in  the  introduction  into  the  tube  leading  from  the  heart  to  the  mercury 
column,  of  a  (modified  cup-and-ball)  valve,  opening,  like  the  aortic 
semilunar  valves,  easily  from  the  heart,  but  closing  firmly  when  fluid 
attempts  to  return  to  the  lieart.  The  higliest  pressure  is  that  whicli 
drives  the  longest  column  of  fluid  past  the  valve,  raising  tlie  naercury 
column  to  a  corresponding  height.  Since  this  column,  once  past  the 
valve,  cannot  return,  tlie  mercury  remains  at  the  height  to  which  it  was 
raised  by  it,  and  thus  records  the  maximum  pressure.  By  reversing 
the  direction  of  the  valve,  the  manometer  is  converted  from  a  maximum 
into  a  minimum  instrument. 

A  simpler  form  of  maximum  and  minimum  manometer  is  that  of 
Hilrthle,  which  consists  of  a  small  chamber  connected  with  two  mano- 
meters, the  opening  of  eacli  manometer  into  the  chamber  being  armed 
with  a  valve  of  tliin  membrane,  so  arranged  that  it  permits  in  the  case 
of  one  manometer,  the  maximum  one,  the  entrance  only  of  the  mercury, 
in  the  case  of  tlie  other,  the  minimum  one,  tlie  exit  only. 

By  means  of  the  maximum  manometer  the  pressure  in  the 
left  ventricle  in  the  dog  has  been  observed  to  reach  a  maximum 
of  about  140  mm.  (mercury),  in  the  right  ventricle  of  about 
60  mm.  and  in  the  right  auricle  of  about  20  mm.  These  figures, 
however,  are  given  as  examples,  and  not  as  averages.  Simi- 
larly negative  pressures  of  from  —  50  mm.  to  —  20  in  the  left 
ventricle  of  the  dog,  of  about  —  15  mm.  in  the  right  ventricle,  and 
of  from  —12  mm.  to  —  7  mm.  in  the  right  auricle,  have  been 
observed  by  the  minimum  manometer.  Part  of  this  diminution  of 
pressure  in  the  cardiac  cavities  is  due,  as  will  be  explained  in  a 
later  part  of  this  work,  to  the  aspiration  of  the  thorax  in  the 
respiratory  movements.  But  even  when  the  thorax  is  opened,  and 
artificial  respiration  kept  up,  under  which  circumstances  no  such 
aspiration  takes  place,  a  negative  pressure  may  be  still  observed, 
the  pressure  in  the  left  ventricle  sinking  according  to  some  obser- 
vations as  low  as  —  24  mm.  Now,  what  the  instrument  actually 
shews  is  that  at  some  time  or  other  during  the  number  of  beats 
which  took  place  while  the  instrument  was  applied  (and  these  may 
have  been  very  few),  the  pressure  in  the  ventricle  sank  so  many 
mm.  below  that  of  the  atmosphere.  Since,  however,  the  negative 
pressure  may  be  observed  when  the  heart  is  beating  quite  regularly, 
each  beat  being  exactly  like  the  others,  we  may  infer  that  the  negative 
pressure  is  repeated  at  some  period  or  other  of  each  cardiac  cycle. 
The  instrument  itself  gives  us  no  information  as  to  the  exact  phase 
of  the  beat  in  which  the  negative  pressure  occurs ;  but  it  is  clear 
from  what  we  have  already  seen  that  when  it  occurs,  it  must 
take  place  at  the  end  of  the  systole,  at  the  beginning   of   the 


212  DUKATIOK   OF   CAEDIAC   PHASES.         [Book  i. 

diastole.  It  is  obvious,  moreover,  from  what  has  gone  before,  that 
the  semilunar  valves  are  closed  before  it  occurs,  and  we  may- 
dismiss  the  view  which  has  been  put  forward  that  it  is  of  the  same 
nature  as  the  negative  pressure  which  makes  its  appearance  behind 
a  column  of  fluid  moving  rapidly  and  suddenly  ceasing,  as  when  a 
rapid  flow  of  water  through  a  tube  is  suddenly  stopped  by  turning 
a  tap.  We  may  probably  attribute  it  to  the  relaxation  of  the 
ventricular  walls.  This,  as  all  the  curves  shew,  is  a  rapid  process, 
something  quite  distinct  from  the  mere  filling  of  the  ventricular 
cavities  with  blood  by  the  venous  inflow;  and,  though  some 
have  objected  to  the  view,  it  may  be  urged  that  this  return 
of  the  ventricle  from  its  contracted  condition  to  its  normal  form 
would  develop  a  negative  pressure.  This  return  we  may  probably 
regard  as  simply  the  total  result  of  the  return  of  each  fibre  to 
its  natural  condition,  though  some  have  urged  that  the  extra 
quantity  of  blood  thrown  into  the  coronary  arteries  at  the  systole 
helps  to  unfold  the  ventricles  somewhat  in  the  way  that  fluid 
driven  between  the  two  walls  of  a  double-walled  collapsed  ball  or 
cup  will  unfold  it. 

We  may  further  conclude  that  such  a  negative  pressure,  when 
it  occurs,  will  assist  the  circulation  (and  it  may  be  remarked  that 
the  return  of  the  thick- walled  left  ventricle  naturally  exerts  a 
greater  negative  pressure  than  the  thin-walled  right  ventricle)  by 
sucking  the  blood  which  has  meanwhile  been  accumulated  in  the 
auricle  from  that  cavity  into  the  ventricle,  the  auriculo-ventricular 
valves  easily  giving  way.  At  the  same  time  this  very  flow  from 
the  auricle  will  at  once  put  an  end  to  the  negative  pressure,  which 
obviously  can  be  of  brief  duration  only. 

It  should,  however,  be  added  that  many  observers  find  the 
development  of  a  negative  pressure  to  be  by  no  means  of  such 
constant  occurrence,  and  not  to  reach  such  marked  limits  as  might 
be  inferred  from  the  numbers  given  above,  at  least  in  the  unopened 
chest.  If  so  it  cannot  be  an  important  factor  in  the  work  of  the 
circulation. 

§  118.  The  duration  of  the  several  'phases.  We  may  first  of  all 
distinguish  certain  main  phases  :  (1)  The  systole  of  the  auricles. 
(2)  The  systole,  proper,  of  the  ventricles,  during  which  their  fibres 
are  in  a  state  of  contraction.  (3)  The  diastole  of  the  ventricles, 
that  is  to  say,  the  time  intervening  between  their  fibres  ceasing  to 
contract,  and  commencing  to  contract  again.  To  these  we  may 
add;  (4)  The  pause  or  rest  of  the  whole  heart,  comprising  the 
period  from  the  end  of  the  relaxation  of  the  ventricles  to  the 
beginning  of  the  systole  of  the  auricles  ;  during  this  time  the  walls 
are  undergoing  no  active  changes,  neither  contracting  nor  relaxing, 
their  cavities  being  simply  passively  filled  by  the  influx  of  blood. 

The  mere  inspection  of  almost  any  series  of  cardiac  curves 
however  taken,  those,  for  instance,  which  we  have  just  discussed, 
will  shew,  apart  from  any  accurate  measurements,  that  the  systole 


Chap,  iv.]  THE  VASCULAR  MECHANISM.  213 

of  the  auricles  is  always  very  brief,  that  the  systole  of  the  ven- 
tricles is  always  very  prolonged,  always  occupying  a  consider- 
able portion  of  the  whole  cycle,  and  that  the  diastole  of  the 
whole  heart,  reckoned  from  the  end  either  of  the  systole,  or 
of  the  relaxation  of  the  ventricle,  is  very  various,  being  in  quickly 
beating  hearts  very  short  and  in  slowly  beating  hearts  decidedly 
longer. 

When  we  desire  to  arrive  at  more  complete  measurements, 
we  are  obliged  to  make  use  of  calculations  based  on  various  data  ; 
and  the  value  of  some  of  these  has  been  debated.  Naturally  the 
most  interest  is  attached  to  the  duration  of  events  in  the  human 
heart. 

A  datum  which  has  b3en  very  largely  used  is  the  interval 
between  the  beginning  of  the  first  and  the  occurrence  of 
the  second  sound.  This  may  be  determined  with  approximative 
correctness,  and  is  found  to  vary  from  '301  to  -327  sec,  occupying 
from  40  to  46  p.c.  of  the  whole  period,  and  being  fairly  constant 
for  different  rates  of  heart  beat.  That  is  to  say  in  a  rapidly  beating 
heart  it  is  the  pauses  which  are  shortened  and  not  the  duration 
of  the  actual  beats. 

The  observer,  listening  to  the  sounds  of  the  heart,  makes  a  signal  at 
each  event  on  a  recording  surface,  the  difference  in  time  between  the 
marks  being  measured  by  means  of  the  vibrations  of  a  tuning-fork 
recorded  on  the  same  surface.  By  practice  it  is  found  possible  to 
reduce  the  errors  of  observation  within  very  small  limits. 

Now  whatever  be  the  exact  causation  of  the  first  sound, 
it  is  undoubtidly  coincident  with  the  systole  of  the  ventricles, 
though  possibly  the  actual  commencement  of  its  becoming  audible 
may  be.  slightly  behind  the  actual  beginning  of  the  muscular  con- 
tractions. Similarly  the  occurrence  of  the  second  sound,  which, 
as  we  have  seen,  is  certainly  due  to  the  closure  of  the  semilunar 
valves,  may  in  accordance  with  the  view  expounded  a  little 
while  back,  be  taken  to  mark  the  close  of  the  ventricular  systole. 
And  on  this  view  the  interval  between  the  beginning  of  the 
first  and  the  occurrence  of  the  second  sound  may  be  regarded 
as  indicating  approximatively  the  duration  of  the  ventricular 
systole,  i.e.  the  period  during  which  the  ventricular  fibres  are 
contracting. 

By  an  ingenious  arrangement  a  microphone  attached  to  a 
stethoscope  may  be  made  to  record  the  heart  sounds  through  the 
stimulation  of  a  muscle-nerve  preparation  ;  and  the  record  so 
obtained  may  be  compared  with  the  various  cardiac  curves.  "When 
this  is  done,  the  first  sound  is  found  to  begin  somewhere  on  the 
systolic  ascent  of  the  ventricular  curve,  the  exact  point  varying, 
and  the  second  sound  to  occur  just  as  the  ventricular  curve  begins 
its  diastolic  descent. 

There  has  been  liowever  as  we  stated  above  sireat  diversence  of 


214  DURATION  OF   CAEDIAC  PHASES.         [Book  i. 

opinion  and  much  discussion  as  to  the  exact  time  of  the  closure  of 
the  semilunar  valves ;  the  view  given  in  the  text  above,  though  it 
seems  to  be  supported  by  adequate  arguments,  is  not  the  only  one 
which  is  held. 

Accepting  the  view  given  in  the  text,  we  may  make  the 
following  statement.  In  a  heart  beating  72  times  a  minute, 
which  may  be  taken  as  the  normal  rate,  each  entire  cardiac  cycle 
would  last  about  0-8  sec,  and  taking  0"3  sec.  as  the  duration  of 
the  ventricular  systole,  the  deduction  of  this  would  leave  0'5  sec. 
for  the  whole  diastole  of  the  ventricle  including  its  relaxation,  the 
latter  occupying  less  than  -1  sec.  At  the  end  of  the  diastole  of 
the  ventricle  there  occurs  the  systole  of  the  auricle,  the  exact 
duration  of  which  it  is  difficult  to  determine,  it  being  hard  to  say 
when  it  really  begins,  but  which,  if  the  contraction  of  the  great 
veins  be  included,  may  perhaps  be  taken  as  lasting  on  an  average 
O'l  sec.  The  'passive  interval,'  therefore,  during  which  neither 
auricle  nor  ventricle  is  undergoing  contraction,  lasts  about  4  sec, 
and  the  absolute  pause  or  rest,  during  which  neither  auricle  nor 
ventricle  is  contracting  or  relaxing,  about  -3  sec.  The  systole 
of  the  ventricle  follows  so  immediately  upon  that  of  the  auricle, 
that  practically  no  interval  exists  between  the  two  events.  In 
the  systole  of  the  ventricle  we  may  distinguish  the  phase  during 
which  pressure  is  being  got  up  before  the  semilunar  valves  are 
opened ;  this  is  exceedingly  short,  probably  from  -02  to  -03  sec. 
During  the  rest  of  the  -3  sec.  of  the  systole,  the  contents  of  the 
ventricle  are  being  pressed  into  the  aorta. 

The  duration  of  the  several  phases  may  for  convenience  sake 
be  arranged  in  a  tabular  form  as  follows : 

sees.        sees. 

Systole  of  ventricle  before  the  open- 
ing of  the  semilunar  valves,  while  "^ 
pressure  is  still  getting  up                    '03 

Continued  contraction  of  the  ventricle,         y 
and  I 

Escape  of  blood  into  aorta  '27  J 

Total  systole  of  the  ventricle  '3 

Diastole  of  both  auricle  and  ventricle, 
neither  contracting,  or  "  passive  in- 
terval " 

Systole  of  auricle  (about  or  less  than) 

Diastole  of  ventricle,  including  relaxa- 
tion and  filling,  up  to  the  beginning 
of  the  ventricular  systole  '5 

Total  Cardiac  Cycle  '8 


Chap.  iv.J  THE  VASCULAR   MECHAXISM.  215 


Summary. 

§  119.  We  may  now  briefly  recapitulate  the  main  facts  con- 
nected with  the  passage  of  blood  through  the  heart.  The  right 
auricle  during  its  diastole,  by  the  relaxation  of  its  muscular  fibres, 
and  by  the  fact  that  all  backward  pressure  from  the  ventricle  is 
prevented  by  the  closing  of  the  tricuspid  valves,  offers  but  little 
resistance  to  the  ingress  of  blood  from  the  veins.  On  the  other 
hand,  the  blood  in  the  trunks  of  both  the  superior  and  inferior 
vena  cava  is  under  a  pressure,  which,  though  diminishing  towards 
the  heart,  remains  higher  than  the  pressure  obtaining  in  the 
interior  of  the  auricle ;  the  blood  in  consequence  flows  into  the 
empty  auricle,  its  progress  in  the  case  of  the  superior  vena  cava 
being  assisted  by  gravity.  At  each  inspiration  this  flow  (as  we 
shall  see  in  speaking  of  respiration)  is  favoured  by  the  diminution 
of  pressure  in  the  heart  and  great  vessels  caused  by  the  respiratory 
movements.  Before  this  flow  has  gone  on  very  long,  the  diastole 
of  the  ventricle  begins,  its  cavity  dilates,  the  flaps  of  the  tricuspid 
valve  fall  back,  and  blood  for  some  little  time  flows  in  an  un- 
broken stream  from  the  vense  cavse  into  the  ventricle.  How  far 
the  entrance  of  blood  from  the  auricle  into  the  ventricle  is,  under 
ordinary  circumstances,  aided  by  the  negative  pressure  in  the 
ventricle  following  the  close  of  the  systole,  must,  as  we  have  said, 
be  left  for  the  present  uncertain.  In  a  short  time,  probably  before 
very  much  blood  has  had  time  to  enter  the  ventricle,  the  auricle  is 
full  ;  and  forthwith  its  sharp,  sudden  systole  takes  place.  Partly 
by  reason  of  the  backward  pressiire  in  the  veins,  which  increases 
rapidly  from  the  heart  towards  the  capillaries,  and  which  at  some 
distance  from  the  heart  is  assisted  by  the  presence  of  valves  in  the 
venous  trunks,  but  still  more  from  the  fact  that  the  systole  begins 
at  the  great  veins  themselves,  and  spreads  thence  over  the  auricle, 
the  force  of  the  auricular  contraction  is  spent  in  driving  the  blood, 
not  back  into  the  veins,  but  into  the  ventricle,  where  the  pressure 
is  still  exceedingly  low.  Whether  there  is  any  backward  flow  at 
all  into  the  great  veins,  or  whether  by  the  progressive  character  of 
the  systole,  the  flow  of  blood  continues,  so  to  speak,  to  follow  up 
the  systole  without  break,  so  that  the  stream  from  the  veins  into 
the  auricle  is  really  continuous,  is  at  present  doubtful ;  though  a 
slight  positive  wave  of  pressure  synchronous  with  the  auricular 
systole,  travelling  backward  along  the  great  veins,  has  been 
observed  at  least  in  cases  where  the  heart  is  beating  vigorously. 

The  ventricle  thus  being  filled  by  the  auricular  systole,  the 
play  of  the  tricuspid  valves  described  above  comes  into  action, 
the  auricular  systole  is  followed  by  that  of  the  ventricle,  and  the 
pressure  within  the  ventricle,  cut  off  from  the  auricle  by  the 
tricuspid  valves,  is  brought  to  bear  on  the  pulmonary  semilunar 
valves,  and  the  column  of  blood  on  the  other  side  of  those  valves. 


216  SUMMAEY   OF   HEART   BEAT.  [Book  i. 

As  soon  as  by  the  rapidly  increasing  shortening  of  the  ventricular 
fibres  the  pressure  within  the  ventricle  becomes  greater  than 
that  in  the  pulmonary  artery,  the  semilunar  valves  open,  and  the 
still  continuing  systole  discharges  the  contents  of  the  ventricle 
into  that  vessel. 

During  the  whole  of  this  time  the  left  side  has  with  still 
greater  energy  been  executing  the  same  manoeuvre.  At  the  same 
time  that  the  vense  cavae  are  filling  the  right  auricle,  the  pulmonary 
veins  are  tilling  the  left  auricle.  At  the  same  time  that  the  right 
auricle  is  contracting,  the  left  auricle  is  contracting  too.  The 
systole  of  the  left  ventricle  is  synchronous  with  that  of  the  right 
ventricle,  but  executed  with  greater  force ;  and  the  flow  of  blood 
is  guided  on  the  left  side  by  the  mitral  and  aortic  valves  in  the 
same  way  that  it  is  on  the  right  by  the  tricuspid  valves  and  the 
valves  of  the  pulmonary  artery. 

As  the  ventricles  become  filled  with  blood,  and  so  increased 
in  volume,  the  apex  begins  to  press  steadily  on  the  chest-wall, 
as  may  be  often  seen  in  the  cardiogram,  the  curve  of  the 
cardiac  impulse.  The  fuller  distension  due  to  the  auricular 
systole  is  more  obvious  in  the  same  curve ;  but  both  these 
changes  are  insignificant  compared  to  the  effect  of  the  change  of 
form,  and  of  the  position  of  the  apex  during  the  ventricular 
systole,  by  which  the  lever  of  the  cardiograph  is  rapidly  and 
forcibly  moved. 

With  this  systole  of  the  ventricles  the  first  sound  is  heard. 

We  may  more  conveniently  follow  the  remaining  events  in  the 
left  ventricle. 

The  effect  of  the  discharge  of  the  contents  of  the  left  ventricle 
is  to  raise  the  pressure  at  the  root  of  the  aorta  to  nearly  the  same 
height  as  that  in  the  ventricle  itself.  The  ventricular  pressure 
continues  for  some  time,  giving  rise  to  the  "  systolic  plateau  "  of 
the  various  cardiac  curves.  In  some  cases  this  pressure  soon 
reaches  a  maximum,  after  which  it  gradually  declines,  the  curve  of 
pressure  sloping,  with  some  secondary  undulations,  gently  down- 
wards. In  other  cases  where  there  is  great  resistance  to  the 
outflow  along  the  arterial  system,  the  pressure  may  continue  to 
rise  during  the  whole  of  the  ventricular  systole.  In  both  cases 
the  curves  of  the  ventricular  pressure  and  of  the  aortic  pressure 
are  similar. 

Then  comes  the  sudden  cessation  of  contraction,  the  sudden 
relaxation  of  the  ventricular  fibres.  The  pressure  in  the  ventricle 
becomes  less  than  that  which  it  itself  has  generated  in  the  aorta, 
and  the  semilunar  valves  suddenly  close  as  the  blood  fiows  back 
from  the  region  of  high  pressure,  the  aorta,  towards  the  region  of 
low  pressure,  the  ventricle.  At  this  moment  the  second  sound  is 
heard. 

Owing  to  the  semilunar  valves  being  closed,  the  pressures  in 
the  ventricle  and  in  the  aorta,  which  before  were  following  the 


Chap.  iv.J  THE  VASCULAR   MECHANISM.  217 

same  course,  now  become  different.  While  the  pressure  sinks 
rapidly  in  the  ventricle,  falling  it  may  be  below  that  of  the  atmos- 
sphere,  and  thus  becoming  a  negative  pressure,  which  in  some  cases 
may  possibly  be  considerable,  that  in  the  aorta  does  not  sink  to 
a  corresponding  degree ;  in  fact,  as  we  shall  see,  it  is  reinforced  to 
a  certain  extent  in  a  secondary  rise,  the  so-called  dicrotic  rise. 

We  have  reason  to  believe  not  only  that  the  quantity  of  blood 
ejected  at  the  systole  may  vary  from  time  to  time,  but  also  that 
at  times  at  all  events  if  not  normally,  the  whole  of  the  blood 
present  in  the  ventricle  at  the  systole  may  fail  to  leave  the 
ventricle  during  the  systole,  more  or  less  remaining  behind  at  the 
close  ;  the  ventricle  in  such  cases  does  not  completely  empty  itself. 
On  the  other  hand,  we  may  perhaps  admit  that,  at  least  under  cer- 
tain circumstances,  when,  for  instance,  the  contents  of  the  ventricle 
are  small,  and  the  ventricle  vigorous  or  the  systole  prolonged,  the 
whole  of  the  contents  may  be  discharged  in  the  earlier  part  of  the 
systole,  the  ventricle  remaining  contracted  for  some  little  time  after 
it  has  emptied  itself. 

The  Worh  done. 

§  120.  We  have  already  (§  114)  spoken  of  that  most  important 
factor  in  the  determination  of  the  work  of  the  heart,  the  pulse- 
volume,  or  the  quantity  ejected-  from  the  ventricle  into  the  aorta 
at  each  systole,  and  of  the  various  methods  by  which  it  may  be 
estimated.  We  have  seen  that  it  probably  varies  -svithin  very 
considerable  limits. 

We  may  here  repeat  the  remark  that  exactly  the  same  quantity 
must  issue  at  a  beat  from  each  ventricle  ;  for  if  the  right  ventricle 
at  each  beat  gave  out  rather  less  than  the  left,  after  a  certain 
number  of  beats  the  whole  of  the  blood  would  be  gathered  in  the 
systemic  circulation.  Similarly,  if  the  left  ventricle  gave  out  less 
than  the  right,  all  the  blood  would  soon  be  crowded  into  the 
lungs.  The  fact  that  the  pressure  in  the  right  ventricle  is  so 
much  less  than  tliat  in  the  left  (probably  30  or  40  mm.  as 
compared  with  200  mm.  of  mercury),  is  due,  not  to  differences  in 
the  quantity  of  blood  in  the  cavities,  but  to  the  fact  that  the 
peripheral  resistance  which  has  to  be  overcome  in  the  lungs  is  so 
much  less  than  that  in  the  rest  of  the  body. 

Not  only  does  the  amount  ejected  vary,  but  the  pressure  under 
which  it  is  ejected  also  varies  within  very  considerable  limits. 
Moreover,  the  number  of  times  tlie  systole  is  nqieated  within  a 
given  period  may  also  vary  considerably.  The  work  done,  therefore, 
varies  very  much.  But  it  may  be  interesting  and  instructive  to 
note  the  results  of  calculating  out  a  very  high  estimate.  Thus 
if  we  take  180  grms.  as  tlie  quantity,  in  man,  ejected 
at  each  stroke  at  a  pressure   of  250  mm.  of  mercury,  which  is 


218  THE   WOEK  DONE.  [Book  i. 

equivalent  to  3-21  meters  of  blood,  this  means  that  the  left 
ventricle  is  capable  at  its  systole  of  lifting  180  grms.  3-21  m.  high, 
i.  e.  it  does  578  gram-meters  of  work  at  each  beat.  Supposing  the 
heart  to  beat  72  times  a  minute,  this  would  give  for  the  day's 
work  of  the  left  ventricle  nearly  60^000  kilogram-meters.  Calcu- 
lating the  work  of  the  right  ventricle  at  one-fourth  that  of  the 
left,  the  work  of  the  whole  heart  during  the  day  would  amount  to 
75,000  kilogram-meters,  which  is  just  about  the  amount  of  work 
done  in  the  ascent  of  Snowdon  by  a  tolerably  heavy  man. 


SEC.   4.    THE   PULSE. 


§  121.  We  have  seen  that  the  arteries,  though  always  dis- 
tended, undergo,  each  time  that  the  systole  of  the  ventricle  drives 
the  contents  of  the  ventricle  into  the  aorta,  a  temporary  additional 
expansion  so  that  when  the  finger  is  placed  on  an  artery,  such 
as  the  radial,  an  intermittent  pressure  on  the  finger,  coming  and 
going  with  the  beat  of  the  heart,  is  felt,  and  when  a  light  lever 
is  placed  on  the  artery,  the  lever  is  raised  at  each  beat,  falling 
between. 

This  intermittent  expansion,  which  we  call  the  pulse,  cor- 
responding to  the  jerking  outflow  of  blood  from  a  severed  artery, 
is  present  in  the  arteries  only,  being,  except  under  particular 
circumstances,  absent  from  the  veins  and  capillaries.  The  expan- 
sion is  frequently  visible  to  the  eye,  and  in  some  cases,  as  where 
an  artery  has  a  bend,  may  cause  a  certain  amount  of  locomotion 
of  the  vessel. 

We  may,  by  applying  various  instruments  to  the  interior  of  an 
artery,  study  the  temporary  increase  of  pressure. which  is  the  cause 
of  the  temporary  increase  of  expansion.  This  makes  itself  felt,  as 
we  have  seen,  in  the  curve  of  arterial  pressure  taken  by  the  mercury 
manometer ;  but  the  inertia  of  the  mercury  prevents  the  special 
characters  of  each  increase  becoming  visible.  In  order  to  obtain 
an  adequate  record  of  these  special  characters  we  must  have 
recourse  to  other  instruments. 

The  membrane-manometer,  of  which  we  have  already  spoken  (§  113), 
and  on  the  results  gained  by  wliich  when  applied  to  the  root  of  the 
aorta  by  means  of  a  catheter  we  have  dwelt  (§  116),  may  also  be  applied 
to  other  arteries,  the  tube  leading  to  the  tambour  of  the  manometer  being 
connected  with  the  artery  by  means  of  a  cannula  in  the  ordinary  way. 

In  Fick's  spring-manometer,  in  its  original  form,  Fig.  50,  the  artery 
is  connected  by  means  of  a  cannula  and  a  rigid  tube  containing  fluid 
with  the  interior  of  a  curved  spring  ;  an  increase  of  ])ressure  unfc>lds 
the  curve  of  the  spring,  the  movements  of  the  end  of  wliich  may  be 
recorded  by  means  of  a  lever.  In  Fick's  improved  form  the  membrane 
of  a  small  air-tambour  works  against  a  horizontal  slip  of  steel  -which 
acts  as  a   spring ;   this  instrument,  like   Frey  and    Krehl's  manometer 


220 


METHODS   OF   EECORDING   PULSE.        [Book  i. 


which  is  only  a  modification  of  it  (see  §  113),  can  he  applied  to  an  artery 
by  a  cannula  in  the  ordinary  way. 

The  "  sphygmoscope  "  consists  of  a  small  elastic  hag,  the  end  of  an 
India  ruhher  tinger,  for  instance,  fitted  on  to  a  conical  cork,  through 
which  passes  a  tube  opening  into  the  bag,  and  connected  by  a  cannula 
with  the  arterj'^ ;  both  bag  and  tube  are,  before  being  connected  with 
the  artery,  filled  with  fluid  of  a  nature  to  hinder  clotting.  The  bag,  by 
means  of  the  conical  cork,  is  firmly  fitted  into  the  end  of  a  small  glass 
tube,  the  cavity  of  which  filled  with  air  is  connected  with  a  recording  air 
tambour.  The  changes  of  pressure  within  the  artery  are  transmitted  to 
the  elastic  bag,  and  through  this  to  the  air  of  the  glass  tube  and  so  to  the 
recording  tambour. 

The  tambour-sphygmoscope  of  Hurthle  is  a  combination  of  the 
membrane-maiiometer  with  a  tambour.  The  membrane  of  the  manometer 
works  not  directly  on  a  lever,  but  on  a  recording  air  tambour,  the  move- 
ments of  which  are  recorded  in  the  usual  way. 

In  the  sphygmotonometer  of  Eoy,  the  artery  is,  by  means  of  a 
cannula,  and  rigid  tube  filled  with  fluid,  connected  with  a  cylinder  in 
which  a  light  piston  works  by  means  of  a  delicate  membrane. 


Fig.  56.   Fick's  Spring  Manometer. 

The  flattened  tube  in  the  form  of  a  hoop  is  firmly  fixed  at  one  end,  while  the 
other  free  end  is  attached  to  a  lever.  The  interior  of  the  tube,  filled  with  spirit,  is 
brought,  by  means  of  a  tube  containing  sodium  carbonate  solution,  into  connection 
with  an' artery,  in  much  the  same  way  as  in  the  case  of  the  mercury  manometer. 
The  increase  of  pressure  in  the  artery  being  transmitted  to  the  hollow  hoop,  tends 
to  straighten  it,  and  correspondingly  moves  the  attached  lever. 


Chap,  iv.]  THE   VASCULAR   MECHANISM. 


221 


And  there  are  still  other  instruments  which  may  be  used  in  a 
similar  way. 

It  is  not  necessary,  however,  to  open  the  artery ;  we  may  study 
indirectly  the  changes  of  pressure  by  recording  the  expansions  and 
retractions  of  the  artery,  the  changes  in  its  diameter,  which  are 
produced  by  the  changes  of  pressure. 

The  most  common  method  of  registering  the  expansion  of  an  artery 
and  at  the  same  time  one  of  tlie  simplest,  is  that  of  bringing  a  light  lever 
to  bear  on  the  outside  of  the  artery. 

A  lever  specially  adapted  to  record  a  pulse  tracing  is  called  a 
sphygmograph,  the  instrument  generally  comprising  a  small  travelling 
recording  surface  on  which  the  lever  writes.  There  are  many  different 
forms  of  sphygmograph,  but  the  general  plan  of  structure  is  the  same. 
Fig.  57  represents  in  a  diagrammatic  form  the  essential  parts  of  the 
sphygmograph  known  as  Dudgeon's,  which  we  have  chosen  for  repre- 
sentation, not  because  it  is  best,  but  because  it  is  one  very  largely 
employed  in  medical  practice.  The  instrument  is  generally  applied  to 
the  radial  artery  because  the  arm  affords  a  convenient  support  to  the 
fulcrum  of  the  lever,  and   because  the  position  of  the  artery,  near  to  the 


h      d        a' 


Fig.  57.    Diagram  of  a  Sphygmograph  (Dudgeon's). 

Certain  supporting  parts  are  omitted  so  that  tlie  multiplying  levers  may  he 
displayed. 

a  is  a  small  metal  plate  which  is  kept  pressed  on  the  artery  hy  the  spring  h. 
The  vertical  movements  of  a  cause  to-and-fro  movements  of  the  lever  c  about  tlie 
fixed  point  d.  Those  are  communicated  to  and  magnified  liy  the  lever  e,  which 
moves  round  the  fixed  point  f.  The  free  end  of  this  lever  carries  a  liglit  steel 
marker  wliich  rests  on  a  strip  of  smoked  paper  //.  Tlie  paper  is  ]daced  beneath  two 
small  wliei'ls,  and  rests  on  a  roller  which  can  be  rotated  by  means  of  clock-work 
contained  in  the  box  h.  The  pa]ier  is  tlius  cansed  to  travel  at  a  uniform  rate. 
The  screw  graduated  in  ounces  Troy  is  bronght  to  bear  on  the  spring  h  by  means  of 
a  camm,  and  by  this  the  pressure  put  on  the  artery  can  be  regulated.  The  levers 
magnify  the  pulse  movements  fifty  times. 


222  METHODS   OF  KECOEDING  PULSE.        [Book  i. 

surface  and  with  the  support  of  the  radius  below  so  that  adequate 
pressure  can  he  brought  to  bear  by  the  lever  on.  the  artery,  is  favour- 
able for  making  observations.  It  can,  of  course,  be  applied  to  other 
arteries. 

The  membrane-manometer  of  Hiirthle  may  also  be  applied  directly 
to  an  unopened  artery.  The  cannula  is  replaced  by  a  small  funnel,  the 
mouth  of  which  is  covered  by  membrane  bearing  at  its  centre  a  small 
block  of  cork.  If  the  cork  be  pressed  lightly  on  an  arter}',  the  expansions 
of  the  artery  move  the  membrane  of  the  funnel,  and  the  movements 
of  this  are  transmitted  along  the  fluid  of  a  rigid  tube  to  the  recording 
tambour. 

A  pulse  tracing  may  also  be  indirectly  obtained  by  the  plethysiuo- 
graphic  method.  If  the  arm  be  introduced  into  a  plethysmograph 
(§  104),  a  tracing  may  be  obtained  of  the  rhythmic  expansions  of  the 
arm,  that  is,  of  the  rhythmic  expansions  of  the  arteries  of  the  arm,  due 
to  the  heart  beats.  If  the  plethysmograph  chamber  be  filled  with  air 
instead  of  fluid,  the  changes  of  pressure  in  the  chamber  may  be  brought 
to  bear  on  a  sensitive  flame,  the  changes  of  which  in  turn  may  be 
photographed. 

If  the  artery  be  laid  bare,  other  methods  may  be  adopted.  In  some 
cases,  in  that  of  the  aorta,  for  instance,  it  is  sufiicient  to  attach  a  light 
hook  into  the  outer  coat  of  the  artery,  arid  to  connect  the  hook  by 
means  of  a  thread  with  a  carefully  balanced  lever.  The  movements  of 
the  coat  of  the  artery  are  then  recorded  by  the  lever. 

The  sphygmotonometer  of  Eoy  may  also  be  used  without  opening 
the  artery.  For  this  purpose  a  length  of  the  artery  is  enclosed  in  a 
tube  with  rigid  walls,  filled  with  fluid,  which  acts  as  a  plethysmograph, 
the  movements  of  the  fluid  around  the  artery  being  recorded  by  means 
of  a  piston  working  a  lever.  If  the  artery  be  hgatured  and  divided, 
one  end  may  be  drawn  into  the  tube  for  the  distance  required.  The 
tube  may  also  be  made  of  two  halves,  one  of  which  is  slipped  under  the 
artery  simply  laid  bare,  the  other  placed  above  it,  and  the  two  halves 
are  brought  together  round  the  artery,  the  two  ends  of  the  tube  being 
closed  with  membrane. 

And  still  other  methods  may  be  employed. 

The  several  tracings  obtained  by  these  several  methods  differ 
of  course  in  minor  features,  but  they  agree  in  general  features  ; 
and  from  a  comparative  study  of  the  results  obtained  by  different 
methods  we  are  able,  in  many  cases  at  all  events,  to  form  conclu- 
sions as  to  which  of  the  minor  features  of  a  curve  are  due  to  the  in- 
strument itself,  and  which  represent  events  actually  taking  place 
in  the  artery.  On  the  whole,  the  curve  obtained  by  directly  record- 
ing the  pressure  within  the  artery  is  concordant  with  that  obtained 
by  recording  the  expansions  of  the  artery ;  the  curve  obtained  by 
the  manometer  or  by  the  sphygmoscope  very  closely  resembles 
that  obtained  by  the  sphygmograph,  and  the  more  completely  the 
incidental  errors  of  each  instrument  are  avoided,  the  more  closely 
do  the  two  curves  agree.  We  may  accordingly  in  treating  of  the 
pulse  confine  ourselves  largely  to  the  results  obtained  by  the  sphyg- 
mograph.    Any  of  the  various  instruments  applied  to  the  radial 


Chap,  iv.]         THE  VASCULAE  MECHANISM.  223 

artery  would  give  some  such  tracing  as  that  shewn  in  Fig.  58  which 
is  obtained  by  means  of  the  sphygmograph.    At  each  heart  beat  the 


Fig.  58.    Pulse  tracing  from  the  Eadial  Artery  of  Man. 

The  vertical  curved  line,  L,  gives  the  tracing  wliich  the  recording  lever  made 
when  the  blackened  paper  was  motionless.  Tlie  curved  interrupted  lines  shew  the 
distance  from  one  another  in  time  of  the  chief  phases  of  the  pulse-wave,  viz. 
X  =  commencement,  and  A  end  of  expansion  of  artery,  p,  predicrotic  notch,  d,  di- 
crotic notch.  C,  dicrotic  crest.  D,  post-dicrotic  crest.  /,  the  post-dicrotic  notch. 
These  terms  are  explained  in  the  text  later  on. 

curve  rises  rapidly,  and  then  falls  more  gradually  in  a  line  which 
is  more  or  less  uneven. 

§  122.  We  have  now  to  study  the  nature  and  characters  of 
the  pulse  in  greater  detail. 

We  may  say  at  once,  and,  indeed,  have  already  incidentally 
seen,  that  the  pulse  is  essentially  due  to  physical  causes ;  it  is 
the  physical  result  of  the  sudden  injection  of  the  contents  of  the 
ventricle  into  the  elastic  tubes  called  arteries.  Its  features 
depend  on  the  one  hand  on  the  systole  of  the  ventricle,  on  the 
quantity  of  blood  which  is  thereby  discharged  into  the  aorta,  and 
on  the  manner  in  which  it  is  discharged,  and  on  the  other  hand 
on  the  elasticity  of  the  arterial  walls.  The  more  important  of 
these  features  may  be  explained  on  physical  principles,  and  may 
be  illustrated  by  means  of  an  artificial  model,  so  far  at  least  as 
we  can  imitate  the  action  of  the  heart. 

We  may  confine  ourselves,  in  the  first  instance,  to  the  simple 
expansion  of  the  arterial  tube  and  its  return  to  its  previous 
condition,  neglecting  for  the  present  all  secondary  events. 

If  two  levers  be  placed  on  the  arterial  tubes  of  an  artificial 
model  Fig.  30,  ^S^.  a.,  S'.  a.,  one  near  to  the  pump,  and  the  other 
near  to  the  peripheral  resistance,  with  a  considerable  length  of 
tubing  between  them,  and  both  levers  be  made  to  write  on  a 
recording  surface,  one  immediately  below  the  otlier,  so  that  their 
curves  can  be  more  easily  compared,  tlie  following  facts  may  be 
observed,  when  the   pump  is  set  to  work  regularly.     They  are 


224 


AETIFICIAL  PULSE. 


[Book  i. 


perhaps   still  better   seen  if   a   number   of   levers   be  similarly 
arranged  at  different  distances  from  the  pump  as  in  Fig.  59. 


so 


^AiVV\A/\AAAAAAAAA/ 


Fig.  59.  Pulse-curves  described  by  a  series  of  sphygmographic  levers  placed  at 
intervals  of  20  cm.  from  each  other  along  an  elastic  tube,  into  wliich  fluid  is  forced 
by  the  sudden  stroke  of  a  pump.  The  pulse-wave  is  travelling  from  left  to  right,  as 
indicated  by  the  arrows  over  tlie  primary  (n)  and  secondary  (6,  c)  pulse-waves.  Ihe 
dotted  vertical  lines  drawn  from  the  summit  of  the  several  primary  waves  to  the 
tuning-fork  curve  below,  each  complete  vibration  of  which  occupies  -5^^  sec,  allow  the 
time  to  be  measured  which  is  taken  up  by  the  wave  in  passing  along  20  cm.  of  the 
tubing.  The  waves  a'  are  waves  rcflprted  from  the  closed  distal  end  of  the  tubing; 
this  is  indicated  by  the  direction  of  the  arrows.  It  will  he  observed  that  in  the 
more  distant  lever  VI.  the  reflected  wave,  having  but  a  slight  distance  to  travel, 
becomes  fused  with  the  primary  wave.     (From  Marey.) 

At  each  stroke  of  the  pump,  each  lever  rises  until  it  reaches 
a  maximum  (Fig.  59,  la,  2a,  &c.),  and  then  falls  again,  thus 
describing  a  curve.  The  rise  is  due  to  the  expansion  of  the  part 
of  the  tube  under  the  lever,  and  the  fall  is  due  to  that  part  of  the 


Chap,  iv.]  THE   VASCULAR   MECHANISM. 


•221 


tube  returning  after  the  expansion  to  its  previous  calibre.  The 
curve  is  therefore  the  curve  of  the  expansion  (and  return)  of 
the  tube  at  the  point  on  which  the  lever  rests.  We  may  call  it 
the  pulse-curve.  It  is  obvious  that  the  expansion  passes  by  the 
lever  in  the  form  of  a  wave.  At  one  moment  the  lever  is  at  rest: 
the  tube  beneath  it  is  simply  distended  to  the  normal  amount 
indicative  of  the  mean  pressure  which  at  the  time  obtains  in  the 
arterial  tubes  of  the  model ;  at  the  next  moment  the  pulse  expan- 
sion reaches  the  lever,  and  the  lever  begins  to  rise ;  it  continues 
to  rise  until  the  top  of  the  wave  reaches  it,  after  which  it  falls 
again  until  finally  it  comes  to  rest,  the  wave  having  completely 
passed  by. 

It  may  perhaps  be  as  well  at  once  to  warn  the  reader  that  the 
figure  which  we  call  the  pulse-curve  is  not  a  representation  of  the 
pulse-wave  itself ;  it  is  simply  a  representation  of  the  movements, 
up  and  down,  of  the  piece  of  the  wall  of  the  tubing  at  the  spot  on 
which  the  lever  rests  during  the  time  that  the  wave  is  passing 
over  that  spot.  We  may  roughly  represent  the  wave  by  the 
diagram  Fig.  60,  in  which  the  wave  shewn  by  the  dotted  line  is 


,.-"'1 


/\ 


A 

Fig.  60.    A  rough  diagrammatic  Representation  of  a  Pulse-Wave  passing 

OVER  AN  Artery. 

passing  over  the  tube  (shewn  in  a  condition  of  rest  by  the  thick 
double  line)  in  the  direction  from  ^to  C.  It  must,  however,  be 
remembered  that  the  wave  thus  figured  is  a  much  shorter  wave 
than  is  the  pulse-wave  in  reality  (that  being,  as  we  shall  see, 
about  6  meters  long),  i.e.  occupies  a  smaller  length  of  the  arterial 
system  from  the  heart  H  towards  the  capillaries  C.  Moreover,  the 
actual  pulse-wave  has  secondary  features,  which  we  are  neglecting 
for  the  present,  and  which,  therefore,  we  do  not  attempt  to  shew 
in  the  tigure. 

The  curves  below,  X,  Y,  Z,  represent,  in  a  similarly  diagram- 
matic fashion,  the  curves  described,  during  the  passage  of  the  wave, 

15 


226 


AETIFICIAL  PULSE. 


[Book  i. 


by  levers  placed  on  the  points  x,  y,  z.  At  Z  the  greater  part  of 
the  wave  has  already  passed  under  the  lever,  which,  during  its 
passage,  has  already  described  the  greater  part  of  its  curve,  shewn 
by  the  thick  line,  and  has  only  now  to  describe  the  small  part, 
shewn  by  the  dotted  line,  corresponding  to  the  remainder  of  the 
wave  from  ^to  H.  At  Fthe  lever  is  at  the  summit  of  the  wave. 
At  X  the  lever  has  only  described  a  small  part  of  the  beginning 
of  the  wave,  viz.  from  C  to  x,  the  rest  of  the  curve,  as  shewn  by 
the  dotted  line,  having  yet  to  be  described. 
But  to  return  to  the  consideration  of  Fig.  59. 
§  123.  The  rise  of  each  lever  is  somewhat  sudden,  but  the  fall 
is  more  gradual,  and  is  generally  marked  with  some  irregularities 
which  we  shall  study  presently.  The  rise  is  sudden  because  the 
sharp  stroke  of  the  pump  suddenly  drives  a  quantity  of  fluid  into 
the  tubing,  and  so  suddenly  expands  the  tube ;  the  fall  is  more 
gradual  because  the  elastic  reaction  of  the  walls  of  the  tube,  which, 
after  the  expanding  power  of  the  pump  has  ceased,  brings  about 
the  return  of  the  tube  to  its  former  calibre  driving  the  fluid 
onwards  to  the  periphery,  is  more  gradual  in  its  action. 

These  features,  the  suddenness  of  the  rise  or  up-stroke,  and  the 
more  gradual  slope  of  the  fall  or  down-stroke,  are  seen  also  in 
natural  pulse-curves  taken  from  living  arteries  (Figs.  58,  61  &c.). 
We  shall  see,  however,  that  under  certain  circumstances  this 
contrast  between  the  up-stroke  and  the  down-stroke  is  not  so 
marked. 

It  may  here  be  noted  that  the  actual  size  of  the  curve,  that  is 
the  amount  of  excursion  of  the 
lever,  depends  in  part  (as  does  also 
to  a  great  extent  the  form  of  the 
curve)  on  the  amount  of  pressure 
exerted  by  the  lever  on  the  tube. 
If  the  lever  only  just  touches  the 
tube  in  its  expanded  state,  the  rise 
will  be  insignificant.  If,  on  the 
other  hand,  the  lever  be  pressed 
down  too  firmly,  the  tube  beneath 
will  not  be  able  to  expand  as  it 
otherwise  would,  and  the  rise  of  the 
lever  will  be  proportionately  dimin- 
ished. There  is  a  certain  pressure 
which  must  be  exerted  by  the  lever 
on  the  tube,  the  exact  amount 
depending  on  the  expansive  power 
of  the  tubing,  and  on  the  pressure 
exerted  by  the  fluid  in  the  tube, 
in  order  that  the  tracing  may  be 
best  marked.  This  is  shewn  in 
Fig.  61,  in  which  are  given  three  tracings  taken  from  the  same 


ABC 


Fig.  61.  Pulse  tracings  from  the 
same  radial  artery  u.mjel!  v)\v 
eerent  pressures  of  the  lever. 

The  letters  are  explained  in  a  later 
part  of  the  text.  Taken  with 
Dudgeon's  sphygmograph. 


Chap,  iv.]  THE   VASCULAR   MECHANISM.  227 

radial  artery  with  the  same  instrument ;  in  the  lower  curve  the 
pressure  of  the  lever  is  too  great,  in  the  upper  curve  too  small,  to 
bring  out  the  proper  characters  of  the  pulse  ;  these  are  seen  more 
distinctly  in  the  middle  curve  with  a  medium  pressure. 

§  124.  It  will  be  observed  that  in  Eig.  59,  curve  I.,  which  is 
nearer  the  pump,  rises  more  rapidly  and  rises  higher  than  curve  II., 
which-  is  farther  away  from  the  pump ;  that  is  to  say,  at  the  lever 
farther  away  from  the  pump  the  expansion  is  less  and  takes  place 
more  slowly  than  at  the  lever  nearer  the  pump.  Similarly  in 
curve  IV.  the  rise  is  still  less,  and  takes  place  still  less  rapidly 
than  in  II.,  and  the  same  change  is  seen  still  more  marked  in  V. 
as  compared  with  IV.  In  fact  if  a  number  of  levers  were  placed 
at  equal  distances  along  the  arterial  tubing  of  the  model,  and  the 
model  were  working  properly,  with  an  adequate  peripheral  resist- 
ance, we  might  trace  out  step  by  step  how  the  expansion,  as  it 
travelled  along  the  tube,  got  less  and  less  in  amount,  and  at  the 
same  time  became  more  gradual  in  its  development,  the  curve 
becoming  lower  and  more  flattened  out,  until,  in  the  neighbourhood 
of  the  artificial  capillaries,  there  was  hardly  any  trace  of  it  left. 
In  other  words,  we  might  trace  out  step  by  step  the  gradual 
disappearance  of  the  pulse. 

The  same  changes,  the  same  gradual  lowering  and  flattening 
of  the  curve,  may  be  seen  in  natural  pulse  tracings ;  compare,  for 
instance,  Fig.  62,  which  is  a  trac- 
ing from  the  dorsalis  pedis  artery,       /),    c 
with  the  tracing  from  the  radial 
artery    Fig.    61,    taken    from    the 

same   individual    with    the    same 

instrument  on  the  same  occasion. 

This  feature  is,  of  course,  not  ob-  Fig.  62.  Pulse  tracixg  from  Dor- 
,,:-., ,r.  :^  r.^^  ^,,i^^  ^,,^,r^^  4-«u^»,  salis  Pedis  taken  from  the  same 
vious    m    all    pulse-curves    taken       i^^dividual  as  f,g.  6i. 

from     different    individuals     with 

different  instruments  and  under  varied  circumstances  ;  but  if 
a  series  of  curves  from  different  arteries  were  carefully  taken 
under  the  same  conditions,  it  would  be  found  that  the  aortic 
tracing  is  higher  and  more  sudden  than  the  carotid  tracing, 
which  again  is  higher  and  more  sudden  than  the  radial  tracing, 
the  tibial  tracing  being  in  turn  still  lower  and  more  flattened. 
The  pulse-curve  dies  out  by  becoming  lower  and  lower,  and  more 
and  more  flattened  out. 

And  a  little  consideration  will  shew  us  that  this  must  be  so. 
The  systole  of  the  ventricle  drives  a  quantity  of  bUiod  into  the 
already  full  aorta.  The  sudden  injection  of  this  quantity  of  blood 
expands  the  portion  of  the  aorta  next  to  the  heart,  the  part 
immediately  adjacent  to  the  semilunar  valves  beginning  to  expand 
first,  and  the  expansion  travelling  thence  on  to  the  end  of  this 
portion.  In  the  same  way  the  expansion  travels  on  from  this 
portion  through  all  the  succeeding  portions  of  the  arterial  system. 


228  DISAPPEARANCE   OF  PULSE.  [Book  i. 

For  the  total  expansion  required  to  make  room  foi  the  new 
quantity  of  blood  is  not  provided  by  that  portion  alone  of  the 
aorta  into  which  the  blood  is  actually  received ;  it  is  supplied  by 
the  whole  arterial  system  :  the  old  quantity  of  blood  which  is 
replaced  by  the  new  in  this  first  portion  has  to  find  room  for  itself 
in  the  rest  of  the  arterial  space.  As  the  expansion  travels  onward, 
however,  the  increase  of  pressure,  which  each  portion  transmits  to 
the  succeeding  portion,  will  be  less  than  that  which  it  received 
from  the  preceding  portion.  For  the  whole  increase  of  pressure 
due  to  the  systole  of  the  ventricle  has  to  be  distributed  over  the 
whole  of  the  arterial  system  ;  the  general  mean  arterial  pressure 
is,  as  we  have  seen,  maintained  by  repeated  systoles,  and  any  one 
systole  has  to  make  its  contribution  to  that  mean  pressure  ;  the 
increase  of  pressure  which  starts  from  the  ventricle  must  there- 
fore leave  behind  at  each  stage  of  its  progress  a  fraction  of  itself ; 
that  is  to  say,  the  expansion  is  continually  growing  less,  as  the 
pulse  travels  from  the  heart  to  the  capillaries.  Moreover,  while 
the  expansion  of  the  aorta  next  to  the  heart  is,  so  to  speak,  the 
direct  effect  of  the  systole  of  the  ventricle,  the  expansion  of  the 
more  distant  artery  is  the  effect  of  the  systole  transmitted  by  the 
help  of  the  elastic  reaction  of  the  arterial  tract  between  the  heart 
and  the  distant  artery ;  and  since  this  elastic  reaction  is  slower  in 
development  than  the  actual  systole,  the  expansion  of  the  more 
distant  artery  is  slower  than  that  of  the  aorta,  the  up-stroke  of 
the  pulse-curve  is  less  sudden,  and  the  whole  pulse-curve  is  more 
flattened. 

The  object  of  the  systole  is  to  supply  a  contribution  to  the 
mean  pressure,  and  the  pulse  is  an  oscillation  above  and  below 
that  mean  pressure,  an  oscillation  which  diminishes  from  the  heart 
onwards,  being  damped  by  the  elastic  walls  of  the  arteries,  and  so, 
little  by  little,  converted  into  mean  pressure  until  in  the  capillaries 
the  mean  pressure  alone  remains,  the  oscillations  having  dis- 
appeared. 

§  125.  If  in  the  model  the  points  of  the  two  levers  at  different 
distances  from  the  pump  be  placed  exactly  one  under  the  other 
on  the  recording  surface,  it  is  obvious  that,  the  levers  being  alike 
except  for  their  position  on  the  tube,  any  difference  in  time 
between  the  movements  of  the  two  levers  will  be  shewn  by  an 
interval  between  the  beginnings  of  the  curves  they  describe,  the 
recording  surface  being  made  to  travel  sufficiently  rapidly. 

If  the  movements  of  the  two  levers  be  thus  compared,  it  will  be 
seen  that  the  far  lever  (Fig.  59,  II.)  commences  later  than  the  near 
one  (Fig.  59,  I.) ;  the  farther  apart  the  two  levers  are,  the  greater 
is  the  interval  in  time  between  their  curves.  Compare  the  series 
I.  to  VI.  (Fig.  59).  In  the  same  way  it  would  be  found  that  the 
rise  of  the  near  lever  began  some  fraction  of  a  second  after  the 
stroke  of  the  pump.  This  means  that  the  wave  of  expansion,  the 
pulse-wave,  takes  some  time  to  travel  along  the  tube. 


Chap.  iv.J  THE  VASCULAR  MECHANISM. 


229 


The  velocity  with  which  the  pulse-wave  travels  depends  chiefly 
on  the  amount  of  rigidity  possessed  by  the  tubing.  The  more 
extensible  (with  corresponding  elastic  reaction)  the  tube,  the  slower 
is  the  wave ;  the  more  rigid  the  tube  becomes,  the  faster  the  wave 
travels ;  in  a  perfectly  rigid  tube,  what  in  the  elastic  tube  would 
be  the  pulse,  becomes  a  mere  shock  travelling  with  very  great 
rapidity.  The  width  of  the  tube  is  of  much  less  influence,  though 
according  to  some  observers  the  wave  travels  more  slowly  in  the 
wider  tubes. 

The  rate  at  which  the  normal  pulse-wave  travels  in  the  human 
body  has  been  variously  estimated  at  from  10  to  5  meters  per 
second.  In  all  probability  we  may  take  6  meters  as  an  average 
rate  ;  but  it  must  be  remembered  that  the  rate  may  vary  very 
considerably  under  different  conditions.  According  to  all  observers 
the  velocity  of  the  wave  in  passing  from  the  groin  to  the  foot  is 
greater  than  that  in  passing  from  the  axilla  to  the  wrist  (6  m. 
against  5  m.).  This  is  probably  due  to  the  fact  that  the  femoral 
artery  with  its  branches  is  more  rigid  than  the  axillary  and  its 
branches.  So,  also,  the  wave  travels  more  slowly  in  the  arteries 
of  children  than  in  the  more  rigid  arteries  of  the  adult.  The 
velocity  is  also  increased  by  circumstances  which  heighten,  and 
decreased  by  those  which  lower  the  mean  arterial  pressure,  since 
with  increasing  pressure  the  arterial  walls  become  more,  and  with 
diminishing  pressure  less  rigid.  Probably  also  the  velocity  of  the 
pulse-wave  depends  on  conditions  of  the  arterial  walls,  which  we 
cannot  adequately  describe  as  mere  differences  in  rigidity.  In 
experimenting  with  artificial  tubes  it  is  found  that  different 
qualities  of  india  rubber  give  rise  to  very  different  results. 

Care  must  be  taken  not  to  confound  the  progress  of  the  pulse- 
wave,  i.e.  of  the  expansion  of  the  arterial  walls,  with  the  actual 
onward  movement  of  the  blood  itself.  The  pulse-wave  travels 
over  the  moving  blood  somewhat  as  a  rapidly  moving  natural, 
wave  travels  along  a  sluggishly  flowing  river.  Thus  while  the  ■ 
velocity  of  the  pulse-wave  is  6  or  possibly  even  10  meters  per  sec, 
that  of  the  current  of  blood  is  not  more  than  half  a  meter  per  sec, 
even  in  the  large  arteries,  and  is  still  less  in  the  smaller  ones. 

§  126.  Referring  again  to  the  caution  given  above,  not  to 
regard  the  pulse-curve  as  a  picture  of  the  pulse-wave,  we  may  now 
add  that  the  pulse-wave  is  of  very  considerable  length.  If  we  know 
how  long  it  takes  for  the  pulse-wave  to  pass  over  any  point  in  the 
arteries  and  how  fast  it  is  travelling,  we  can  easily  calculate  the 
length  of  the  wave.  In  an  ordinary  pulse-curve  the  artery,  owing  to 
the  slow  return,  is  seen  not  to  regain  the  calibre  which  it  had  before 
the  expansion,  until  just  as  the  next  expansion  begins,  that  is  to 
say,  the  pulse-wave  takes  the  whole  time  of  a  cardiac  cycle,  viz. 
rj^ths  sec,  to  pass  by  the  lever.  Taking  the  velocity  of  the  pulse- 
wave  as  6  meters  per  sec,  the  length  of  the  wave  will  be  ^^jths  of 
6  m.,  that  is,  nearlv  5  meters.     And  even  if  we  took  a  smaller 


230 


VELOCITY  OF 


WAVE. 


[Book  i. 


estimate,  by  supposing  that  the  rearMpansion  and  return  of  the 
artery  at  any  point  took  much  less  time,  say  ^%th  sec,  the  length 
of  the  pulse-wave  would  still  be  more  than  2  meters.  But  even 
in  the  tallest  man  the  capillaries  farthest  from  the  heart,  those  in 
the  tips  of  the  toes,  are  not  2  m.  distant  from  the  heart.  In  other 
words,  the  length  of  the  pulse-wave  is  much  greater  than  the 
whole  length  of  the  arterial  system,  so  that  the  beginning  of 
each  Avave  has  become  lost  in  the  small  arteries  and  capillaries 
some  time  before  the  end  of  it  has  finally  passed  away  from  the 
beginning  of  the  aorta. 

We  must  now  return  to  the  consideration  of  certain  special 
features  in  the  pulse,  which,  from  the  indications  they  give  or 
suggest  of  the  condition  of  the  vascular  system,  are  often  of  great 
interest. 

§  127.  Secondary  waves.  In  nearly  all  pulse  tracings,  the 
curve  of  the  expansion  and  recoil  of  the  artery  is  broken  by  two, 
three,  or  several  smaller  elevations  and  depressions :  secondary 
waves  are  imposed  upon  the  fundamental  or  primary  wave.  In 
the  sphygmographic  tracing  from  the  carotid,  Fig.  63,  and  in  many 
of  the  other  tracings  given,  these  secondary  elevations  are  marked 


T"iG.  63.    Pulse  tracing  from  carotid  artery  of  healthy  man  (Moens). 

X,  commencement  of  expansion  of  the  artery.  A,  summit  of  the  first  rise.  C, 
dicrotic  secondary  wave.  B,  predicrotic  secondary  wave  ;  p,  notch  preceding  this. 
D,  succeeding  secondary  wave.  The  curve  above  is  that  of  a  tuning-fork  with  ten 
double  vibrations  in  a  second. 

as  B,  C,  D.  When  one  such  secondary  elevation  only  is  conspic- 
uous, so  that  the  pulse-curve  presents  two  notable  crests  only, 
the  primary  crest  and  a  secondary  one,  the  pulse  is  said  to  be 
"  dicrotic  "  ;  when  two  secondary  crests  are  prominent,  the  pulse  is 
often  called  "  tricrotic  "  ;  when  several,  "  polycrotic."  As  a  general 
rule,  the  secondary  elevations  appear  only  on  the  descending  limb 
of  the  primary  wave  as  in  most  of  the  curves  given,  and  the  curve 
is  then  spoken  of  as  "  katacrotic."  Sometimes,  however,  the  first 
elevation  or  crest  is  not  the  highest,  but  appears  on  the  ascending 
portion  of  the  main  curve  :  such  a  curve  is  spoken  of  as  "  anacrotic  " 
Fig.  64.  As  we  have  already  seen  (§  116)  the  curve  of  pressure 
at  the  root  of  the  aorta,  nnd,  indeed,  that  of  endocardiac  pressure 
may  be  in  like  manner  "  anacrotic  "  (Figs.  53,  54). 


Chap,  iv.]         THE  VASCULAE   MECHANISM. 


231 


Of  these  secondary  elevations,  the  most  frequent,  conspicuous 
and  important  is  the  one  which  appears 
some  way  down  on  the  descending  limb, 
and  is  marked  C  on  Fig.  63  and  on  most 
of  the  curves  here  given.  It  is  more  or 
less  distinctly  visible  on  all  sphygmograms, 
and  may  be  seen  in  those  of  the  aorta 
as  well  as  of  other  arteries.  Sometimes 
it  is  so  slight  as  to  be  hardly  discernible ; 
at  other  times  it  may  be  so  marked  as 
to  give  rise  to  a  really  double  pulse 
(Fig.  65),  i.e.  a  pulse  which  can  be  felt 
as  double  by  the  finger :  hence  it  has  been 
called  the  dicrotic  elevation  or  the  dicrotic  wave,  the  notch 
preceding  the  elevation  being  spoken  of  as  the  "  dicrotic  notch." 


Fig.  64,  Anacrotic  spHYCr- 

JIOGRAl'H    TRACING    FROM 

THE    Ascending   aorta 
(Aneurism). 


Fig.  65.    Two  grades  of  marked   dicrotism   in  radial  pulse  of  man. 
(Typhoid  Fever.) 

Neither  it  nor  any  other  secondary  elevations  can  be  recognized 
in  the  tracings  of  blood  pressure  taken  with  a  mercury  manometer. 
This  may  be  explained,  as  we  have  said  §  121,  by  the  fact  that 
the  movements  of  the  mercury  column  are  too  sluggish  to  repro- 
duce these  finer  variations.  Moreover,  when  the  normal  pulse 
is  felt  by  the  finger,  most  persons  find  themselves  unable  to  detect 
any  dicrotism.  But  that  it  does  really  exist  in  the  normal  pulse 
is  shewn  by  the  fact  that  it  appears,  sometimes  to  a  marked 
extent,  sometimes  to  a  less  extent,  not  only  in  sphygmograms  and 
in  curves  of  arterial  pressure  taken  by  adequate  instruments,  but 
also  and  in  a  most  unmistakeable  manner  in  the  tracing  obtained 
by  allowing  the  blood  to  spirt  directly  from  an  opened  small 
artery,  such  as  the  dorsalis  pedis,  upon  a  recording  surface. 

Less  constant  and  conspicuous  than  the  dicrotic  wave,  but  yet 
appearing  in  most  sphygmograms,  is  an  elevation  which  appears 
higher  up  on  the  descending  limb  of  the  main  W'ave ;  it  is  marked 
B  in  Fig.  63,  and  on  several  of  the  other  curves,  and  is  frequently 
called  the  prcdicrotic  wave  ;  it  may  become  very  ])rominent.  Some- 
times other  secondary  waves,  often  called  '  post-dicrotic,'  are  seen 
following  the  dicrotic  wave,  as  at  D  in  Fig.  63,  and  some  other 
curves ;  but  these  are  not  often  present,  and  usually  even  when 
present  inconspicuous. 

When  tracings  are  taken  from  several  arteries,  or  from  the  same 
artery  under  different  conditions  of  tl\e  body,  these  secondary 
waves  are  found  to  vary  very  considerably,  giving  rise  to  many 


232  THE   DICEOTIC  WAVE.  [Book  i. 

characteristic  forms  of  pulse-curve.  Were  we  able  with  certainty 
to  trace  back  the  several  features  of  the  curves  to  their  respective 
causes,  an  adequate  examination  of  sphygmographic  tracings 
would  undoubtedly  disclose  much  valuable  information  concerning 
the  condition  of  the  body  presenting  them.  The  problems,  how- 
ever, of  the  origin  of  these  secondary  waves  and  of  their  variations 
are  complex  and  difficult;  so  much  so  that  the  detailed  interpre- 
tation of  a  sphygmographic  tracing  is  still  in  many  cases  and  in 
many  respects  very  uncertain. 

§  128.  The  Dicrotic  Wave.  The  chief  interest  attaches  to 
the  nature  and  meaning  of  the  dicrotic  wave.  In  general  the 
main  conditions  favouring  the  dicrotic  wave  are  (1)  a  highly 
extensible  and  elastic  arterial  wall ;  (2)  a  comparatively  low  mean 
pressure,  leaving  the  extensible  and  elastic  reaction  of  the  arterial 
wall  free  scope  to  act ;  and  (3)  a  vigorous  and  rapid  stroke  of  the 
ventricle,  discharging  into  the  aorta  a  considerable  quantity  of 
blood. 

The  origin  of  this  dicrotic  wave  has  been  and  indeed  still  is 
much  disputed. 

In  the  first  place,  observers  are  not  agreed  as  to  the  part  of 
the  arterial  system  in  which  it  first  makes  its  appearance.  In 
such  a  system  as  that  of  the  arteries  we  have  to  deal  with  two 
kinds  of  waves.  There  are  the  waves  which  are  generated  at  the 
pump,  the  heart,  and  travel  thence  onwards  towards  the  periphery  ; 
the  primary  pulse-wave  due  to  the  discharge  of  the  contents  of 
the  ventricle  is  of  this  kind.  But  there  may  be  other  waves 
which,  started  somewhere  in  the  periphery,  travel  backwards 
towards  the  central  pump ;  such  are  what  are  called  '  reflected ' 
waves.  For  instance,  when  the  tube  of  the  artificial  model,  bear- 
ing two  levers,  is  blocked  just  beyond  the  far  lever,  the  primary 
wave  is  seen  to  be  accompanied  by  a  second  wave,  which  at  the 
far  lever  is  seen  close  to,  and  often  fused  into,  the  primary  wave 
(Fig.  59,  VI.  a'),  but  at  the  near  lever  is  at  some  distance  from  it 
(Fig.  59,  I.  a'),  being  the  farther  from  it  the  longer  the  interval 
between  the  lever  and  the  block  in  the  tube.  The  second  wave  is 
evidently  the  primary  wave  reflected  at  the  block  and  travelling 
backwards  towards  the  pump.  It  thus,  of  course,  passes  the  far 
lever  before  the  near  one.  And  it  has  been  argued  that  the 
dicrotic  wave  of  the  pulse  is  really  such  a  reflected  wave,  started 
either  at  the  minute  arteries  and  capillaries,  or  at  the  several 
points  of  bifurcation  of  the  arteries,  and  travelling  backwards  to 
the  aorta.  But  if  this  were  the  case  the  distance  between  the 
primary  crest  and  the  dicrotic  crest  ought  to  be  less  in  arteries 
more  distant  from,  than  in  those  nearer  to  the  heart,  just  as  in 
the  artificial  scheme  the  reflected  wave  is  fused  with  a  primary 
wave  near  the  block  (Fig.  59,  VI.  6  a.  a'),  but  becomes  more  and 
more  separated  from  it  the  farther  back  towards  the  pump  we  trace 
it  (Fig.  59,  I.  1.  a.  a').     Now  this  is  not  the  case  with  the  dicrotic 


Chap.  iv.J  THE   VASCULAR   MECHANISM.  233 

wave ;  careful  measurements  shew  that  the  distance  between 
the  primary  and  dicrotic  crests  is  either  the  same  or  certainly  not 
less  in  the  smaller  or  more  distant  arteries  than  in  the  larger  or 
nearer  ones.  This  feature  indeed  proves  that  the  dicrotic  wave 
cannot  be  due  to  reflection  at  the  periphery  or  indeed  in  any  way 
a  retrograde  wave.  Besides  the  multitudinous  peripheral  division 
would  probably  render  one  large  peripherically  reflected  wave  im- 
possible. Again,  the  more  rapidly  the  primary  wave  is  oblite- 
rated or  at  least  diminished  on  its  way  to  the  periphery  the  less 
conspicuous  should  be  the  dicrotic  wave.  Hence  increased  ex- 
tensibility and  increased  elastic  reaction  of  the  arterial  walls 
which  tend  to  use  up  rapidly  the  primary  wave,  should  also  lessen 
the  dicrotic  wave.  But  as  a  matter  of  fact  these  conditions,  as  we 
have  said,  are  favourable  to  the  prominence  of  the  dicrotic  wave. 

We  may  conclude  then  that  the  dicrotic  wave  like  tbe  primary 
wave  begins  at  the  heart,  and  travels  thence  towards  the  periphery. 
But  even  if  this  be  admitted  observers  are  not  agreed  as  to  the 
mechanism  of  its  production.  The  following  view  is  the  one 
which  seems  the  most  satisfactory,  though  it  is  not  accepted 
by  all  inquirers. 

The  simultaneous  curves  of  endocardiac  and  aortic  pressure 
(Fig.  54  and  others)  shew  us  that  the  dicrotic  notch  as  it  is  called, 
the  depression  immediately  preceding  the  dicrotic  wave  is,  in  a 
normal  beat,  coincident  with  the  end  of  the  systole.  The  curve 
of  the  differential  manometer  further  shews  us  that  this  is  the 
point  at  which  the  pressure  in  the  ventricle  begins  to  become  less 
than  in  the  aorta.  We  may  therefore  reason  in  the  following 
way.  The  flow  from  the  ventricle  into  the  aorta  ceases  because 
the  systole  ceases  ;  the  cessation  takes  place  while  the  two  cavi- 
ties are  still  open  to  each  other,  and  probably,  in  most  cases  at 
least,  while  there  is  still  more  or  less  blood  in  the  ventricle.  The 
pressure  in  the  ventricle  tends  to  become  less  than  that  in  the  aorta, 
and  the  blood  in  the  aorta  tends  to  flow  back  into  the  ventricle. 
But  the  first  effect  of  this  is  to  close  firmly  the  semilunar  valves. 
The  expansion  of  the  aorta,  (which  in  many  cases  had  been  lessen- 
ing even  during  the  systole  owing  to  the  flow  through  the  periphery 
ofthe  arterial  system  being  more  rapid  than  the  flow  from  the 
ventricle,  but  in  some  cases,  in  the  anacrotic  instances,  had  not,) 
lessens  with  tlie  cessation  of  the  flow  ;  the  aorta  shrinks,  press- 
ing upon  its  contents,  liut  part  of  this  pressure  is  spent  on  the 
closed  semilunar  valves,  and  the  resistance  offered  by  tbese  starts 
a  new  wave  of  expansion,  the  dicrotic  wave,  which  travels  thence 
onwards  towards  the  periphery  in  the  wake  of  the  primary  wave. 
If  w^e  admit  that  the  blood  is  flowing  from  the  ventricle  during 
the  whole  of  the  systole,  we  must  also  admit  that  the  semilunar 
valves  do  not  close  until  the  end  of  the  systole,  and  this  being,  as 
shewn  by  the  curves,  just  antecedent  to  the  dicrotic  wave,  we  may 
attribute  this  wave  to  the  rebound  from  the  closed  valves.      It  is 


234  THE  DICROTIC   WAVE.  [Book  i. 

not  necessary  that  the  valves  should  act  perfectly,  and  the  dicrotic 
wave  may  occur  in  cases  where  the  valves  are  more  or  less  in- 
competent ;  all  that  is  required  for  its  production  is  an  adequate 
obstacle  to  the  return  of  blood  from  the  aorta  to  the  ventricle, 
and  without  such  an  obstacle  the  circulation  could  not  be  carried 
on. 

§  129.  Moreover  it  must  be  remembered  that  though  we  may 
thus  regard  the  closed  valves  as  so  to  speak  the  determining  cause 
of  the  dicrotic  wave,  the  wave  itself  is  an  oscillation  of  the  arterial 
walls,  and  the  remarks  made  a  little  while  back  concerning  the 
inertia  of  the  walls  hold  good  for  this  explanation  also.  Hence 
the  conditions  which  determine  the  prominence  or  otherwise  of 
the  dicrotic  wave  are  conditions  relating  to  the  elasticity  of  the 
arterial  walls,  and  to  the  circumstances  which  call  that  elasticity 
into  play.  For  instance,  the  dicrotic  wave  is  less  marked  in  rigid 
arteries  (such  as  those  of  old  people)  than  in  healthy  elastic  ones ; 
the  rigid  wall  neither  expands  so  readily  nor  shrinks  so  readily, 
and  hence  does  not  so  readily  give  rise  to  secondary  waves.  Again, 
the  dicrotic  wave  is,  other  things  being  equal,  more  marked  when 
the  mean  arterial  pressure  is  low  than  when  it  is  high ;  indeed  it 
may  be  induced  when  absent,  or  increased  when  slightly  marked, 
by  diminishing,  in  one  way  or  another,  the  mean  pressure.  Now 
when  the  pressure  is  high,  the  arteries  are  kept  continually  much 
expanded,  and  are  therefore  the  less  capable  of  further  expansion, 
that  is  to  say,  are,  so  far,  more  rigid.  Hence  the  additional 
expansion  due  to  the  systole  is  not  very  great ;  there  is  a  less 
tendency  for  the  arterial  walls  to  swing  backwards  and  forwards, 
so  to  speak,  and  hence  a  less  tendency  to  the  development  of 
secondary  waves.  When  the  mean  pressure  is  low,  the  opposite 
state  of  things  exists  ;  supposing  of  course  that  the  ventricular 
stroke  is  adequately  vigorous  (the  low  pressure  being  due,  not 
to  a  diminished  cardiac  stroke  but  to  diminished  peripheral 
resistance)  the  relatively  empty  but  highly  distensible  artery 
is  rapidly  expanded,  and  falling  rapidly  back  enters  upon  a 
secondary   (dicrotic)   expansion,  and   may  even  exhibit  a  third. 

Moreover  the  same  principles  may  be  applied  to  explain  why 
sometimes  dicrotism  will  appear  marked  in  a  particular  artery 
while  it  remains  little  marked  in  the  rest  of  the  system.  In 
experimenting  with  an  artificial  tubing  such  as  the  arterial  model, 
the  physical  characters  of  which  remain  the  same  throughout, 
both  the  primary  and  the  secondary  waves  retain  the  same 
characters  as  they  travel  along  the  tubing  save  only  that  both 
gradually  diminish  towards  the  periphery ;  and  in  the  natural 
circulation,  when  the  vascular  conditions  are  fairly  uniform 
throughout,  the  pulse-curve,  as  a  rule,  possesses  the  same  general 
characters  throughout,  save  that  it  is  gradually  '  damped  off.' 
But  suppose  we  were  to  substitute  for  the  first  section  of  the 
tubing  a  piece  of  perfectly  rigid  tubing ,  this  at  the  stroke  of  the 


Chap,  iv.]  THE   VASCULAR   MECHANISM.  235 

pump  on  account  of  its  being  rigid  would  shew  neither  primary 
nor  secondary  expansion,  but  the  expanding  force  of  the  pump's 
stroke  would  be  transmitted  through  it  to  the  second,  elastic 
section,  and  here  the  primary  and  secondary  waves  would  at  once 
become  evident.  This  is  an  extreme  case,  but  the  same  thing 
would  be  seen  to  a  less  degree  in  passing  from  a  more  rigid,  that 
is  less  extensible  and  elastic  section,  to  a  less  rigid,  more  exten- 
sible and  elastic  section  ;  the  primary  and  secondary  expansions, 
in  spite  of  the  general  damping  effect,  would  suddenly  increase. 
Similarly  in  the  living  body  a  pulse-curve  which  so  long  as  it  is 
travelling  along  arteries  in  which  the  mean  pressure  is  high,  and 
which  are  therefore  practically  somewhat  rigid,  is  not  markedly 
dicrotic,  may  become  very  markedly  dicrotic  when  it  comes  to  a 
particular  artery,  in  which  the  mean  pressure  is  low  (we  shall  see 
presently  that  such  a  case  may  occur),  and  the  walls  of  which 
are  therefore  for  the  time  being  relatively  more  distensible  than 
the  rest. 

Lastly  we  may  recall  the  observation  made  above  §  123  tliat 
the  curve  of  expansion  of  an  elastic  tube  is  modified  by  the  pres- 
sure exerted  by  the  lever  employed  to  record  it,  and  that  hence, 
in  the  same  artery,  and  with  the  same  instrument,  the  size,  form, 
and  even  the  special  features  of  the  curve  vary  according  to  the 
amount  of.  pressure  with  which  the  lever  is  pressed  upon  the 
artery.  Accordingly  the  amount  of  dicrotism  apparent  in  a  pulse 
may  be  modified  by  the  pressure  exerted  by  the  lever.  In  Fig.  61 
for  instance  the  dicrotic  wave  is  more  evident  in  the  middle  than 
in  the  upper  tracing. 

§  130.  Concerning  the  other  secondary  waves  on  the  pulse-curve 
such  as  that  which  has  been  called  the '  predicrotic '  wave  {B  on  Fig. 
63  and  on  some  of  the  other  pulse-curves)  it  will  not  be  desirable 
to  say  much  here.  They  have  been  the  occasion  of  much  discus- 
sion, especially  when  considered  under  the  view  that  the  ventricle 
rapidly  emptied  itself  at  the  earlier  part  of  the  systole.  We  will 
content  ourselves  with  the  following  remark.  The  predicrotic 
and  the  other  secondary  waves  in  question  are,  like  the  dicrotic 
wave,  propagated  from  the  heart  towards  the  periphery,  they  are 
seen  in  sphygmograms  taken  from  the  root  of  the  aorta  as  well  as 
from  more  peripheral  arteries,  and  some  are  also  seen  in  tlie  curves 
of  ventricular  pressure.  Some  of  the  features  of  these  secondary 
waves  may  be  due  to  imperfections  in  the  instruments  used,  to 
inertia  and  the  like,  but  the  main  features  undoubtedly  represent 
events  taking  place  in  the  vascular  system  itself.  When  we  com- 
pare the  curve  of  pressure  in  tlie  aorta  with  tliat  in  the  ventricle, 
we  observe  that  up  to  the  dicrotic  notch,  (in  wliat  may  be  called 
the  systolic  part  of  the  pulse-curve,  the  part  which  corresponds  to 
the  systole  of  the  ventricle,  in  contrast  to  the  diastolic  part  which 
follows  and  which  includes  the  dicrotic  wave) ,  the  variations  seen 
in  the  aortic  curve,  the  secondary  waves  of  which  we  are  speaking, 


236  THE  VENOUS   PULSE.  [Book  i. 

are  exactly  reproduced  in  the  ventricular  curve.  And  it  has,  with 
considerable  reason,  been  urged  that  both  in  the  aorta  (and  so  in  the 
other  arteries)  and  in  the  ventricle  they  are  due  to  events  taking 
place  in  the  ventricle,  the  systole  for  instance  not  being  equally 
sustained. 

We  may  further  call  once  to  mind  the  fact  to  which  we  have 
already  called  attention  that,  while  sometimes  the  curve  of  ven- 
tricular pressure  reaches  its  maximum  at  the  very  beginning  of 
the  systole,  declining  more  or  less  slowly  afterwards,  at  other  times 
the  maximum  is  reached  at  the  end  of  the  systole,  the  curve  of 
pressure  being  anacrotic ;  we  may  add  that  the  maximum  may 
also  occur  at  any  intermediate  point.  Further,  and  this  is  the 
matter  to  which  we  wish  to  call  attention,  the  curve  of  aortic 
pressure  follows  that  of  the  ventricular  pressure,  both  being  kata- 
crotic  or  anacrotic  as  the  case  may  be.  As  we  hav^e  urged,  the 
anacrotic  curve  is  seen  when  the  peripheral  resistance  is  such  that, 
for  some  time  during  the  systole,  the  flow  from  the  aorta  towards 
the  periphery  is  slower  than  the  flow  from  the  ventricle  into  the 
aorta.  Such  a  condition  is  apt  to  be  met  with  when  the  arteries 
are  more  rigid  than  normal,  and  under  these  circumstances  the 
anacrotic  characters  of  the  pulse  may  become  prominent. 

§  131.  Venous  Pulse.  Under  certain  circumstances  the  pulse 
may  be  carried  on  from  the  arteries  through  the  capillaries  into  the 
veins.  Thus,  as  we  shall  see  later  on,  when  the  salivary  gland  is 
actively  secreting,  the  blood  may  issue  from  the  gland  through  the 
veins  in  a  rapid  pulsating  stream.  The  nervous  events  which  give 
rise  to  the  secretion  of  saliva,  lead  at  the  same  time,  by  the  agency 
of  vaso-motor  nerves,  of  which  we  shall  presently  speak,  to  a  widen- 
ing of  the  small  arteries  of  the  gland.  When  the  gland  is  at  rest, 
the  minute  arteries  are,  as  we  shall  see,  somewhat  constricted  and 
narrowed,  and  thus  contribute  largely  to  the  peripheral  resistance 
in  the  part ;  this  peripheral  resistance  throws  into  action  the 
elastic  properties  of  the  small  arteries  leading  to  the  gland,  and 
the  remnant  of  the  pulse  reaching  these  arteries  is,  as  we  before 
explained,  finally  destroyed.  When  the  minute  arteries  are  dilated, 
their  widened  channels  allow  the  blood  to  flow  more  easily  through 
them  and  with  less  friction ;  the  peripheral  resistance  which  they 
normally  offer  is  thus  lessened.  In  consequence  of  this  the  elasti- 
city of  the  walls  of  the  small  arteries  is  brought  into  play  to  a 
less  extent  than  before,  and  these  small  arteries  cease  to  do  their 
share  in  destroying  the  pulse  which  comes  down  to  them  from  the 
larger  arteries.  As  in  the  case  of  the  artificial  model,  where  the 
'  peripheral '  tubing  is  kept  open,  not  enough  elasticity  is  brought 
into  play  to  convert  the  intermittent  arterial  flow  into  a  con- 
tinuous one,  and  the  pulse  which  reaches  the  arteries  of  the  gland 
passes  on  through  them  and  through  the  capillaries,  and  is  con- 
tinued on  into  the  veins.  A  similar  venous  pulse  is  also  some- 
times seen  in  other  organs. 


Chap,  iv.]  THE   VASCULAR   MECHANISM.  237 

Careful  tracings  of  the  oreat  veins  in  the  neii^hljourhood  of  the 
heart  shew  elevations  and  depressions,  which  appear  due  to  the 
variations  of  endocardiac  pressure,  and  which  may  perhaps  be 
spoken  of  as  constituting  a  '  venous  pulse,'  though  they  have 
a  quite  different  origin  from  the  venous  pulse  just  described 
in  the  salivary  gland.  In  such  a  pulse  it  is  the  depression  of 
the  wave,  not  the  elevation,  which  corresponds  to  the  systole 
of  the  ventricle,  the  pulse-wave  is  the  negative  of  the  arterial 
pulse-wave ;  the  matter  however  needs  further  study.  In  cases 
again  of  insufficiency  of  the  tricuspid  valves,  the  systole  of  the 
ventricle  makes  itself  distinctly  felt  in  the  great  veins ;  and  an 
expansion  travelling  backwards  from  the  heart  becomes  very 
visible  in  the  veins  of  the  neck.  This,  in  which  the  elevation  of 
the  wave  like  that  of  the  arterial  pulse-wave  corresponds  to  the 
ventricular  systole,  is  also  spoken  of  as  a  venous  pulse. 

Variations  of  pressure  in  the  great  veins  due  to  the  respiratory 
movements  are  also  sometimes  spoken  of  as  a  venous  pulse  ;  the 
nature  of  these  variations  will  be  explained  in  treating  of  respi- 
ration. 


SEC.  5.  THE  REGULATION  AND  ADAPTATION  OF 
THE  VASCULAR  MECHANISM. 


The  Regulation  of  the  Beat  of  the  Heart. 

§  132.  So  far  the  facts  with  which  we  have  had  to  deal, 
with  the  exception  of  the  heart's  beat  itself,  have  been  simply 
physical  facts.  All  the  essential  phenomena  which  we  have 
studied  may  be  reproduced  on  a  dead  model.  Such  an  unvary- 
ing mechanical  vascular  system  would  however  be  useless  r,o  a 
living  body  whose  actions  were  at  all  complicated.  The  promi- 
nent feature  of  a  living  mechanism  is  the  power  of  adapting  itself 
to  changes  in  its  internal  and  external  circumstances.  And  the 
vascular  mechanism  in  all  animals  in  which  it  is  present  is  capable 
of  local  and  general  modifications,  adapting  it  to  local  and  general 
changes  of  circumstance.  These  modifications  fall  into  two  great 
classes : 

1.  Changes  in  the  heart's  beat.  These,  being  central,  have  of 
course  a  general  effect ;  they  influence  or  may  influence  the  whole 
body. 

2.  Changes  in  the  peripheral  resistance,  due  to  variations  in 
the  calibre  of  the  minute  arteries,  brought  about  by  the  agency  of 
their  contractile  muscular  coats.  These  changes  may  be  either 
local,  affecting  a  particular  vascular  area  only,  or  general,  affecting 
all  or  nearly  all  the  blood  vessels  of  the  body. 

These  two  classes  of  events  are  chieiiy  governed  by  the 
nervous  system.  It  is  by  means  of  the  nervous  system  that  the 
heart's  beat  and  the  calibre  of  the  minute  arteries  are  brought 
into  relation  with  each  other,  and  with  almost  every  part  of  the 
body.  It  is  by  means  of  the  nervous  system  acting  either  on 
the  heart,  or  on  tlie  small  arteries,  or  on  both,  that  a  change  of 


Chap,  iv.]  THE   VASCULAR  MECHANISM.  239 

circumstances  affecting  either  the  whole  or  a  part  of  the  body  is 
met  by  compensating  or  regulative  changes  in  the  flow  of  blood. 
The  study  of  these  changes  becomes  therefore  to  a  large  extent 
a  study  of  nervous  actions. 

The  circulation  may  also  be  modified  by  events  not  belonging 
to  either  of  the  above  two  classes.  Thus,  in  this  or  that  peripheral 
area,  changes  in  the  capillary  walls  and  the  walls  of  the  minute 
arteries  and  veins  may  lead  to  an  increase  of  the  tendency  of  the 
blood  corpuscles  to  adhere  to  the  vascular  walls,  and  so,  quite 
apart  from  any  change  in  the  calibre  of  the  blood  vessels,  may 
lead  to  increase  of  the  peripheral  resistance.  This  is  seen  in  an 
extreme  case  in  inflammation,  but  may  possibly  intervene  to  a  less 
extent  in  the  ordinary  condition  of  the  circulation,  and  may  also 
be  under  the  influence  of  the  nervous  system.  Further,  any 
decided  change  in  the  quantity  of  blood  actually  in  circulation 
must  also  influence  the  working  of  the  vascular  mechanism.  But 
both  these  changes  are  unimportant  compared  with  the  other  two 
kinds  of  changes.  Hence,  the  two  most  important  problems  for 
us  to  study  are,  1,  how  the  nervous  system  regulates  the  beat  of 
the  heart,  and  2,  how  the  nervous  system  regulates  the  calibre  of 
the  blood  vessels.     We  will  first  consider  the  former  problem. 


The   Development  of  the   Normal   Beat. 

§  133.  The  heart  of  a  mammal  or  of  a  warm  blooded  animal 
generally  ceases  to  beat  within  a  few  minutes  after  being  removed 
from  the  body  in  the  ordinary  way,  the  hearts  of  newly-born 
animals  continuing  however  to  beat  for  a  longer  time  than  those 
of  adults.  Hence,  though  by  special  precautions  and  by  means  of 
an  artificial  circulation  of  blood,  an  isolated  mammalian  heart  may 
be  preserved  in  a  pulsating  condition  for  a  much  longer  time,  our 
knowledge  of  the  exact  nature  and  of  the  causes  of  the  cardiac 
beat  is  as  yet  very  largely  based  on  the  study  of  the  hearts  of 
cold  blooded  animals,  which  will  continue  to  beat  for  hours,  or 
under  favourable  circumstances  even  for  days,  after  they  have 
been  removed  from  the  body  with  only  ordinary  care.  We  have 
reason  to  think  that  the  mechanism  by  which  the  beat  is  carried 
on  varies  in  some  of  its  secondary  features  in  different  kinds  of 
animals :  that  the  hearts,  for  instance,  of  the  eel,  the  snake,  the 
tortoise  and  the  frog,  differ  in  some  minor  details  of  behaviour, 
both  from  each  other  and  from  those  of  the  bird  and  of  the  mammal ; 
but  we  may,  at  first  at  all  events,  take  the  heart  of  the  frog  as 
illustrating  the  main  and  important  truths  concerning  the  causes 
and  mechanism  of  the  beat. 


240  GRAPHIC   RECORD   OF  HEART  BEAT.     [Book  i. 

In  studying  closely  the  phenomena  of  the  beat  of  the  heart  it  becomes 
necessary  to  obtain  a  graphic  record  of  the  various  movements. 

1.  In  the  frog,  or  other  cold  blooded  animal^  a  light  lever  may  be 
placed  directly  on  the  ventricle  (or  on  an  auricle,  &c.),  and  changes  of 
form,  due  either  to  distension  by  the  influx  of  bloud,  or  to  the  systole, 
will  cause  movements  of  the  lever,  which  may  be  recorded  on  a  travel- 
ling surface.  The  same  method  as  we  have  seen  may  be  applied  to  the 
mammalian  heart. 

2.  Or,  as  in  Gaskell's  method,  the  heart  may  be  fixed  by  a  clamp 
carefully  adjusted  round  the  auriculo-ventricular  groove,  while  the  apex 
of  the  ventricle  and  some  portion  of  one  auricle  are  attached  by  threads 
to  horizontal  levers,  placed  respectively  above  and  below  the  heart. 
The  auricle  and  the  ventricle  each  in  its  systole  pulls  at  the  lever 
attached  to  it ;  and.  the  times  and  extent  of  the  contractions  may  thus 
be  recorded.  Or  the  thread  may  be  attached  to  the  apex  of  the  ven- 
tricle only,  the  heart  being  fixed  by  the  aorta  or  left  in  position  in  the 
body. 

3.  A  record  of  endo-cardiac  pressure  may  be  taken  in  the  frog  or 
tortoise,  as  in  the  mammal,  by  means  of  an  appropriate  manometer. 
And  in  these  animals,  at  all  events,  it  is  easy  to  keep  np  an  artificial 
circulation.  A  cannula  is  introduced  into  the  sinus  venosus,  and  another 
into  the  ventricle  through  the  aorta.  Serum  or  dilute  blood  (or  any 
other  fluid  which  it  may  be  desired  to  employ)  is  driven  by  moderate 
pressure  through  the  former ;  to  the  latter  is  attached  a  tube  connected 
by  means  of  a  side  piece  with  a  small  mercury  or  other  manometer.  So 
long  as  the  exit-tube  is  open  at  the  end,  fluid  flows  freely  through  the 
heart  and  apparatus.  Upon  closing  the  exit-tube  at  its  far  end,  the 
force  of  tlie  ventricular  systole  is  brought  to  bear  on  the  manometer, 
the  index  of  which  registers  in  the  usual  way.  Newell  Martin  has 
succeeded  in  applying  a  modification  of  this  method  to 

the  mammalian  heart. 

4.  The  movements  of  the  ventricle  may  be  regis- 
tered by  introducing  into  it,  through  the  auriculo- 
ventricular  orifice,  a  so-called  '  perfusion  '  cannula,  Figs. 
66  and  67  I.,  with  a  double  tube,  one  inside  the  other, 
and  tying  the  ventricle  on  to  the  cannula  at  the 
auriculo-ventricular  groove,  or  at  any  level  below  that 
which  may  be  desired.  The  blood  or  other  fluid  is 
driven  at  an  adequate  pressure  through  tlie  tube  a, 
enters  the  ventricle,  and  returns  by  the  tube  h.  If  h 
be  connected  with  a  manometer,  as  in  method  3,  the 
movements  of  the  ventricle  may  be  registered.  y^g.  66  A  Per- 
fusion Cannula. 

5.  In  the  apparatus  of  Roy,  Fig.  67  II.,  the  exit- 
tube  is  free,  but  the  ventricle  (the  same  method  may  be  adopted  for  the 
whole  heart)  is  placed  in  an  air-tight  chamber,  filled  with  oil,  or  partly 
with  normal  saline  solution  and  partly  with  oil.  By  means  of  the  tube 
b  the  interior  of  the  chamber  a  is  continuous  with  that  of  a  small  cylinder 
c,  in  which  a  piston  d,  secured  by  thin,  flexible,  animal  membrane,  works 


Chap,  iv.]        THE   VASCULAR  MECHANISM. 


241 


up  and  down.  The  piston  again  bears  on  a  lever  e  by  means  of  which 
its  movements  may  be  registered.  Wlien  tlie  ventricle  contracts,  and 
by  contracting  diminishes  in  volume,  there  is  a  lessening  of  pressure  in 


Fig.  67,    Porelt  diagrammatic  figures  of 

I.  Perfusion  canuula  tied  into  frog's  ventricle,  a,  entrance,  6,  exit-tube ;  a,  wall 
of  ventricle  ;  j8,  ligature. 

II.  lioy's  apparatus  modified  by  Gaskell.  a,  chamber  filled  with  saline  solution 
and  oil,  containiug  the  ventricle  a  tied  on  to  the  profusion  cannula/  h,  tube  leading 
to  cylinder  c,  in  which  moves  piston  d,  working  the  lever  e. 

the  interior  of  the  chamber  ;  this  is  transmitted  to  the  cylinder,  and 
the  piston  correspondingly  rises,  carrying  with  it  the  lever.  As  the 
ventricle  subsequently  becomes  distended,  the  pressure  in  the  chamber 
is  increased,  and  the  piston  and  lever  sink.  In  this  way  variations  in 
the  volume  of  the  ventricle  may  be  recorded,  without  any  great  inter- 
ference with  the  flow  of  blood  or  fluid  through  it. 

The  heart  of  the  frog,  as  we  have  just  said,  will  continue  to 
beat  for  hours  after  removal  from  the  body,  even  though  the  cavi- 
ties have  been  cleared  of  blood,  and,  indeed,  when  they  are  almost 
empty  of  all  fluid.  The  beats  thus  carried  out  are  in  all  import- 
ant respects  identical  with  the  beats  executed  by  the  heart  in  its 
normal  condition  within  the  living  body.  Hence  we  may  infer 
that  the  beat  of  the  heart  is  an  automatic  action :  the  muscular 
contractions  which  constitute  the  beat  are  due  to  causes  which 
arise  spontaneously  in  the  heart  itself. 

In  the  frog's  heart,  as  in  that  of  the  mammal,  §  108,  there  is  a 
distinct  sequence  of  events  which  is  the  same  whether  the  heart  be 
removed  from,  or  be  still  in  its  normal  condition  within  the  body. 
First  comes  the  beat  of  the  sinus  venosus,  preceded  by  a  more  or 
less  peristaltic  contraction  of  the  large  veins  leading  into  it ;  next 
follows  the  sharp  beat  of  the  two  auricles  together ;  then  comes  the 
longer  beat  of  the  ventricle  ;  and  lastly  the  cycle  is  completed  by  the 

16 


242  GRAPHIC   RECORD    OF   HEART   BEAT.     [Book  i. 

beat  of  the  bulbus  arteriosus,  which  does  not,  Hke  the  mammalian 
aorta,  simply  recoil  by  elastic  reaction  after  distension  by  the 
ventricular  stroke  but  carries  out  a  distinct  muscular  contraction 
passing  in  a  wave  from  the  ventricle  outwards. 

When  the  heart  in  dying  ceases  to  beat,  the  several  move- 
ments cease,  as  a  rule,  in  an  order  the  inverse  of  the  above. 
Omitting  the  bulbus  arteriosus,  which  sometimes  exhibits  great 
rhythmical  power,  we  may  say  that  first  the  ventricle  fails,  then 
the  auricles  fail,  and  lastly  the  sinus  venosus  fails. 

The  heart  after  it  has  ceased  to  beat  spontaneously  remains- 
for  some  time  irritable,  that  is  capable  of  executing  a  beat,  or 
a  short  series  of  beats,  when  stimulated  either  mechanically  as 
by  touching  it  with  a  blunt  needle  or  electrically  by  an  induction 
shock  or  in  other  ways.  The  artificial  beat  so  called  forth  may 
be  in  its  main  features  identical  with  the  natural  beat,  all  the 
divisions  of  the  heart  taking  part  in  the  beat,  and  the  sequence 
of  events  being  the  same  as  in  the  natural  beat.  Thus  when  the 
sinus  is  pricked  the  beat  of  the  sinus  may  be  followed  by  a  beat 
of  the  auricles  and  of  the  ventricle ;  and  even  when  the  ventricle 
is  stimulated,  the  directly  following  beat  of  the  ventricle  may  be 
succeeded  by  a  complete  beat  of  the  whole  heart.  Under  certain 
circumstances  however  the  division  directly  stimulated  is  the 
only  one  to  beat ;  when  the  ventricle  is  pricked  for  instance  it 
alone  may  beat,  or  when  the  sinus  is  pricked  it  alone  may  beat. 
The  results  of  stimulation  moreover  may  differ  according  to  the 
condition  of  the  heart  and  according  to  the  particular  spot  to 
which  the  stimulus  is  applied. 

With  an  increasing  loss  of  irritability,  the  response  to  stimu- 
lation ceases  in  the  several  divisions  in  the  same  order  as  that  of 
the  failure  of  the  natural  beat ;  the  ventricle  ceases  to  respond 
first,  then  the  auricles,  and  lastly  the  sinus  venosus,  which  fre- 
quently responds  to  stimulation  long  after  the  other  divisions 
have  ceased  to  make  any  sign. 

It  would  appear  as  if  the  sinus  venosus,  auricles,  and  ventricle 
formed  a  descending  series  in  respect  to  their  irritability  and  to 
the  power  they  possess  of  carrying  on  spontaneous  rhythmic 
beats,  the  sinus  being  the  most  potent.  This  is  also  seen  in  the 
following  experiments. 

In  order  that  the  frog's  heart  may  beat  after  removal  from  the 
body  with  the  nearest  approach  in  rapidity,  regularity,  and  en- 
durance to  the  normal  condition,  the  removal  must  be  carried  out 
so  that  the  excised  heart  still  retains  the  sinus  venosus  intact. 

When  the  incision  is  carried  through  the  auricles  so  as  to 
leave  the  sinus  venosus  behind  in  the  body,  the  sinus  venosus 
beats  forcibly  and  regularly,  having  suffered  hardly  any  inter- 
ruption from  the  operation ;  but  the  auricles  and  ventricle  remain 
motionless,  often  for  a  considerable  time,  and  when  they  do  re- 
sume spontaneous  beats  these  have  a  rhythm  different  from  that 
of  the  sinus,  and  are  less  vigorous  and  lasting  than  those  of  the 


Chap,  iv.]  THP:   VASCULAR   MECHANISM.  243 

entire  heart.  If  the  incision  be  carried  between  the  auricles  and 
ventricle,  the  former  with  the  sinus  beat  regularly  and  forcibly 
while  the  latter  often  exhibits  no  spontaneous  beats  at  all,  or  if 
these  do  appear  they  last  for  a  short  time  only.  Lastly  if  the 
ventricle  be  cut  across  leaving  the  upper  third  attached  to  the 
auricles,  this  beats  regularly  with  the  sinus  and  auricles,  but 
tlie  detached  lower  two-thirds  do  not  beat  spontaneously  at  all. 

Now,  while  ganglia  are  abundant  over  the  sinus,  are  numer- 
ous over  the  auricles,  and,  as  Bidder's  ganglia,  are  present  at  the 
auriculo-ventricular  junction,  no  nerve  cells  are  present  in  the 
lower  part  of  the  ventricle.  Hence  the  view  has  suggested  itself 
that  the  rhythmic  spontaneous  beating  is  due  to  impulses  pro- 
ceeding rhythmically  from  the  nerve  cells  of  the  ganglia.  But 
serious  objections  may  be  urged  against  this  view.  Even  in  the 
case  of  the  frog,  the  lower  part  of  the  ventricle,  the  mere  tip 
almost,  may  under  specially  favourable  circumstances  beat  in  an 
apparently  spontaneous  manner ;  this  occurs  when  it  is  tied 
round  the  end  of  a  perfusion  cannula  (Fig.  66),  and  fed  with  blood 
or  serum  at  a  somewhat  high  pressure.  And  in  the  case  of  the 
tortoise  a  mere  strip  of  muscle,  quite  free  from  nerve  cells,  cut 
out  of  the  ventricle  may  be  made  to  beat,  in  an  apparently  spon- 
taneous manner,  with  great  regularity  for  a  considerable  time. 

Without  entering  into  any  lengthy  discussion  concerning  a 
matter  which  is  and  has  been  much  debated,  we  may  say  that 
the  cardiac  muscular  fibre  differs  in  properties,  as  it  does  in  struc- 
ture, from  the  skeletal  muscular  fibre,  that  it  is  not  to  the  same 
extent  as  that,  a  mere  instrument,  so  to  speak,  in  the  hands  of  a 
motor  nerve  fibre,  but  has,  itself,  largely  to  do  with  originating 
its  own  contraction.  The  muscular  contraction,  we  may  here 
observe,  of  which  the  beat  is  a  development,  is  not  a  tetanus, 
but  a  somewhat  long  continued  simple  contraction.  This  may 
be  readily  shewn  to  be  the  case  on  the  slip  of  the  tortoise  ven- 
tricle just  referred  to.  Such  a  slip,  when  attached  to  a  lever,  and 
stimulated  with  a  single  induction  shock,  gives  what  is  obviously 
a  simple  contraction,  and  a  beat  of  the  slip  occurring  naturally 
has  exactly  the  same  features.  And  the  electric  change  shewn 
at  any  part  of  the  heart  during  a  beat  natural  or  induced  by 
stimulation  is  that  characteristic  of  a  simple  contraction.  The 
intact  ventricle  at  rest  is  as  we  have  already  said  (§  63)  isoelec- 
tric, but  each  part  just  as  it  is  entering  into  a  state  of  contraction 
becomes  negative  towards  the  rest.  Hence  when  the  electrodes 
of  a  galvanometer  are  placed  on  two  points  A,  B  oi  the  surface 
of  the  ventricle  a  diphasic  variation  of  the  galvanometer  needle 
is  seen  when  a  beat,  natural  or  excited,  occurs.  Supposing  that 
the  wave  of  contraction  reaches  A  first,  this  will  become  negative 
towards  the  rest  of  the  ventricle,  including  B,  but  when  the  wave 
sometime  afterwards  reaches  B,  B  will  become  negative  towards 
the  rest  of  the  ventricle,  including  A.     Compare  §  64. 

But  the  contraction  of  cardiac  muscle  differs  from  that  of  a 


244       FEATURES   OF   CARDIAC   CONTRACTION.     [Book  i. 

skeletal  muscle  in  the  following  important  feature.  When  we 
stimulate  a  skeletal  muscle  with  a  strong  stimulus  we  get  a  large 
contraction,  when  we  apply  a  weak  stimulus  we  get  a  small 
contraction ;  within  certain  limits  (see  §  74)  the  contraction  is 
proportional  to  the  stimulus.  This  is  not  the  case  with  the  qui- 
escent ventricle  or  heart.  When  we  apply  a  strong  induction- 
shock  we  get  a  beat  of  a  certain  strength ;  if  we  now  apply  a 
weak  shock  we  get  either  no  beat  at  all  or  quite  as  strong  a  beat 
as  with  a  stronger  stimulus.  That  is  to  say  the  magnitude  of 
the  beat  depends  on  the  condition  of  the  ventricle  (or  heart)  and 
not  on  the  magnitude  of  the  stimulus.  If  the  stimulus  can  stir 
the  ventricle  up  to  beat  at  all,  the  beat  is  the  best  which  the 
ventricle  can  at  the  time  accomplish  ;  the  stimulus  produces 
either  its  maximum  effect  or  none  at  all.  It  would  seem  as  if 
the  stimulus  does  not  produce  a  contraction  in  the  same  way 
that  it  does  when  it  is  brought  to  bear  on  a  skeletal  muscle,  but 
rather  stirs  up  the  heart  in  such  a  way  as  to  enable  it  to  execute 
a  spontaneous  beat  which,  without  the  extra  stimulus,  it  could 
not  bring  about.  And  we  have  reason  to  think  that  the  normal 
beat  of  the  heart  within  the  body  is  the  maximum  beat  of  which 
it  is  capable  at  the  moment.  These  and  other  special  features  of 
the  contraction  of  cardiac  muscle  lead  to  the  conclusion  that  the 
rhythmic  power  does  not  reside  wholly  in  the  ganglia ;  but  we 
must  not  here  discuss  the  question  further,  nor  enter  upon  the 
difficult  problem  of  how  the  remarkable  sequence  in  contraction 
of  the  several  parts  is  developed  and  as  a  rule  maintained. 

§  134.  In  the  above  we  have  dealt  chiefly  with  the  heart  of 
the  cold  blooded  animal,  but  so  far  as  we  know  the  same  general 
conclusions  hold  good  for  the  mammalian  heart  also.  There  is,  it 
is  true,  in  the  mammal,  no  prepotent  sinus  venosus,  but  as  in  the 
frog  the  auricles  are  dominant,  and  their  beat  determines  the  beat 
of  the  ventricles.  Even  more  clearly  than  in  the  frog  however, 
the  ventricles,  though  they  normally  follow  the  aviricles  in  their 
beat,  being  initiated  as  it  were  by  them,  possess  an  independent 
rhythmic  power  of  their  own.  By  a  mechanical  contrivance  all 
conduction  of  nervous  or  muscular  impulses  between  the  auricles 
and  ventricles  may  be  abolished,  though  the  blood  may  continue  to 
flow  from  the  cavities  of  the  former  to  those  of  the  latter.  When 
this  is  done  the  ventricles  go  on  beating  rhythmically,  but  at  a 
rate  which  is  quite  independent  of  that  of  the  auricular  beats. 

We  may  now  turn  to  the  nervous  mechanisms  by  which  the 
beat  of  the  heart,  thus  arising  spontaneously  within  the  tissues  of 
the  heart  itself,  is  modified  and  regulated  to  meet  the  require- 
ments of  the  rest  of  the  body. 

9 

The  Government  of  the  Heart  Beat  hj  the  Nervous  System. 

§  135.  It  will  be  convenient  to  begin  with  the  heart  of  the 
frog.    This  is  connected  with  the  central  nervous  system  through, 


Chap,  iv.j  THE   VASCULAR  MECHANISM. 


245 


and  therefore  governed  by,  the  two  vagus  nerves,  each  of  which 
though  apparently  a  single  nerve  contains,  as  we  shall  see,  fibres 
of  different  origin  and  nature. 

If  while  the  beats  of  the  heart  of  a  frog  are  being  carefully 
registered  an  interrupted  current  of  moderate  strength  be  sent 
through  the  vagus  nerve,  the  heart  is  seen  to  stop  beating.  It 
remains  for  a  time  in  diastole,  perfectly  motionless  and  flaccid  ; 
all  the  muscular  fibres  of  the  several  chambers  are  for  the  time 
being  in  a  state  of  relaxation.  The  heart  has  been  inhibited  by 
the  impulses  descending  the  vagus  from  the  part  of  the  nerve 
stimulated. 

If  the  duration  of  the  stimulation  be  short  and  the  strength  of 
the  current  great,  the  standstill  may  continue  after  the  current  has 
been  shut  off;  the  beats,  when  they  reappear,  are  generally  at  first 
feeble  and  infrequent,  but  soon  reach  or  even  go  beyond  their 
previous  vigour  and  frequency.  If  the  duration  of  the  stimulation 
be  very  long,  the  heart  may  recommence  beating  while  the  stimula- 
tion is  still  going  on,  but  the  beats  are  feeble  and  infrequent 
though  gradually  increasing  in  strength  and  frequency.  The  effect 
of  the  stimulation  is  at  its  maximum  at  or  soon  after  the  com- 
mencement of  the  application  of  the  stimulus,  gradually  declining 
afterwards ;  but  even  at  the  end  of  a  very  prolonged  stimulation 
the  beats  may  still  be  less  in  force  or  in  frequency,  or  in  both,  than 
they  were  before  the  nerve  was  stimulated,  and  on  the  removal  of 
the  current  may  shew  signs  of  recovery  by  an  increase  in  force  and 
frequency.  The  effect  is  not  produced  instantaneously  ;  if  on  the 
curve  the  point  be  exactly  marked  when  the  current  is  thrown 
in,  as  at  on  Fig.  68,  it  will  frequently  be  found  that  one  beat  at 


off 


AAAAi' 


Fig.  68.   Inhibition  op  Frog's  Heart  by  stimulation  of  Vagus  Nerve. 


on  marks  the  time  at  which  the  interrupted  current  was  thrown  into  tlie  vagus, 
o/T'whcn  it  was  slint  off.  The  time  marker  below  marks  seconds.  The  beats  were 
roi;;ist(^rcd  by  sns])eiidiug  the  ventricle  from  a  clamp  attached  to  tiie  aorta  and 
attacliini;  a  li_c;ht  lever  to  the  tip  of  the  ventricle. 

least  occurs  after  the  current  has  passed  into  the  nerve ;  the 
development  of  that  beat  has  taken  place  before  the  impulses 
descending  the  vagus  have  had  time  to  affect  the  heart. 


246  AUGMENTATION   OF   THE   BEAT.  [Book  i. 

The  stimulus  need  not  necessarily  be  the  interrupted  current ; 
mechanical,  chemical  or  thermal  stimulation  of  the  vagus  will 
also  produce  inhibition  ;  but  in  order  to  get  a  marked  effect  it  is 
desirable  to  make  use  of  not  a  single  nervous  impulse  but  a  series 
of  nervous  impulses ;  thus  it  is  difficult  to  obtain  any  recognisable 
result  by  employing  a  single  induction  shock  of  moderate  intensity 
only.  As  we  shall  see  later  on  '  natural '  nervous  impulses  descend- 
ing the  vagus  from  the  central  nervous  system,  and  started  there, 
by  afferent  impulses  or  otherwise,  as  parts  of  a  reflex  act,  may 
produce  inhibition. 

The  stimulus  may  be  applied  to  any  part  of  the  course  of  the 
vagus  from  high  up  in  the  neck  right  down  to  the  sinus  ;  indeed, 
very  marked  results  are  obtained  by  applying  the  electrodes 
directly  to  the  sinus  where  as  we  have  seen  the  two  nerves  plunge 
into  the  substance  of  the  heart.  The  stimulus  may  also  be  applied 
to  either  vagus,  though  in  the  frog,  and  some  other  animals,  one 
vagus  is  sometimes  more  powerful  than  the  other.  Thus  it  not 
unfrequently  happens  that  even  strong  stimulation  of  the  vagus  on 
one  side  produces  no  change  of  the  rhythm,  while  even  moderate 
stimulation  of  the  nerve  on  the  other  side  of  the  neck  brings  the 
heart  to  a  standstill  at  once. 

If  during  the  inhibition  the  ventricle  or  other  part  of  the  heart 
be  stimulated  directly,  for  instance  mechanically  hj  the  prick  of  a 
needle,  a  beat  may  follow;  that  is  to  say,  the  impulses  descending 
the  vagus,  while  inhibiting  the  spontaneous  beats,  have  not  wholly 
abolished  the  actual  irritability  of  the  cardiac  tissues. 

With  a  current  of  even  moderate  intensity,  such  a  current  for 
instance  as  would  produce  a  marked  tetanus  of  a  muscle-nerve 
preparation,  the  standstill  is  complete,  that  is  to  say,  a  certain 
number  of  beats  are  entirely  dropped ;  but  with  a  weak  current 
the  inhibition  is  partial  only,  the  heart  does  not  stand  absolutely 
still  but  the  beats  are  slowed,  the  intervals  between  them  being 
prolonged,  or  weakened  only  without  much  slowing,  or  both 
slowed  and  weakened.  Sometimes  the  slowing  and  sometimes 
the  weakening  is  the  more  conspicuous  result. 

§  136.  It  sometimes  happens  that,  when  in  the  frog  the  vagus 
is  stimulated  in  the  neck,  the  effect  is  very  different  from  that 
just  described  ;  for  the  beats  are  increased  in  frequency,  though 
they  may  be  at  first  diminished  in  force.  And,  occasionally,  the 
beats  are  increased  both  in  force  and  in  frequency  :  the  result 
is  augmentation,  not  inhibition.  But  this  is  due  to  the  fact  that 
in  the  frog  the  vagus  along  the  greater  part  of  its  course  is  a  mixed 
nerve  and  contains  fibres  other  than  those  of  the  vagus  proper. 

If  we  examine  the  vagus  nerve  closely,  tracing  it  up  to  the 
brain,  we  find  that  just  as  the  nerve  has  pierced  the  cranium, 
just  where  it  passes  through  the  ganglion  (GV,  Fig.  69),  certain 
fibres  pass  into  it  from  the  sympathetic  nerve  of  the  neck,  Sy,  of 
the  further  connections  of  which  we  shall  speak  presently. 


Chap,  iv.]  THE   VASCULAR   MECHANISM. 


247 


This  being  the  case,  we  may  expect  that  we  should  get  difi'erent 
results  according  as  we  stimulated  (1)  the  vagus  in  the  cranium, 


Fig. 


69.      Diagrammatic     Eepkesextation     of    the     course 
AiTGMENTOK   Fibres   in  the   Frog. 


OF    Cardiac 


Vr.  roots  of  vagus  (and  ixth)  nerve.  GV.  ganglion  of  same.  Cr.  line  of  cranial 
wall.  Vg.  vagus  trunk,  ix.  ninth,  glosso-pharyngeal  nerve.  S.V.C.  superior  vena 
cava.  Sy.  sympathetic  nerve  in  neck.  G.C.  junction  of  sympathetic  ganglion  with 
vagus  ganglion,  sending  i.e.  intracranial  fibres  passing  to  Gasserian  ganglion.  The 
rest  of  the  filires  pass  along  the  vagus  trunk.  6'-'^  sym])atiietic  ganglion  connected 
with  the  first  spinal  nerve.  G"^  sympathetic  ganglion  of  the  second  spinal  nerve. 
An.V.  annulus  of  Vieussens.  A.sh.  subclavian  artery.  6"^^^  sympathetic  ganglion  of 
the  third  spinal  nerve.     ///.  third  spinal  nerve,     r.c.  ramus  communicans. 

The  course  of  the  augmentor  fibres  is  shewn  by  the  thick  black  line.  They  may 
be  traced  from  the  spinal  cord  by  the  anterior  root  of  tiie  third  spinal  nerve,  through 
the  ramus  communicans  to  the  corresponding  sympatlietic  ganglion  6'^^^  and  thence 
by  the  second  ganglion  G",  the  annulus  of  Vieussens,  and  the  first  ganglion  G'  to 
tiie  cervical  sympathetic  Si/,  and  so  bv  the  vagus  trunk  to  the  superior  vena  cava 
S.V.C. 

before  it  was  joined  by  the  sympathetic,  (2)  the  sympathetic  fibres 
before  they  join  the  vagus,  and  (3)  the  vagus  trunk,  containing  both 
the  real  vagus  and  the  sympathetic  fibres.  AVhat  we  have  pre- 
viously described  are  the  ordinary  results  of  stimulating  the  mixed 


248  AUGMENTATION   OF   THE   BEAT.  [Book  r, 

trunk,  and  these,  as  we  have  said,  are  not  wholly  constant,  though, 
usually  and  in  the  main,  most  distinct  inhibitory  results  follow. 

If  we  stimulate  the  sympathetic  in  the  neck  as  at  Si/,  Fig.  69, 
cutting  the  nerve  below  so  as  to  block  all  impulses  from  passing 
downwards,  and  only  allow  impulses  to  pass  up  to  the  vagus  and 
thence  down  the  mixed  vagus  trunk  to  the  heart,  we  get  very 
remarkable  results.  The  beat  of  the  heart  instead  of  being  inhib- 
ited is  augmented,  the  beats  are  increased  either  in  frequency  or  in 
force,  or  most  generally  both  in  frequency  and  in  force.  The  effect 
is  perhaps  best  seen  when  the  heart  before  stimulation  is  beating 
slowly  and  feebly ;  upon  stimulation  of  the  cervical  sympathetic 
the  beats  at  once  improve  in  vigour  and  frequency ;  indeed,  a  heart 
which  for  one  reason  or  another  has  almost  ceased  to  beat  may, 
by  proper  stimulation  of  the  sympathetic,  be  called  back  into 
vigorous  activity. 

If,  on  the  other  hand,  we  stimulate  the  vagus  before  it  has  been 
joined  by  the  sympathetic  fibres  (and  to  ensure  the  result  not 
being  marred  by  any  escape  of  the  stimulating  current  on  to  the 
sympathetic  fibres  it  is  necessary  to  stimulate  the  vagus  within  the 
cranium)  we  get  pure  and  constant  inhibitory  results,  the  beats  are 
for  a  time  wholly  abolished,  or  are  slowed,  or  are  weakened,  or  are 
both  slowed  and  weakened. 

Obviously,  then,  the  heart  of  the  frog  is  supplied  through  the 
vagus  by  two  sets  of  fibres  coming  from  the  central  nervous  system, 
the  one  by  the  vagus  proper  and  the  other  by  the  cervical  sym- 
pathetic nerve,  and  these  two  sets  have  opposite  and  antagonistic 
effects  upon  the  heart. 

The  one  set,  those  belonging  to  the  vagus  proper,  are  inhibitory; 
they  weaken  the  systole  and  prolong  the  diastole,  the  effect  with  a 
strong  stimulation  being  complete,  so  that  the  heart  is  for  a  time 
brought  to  a  standstill.  Sometimes  the  slowing,  sometimes  the 
weakening  is  the  more  prominent.  When  the  nerve  and  the  heart 
are  in  good  condition,  it  needs  only  a  slight  stimulus,  a  weak 
current,  to  produce  a  marked  effect,  and  it  may  be  mentioned  that 
the  more  vigorous  the  heart,  the  more  rapidly  it  is  beating,  the 
easier  is  it  to  bring  about  inhibition.  Although,  as  we  have  said, 
the  effect  is  at  its  maximum  soon  after  the  beginning  of  stimula- 
tion, a  very  prolonged  inhibition  may  be  produced  by  prolonged 
stimulation ;  indeed,  by  rhythmical  stimulation  of  the  vagus  the 
heart  may  be  kept  perfectly  quiescent  for  a  very  long  time  and 
yet  beat  vigorously  upon  the  cessation  of  the  stimulus.  In  other 
words,  the  instruments  of  inhibition,  that  is,  the  fibres  of  the  vagus 
and  the  part  or  substance  of  the  heart  upon  which  these  act  to 
produce  inhibition,  whatever  that  part  or  substance  may  be,  are 
not  readily  exhausted.  Further,  the  inhibition  when  it  ceases  is, 
frequently  at  all  events,  followed  by  a  period  of  reaction,  during 
which  the  heart  for  a  while  beats  more  vigorously  and  rapidly 
than    before.     Indeed  the  total  effect  of   stimulating*  the  vagus 


Chap,  iv.]  THE   VASCULAR   MECHANISM.  249 

fibres  is  not  to  exhaust  the  heart,  but  rather  to  strengthen  it ;  and 
by  repeated  inhibitions  carefully  administered,  a  feebly  beating 
heart  may  be  nursed  into  vigorous  activity. 

The  other  set,  those  joining  the  vagus  from  the  sympathetic, 
are  'augmentor'  or  'accelerating'  fibres;  the  latter  name  is  the 
more  common  but  the  former  is  more  accurate,  since  the  effect  of 
stimulating  these  fibres  is  to  increase  not  only  the  rapidity  but 
the  force  of  the  beat ;  not  only  is  the  diastole  shortened  but  the 
systole  is  strengthened,  sometimes  the  one  result  and  sometimes 
the  other  being  the  more  prominent.  These  augmentor  fibres 
need  a  somewhat  strong  stimulation  to  produce  an  effect,  the  time 
required  for  the  maximum  effect  to  be  produced  is  long,  and  the 
effect,  when  produced,  may  last  for  some  time.  A  slowly  or 
weakly  beating  heart  is  more  easily  augmented  than  is  a  strong 
one.  Further,  the  augmentation  is  followed  by  a  period  of  reac- 
tion in  which  the  beats  are  feebler,  by  a  stage  of  exhaustion ; 
and  indeed  by  repeated  stimulation  of  these  sympathetic  fibres  a 
fairly  vigorous  heart,  especially  a  bloodless  one,  may  be  reduced 
to  a  very  feeble  condition. 

By  watching  the  effects  of  stimulating  the  sympathetic  nerve 
at  various  points  of  its  course  we  may  trace  these  augmentor 
fibres  from  their  junction  with  the  vagus  down  the  short  sympa- 
thetic of  the  neck  through  the  sympathetic  ganglion  connected 
with  the  first  spinal  nerve,  G^,  Fig.  69,  through  one  or  both  the 
loops  of  the  annulus  of  Vieussens,  An.  V,  through  the  second 
ganglion,  connected  with  the  second  spinal  nerve,  G'\  to  the  third 
ganglion  connected  with  the  third  spinal  nerve,  G^^^,  and  thence 
through  the  ramus  communicans  or  visceral  branch  of  that 
ganglion,  r.c,  to  the  third  spinal  nerve,  ///,  by  the  anterior  root 
of  wliich  they  reach  the  spinal  cord. 

§  137.  Both  sets  of  fibres,  then,  may  be  traced  to  the  central 
nervous  system  ;  and  we  find  accordingly  that  the  heart  may  be 
inhibited  or  augmented  by  nervous  impulses  which  are  started  in 
the  nervous  system  either  by  afferent  impulses  as  part  of  a  reflex 
act  or  otherwise,  and  which  pass  to  the  heart  by  the  inhibitory  or 
by  the  augmenting  tract. 

Thus  if  the  spinal  bulb  or  a  particular  part  of  the  spinal  bulb 
which  is  specially  connected  with  the  vagus  nerve  be  stimulated, 
the  heart  is  inhibited ;  if,  for  instance,  a  needle  be  thrust  into 
this  part  the  heart  stands  still.  This  nervous  area  may  be 
stirred  to  action,  in  a  'reflex'  manner,  by  afferent  impulses 
reaching  it  from  various  parts  of  the  body.  Thus  if  the  abdomen 
of  a  frog  be  laid  bare,  and  the  intestine  be  struck  sharply  with  the 
handle  of  a  scalpel,  the  heart  will  stand  still  in  diastole  with  all 
the  phenomena  of  vagus  inhibition.  If  the  nervi  mcsentcrici  or 
the  connections  of  these  nerves  with  the  spinal  cord  be  stimulated 
with  the  interrupted  current,  cardiac  inhibition  is  similarly  pro- 
duced.    If  in  these  two  experiments  both  vagi  are  divided,  or  the 


250  INHIBITION   IN   THE   MAMMAL.  [Book  i. 

spinal  bulb  is  destroyed,  inhibition  is  not  produced,  however  much 
either  the  intestine  or  the  mesenteric  nerves  be  stimulated.  This 
shews  that  the  phenomena  are  caused  by  impulses  ascending 
along  the  mesenteric  nerves  to  the  spinal  bulb,  and  so  affecting  a 
portion  of  that  organ  as  to  give  rise  by  reflex  action  to  impulses 
which  descend  the  vagus  nerve  or  nerves  as  inhibitory  impulses. 
The  portion  of  the  spinal  bulb  thus  mediating  between  the  afferent 
and  efferent  impulses  may  be  spoken  of  as  the  cardio-inliibitory 
centre.  This  centre  may  be  thrown  into  activity,  and  so  inhibition 
produced,  by  afferent  impulses  reaching  it  along  various  nerves ; 
by  means  of  it  reflex  inhibition  through  one  vagus  may  be  brought 
about  by  stimulation  of  the  central  end  of  the  other. 

And  we  have  reason  to  think  that  in  a  similar  manner 
augmentor  impulses  are  developed  in  the  central  nervous  system 
either  as  part  of  a  reflex  chain  or  otherwise. 

§  138.  So  far  we  have  been  dealing  with  the  heart  of  the 
frog,  but  the  main  facts  w^hich  we  have  stated  regarding  inhi- 
bition and  augmentation  of  the  heart  beat  apply  also  to  other 
vertebrate  animals  including  mammals,  and,  indeed,  we  meet 
similar  phenomena  in  the  hearts  of  invertebrate  animals. 

If  in  a  mammal  the  heart  be  exposed  to  view  by  opening  the 
thorax,  and  the  vagus  nerve  be  stimulated  in  the  neck,  the  heart 
may  be  seen  to  stand  still  in  diastole,  with  all  the  parts  flaccid 
and  at  rest.  If  the  current  employed  be  too  weak,  the  result,  as 
in  the  frog,  is  not  an  actual  arrest  but  a  slowing  or  weakening  of 
the  beats.  By  placing  a  light  lever  on  the  heart  or  by  other 
methods,  a  graphic  record  of  the  standstill,  or  of  the  slowing,  of 
the  complete  or  incomplete  inhibition  may  be  obtained.  The 
result  of  stimulating  the  vagus  is  also  well  shewn  on  the  blood 


Fig.  70.     Tracing,  shewing  the  influence  of  Cardiac  Inhibition  on  Blood 
Pressure.     From  a  Rabbit. 

X  the  marks  on  the  signal  line  when  tlie  current  is  thrown  into,  and  y  shut  off 
from  the  vagus.  The  time  marker  below  marks  seconds,  the  heart,  as  is  frequently 
the  case  in  the  rabbit,  beating  very  rapidly. 


Chap,  iv.]  THE   VASCULAR   MECHANISM.  251 

pressure  curve,  the  effect  of  complete  cardiac  inhibition  on  blood 
pressure  being  most  striking.  If,  while  a  tracing  of  arterial 
pressure  is  being  taken,  the  beat  of  the  heart  be  suddenly 
arrested  by  vagus  stimulation,  some  such  curve  as  that  represented 
in  Fig.  70  will  be  obtained.  It  will  be  observed  that  two  beats 
follow  the  application  of  the  current  marked  by  the  point  a, 
which  corresponds  to  the  signal  x  on  the  line  below.  Then  for  a 
space  of  time  no  beats  at  all  are  seen,  the  next  beat  h  taking 
place  almost  immediately  after  the  shutting  off  the  current  at  y. 
Immediately  after  the  last  beat  following  a,  there  is  a  sudden  fall 
of  the  blood  pressure.  At  the  pulse  due  to  the  last  systole,  the 
arterial  system  is  at  its  maximum  of  distention ;  forthwith  the 
elastic  reaction  of  the  arterial  walls  propels  the  blood  forward  into 
the  veins,  and,  there  being  no  fresh  fluid  injected  from  the  heart, 
the  fall  of  the  mercury  is  unbroken,  being  rapid  at  first,  but 
slower  afterwards,  as  the  elastic  force  of  the  arterial  walls  is 
more  and  more  used  up.  With  the  returning  beats  the  pressure 
correspondingly  rises  in  successive  leaps  until  the  normal  mean 
pressure  is  regained.  The  size  of  these  returning  leaps  of  the 
mercury  may  seem  disproportionately  large,  but  it  must  be  re- 
membered that  by  far  the  greater  part  of  the  force  of  the  first 
few  strokes  of  the  heart  is  expended  in  distending  the  arterial 
system,  a  small  portion  only  of  the  blood  which  is  ejected  into  the 
arteries  passing  on  into  the  veins.  As  the  arterial  pressure  rises, 
more  and  more  blood  passes  at  each  beat  through  the  capillaries, 
and  the  rise  of  the  pressure  at  each  beat  becomes  less  and  less, 
until  at  last  the  whole  contents  of  the  ventricle  pass  at  each 
stroke  into  the  veins,  and  the  mean  arterial  pressure  is  established. 
To  this  it  may  be  added,  that,  as  we  have  seen,  the  force  of  the 
individual  beats  may  be  somewhat  greater  after  than  before  inhi- 
bition. Besides,  when  the  mercury  manometer  is  used,  the  inertia 
of  the  mercury  tends  to  magnify  the  effects  of  the  initial  beats. 

The  above  is  an  example  of  complete  inhibition,  of  a  total  stand- 
still for  a  while  of  the  whole  heart,  such  as  may  be  obtained  by 
powerful  stimulation  of  the  vagus  ;  both  auricles  and  ventricles 
remain  for  a  period  free  from  all  contractions  ;  and  as  the 
previously  existing  arterial  pressure  drives  the  blood  onward  from 
the  arteries  through  the  capillaries  and  veins  towards  the  heart, 
the  cavities  of  the  heart  become  distended  with  blood,  especially 
on  the  right  side. 

A  weaker  stimulation  of  the  vagus  produces  an  incomplete 
inhibition,  the  heart  continues  to  beat  but  with  a  different 
rhythm  and  stroke,  and  by  careful  observation  many  interesting 
features  may  be  observed.  If  a  record  be  obtained,  by  one  or 
other  of  the  methods  mentioned  in  §  113  or  elsewhere,  of  the 
behaviour  of  the  auricles  and  ventricles  respectively,  it  will  be 
observed  that  the  inhibition  tells  much  more  on  the  auricles  than 
on  the  ventricles.     The  extent  of  the  auricular  contractions  is 


252  INHIBITION    IN   THE   MAMMAL.  [Book  i. 

especially  affected,  more  so  than  that  of  the  ventricles,  and  it  may 
sometimes  be  observed  that  the  auricles  are  brought  to  complete 
quiescence  while  the  ventricles  still  continue  to  beat ;  the  latter 
now  exhibit  that  independent  rhythm  of  which  we  spoke  in  §  134.' 
In  a  somewhat  similar  manner  the  stimulation  of  the  vagus,  by 
affecting  the  rhythm  of  the  auricles  more  than  that  of  the  ventricles, 
may  lead  to  a  want  of  coordination  between  the  two,  the  especially 
slowed  auricles  beating  at  one  rate,  the  ventricles  at  another. 
It  is  indeed  maintained  by  some  that  the  vagus  acts  directly  on 
the  auricles  only,  the  changes  in  the  ventricles  being  of  a  secondary 
nature,  caused  by  the  changes  in  the  auricles. 

When  the  output  from  the  ventricles  during  vagus  stimulation 
is  measured,  by  the  cardiometer  or  otherwise,  it  is  found,  as  might 
be  expected,  that  this  is  lessened.  The  diminution  during  a  given 
period  may  be  due  to  the  mere  slowing  of  the  beat;  but  the 
individual  pulse  volume  is  in  some  cases,  at  least,  also  lessened. 
It  may  by  the  same  method  be  observed  that  the  quantity  remain- 
ing in  the  ventricle  at  the  end  of  the  systole  is  increased ;  the 
ventricle  appears  to  expand  more  during  diastole.  Of  the  effects 
thus  produced  on  the  circulation  we  shall  speak  later  on. 

We  may  now  turn  to  some  further  details  concerning  the 
course  of  these  inhibitory  fibres.  They  run  in  the  trunk  of  the 
vagus ;  this  is  clear  not  only  in  the  case  of  an  animal  like  the 
rabbit,  in  which  the  vagus  runs  separate  from  the  cervical  sym- 
pathetic but  also  in  the  case  of  the  dog,  in  which  the  two  nerves 
are  more  or  less  bound  up  together.  Leaving  the  vagus  by  the 
cardiac  branches,  they  reach  the  cardiac  tissues  by  the  cardiac 
plexuses.  When  we  trace  the  fibres  in  the  other  direction  to- 
wards the  central  nervous  system,  we  have  to  bear  in  mind  that 
the  fibres  which  compose  the  trunk  of  the  vagus  have,  as  we  shall 
see  in  studying  the  central  nervous  system,  two  distinct  central 
orif^ins.  On  the  one  hand,  there  are  the  fibres  which  are  the 
proper  vagus  fibres  which,  leaving  the  spinal  bulb,  pass  through 
both  the  jugular  ganglion  and  trunk  ganglion  (Fig.  71  r.  GJ. 
G.  Tr.  Vg.).  On  the  other  hand,  there  are  fibres  which,  belonging 
to  the  spinal  accessory  nerve  {Bip.  Ac.)  and  to  what  we  shall  learn 
to  speak  of  as  the  bulbar  division  of  that  nerve,  pass  after  leaving 
the  spinal  bulb  to  the  trunk  ganglion  of  the  vagus,  and  thence 
form  part  of  the  vagus  trunk.  Now,  it  is  these  fibres  of  the  spinal 
accessory  nerve  and  not  the  proper  vagus  fibres  which  supply  the 
inhibitory  fibres  to  the  heart.  Thus,  if  the  bulbar  roots  of  the 
spinal  accessory  be  divided,  those  of  the  vagus  proper  being  left 
intact,  the  spinal  accessory  fibres  in  the  vagus  trunk  degenerate, 
and  when  this  has  taken  place  stimulation  of  the  vagus  fails  to 
produce  the  ordinary  inhibitory  effect. 

Within  the  spinal  bulb  these  inhibitory  fibres  are  connected, 
in  the  mammal  as  in  the  frog,  with  a  cardio-inhibitory  centre  ;  and 
in  the  mammal  as  in  the  frog  inhibition  may  be  brought  about 


Chap,  iv.]  THE   VASCULAR  MECHANISM.  253 

not  only  by  artificial  stimulation  of  the  vagus,  but  by  stimulation 
in  a  retlex  manner  or  otherwise  of  the  cardio-inhibitory  centre. 
Thus  the  fainting  which  often  follows  upon  a  blow  on  the  stomach 
is  a  repetition  of  the  result  mentioned  a  little  while  ago  as  obtained 
on  the  frog  by  striking  the  stomach  or  stimulating  the  nervi 
mesenterici.  So  also  the  fainting,  complete  or  partial,  which 
accompanies  severe  pain  or  mental  emotion,  is  an  illustration  of 
cardiac  inhibition  by  the  vagus.  These  are  familiar  examples  of 
more  or  less  complete  inhibition  ;  but  simple  slowing  or  weakening 
of  the  beat  through  the  inhibitory  mechanism  is  probably  an 
event  of  much  more  common  occurrence.  For  instance,  a  rise  of 
general  blood  pressure,  or,  and  perhaps  more  especially,  a  rise  in 
the  blood  pressure  of  the  vessels  of  the  brain,  sets  going  inhibitory 
impulses  by  which  the  work  of  the  heart  is  lessened,  and  the  high 
blood  pressure  lowered,  the  dangers  of  a  too  high  pressure  being 
thus  averted.  Again,  the  inhibition  may  be  brought  about  in  a 
reflex  manner  by  impulses  started  in  the  heart  itself  and  ascending 
to  the  central  nervous  system  along  afferent  fibres  which  run  in 
the  vagus  trunk  from  the  heart  to  the  spinal  bulb.  In  this  way  the 
heart  regulates  its  own  action  according  to  its  condition  and  its 
needs. 

There  is  also  some  reason  for  thinking  that,  in  some  animals 
at  least,  the  central  nervous  system  by  means  of  the  cardiac 
inhibitory  fibres  keeps,  as  it  were,  a  continual  rein  on  the  heart, 
for,  in  the  dog  for  example,  section  of  both  vagi  causes  a  quickening 
of  the  heart's  beat.  But  we  shall  have  to  speak  of  these  matters 
more  than  once  later  on.  Meanwhile  we  may  turn  to  the  augmentor 
fibres. 

So  much  of  our  knowledfre  of  the  nervous  work  of  the  heart  and 
■especially  of  the  action  of  the  augmentor  fibres  has  been  gained  by 
experiments  on  dogs  that  it  may  be  desirable  to  give  a  few  details  con- 
cerning the  nerves  of  the  heart  in  this  animal. 

In  the  dog  the  vagus  soon  after  it  issues  from  its  trunk  gangUon 
(6r.  Tr.  Vg.,  Fig.  71)  is  joined  by  the  sympatlietic  nerve  proceeding  from 
the  superior  cervical  gangHon,  the  two  forming  the  vago-symputhetic 
trunk.  As  this  trunk  enters  the  thorax,  the  sympathetic  portion  bears 
a  ganglion  (6r.  C.)  usually  called  the  lower  cervical  ganglion.  To  this 
ganglion  there  pass  from  the  stellate  ganglion  (C.St.)  of  the  thoracic 
sympathetic  chain,  two  nerves,  one  running  ventral  to,  the  other  dorsal 
to  the  subclavian  artery,  and  thus  forming  with  the  two  ganglia,  the 
annulus  of  Vieussens  (A71.V.). 

A  very  large  number  of  the  cardiac  nerves  spring  from  the  lower 
cervical  ganglion  and  from  the  vagus  trunk  lying  in  contact  with  it, 
from  the  vagus  trunk  below  this  ganglion,  from  the  annulus  of  Vieus- 
sens, chiefly  at  least  from  the  ventral  limb,  and  sometimes  from  the 
stellate  ganglion.  There  are  besides  cardiac  branches  passing  from 
the  vago-sympathetic  trunk  between  the  levels  of  the  superior  and 
of  the  inferior  cervical  ganglia,  cardiac  branches  of  the  recurrent 
lar^'ugeal,    a   cardiac    branch    of    the    superior   laryngeal,    and    a    long 


254  AUGMENTOR   FIBRES   IN   MAMMAL.  [Book  i. 


GTh?  - 


Fig.  71.    Diagrammatic  Eepresentation  of  the  Cardial  Inhibitory  and 
augmentor  fibkes  in  the  dog. 


fibres. 


The  upper  portion  of  the  figure  represents  the  inhibitory,  the  lower  the  augmentor 


Chap,  iv.]  THE   VASCULAR   MECHANISM.  255 

r.Vff.  roots  of  the  vagus ,  r.Sp.Ac.  roots  of  the  spinal  accessory;  both  drawn 
very  diagrammatically.  G.J.  gaogliou  jugulars.  G.Tr.Vg.  gangliou  trunci  vagi. 
Sp.Ac.  spinal  accessory  trunk.  Ext.Sp.Ac.  external  spinal  accessory.  i.Sp.Ac. 
internal  spinal  accessory.  Vg.  trunk  of  vagus  nerve,  n.c.  branches  going  to 
heart.  C.Si/.  cervical  sympathetic.  G.C.  lower  cervical  ganglion.  A.sb.  sub- 
clavian artery.  Jin.KAnnulus  of  Vieussens.  6'. 5/.  stellate  ganglion,  correspond- 
ing to  the  first,  second,  and  third  ganglia  of  the  thoracic  chain.  G.Th.*,  G.'J'li°, 
fourth  and  fifth  thoracic  ganglia.  2>.i.,  D.n.,  Z).iii.,  Z>.iv.,  D.v.,  first,  second,  third, 
fourth  and  fifth  thoracic  spinal  nerves,  r,  c.  ramus  communicans.  n.  c.  nerves 
(cardiac)  passing  to  the  heart  from  the  cervical  ganglion  and  from  the  annulus  of 
Vieussens. 

The  inhibitory  fibres,  shewn  by  black  lines,  run  in  the  upper  (bulbar)  roots  of 
the  spinal  accessory,  by  the  internal  branch  of  the  spinal  accessory,  past  the 
ganglion  trunci  vagi,  along  the  trunk  of  the  vagus,  and  so  by  branches  to  the 
heart. 

The  augmentor  fibres,  also  shewn  by  black  lines,  pass  from  the  spinal  cord  by  the 
anterior  roots  of  the  second  and  third  thoracic  nerves  (possibly  also  from  the  first, 
fourth  and  fifth  as  indicated  by  broken  black  lines),  pass  the  stellate  ganglion  by 
the  annulus  of  Vieussens  to  the  lower  cervical  ganglion,  from  whence,  as  also  from 
the  annulus  itself,  they  pass  along  the  cardiac  nerves  to  the  heart.  An  occasional 
tract  from  the  stellate  ganglion  itself  is  not  shewn  in  the  figure. 

slender  nerve  from  the  superior  cervical  ganglion  passing  independently 
to  the  heart.  The  arrangement  is  not  exactly  the  same  on  the  two 
sides  of  the  body,  and  the  minor  details  difi'er  in  different  individuals. 
As  in  other  animals  the  various  cardiac  nerves  mingle  in  the  cardiac 
plexuses. 

In  the  dog  it  has  been  ascertained  by  separate  stimulation  of 
these  several  cardiac  nerves,  that  augmentor  fibres  are  contained  in 
some  or  other  of  the  nerves  passing  from  the  lower  cervical  ganglion 
and  the  adjoining  vagus  trunk,  from  the  annulus  of  Vieussens, 
especially  the  lower,  ventral,  limb,  and  sometimes  from  the  stellate 
ganglion    itself.     The    results  differ  a  good  deal  in  different  in- 

DO  O 

dividuals,  and  there  are  reasons  for  thinking  that  the  nerves  in 
question  may  contain  efferent  fibres  other  than  augmentor  fibres, 
by  reason  of  which  stimulation  of  them  may  give  rise  to  other 
than  pure  augmentor  effects.  Speaking  broadly,  however,  we  may 
say  that  we  may  trace  the  augmentor  fibres  back  from  the  cardiac 
plexuses  through  the  lower  cervical  ganglion  and  the  annulus  of 
Vieussens  to  the  stellate  ganglion. 

This  ganglion  is  in  reality  several  sympathetic  ganglia  fused 
together.  It  undoubtedly,  in  the  dog,  represents  the  first,  second 
and  third  thoracic  sympathetic  ganglia,  receiving,  as  it  does, 
branches,  rami  communicantes,  from  the  first,  second  and  third 
thoracic  spinal  nerves.  Since  it  also  receives  branches  from  the 
eighth  and  seventh  cervical  nerves,  it  has  been  argued  that  it 
represents  not  only  the  three  thoracic  sympathetic  ganglia,  but 
also  what  in  man  and  other  animals  is  called  the  lower  cervical 
gangliou  ;  if  so,  what  has  been  called  above  the  lower  cervical 
ganglion  should  be  regarded  as  the  middle  cervical  ganglion. 
From  the  stellate  ganglion  the  sympathetic  cord  passes  to  the 
ganglion,  which  is  connected  by  a  ramus  communicans  with  the 


•256  AUGMENTOR   FIBRES   IN   MAMMAL.  [Book  i. 

fourth  thoracic  spinal  nerve,  and  which  is  therefore,  in  reality,  the 
fourth  thoracic  ganglion,  and  so  on  to  the  rest  of  the  thoracic  chain. 

Now,  when  the  several  rami  communicantes,  or  the  anterior 
roots,  of  the  lower  cervical  and  upper  thoracic  nerves  are  separately 
stimulated,  it  is  found  that  augmentor  effects  make  their  appear- 
ance with  considerable  constancy  when  the  second  and  third 
thoracic  nerves  are  stimulated  ;  the  effects  are  less  constant  with 
the  first  and  fourth  thoracic  nerves ;  sometimes  some  effect  may 
appear  with  the  fifth  thoracic  nerve,  but  not  with  any  other 
thoracic  nerves,  or  with  any  of  the  cervical  nerves. 

We  may  therefore  say  that,  in  the  dog,  augmentor  impulses 
leave  the  spinal  cord  by  the  anterior  roots  of  the  second  and  third, 
to  some  extent  the  first  and  fourth,  and  possibly  the  fifth 
thoracic  nerves,  travel  by  the  several  rami  communicantes  to  the 
stellate  ganglion,  and  pass  thence  to  the  cardiac  plexuses,  and  so  to 
the  heart,  by  nerves  from  the  stellate  ganglion  itself,  or  from  the 
annulus  of  Vieussens,  or  from  the  so-called  lower  cervical  ganglion. 
In  the  cat  the  path  of  the  augmentor  impulses  is  very  similar,  and 
we  may  regard  the  statement  just  made  as  representing,  in  a  broad 
way,  the  path  of  these  impulses  in  the  mammal  generally.  They 
leave  the  spinal  cord  by  the  upper  thoracic  nerves,  and  pass  to  the 
heart  through  the  lower  cervical  and  upper  thoracic  sympathetic 
gansjlia. 

The  effect  of  stimulating  these  augmentor  fibres  is,  in  some 
cases,  to  increase  the  rapidity  of  the  rhythm.  When  the  heart  is 
beating  very  slowly  this  acceleration  may  be  very  conspicuous,  but 
when  the  heart  is  beating  quickly,  or  even  at  what  may  be  called 
a  normal  rate,  the  acceleration  observed  may  be  very  slight.  A 
more  constant  and  striking  effect  is  the  increase  in  the  force  of  the 
beat.  When  tracings  are  taken  of  the  movements  of  the  auricles 
and  ventricles  separately,  it  is  observed  that  in  the  case  both  of  the 
auricles  and  of  the  ventricles,  the  extent  of  the  systole  is  increased ; 
moreover,  it  would  seem  also  that  both  cavities  undergo  a  larger 
expansion :  they  are  filled  with  a  larger  quantity  of  blood  during 
the  diastole.  This  means  that  the  output  of  the  heart  is  increased 
by  the  action  of  the  augmentor  nerves,  and  that  such  is  the  effect 
may  be  directly  shewn  by  the  cardiometer.  Moreover,  this  increase 
of  the  output  may  take  place  in  spite  of  a  concomitant  rise  of 
arterial  pressure,  so  that  the  effect  of  the  action  of  the  augmentor 
nerves  is  distinctly  to  increase  the  work  of  the  heart ;  and  this  may 
take  place  even  though  no  marked  acceleration  occurs. 

In  the  mammal  as  in  the  case  of  the  frog,  when  the  augmentor 
fibres  are  stimulated,  some  time  elapses  before  the  maximum  effect 
is  witnessed  and  the  influence  of  the  stimulation  may  last  some 
considerable  time  after  the  stimulation  has  ceased. 

When  records  are  taken  of  the  behaviour  of  the  heart  during 
the  stimulation  of  afferent  nerves,  such  as  the  sciatic  or  the 
splanchnic,  the  records  shew  that  the  heart  may  behave  very  much 


Chap,  iv.]  THE   VASCULAR   MECHANISM.  257 

in  the  same  way  as  when  the  aiigmentor  fibres  are  directly  stimu- 
lated ;  there  is  a  marked  increase  in  the  force  of  the  auricular  and 
of  the  ventricular  systole,  and  at  times  an  obvious  acceleration  of 
the  rhythm.  We  may  infer  that  in  such  a  case  the  augmentor 
fibres  are  thrown  into  activity  through  the  afferent  impulses  as 
part  of  a  reflex  act.  At  the  same  time  it  must  be  remembered, 
that  afferent  impulses  may  increase  the  beat  of  the  heart  not  by 
exciting  the  augmentor  mechanism,  but  by  depressing,  that  is 
by  inhibiting  a  previously  existing  activity  of  the  cardio-inhibitory 
centre ;  to  this  point  we  shall  again  have  to  refer. 

We  may  however  conclude  that  both  the  inhibitory  and  the 
augmentor  mechanisms  of  the  heart  can  be  brought  into  action  by 
means  of  the  central  nervous  system.  Speaking  broadly  the  effect 
of  the  former  is  to  diminish  the  work  of  the  heart,  and  so  to  lower 
the  blood  pressure,  and  that  of  the  latter  to  increase  the  work  of 
the  heart,  and  so  to  heighten  the  blood  pressure. 

§  139.  If,  either  in  a  frog  or  a  mammal,  or  other  animal,  after 
the  vagus  fibres  have  been  proved,  by  trial,  to  produce,  upon  stimu- 
lation, the  usual  inhibitory  effects,  a  small  quantity  of  atropin 
be  introduced  into  the  circulation  (when  the  experiment  is  con- 
ducted on  a  living  animal,  or  be  applied  in  a  weak  solution  to 
the  heart  itself  when  the  experiment  is  conducted,  in  the 
frog  for  instance,  on  an  excised  heart  or  after  the  circulation  has 
ceased),  it  will  after  a  short  time  be  found,  not  only  that  the  stimu- 
lation, the  application  of  a  current  for  instance,  which  previously 
when  applied  to  the  vagus  produced  marked  inhibition,  now 
produces  no  inhibition,  but  even  that  the  strongest  stimulus,  the 
strongest  current  applied  to  the  vagus,  will  wholly  fail  to  affect 
the  heart,  provided  that  there  be  no  escape  of  current  on  to  the 
cardiac  tissues  themselves ;  under  the  influence  of  even  a  small 
dose  of  atropin,  the  strongest  stimulation  of  the  vagus  will  not 
produce  standstill  or  appreciable  slowing  or  weakening  of  the  beat. 

Further,  this  special  action  of  atropin  on  the  heart  is  so 
to  speak  complemented  by  the  action  of  muscarin,  the  active 
principle  of  many  poisonous  mushrooms.  If  a  small  quantity  of 
muscarin  be  introduced  into  the  circulation,  or  applied  diiectly  to 
the  heart,  the  beats  become  slow  and  feeble,  and  if  the  dose  be 
adequate  the  heart  is  brought  to  a  complete  standstill.  The  effect 
is  in  some  respects  like  that  of  powerful  stimulation  of  the  vagus. 
Now  if,  in  a  frog,  the  heart  be  brought  to  a  standstill  by  a  dose  of 
muscarin,  the  application  of  an  adequate  quantity  of  atropin  will 
bring  back  the  beats  to  quite  their  normal  strength  and  rhythm. 
The  one  drug  is  so  far  as  the  heart  is  concerned  (and  indeed  in 
many  other  respects)  the  antidote  of  the  other.  These  and  other 
results  have  been  taken  to  indicate  that  there  exists  in  the  heart 
a  special  inhibitory  mechanism,  and  that  it  is  through  this  special 
mechanism  that  the  inhibitory  fibres  of  the  vagus  produce  inhibi- 
tion, while  atropin  produces  the  effect  just  mentioned  by  paralys- 

17 


258  INHIBITION  AND  AUGMENTATION.        [Book  i. 

iug,  by  rendering  incapable  of  activity,  and  muscarin  its  effect  by 
exciting,  stimulating  into  activity,  this  same  inhibitory  mechan- 
ism. It  has  further  been  suggested  that  some  of  the  ganglia  in  the 
heart  furnish  the  mechanism  in  question.  And  it  has  been  sup- 
posed that  there  is  a  corresponding  augmenting  mechanism.  But 
objections  may  be  urged  against  this  view,  and  it  is  safer  to  leave 
as  an  open  question  the  exact  manner  in  which  inhibition  and 
augmentation  are  brought  about. 

One  point  is  perhaps  worthy  of  mention.  We  have  seen 
that  inhibition  may  be  followed  by  a  phase  of  increased  activity, 
and  that  on  the  whole  the  heart  is  strengthened  rather  than 
weakened  by  the  process,  while  on  the  other  hand  augmentation  is 
followed  by  depression  and  the  process  is  distinctly  an  exhausting 
one.  Hence  whatever  be  the  exact  mechanism  of  inhibition  and  of 
augmentation,  whatever  be  the  particular  elements  of  the  cardiac 
structures  which  are  concerned  in  the  one  or  the  other,  augmenta- 
tion means  increased  expenditure,  inhibition  means  a  lessened  ex- 
penditure, of  energy  on  the  part  of  the  muscular  tissue  of  the 
heart.  Whatever  the  manner  in  which  the  respective  fibres  act, 
the  effect  of  the  activity  of  the  augmentor  fibres  is  to  hurry  on 
the  downward,  catabolic  changes  of  the  cardiac  tissue,  while  that 
of  the  inhibitory  fibres  is  an  opposite  one,  and  we  may  probably 
say  that  the  latter  assists  the  constructive,  anabolic,  changes. 


Other  Infiuences  regulating  or  modifying  the  Beat  of  the  Heart. 

§  140.  Important  as  is  the  regulation  of  the  heart  by  the 
nervous  system,  it  must  be  borne  in  mind  that  other  influences 
are  or  may  be  at  work.  The  beat  of  the  heart  may  for  instance 
be  modified  by  influences  bearing  directly  on  the  nutrition  of  the 
heart.  The  tissues  of  the  heart,  like  all  other  tissues,  need  an 
adequate  supply  of  blood  of  a  proper  quality ;  if  the  blood  vary 
in  quality  or  quantity  the  beat  of  the  heart  is  correspondingly 
affected.  The  excised  frog's  heart,  as  we  have  seen,  continues  to 
beat  for  some  considerable  time,  though  apparently  empty  of  blood. 
After  a  while  however  the  beats  diminish  and  eventually  disappear ; 
and  their  disappearance  is  greatly  hastened  by  washing  out  the 
heart  with  normal  saline  solution,  which  when  allowed  to  flow 
through  the  cavities  of  the  heart  readily  permeates  the  tissues  on 
account  of  the  peculiar  construction  of  the  ventricular  walls.  If 
such  a  '  washed  out '  quiescent  heart  be  fed  by  means  of  a  perfu- 
sion cannula,  in  the  manner  described  (§  133),  with  diluted  blood 
(of  the  rabbit,  sheep,  &c.),  it  may  be  restored  to  functional  activity. 
A  similar  but  less  complete  restoration  may  be  witnessed  if  serum 
be  used  instead  of  blood  ;  and  a  heart  fed  regularly  with  fresh, 
supplies  of  blood  or  even  of  serum  may  be  kept  beating  for  a 
very  great  length  of  time. 


Chap,  iv.]  THE   VASCULAR   MECHANISM.  259 

Now,  serum  is  as  we  have  seen  a  very  complex  tluid  containing 
several  proteids,  many  '  extractives '  and  various  inorganic  salts. 
As  regards  proteids  experiments  have  shewn  that  peptone  and 
albumose  so  far  from  being  beneiicial  are  directly  poisonous  to  the 
heart,  that  paraglobulin  is  without  effect,  but  that  serum-albumin 
will  maintain  the  beats  for  a  long  time  and  will  restore  the  beats 
of  a  '  washed-out '  heart.  We  might  infer  from  this  that  serum- 
albumin  is  directly  concerned  in  the  nutrition  of  the  cardiac  tissue ; 
but  we  are  met  with  the  striking  fact  that  a  frog's  heart  may  be 
maintained  in  vigorous  pulsation  for  many  hours,  and  that  a 
'  washed-out '  frog's  heart  may  be  restored  to  vigorous  pulsation  by 
being  fed  with  normal  saline  fluid  to  which  a  calcium  salt  with  a 
trace  of  a  potassium  salt  has  been  added.^  On  the  other  hand, 
serum  from  which  the  calcium  salts  have  been  removed  by 
precipitation  with  sodium  oxalate  is  powerless  to  maintain  or  to 
restore  cardiac  pulsations.  Obviously  in  the  changes,  whatever 
they  may  be,  through  which  such  fluids  as  serum,  milk  and  the  like 
(for  milk  and  other  fluids  have  been  found  efficient  in  this  respect) 
maintain  the  beat  of  the  heart,  calcium  salts  play  an  important 
part ;  and  it  is  tempting  to  connect  this  with  the  relation  of  calcium 
salts  to  the  clotting  of  blood  (§  20).  We  are  not  however  justified 
in  inferring  because  serum  is  ineffective  in  the  absence  of  calcium 
salts,  that  the  serum  albumin  is  useless ;  and,  indeed  the  beneficial 
effects  of  the  calcic  saline  fluid  are  not  so  complete  as  those  of  serum 
or  of  blood ;  moreover  the  possible  influences  of  the  various  extrac- 
tives, such  as  sugar  for  instance,  present  in  the  serum  have  to  be  con- 
sidered. We  may  in  addition  call  to  mind,  what  we  said  in  treating 
of  the  skeletal  muscles  (§  81),  that  fatigue  or  exhaustion  may  have 
a  double  nature,  the  using  up  of  contractile  material  on  the  one 
hand  and  on  the  other  hand  the  accumulation  of  waste  products  ; 
and  the  nutritive  or  restorative  influence  over  the  heart  of  any 
material  may  bear  on  the  one  or  the  other  of  these.  Thus  the 
beneficial  effect  of  alkalies  is  probably  in  part  due  to  their 
antagonizing  the  acids  which  as  we  have  seen  are  being  constantly 
produced  during  muscular  contraction. 

In  the  various  experiments  which  have  been  made  in  thus 
feeding  hearts  with  nutritive  and  other  fluids  two  facts  worthy  of 
notice  have  been  brought  to  licjht. 

One  is  that  various  substances  have  an  effect  on  the  mus- 
cular walls,  apart  from  the  direct  modification  of  the  contractions. 
The  muscular  fibres  of  the  heart  over  and  above  their  rhythmic 
contractions  are  capable  of  varying  in  length,  so  that  at  one  time 
they  are  lunger,  and  the  chambers  when  pressure  is  applied  to 
them  internally  are  dilated  beyond  the  normal,  while  at  another 
time  they  are  shorter,  and  the  chambers,  with  the  same  internal 

^  Ey  ■Ringer's  Ileart-Fluirl,  for  instance,  wliich  is  made  l>y  saturatinc;  in  tlie  cold 
normal  saline  soliiiion  (-(35  p.  c.  sodium  cliloride)  with  calcium  phosphate,  and 
adding  to  100  c.c.  of  the  mixture,  2  c.c.  of  a  1  p.  c.  solution  of  potassium  chloride. 


260  EEGULATION   BY   NUTRITION.  [Book  i. 

pressure,  are  contracted  beyond  the  normal.  In  other  words,  the 
heart  possesses  what  we  shall  speak  of  in  reference  to  arteries  as 
tonicity  or  tonic  contraction,  and  the  amount  of  this  tonic  contrac- 
tion, and  in  consequence  the  capacity  of  the  chambers,  varies  accord- 
ing to  circumstances.  The  presence  of  some  substances  appears  to 
increase,  of  others  to  diminish  this  tonicity  and  thus  to  diminish  or 
increase  the  capacity  of  the  chambers  during  diastole.  This  of 
course  would  have  an  effect,  other  things  being  equal,  on  the 
output  from  the  heart  and  so  on  its  work ;  and  indeed  there  is 
some  evidence  that  the  augmentor  and  inhibitory  impulses  may 
also  affect  this  tonicity,  but  observers  are  not  agreed  as  to  the 
manner  in  which  and  extent  to  which  they  may  thus  act. 

Another  fact  worthy  of  notice  is  when  the  heart  is  thus  artifi- 
cially fed  with  serum,  or  other  fluids  or  even  with  blood,  the  beats, 
whether  spontaneous  or  provoked  by  stimulation,  are  apt  to  become 
intermittent  and  to  arrange  themselves  into  groups.  This  intermit- 
tence  is  possibly  due  to  the  fluid  employed  being  unable  to  carry  on 
nutrition  in  a  completely  normal  manner,  and  to  the  consequent 
production  of  abnormal  chemical  substances  ;  and  it  is  probable  that 
cardiac  intermittences  seen  during  life  are  in  certain  cases  thus 
brought  about  by  some  direct  interference  with  the  nutrition  of  the 
cardiac  tissue  and  not  through  extrinsic  nervous  impulses  descend- 
ing to  the  heart  from  the  central  nervous  system.  Various  chemical 
substances  in  the  blood,  arising  within  the  body  or  introduced  as 
drugs,  may  thus  affect  the  heart's  beat  by  acting  on  its  muscular 
fibres,  or  its  nervous  elements,  or  both,  and  that  probably  in  various 
ways,  modifying  in  different  directions  the  rhythm,  or  the  individual 
contractions,  or  both. 

Concerning  the  effect  on  the  heart  of  blood  which  has  not  been 
adequately  changed  in  the  lungs  we  shall  speak  when  we  come  to 
treat  of  respiration. 

The  physical  or  mechanical  circumstances  of  the  heart  also 
affect  its  beat ;  of  these  perhaps  the  most  important  is  the  amount 
of  the  distention  of  its  cavities.  The  contractions  of  cardiac 
muscle,  like  those  of  ordinary  muscle  (see  §  76),  are  increased  up 
to  a  certain  limit  by  the  resistance  which  they  have  to  overcome ; 
a  full  ventricle  will,  other  things  being  equal,  contract  more 
vigorously  than  one  less  full ;  though,  as  in  ordinary  muscle,  the 
limit  at  which  resistance  is  beneficial  may  be  passed,  and  an  over- 
full ventricle  will  fail  to  beat  at  all.  Hence  an  increase  in  the 
quantity  of  blood  in  the  ventricle  will  augment  the  work  done  in 
two  ways ;  the  quantity  thrown  out  will,  unless  antagonistic 
influences  intervene,  be  greater,  and  the  increased  quantity  will  be 
ejected  with  greater  force.  Further,  since  the  distention  of  the 
ventricle  at  the  commencement  of  the  systole  at  all  events  is 
dependent  on  the  auricular  systole,  the  work  of  the  ventricle  (and 
so  of  the  heart  as  a  whole)  is  in  a  measure  governed  by  the 
auricle. 


Chap,  iv.]  THE   VASCULAR   MECHANISM.  261 

An  interesting  combination  of  direct  mechanical  effects  and 
indirect  nervous  effects  is  seen  in  the  relation  of  the  heart's 
beat  to  blood  pressure.  When  the  blood  pressure  is  high,  not 
only  is  the  resistance  to  the  ventricular  systole  increased,  but, 
other  things  being  equal,  more  blood  flows  (in  the  mammalian 
heart)  through  the  coronary  arteries.  Both  these  events  would 
increase  the  activity  of  the  heart,  and  we  might  expect  that  the 
increase  would  be  manifest  in  the  rate  of  the  rhythm  as  well  as  in 
the  force  of  the  individual  beats.  As  a  matter  of  fact,  however, 
we  do  not  find  this.  On  the  contrary,  the  relation  of  heart  beat  to 
pressure  may  be  put  almost  in  the  form  of  a  law,  that  "  the  rate 
of  the  beat  is  in  inverse  ratio  to  the  arterial  pressure ; "  a  rise  of 
pressure  being  accompanied  by  a  diminution,  and  fall  of  pressure 
by  an  increase  of  the  rate  of  the  rhythm.  This  however  only  holds 
good  if  the  vagus  nerves  be  intact.  If  these  be  previously  divided, 
then  in  whatever  way  the  blood  pressure  be  raised  —  wdiether  by 
injecting  blood  or  clamping  the  aorta,  or  increasing  the  peripheral 
resistance,  through  an  action  of  the  vaso-motor  nerves  which  we 
shall  have  to  describe  directly  —  or  in  whatever  way  it  be  lowered, 
no  such  clear  and  decided  inverse  relation  between  blood  pressure 
and  pulse-rate  is  observed.  It  is  inferred  therefore  that  increased 
blood  pressure  causes  a  slowing  of  the  beat,  when  the  vagus  nerves 
are  intact,  because  the  cardio-inhibitory  centre  in  the  medulla  is 
stimulated  by  the  high  pressure,  either  directly  by  the  pressure 
obtaining  in  the  blood  vessels  of  the  medulla,  or  in  some  indirect 
manner,  and  the  heart  in  consequence  more  or  less  inhibited. 


SEC.    6.      CHANGES   IN  THE   CALIBRE   OF  THE  MINUTE 
ARTERIES.     VASO-MOTOR  ACTIONS. 


§  141.  All  arteries  contain  plain  muscular  fibres,  for  the  most 
part  circularly  disposed,  and  most  abundant  in,  or  sometimes  al- 
most entirely  confined  to,  the  middle  coat.  Further  as  the  arteries 
become  smaller,  the  muscular  element  as  a  rule  becomes  more  and 
more  prominent  as  compared  with  the  other  elements,  until,  in  the 
minute  arteries,  the  middle  coat  consists  almost  entirely  of  a  series 
of  plain  muscular  fibres  wrapped  round  the  internal  coat.  Nerve 
fibres,  of  whose  nature  and  course  we  shall  presently  speak,  are 
distributed  largely  to  the  arteries,  and  appear  to  end  chiefly  in  fine 
plexuses  round  the  muscular  fibres,  but  their  exact  terminations 
have  not  as  yet  been  clearly  made  out.  By  mechanical,  electrical, 
or  other  stimulation,  this  muscular  coat  may,  in  the  living  artery, 
be  made  to  contract.  During  this  contraction,  which  has  the  slow 
character  belonging  to  the  contractions  of  all  plain  muscle,  the 
calibre  of  the  vessel  is  diminished.  The  veins  also  as  we  have 
seen  possess  muscular  elements,  but  these  vary  in  amount  and 
distribution  very  much  more  in  the  veins  than  in  the  arteries. 
Most  veins  however  are  contractile,  and  may  vary  in  calibre 
according  to  the  condition  of  their  muscular  elements.  Veins 
are  also  supplied  with  nerves.  It  will  be  of  advantage  however 
to  consider  separately  the  little  we  know  concerning  the  changes 
in  the  veins  and  to  confine  ourselves  at  present  to  the  changes  in 
the  arteries. 

If  any  individual  small  artery  in  the  web  of  a  frog's  foot  be 
watched  under  the  microscope,  it  will  be  found  to  vary  considerably 
in  calibre  from  time  to  time,  being  sometimes  narrowed  and 
sometimes  dilated;  and  these  changes  may  take  place  without 
any  obvious  changes  either  in  the  heart  beat  or  in  the  general 
circulation ;  they  are  clearly  changes  of  the  artery  itself.  During 
the  narrowing,  which  is  obviously  due  to  a  contraction  of  the 
muscular  coat  of  the  artery,  the  capillaries  fed  by  the  artery  and 
the  veins  into  which  these  lead  become  less  filled  with  blood,  and 


Chap,  iv.]  THE  VASCULAR  MECHANISM.  263 

therefore  paler.  During  the  widening,  which  corresponds  to  the 
relaxation  of  the  muscular  coat,  the  same  parts  are  fuller  of  blood, 
and  redder.  It  is  obvious  that,  the  pressure  at  the  entrance  into 
any  given  artery  remaining  the  same,  more  blood  will  enter  the 
artery  when  relaxation  takes  place,  and  consequently  the  resistance 
offered  by  the  artery  is  diminished,  and  less  when  contraction 
occurs,  and  the  resistance  is  consequently  increased ;  the  blood 
flows  in  the  direction  of  least  resistance. 

The  extent  and  intensity  of  the  narrowing  or  widening,  of  the 
constriction  or  dilation  which  may  thus  be  observed  in  the  frog's 
web,  vary  very  largely.  Variations  of  slight  extent,  either  more  or 
less  regular  and  rhythmic  or  irregular,  occur  even  when  the  animal 
is  apparently  subjected  to  no  disturbing  causes,  and  may  be  spoken 
of  as  spontaneous ;  larger  changes  may  follow  events  occurring  in 
various  parts  of  the  body ;  while  as  the  result  of  experimental 
interference  the  arteries  may  become  either  constricted,  in  some 
cases  almost  to  obliteration,  or  dilated  until  they  acquire  double 
or  more  than  double  their  normal  diameter.  This  constriction  or 
dilation  may  be  brought  about  not  only  by  treatment  applied 
directly  to  the  web,  but  also  by  changes  affecting  the  nerves  of 
the  leg  or  other  parts  of  the  body.  Thus  section  of  the  nerves  of  the 
leg  is  generally  followed  by  a  widening  which  may  be  slight  or 
which  may  be  very  marked,  and  which  is  sometimes  preceded  by 
a  passing  constriction  ;  while  stimulation  of  the  peripheral  stump 
of  a  divided  nerve  by  an  interrupted  current  of  moderate  in- 
tensity gives  rise  to  constriction,  often  so  great  as  almost  to 
obliterate  some  of  the  minute  arteries. 

Obviously,  then,  the  contractile  muscular  elements  of  the  minute 
arteries  of  the  web  of  the  frog's  foot  are  capable  by  contraction  or 
relaxation  of  causing  decrease  or  increase  of  the  calibre  of  the 
arteries ;  and  this  condition  of  constriction  or  dilation  may  be 
brought  about  through  the  agency  of  nerves.  Indeed,  not  only  in 
the  frog,  but  also,  and  still  more  so,  in  warm  blooded  animals,  have 
we  evidence  that  in  the  case  of  a  very  large  number  of,  if  not  all,  the 
arteries  of  the  body,  the  condition  of  the  muscular  coat,  and  so  the 
calibre  of  the  artery,  is  governed  by  means  of  nerves  ;  these  nerves 
have  received  the  general  name  of  vaso-motor  nerves. 

§  142.  If  the  ear  of  a  rabbit,  preferably  a  light  coloured  one, 
be  held  up  before  the  light,  a  fairly  conspicuous  artery  will  be  seen 
running  up  the  middle  line  of  the  ear,  accompanied  hj  its  broader 
and  more  obvious  veins.  If  this  artery  be  carefully  watched  it  will 
be  found,  in  most  instances,  to  be  undergoing  rhythmic  changes  of 
calibre,  constriction  alternating  with  dilation.  At  one  moment  the 
artery  appears  as  a  delicate,  hardly  visible  pale  streak,  the  whole 
ear  being  at  the  same  time  pallid.  After  a  while  the  artery  slowly 
widens  out,  becomes  broad  and  red,  the  whole  ear  blushing,  and 
many  small  vessels  previously  invisible  coming  into  view.  Again 
the  artery  narrows  and  the  blush  fades  away ;  and  this  may  be 


264  CHANGES   IN   CALIBEE   OF  ARTERIES.      [Book  i. 

repeated  at  somewhat  irregular  intervals  of  a  minute,  more  or  less. 
The  extent  and  regularity  of  the  rhythm  are  usually  markedly 
increased  if  the  rabbit  be  held  up  by  the  ears  for  a  short  time 
previous  to  the  observation.  Similar  rhythmic  variations  in  the 
calibre  of  the  arteries  have  been  observed  in  several  regions  of  the 
body,  ex.  gr.  in  the  vessels  of  the  mesentery  and  elsewhere  ; 
probably  they  are  widely  spread. 

Sometimes  no  such  variations  are  seen,  the  artery  remains 
constant  in  a  condition  intermediate  between  the  more  extreme 
widening  and  extreme  narrowing  just  described.  In  fact,  we  may 
speak  of  an  artery  as  being  at  any  given  time  in  one  of  three 
phases.  It  may  be  very  constricted,  in  which  case  its  muscular 
fibres  are  very  much  contracted ;  or  it  may  be  very  dilated,  in 
which  case  its  muscular  fibres  are  relaxed ;  or  it  may  be  mode- 
rately constricted,  the  muscular  fibres  being  contracted  to  a  certain 
extent,  and  remaining  in  such  a  condition  that  they  may  on  the 
one  hand  pass  into  stronger  contraction,  leading  to  marked  con- 
striction, or,  on  the  other  hand,  into  distinct  relaxation,  leading 
to  dilation.  We  have  reason  to  think,  as  we  shall  see,  that  many 
arteries  of  the  body  are  kept  habitually,  or  at  least  for  long 
periods  together,  in  this  intermediate  condition,  which  is  fre- 
quently spoken  of  as  tonic  contraction  or  tonus,  or  arterial  tone. 

§  143.  If,  now,  in  a  vigorous  rabbit,  in  which  the  heart  is 
beating  with  adequate  strength,  and  the  whole  circulation  is  in  a 
satisfactory  condition,  the  cervical  sympathetic  nerve  be  divided  on 
one  side  of  the  neck,  remarkable  changes  may  be  observed  in  the 
blood  vessels  of  the  ear  of  the  same  side.  The  arteries  and  veins 
widen,  they,  together  with  the  small  veins  and  the  capillaries, 
become  full  of  blood,  many  vessels  previously  invisible  come  into 
view,  the  whole  ear  blushes,  and  if  the  rhythmic  changes  described 
above  were  previously  going  on,  these  now  cease ;  in  conse- 
quence of  the  extra  supply  of  warm  blood  the  whole  ear  becomes 
distinctly  warmer.  Now,  these  changes  take  place,  or  may  take 
place,  without  any  alteration  in  the  heart  beat  or  in  the  general 
circulation.  Obviously  the  arteries  of  the  ear  have,  in  conse- 
quence of  the  section  of  the  nerve,  lost  the  tonic  contraction 
which  previously  existed ;  their  muscular  coats  previously  some- 
what contracted  have  become  quite  relaxed,  and  whatever  rhythmic 
contractions  were  previously  going  on  have  ceased.  The  more 
marked  the  previous  tonic  contraction,  and  the  more  vigorous  the 
heart  beats,  so  that  there  is  an  adequate  supply  of  blood  to  fill  the 
widened  channels,  the  more  striking  the  result.  Sometimes,  as 
when  the  heart  is  feeble,  or  the  pre-existing  tonic  contraction  is 
slight,  the  section  of  the  nerve  produces  no  very  obvious  change. 

If  now  the  upper  segment  of  the  divided  cervical  sympathetic 
nerve,  that  is  the  portion  of  the  nerve  passing  upwards  to  the  head 
and  ear,  be  laid  upon  the  electrodes  of  an  induction  machine,  and  a 
gentle  interrupted  current  be  sent  through  the  nerve,  fresh  changes 


Chap,  iv.]  THE   VASCULAR   MECHANISM.  265 

take  place  in  the  blood  vessels  of  the  ear.  A  short  time  after  the 
application  of  the  current,  for  in  this  effect  there  is  a  latent  period 
of  very  appreciable  duration,  the  ear  grows  paler  and  cooler,  many 
small  vessels,  previously  conspicuous,  become  again  invisible,  the 
main  artery  shrinks  to  the  tliinnest  thread,  and  the  main  veins 
become  correspondingly  small.  When  the  current  is  shut  off  from 
the  nerve,  these  effects  still  last  some  time,  but  eventually  pass 
off;  the  ear  reddens,  blushes  once  more,  and,  indeed,  may  become 
even  redder  and  hotter,  with  the  vessels  more  filled  with  blood 
than  before.  Obviously  the  current  has  generated  in  the  cervical, 
sympathetic,  nerve  impulses  which,  passing  upward  to  the  ear  and 
finding  their  way  to  the  muscular  coats  of  the  arteries  of  the  ear, 
have  thrown  the  muscles  of  those  coats  into  forcible  contractions, 
and  have  thus  brought  about  a  forcible  narrowing  of  the  calibre  of 
the  arteries,  a  forcible  constriction.  Through  the  narrowed  con- 
stricted arteries  less  blood  finds  its  way,  and  hence  the  paleness 
and  coldness  of  the  ear.  If  the  impulses  thus  generated  be  very 
strong,  the  constriction  of  the  arteries  may  be  so  great  that  the 
smallest  quantity  only  of  blood  can  make  its  way  through  them, 
and  the  ear  may  become  almost  bloodless.  If  the  impulses  be 
weak  the  constriction  induced  may  be  slight  only  ;  and,  indeed,  by 
careful  manipulation  the  nerve  may  be  induced  to  send  up  to  the 
ear  impulses  only  just  sufficiently  strong  to  restore  the  moderate 
tonic  constriction  which  existed  before  the  nerve  was  divided. 

We  infer  from  these  experiments  that  among  the  various  nerve 
fibres  making  up  the  cervical  sympathetic,  there  are  certain  fibres 
which,  passing  upwards  to  the  head,  become  connected  with  the 
arteries  of  the  ear,  and  that  these  fibres  are  of  such  a  kind  that 
impulses,  generated  in  them  and  passing  upwards  to  the  ear,  lead 
to  marked  contraction  of  the  muscular  fibres  of  the  arteries,  and 
thus  produce  constriction.  These  fibres  are  vaso-motor  fibres  for 
the  blood  vessels  of  the  ear.  From  the  loss  of  tone,  so  frequently 
following  section  of  the  cervical  sympathetic,  we  may  further  infer, 
that,  normally  during  life,  impulses  of  a  gentle  kind  are  continually 
passing  along  these  fibres,  upwards  through  the  cervical  sympathe- 
tic, which  impulses,  reaching  the  arteries  of  the  ear,  maintain  the 
normal  tone  of  those  arteries.  But,  as  we  said,  the  existence  of  this 
tone  is  not  constant,  and  the  effects  of  these  tonic  impulses 
are  not  so  conspicuous  as  those  of  the  artificial  constrictor  im- 
pulses generated  by  stimulation  of  the  nerve. 

§  144.  The  above  results  are  obtained  whatever  be  the  region 
of  the  cervical  sympathetic  which  we  divide  or  stimulate  between  the 
upper  and  the  lower  cervical  ganglion.  We  may  therefore  describe 
these  vaso-motor  impulses  as  passing  upwards  from  the  lower  cer- 
vical ganglion  along  the  cervical  sympathetic,  to  the  upper  cervical 
ganglion,  from  which  they  issue  by  branches  wdiich  ultimately  find 
their  way  to  the  ear.  But  these  impulses  do  not  start  from  the 
lower  cervical  ganglion .;  on  the  contrary,  by  repeating  the  experi- 


266 


VASO-MOTOR   FIBRES   OF  THE   EAR.       [Book  i. 


Any. 


merits  of  division  and  stimulation  in  a  series  of  animals,  we  may 
trace  the  path  of  these  impulses  from  the  lower  cervical  ganglion, 
Fig.  72,  through  the  annulus  of  Vieussens  to 
the  ganglion  stellatum  and  upper  part  of  the 
thoracic  sympathetic  chain,  and  thence  along 
the  rami  communicantes  of  some  or  other 
of  the  upper  thoracic  spinal  nerves  to  the 
anterior  roots  of  those  nerves,  and  so  to  the 
spinal  cord.  In  the  cat  and  the  dog,  and 
probably  in  other  higher  mammals,  the  chief 
path  of  the  impulses  lies  in  the  second  and 
third  thoracic  nerves,  though  some  pass  by 
the  fourth,  and  a  variable  small  number  by 
the  fifth  and  the  first ;  in  the  rabbit  the 
path  is  more  widespread,  and  reaches  lower 
down,  for  while  the  impulses  pass  chiefly  by 
the  fourth  and  fifth  thoracic  nerves,  some 
pass  by  the  second  and  third,  and  others  by 
the  sixth,  seventh,  and  even  eighth  nerves. 
The  exact  path  also  differs  in  different  indi- 
viduals of  the  same  species.  It  will  be 
observed  that  from  the  spinal  cord  up  to  the 
annulus  of  Vieussens,  and  the  lower  cervical 
ganglion,  these  vaso-motor  impulses  for 
the  ear,  and  the  augmentor  impulses  for  the 
heart,  (cf.  Fig.  71)  follow  much  the  same 
path ;  but  there  they  part   company.     We 

Fig.  72.    Diagram  Illustrating  the  Paths  of  Vaso-constrictor  Fibres  along 
THE  Cervical  Sympathetic  and  (part  of)  the  Abdominal  Splanchnic. 

Aur.  artery  of  ear.  G.C.S.  superior  cervical  ganglion.  Abd.  Spl.  upper  roots 
of  and  part  of  abdominal  splanchnic  nerve.  V.M.C.  vasomotor  centre  in  spinal 
bulb.  The  other  references  are  the  same  as  in  Fig.  71,  §  138.  The  paths  of  the 
constrictor  fibres  are  shewn  by  the  arrows.  The  dotted  line  along  the  middle  of 
the  spinal  cord,  Sp.  C,  is  to  "indicate  the  passage  of  constrictor  impulses  down 
the  cord  from  the  vaso-motor  centre  in  the  spinal  bulb. 

can  thus  trace  these  vaso-motor  impulses  backwards  along  the  cer- 
vical sympathetic  to  the  anterior  roots  of  certain  thoracic  nerves,  and 
through  these  to  the  thoracic  region  of  the  spinal  cord,  where  we 
will  for  the  present  leave  them.  We  may,  accordingly,  speak  of 
vaso-motor  fibres  for  the  ear  as  passing  from  the  thoracic  spinal 
cord  to  the  ear  along  the  track  just  marked  out ;  stimulation  of  these 
fibres  at  their  origin  from  the  spinal  cord,  or  at  any  part  of  their 
course  (along  the  anterior  roots  of  the  second,  third  or  other  upper 
thoracic  nerves,  visceral  branches  [rami  communicantes]  of  those 
nerves,  ganglion  stellatum  or  upper  part  of  thoracic  sympathetic 
chain,  annulus  of  Vieussens,  &c.  &c.),  leads  to  constriction  in  the 
blood  vessels  of  the  ear  of  that  side ;  and  section  of  these  fibres 
at  any  part  of  the  same  course  tends  to  abolish  any  previously 


Chap,  iv.] 


THE  VASCULAR  MECHANISM. 


267 


existing  tonic  constriction  of  the  blood  vessels  of  the  ear,  though 
the  effect  of  section  is  not  so  constant  or  striking  as  that  of 
stimulation. 

§  145.  We  must  now  turn  to  another  case.  In  dealing  with 
digestion  we  shall  have  to  study  the  submaxillary  salivary  gland. 
We  may  for  the  present  simply  say  that  this  is  a  glandular  mass 
well  supplied  with  blood  vessels,  and  possessing  a  double  nervous 
supply.  On  the  one  hand  it  receives  fibres  from  the  cervical 
sympathetic,  Fig.  73  v.  sym.  (in  the  dog,  in  which  the  effects  which 
we  are  about  to  describe  are  best  seen,  the  vagus  and  cervical 


V^aym. 


Fig.  73. 


DiAGKAMMATIC    REPRESENTATION    OF    THE    SUBMAXILLARY    GlAND   OF 
THE    UOG    WITH    ITS    NeKVE    AND    BlOOD    VESSELS. 


(The  di.ssection  has  been  made  on  an  animal  lying  on  its  hack,  but  since  all  the 
parts  shewn  in  the  figure  cannot  be  seen  from  any  one  point  of  view,  the  figure  does 
not  give  the  exact  anatomical  relations  of  the  several  structures.) 

sm.  (fid.  The  submaxillary  gland,  into  the  duct  (sm.  d.)  of  which  a  cannula  has 
been  tied  The  subliugual  gland  and  duct  are  not  sliewn.  n.l.,n.l'.  The  lingual 
branch  of  the  fifth  nerve,  the  part  n.  I.  is  going  to  the  tongue,  c/i.  t.,  ch.  t'.,  c/i.  I". 
The  churchi  tympani  The  part  cli.  t".  is  proceeding  from  the  facial  nerve  ;  at  ch.  t'. 
it  becomes  conjoined  with  the  lingual  n  r  and  afterwards  diverging  passes  as  ch.  t. 
to  the  gland  along  the  duct ;  the  continuation  of  tlie  nerve  in  company  with  the 
lingual  n.  l.  is  not  shewn,  sm.  r/l.  The  submaxillary  ganglion  with  its  several 
roots.  (I.  car.  The  carotid  artery,  two  small  branches  of  which,  a.  sm.  a.  and  r.  sm.  p., 
pass  to  the  anterior  and  posterior  ])arts  of  the  gland,  r.s.m.  The  anterior  and  pos- 
terior veins  from  the  gland,  falling  intor. ./.  the  jugular  vein.  v.  si/m.  The  con- 
joined vagus  and  sympathetic  trunks.  (/.  cer.  s.  Tlie  upper  cervical  ganglion,  two 
branches  of  which  forming  a  plexus  (a./.)  over  the  facial  artery,  are  distributed 
(h.  si/m.  sm.)  along  the  two  glandular  arteries  to  the  anterior  and  posterior  portions 
of  the  gland. 

The  arrows  indicate  the  direction  taken  by  the  nervous  impulses  during  reflex 
stimulation  of  the  gland.  They  ascend  to  the  brain  by  the  lingual  and  descend  by 
the  chorda  tympani. 


268  CONSTRICTOR  AND  DILATOR   FIBRES.      [Book  i. 

sympathetic  are  enclosed  in  a  common  sheath  so  as  to  form  what 
appears  to  be  a  single  trunk),  which  reach  the  gland  in  company 
with  the  arteries  supplying  the  gland  (n.  sym.  sm.).  On  the 
other  hand  it  receives  fibres  from  a  small  nerve  called  the  chorda 
tympani  (ch.  t),  which,  springing  from  the  7th  cranial  (facial) 
nerve,  crosses  the  tympanum  of  the  ear  (hence  the  name),  and, 
joining  the  lingual  branch  of  the  5th  nerve,  runs  for  some  distance 
in  company  with  that  nerve,  and  then  ends  partly  in  the  tongue, 
and  partly  in  a  small  nerve  which,  leaving  the  lingual  nerve  before 
reaching  the  tongue,  runs  along  the  duct  of  the  submaxillary 
gland,  and  is  lost  in  the  substance  of  the  gland ;  a  small  branch 
is  also  given  off  to  the  sublingual  gland. 

Now,  when  the  chorda  tympani  is  simply  divided,  no  very 
remarkable  changes  take  place  in  the  blood  vessels  of  the  gland, 
but  if  the  peripheral  segment  of  the  divided  nerve,  that  still  in 
connection  with  the  gland,  be  stimulated,  very  marked  results 
follow.  The  small  arteries  of  the  gland  become  very  much  dilated, 
and  the  whole  gland  becomes  flushed.  (As  we  shall  see  later  on 
the  gland  at  the  same  time  secretes  saliva  copiously,  but  this  does 
not  concern  us  just  now.)  Changes  in  the  calibre  of  the  blood 
vessels  are,  of  course,  not  so  readily  seen  in  a  compact  gland  as  in 
a  thin  extended  ear ;  but  if  a  fine  tube  be  placed  in  one  of  the 
small  veins  by  which  the  blood  returns  from  the  gland,  the  effects 
on  the  blood  flow  of  stimulating  the  chorda  tympani  become 
very  obvious.  Before  stimulation  the  blood  trickles  out  in  a  thin, 
slow  stream  of  a  dark  venous  colour  ;  during  stimulation  the  blood 
rushes  out  in  a  rapid  full  stream,  often  with  a  distinct  pulsation, 
and  frequently  of  a  colour  which  is  still  scarlet  and  arterial  in 
spite  of  the  blood  having  traversed  the  capillaries  of  the  gland ; 
the  blood  rushes  so  rapidly  through  the  widened  blood  vessels  that 
it  has  not  time  to  undergo  completely  that  change  from  arterial  to 
venous  which  normally  occurs  while  the  blood  is  traversing  the 
capillaries  of  the  gland.  This  state  of  things  may  continue  for 
some  time  after  the  stimulation  has  ceased,  but  before  long  the 
flow  from  the  veins  slackens,  the  issuing  blood  becomes  darker 
and  venous,  and  eventually  the  circulation  becomes  normal. 

We  shall  have  occasion  later  on  to  speak  of  the  nervi  erigentes, 
the  stimulation  of  which  gives  rise  to  the  erection  of  the  penis.  The 
erection  of  the  penis  is  partly  due  to  a  widening  of  the  arteries 
supplying  the  peculiar  erectile  tissue  of  that  organ,  whereby  that 
tissue  becomes  distended  with  blood,  and  the  widening  is  brought 
about  by  impulses  passing  along  the  nerves  in  question.  Obviously 
the  chorda  tympani  and  the  nervi  erigentes  contain  fibres  which 
we  may  speak  of  as  '  vaso-motor '  since  stimulation  of  them 
produces  a  change  in,  brings  about  a  movement  in  the  blood 
vessels ;  but  the  change  produced  is  of  a  character  the  very 
opposite  to  that  produced  in  the  blood  vessels  of  the  ear  by 
stimulation  of  the  cervical  sympathetic.    There  stimulation  of  the 


Chap,  iv.]  THE   VASCULAR   MECHANISM.  269 

nerve  caused  contraction  of  the  muscular  fibres,  constriction  of 
the  small  arteries  ;  here  stimulation  of  the  nerve  causes  a  widen- 
ing of  the  arteries,  which  widening  is  undoubtedly  due  to  relaxa- 
tion of  the  muscular  fibres.  Hence  we  must  distinguish  between 
two  kinds  of  vaso-motor  fibres,  fibres  the  stimulation  of  which 
produces  constriction,  vaso-constrictor  fibres,  and  fibres  the  stimu- 
lation of  which  causes  the  arteries  to  dilate,  raso-dilator  fibres, 
the  one  kind  being  the  antagonist  of  the  other. 

§  146.  In  the  chorda  tympani,  the  vaso-motor  fibres  are 
exclusively  vaso-dilator  fibres,  and  this  is  true  both  of  the  part 
of  the  nerve  ending  in  the  submaxillary  and  sublingual  glands, 
and  the  rest  of  the  ending  of  the  nerve  in  the  tongue.  Stimula- 
tion of  the  chorda  tympani  (so  far  as  the  vaso-motor  functions  of 
the  nerve  are  concerned,  for  it  has,  as  we  shall  see,  other  func- 
tions), at  any  part  of  its  course  from  its  leaving  the  facial  nerve 
to  its  endings  in  the  gland  or  tongue,  produces  only  vaso-dilator 
effects,  never  vaso-constrictor  effects.  The  cervical  sympathetic 
on  the  other  hand  is  not  exclusively  vaso-constrictor.  It  con- 
tains as  we  have  seen  vaso-constrictor  fibres  for  the  ear.  It  also 
contains  vaso-constrictor  fibres  for  other  regions  of  the  head  and 
face.  For  instance  the  branches  of  the  cervical  sympathetic 
going  to  the  submaxillary  gland  of  which  we  just  spoke  (Fig.  73 
n.  sym.  sm.),  contain  vaso-constrictor  fibres  for  the  vessels  of  the 
gland ;  stimulation  of  these  fibres  produces,  on  the  vessels  of  the 
gland,  an  effect  exactly  the  opposite  of  that  produced  by  stimula- 
tion of  the  chorda  tympani;  to  this  point  we  shall  have  to  return 
when  we  deal  with  the  gland  in  connection  with  digestion.  And 
we  misht  give  other  instances  ;  in  fact  the  dominant  effect  on 
the  blood  vessels  of  stimulating  the  cervical  sympathetic  is  a 
vaso-constrictor  effect.  There  are  however  certain  cases  in  which 
the  opposite  effect,  a  vaso-dilator  effect,  in  certain  regions  has 
been  observed  as  the  result  of  stimulating  the  cervical  sympa- 
thetic. And  we  may  now  turn  to  other  nerves  in  which  such  a 
double  effect,  now  a  vaso-constrictor,  now  a  vaso-dilator  effect, 
may  be  more  readily  obtained. 

In  the  frog  as  we  have  seen,  division  of  the  nerves  of  the  leg 
leads  to  a  widening  of  the  arteries  of  the  web  of  the  foot  of  the 
same  side,  and  stimulation  of  the  peripheral  end  of  the  nerve 
causes  a  constriction  of  the  vessels,  which,  if  the  stimulation  be 
strong,  may  be  so  great  that  the  web  appears  for  the  time  being 
to  be  devoid  of  blood.  Also  in  a  mammal  division  of  the  sciatic 
nerve  causes  a  similar  widening  of  the  small  arteries  of  the  skin 
of  the  leg.  Where  the  condition  of  the  circulation  can  be  readily 
examined,  as  for  instance  in  the  hairless  balls  of  the  toes,  espe- 
cially when  these  are  not  pigmented,  the  vessels  are  seen  to  be 
dilated  and  injected ;  and  a  thermometer  placed  between  the  toes 
shews  a  rise  of  temperature  amounting,  it  may  be,  to  several 
degrees.     If  moreover  the  peripheral  end  of  the  divided  nerve  be 


270        VASO-MOTOR  NERVES   OF  THE  LIMBS.      [Book  i. 

stimulated,  the  vessels  of  the  shin  become  constricted,  the  skin 
grows  pale,  and  the  temperature  of  the  foot  falls.  And  very  similar 
results  are  obtained  in  the  forelimb  by  division  and  subsequent 
stimulation  of  the  nerves  of  the  brachial  plexus. 

The  quantity  of  blood  present  in  the  blood  vessels  of  a  part  of  the 
body  or  of  an  organ  of  the  mammal  may  sometimes  be  observed 
direct]}^  by  means  of  the  ijletliysviograph,  of  which  we  have  already 
spoken  (§  104),  but  has  frequently  to  be  determined  indirectly.  The 
temperature  of  a  passive  structure  subject  to  cooling  influences,  such 
as  the  skin,  is  largely  dependent  on  the  supply  of  blood:  the  more 
abundant  the  supply,  the  warmer  the  part.  Hence  in  these  parts 
variations  in  the  quantity  of  blood  may  be  inferred  from  variations  of 
temperature;  but  in  dealing  with  more  active  structures  such  as 
muscles  there  are  obviously  sources  of  error  in  the  possibility  of  the 
treatment  adopted,  such  as  the  stimulation  of  a  nerve,  giving  rise  to 
an  increase  of  temperature  due  to  increased  metabolism,  independent 
of  variations  in  blood  supply. 

So  far  the  results  are  quite  like  those  obtained  by  division  and 
stimulation  of  the  cervical  sympathetic,  and  we  might  infer  that 
the  sciatic  nerve  and  brachial  plexus  contain  vaso-constrictor 
fibres  only  for  the  vessels  of  the  skin  of  the  hind  limb  and  fore 
limb,  vaso-dilator  fibres  being  absent.  But  sometimes  a  different 
result  is  obtained ;  on  stimulating  the  divided  sciatic  nerve  the 
vessels  of  the  foot  are  not  constricted  but  dilated,  perhaps  widely 
dilated.  And  this  vaso-dilator  action  is  almost  sure  to  be  mani- 
fested when  the  nerve  is  divided,  and  the  peripheral  stump  stimu- 
lated some  time,  two  to  four  days,  after  division,  by  which  time 
commencing  degeneration  has  begun  to  modify  the  irritability  of 
the  nerve.  For  example,  if  the  sciatic  be  divided,  and  some  days 
afterwards,  by  which  time  the  flushing  and  increased  tempera- 
ture of  the  foot,  following  upon  the  section,  has  wholly  or  largely 
passed  away,  the  peripheral  stump  be  stimulated  with  an  inter- 
rupted current  a  renewed  flushing  and  rise  of  temperature  is  the 
result.  We  are  led  to  conclude  that  the  sciatic  nerve  (and  the 
same  holds  good  for  the  brachial  plexus)  contains  both  vaso-con- 
strictor and  vaso-dilator  fibres,  and  to  interpret  the  varying  result 
as  due  to  variations  in  the  relative  irritability  of  the  two  sets  of 
fibres.  The  constrictor  fibres  appear  to  predominate  in  these 
nerves,  and  hence  constriction  is  the  more  common  result  of 
stimulation ;  the  constrictor  fibres  also  appear  to  be  more  readily 
affected  by  a  tetanizing  current  than  do  the  dilator  fibres.  When 
the  nerve  after  division  commences  to  degenerate  the  constrictor 
fibres  lose  their  irritability  earlier  than  the  dilator  fibres,  so  that 
at  a  certain  stage  a  stimulus,  such  as  the  interrupted  current, 
while  it  fails  to  affect  the  constrictor  fibres,  readily  throws  into 
action  the  dilator  fibres.  The  latter,  indeed,  appear  to  retain  their 
irritability  after  section  of  the  nerve  for  a  much  longer  time  than 


Chap,  iv.]  THE   VASCULAR   MECHANISM.  271 

do  ordinary  motor  nerves  (§  78).  The  result  is  perhaps  even  still 
more  striking  if  a  mechanical  stimulus,  such  as  that  of  "  crimp- 
ing "  the  nerve  by  repeated  snips  with  the  scissors,  be  employed. 
Exposure  to  a  low  temperature  again  seems  to  depress  the  con- 
strictors more  than  the  dilators  ;  hence  when  the  leg  is  placed  in 
ice-cold  water  stimulation  of  the  sciatic,  even  when  the  nerve  has 
been  but  recently  divided,  throws  the  dilator  only  into  action  and 
produces  flushing  of  the  skin  with  blood.  Slow  rhythmical  stimu- 
lation moreover  of  even  a  freshly  divided  nerve  may  produce  dila- 
tion. And  there  are  other  facts  which  support  the  same  view 
that  the  sciatic  nerve  (and  brachial  plexus)  contains  both  vaso- 
constrictor and  vaso-dilator  fibres  which  are  differently  afi'ected  by 
different  circumstances. 

In  the  splanchnic  nerves  which  supply  fibres  to  the  blood  ves- 
sels of  so  large  a  part  of  the  abdominal  viscera,  there  is  abundant 
evidence  of  the  presence  of  vaso-constrictor  fibres.  Division  of 
this  nerve  leads  to  a  widening  of  the  blood  vessels  of  the  abdo- 
minal viscera,  stimulation  of  the  nerve  to  a  constriction  ;  and  as 
we  shall  see,  since  the  amount  of  blood  vessels  thus  governed  by 
this  nerve  is  very  large  indeed,  interference  either  in  the  one 
direction  or  the  other  with  its  vaso-motor  functions  produces  very 
marked  results,  not  only  on  the  circulation  in  the  abdomen  but 
on  the  whole  vascular  system.  There  is  some  evidence  that  the 
splanchnic  nerves  also  contain  vaso-dilator  fibres,  but  this  evi- 
dence is  of  a  more  or  less  indirect  character,  and  in  any  case,  the 
number  of  such  fibres  must  be  small. 

So  far  as  we  know,  the  vaso-motor  fibres  contained  in  the 
sciatic  and  the  like  spinal  nerves  are  distributed  chiefly  at  least 
to  the  blood  vessels  of  the  skin.  Though  so  large  a  part  of  the 
fibres  of  these  nerves  end  in  the  muscles,  the  evidence  of  vaso- 
motor fibres  passing  to  the  blood  vessels  of  the  muscles  is  by  no 
means  clear  and  undisputed.  The  blood  vessels  of  a  muscle  un- 
doubtedly may  change  in  calibre.  For  instance,  when  a  muscle 
contracts  there  is  always  an  increased  flow  of  blood  through  the 
muscle ;  this  may  be  in  part  a  mere  mechanical  result  of  the 
change  of  form,  the  shortening  and  thickening  of  the  fibres  open- 
ing out  the  minute  blood  vessels,  but  is  also,  if  not  chiefly,  due  to 
the  widening  of  the  arteries  hj  relaxation  of  their  muscular  walls. 
Such  a  widening  may  be  seen  when  a  thin  muscle  of  a  frog  is 
made,  in  the  living  body,  to  contract  under  the  microscope.  But 
this  widening  has  not  been  proved  beyond  dispute  to  be  due  to 
the  action  of  vaso-dilator  fibres.  Indeed  it  has  been  argued  that 
when  a  muscle  contracts,  some  of.  the  chemical  products  of  the 
metaliolism  of  the  muscle  may,  by  direct,  local  action  on  the 
minute  blood  vessels,  lead  to  a  widening  of  those  blood  vessels. 
And  in  some  other  organs,  the  brain  and  the  kidney  for  instance, 
we  find  functional  activity  accompanied  by  a  widening  of  the 
blood  vessels  under  circumstances  which  seem  to  preclude  the 


272         THE   COURSE   OF  VASO-MOTOR  FIBRES.     [Book  i. 

possibility  of  the  widening  being  due  to  vaso-dilator  impulses 
reaching  the  organ  from  without ;  in  such  instances  it  is  sug- 
gested  that  the  widening  is  due  to  a  local  effect  of  the  products 
of  the  activity  of  the  organ.  To  this  point  we  shall  return.  With 
regard  to  vaso-constrictor  fibres  also  the  evidence  that  they  are 
supplied  to  muscles  is,  in  like  manner,  not  beyond  dispute. 
Section  or  stimulation  of  the  nerves  induces  it  is  true  changes  in 
the  temperature  of  the  muscles  as  it  does  in  that  of  the  skin. 
But,  as  we  urged  just  now,  to  argue  from  this  that  changes  in  the 
blood  supply  have  taken  place  is  not  wholly  safe ;  moreover  the 
changes  in  temperature  observed  are  slight.  Again,  the  fact  that 
when  the  nerve  of  a  muscle  is  divided  the  blood  vessels  of  the 
muscle  widen,  somewhat  like  the  blood  vessels  of  the  ear  after 
division  of  the  cervical  sympathetic,  has  been  brought  forward  as 
indicating  the  presence  of  vaso-constrictor  fibres  carrying  the  kind 
of  influence  which  we  called  tonic,  leading  to  an  habitual  moder- 
ate constriction.  •  Neither  arguments  can  be  regarded  as  abso- 
lutely conclusive.  The  knowledge  we  possess  at  present  leaves 
us  in  fact  in  doubt  whether  the  blood-flow  through  the  muscles, 
though  these  form  so  large  a  part  of  the  body,  is  really  governed 
by  the  central  nervous  system. 

The  two  parts  of  the  body  undoubtedly  and  pre-eminently  sup- 
plied by  vaso-constrictor  fibres  proceeding  from  and  governed  by 
the  central  nervous  system  are  on  the  one  hand  the  skin  and  on 
the  other  hand  the  abdominal  viscera.  As  we  shall  see,  the  vari- 
ations in  the  blood  supply  to  the.  skin  are  more  strikingly  of  use 
to  the  body  at  large,  in  regulating  the  temperature  of  the  body 
for  instance,  than  they  are  to  the  skin  itself.  The  variations  in 
the  blood  supply  to  the  abdominal  viscera  also  serve  important 
general  purposes  ;  they  play  their  part  in  the  regulation  of  the 
temperature  of  the  body,  and  through  them  the  viscera  serve  as 
a  reservoir  to  which  blood  may  without  harm  be  shunted  when 
occasion  demands.  It  would  appear  as  if  the  vaso-constrictor 
mechanism  were  chiefly  used  for  the  general  purposes  of  the 
economy. 

Accepting  the  view  that  the  presence  of  vaso-dilator  fibres  in 
the  nerves  going  to  muscles  is  not  definitely  proved  and  disregard- 
ing the  scanty  and  more  or  less  obscure  vaso-dilators  of  the  sciatic 
and  other  spinal  nerves,  we  find  that  in  special  cases  only,  in 
cases  where  it  would  seem  that  special  means  are  needed  to 
secure  an  ample  flow  of  blood  through  a  particular  part,  unmis- 
takably vaso-dilator  fibres  are  present. 


The  Course  of  Vaso-motor  Fibres. 

§  147.     Both  the  vaso-constrictor  and  the  vaso-dilator  fibres 
have  their  origin  in  the  central  nervous  system,  the  spinal  cord 


Chap,  iv.]  THE   VASCULAR   MECHANISM.  273 

or  the  brain,  but  it  will  be  desirable  to  speak  of  the  course  of  the 
two  sets  separately. 

Vaso-constrictor  Fibres.  In  the  mammal,  so  far  as  we  know 
at  present,  all  the  vaso-constrictor  fibres  for  the  whole  body  take 
their  origin  in  the  middle  region  of  the  spinal  cord,  or  rather, 
leave  the  spinal  cord  by  the  nerves  belonging  to  this  middle 
region.  Thus  in  the  dog  the  vaso-constrictor  fibres,  not  only  for 
the  trunk  but  for  the  limbs,  head,  face  and  tail,  leave  the  spinal 
cord  by  the  anterior  roots  of  the  spinal  nerves  reaching  from 
about  the  second  thoracic  to  the  fourth  lumbar  nerve,  both  inclu- 
sive, though  some  few  may  pass  by  the  first  thoracic  and  by  the 
fifth  lumbar. 

Those  for  the  head  and  neck  leave  the  spinal  cord  as  we  have 
seen,  §  144,  chietty  by  the  second  and  third  thoracic  nerves, 
though  some  leave  by  the  fourth  and  a  variable  small  number  by 
the  fifth  and  by  the  first ;  those  for  the  fore  limbs  leave  by  a 
number  of  thoracic  nerves  reaching  from  the  fourth  to  the  ninth 
or  even  the  tenth,  those  by  the  seventh  being  the  most  numerous. 
Those  for  the  hind  limbs  leave  by  the  nerves  reaching  from  the 
eleventh  thoracic  to  the  third  lumbar,  some  passing  by  the  tenth 
thoracic  and  the  fourth  lumbar.  Those  for  the  tail  leave  by  the 
first,  second  and  third  lumbar.  And  those  for  the  trunk  leave 
by  the  successive  spinal  nerves  supplying  the  trunk.  This  ar- 
rangement may  be  taken  as  indicating  generally  how  these  fibres 
leave  the  spinal  cord,  bearing  in  mind  that  the  fourth  lumbar 
nerve  of  the  dog  corresponds  to  about  the  second  lumbar  of  man, 
and  that  the  details  differ  in  different  kinds  of  animals  and  indeed 
in  different  individuals. 

Running  in  the  case  of  each  nerve  root  to  the  mixed  nerve 
trunk  these  vaso-constrictor  fibres  pass  along  the  visceral  branch, 
white  ramus  communicans,  to  the  thoracic  and  abdominal  sympa- 
thetic ganglia  (Fig.  72).  From  thence  they  reach  their  destina- 
tion in  various  ways.  Thus,  those  going  to  the  head  and  neck  pass 
upward  through  the  annulus  of  Vieussens  to  the  lower  cervical 
ganglion  and  thence,  as  we  have  seen,  up  the  cervical  sympa- 
thetic; many  of  the  fibres  for  the  neck  however  pass  directly 
from  the  stellate  ganglion.  Those  for  the  abdominal  viscera  pass 
off  in  a  similar  way  by  the  splanchnic  nerves,  Fig.  72,  aid.  spl. 
and  by  smaller  nerves  joining  the  inferior  mesenteric  ganglion. 
Those  destined  for  the  arm,  making  their  way  backwards  by  grey 
rami  commnnicantes  (Fig.  23  r.  v.),  join  the  nerves  of  the  bracliial 
plexus;  while  those  for  the  hind  leg  pass  in  a  similar  way  through 
some  portion  of  the  abdominal  sympathetic  before  they  join  the 
nerves  of  the  sciatic  plexus.  These  as  we  have  seen  are  dis- 
tributed chiefiy  to  the  skin,  and  the  constrictor  fil»res  of  the  skin 
of  the  trunk  probably  reach  the  spinal  nerves  in  which  they 
ultimately  run  in  a  similar  manner.  All  the  vaso-constrictor 
fibres,  whatever  their  destination,  leave  the  spinal  cord  bv  the 

18 


'?74     COURSE   OF  VASO-CONSTRICTOR  FIBRES.     [Book  i. 

anterior  roots  of  spinal  nerves,  and  then  passing  through  the 
appropriate  visceral  branches,  join  the  thoracic  or  abdominal 
sympathetic  ganglia.  In  their  course  the  fibres  undergo  a  re- 
markable change. 

Along  the  anterior  root  and  along  the  visceral  branch  they  are 
medullated  fibres,  but  before  they  reach  the  blood  vessels  for 
which  they  are  destined  they  become  non-medullated  fibres ;  they 
appear  to  lose  their  medulla  in  some  or  other  of  the  ganglia. 

We  are  in  many  cases  able  to  determine  experimentally  by  the 
following  method,  the  ganglion  or  ganglia  in  which  particular 
fibres  end,  that  is  to  say  in  which  they  become  connected  with 
nerve  cells.  It  is  found  that  the  drug  nicotin  abolishes  or  sus- 
pends the  action  of  vaso-motor  fibres  and  of  other  fibres  running 
in  the  sympathetic  system.  Thus  in  a  rabbit,  after  a  certain  dose 
of  nicotin  has  been  given,  stimulation  of  the  cervical  sympathetic 
nerve  in  the  neck  no  longer  causes  constriction  of  the  vessels  of 
the  ear.  But  it  is  found  in  such  cases  that  though  stimulation  of 
the  trunk  of  the  nerve  in  the  neck  is  without  effect,  stimulation 
of  the  appropriate  nerve  branches  passing  off  from  the  superior 
cervical  ganglion  on  their  way  to  the  ear,  does  produce  constric- 
tion of  tiie  vessels  of  the  ear.  Obviously  the  nicotin  does  not 
affect  the  peripheral  fibres  and  endings  of  the  nerve,  but  some 
part  of  the  nerve  more  central  than  the  branches  proceeding  from 
the  superior  cervical  ganglion.  Further,  if  the  ganglion  itself  be 
cautiously  painted  with  a  weak  (1  p.c.)  solution  of  nicotin,  care 
being  taken  to  avoid  excess,  stimulation  of  the  nerve  in  the  neck 
has  no  effect  on  the  vessels  of  the  ear,  whereas  if  the  nicotin  be 
applied  to  a  corresponding  extent  to  the  trunk  of  the  nerve  in  the 
neck,  none  being  allowed  to  have  access  to  the  oano-lion,  stimu- 
lation  of  the  trunk  in  the  neck,  even  if  applied  to  the  very  spot 
on  which  the  nicotin  has  been  placed,  produces  the  usual  con- 
striction of  the  vessels  of  the  ear.  Obviously  the  nicotin  produces 
its  paralysing  effects  by  acting  on  the  nerve  cells,  or  on  the  fibres 
just  as  they  are  becoming  connected  with  nerve  cells.  If  the 
solution  of  nicotin  be  applied  not  to  the  upper,  but  to  the  middle 
or  to  the  lower  cervical  ganglion,  stimulation  of  the  nerve  between 
the  ganglion  and  the  spinal  cord  produces  the  usual  constrictor 
effects.  This  shews  that  the  constrictor  fibres  pass  through  the 
lower  and  the  middle  ganglion  as  fibres,  not  connected  with  cells, 
otherwise  they  would  he  here  affected  by  nicotin  ;  they  are  affected 
by  nicotin  in  the  upper  ganglion,  and  we  therefore  infer  that  they 
end  in,  that  is,  are  connected  with  cells  in  that  ganglion.  In  the 
same  way  it  may  be  found  that  the  vaso-constrictor  fibres  of  the 
abdominal  splanchnic  are  connected  with  cells  in  the  solar  plexus. 
Indeed  by  this  method  we  may  determine  in  what  ganglia  the 
vaso-constrictor  and  other  fibres  of  the  sympathetic  system  end ; 
and  a  remarkable  distribution,  determined  by  morphological 
causes  among  others,  has  in  this  way  been  made  out,  some  fibres 


Cpiap.  IV.]  THE   VASCULAR   MECHANISM.  275 

very  speedily  becoming  connected  with  nerve  cells,  others  run- 
ning a  very  long  course  before  they  so  end. 

§  148.  Vaso-dilator  Fibres.  Some  jf  these  appear  to  run 
much  the  same  course  as  the  vaso-constrictors.  Such  are  the 
vaso-dilator  fibres  running  in  spinal  nerves  like  the  sciatic  and 
brachial,  those  which  seem  to  be  present  in  the  splanchnic,  and 
certain  fibres  of  the  cervical  sympathetic  which  in  some  animals 
at  least  act  as  vaso-dilators  towards  certain  parts  of  the  mouth 
and  face.  With  regard  to  these,  the  evidence  of  whose  existence, 
as  we  have  seen,  is  at  least  in  most  cases,  difficult,  special  or 
indirect,  we  have  at  present  no  proof  that  their  general  course  is 
essentially  different  from  that  of  the  constrictors. 

The  more  distinct  and  notable  vaso-dilators  however  do  run  a 
different  course.  These  are  found  in  the  nerves  coming  from  the 
cranial  and  sacral  regions  of  the  central  nervous  system  whence, 
as  we  have  seen,  no  vaso-constrictor  fibres  are  known  to  issue. 
Thus  the  vaso-dilator  fibres  for  the  sub-maxillary  gland  running 
in  the  chorda  tympani  may  be  traced  as  we  have  seen  back  to 
the  facial  or  seventh  nerve ;  and  the  continuation  of  the  chorda 
tympani  along  the  lingual  nerve  to  the  tongue  contains  vaso-dila- 
tor fibres  for  that  organ ;  when  the  linoual  is  stimulated,  the 
blood  vessels  of  the  tongue  dilate  owing  to  the  stimulation  of  the 
conjoined  chorda  tympani  fibres.  The  ramus  tympanicus  of 
the  glossopharyngeal  nerve  contains  vaso-dilator  fibres  for  the 
parotid  gland,  and  it  appears  probable  that  the  trigeminal  nerve 
contains  vaso-dilator  fibres  for  the  eye  and  nose  and  possibly  for 
other  parts.  The  vaso-dilator  fibres  which  pass  into  the  ncrvi 
erigentes,  leave  the  sacral  region  of  the  cord  by  the  anterior  roots 
of  the  sacral  nerves,  the  particular  nerves  differing  in  difi'erent 
animals;  thus  in  the  dog  and  cat  they  pass  by  the  first,  second 
and  third,  in  the  rabbit  by  the  second,  third  and  fourth,  in  man 
by  the  third,  fourth  and  fifth  sacral  nerves. 

In  these  instances  the  vaso-dilator  fibres,  as  they  leave  the 
central  nervous  system,  are,  like  the  vaso-constrictor  fibres,  fine 
medullated  fibres,  but  unlike  the  majority  at  least  of  the  vaso- 
constrictors they  retain  their  medulla  for  the  greater  part  of 
their  course  and  only  lose  it  near  their  termination  in  the  tissue 
whose  blood  vessels  they  supply. 


Tlie  Effects  of  Vaso-motor  Actions. 

§  149.  A  very  little  consideration  will  shew  that  vaso-motor 
action  is  a  most  important  factor  in  the  circulation.  In  the  first 
place  the  wdiole  flow  of  blood  in  the  body  is  adapted  to  and 
governed  by  what  we  may  call  the  general  tone  of  the  arteries 
of  the  body  at  large.  In  a  normal  condition  of  the  body,  tlie 
muscular  fibres  of  a  verv  large  number  of  the  minute  arteries 


276  EFFECTS   OF   VASO-MOTOR   ACTIONS.       [Book  i. 

of  the  body  are  in  a  state  of  tonic,  i.  e.  of  moderate,  contraction, 
and  it  is  the  narrowing  due  to  this  contraction  which  forms 
a  large  item  of  that  peripheral  resistance  which  we  have  seen 
to  be  one  of  the  great  factors  of  blood  pressure.  The  nor- 
mal general  blood  pressure,  and  therefore  the  normal  How  of 
blood,  is  in  fact  dependent  on  the  '  general  tone '  of  the  minute 
arteries. 

In  the  second  place  local  vaso-motor  changes  in  the  condi- 
tion of  the  minute  arteries,  changes,  that  is  to  say,  of  any  par- 
ticular vascular  area,  have  very  decided  effects  on  the  circulation. 
These  changes,  though  local  themselves,  may  have  effects  which 
are  both  local  and  general,  as  the  following  considerations  will 
shew. 

Let  us  suppose  that  the  artery  ^  is  in  a  condition  of  normal 
tone,  is  midway  between  extreme  constriction  and  dilation.  The 
flow  through  A  is  determined  by  the  resistance  in  A  and  in  the 
vascular  tract  which  it  supplies,  in  relation  to  the  mean  arterial 
pressure,  which  again  is  dependent  on  the  way  in  which  the  heart 
is  beating  and  on  the  peripheral  resistance  of  all  the  small  arteries 
and  capillaries,  A  included.  If,  while  the  heart  and  the  rest  of 
the  arteries  remain  unchanged,  A  be  constricted,  the  peripheral 
resistance  in  A  will  increase,  and  this  increase  of  resistance  will 
lead  to  an  increase  of  the  general  arterial  pressure.  Since,  as  we 
have  seen,  §  101,  it  is  arterial  pressure  which  is  the  immediate 
cause  of  the  flow  from  the  arteries  to  the  veins,  this  increase  of 
arterial  pressure  will  tend  to  drive  more  blood  from  the  arteries 
into  the  veins.  The  constriction  of  A  however,  by  increasing  the 
resistance,  opposes  any  increase  of  the  flow  through  A  itself,  in  fact 
will  make  the  flow  through  A  less  than  before.  The  whole  increase 
of  discharge  from  the  arterial  into  the  venous  system  will  take 
place  through  the  arteries  in  which  the  resistance  remains  un- 
changed, that  is,  through  channels  other  than  A.  Thus,  as  the 
result  of  the  constriction  of  any  artery  there  occur,  (1)  diminished 
flow  through  the  artery  itself,  (2)  increased  general  arterial 
pressure,  leading  to  (3)  increased  flow  through  the  other  arteries. 
If,  on  the  other  hand,  A  be  dilated,  while  the  heart  and  other 
arteries  remain  unchanged,  the  peripheral  resistance  in  A  is 
diminished.  This  leads  to  a  lowering  of  the  general  arterial 
pressure,  which  in  turn  tends  to  drive  less  blood  from  the  arteries 
into  the  veins.  The  dilation  of  A  however,  by  diminishing  the 
resistance,  permits,  even  with  the  lowered  pressure,  more  blood  to 
pass  through  A  itself  than  before.  Hence  the  diminished  flow 
tells  all  the  more  on  the  rest  of  the  arteries  in  which  the  resistance 
remains  unchanged.  Thus,  as  the  result  of  the  dilation  of  any 
artery,  there  occur  (1)  increased  flow  of  blood  through  the  artery 
itself,  (2)  diminished  general  pressure,  and  (3)  diminished  flow 
through  the  other  arteries.  Where  the  artery  thus  constricted  or 
dilated  is  small,  the  local  effect,  the  diminution  or  increase  of  flow 


Chap,  iv.]  THE   VASCULAR   MECHANISM.  277 

through  itself,  is  much  more  marked  than  the  general  effects,  the 
change  in  blood  pressure  and  the  flow  through  other  arteries. 
When,  however,  the  area  the  arteries  of  which  are  affected  is  large, 
the  general  effects  are  very  striking.  Thus  if  while  a  tracmg  of 
the  blood  pressure  is  being  taken  by  means  of  a  manometer 
connected  with  the  carotid  artery,  the  abdominal  splanchnic  nerves 
be  divided,  a  conspicuous  but  steady  fall  of  pressure  is  observed, 
very  similar  to  but  more  marked  than  that  which  is  shewn  in 
Fig.  74.  The  section  of  the  abdominal  splanchnic  nerves  causes 
the  arteries  of  the  abdominal  viscera  to  dilate,  and  these  being 
very  numerous,  a  large  amount  of  peripheral  resistance  is  taken 
away,  and  the  blood  pressure  falls  accordingly ;  a  large  increase 
of  flow  into  the  portal  veins  takes  place,  and  the  supply  of  blood 
to  the  face,  arms,  and  legs  is  proportionally  diminished.  It  will 
be  observed  that  the  dilation  of  the  arteries  is  not  instantaneous 
but  somewhat  gradual,  as  shewn  by  the  pressure  sinking  not 
abruptly  but  with  a  gentle  curve. 

The  general  effects  on  blood  pressure  by  vaso-motor  changes 
are  so  marked  that  the  manometer  may  be  used  to  detect  vaso- 
motor actions.  Thus,  if  the  stimulation  of  a  particular  nerve  or 
any  other  operation  leads  to  a  marked  rise  of  the  mean  blood 
pressure,  unaccompanied  by  any  notable  changes  in  the  heart  beat, 
we  may  infer  that  constriction  has  taken  place  in  the  arteries  of 
some  considerable  vascular  area;  and  similarly,  if  the  effect  be 
a  fall  of  blood  pressure,  we  may  infer  that  constriction  has  given 
way  to  dilation. 


Vaso-motor  Functions  of  the  Central  Nervous  System. 

§  150.  The  central  nervous  system,  to  which  we  have  traced 
the  vaso-motor  nerves,  makes  use  of  these  nerves  to  regulate  the 
flow  of  blood  through  the  various  organs  and  parts  of  the  body ; 
by  the  local  effects  thus  produced  it  assists  or  otherwise  influences 
the  functional  activity  of  this  or  that  organ  or  tissue ;  by  the 
general  effects  it  secures  the  well  being  of  the  body.  When  the 
vaso-dilators  are  brought  into  play  the  chief  effect  is  a  local 
one ;  when  a  general  effect  has  to  be  produced  the  vaso-con- 
strictors  are  employed,  though  these  of  course  also  bring  about 
local  effects.     And  we  may  consider  the  two  separately. 

The  vaso-dilator  nerves,  the  use  of  which  is  more  simple 
than  that  of  the  vaso-constrictors  in  so  far  as  it  appears  not 
to  be  complicated  by  the  presence  of  habitual  tonic  influences, 
occur  as  parts  of  distinct  mechanisms  used  chiefly  at  least  as 
reflex  mechanisms,  with  centres  placed  in  different  regions  of  the 
central  nervous  system.  Thus,  when  food  is  placed  in  the  mouth 
afferent  impulses,  generated  in  the  nerves  of  taste,  give  rise  in 
the  central  nervous  system  to  efferent  impulses,  which  descend 


278  USE    OF   VASO-DILATOR   FIBRES.  [Book  i. 

the  chorda  tympani  and  other  nerves  to  the  salivary  glands  and, 
by  dilating  the  blood  vessels,  secuie  a  copious  liow  of  blood 
through  the  glands  while,  as  we  shall  see  later  on,  they  excite 
the  glands  to  secrete.  The  centre  of  this  retlex  action  appears 
to  lie  in  the  spinal  bulb  and  may  be  thrown  into  activity  not 
only  by  impulses  reaching  it  along  the  specific  nerves  of  taste, 
but  also  by  impulses  passing  along  other  channels ;  thus,  emotions 
started  in  the  brain  by  the  sight  of  food  or  otherwise  may  give 
rise  to  impulses  passing  down  along  the  central  nervous  system 
itself  to  the  spinal  bulb,  or  events  in  the  stomach  may  send 
impulses  up  the  vagus  nerve,  or  stimulation  of  one  kind  or  another 
may  send  impulses  up  almost  any  sentient  nerve,  and  these 
various  impulses  reaching  the  spinal  bulb  may,  by  reflex  action, 
throw  into  activity  the  vaso-dilator  fibres  of  the  chorda  tympani 
and  other  analogous  nerves,  and  bring  about  a  flushing  of  the 
salivary  glands,  while  at  the  same  time  they  cause  the  glands  to 
secrete. 

The  vaso-dilator  fibres  of  the  nervi  erigentes  may  be  thrown 
into  activity  in  a  similar  re  fie  x  way,  the  centre,  which  is  also 
easily  thrown  into  activity  by  impulses  descending  down  the  spinal 
cord  from  the  brain,  being  placed  in  the  sacral  and  perhaps  also 
in  the  upper  lumbar  or  lower  thoracic  region  of  the  spinal  cord. 
That  such  a  centre  does  exist  is  shewn  by  the  fact  that,  when 
in  a  dog  the  spinal  cord  is  completely  divided  in  the  thoracic 
region,  erection  of  the  penis  may  readily  be  brought  about  by 
stimulation  of  appropriate  sentient  surfaces.  And  other  instances 
might  be  quoted  in  which  vaso-dilator  fibres  appear  as  part  of  a 
refiex  mechanism  the  centre  of  which  is  placed  in  the  central 
nervous  system  not  far  from  the  origin  of  the  nerves  in  which  the 
vaso-dilator  fibres  run. 

§  151.  Turning  now  to  the  vaso-constrictor  fibres  we  find 
that  these  form  a  more  coherent  system  ;  and  this  is  in  accordance 
with  the  feature  of  the  vaso-constrictor  mechanisms,  that  they  are 
largely  employed  to  produce  general  effects  Moreover  their  utility 
is  increased,  though  at  the  same  time  their  use  becomes  somewhat 
more  complicated,  by  reason  of  the  existence  of  tonic  influences ; 
since  the  same  fibres  may,  on  the  one  hand,  by  an  increase  in  the 
impulses  passing  along  them,  be  the  means  of  constriction,  and 
on  the  other  hand,  by  the  removal  or  diminution  of  the  tonic 
influences  passing  along  them,  be  the  means  of  dilation.  We  have 
already  traced  all  the  vaso-constrictor  fibres  from  the  middle 
region  of  the  spinal  cord  to  the  sympathetic  system  in  the  thorax 
and  abdomen  ;  from  thence  they  pass  (1)  by  the  splanchnic, 
hypogastric,  and  other  nerves  to  the  viscera  of  the  abdomen  and 
pelvis,  (concerning  the  vaso-motor  nerves  of  the  thoracic  viscera 
we  know  at  present  very  little),  (2)  by  the  cervical  sympathetic 
to  the  skin  of  the  head  and  neck,  the  salivary  glands  and  mouth, 
the  eyes  and  other  parts,  and  possibly  the  brain   including   its 


Chap,  iv.]  THE   VASCULAR   MECHANISM,  279 

membranes,  though  the  presence  of  vaso-motor  fibres  in  the 
brain  itself  is  much  disputed,  (3)  by  the  brachial  and  sciatic 
plexuses  to  the  skin  of  the  fore-  and  hind-limbs,  and  by  various 
other  nerves  to  the  skin  of  the  trunk.  The  chief  parts  of  the 
body  supplied  by  vaso-constrictor  fibres  appear  to  be  the  skin 
with  its  appendages,  and  the  alimentary  canal  with  its  appendages, 
glandular  and  other ;  the  great  mass  of  skeletal  muscles  appears, 
as  we  have  seen,  to  receive  a  relatively  small  supply  of  vaso-con- 
strictor fibres. 

If  in  an  animal  the  spinal  cord  be  divided  in  the  lower  thoracic 
region,  the  skin  of  the  legs  becomes  flushed,  their  temperature 
frequently  rises,  and  tliere  is  a  certain  amount  of  fall  in  the 
general  blood  pressure  as  measured,  for  instance,  in  the  carotid ; 
and  this  state  of  things  may  last  for  some  considerable  time. 
Obviously  the  section  of  the  spinal  cord  has  cut  off  the  usual  tonic 
influences  descending  to  the  lower  limbs  ;  in  consequence  the 
blood  vessels  have  become  dilated,  in  consequence  the  general 
peripheral  resistance  has  become  proportionately  diminished,  and 
in  consequence  the  general  blood  pressure  has  fallen.  The  tonic 
vaso-constrictor  impulses  for  the  lower  limbs,  therefore,  have  their 
origin  in  the  central  nervous  system  higher  up  than  the  lower 
thoracic  region  of  the  spinal  cord. 

If  the  spinal  cord  be  divided  higher  up,  say  above  the  roots  of 
the  fifth  or  sixth  thoracic  nerves,  the  cutaneous  blood  vessels  of 
the  lower  limbs  dilate,  as  in  the  former  case,  and  on  examination 
it  will  be  found  that  the  blood  vessels  of  the  abdomen  are  also 
largely  dilated ;  at  the  same  time  the  blood  pressure  undergoes  a 
very  marked  fall,  it  may  indeed  be  reduced  to  a  very  few  milli- 
meters of  mercury.  Obviously  the  tonic  vaso-constrictor  impulses 
passing  to  the  abdomen  and  to  the  lower  limbs  take  origin  in  the 
central  nervous  system  higher  up  than  the  level  of  the  fifth 
thoracic  nerve. 

If  the  section  of  the  spinal  cord  be  made  above  the  level  of 
the  second  thoracic  nerve,  in  addition  to  the  abovementioned 
results  the  vessels  of  the  head  and  face  also  become  dilated ;  but 
in  consequence  of  the  fall  of  general  blood  ])ressure  just  mentioned, 
these  vessels  never  become  so  full  of  blood,  the  loss  of  tone  is  not 
so  obvious  in  them  as  after  simple  division  of  the  cervical  sym- 
pathetic, since  the  latter  operation  produces  little  or  no  effect  on 
the  general  blood  pressure. 

Obviously  then  the  tonic  vaso-constrictor  impulses,  which 
passing  to  the  skin  and  viscera  of  tlie  body  maintain  that  tonic 
narrowing  of  so  many  small  arteries  by  which  the  general  peri- 
pheral resistance,  and  so  the  general  blood  pressure,  is  maintained, 
proceed  from  some  part  of  the  central  nervous  system  higher  up 
than  the  upper  thoracic  region  of  the  spinal  cord.  And,  since 
exactly  the  same  results  follow  upon  section  of  the  spinal  cord  in 
the  cervical   region    right  up    to  the  lower   limit  of   the  spinal 


280  VASO-MOTOR  CENTRE.  [Book  i. 

bulb,  we  infer  that  these  tonic  impulses  proceed  from  the  spinal 
bulb. 

On  the  other  hand  we  may  remove  the  whole  of  the  brain 
ri^ht  down  to  the  upper  limits  of  the  spinal  bulb,  and  yet  produce 
no  ilushing,  or  only  a  shght  transient  flushing,  of  any  part  of  the 
body  and  no  fall  at  all,  or  only  a  slight  transient  fall,  of  the 
general  blood  pressure.  We  therefore  seem  justified  in  assuming 
the  existence  in  the  spinal  bulb  of  a  nervous  centre,  which  we 
may  speak  of  as  a  vaso-motor  centre,  or  the  bulbar  vaso-motor 
centre,  from  which  proceed  tonic  vaso-constrictor  impulses,  or 
which  regulates  the  emission  and  distribution  of  such  tonic  vaso- 
constrictor impulses  or  influences  over  various  parts  of  the  body. 

§  152.  The  existence  of  this  vaso-motor  centre  may,  moreover, 
be  shewn  in  another  way.  The  extent  or  amount  of  the  tonic 
constrictor  impulses  proceeding  from  it  may  be  increased  or 
diminished,  the  activity  of  the  centre  may  be  augmented  or 
inhibited,  by  impulses  reaching  it  along  various  afferent  nerves ; 
and  provided  no  marked  changes  in  the  heart  beat  take  place  at 
the  same  time,  a  rise  or  fall  of  general  blood  pressure  may  be 
taken  as  a  token  of  an  increase  or  decrease  of  the  activity  of  the 
centre. 

In  the  rabbit  there  is  found  in  the  neck,  lying  side  by  side 
with  the  cervical  sympathetic  nerve  and  running  for  some  distance 
in  company  with  it,  a  slender  nerve  which  may  be  ultimately 
traced  down  to  the  heart,  and  which,  if  traced  upwards,  is  found  to 
come  off  somewhat  high  up  from  the  vagus,  by  two  or  more  roots, 
one  of  which  is  generally  a  branch  of  the  superior  laryngeal  nerve. 
This  nerve  (the  fibres  constituting  which  are  in  the  dog  bound  up 
with  the  vagus,  and  do  not  form  an  independent  nerve)  appears 
to  be  exclusively  an  afferent  nerve ;  when  after  division  of  the 
nerve  the  peripheral  end,  the  end  still  in  connection  with  the 
heart,  is  stimulated  no  marked  results  follow.  The  beginnings  of 
the  nerve  in  the  heart  are  therefore  quite  different  from  the 
endings  of  the  inhibitory  fibres  of  the  vagus,  or  of  the  augmentor 
fibres  of  the  sympathetic  system ;  the  nerve  has  nothing  to  do 
with  the  nervous  regulation  of  the  heart  treated  of  in  Sec.  5. 
If  now,  while  the  pressure  in  an  artery  such  as  the  carotid  is  being 
registered,  the  central  end  of  the  nerve  (i.e.  the  one  connected 
with  the  brain)  be  stimulated  with  the  interrupted  current,  a 
gradual  but  marked  fall  of  pressure  (Fig.  74)  in  the  carotid  is 
observed,  lasting,  when  the  period  of  stimulation  is  short,  some 
time  after  the  removal  of  the  stimulus.  Since  the  beat  of  the 
heart  is  not  markedly  changed,  the  fall  of  pressure  must  be  due  to 
the  diminution  of  peripheral  resistance  occasioned  by  the  dilation 
of  some  arteries.  And  it  is  probable  that  the  arteries  thus 
dilated  are  chiefly  if  not  exclusively  those  arteries  of  the  ab- 
dominal viscera  which  are  governed  by  the  splanchnic  nerves;  for 
if  these  nerves  are  divided  on  both  sides  previous  to  the  experi- 


Chap,  iv.]  THE  VASCULAR  MECHANISM.  281 

ment,  the  fall  of  pressure  when  the  nerve  is  stimulated  is  very- 
small,  in  fact  almost  insignificant.  The  inference  we  draw  is  as 
follows.     The  afferent  impulses  passing  upwards  along  the  nerve 


■•''^'\AAW-'^^'^Ar'^ 


Fig.  74.    Tracing,  shewing  the  Effect  on  Blood  Pressure  of  stimulating 

THE    CENTRAL    END   OF    THE    DEPRESSOR    NeRVB    IN    THE    RaBBIT. 

On  the  time  marker  below  the  intervals  correspond  to  seconds.     At  x  an  interrupted 
current  was  thrown  into  the  nerve. 

in  question  have  so  affected  some  part  of  the  central  nervous 
system  that  the  influences  which,  in  a  normal  condition  of  things, 
passing  along  the  splanchnic  nerves  keep  the  minute  arteries  of 
the  abdominal  viscera  in  a  state  of  moderate  tonic  constriction, 
fail  altogether,  and  those  arteries  in  consequence  dilate  just  as 
they  do  when  the  splanchnic  nerves  are  divided,  the  effect  being 
possibly  increased  by  the  similar  dilation  of  other  vascular  areas. 
Since  stimulation  of  the  nerve  of  which  we  are  speaking  always 
produces  a  fall,  never  a  rise  of  blood  pressure,  the  amount  of  fall 
of  course  being  dependent  on  circumstances,  such  as  the  condition 
of  the  nervous  system,  state  of  blood  pressure  and  the  like,  the 
nerve  is  known  by  the  name  of  the  depressor  nerve.  As  we  shall 
point  out  later  on,  by  means  of  this  afferent  nerve  from  the 
heart  the  peripheral  resistance  is,  in  the  living  body,  lowered  to 
suit  the  weakened  powers  of  a  labouring  heart. 

This  gradual  lowering  of  blood  pressure  by  diminution  of 
peripheral  resistance  affords  a  marked  contrast  to  the  sudden 
lowering  of  blood  pressure  by  cardiac  inhibition ;  compare  Fig.  74 
with  Fig.  70. 

§  153.  But  the  general  blood  pressure  may  be  modified  by 
afferent  impulses  passing  along  other  nerves  than  the  depressor, 
the  modification  taking  on,  according  to  circumstances,  the  form 
either  of  decrease  or  of  increase. 

Thus,  if  in  an  animal  placed  under  the  influence  of  urari 
(some  anesthetic  other  than  chloral  &c.  being  used),  the  central 
stump  of  the  divided  sciatic  nerve  be  stimulated,  an  increase 
of  blood  pressure  (Fig.  75)  almost  exactly  the    reverse    of   the 


282  DEPRESSOR  NERVE.  [Book  i. 

decrease  brought  about  by  stimulating  the  depressor,  is  observed. 
The  curve  of  tlie  blood  pressure,  after  a  latent  period  during  which 
no  changes  are  visible,  rises  steadily,  reaches  a  maximum  and 


JW\/VW\MAfVWW^' 


~f^^f' 


.^...^vw.--^*^ 


Fig.  75.     Effect  on  Blood  Pressure  Curve  of  stimulating  Sciatic  Nerve 

UNDER  Urari  (Cat). 

X  marks  the  moment  in  which  the  current  was  thrown  into  the  nerve.     Artificial 
respiration  was  carried  on,  and  the  usual  respiratory  undulations  are  absent. 

soon  slowly  falls  again,  the  fall  sometimes  beginning  to  appear 
before  the  stimulus  has  been  removed.  This  rise  of  pressure, 
since  it  may  be  observed  in  the  absence  of  any  increase  in  the 
heart  beat,  such  at  least  as  could  give  rise  to  it,  must  be  due  to 
the  constriction  of  certain  arteries ;  the  arteries  in  question  being 
those  of  the  splanchnic  area  certainly,  and  possibly  those  of  other 
vascular  areas  as  well.  The  effect  is  not  confined  to  the  sciatic ; 
stimulation  of  any  nerve  containing  afferent  fibres  may  produce 
the  same  rise  of  pressure,  and  so  constant  is  the  result  that  the 
experiment  has  been  made  use  of  as  a  method  for  determining  the 
existence  of  afferent  fibres  in  any  given  nerve  and  even  the  paths 
of  centripetal  impulses  through  the  spinal  cord. 

If,  on  the  other  hand,  the  animal  be  under  the  influence 
not  of  urari  but  of  a  large  dose  of  chloral,  instead  of  a  rise  of 
blood  pressure  a  fall,  very  similar  to  that  caused  by  stimulating 
the  depressor,  is  observed  when  an  afferent  nerve  is  stimulated. 
The  condition  of  the  central  nervous  system  seems  to  determine 
whether  the  effect  of  afferent  impulses  on  the  central  nervous 
system  is  one  leading  to  an  augmentation  of  vaso-constrictor 
impulses,  and  so  to  a  rise,  or  one  leading  to  a  diminution  of  vaso- 
constrictor impulses  and  so  to  a  fall  of  blood  pressure. 

§  154.  We  have  used  the  words  '  central  nervous  system  '  in 
speaking  of  the  above ;  we  have  evidence,  however,  that  the  part 
of  the  central  nervous  system  acted  on  by  the  afferent  impulses 
is  the  vaso-raotor  centre  in  the  spinal  bulb,  and  that  the  effects  in 
the  way  of  diminution  (depressor)  or  of  augmentation  (pressor)  are 
the  results  of  afferent  impulses  inhibiting  or  augmenting  the  tonic 
activity  of  this  centre  or  of  a  part  of  this  centre  especially 
connected  with  the  splanchnic  nerves.  The  whole  brain  may  be 
removed  right  down  to  the  bulb,  and  yet  the  effects  of  stimulation 
in  the  direction  either  of  diminution  or  of  augmentation  may  still 
be  brought  about.     If  the  bulb  be  removed,  these  effects  are  no 


CiiAP.  IV.]  THE   VASCULAR   MECHANISM.  283 

longer  seen,  though  all  the  rest  of  the  nervous  system  be  left  intact. 
Nay,  more,  by  partially  interfering  with  the  bulb,  we  may  partially 
diminish  these  effects  and  mark  out,  so  to  speak,  the  limits  of 
the  centre  in  question  within  the  bulb  itself.  Thus,  in  an  intact 
animal  under  urari,  stimulation  of  the  sciatic  nerve  with  a  stimulus 
of  a  certain  strength  will  produce  a  rise  of  blood  pressure  up  to 
a  certain  extent.  After  removal  of  the  whole  brain  right  down 
to  the  bulb,  the  same  stimulation  will  produce  the  same  rise  as 
before  ;  the  vaso-motor  centre  has  not  been  interfered  with.  Pro- 
ceeding downwards,  however,  and  removing  the  bulb  piecemeal 
by  successive  transverse  sections  a  level  is  soon  met  with,  beyond 
which  removal  of  the  nervous  substance  causes  an  obvious  dim- 
inution in  the  effect  produced  by  the  stimulation  of  the  sciatic  ; 
this  marks  the  upper  limit  of  the  centre.  Proceeding  still  further 
downwards  with  successive  slices,  stimulation  of  the  sciatic  pro- 
duces less  and  less  rise  of  blood  pressure,  until  at  last  a  level  is 
reached,  at  which  even  strong  stimulation  of  the  sciatic  or  any 
other  afferent  nerve  produces  no  effect  at  all  on  blood  pressure ; 
this  marks  the  lower  limit  of  the  centre.  In  this  way  the  lower 
limit  of  the  bulbar  vaso-motor  centre  has  been  determined  in 
the  rabbit  at  a  horizontal  line  drawn  about  4  or  5  mm.  above  the 
point  of  the  calamus  scriptorius,  and  the  upper  limit  at  about 
4  mm.  higher  up,  i.e.  about  1  or  2  mm.  below  the  corpora  quadri- 
gemiiia.  We  may  add  that  the  centre  appears  to  be  bilateral, 
the  halves  being  placed  not  in  the  middle  line  but  more  sideways 
and  rather  nearer  the  anterior  than  the  posterior  surface.  But 
we  will  reserve  what  we  have  to  say  as  to  the  structural  features 
of  this  centre  until  we  come  to  study  the  spinal  bulb  in  detail. 

§  155.  The  above  experiments  appear  to  afford  adequate  evi- 
dence that,  in  a  normal  state  of  the  body,  the  integrity  of  the 
bulbar  vaso-motor  centre  is  essential  to  the  production  and  dis- 
tribution of  those  continued  constrictor  impulses  by  which  the 
general  arterial  tone  of  the  body  is  maintained,  and  that  an 
increase  or  decrease  of  vaso-constrictor  action  in  particular  arteries, 
or  in  the  arteries  generally,  is  brought  about  by  means  of  the  same 
bulbar  vaso-motor  centre.  But  we  must  not  therefore  conclude 
that  this  small  portion  of  the  spinal  bulb  is  the  only  part  of 
the  central  nervous  system  which  can  act  as  a  centre  for  vaso-con- 
strictor fibres ;  and,  so  we  have  seen,  there  is  no  evidence  at 
present  that  the  vaso-dilator  fibres  are  connected  with  either  this 
or  any  other  one  centre.  In  the  frog  reflex  vaso-motor  effects  may 
be  obtained  by  stimulating  various  afferent  nerves  after  the  whole 
spinal  bulb  has  been  removed,  and,  indeed,  even  when  only  a  com- 
paratively small  portion  of  the  spinal  cord  has  been  left  intact,  and 
connected,  on  the  one  hand,  with  the  afferent  nerve  which  is  being 
stimulated,  and,  on  the  other,  with  the  efferent  nerves  in  which 
run  the  vaso-motor  fibres,  whose  action  is  being  studied.  In  the 
mammal  such  effects  do  not  so  readily  appear,  but  may  with  care 


284  SUMMARY   OF   VASO-MOTOR  ACTIONS.     [Book  i. 

and  under  special  conditions  be  obtained.  Thus  in  the  dog,  when 
the  spinal  cord  is  divided  in  the  thoracic  region,  the  arteries  of 
the  hind  limbs  and  hinder  part  of  the  body,  as  we  have  already 
said,  §  150,  become  dilated.  This  one  would  naturally  expect  as 
the  result  of  their  severance  from  the  bulbar  vaso-motor  centre. 
But  if  the  animal  be  kept  in  good  condition  for  some  time,  a 
normal  or  nearly  normal  arterial  tone  is  after  a  while  re-estab- 
lished; and  the  tone  thus  regained  may,  by  afferent  impulses 
reaching  the  cord  below  the  section,  be  modified  in  the  direction 
certainly  of  diminution,  i.  e.  dilation,  and  possibly,  but  this  is  not 
so  certain,  of  increase,  i.  e.  constriction ;  dilation  of  various  cutane- 
ous vessels  of  the  limbs  may  be  readily  produced  by  stimulation 
of  the  central  stump  of  one  or  another  nerve. 

These  and  other  results  lead  to  the  conclusion  that  the  bulbar 
vaso-motor  centre  is  not  to  be  regarded  as  the  sole  vaso-motor 
centre,  whence  alone  can  issue  tonic  constrictor  impulses  or 
whither  afferent  impulses  from  this  or  that  part  of  the  body  must 
always  travel  before  they  can  affect  the  vaso-constrictor  impulses 
passing  along  this  or  that  nerve.  We  are  rather  to  suppose  that 
the  spinal  cord  along  its  whole  length  contains,  interlaced  with 
the  reflex  and  other  mechanisms  by  which  the  skeletal  muscles 
are  governed,  vaso-motor  centres  and  mechanisms  of  varied  com- 
plexity, the  details  of  whose  functions  and  topography  have  yet 
largely  to  be  worked  out.  As  in  the  absence  of  the  sinus  venosus 
the  auricles  and  ventricle  of  the  frog's  heart  may  still  continue  to 
beat,  so  in  the  absence  of  the  spinal  bulb  these  spinal  vaso-motor 
centres  provide  for  the  vascular  emergencies  which  arise. 

§  156.  We  may  sum  up  the  history  of  vaso-motor  actions 
somewhat  as  follows. 

In  the  case  of  at  least  a  very  large  number  of  the  arteries  of 
the  body  we  have  direct  experimental  evidence  that  these  arteries 
are  connected  with  the  central  nervous  system  by  nerve  fibres, 
called  vaso-motor  fibres,  the  action  of  which  varies  the  amount  of 
contraction  of  the  muscular  coats  of  the  arteries  and  so  leads  to 
changes  in  calibre.  The  action  of  these  vaso-motor  fibres  is  more 
manifest,  and  probably  more  important  in  the  case  of  small  and 
minute  arteries  than  in  the  case  of  large  ones. 

These  vaso-motor  fibres  are  of  two  kinds.  The  one  kind,  vaso- 
constrictor fibres,  are  of  such  a  nature  or  have  such  connections 
at  their  peripheral  endings  that  stimulation  of  them  produces 
narrowing,  constriction  of  the  arteries.  During  life  these  fibres 
appear  to  be  the  means  by  which  the  central  nervous  system 
exerts  a  continued  tonic  influence  on  the  arteries  and  maintains 
an  arterial  '  tone ; '  and  this  arterial  tone  may  be  modified  by  the 
action  of  the  central  nervous  system,  so  as  to  give  place  on  the 
one  hand  to  constriction  and  on  the  other  to  widening.  The  other 
kind,  vaso-dilator  fibres,  are  of  such  a  kind,  or  have  such  connec- 
tions, that  stimulation  of  them  produces  widening,  dilation  of  the 


Chap,  iv.]  THE   VASCULAR   MECHANISM.  285 

arteries.     There  is  no  adequate  evidence  that  these  vaso-dilator 
fibres  serve  as  channels  for  tonic  dilating  impulses  or  influences. 

The  vaso-constrictor  fibres  leave  the  spinal  cord  by  the  anterior 
roots  of  the  nerves  coming  from  the  middle  region  only  of  the 
spinal  cord.  In  the  dog,  this  region  extends  from  about  the  first 
or  second  thoracic  to  the  fourth  or  fifth  lumljar  nerve ;  and  in 
other  animals  is  probably  of  corresponding  extent.  Leaving  the 
spinal  nerves  by  the  respective  visceral  branches,  rami  communi- 
cantes,  the  fibres  pass  into  the  sympathetic  system,  the  majority 
joining  the  main  sympathetic  chain  of  ganglia  in  the  thorax  and 
abdomen,  but  some,  for  instance  those  going  to  certain  parts  of 
the  intestine  and  some  other  viscera,  leaving  that  chain  on  one 
side  and  passing  directly  to  more  peripheral  ganglia,  such  as  the 
solar  plexus  and  the  inferior  mesenteric  ganglia.  From  the  sym- 
pathetic chain  the  fibres  run  to  their  destination  in  such  nerves 
as  the  cervical  sympathetic  and  splanchnic,  those  allotted  to  the 
skin  of  the  limbs  and  trunk  running  back  again  to  join  the  respec- 
tive spinal  nerves.  In  the  ganglia  of  the  sympathetic  chain  or  in 
other  more  peripheral  ganglia  the  fibres  lose  their  medulla,  and 
continue  their  course  as  non-medullated  fibres. 

In  the  intact  organism  the  emission  and  distribution  along 
these  vaso-constrictor  fibres  of  tonic  constrictor  impulses,  by  which 
general  and  local  arterial  tone  is  maintained  and  regulated,  is 
governed  by  a  limited  portion  of  the  spinal  bulb  known  as  the 
bulbar  vaso-motor  centre ;  and  when  some  change  of  conditions  or 
other  natural  stimulus  brings  about  a  change  in  the  activity  of  the 
vaso-constrictor  fibres  of  one  or  more  vascular  areas,  or  of  all  the 
arteries  supplied  with  vaso-constrictor  fibres,  this  same  bulbar 
vaso-motor  centre  appears  in  such  cases  to  play  the  part  of  a 
centre  of  reflex  action.  Nevertheless,  in  cases  where  the  nervous 
connections  of  this  bulbar  vaso-motor  centre  with  a  vascular  area 
are  cut  off  by  an  operation,  as  by  section  of  the  cord,  other  parts 
of  the  spinal  cord  may  act  as  centres  for  the  vaso-constrictor 
fibres  of  the  area,  and  possibly  these  subordinate  centres  may  be 
to  a  certain  extent  in  action  in  the  intact  organism. 

The  vaso-dilator  fibres  of  whose  existence  we  have  clear  and 
undisputed  experimental  evidence,  are  very  limited  in  distribution. 
In  the  cases  best  known,  the  fibres  leave  certain  regions  of  the 
central  nervous  system  and  proceed  to  their  destination  along 
certain  cerebro-spinal  nerves  ;  they  do  not  lose  their  medulla 
until  they  approach  their  termination.  But  as  we  have  seen  there 
is  evidence  of  vaso-dilator  fibres  also  running  in  nerves  of  the 
sympathetic  system.  The  vaso-dilator  fibres  are  generally  thrown 
into  action  as  part  of  a  reflex  act,  and  the  centre,  in  the  reflex  act, 
appears  in  each  case  to  lie  in  the  central  nervous  system  not  far 
from  the  origin  of  the  ordinary  motor  fibres  which  the  dilator 
fibres  accompany. 

The  effects  of  the  activity  of  the  vaso-dilator  fibres  appear  to  be 


286  EXAMPLES   OF   VASO-MOTOR   ACTIONS.     [Book  i. 

essentially  local  in  nature.  When  any  set  of  the  fibres  come  into 
action  the  vascular  area  which  these  govern  is  dilated ;  and  the 
vascular  areas  so  governed  are  relatively  so  small  that  changes  in 
them  produce  little  or  no  effect  on  the  vascular  system  in  general ; 
the  fibres  are  called  into  play  to  produce  special  efi'ects  in  special 
organs. 

The  effects  of  changes  in  the  activity  of  the  vaso-constrictor 
fibres  are  both  local  and  general.  They  are  also  double  in  nature ; 
by  an  inhibition  of  tonic  constrictor  impulses  a  certain  amount  of 
dilation  may  be  effected ;  by  an  augmentation  of  constrictor  im- 
pulses, constriction,  it  may  be  of  considerable  extent,  may  be 
brought  about.  When  the  vascular  area  so  afl'ected  is  small  the 
effects  are  local,  more  or  less  blood  is  distributed  through  the  area  ; 
when  the  vascular  area  affected  is  large,  the  inhibition  of  constric- 
tion may  lead  to  a  marked  fall,  and  an  augmentation  of  constric- 
tion to  a  marked  rise  of  general  blood  pressure.  Broadly  speaking, 
we  may  say  that  whenever  a  vascular  change  is  needed  for  the 
general  well-being  of  the  economy,  it  is  this  vaso-constrictor 
system  which  is  called  into  play. 

The  distribution  of  clearly  proved  vaso-dilator  fibres  is  as  we 
have  said  very  limited,  and  even  the  vaso-constrictor  fibres  are 
most  abundant  in  the  nerves  going  to  the  skin  and  to  the  viscera. 
In  respect  to  the  arteries  supplying  the  numerous  skeletal  mus- 
cles, there  is  much  dispute  as  to  whether  they  are  supplied  by 
vaso-dilator  fibres ;  and  the  supply  of  vaso-constrictor  fibres  to 
them  is  at  least  not  large.  We  may  perhaps  infer  that  the  vascu- 
lar changes  in  the  muscles  are  intended  chiefiy  for  the  benefit  of 
the  muscles  themselves,  and  are  not  to  any  great  extent,  like  those 
of  the  skin  and  viscera,  utilized  for  the  more  general  purposes  of' 
the  economy. 

§  157.  We  shall  have  occasion  later  on  again  and  again  to 
point  out  instances  of  the  effects  of  vaso-motor  action  both  local 
and  general,  but  we  may  here  quote  one  or  two  characteristic 
examples.  "  Blushing "  is  one.  Nervous  impulses  started  in 
some  parts  of  the  brain  by  an  emotion  produce  a  powerful  inhibi- 
tion of  that  part  of  the  bulbar  vaso-motor  centre  which  governs 
the  vascular  areas  of  the  head  supplied  by  the  cervical  sympa- 
thetic, and  hence  has  an  effect  on  the  vaso-motor  fibres  of  the 
cervical  sympathetic  almost  exactly  the  same  as  that  produced  by 
section  of  the  nerve.  In  consequence  the  muscular  walls  of  the 
arteries  of  the  head  and  face  relax,  the  arteries  dilate  and  the 
whole  region  becomes  suffused.  Sometimes  an  emotion  gives  rise 
not  to  blushing,  but  to  the  opposite  effect,  viz.  to  pallor  of  the  face. 
In  a  great  number  of  cases  this  has  quite  a  different  cause,  being 
due  to  a  sudden  diminution  or  even  temporary  arrest  of  the  heart's 
beats ;  but  in  some  cases  it  may  occur  without  any  change  in  the 
beat  of  the  heart,  and  is  then  due  to  a  condition  the  very  converse 
of  that  of  blushing,  that  is,  to  an  increased  arterial  constriction ; 


Chap,  iv.]  THE  VASCULAR   MECHANISM.  287 

and  this  increased  constriction,  like  the  dilation  of  blushing,  is 
effected  through  the  agency  of  the  central  nervous  system  and  the 
cervical  sympathetic.  Blushing  and  its  opposite  pallor  are  most 
marked  in  the  face ;  but  other  parts  of  the  body  may  blush  (or 
grow  pale)  the  change  lieing  Ijrought  about  by  appropriate  nerves. 

The  vascular  condition  of  the  skin  at  large  aftbrds  another 
instance.  When  tlie  temperature  of  the  air  is  low  the  vessels  of  the 
skin  are  constricted,  and  the  skin  is  pale  ;  when  the  temperature  of 
the  air  is  high  the  vessels  of  the  skin  are  dilated,  and  the  skin  is 
red  and  Hushed.  In  both  these  cases  the  effect  is  mainly  a  reflex  one, 
it  being  the  central  nervous  system  which  brings  about  augmen- 
tation of  constriction  in  the  one  case  and  inhilntion  in  the  other ; 
though  possibly  some  slight  effect  is  produced  Ijy  the  direct  local 
action  of  the  cold  or  heat  on  the  vessels  of  the  skin.  Moreover 
the  vascular  changes  in  the  skin  are  accompanied  by  corresponding 
vascular  changes  in  the  viscera  (chiefly  abdominal)  of  a  reverse 
kind.  When  the  vessels  of  the  skin  are  dilated  those  of  the 
viscera  are  constricted,  and  vice  versa;  so  that  a  considerable 
portion  of  the  whole  blood  ebbs  and  flows,  so  to  speak,  according 
to  circumstances  from  skin  to  viscera  and  from  viscera  to  skin. 
By  these  changes,  as  we  shall  see  later  on,  the  maintenance  of  the 
normal  temperature  of  the  body  is  in  large  measure  secured. 

We  shall  see  later  on  that  the  secretion  of  urine  is  in  a  peculiar 
way  dependent  on  the  flow  of  blood  through  the  kidney.  A  very 
favourable  condition  for  this  flow  is  a  dilated  condition  of  the  renal 
arteries  coincident  with  a  high  general  blood  pressure,  and  this 
condition  as  we  shall  see  leads  to  a  copious  secretion  of  urine. 
The  high  general  blood  pressure  in  this  case  is  largely  caused 
by  very  general  arterial  constriction,  leading  to  great  increase 
of  peripheral  resistance,  while  the  dilated  state  of  the  renal  arteries 
appears  to  be  due  to  a  lack  of  the  usual  tonic  constrictor  impulses ; 
though  these  constrictor  impulses  are  increased  in  respect  to  other 
arteries,  they  are  diminished  in  respect  to  the  renal  arteries 
themselves. 

When  food  is  placed  in  the  mouth  the  blood  vessels  of  the 
silivary  glands  as  we  have  seen  are  flushed  with  blood  as  an 
adjuvant  to  the  secretion  of  digestive  fluid ;  and  as  the  food 
passes  along  the  alimentary  canal  each  section  in  turn,  with 
the  glandular  appendages  belonging  to  it,  welcomes  its  advent  by 
flushing  with  blood.  As  we  have  already  said,  we  have,  at  present, 
no  satisfactory  evidence,  except  in  the  case  of  the  salivary  glands, 
that  this  flushing  is  carried  out  by  special  vaso-dilator  nerves.  Along 
the  rest  of  the  alimentary  canal  the  widening  of  the  arteries  and 
thus  the  increased  flow  seems  to  be  brought  about  by  diminution 
of  vaso-constrictor  impulses,  so  far  at  least  as  it  is  ensured  by  the 
intervention  of  the  central  nervous  system.  We  say  '  so  far' 
because  as  we  shall  see  we  have  evidence  that  the  vessels  of  the 
kidney  may  change  in  calibre  independently  of  any  influences 


288  VASO-MOTOR  NERVES   OF  THE  VEINS.     [Book  i. 

proceeding  from  the  central  nervous  system,  after  for  instance  all 
the  nerves  going  to  the  kidney  have  been  divided ;  in  such  cases 
the  changes  in  the  calibre  of  the  renal  vessels  seem  to  be  due  to 
some  direct  local  action  ;  and  it  is  possible  that  the  flushing  of  the 
alimentary  canal  when  food  enters  it  is  similarly,  in  part  or  at 
times,  the  result  of  some  local  action  on  the  blood  vessels. 

§158.  Vaso-viotor  nerves  of  the  Veins.  Although  the  veins  are 
provided  with  muscular  fibres  and  are  distinctly  contractile,  and 
although  rhythmic  variations  of  calibre  due  to  contractions  may 
be  seen  in  the  great  veins  opening  into  the  heart,  in  the  veins  of 
the  bat's  wing,  and  elsewhere,  our  knowledge  as  to  any  nervous 
arrangements  governing  the  veins  is  at  present  very  limited.  The 
portal  vein,  the  walls  of  which  are  conspicuously  muscular,  the 
muscular  fibres  being  arranged  both  as  a  circular  and  as  a  longi- 
tudinal coat,  is  like  the  veins  just  mentioned  subject  to  rhythmic 
variations  of  calibre ;  these  might  be  due  to  active  rhythmic 
contractions  of  the  portal  vein  itself  or  might  be  of  a  passive 
nature,  due  to  a  rhythmic  rise  and  fall  in  the  quantity  of  blood 
discharged  into  it  from  the  vessels  of  the  viscera.  The  former 
view  is  supported  by  the  observation  that  after  the  aorta  has  been 
obstructed,  so  that  no  blood  can  pass  into  the  portal  vein  from  the 
mesenteric  and  other  arteries,  contractions  of  the  portal  vein  may 
be  obtained  by  stimulating  the  splanchnic  nerves.  The  great 
distention  of  the  venous  system  with  blood  which  occurs  in  the 
frog  when  the  brain  and  spinal  cord  are  destroyed,  and  which 
renders  the  heart  almost  bloodless,  the  greater  part  of  the  blood 
being  lodged  in  the  veins,  has  also  been  supposed  to  point  to  some 
normal  tone  of  the  veins  dependent  on  the  central  nervous 
system. 


SEC.  7.     THE   CAPILLARY   CIRCULATION. 


§  159.  We  have  already,  some  time  back  (§  99),  mentioned 
some  of  the  salient  features  of  the  circulation  through  the  capil- 
laries, viz.  the  difficult  passage  of  the  corpuscles  (generally  in 
single  file,  though  sometimes  in  the  larger  channels  two  or 
more  abreast)  and  plasma  through  the  narrow  channels,  in  a 
stream  which  though  more  or  less  irregular  is  steady  and  even,  not 
broken  by  pulsations,  and  slower  than  that  in  either  the  arteries 
or  the  veins.  We  have  further  seen  (§  9-1)  that  the  capillaries 
vary  very  much  in  width  from  time  to  time ;  and  there  can  be 
no  doubt  that  the  changes  in  their  calibre  are  chiefly  of  a  passive 
nature.  They  are  expanded  when  a  large  supply  of  blood  reaches 
them  through  the  supplying  arteries,  and,  by  virtue  of  their 
elasticity,  shrink  again  when  the  supply  is  lessened  or  withdrawn  ; 
they  may  also  become  expanded  by  an  obstacle  to  the  venous 
outflow. 

On  the  other  hand,  there  is  a  certain  amount  of  evidence  that, 
in  young  animals  at  all  events,  the  calibre  of  a  capillary  canal 
may  vary,  quite  independently  of  the  arterial  supply  or  the 
venous  outflow,  in  consequence  of  changes  in  the  form  of  the 
epithelioid  cells,  allied  to  the  changes  which  in  a  muscle-fibre  or 
muscle-cell  constitute  a  contraction ;  and  though  the  matter  re- 
quires further  investigation,  it  is  possible  that  these  active  changes 
play  an  important  part  in  determining  the  quantity  of  blood  pass- 
ing through  a  capillary  area ;  but  there  is  as  yet  no  satisfactory 
evidence  that  they,  like  the  corresponding  changes  in  the  arteries, 
are  governed  by  the  nervous  system. 

Over  and  above  these  changes  of  form,  the  capillaries  and 
minute  vessels  are  subject  to  still  other  changes  and  so  exert 
influences  by  virtue  of  which  they  play  an  important  part  in  the 
work  of  the  circulation.  Their  condition  determines  the  amount 
of  resistance  offered  by  their  channels  to  the  flow  of  blood  through 
those  channels,  and  determines  the  amount  and  character  of  that 
interchange  between  the  blood  and  the  tissues  which  is  the  main 
fact  of  the  circulation. 

19 


290  IXFLAMMATIOK  [Book  i. 

If  the  web  of  tlie  frog's  foot,  or,  better  still,  if  some  transparent 
tissue  of  a  mammal  be  watched  under  the  microscope,  it  will  be  ob- 
served that,  while  in  the  small  capillaries  the  corpuscles  are  pressed 
through  the  channel  in  single  file,  one  after  the  other,  each  corpuscle 
as  it  passes  occupying  the  whole  bore  of  the  capillary,  in  the  larger 
capillaries  (of  the  mammal),  and  especially  in  the  small  arteries 
and  veins  which  permit  the  passage  of  more  than  one  corpuscle 
abreast,  the  red  corpuscles  run  in  the  middle  of  the  channel,  forming 
a  coloured  core,  between  which  and  the  sides  of  the  vessels  all 
round  is  a  colourless  layer,  containing  no  red  corpuscles,  called 
the  '  plasmatic  layer '  or  '  peripheral  zone.'  This  division  into  a 
peripheral  zone  and  an  axial  stream  is  due  to  the  fact  that  in  any 
stream  passing  through  a  closed  channel  the  friction  is  greatest 
at  the  sides,  and  diminishes  towards  the  axis.  The  corpuscles 
pass  where  the  friction  is  least,  in  the  axis.  A  quite  similar  axial 
core  is  seen  when  any  fine  particles  are  driven  with  a  sufficient 
velocity  in  a  stream  of  fluid  through  a  narrow  tube.  As  the 
velocity  is  diminished  the  axial  core  becomes  less  marked  and 
disappears. 

In  the  peripheral  zone,  especially  in  that  of  the  veins,  are 
frequently  seen  white  corpuscles,  sometimes  clinging  to  the  sides 
of  the  vessel,  sometimes  rolling  slowly  along,  and  in  general  moving 
irregularly,  stopping  for  a  while  and  then  suddenly  moving  on. 
The  greater  the  velocity  of  the  flow  of  blood,  the  fewer  the  white 
corpuscles  in  the  peripheral  zone,  and  with  a  very  rapid  flow  they, 
as  well  as  the  red  corpuscles,  may  be  all  confined  to  the  axial 
stream.  The  presence  of  the  white  corpuscles  in  the  peripheral 
zone  has  been  attributed  to  their  being  specifically  lighter  tban 
the  red  corpuscles,  since  when  fine  particles  of  two  kinds,  one  lighter 
than  the  other,  are  driven  through  a  narrow  tube,  the  heavier 
particles  flow  in  the  axis  and  the  lighter  in  the  more  peripheral 
portions  of  the  stream.  But,  besides  this,  the  white  corpuscles 
have  a  greater  tendency  to  adhere  to  surfaces  than  have  the  red, 
as  is  seen  by  the  manner  in  which  the  former  become  fixed  to 
the  glass  slide  and  cover-slip  when  a  drop  of  blood  is  mounted 
for  microscopical  examination.  They  probably  thus  adhere  by 
virtue  of  the  amoeboid  movements  of  their  protoplasm,  so  that  the 
adhesion  is  to  be  considered  not  so  much  a  mere  physical  as  a 
physiological  process,  and  hence  may  be  expected  to  vary  with  the 
varying  nutritive  conditions  of  the  corpuscles  and  of  the  blood 
vessels.  Thus  while  the  appearance  of  the  white  corpuscles  in  the 
peripheral  zone  may  be  due  to  their  lightness,  their  temporary 
attachment  to  the  sides  of  the  vessels  and  characteristic  progression 
is  the  result  of  their  power  to  adhere ;  and  as  we  shall  presently 
see  their  amoeboid  movements  may  carry  them  on  beyond  mere 
adhesion. 

§  160.  These  are  the  phenomena  of  the  normal  circulation, 
and  may  be  regarded  as  indicating  a  state  of  equilibrium  between 


Chap,  iv.]  THE   YASCULAK   MECHANISM.  291 

the  blood  on  the  one  hand  and  the  blood  vessels  with  the  tissues 
on  the  other  ;  but  a  different  state  of  things  sets  in  when  that 
equilibrium  is  overthrown  by  causes  leading  to  what  is  called 
inflammation  or  to  allied  conditions. 

[f  an  irritant,  such  as  a  drop  of  chloroform  or  a  little  diluted 
oil  of  mustard,  be  applied  to  a  small  portion  of  a  frog's  web,  tongue, 
mesentery,  or  some  uther  transparent  tissue,  the  following  changes 
may  be  observed  under  the  microscope  ;  they  may  be  still  better  seen 
in  the  mesentery  or  other  transparent  tissue  of  a  mammal.  The 
first  effect  that  is  noticed  is  a  dilation  of  the  arteries,  accompanied 
by  a  quickening  of  the  stream.  The  irritant,  probably  by  a  direct 
action  on  the  muscular  fibres  of  the  arteries,  has  led  to  a  re- 
laxation of  the  muscular  coat,  and  hence  to  a  widening ;  and  we 
have  already,  §  105,  explained  how  such  a  widening  in  a  small 
artery  may  lead  to  a  temporary  quickening  of  the  stream.  In 
consequence  of  the  greater  flow  through  the  arteries,  the  capillaries 
become  filled  with  corpuscles,  and  many  passages,  previously 
invisible  or  nearly  so  on  account  of  their  containing  no  corpuscles, 
now  come  into  view.  The  veins  at  the  same  time  appear  enlarged 
and  full.  If  the  stimulus  be  very  slight,  this  may  all  pass  away, 
the  arteries  gaining  their  normal  constriction,  and  the  capillaries 
and  veins  returning  to  their  normal  condition ;  in  other  words,  the 
effect  of  the  stimulus  in  such  a  case  is  simply  a  temporary  blush. 
Unless,  however,  the  chloroform  or  mustard  be  applied  with  especial 
care,  the  effects  are  much  more  profound,  and  a  series  of  remarkable 
changes  set  in. 

In  the  normal  circulation,  as  we  have  just  said,  white  corpuscles 
may  be  seen  in  the  peripheral,  plasmatic  zone,  but  they  are  scanty 
in  number,  and  each  one,  after  staying  for  a  little  time  in  one  spot, 
suddenly  gets  free,  sometimes  almost  by  a  jerk  as  it  were,  and  then 
rolls  on  for  a  greater  or  less  distance.  In  the  area  now  under 
consideration  a  large  number  of  white  corpuscles  soon  gather  in 
the  peripheral  zones,  especially  of  the  veins  and  venous  capillaries 
(that  is  of  the  larger  capillaries  which  are  joining  to  form  veins), 
but  also,  of  the  other  capillaries,  and,  to  a  less  extent,  of  the  arteries ; 
and  this  takes  place  although  the  vessels  still  remain  dilated  and  the 
stream  still  continues  rapid,  though  not  so  rapid  as  at  first.  Each 
white  corpuscle  appears  to  exhibit  a  greater  tendency  to  stick  to 
the  sides  of  the  vessels,  and  though  driven  away  from  the  arteries 
by  the  stronger  arterial  stream,  becomes  lodged  so  to  speak  in  the 
veins.  Since  new  white  corpuscles  are  continually  being  brought 
by  the  blood  stream  on  to  the  scene,  the  number  of  them  in  the 
peripheral  zones  of  the  veins  increases  more  and  more,  and  this 
may  go  on  until  the  inner  surface  of  the  veins  and  venous 
capillaries  appears  to  be  lined  with  a  layer  of  white  corpuscles. 
The  small  capillaries  too  contain  more  white  corpuscles  than 
usual,  and  even  in  the  arteries  these  are  abundant,  though  not 
forming  the  distinct  layer  seen  in  the    veins.     The    white    cor- 


292  MIGKATIOX   OF   WHITE   CORPUSCLES.     [Book  i. 

puscles,  however,  are  not  the  only  bodies  present  in  the  peri- 
pheral zone.  Though  in  the  normal  circulation  blood-platelets 
(see  §  33)  cannot  be  seen  in  the  peripheral  zone,  and  hence  (on 
the  view,  which  has  the  greater  support,  that  these  bodies  are 
really  present  in  quite  normal  blood)  must  be  confined  to  the  axial 
stream,  they  make  their  appearance  in  that  zone  during  the 
changes  which  we  are  now  describing.  Indeed,  in  many  cases  they 
are  far  more  abundant  than  the  white  corpuscles,  the  latter  appear- 
ing imbedded  at  intervals  in  masses  of  the  former.  Soon  after 
their  appearance  the  individual  platelets  lose  their  outline,  and  run 
together  into  formless  masses. 

§  161.  This  much,  the  appearance  of  numerous  white  cor- 
puscles and  platelets  in  the  peripheral  zones,  may  take  place  while 
the  stream,  though  less  rapid  than  at  the  very  first,  still  remains 
rapid  ;  so  rapid  at  all  events  that,  owing  to  the  increased  width 
of  the  passages,  in  spite  of  the  obstruction  offered  by  the  adherent 
white  corpuscles,  the  total  quantity  of  blood  flowing  in  a  given 
time  through  the  inflamed  area  is  greater  than  normal.  But 
soon,  though  the  vessels  still  remain  dilated,  the  stream  is  observed 
most  distinctly  to  slacken,  and  then  a  remarkable  phenomenon 
makes  its  appearance.  The  white  corpuscles  lying  in  contact  with 
the  walls  of  the  veins  or  of  the  capillaries  are  seen  to  thrust  processes 
through  the  walls  ;  and,  the  process  of  a  corpuscle  increasing  at  the 
expense  of  the  rest  of  the  body  of  the  corpuscle,  the  whole  cor- 
puscle, by  what  appears  to  be  an  example  of  amoeboid  movement, 
makes  its  way  through  the  wall  of  the  vessel  into  the  lymph 
space  outside  ;  the  perforation  appears  to  take  place  in  the  cement 
substance  joining  the  epithelioid  plates  together.  This  is  the 
migration  of  the  white  corpuscles  to  which  we  alluded  in  §  32,  and 
takes  place  chiefly  in  the  veins  and  capillaries,  not  at  all  or  to  a 
very  slight  extent  in  the  arteries.  Through  this  migration  the 
lymph  spaces  around  the  vessels  in  the  inflamed  area  become 
crowded  with  white  corpuscles.  At  the  same  time  fluid  passes 
from  the  interior  of  the  blood  vessels  through  the  altered  walls 
into  the  lymph  spaces  more  rapidly  than  it  escapes  from  the 
lymph  spaces  along  the  lymphatic  channels ;  these  lymph  spaces 
become  distended  with  lymph,  which  also  changes  somewhat  in  its 
chemical  characters  ;  it  tends  to  clot  more  readily  and  more  firmly, 
and  is  sometimes  spoken  of  as  'exudation  fluid,'  or  by  the  older 
writers  as  'coagulable  lymph.'  This  turgescence  of  the  lymph 
spaces,  together  with  the  dilated  crowded  condition  of  the  blood 
vessels,  fjives  rise  to  the  swelling  which  is  one  of  the  features  of 
inflammation. 

If  the  inflammation  now  passes  off  the  white  corpuscles  cease  to 
emigrate,  cease  to  stick  for  any  length  of  time  to  the  sides  of  the 
vessels,  the  stream  of  blood  through  the  vessels  quickens  again,  and 
the  vessels  themselves,  though  they  may  remain  for  a  long  time 
dilated,  eventually  regain  their  calibre,  and  a  normal  circulation  is 


Chap,  iv.]  THE   VASCULAR   MECHANISM. 


293 


re-established.  The  migrated  corpuscles  move  away  from  the 
region  along  the  labyrinth  of  lymph  spaces,  and  the  surplus  lymph 
also  passes  away  along  the  lymph  spaces  and  lymphatic  vessels. 

A  more  powerful  action  of  the  irritant  may  lead  to  still  other 
events.  More  and  more  white  corpuscles,  arrested  in  their  passage, 
crowd  the  channels  and  block  the  way,  so  that  though  the  vessels 
remain  dilated,  the  stream  becomes  slower  and  slower,  until  at  last 
it  stops  altogether,  and  '  stagnation  '  or  '  stasis  '  sets  in.  The  red 
corpuscles  are  driven  in,  often  in  masses,  among  the  white  cor- 
puscles and  platelets,  the  distinction  between  axial  stream  and 
peripheral  zone  becoming  lost ;  and  arteries,  veins  and  capillaries, 
all  distended,  sometimes  enormously  so,  are  filled  with  a  mass  of 
mingled  red  and  white  corpuscles  and  platelets.  The  red  corpuscles 
run  together  so  that  their  outlines  are  no  longer  distinguishable  ; 
they  appear  to  become  fused  into  a  homogeneous  red  mass.  And 
it  may  now  be  observed  that,  not  only  white  corpuscles  but  also 
red  corpuscles,  make  their  way  through  the  distended  and  altered 
walls  of  the  capillaries,  chiefly,  at  all  events,  at  the  junctions  of 
the  epithelioid  plates,  into  the  lymph  spaces  beyond.  This  is 
spoken  of  as  the  diapedesis  of  the  red  corpuscles. 

This  condition  of  'stasis'  may  be  the  prelude  to  further 
mischief,  and,  indeed,  to  the  death  of  the  tissue,  but  it,  too,  like  the 
earlier  stage  of  inflammation,  may  pass  away.  As  it  passes  away 
the  outlines  of  the  corpuscles  become  once  more  distinct,  those  on 
the  venous  side  of  the  block  gradually  drop  away  into  the  neigh- 
bouring currents,  little  by  little  the  whole  obstruction  is  removed, 
and  the  current  through  the  area  is  re-established. 

§  162.  The  slowing  or  the  arrest  of  the  blood  current  described 
above  is  not  due  to  any  lessening  of  the  heart's  beat ;  the  arterial 
pulsations,  or  at  least  the  arterial  flow,  may  be  seen  to  be  continued 
in  full  force  down  to  the  affected  area,  and  there  to  cease  very 
suddenly.  It  is  not  due  to  the  peripheral  resistance  being 
increased  by  any  constriction  of  the  small  arteries,  for  these 
continue  dilated,  sometimes  exceedingly  so.  It  must,  therefore,  be 
du3  to  some  new  and  unusual  resistance  occurring  in  the  area  itself, 
and  this  we  are  by  many  reasons  led  to  attribute  to  an  increased 
tendency  of  the  corpuscles,  especially  of  the  white  corpuscles,  to 
stick  to  the  sides  of  the  vessels.  The  increase  of  adhesiveness  is 
not  caused  by  any  change  confined  to  the  corpuscles  themselves ; 
for  if  after  a  temporary  delay  one  set  of  corpuscles  has  managed  to 
pass  away  from  the  affected  area,  the  next  set  of  corpuscles  brought 
to  the  area  in  the  blood  stream  is  subjected  to  the  same  delay. 
The  cause  of  the  increased  adhesiveness  must  therefore  lie  in  the 
walls  of  the  blood  vessels,  or  in  the  tissue  of  which  these  form  a 
part.  That  the  increased  adhesion  is  due  to  the  vascular  walls  and 
not  primarily  to  the  corpuscles  themselves  is  further  shewn  by  the 
fact  that  if,  in  the  frog,  an  artificial  blood  of  normal  saline  solution, 
to  which  milk  has  been  added,  be  substituted  for  normal  blood,  a 


294  INFLAMMATION.  [Book  i. 

stasis  may  by  irritants  be  induced  in  which  oil-globules  play  the 
part  of  corpuscles,  and  by  their  aggregation  bring  about  an  arrest 
of  the  flow. 

We  are  led  to  conclude  that  there  exist  in  health  certain 
relations  between  the  blood  on  the  one  hand,  and  the  walls  of  the 
vessels  on  the  other,  by  which  the  tendency  of  the  corpuscles  to 
adhere  to  the  blood  vessels  is  kept  within  certain  limits  ;  these 
relations  consequently  determine  the  normal  flow,  with  its  axial 
stream  and  peripheral  zone,  and  the  normal  amount  of  peripheral 
resistance ;  in  inflammation,  these  relations,  in  a  manner  we 
cannot  as  yet  fully  explain,  are  disturbed  so  that  the  tendency 
of  the  corpuscles  to  adhere  to  the  sides  of  the  vessels  is  largely 
and  progressively  increased.  Hence  the  tan-ying  of  the  corpuscles 
in  spite  of  the  widening  of  their  path,  and  finally  their  agglomera- 
tion and  fusion  in  the  distended  channels. 

The  changes  occurring  in  the  vascular  walls  at  the  same  time 
also  modify  the  passage  from  the  blood  to  the  tissue  of  the  fluid 
parts  of  the  blood,  the  lymph  of  inflamed  areas  being  more 
abundant  and  richer  in  proteids  than  normal  lymph.  There  is  a 
greater  outflow  from  the  interior  of  the  blood  vessel  into  the 
lymph  spaces  outside,  and,  indeed,  it  has  been  urged  that  this, 
carrying  the  blood  corpuscles  with  it,  mechanically  promotes  the 
gathering  of  the  white  corpuscles  at  the  sides  of  the  vessel  and 
their  subsequent  passage  through  the  walls. 

We  must  not,  however,  pursue  this  subject  of  inflammation  any 
further.  We  have  said  enough  to  shew  that  the  peripheral  re- 
sistance (and  consequently  all  that  depends  on  that  peripheral 
resistance)  is  not  wholly  determined  by  the  varying  width  of  the 
minute  passages,  but  is  also  dependent  on  the  vital  condition  of 
the  tissue  of  which  the  walls  of  the  passages  form  a  part.  When 
the  tissue  is  in  health,  a  certain  resistance  is  offered  to  the 
passage  of  blood  through  the  capillaries  and  other  minute  vessels, 
and  the  whole  vascular  mechanism  is  adapted  to  overcome  this 
resistance  to  such  an  extent  that  a  normal  circulation  can  take 
place.  When  the  tissue  becomes  affected,  the  disturbance  of  the 
relations  between  the  tissue  and  the  blood  may  so  augment  the  re- 
sistance that  the  passage  of  the  blood  becomes,  as  in  inflammation, 
difficult,  or,  as  in  stasis,  impossible.  And  it  is  quite  open  to  us  to 
suppose  that  under  certain  circumstances  the  reverse  of  the  above 
may  occur  in  this  or  that  area,  that  conditions  may  arise  in  which 
the  resistance  is  lowered  below  the  normal,  and  the  circulation  in 
the  area  quickened.  Thus  the  vital  condition  of  the  tissue  becomes 
a  factor  in  the  maintenance  of  the  circulation  ;  and  it  is  possible, 
though  not  yet  proved,  that  these  vital  conditions  are  directly 
under  the  dominion  of  the  nervous  system. 

§  163.  Changes  in  the  peripheral  resistance  may  also  be 
brought  about  by  changes  in  the  character  of  the  blood,  especially 
by  changes  in  the  relative  amount  of  gases  present.     When  a 


Chap,  it.]  THE   VASCULAR   MECHANISM.  295 

stream  of  defibrinated  blood  is  artificially  driven  through  a 
perfectly  fresh  excised  organ,  such  as  the  kidney,  it  is  found  that 
the  resistance  to  the  tiow  of  blood  through  the  organ,  measured, 
for  instance,  by  the  amount  of  outflow  in  relation  to  the  pressure 
exerted,  varies  considerably  owing  to  changes  taking  place  in  the 
organ,  and  may  be  increased  by  increasing  the  venuus  character, 
and  diminished  by  increasing  the  arterial  character  of  the  blood. 
Remarkable  changes  in  the  resistance  are  also  brought  about  by 
the  addition  of  small  quantities  of  certain  drugs  such  as  chloral, 
atropin  &c.  to  the  blood. 

These  chamres  have  been  attributed  to  the  altered  blood  acting 
on  the  walls  of  the  vessels,  inducing,  for  instance,  constriction  or 
widening  of  the  small  arteries,  or,  it  may  be,  affecting  the  capil- 
laries, for  it  has  been  asserted  that  the  epithelioid  plates  of  the 
capillaries  vary  in  form  according  to  the  relative  quantities  of 
carbonic  acid  and  oxygen  present  in  the  blood.  But  this  is  not 
the  whole  explanation  of  the  matter,  since  similar  variations  in 
resistance  are  met  with  when  blood  is  driven  through  fine  capil- 
lary tubes  of  inert  matter.  In  such  experiments  it  is  found  that 
the  resistance  to  the  flow  increases  with  a  diminution  of  the 
oxygen  carried  by  the  red  corpuscles,  and  is  modified  by  the 
addition  to  the  blood  of  even  small  qviantities  of  certain  drugs. 

It  is  obvious,  then,  that  in  the  living  body  the  peripheral 
resistance,  being  the  outcome  of  complex  conditions,  may  be 
modified  in  many  ways.  Experiment  teaches  us  that,  even  in 
dealing  with  non-living  inert  matter,  the  flow  of  fluid  through 
capillary  tubes  may  be  modified  on  the  one  hand  by  changes  in 
the  substance  of  which  the  tubes  are  composed,  and  on  the  other 
hand  by  changes  in  the  chemical  nature  (even  independent  of  the 
specific  gravity)  of  the  fluid  which  is  used.  In  the  living  body 
both  the  fluid  and  the  tubes,  both  the  blood  and  the  walls  of  the 
minute  vessels,  are  subject  to  incessant  change ;  the  vessels  are 
continually  changing  because  they  are  living  structures,  and  the 
blood  is  continually  changing  not  only  because  it  too  is  in  part  at 
least  alive,  but  also  because  all  the  tissues  of  the  body  are  working 
upon  it.  The  changes  in  the  one,  moreover,  are  capable  of  reacting 
upon  and  inducing  changes  in  the  other ;  and,  lastly,  the  changes 
both  of  the  one  and  of  the  other  may  be  primarily  set  going  by 
events  taking  place  in  some  part  of  the  l)ody  far  away  from  the 
region  in  which  these  changes  are  modifying  the  resistance  to  the 
flow. 


SEC.  8.     CHANGES  IN  THE  QUANTITY  OF   BLOOD. 


§  164.  In  an  artificial  scheme,  changes  in  the  total  quantity  of 
fluid  in  circulation  will  have  an  immediate  and  direct  effect  on  the 
arterial  pressure,  increase  of  the  quantity  heightening  and  decrease 
diminishing  it.  This  effect  will  be  produced  partly  by  the  pump 
being  more  or  less  filled  at  each  stroke,  and  partly  by  the  peri- 
pheral resistance  being  increased  or  diminished  by  the  greater 
or  less  fulness  of  the  small  peripheral  channels.  The  pressure 
along  the  whole  system  and  hence  the  venous  pressure  will  under 
all  circumstances  be  raised  with  the  increase  of  fluid,  but  an 
increase  of  the  arterial  pressure  beyond  that  of  the  venous  pressure 
will  be  observed  only  so  long  as  the  elasticity  of  the  arterial  tubes 
can  be  brought  into  play. 

In  the  natural  circulation,  the  direct  results  of  change  of  quan- 
tity are  modified  by  compensatory  arrangements.  Thus  experi- 
ment shews  the  following  when  an  animal  with  normal  blood 
pressure  is  bled  from  one  carotid.  The  pressure  in  the  other 
carotid  sinks  so  long  as  the  bleeding  is  going  on  ;  this  is  chiefly 
because  the  free  opening  in  the  vessel,  from  which  the  bleeding  is 
going  on,  cuts  off  a  great  deal  of  the  peripheral  resistance,  and  so 
leads  to  a  general  lowering  of  the  blood  pressure.  It  remains 
depressed  for  a  brief  period  after  the  bleeding  has  ceased,  but 
in  a  short  time  regains  or  nearly  regains  the  normal  height. 
This  recovery  of  blood  pressure,  after  haemorrhage,  is  witnessed  so 
long  as  the  loss  of  blood  does  not  amount  to  more  than  about  3  per 
cent,  of  the  body-weight.  Beyond  that,  a  large  and  frequently  a 
sudden  dangerous  permanent  depression  is  observed. 

The  restoration  of  the  pressure  after  the  cessation  of  the 
bleeding  is  too  rapid  to  permit  us  to  suppose  that  the  quantity  of 
fluid  in  the  blood  vessels  is  replaced  by  the  withdrawal  of  lymph 
from  the  extra-vascular  elements  of  the  tissues.  In  all  probability 
the  result  is  gained  by  an  increased  action  of  the  vaso-constrictor 
nerves  increasing  the  peripheral  resistance,  the  vaso-motor  centre 
being  thrown  into  increased  action  by  the  diminution  of  its 
blood  supply  ;  when  the  blood  by  ligature  of  the  arteries  in  the 


Chap,  iv.]  THE   VASCULAR   MECHANISM.  297 

neck  is  suddenly  cut  off  from  the  brain  and  so  from  the  spinal 
bulb,  a  rise  of  blood  pressure  is  observed.  When  the  loss  of  blood 
has  gone  beyond  a  certain  limit,  this  vaso-constrictor  action  is 
insufficient  to  compensate  the  diminished  quantity  (possibly  the 
vaso-motor  centre  in  part  becomes  exhausted),  and  a  considerable 
depression  takes  place ;  but  at  this  epoch  the  loss  of  blood 
frequently  causes  antemic  convulsions. 

Similarly,  when  an  additional  quantity  of  blood  is  injected  into 
the  vessels,  no  marked  increase  of  blood  pressure  is  observed  so 
long  as  the  vaso-motor  centre  in  the  spinal  bulb  is  intact.  If, 
however,  the  cervical  spinal  cord  be  divided  previous  to  the  in- 
jection, the  pressure,  which,  on  account  of  the  removal  of  the 
bulbar  vaso-motor  centre,  is  very  low,  is  permanently  raised  by  the 
injection  of  blood.  At  each  injection  the  pressure  rises  ;  it  falls 
somewhat  afterwards,  but  eventually  remains  at  a  higher  level  than 
before.  This  rise  is  stated  to  continue  until  the  amount  of  blood 
in  the  vessels  above  the  normal  quantity  reaches  from  2  to  3 
per  cent,  of  the  body-weight,  beyond  which  point  it  is  said  no 
further  rise  of  pressure  occurs.  The  absence  of  any  marked  rise 
of  blood  pressure,  so  long  as  the  bulbar  vaso-motor  centre  is  intact, 
shews  that  the  addition  of  the  extra  quantity  of  blood  stimulates 
that  centre  to  increased  activity.  But  while  a  diminution  of  blood 
supply  seems  to  affect  the  centre  directly,  an  increase  of  blood 
supply  probably  acts  in  an  indirect  manner.  When  the  arteries 
in  the  neck  are  ligatured,  the  rise  of  blood  pressure  is  much  more 
marked  if  the  depressor  nerves  be  divided  ;  so  long  as  these 
nerves  are  intact  impulses  passing  along  them  from  the  heart 
withstand  the  stimulating  effects  on  the  vaso-motor  centre  of  the 
loss  of  blood.  And  we  may  perhaps  infer  that  when  an  extra 
quantity  of  blood  is  injected,  the  greater  fulness  stimulates 
the  endings  of  the  depressor  nerves  in  the  heart,  and  so  by 
developing  depressor  impulses  lessens  the  activity  of  the  vaso- 
motor centre. 

The  facts  stated  seem,  then,  to  shew,  in  the  first  place,  that  when 
the  volume  of  the  blood  is  increased,  compensation  is  effected  by 
a  lessening  of  the  peripheral  resistance  by  means  of  a  diminished 
action  of  the  vaso-motor  centre,  so  that  the  normal  blood  pressure 
remains  constant.  They  further  shew  that  a  much  greater  quantity 
of  blood  can  be  lodged  in  the  blood  vessels  than  is  normally  present 
in  them.  That  the  additional  quantity  injected  does  remain  in 
the  vessels  is  proved  by  the  absence  of  extravasations,  and  of  any 
considerable  increase  of  the  extra-vascular  lymphatic  fluids.  It 
has  already  been  insisted  that,  in  henlth,  the  veins  and  capillaries 
must  be  regarded  as  being  far  from  filled  ;  for  were  thev  to  receive 
all  the  blood  which  they  can,  even  at  a  low  pressure,  hold,  the 
whole  quantity  of  blood  in  the  body  would  be  lodged  in  them 
alone.  In  these  cases  of  large  addition  of  blood,  the  extra  quantity 
appears  to  be  lodged  in  the  small  veins  and  capillaries  (especially 


298  CHANGES   IX   QUANTITY   OE  BLOOD.      [Book  i. 

of  the  internal  organs),  which  are  abnormally  distended  to  contain 
the  surplus. 

We  learn,  also,  from  these  facts  the  two  practical  lessons  :  first, 
that  blood  pressure  cannot  be  lowered  directly  in  a  mechanical 
manner  by  bleeding,  unless  the  quantity  removed  be  dangerously 
large  ;  and  secondly,  that  there  is  no  necessary  connection  between 
a  high  blood  pressure  and  fulness  of  blood  or  plethora,  since  an 
enormous  quantity  of  blood  may  be  driven  into  the  vessels  without 
any  marked  rise  of  pressure. 

"When  a  quantity  of  blood  or,  indeed,  of  fluid  is  injected  into 
the  veins,  the  output  from  the  heart  is  increased  and  observations 
seem  to  shew  that  the  increase  in  the  output  is  out  of  proportion 
to  the  quantity  of  fluid  injected,  indicating  that  the  result  is  of 
complex  origin.  In  spite  of  this  increased  output,  the  blood 
pressure  is  not  raised ;  the  effect  is  compensated  by  vascular 
dilation  somewhere.  Conversely  when  blood  is  withdrawn,  the 
output  is  diminished,  but  here  again  the  effect  on  the  blood 
pressure  is  soon  compensated,  this  time  by  vascular  constriction. 


SEC.   9.     A  REVIEW   OF  SOME   OF   THE  FEATURES   OF 
THE   CIRCULATION. 


§  165.  The  facts  dwelt  on  in  the  foregoing  sections  have 
shewn  us  that  the  factors  of  the  vascular  mechanism  may  be 
regarded  as  of  two  kinds :  one  constant,  or  approximately  constant; 
the  other  variable. 

The  constant  factors  are  supplied  by  the  length,  natural  bore, 
and  distribution  of  the  blood  vessels,  by  the  extensibility  and 
elastic  reaction  of  their  walls,  and  by  such  mechanical  contrivances 
as  the  valves.  By  the  natural  bore  of  the  various  blood  vessels  is 
meant  the  diameter  which  each  would  assume  if  the  muscular 
fibres  were  wholly  at  rest,  and  the  pressure  of  iiuid  within  the 
vessel  were  equal  to  the  pressure  outside.  It  is  obvious,  however, 
that  even  these  factors  are  only  approximately  constant  for  the 
life  of  an  individual.  The  length  and  distribution  of  the  vessels 
change  with  the  growth  of  the  whole  body  or  parts  of  the  body, 
and  the  physical  qualities  of  the  walls,  especially  of  the  arterial 
walls,  their  extensibility  and  elastic  reaction  change  continually 
with  the  age  of  the  individual.  As  the  body  grows  older,  the  once 
supple  and  elastic  arteries  become  more  and  more  stiff  and  rigid, 
and  often  in  middle  life,  or  it  may  be  earlier,  a  lessening  of  arterial 
resilience  which  proportionately  impairs  the  value  of  the  vascular 
mechanism  as  an  agent  of  nutrition,  marks  a  step  towards  the 
grave. 

The  chief  variable  factors  are  on  the  one  hand  the  beat  of  the 
heart,  and  on  the  other  the  peripheral  resistance,  the  variations  in 
the  latter  being  chieHy  brought  about  by  muscular  contraction  or 
relaxation  in  the  minute  arteries,  but  also,  though  to  what  extent 
has  not  yet  been  accurately  determined,  by  the  condition  of  the 
minute  vessels  according  to  which  the  blnod  can  pass  through 
them  with  less  or  with  greater  ease,  as  well  as  by  the  character 
of  the  circulating  blood. 

These  two  chief  variables,  the  beat  of  the  heart  and  the  width 
of  the  minute  arteries,  are  known  to  be  governed  and  regulated  by 
the  central  nervous  system,  which  adapts  each  to  the  circumstances 


300    INTRINSIC   REGULATION  OF  HEART  BEAT.   [Book  i. 

of  the  moment,  and  at  the  same  time  brings  the  two  into  mutual 
dependence  ;  but  the  central  nervous  system  is  not  the  only  means 
of  government :  there  are  other  modes  of  regulation,  and  so  other 
safeguards. 

§  166.  Let  us  first  consider  the  heart.  The  object,  if  we  may 
use  the  expression,  of  the  systole  of  the  ventricle  is  to  secure 
the  needed  arterial  pressure  ;  it  is  this,  as  we  have  seen,  which 
drives  the  blood  through  the  capillaries  back  to  the  heart.  To  do 
this  the  ventricle  must  deliver  at  the  stroke  a  certain  quantity  of 
blood,  exerting  the  pressure  required  to  lodge  the  blood  in  the 
arteries,  and  repeating  the  stroke  at  appropriate  intervals.  Hence 
the  work  done  will,  in  part,  depend  on  the  quantity  of  blood 
collected  in  the  ventricle  during  the  diastole,  that  is,  on  the  inflow 
from  the  venous  system.  If  the  quantity  brought  be  too  small, 
then  though  the  whole  contents  of  the  ventricle  be  ejected  with 
adequate  force  at  each  stroke,  and  the  stroke  repeated  regularly, 
the  ventricle  will  fail  in  its  object ;  speaking  generally  we  may 
say  that  a  lessened  venous  inflow  will  tend  to  lessen,  and  an 
increased  venous  inflow  will  tend  to  increase  the  work  of  the  heart. 
This  venous  inflow  is  dependent  on  various  causes,  and  may  be 
variously  modified  by  various  events. 

The  blood  in  filling  the  ventricle  distends  its  walls,  and  this 
distension,  especially  the  fuller  distension  resulting  from  the 
auricular  systole,  also  influences  the  ventricular  stroke ;  for  the 
contraction  of  the  cardiac  fibre,  like  that  of  the  skeletal  muscular 
fibre,  is  increased  up  to  a  certain  limit  by  the  fibre  being  put  on 
the  stretch  (§  140).  This  influence,  however,  is  more  distinctly 
seen  on  the  arterial  side.  The  greater  the  arterial  pressure 
against  which  the  ventricle  has  to  deliver  its  contents,  the  greater 
the  tension  of  the  ventricular  walls ;  and  hence,  a  high  arterial 
pressure  tends  of  itself  to  enforce  the  ventricular  systole.  As  in 
the  skeletal  muscle,  however,  this  beneficial  influence  soon  reaches 
its  limit. 

§  167.  The  spontaneous  beat  of  the  heart  is  the  outcome  of 
the  nutrition  of  the  cardiac  tissues.  In  the  absence  of  all  inter- 
ference by  inhibitory  or  augmentor  fibres,  the  heart  will  continue 
beating  with  a  certain  rhythm  and  force,  determined  by  the 
metabolism  going  on  in  its  muscular  and  nervous  elements.  The 
beat  therefore  will  be  influenced  by  anything  which  affects  that 
metabolism.  And  the  obvious  and  direct  cause  of  changes  in  the 
nutrition  and  so  in  the  behaviour  of  the  heart  lies  in  changes  in 
the  quantity  and  quality  of  the  blood  supplied  to  the  cardiac  tis- 
sues. In  the  mammal  this  means  the  quantity  and  quality  of  the 
blood  flowing  through  the  coronary  arteries. 

If  in  a  mammal  the  coronary  arteries  be  tied  or  otherwise 
occluded  the  heart  in  a  few  seconds  comes  to  a  standstill ;  this, 
which  always  results  if  both  arteries  be  tied,  sometimes  if  one 
only  be  tied,  is  preceded  by  an  irregularity  or  by  changes  in  the 


Chap.  iv.J  THE  VASCULAR  MECHANISM.  301 

beat  and  is  followed  by  fibrillar  contractions  of  parts  of  the  ven- 
tricles. This  is  an  extreme  case,  but  it  illustrates  in  a  striking 
manner  how  closely  the  rhythmic  contraction  of  the  cardiac  fibres 
is  dependent  on  the  blood  supply. 

The  quantity  of  blood  flowing  through  the  coronary  arteries  is 
dependent  on  the  pressure  in  the  aorta,  or  rather  on  the  difference 
between  that  pressure  and  the  pressure  in  the  right  auricle  into 
which  the  coronary  veins  open,  and  on  the  resistance  offered  by 
the  coronary  vessels.  Hence  with  a  high  aortic  pressure,  more 
blood  passes  to  the  cardiac  tissue.  This  is  at  least  favourable  to 
the  development  of  the  beat,  and  may  be  the  direct  cause  of  a 
stronger  stroke  ;  indeed  observations  seem  to  shew  this.  Thus  a 
high  aortic  pressure  itself  helps  the  heart  to  the  effort  necessary 
to  overcome  that  high  pressure.  Conversely  a  low  aortic  pressure 
would  similarly  tend  to  spare  the  heart  an  unnecessary  exertion. 
As  to  how  the  heart  may  be  influenced  by  changes  in  the  w^idth 
of  the  coronary  arteries  brought  about  by  vaso-motor  action,  we 
have  at  present  but  little  definite  knowledge. 

More  important  still  than  the  quantity  is  the  quality  of  the 
blood  flowing  through  the  coronary  vessels.  We  shall  have 
occasion  in  treating  of  respiration  to  speak  of  the  manner  in 
which  blood  deficient  in  oxygen  or  overladen  with  carbonic  acid 
affects  the  beat  of  the  heart ;  and  we  may  here  be  content  to  point 
out  that  every  change  in  the  constitution  of  the  blood,  whether 
arising  from  the  presence  of  substances  such  as  drugs  and  poisons, 
introduced  from  without,  or  of  substances  manufactured  in  this 
or  that  tissue  of  the  body  or  resulting  from  the  absence  or  paucity 
or  from  excess  of  one  or  more  of  the  normal  constituents,  may 
unfavourably  or,  it  may  be,  favourably  affect  the  heart  beat,  by 
directly  influencing  the  cardiac  tissues  through  the  coronary 
arteries.  These  changes  in  the  blood  may  of  course  also  work 
upon  the  heart  through  the  central  nervous  system,  and  this 
indirect  effect  may  possibly  be  different  from  the  direct  effect. 
Thus,  when  the  breathing  is  interfered  with,  the  too  highly 
venous  blood,  while  it  acts  directly  on  the  cardiac  tissues  and  that 
unfavourably,  also  stimulates  the  cardio-inhibitory  centre,  whereby 
the  heart  is  slowed  and  its  expenditure  of  energy  lessened. 

§  168.  As  is  well  known,  the  beat  of  the  heart  may  become 
temporarily  or  permanently  irregular.  That  many  hearts  go  on 
beating  day  after  day,  year  after  year,  without  any  such  irregu- 
larity is  a  striking  proof  of  tlie  complete  balance  which  usually 
ol)tains  between  the  several  factors  of  which  we  are  speaking. 
Sometimes  such  cardiac  irregularities,  those  of  a  transient  nature 
and  brief  duration,  are  the  results  of  extrinsic  nervous  influences. 
Some  events  taking  place  in  the  stomach,  for  instance,  give  rise  to 
afferent  impulses  which  ascending  from  the  mucous  membrane  of 
the  stomach  along  certain  aflerent  fibres  of  the  vagus  to  tlie 
spinal  bulb,  so  augment  the  action  of  the  cardio-inhibitory  centre 


302  IRREGULAR  HEART   BEAT.  [Book  i. 

as  to  stop  the  heart  for  a  beat  or  two,  the  stoppage  being  fre- 
quently followed  by  a  temporary  increase  in  the  rapidity  and  force 
of  the  beat.  Such  a  passing  failure  of  the  heart  beat,  in  its 
sudden  onset,  in  its  brief  duration,  and  in  the  reaction  which  fol- 
lows, very  closely  resembles  the  complete  but  temporary  inhibition 
brought  about  by  artificial  stimulation  of  the  vagus.  And  as  we 
have  seen  the  inhibitory  action  of  the  vagus  is  especially  prone  to 
be  set  going  by  afferent  impulses  passing  up  to  the  central  ner- 
vous system  from  the  viscera. 

The  effects  however  which  we  produce  by  our  rough  means  of 
direct  stimulation  of  the  trunk  of  the  vagus  do  not  afford  a  true 
picture  of  the  action  of  the  cardio-inhibitory  mechanism  in  the 
living  body;  v/e  come  nearer  to  this  when  we  obtain  inhibition  in 
a  reliex  manner.  From  the  knowledge  gained  in  this  way  we 
may  infer  that  the  fainting  which  comes  from  pain,  emotions  and 
the  like,  is  due  to  the  action  of  the  inhibitory  mechanism. 
Several  forms  of  irregular  heart  beat  are  probably  brought  about 
by  the  same  mechanism ;  we  may  in  this  relation  call  to  mind 
that  one  effect  of  the  action  of  the  inhibitory  fibres  is  tQ  produce 
not  merely  slowing  or  weakening  but  distinct  irregularity  of  the 
heart  beat. 

Many  observations  shew  that  the  cardio-inhibitory  mechanism 
may  be  affected  by  afferent  impulses  or  otherwise  in  two  different 
ways.  On  the  one  hand  the  cardio-inhibitory  centre  may  be 
thrown  into  action,  or  when  already  in  action  may  have  its  action 
increased  ;  on  the  other  hand  if  already  in  action,  that  action  may 
be  lessened  ;  the  inhibition  may  itself  be  inhibited.  The  division 
of  both  vagus  nerves  in  the  dog  affords  an  instance  of  the  effect 
on  the  heart  of  arresting  previously  existing  inhibitory  impulses. 
Hence  it  becomes  difficult  in  the  complex  living  body  to  distin- 
guish between  an  augmentation  due  to  activity  of  the  augmentor 
mechanism  and  one  due  to  suspension  of  the  previously  active 
inhibitory  mechanism.  The  two  may  probably  be  distinguished 
by  studying  the  details  of  the  behaviour  of  the  heart  in  the  two 
cases.  Failing  this  it  is  difficult  to  say  whether  a  case  of  that 
irregularity  of  the  heart  which  we  call  '  palpitation  '  has  been 
brought  about  positively  by  the  one  mechanism  or  negatively  by 
the  other. 

We  must  remember,  moreover,  that  irregularity  in  the  heart 
beat  in  at  least  a  large  number  of  cases  is  the  result  not  of  ner- 
vous influences  from  without,  but  of  intrinsic  events.  For  in- 
stance, in  many  cases  the  irregularity  of  the  heart  beat  is  wholly 
unaffected  by  atropin,  and  therefore  cannot  be  due  to  vagus 
action.  It  is  very  often  the  result  of  what  we  may  call  a  dis- 
ordered nutrition  of  the  cardiac  substance,  though  we  cannot  state 
the  exact  nature  of  the  disorder. 

§  169.  We  may  repeat  that  the  effect  of  inhibitory  action  is 
to  lessen  the  expenditure  of  energy  and  so  to  assist  the  heart  for 


Chap,  iv.]  THE   VASCULAR  MECHANISM.  303 

future  efforts  ;  it  saves  the  heart  at  the  expense  of  the  rest  of  the 
economy.  The  heart,  so  far  as  we  know,  cannot  in  the  working 
of  the  living  economy  be  brought  to  a  final  arrest  by  the  simple 
action  of  the  vagus.  The  effect  of  the  augmentor  action  on  the 
other  hand  is  to  increase  the  expenditure  of  energy ;  it  saves  the 
rest  of  the  economy  at  the  expense  of  the  heart.  And  probably 
in  some  cases  augmentor  action  may  bring  about  the  cessation  of 
the  heart  beat.  Disordered  cardiac  nutrition  shews  itself  fre- 
quently in  a  dilated  condition  of  the  ventricles  ;  the  systole  is 
inadequate  to  secure  an  adequate  discharge  into  the  arteries,  the 
residual  blood  in  the  ventricles  is  increased.  If  the  augmentor 
mechanism  be  brought  to  bear  on  such  a  weakened  and  dilated 
ventricle,  it  may  induce  a  fruitless  expenditure  of  energy ;  the 
beat  though  increased  is  still  inadequate  to  secure  the  needed 
discharge  of  the  contents,  while  the  fibre  is  exhausted  by  the 
increased  metabolism.  And  the  final  result  of  such  an  effort  may 
be  the  cessation  of  the  beat. 

§  170.  Turning  now  to  the  minute  arteries  and  the  peripheral 
resistance  which  they  regulate,  we  may  call  to  mind  the  existence 
of  the  two  kinds  of  mechanism,  the  vaso-con stricter  mechanism, 
which,  owing  to  the  maintenance  by  the  central  nervous  system 
of  a  tonic  influence,  can  be  worked  both  in  a  positive  constrictor, 
and  in  a  negative  dilator  direction,  and  the  vaso-dilator  mechanism, 
which,  so  far  as  we  know,  exerts  its  influence  in  one  direction 
only,  viz.  to  dilate  the  blood  vessels.  The  latter,  dilator  mechan- 
ism seems,  as  we  have  seen,  to  be  used  in  special  instances  only, 
as  seen  in  the  cases  of  the  chorda  tympani  and  nervi  erigentes  ; 
the  use  of  the  former,  constrictor  mechanism  appears  to  be  more 
general.  Thus  the  relaxation  of  the  cutaneous  arteries  of  the  head 
and  neck,  which  is  the  essential  feature  in  blushing,  seems  due  to 
mere  loss  of  tone,  to  the  removal  of  constrictor  influences  previ- 
ously exerted  through  the  vaso-constrictor  fibres  of  the  cervical 
sympathetic.  Though  probably  dilator  fibres  pass  directly  along 
the  roots  of  the  cervical  and  of  certain  cranial  nerves  to  the  nerves 
of  the  head  and  neck,  we  have  no  evidence  that  these  come  into 
play  in  blushing;  as  we  have  .seen,  blushing  may  be  imitated  by 
mere  section  of  the  cervical  sympathetic.  So  also  the  '  glow '  and 
redness  of  the  skin  of  the  whole  body,  i.  e.  general  dilation  of  the 
cutaneous  arteries,  which  is  produced  by  external  warmth,  is 
probably  another  instance  of  diminished  activity  of  tonic  con- 
strictor influences;  though  the  result,  that  the  dilation  produced 
by  warming  an  animal  in  an  oven  is  greater  than  that  produced 
by  section  of  nerves,  seems  to  point  to  the  dilator  fibres  for  the 
cutaneous  vessels  which,  as  we  have  seen,  probably  exist  in  the 
sciatic  and  brachial  plexu.ses  and  possibly  in  all  the  spinal  nerves, 
also  taking  part  in  the  action.  A  similar  loss  of  constrictor  action 
in  the  cutaneous  vessels  may  be  the  result  of  certain  emotions, 
whether  going  so  far  as  actual  blushing  of  the  body,  or  merely 


304  THE  EFFECTS   OF  BODILY   EXERCISE.     [Book  i. 

producing  a  '  glow.'  The  warm  and  flushed  condition  of  the  skin, 
which  follows  the  drinking  of  alcoholic  fluids,  is  probably,  in  a 
similar  manner  the  result  of  an  inhibition  of  that  part  of  the  vaso- 
motor centre  which  governs  the  cutaneous  arteries.  The  effect  of 
cold  on  the  other  hand,  and  of  certain  emotions,  or  of  emotions 
under  certain  conditions,  is  to  increase  the  constrictor  action  on 
the  cutaneous  vessels,  and  the  skin  grows  pale.  It  may  be  worth 
while  to  point  out,  that  in  both  the  above  cases,  while  both  the 
cold  and  the  warmth  produce  their  effects,  chiefly  at  all  events 
through  the  central  nervous  system,  and  very  slightly,  if  at  all, 
by  direct  action  on  the  skin,  their  action  on  the  central  nervous 
system  is  not  simply  a  general  augmentation  or  inhibition  of  the 
whole  vaso-motor  centre.  On  the  contrary,  the  cold,  while  it 
constricts  the  cutaneous  vessels,  so  acts  on  the  vaso-motor  centre 
as  to  inhibit  that  portion  of  the  vaso-motor  centre  which  governs 
the  abdominal  splanchnic  area ;  while  less  blood  is  carried  to  the 
colder  skin,  by  the  opening  up  of  the  splanchnic  area  more  blood 
is  turned  on  to  the  warmer  regions  of  the  body,  and  the  rise  of 
blood  pressure  which  the  constriction  of  the  cutaneous  vessels 
tended  to  produce,  and  which  might  be  undesirable,  is  hereby 
prevented.  Conversely,  when  warmth  dilates  the  cutaneous  ves- 
sels, it  at  the  same  time  constricts  the  abdominal  splanchnic  area, 
and  prevents  an  undesirable  fall  of  pressure. 

§  171.  The  influence  on  the  body  of  exercise  illustrates  both 
the  manner  in  which  the  two  vascular  factors,  the  heart  beat  and 
the  peripheral  resistance,  are  modified  by  circumstances,  and  the 
mutual  action  of  these  on  each  other.  This  influence  is  exceed- 
ingly complex,  and  we  cannot  treat  it  properly  until  we  have 
studied  several  physiological  matters  to  which  we  shall  come 
later  on.  We  can  here  only  touch  in  a  general  way  on  some 
salient  points. 

We  know  from  superficial  observation  that  during  active 
exertion  the  breathing  is  increased,  the  heart  beats  more  quickly 
and  apparently  with  greater  vigour,  and  the  skin,  flushed  with 
blood,  perspires  freely. 

The  repeated  strong  contractions  of  the  skeletal  muscles  to 
which  we  turn  as  the  ultimate  cause  of  these  events  affect  the 
body  in  two  main  ways,  the  one  chemical,  the  other  physical. 
When  the  muscles  contract  they  take  from  the  blood  a  larger 
amount  of  oxygen,  they  give  up  to  the  blood  a  larger  amount  of 
carbonic  acid,  and  they  discharge  into  the  blood,  either  directly 
into  the  capillaries  of  the  muscles  or  indirectly  through  the  lymph 
stream,  a  quantity  of  substances,  probably  of  several  kinds,  such 
as  sarcolactic  acid  and  the  like,  which  arise  from  the  metabolism  of 
the  muscular  substance.  The  blood  leaving  a  muscle  at  work  is 
chemically  different  from  the  blood  leaving  a  muscle  at  rest. 
There  is  also  a  physical  change.  During  the  contraction  of  a 
muscle  the  blood  vessels  are  dilated  ;  this  when  many  muscles 


Chap,  iv.]  THE   VASCULAR   MECHANISM.  305 

are  at  work  would  lead  unless  compensated  to  a  lessening  of  peri- 
pheral resistance,  and  so  to  a  fall  of  arterial  pressure,  for  the 
minute  vessels  of  the  muscles  form  a  large  part  of  the  whole 
system  of  minute  vessels  of  the  body  ;  at  the  same  time  it  would 
increase  the  venous  inflow  into  the  heart. 

Now  we  shall  later  on  point  out  that  the  increased  breathing 
which  follows  upon  exertion  is  due  to  the  chemical  changes  thus 
produced  in  the  blood,  and  not  only  to  the  diminution  of  oxygen 
and  increase  of  carbonic  acid,  but  also  and  perhaps  especially  to 
the  presence  of  the  other  products  of  metabolism  referred  to 
above.  Indeed  we  have  reason  to  think  that  the  increase  in 
breathing  is  sufficient  to  maintain  the  blood  in  a  normal  condition 
so  far  as  oxygen  and  carbonic  acid  are  concerned ;  the  blood  is  not 
more  venous  during  exertion  than  during  rest,  it  is  possibly  less 
venous.  The  increased  breathing  however,  though  it  clears  the 
blood  of  the  excess  of  carbonic  acid,  leaves  behind  in  the  blood  the 
other  muscular  products,  ready  to  produce  their  effects  on  the  body 
before  they  are  got  rid  of  by  organs  other  than  the  lungs. 

This  increased  breathing  promotes  mechanically,  as  we  shall 
point  out  later  on,  the  How  of  blood  to  the  heart  and  through  the 
lungs.  And  this  together  with  the  increased  venous  flow  from 
the  contracting  muscles  favours  the  beat  of  the  heart,  supplying 
the  means  for  a  greater  output  and  probably  also  tending  to 
increase  the  force  of  the  systole. 

But  there  are  other  influences  at  work  on  the  heart.  The 
changes  in  the  blood  and  probably  the  presence  of  the  above 
mentioned  metabolic  products,  no  less  than  the  excess  of  carbonic 
acid,  so  affect  the  vaso-motor  centre  as  to  lead  to  a  great  widening 
of  the  cutaneous  vessels ;  at  the  same  time  as  we  shall  see  these 
so  affect  other  parts  of  the  central  nervous  system  as  to  lead  to  a 
great  activity  of  the  sweat  glands,  by  means  of  which  the  products 
in  question  are  got  rid  of  or  rendered  inert.  But  the  widening  of 
the  vessels  of  the  skin  and  of  many  muscles  at  the  same  time 
must  unless  compensated  lead  to  a  fall  of  arterial  pressure.  We 
have  evidence  however  that  the  arterial  pressure  does  not  fall,  in 
fact  may  be  higher  than  normal ;  a  very  marked  compensation 
must  therefore  take  place.     This  is  probably  of  a  double  nature. 

On  the  one  liand,  the  altered  blood  increases  the  woi-k  of  the 
heart,  enabling  it  by  a  quicker  rhythm  or  a  stronger  stroke  or  by 
both  combined,  to  avail  itself  of  the  advantages  of  the  greater 
venous  inflow  and  to  increase  its  output,  whereby  the  arterial 
pressure  increases.  We  cannot  suppose  that  tliis  increased  work 
is  due  to  the  direct  effect  of  the  altered  blood  on  the  cardiac 
muscles,  for  the  altered  l)lood,  is  distinctly  injurious  to  muscular 
tissue.  The  increase  of  the  heart's  work  is  gained  in  spite  of  this 
influence  of  the  altered  blood,  and  is  due  to  the  intervention  of 
the  central  nervous  system.  There  are  several  facts  which  seem 
to  support  the  view  that  the  altered  blood  throws  mto  activity  the 


306  THE  EFFECTS   OF   FOOD.  [Book  i. 

augmentor  system,  and  thus  by  increasing  the  work  of  the  heart 
raises  or  maintains  the  arterial  pressure. 

On  the  other  hand,  we  have  reason  to  think  that  while  that 
part  of  the  vaso-motor  centre  which  governs  the  cutaneous  vas- 
cular area  is  being  inhibited,  that  part  which  governs  the  abdominal 
splanchnic  area  is  on  the  contrary  being  augmented.  In  this 
way  a  double  end  is  gained.  On  the  one  hand,  the  mean  blood 
pressure  is  maintained  or  increased  in  a  more  economical  manner 
than  by  increasing  the  heart  beats,  and  on  the  other  hand,  the 
blood  during  the  exercise  is  turned  away  from  the  digestive  organs 
which  at  the  time  are  or  ought  to  be  at  rest  and  therefore 
requiring  comparatively  little  blood.  These  organs  certainly  at 
all  events  ought  not  during  exercise  to  be  engaged  in  the  task  of 
digesting  and  absorbing  food,  and  the  old  saying, "  after  dinner  sit 
awhile,"  may  serve  as  an  illustration  of  the  working  of  the  vascular 
mechanism  with  which  we  are  dealing.  The  duty  which  some  of 
the  digestive  organs  have  during  exercise  to  carry  out  in  the  way 
of  excretion  of  metabolic  waste  products  is  as  we  have  already 
said  probably  taken  on  by  the  flushed  and  perspiring  skin.  It  is 
true  that  at  the  beginning  of  a  period  of  exercise,  before  the  skin 
so  to  speak  has  settled  down  to  its  work,  an  increased  flow  of 
urine,  dependent  on  or  accompanied  by  an  increased  flow  of  blood 
through  the  kidney,  may  make  its  appearance ;  but  in  this  case, 
as  we  shall  see  later  on  in  dealing  with  the  kidney,  the  flow  of 
blood  through  the  kidney  may  be  increased  in  spite  of  constriction 
of  the  rest  of  the  splanchnic  area,  and,  besides,  such  an  initial 
increase  of  urine  speedily  gives  way  to  a  decrease. 

§  172.  The  effect  of  food  on  the  vascular  mechanism  affords  a 
marked  contrast  to  the  effect  of  bodily  labour.  The  most  prominent 
result  is  a  widening  of  the  whole  abdominal  vascular  area,  accom- 
panied by  so  much  constriction  of  the  cutaneous  vascular  area 
and  so  much  increase  of  the  heart's  beat  as  are  sufficient  to  neutra- 
lize the  tendency  of  the  widening  of  the  abdominal  vascular  area 
to  lower  the  mean  pressure,  or  perhaps  even  sufficient  to  raise 
slightly  the  mean  pressure. 

The  widening  of  the  abdominal  vascular  area,  as  we  have 
seen  (§  157),  is  probably  an  inhibition  of  tonic  vaso-constrictor 
impulses  as  a  reflex  act,  assisted  possibly  by  some  local  action 
due  to  the  presence  of  the  food  and  similar  to  that  supposed  to 
take  place  in  the  skeletal  muscles  during  contraction.  We 
have  at  present  no  clear  evidence  that  the  absorbed  products 
of  digestion  y^lay  any  important  part  in  this  splanchnic  dilation  by 
acting  on  the  central  nervous  system ;  but  the  concomitant 
increase  of  the  heart  beat  is  probably  due  to  this  cause.  We  have 
no  exact  knowledge  of  how  the  absorbed  products  bring  this 
about,  and  possibly  the  mode  of  action  differs  with  the  different 
constituents  of  food.  With  regard  to  alcohol,  which  is  so  often 
part  of  a  meal,  we  may  perhaps  say  that  the  character   of   its 


Chap,  iv.]  THE   VASCULAR  MECHANISM.  307 

effects,  the  quickening  and  strengthening  of  the  beats,  seems  to 
point  to  its  setting  in  action  the  augmentor  mechanism,  but  it 
also  probably  acts  directly  on  the  cardiac  tissues.  In  any  case  the 
effects  depend  largely  on  the  dose,  and  if  this  is  large  the  direct 
effects  become  prominent,  and  the  ultimate  result  is  a  deleterious 
weakening. 

Any  large  widening  of  the  cutaneous  area,  especially  if  accom- 
panied by  muscular  labour  and  the  incident  widening  of  the 
arteries  of  the  muscles,  would  tend  so  to  lower  the  general  blood 
pressure  (unless  met  by  a  wasteful  use  of  cardiac  energy)  as 
injuriously  to  lessen  the  flow  through  the  active  digesting  viscera. 
A  moderate  constriction  of  the  cutaneous  vessels  on  the  other  hand, 
by  throwing  more  blood  on  the  abdominal  splanchnic  area  without 
tasking  the  hsart,  is  favourable  to  digestion,  and  is  probably  the 
physiological  explanation  of  the  old  saying,  "  If  you  eat  till  you  're 
cold,  you  '11  live  to  be  old." 

In  fact  during  life  there  seems  to  be  a  continual  give-and-take 
between  the  blood  vessels  of  the  somatic  and  those  of  the  splanchnic 
divisions  of  the  body :  to  fill  the  one  the  other  is  proportionately 
emptied,  and  vice  versa. 

§  173.  In  the  following  sactions  of  this  work  we  shall  see  re- 
peated instances,  similar  to  or  even  more  striking  than  the  above, 
of  the  management  of  the  vascular  mechanism  by  means  of  the 
nervous  system,  and  we  therefore  need  dwell  no  longer  on  the  sub- 
ject. We  may  simply  repeat  that  at  the  centre  lies  the  cardiac 
muscular  fibre,  and  at  the  periphery  the  plain  muscular  fibre  of  the 
minut3  artery.  On  these  two  elements  the  central  nervous  system, 
directed  by  this  or  that  impulse  reachmg  it  along  afferent  nerve 
fibres,  or  affected  directly  by  this  or  that  influence,  is  during  life 
continually  playing,  now  augmenting,  now  inhibiting,  now  the  one, 
now  the  other,  and  so,  by  help  of  the  elasticity  of  the  arteries  and 
the  mechanism  of  the  valves,  directing  the  blood  flow  according  to 
the  needs  of  the  body. 


BOOK  II. 


THE  TISSUES   OF   CHEMICAL   ACTION  WITH   THEIR 
EESPECTIVE  MECHANISMS.      NUTRITION. 


CHAPTER  I. 

THE   TISSUES   AND   MECHANISMS   OF   DIGESTION. 


§  174.  The  food  in  passing  along  the  alimentary  canal  is 
subjected  to  the  action  of  certain  juices  supplied  by  the  secretory 
activity  of  the  epithelium  cells  which  line  the  canal  itself  or 
which  form  part  of  its  glandular  appendages.  These  juices  (viz. 
saliva,  gastric  juice,  bile,  pancreatic  juice,  and  the  secretions  of  the 
small  and  large  intestines),  poured  upon  and  mingling  with  the 
food,  produce  in  it  such  changes,  that  from  being  largely  insoluble 
it  becomes  largaly  soluble,  or  otherwise  modify  it  in  such  a  way 
that  the  larger  part  of  what  is  eaten  passes  into  the  blood,  either 
directly  by  means  of  the  capillaries  of  the  alimentary  canal  or 
indirectly  by  means  of  the  lacteal  system,  while  the  smaller  part 
is  discharged  as  excrement. 

Those  parts  of  the  food  which  are  thus  digested,  absorbed  and 
made  use  of  Ijy  the  body,  are  spoken  of  as  food-stuffs  (they  have 
al-o  been  called  alimentary  principles)  and  may  be  conveniently 
divided  into  four  great  classes. 

1.  Proteicls.  We  have  previously  (§  15)  spoken  of  the  chief 
characters  of  this  class,  and  have  dealt  with  several  members  in 
treating  of  blood  and  muscle.  We  may  here  repeat  that  in  general 
composition  they  contain  in  100  parts  by  wc'ght  "in  round 
numbers  "  rather  more  than  15  parts  of  nitrogen,  rather  more 
than  50  parts  of  carbon,  about  7  parts  of  hydrogen,  and  rather 
more  than  20  parts  of  oxygan ;  though  essentially  the  nitrogenous 
bodies  of  food  and  of  the  body,  they  are  made  u])  of  carbon  to  the 
extent  of  more  than  half  their  weight. 

The  uitrogenous  body  gelatin,  which  occurs  largely  in  animal 
food,  and  some  other  bodies  of  less  importance,  while  more  closely 
alliod  to  ])r()teid  bodies  than  to  any  other  class  of  organic  sub- 
stances, differ  considerably  from  proteids  in  composition  and 
especially  in  their  behaviour  in  the  body  ;  they  are  not  of  sufficient 
importance  to  form  a  class  by  themselves. 


312  FOOD-STUFFS.  [Book  ii. 

2.  i^a^s,  frequently  but  erroneously  called  Hydrocarbons.  These 
vary  very  widely  in  chemical  composition,  ranging  from  such  a 
comparatively  simple  fat  as  butyrin  to  the  highly  complex  lecithin 
(§  66)  ;  they  all  possess,  in  view  of  the  oxidation  of  both  their 
carbon  and  their  hydrogen,  a  large  amount  of  potential  energy. 

3.  Carho-hydrates,  or  sugars  and  starches.  These  possess 
weight  for  weight  relatively  less  potential  energy  than  do  fats ; 
they  already  contain  in  themselves  a  large  amount  of  combined 
oxygen  and  when  completely  oxidised  give  out,  w^eight  for  weight, 
less  heat  than  do  fats. 

4.  Saline  or  Mineral  Bodies,  and  Water.  These  salts  are  for 
the  most  part  inorganic  salts  ;  and  this  class  differs  from  the  three 
preceding  classes  inasmuch  as  the  usefulness  of  its  members  to 
the  body  lies  not  so  much  in  the  amount  of  energy  which  may 
be  given  out  by  their  oxidation,  as  in  the  various  influences  which, 
by  their  presence,  they  exercise  on  the  metabolic  events  of  the 
body. 

These  several  food-stuffs  are  variously  acted  upon  in  the 
several  parts  of  the  alimentary  canal,  and  we  may  distinguish,  as 
the  food  passes  along  the  digestive  tract,  three  main  stages : 
digestion  in  the  mouth  and  stomach,  digestion  in  the  small 
intestine,  and  digestion  in  the  large  intestine.  In  many  animals 
the  first  stage  is,  to  a  large  extent,  preparatory  only  to  the  second 
which  in  all  animals  is  the  stage  in  which  the  food  undergoes  the 
greatest  change;  in  the  third  stage  the  changes  liegun  in  the 
previous  stages  are  completed,  and  this  stage  is  especially  chaiac- 
terised  by  the  absorption  of  fluid  from  the  interior  of  the  alimen- 
tary canal. 

It  will  be  convenient  to  study  these  stages,  more  or  less  apart, 
though  not  wholly  so,  and  it  will  also  be  convenient  to  consider 
the  whole  subject  of  digestion  under  the  following  heads:  — 

First,  the  characters  and  properties  of  the  various  juices,  and 
the  changes  which  they  bring  about  in  the  food  eaten. 

Secondly,  the  nature  of  the  processes  by  means  of  which 
the  epithelium  cells  of  the  various  glands  and  various  tracts  of 
the  canal  are  able  to  manufacture  so  many  various  juices  out  of  the 
common  source,  the  blood,  and  the  manner  in  which  the  secretory 
activity  of  the  cells  is  regulated  and  subjected  to  the  needs  of  the 
economy. 

Thirdly,  the  mechanisms,  here  as  elsewhere  chiefly  of  a  mus- 
cular nature,  by  which  the  food  is  passed  along  the  canal,  and 
most  efficiently  brought  into  contact  with  the  several  juices. 

Fourthly  and  lastly,  the  means  by  which  the  nutritious  digested 
material  is  separated  from  the  undigested  or  excremental  material, 
and  absorbed  into  the  blood. 


SEC.   1.     THE   CHARACTERS   AND   PROPERTIES   OF 
SALIVA  AND   GASTRIC   JUICE. 

Saliva. 


§  175.  Mixed  saliva,  as  it  appears  in  the  mouth,  is  a  thick, 
glairy,  generally  frothy  and  turbid  tluid.  Under  the  microscope 
it  is  seen  to  contain,  besides  the  molecular  debris  of  food,  bacteria 
and  other  organisms  (frequently  cryptogamic  spores),  epithelium- 
scales,  mucus-corpuscles  and  granules,  and  the  so-called  salivary 
corpuscles.  Its  reaction  in  a  healthy  subject  is  alkaline,  espe- 
cially when  the  secretion  is  abundant.  When  the  saliva  is  scanty, 
or  when  the  subject  suffers  from  dyspepsia,  the  reaction  of  the 
mouth  may  be  acid.  Saliva  contains  but  little  solid  matter,  on 
an  average  probably  about  -5  p.c,  the  specific  gravity  varying 
from  1-002  to  1-006.  Of  these  solids,  rather  less  than  half,  about 
•2  p.c,  are  salts  (including  at  times  a  minute  quantity  of  potas- 
sium sulphocyanate).  The  organic  bodies  which  can  be  recognised 
in  it  are  globulin  and  serum-albumin  (see  §§  16,  17)  found  in  small 
quantities  only,  other  obscure  bodies  occurring  in  minute  quantity, 
and  iiiucvi ;  the  latter  is  by  far  the  most  conspicuous  organic  con- 
stituent, the  glairiness  or  ropiness  of  mixed  and  other  kinds  of 
saliva  being  due  to  its  presence. 

Mucin.  If  acetic  acid  be  cautiously  added  to  mixed  saliva 
the  viscidity  of  the  saliva  is  increased,  and  on  further  addition  of 
the  acid  a  semi-opaipie  ropy  mass  separates  out,  leaving  the  rest 
of  the  saliva  limpid.  Tiiis  ropy  mass,  which  is  mucin,  if  stirred 
carefully  with  a  glass  rod,  shrinks,  becoming  opaque,  clings  to  the 
glass  rod  and  may  be  thus  removed  from  the  fluid.  If  the  quan- 
tity of  mucin  be  small  and  the  saliva  be  violently  shaken  or 
stirred  while  the  acid  is  being  added,  the  mucin  is  apt  to  be  pre- 
cipitated in  flakes,  and  may  then  be  separated  by  filtration.  It 
may  be  added  that  the  precipitation  of  mucin  by  acid  is  grently 
influenced  by  the  presence  of  sodium  chloride  and  other  salts  : 
thus  after  the  addition  of  sodium  chloride  acetic  acid  even  in  con- 
siderable excess  will  not  cause  a  precipitate  of  mucin. 


314  MUCIN.  [Book  ii. 

Mucin,  thus  prepared  and  purified  by  washing  with  acetic 
acid,  swells  out  in  water,  without  actually  dissolving ;  it  will 
however  dissolve  into  a  viscid  fluid  readily  in  dilute  (O'l  p.c.) 
solutions  of  potassium  hydrate,  more  slowly  in  solutions  of  alka- 
line salts.  In  order  to  filter  a  mucin  solution,  great  dilution  with 
water  is  necessary.  Mucin  is  precipitated  by  strong  alcohol  and 
by  various  metallic  salts  ;  it  may  also  be  precipitated  by  dilute 
mineral  acids,  but  the  precipitate  is  then  soluble  in  excess  of  the 
acid.  Mucin  gives  the  three  proteid  reactions  mentioned  in  §  15, 
but  it  is  a  very  complex  body,  more  complex  even  than  proteids, 
for  by  treatment  with  dilute  mineral  acids,  and  in  other  ways,  it 
may  be  converted  into  some  form  of  proteid  (acid-albumin  when 
dilute  mineral  acid  is  used),  while  at  the  same  time  there  is 
formed  a  body  which  appears  to  be  a  carbohydrate  and  resembles 
a  sugar  in  having  the  power  of  reducing  cupric  sulphate  solutions. 
Several  kinds  of  mucin  appear  to  exist  in  various  animal  bodies, 
but  they  seem  all  to  agree  in  the  character  that  they  can  by 
appropriate  treatment  be  split  up  into  a  proteid  of  some  kind  and 
into  a  carbohydrate  or  allied  body, 

§  176.  The  chief  purpose  served  by  the  saliva  in  digestion  is 
to  moisten  and  soften  the  food,  and  to  assist  in  mastication  and 
deglutition.  In  some  animals  this  is  its  only  function.  In  other 
animals  and  in  man  it  has  a  specific  solvent  action  on  some  of  the 
food-stuffs.  Such  minerals  as  are  soluble  in  slightly  alkaline 
fluids  are  dissolved  by  it.  On  fats  it  has  no  eflect  save  that  of 
producing  a  very  feeble  emulsion.  On  proteids  it  has  also  no 
specific  action,  though  pieces  of  meat,  cooked  or  uncooked,  appear 
greatly  altered  after  they  have  been  masticated  for  some  time ; 
the  chief  alteration  however  which  thus  takes  place  is  a  change  in 
the  hsemoglobin,  and  a  general  softening  of  the  muscular  fibres 
by  aid  of  the  alkalinity  of  the  saliva.  Of  course  when  particles 
of  food  are  retained  for  a  long  time  in  the  mouth,  as  in  the  inter- 
stices, or  in  cavities  of  the  teeth,  the  bacteria  or  other  organisms 
which  are  always  present  in  the  mouth  may  produce  much  more 
profound  changes,  but  these  are  not  the  legitimate  products  of 
the  action  of  saliva.  The  characteristic  property  of  saliva  is  that 
of  converting  starch  into  some  form  of  sugar. 

Action  of  Saliva  on  Starch  If  to  a  quantity  of  boiled  starch, 
which  is  always  more  or  less  viscid  and  somewhat  opaque  or  tur- 
bid, a  small  quantity  of  saliva  be  added,  it  will  be  found  after  a 
short  time  that  an  important  change  has  taken  place,  inasmuch  as 
the  mixture  has  lost  its  previous  viscidity  and  become  thinner 
and  more  transparent.  In  order  to  understand  this  change,  the 
reader  must  bear  in  mind  the  existence  of  the  following  bodies 
all  belonging  to  the  class  of  carbohydrates. 

1.  Starch,  which  forms  with  water  not  a  true  solution  but  a 
more  or  less  viscid  mixture,  and  gives  a  characteristic  blue  colour 
with  iodine.     The  formula  is  CcHmO^  or  more  correctly  (CcH,o05)„ 


Chap,  i.]  TISSUES  AND  MECHANISMS  OF  DIGESTION.  315 

since  the  molecule  of  starch  is  some  multiple  (?i  being  not  less 
than  5)  of  the  simpler  formula.  A  kind  of  starch,  known  as 
soluble  starch,  while  giving  a  blue  colour  with  iodine,  forms, 
unlike  ordinary  starch,  a  clear  solution. 

2.  Dcxiriiis,  differing  from  starch  in  forming  a  clear  solution. 
Of  these  there  are  at  least  two ;  one  erythrodcxtrin,  often  spoken 
of  simply  as  dextrin,  giving  a  port-wine  red  colour  with  iodine, 
and  a  second,  achrooclcrtrin,  which  gives  no  colour  at  all  with 
iodine.  The  formula  for  dextrin  is  the  same  as  that  for  starch, 
but  has  a  smaller  molecule  and  might  be  represented  by  (CoHioO^),/. 

3.  Dedrose,  also  called  glucose  or  grape-sugar,  giving  no 
color.ition  with  iodine,  but  characterised  by  the  power  of  reducing 
cupric  and  ot.her  metallic  salts ;  thus,  when  dextrose  is  boiled 
with  a  fluid  known  as  Fehling's  fluid,  which  is  a  solution  of 
hydrated  cupric  oxide  in  an  excess  of  caustic  alkali  and  double 
tartrate  of  sodium  and  potassium,  the  cupric  oxide  is  reduced  and 
a  red  or  yellow  dsposit  of  cuprous  oxide  is  thrown  down.  This 
reaction  serves  with  others  as  a  convenient  test  for  dextrose. 
Neither  starch  nor  that  commonest  form  of  sugar  known  as  cane- 
sugar,  give  this  reaction  ;  wh  rither  the  dextrins  do  is  doubtful. 
The  formula  for  dextrose  is  C,;H,oOg ;  it  is  more  simple  than  that 
of  starch  or  dextrin  and  contains  an  additional  HoO  for  every  0,;. 
Unlike  starch  and  dextrin  it  can  be  obtained  in  a  crystalline  form, 
either  from  aqueous  solutions  (it  being  readily  soluble  in  water), 
in  which  case  the  crystals  contain  water  of  crystallisation,  or  from 
its  solutions  in  alcohol  (in  which  it  is  sparingly  soluble),  in  which 
case  the  crystals  have  no  such  water  of  crystallisation.  Solutions 
of  dextrose  have  a  marked  dextrorotatory  power  over  rays  of  light. 

4.  Maltose,  very  similar  to  dextrose,  and  like  it  capable  of 
reducing  cupric  salts.  The  formula  is  somewhat  different,  being 
CioH^.Q,,.  Besides  this,  it  differs  from  dextrose  chiefly  in  its 
smaller  reducing  power,  i.e.  a  given  weight  will  not  convert  so 
much  cupric  oxide  into  cuprous  oxide  as  will  the  same  weight  of 
dextrose,  and  in  having  a  stronger  rotatory  action  on  rays  of  light. 
Like  dextrose  it  can  be  crystallised,  the  crystals  from  aqueous 
solutions  containing  water  of  crystallisation. 

Now  when  a  quantity  of  starch  is  lioiled  with  water  we  may 
recognise  in  the  viscid  imperfect  solution,  on  the  one  hand  the 
presence  of  starch,  by  the  blue  colour  which  the  addition  of  iodine 
gives  rise  to,  and  on  the  other  hand  the  absence  of  sugar  (maltose, 
dextrose),  by  the  fact  that  when  boiled  with  Fehling's  fluid  no 
reduction  takes  place  and  no  cuprous  oxide  is  precipitated. 

If  however  the  boiled  starch  be  submitted  for  a  while  to  the 
action  of  saliva,  especially  at  a  somewhat  high  temperature  such 
as  35*^  or  40"C.,  it  is  found  that  the  subsequent  addition  of  iodine 
gives  no  blue  colour  at  all,  or  very  much  less  colour,  shewing  that 
the  starch  has  disappeared  or  diminished ;  on  the  other  hand  the 
mixture  readily  gives  a  precipitate  of  cuprous  oxide  when  boiled 


316  ACTION   OF   SALIVA.  [Book  ii. 

with  Fehling's  fluid,  shewing  that  maltose  or  dextrose  is  present. 
That  is  to  say  the  saliva  has  converted  the  starch  into  maltose 
or  dextrose.  The  presence  of  the  previously  absent  sugar  may 
also  be  shewn  by  fermentation  and  by  the  other  tests  for  sugar. 
Moreover,  if  an  adequately  large  quantity  of  starch  be  subjected 
to  the  charge,  the  sugar  formed  may  be  isolated,  and  its  charac- 
ters determined.  When  this  is  done  it  is  found  that  while  some 
dextrose  is  formed  the  greater  part  of  the  sugar  which  appears  is 
in  the  form  of  maltose.  As  is  well  known,  starch  may  by  the 
action  of  dilute  acid  be  converted  into  dextrin,  and  by  further 
action  into  sugar  ;  but  the  sugar  thus  formed  is  always  wholly 
dextrose,  and  not  maltose  at  all.  The  action  of  saliva  in  this 
respect  differs  from  the  action  of  dilute  acid. 

While  the  conversion  of  the  starch  by  the  saliva  is  going  on 
the  addition  of  iodine  frequently  gives  rise  to  a  red  or  violet 
colour  instead  of  a  pure  blue,  but  when  the  conversion  is  complete 
no  coloration  at  all  is  observed.  The  appearance  of  this  red 
colour  indicates  the  presence  of  dextrin  (erythrodextrin) ;  the 
violet  colour  is  due  to  the  red  being  mixed  with  the  blue  of  still 
unchanged  starch. 

The  appearance  of  dextrin  shews  that  the  action  of  the  saliva 
on  the  starch  is  somewhat  complex  ;  and  this  is  still  further 
proved  by  the  fact  that  even  when  the  saliva  has  completed  its 
work  the  whole  of  the  starch  does  not  reappear  as  maltose  or 
dextrose.  A  considerable  quantity  of  the  other  dextrin  (achroo- 
dextrin)  always  appears  and  remains  unchanged  to  the  end ;  and 
there  are  probably  several  other  bodies  also  formed  out  of  the 
staich,  the  relative  proportions  varying  according  to  circumstances. 
The  change  therefore,  though  perhaps  we  may  speak  of  it  in  a 
general  way  as  one  of  hydration,  cannot  be  exhibited  under  a 
simple  formula,  and  we  may  rest  content  for  the  present  with  the 
statement  that  starch  when  subjected  to  the  action  of  saliva  is 
converted  chiefly  into  the  sugar  known  as  maltose  with  a  com- 
paratively small  quantity  of  dextrose  and  to  some  extent  into 
achroodextrin  (erythrodextrin  appearing  temporarily  only  in  the 
process),  other  bodies  on  which  we  need  not  dwell  being  formed 
at  the  same  time. 

Eaw  unboiled  starch  undergoes  a  similar  change  but  at  a  much 
slower  rate.  This  is  due  to  the  fact  that  in  the  curiously  formed 
starch  grain  the  true  starch,  or  granulosa,  is  invested  with  coats 
of  cellulose.  This  latter  material,  which  requires  previous  treat- 
ment with  sulphuric  acid  before  it  will  give  the  blue  reaction 
on  the  addition  of  iodine,  is  apparently  not  acted  upon  by  saliva. 
Hence  the  saliva  can  only  get  at  the  granulose  by  traversing  the 
coats  of  cellulose,  and  the  conversion  of  the  former  is  thereby 
much  hindered  and  delayed. 

§  177.  The  conversion  of  starch  into  sugar,  and  this  we  may 
speak  of  as  the  amylolytic  action  of  saliva,  will  go  on  at  the  ordinary 


Chap. I.J  TISSUES  AND  MECHANISMS  OF  DIGESTION.  317 

temperature  of  the  atmosphere.  The  lower  the  temperature  the 
slower  the  change,  and  at  about  0''  C.  the  conversion  is  indefinitely 
prolonged.  After  exposure  to  this  cold  for  even  a  consideraljle 
time  the  action  recommences  when  the  temperature  is  again 
raised.  Increase  of  temperature  up  to  about  35*^ — 40*^,  or  even  a 
little  higher,  favours  the  change,  the  greatest  activity  being  said 
to  be  manifested  at  about  40'1  Much  beyond  this  point,  however, 
increase  of  temperature  becomes  injurious,  markedly  so  at  60^  or 
70*^ ;  and  saliva  which  has  been  boiled  for  a  few  minutes  not  only 
has  no  action  on  starch  while  at  that  temperature,  but  does  not 
regain  its  powers  on  cooling.  By  being  boiled,  the  amylolytic 
activity  of  saliva  is  permanently  destroyed. 

The  action  of  saliva  on  starch  is  most  rapid  when  the  reaction 
of  the  mixture  is  neutral  or  nearly  so ;  it  is  hindered  or  arrested 
by  a  distinctly  acid  reaction.  Indeed  the  presence  of  even  a  very 
small  quantity  of  free  acid,  at  all  events  of  hydrochloric  acid,  at 
the  temperature  of  the  body  not  only  suspends  the  action  but 
speedily  leads  to  permanent  abolition  of  the  activity  of  the  juice. 
The  bearing  of  this  will  be  seen  later  on. 

The  action  of  saliva  is  hampered  by  the  presence  in  a  concen- 
trated state  of  the  product  of  its  own  action,  that  is,  of  sugar.  If 
a  small  quantity  of  saliva  be  added  to  a  thick  mass  of  boiled  starch, 
the  action  will  after  a  while  slacken,  and  eventually  come  to  almost 
a  stand-still  long  before  all  the  starch  has  been  converted.  On 
diluting  the  mixture  with  water,  the  action  will  recommence.  If 
the  products  of  action  be  removed  as  soon  as  they  are  formed,  by 
dialysis  for  example,  a  small  quantity  of  saliva  will,  if  sufficient 
time  be  allowed,  convert  into  sugar  a  very  large,  one  might  almost 
say  an  indefinite,  quantity  of  starch.  Whether  the  particular 
constituent  on  which  the  activity  of  saliva  depends  is  at  all 
consumed  in  its  action  has  not  at  present  been  definitely  settled. 

On  what  constituent  do  the  amylolytic  virtues  of  saliva  depend? 

If  saliva,  filtered  and  thus  freed  from  much  of  its  mucin  and 
from  other  formed  constituents,  be  treated  with  ten  or  fifteen  times 
its  bulk  of  alcohol,  a  precipitate  is  formed  containing  besides  other 
substances  all  the  proteid  matters.  Upon  standing  under  the 
alcohol  for  some  time  (several  days),  the  proteids  thus  precipitated 
become  coagulated  and  ins(ilul)le  in  water.  Hence,  an  aqueous 
extract  of  the  precipitate,  made  after  this  interval,  contains  very 
little  proteid  material;  yet  it  is  exceedingly  active.  Moreover 
by  other  more  elaborate  methods  there  may  be  obtained  from 
saliva  solutions  which  appear  to  1)3  almost  entirely  free  from 
proteids  and  yet  are  intens^ely  amylolytic.  But  even  the.se  probably 
contain  other  bodies  besides  the  really  active  constituent.  AVhat- 
ever  the  active  substance  be  in  itself,  it  exists  in  such  extremely 
small  quantities  that  it  has  never  yet  been  satisfactorily  isolated ; 
and  indeed  the  only  clear  evidence  we  have  of  its  existence  is  the 
manifestation  of  its  peculiar  powers. 


318  THE   AMYLOLYTIC   FERMENT.  [Book  ii. 

The  salient  features  of  this  body,  this  amylolytic  agent,  which 
we  may  call  ijtyalin,  are  then  :  —  1st,  its  presence  in  minute  and 
almost  inappreciable  quantity.  2nd,  the  close  dependence  of  its 
acti%dty  on  temperature.  3rd,  its  permanent  and  total  destruction 
by  a  high  temperature  and  by  various  chemical  reagents.  4th,  the 
want  of  any  clear  proof  that  it  itself  undergoes  any  change  during 
the  manifestation  of  its  powers ;  that  is  to  say,  the  energy  neces- 
sary for  the  transformation  which  it  effects  does  not  come  out  of 
itself ;  if  it  is  all  used  up  in  its  action,  the  loss  is  rather  that 
of  simple  wear  and  tear  of  a  machine  than  that  of  a  substance 
expended  to  do  work.  5th,  the  action  which  it  induces  is  probably 
of  such  a  kind  (splitting  up  of  a  molecule  with  assumption  of 
water)  as  is  afi'ected  by  that  particular  class  of  agents  called 
"  hydrolytic." 

These  features  mark  out  the  amylolytic  active  body  of  saliva 
as  belonging  to  the  class  oi  ferments ; '^  and  we  may  henceforward 
speak  of  the  amylolytic  ferment  of  saliva.  The  fibrin-ferment 
(§  20)  is  so  called  because  its  action  in  many  ways  resembles  that 
of  the  ferment  of  which  we  are  now  speaking. 

§  178.  Mixed  saliva,  whose  properties  we  have  just  discussed, 
is  the  result  of  the  mingling  in  various  proportions  of  saliva  from 
the  parotid,  submaxillary,  and  sublingual  glands  with  the  secretion 
from  the  buccal  glands.  These  constituent  juices  have  their  own 
special  characters,  and  these  are  not  the  same  in  all  animals. 
Moreover  in  the  same  individual  the  secretion  differs  in  composition 
and  properties  according  to  circumstances  ;  thus,  as  we  shall  see  in 
detail  hereafter,  the  saliva  from  the  submaxillary  gland  secreted 
under  the  influence  of  the  chorda  tympani  nerve  is  different  from 
that  which  is  obtained  from  the  same  gland  by  stimulating  the 
sympathetic  nerve. 

In  man  pure  parotid  saliva  may  easily  be  obtained  by  introducing  a 
fine  cannula  into  the  opening  of  the  Stenonian  duct,  and  submaxillary 
saliva  or  rather  a  mixture  of  submaxillary  and  sublingual  saliva,  by 
similar  catheterisation  of  the  Whartonian  duct.  In  animals  the  duct 
may  be  dissected  out  and  a  cannula  introduced. 

Parotid  saliva  in  man  is  clear  and  limpid,  not  viscid ;  the  reaction 
of  the  first  drops  secreted  is  often  acid,  the  succeeding  portions, 

1  Ferments  may.  for  the  present  at  least,  be  divirled  into  two  classes,  commonly 
caller!  orqanii^pcl  and  imorr/amfn'fl.  Of  the  former,  yeast  may  be  taken  as  a  well- 
known  example  The  fermentative  activity  of  veast  which  leads  to  the  conversion 
of  sufjar  into  alcohol,  is  dependent  on  the  life  of  tlie  yeast-cell.  Unless  the  yeast- 
cell  be  livin?  and  functional,  fermentation  does  not  take  place;  when  the  yeast- 
cell  dies  fermentation  ceases  ;  and  no  substance  obtained  from  the  fluid  parts  of 
yeast,  by  precipitation  with  alcohol  or  otherwise,  will  give  rise  to  alcoholic  fermen- 
tation. Tlie  salivary  ferment  belongs  to  the  lalter  class  ;  it  is  a  substance,  not  a 
living  or<xanism  like  yeast.  It  may  be  added  however  that  possibly  the  organised 
ferment,  the  yeast  for  instance,  produces  its  effect  bv  means  of  an  ordinary 
unorganised  ferment  which  it  generates,  but  which  is  immediately  made  away  with. 


Chap,  i.]    TISSUES  AND  MECHANISMS  OF  DIGESTION.  319 

at  all  events  when  the  How  is  at  all  copious,  are  alkaline ;  that  is 
to  say  the  natural  secretion  is  alkaliner-but  this  may  be  obscured 
by  acid  changes  taking  place  ii^-feire'tluid  which  has  been  retained 
in  the  duct,  possibly  by  tlii*^funnation  of  an  excess  of  carbonic  acid. 
On  standing,  the  cle3#^^iid  becomes  turbid  from  a  precipitate  of 
calcic  carbonat!i<due  to  an  escape  of  carbonic  acid.  It  contains 
"lobulin  •atTcTsome  other  forms  of  alljTimin,  with  little  or  no  mucin. 
Potassium  sulphocyanate  may  also  sometimes  be  detected,  but 
structural  elements  are  absent. 

Submaxillary  saliva,  in  man  and  in  most  animals,  differs  from 
parotid  saliva  in  baing  more  alkaline  and,  from  the  presence  of 
mucin,  more  viscid;  it  contains  salivary  corpuscles,  that  is  l)odies 
closely  resembling  if  not  identical  with  leucocytes,  and,  often  in 
abundance,  amorphous  masses.  The  so-called  chorda  saliva  in 
the  dog,  that  is  to  say  saliva  obtained  by  stimulating  the  chorda 
tympani  nerve,  (of  which  we  shall  presently  speak),  is  under 
ordinary  circumstances  thinner  and  less  viscid,  contains  less 
mucin,  and  fewer  structural  elements,  than  the  so-called  sympa- 
thetic saliva,  which  is  remarkable  for  its  viscidity,  its  structural 
elements,  and  for  its  larger  total  of  solids. 

Sublingual  saliva  is  more  viscid,  and  contains  more  salts  (in 
the  dog  about  1  p.c),  than  the  submaxillary  saliva. 

The  action  of  saliva  varies  in  intensity  in  different  animals. 
Thus  in  man,  the  pig,  the  guinea-pig,  and  the  rat,  both  parotid 
and  submaxillary  and  mixed  saliva  are  amylolytic ;  the  sub- 
maxillary saliva  being  in  most  cases  more  active  than  the  parotid 
In  the  rabljit,  while  the  submaxillary  saliva  has  scarcely  any 
action,  that  of  the  parotid  is  energetic.  The  saliva  of  the  cat  is 
much  less  active  than  the  above;  that  of  the  dog  is  still  less 
active,  indeed  is  almost  inert.  In  the  horse,  sheep,  and  ox,  the 
amylolytic  powers  of  either  mixed  saliva,  or  of  any  one  of  the  con- 
stituent juices,  are  extremely  feeble. 

Where  the  saliva  of  any  gland  is  active,  an  aqueous  infusion  of 
the  same  gland  is  also  active.  The  importance  and  bearing  of  this 
statement  will  be  seen  later  on.  From  the  aqueous  infusion  of 
the  gland,  as  from  saliva  itself,  the  ferment  maybe  approximately 
isolated.  In  some  cases  at  least  some  ferment  may  be  extracted 
from  the  gland  even  when  the  secretion  is  itself  inactive.  In  fact 
a  ready  method  of  preparing  a  highly  amylolytic  licjuid  tolerably 
free  from  proteid  and  other  impurities,  is  to  mince  finely  a  gland 
known  to  have  an  active  secretion,  such  for  instance  as  that  of  a 
rat,  to  dehydrate  it  by  allowing  it  to  stand  under  absolute  alcohol 
for  some  days,  and  then,  having  poured  off  most  of  the  alcohol, 
and  removed  the  remainder  l)y  evaporation  at  a  low  tempera- 
ture, to  cover  the  pieces  of  gland  with  strong  glycerine.  Though 
some  of  the  ferment  appears  to  lie  destroyed  by  the  alcohol  a 
mere  drop  of  such  a  glycerine  extract  rapidly  converts  starch  into 
sugnr. 


320  GASTRIC   JUICE.  [Book  ii. 


Gastric   Juice. 

§  179.  There  is  no  difficulty  in  obtaining  what  may  fairly  be 
considered  as  a  normal  saliva ;  but  there  are  many  obstacles  in  the 
way  of  determining  the  normal  characters  of  the  secretion  of  the 
stomach.  When  no  food  is  ^aken  the  stomach  is  at  rest  and  no 
secretion  takes  place.  When  food  is  taken,  the  characters  of  the 
gastric  juice  secreted  are  obscured  by  the  food  with  which  it  is 
mingled.  The  gastric  membrane  may  it  is  true  be  artificially 
stimulated,  by  touch  for  instance,  and  a  secretion  obtained.  This 
we  may  speak  of  as  gastric  juice,  but  it  may  be  doubted  whether 
it  ought  to  be  considered  as  normal  gastric  juice.  And  indeed  as 
we  shall  see  even  the  juice,  which  is  poured  into  the  stomach 
during  a  meal,  varies  in  composition  as  digestion  is  going  on. 
Hence  the  characters  which  we  shall  give  of  gastric  juice  must  be 
considered  as  having  a  general  value  only. 

Gastric  juice,  obtained  in  as  normal  a  condition  as  possible 
from  the  healthy  stomach  of  a  fasting  dog,  by  means  of  a  gastric 
fistula,  is  a  thin  almost  colourless  fiuid  with  a  sour  taste  and 
odour. 

lu  the  operation  for  gastric  fistula,  an  incision  is  made  through 
the  abdominal  walls,  along  the  li7iea  alba,  the  stomach  is  opened,  and  the 
lips  of  the  gastric  wound  secure]}'  sewn  to  those  of  the  incision  in  the 
abdominal  walls.  Union  soon  takes  place,  so  that  a  permanent  opening 
from  the  exterior  into  the  inside  of  the  stomach  is  established.  A  tube 
of  proper  construction,  introduced  at  the  time  of  the  operation,  becomes 
firmly  secured  in  place  by  the  contraction  of  healing.  Through  the 
tube  the  contents  of  the  stomach  can  be  received,  and  the  mucous 
membrane  stimulated  at  pleasure. 

When  obtained  from  a  natural  fistula  in  man,  its  specific 
gravity  has  been  found  to  differ  little  from  that  of  water,  varying 
from  rOOl  to  1-010,  and  the  amount  of  solids  present  to  be 
correspondingly  small.  In  animals,  pure  gastric  juice  seems  to  be 
equally  poor  in  solids,  the  higher  estimates  which  some  observers 
have  obtained  being  probably  due  to  admixture  with  food,  &c. 

Of  the  solid  matters  present  about  half  are  inorganic  salts, chiefly 
alkaline  (sodium)  chlorides,  with  small  quantities  of  phosphates. 
The  organic  material  consists  of  pepsin,  a  body  to  be  described 
immediately,  mixed  with  other  substances  of  undetermined  nature. 
In  a  healthy  stomach  gastric  juice  contains  a  very  small  quantity 
only  of  mucin,  unless  some  submaxillary  saliva  has  been  swallowed. 

The  reaction^  is  distinctly  acid,  and  the  acidity  is  normally 
due  to  free  hydrochloric  acid.  This  is  shewn  by  various  proofs, 
among  which  we  may  mention  the  conclusive  fact  that  the 
amount  of  chlorine  present  in  gastric  juice  is  more  than  would 
suffice  to  form  chlorides  with  all  the  bases  present,  and  that  the 
excess  if  regarded  as  existing  in  the  form  of  hydrochloric  acid 


Chap.  i.J  TISSUES  AND  MPXHANISMS  OF  DIGESTION.  321 

corresponds  exactly  to  the  quantity  of  free  acid  present.  Lactic 
and  butyric  and  other  acids  when  present  are  secondary  products, 
arising  either  by  their  respective  fermentations  from  articles  of 
food,  or  from  the  decomposition  of  their  alkaline  or  other  salts. 
In  man  the  amount  of  free  hydrochloric  acid  in  healthy  juice  may 
be  stated  to  be  about  '2  per  cent.,  but  in  some  animals  it  is 
probably  higher. 

§  180.  On  starch  gastric  juice  has  no  amylolytic  action ;  on 
the  contrary  when  saliva  is  mixed  with  gastric  juice  any  amylo- 
lytic ferment  which  may  be  present  in  the  former  is  at  once 
prevented  from  acting  by  the  acidity  of  the  mixture.  Moreover 
in  a  very  short  time,  especially  at  the  temperature  of  the  body, 
the  amylolytic  ferment  is  destroyed  by  the  acid  so  that  even  on 
neutralisation  the  mixture  is  unable  to  convert  starch  into  sugar. 

On  dextrose  healthy  gastric  juice  has  no  effect.  And  its  power 
of  inverting  cane-sugar  seems  to  be  less  than  that  of  hydrochloric 
acid  diluted  to  the  same  degree  of  acidity  as  itself.  In  an  un- 
healthy stomach  however  containing  much  mucus,  the  gastric 
juice  is  very  active  in  converting  cane-sugar  into  dextrose.  This 
power  seems  to  be  due  to  the  presence  in  the  mucus  of  a  special 
ferment,  analogous  to,  but  quite  distinct  from,  the  ptyalin  of 
saliva.  An  excessive  quantity  of  cane-sugar  introduced  into  the 
stomach  causes  a  secretion  of  mucus,  and  hence  provides  for  its 
own  conversion. 

On  fats  gastric  juice  has  at  most  a  limited  action.  When 
adipose  tissue  is  eaten,  the  chief  change  which  takes  place  in  the 
stomach  is  that  the  proteid  and  gelatiniferous  envelopes  of  the 
fat-cells  are  dissolved,  and  the  fats  set  free.  Though  there  is 
experimental  evidence  that  emulsion  of  fats  to  a  certain  extent 
does  take  place  in  the  stomach,  the  great  mass  of  the  fat  of  a  meal 
is  not  so  changed. 

Such  minerals  as  are  soluble  in  free  hydrochloric  acid  are  for 
the  most  part  dissolved;  though  there  is  a  difference  in  this  and 
in  some  other  rcs])ects  between  gastric  juice  and  simple  free 
hydrochloric  acid  diluted  with  water  to  the  same  degree  of  acidity 
as  the  juice,  the  presence  either  of  the  pepsin  or  of  other  bodies 
apparently  modifying  the  solvent  action  of  the  acid. 

The  essential  property  of  gastric  juice  is  the  power  of  dis- 
solving proteid  matters,  and  of  converting  them  into  a  substance 
called  peptone. 

Action  of  gastric  juice  on  protcicls.  The  results  are  essentially 
the  same  whether  natural  juice  obtained  by  means  of  a  fistula  or 
artificial  juice,  i.e.  an  acid  infusion  of  the  mucous  membrane  of 
the  stomach,  be  used. 

Artificial  gastric  juice  may  be  prepared  in  any  of  the  followinc:  ways. 

1.      The  nuicous  membrane  of  a  pig's  or  dog's  stoniacli  is  removed 

from    the   muscular   coat,   finely    minced,    rubbed    in   a  mortar   with 

21 


322  DIGESTION  OF   PROTEIDS.  [Book  ii. 

pounded  glass  and  extracted  with  water.  The  aqueous  extract  filtered 
and  acidulated  (it  is  in  itself  somewhat  acid)  until  it  has  a  free  acidity 
corresponding  to  '2  p.c.  of  hydrochloric  acid,  contains  but  little  of  the 
products  of  digestion  such  as  peptone,  but  is  fairly  potent. 

2.  The  mucous  membrane  similarly  prepared  and  minced  is 
allowed  to  digest  at  SS'^  C.  in  a  large  quantity  of  hydrochloric  acid 
diluted  to  "2  p.c.  The  greater  part  of  the  membrane  disappears, 
shreds  only  being  left,  and  the  somewhat  opalescent  liquid  can  be 
decanted  and  filtered.  The  filtrate  has  powerful  digestive  (peptic) 
properties,  but  contains  a  considerable  amount  of  the  products  of 
digestion  (peptone,  &c.),  arising  from  the  digestion  of  the  mucous 
membrane  itself.^ 

3.  The  mucous  membrane,  similarly  prepared  and  minced,  is 
thrown  into  a  comparatively  large  quantity  of  concentrated  glj'cerine, 
and  allowed  to  stand.  The  membrane  may  be  previously  dehydrated 
by  being  allowed  to  stand  under  alcohol,  but  this  is  not  necessar}^, 
and  a  too  prolonged  action  of  the  alcohol  injures  or  even  destroys  the 
activity  of  the  product.  The  decanted  clear  glycerine,  in  which  a 
comparatively  small  quantity  of  the  ordinary  proteids  of  the  mucous 
membrane  are  dissolved,  if  added  to  h3'drochloric  acid  of  '2  p.c.  (about 
1  c.c.  of  the  glycerine  to  100  c.c.  of  the  dilute  acid  is  sufficient),  makes 
an  artificial  juice  tolerably  free  from  ordinary  proteids  and  peptone, 
and  of  remarkable  potency,  the  presence  of  the  glycerine  not  interfer- 
ing with  the  results. 

Before  proceeding  to  study  the  action  of  gastric  juice  on  pro- 
teids it  will  be  useful  to  review  very  briefly  the  chief  characters 
of  the  more  important  members  of  the  group. 

The  more  important  proteids  which  we  have  thus  far  studied 
are :  1.  Fibrin,  insoluble  in  water  and  not  really  soluble  {i.e. 
without  change)  in  saline  solutions.  2.  Myosin,  insoluble  in 
water  but  soluble  in  saline  solutions,  provided  these  are  not  too 
dilute  or  too  concentrated.  3.  Globulin  (including  paraglobulin, 
fibrinogen  &c.),  insoluble  in  water,  but  readily  soluble  in  even  very 
dilute  saline  solutions.  4.  Albumin,  serum- albumin,  soluble  in 
water  in  the  absence  of  all  salts.  5.  Acid-alhvmin,  into  which 
globulins  and  myosin  are  rapidly  converted  by  the  action  of  dilute 
acids,  the  particular  acid-albumin  into  which  the  myosin  of  muscle 
is  changed  being  sometimes  called  syntonin.  If  the  reagent  used 
be  not  dilute  acid  but  dilute  alkali,  the  product  is  called  alkali- 
albumin.  The  two  bodies,  acid-albumin  and  alkali-albumin,  are 
very  parallel  in  their  characters,  and  may  readily  be  converted 
the  one  into  the  other  by  the  use  of  dilute  alkali  or  dilute  acid 
respectively.  Their  most  important  common  characters  are  in- 
solubility in  water  and  in  saline  solutions  and  ready  solubility  in 
dilute  acids  and  alkalis.  6.  Coagulated  proteids.  As  we  have 
seen,  when  fibrin  suspended  in  water,  serum-albumin  in  solution, 

1  These  liowever  may  he  removed  by  concentration  at  40'>  C.  and  subsequent 
dialysis. 


Chap,  i.]    TISSUES  AND  MECHANISMS  OF  DIGESTION.  323 

acid-albumin  or  alkali-albumin  suspended  in  water,  or  paraglo- 
bulin  suspended  in  water  or  dissolved  in  a  dilute  saline  solution, 
are  heated  to  a  temperature,  which  for  the  whole  group  may  be 
put  down  at  about  75^  —  80*^  C,  each  of  them  becomes  coagulated, 
and  after  the  change  is  insoluble  in  water,  saline  solutions,  dilute 
acids  &c.,  in  fact  in  everything  but  very  strong  acids.  Myosin 
and  fibrinogen  undergo  a  similar  change  at  a  lower  temperature, 
viz.  about  56*^  C.  We  may,  for  present  purposes,  speak  of  all 
these  proteids  thus  changed  under  the  one  term  of  coagulated 
proteids. 

To  the  above  list  we  may  now  add  two  other  proteids,  viz. : 
7.  A  kind  of  albumin  which  forms  the  great  bulk  of  the  proteid 
matter  present  in  raw  '  white  of  egg,'  and  which,  since  it  differs 
in  minor  characters  from  the  albumin  of  blood  and  of  the  tissues, 
is  called  egg-albumin.  8.  The  peculiar  proteid  casein,  an  impor- 
tant constituent  of  milk.  This  may  perhaps  be  regarded  as  a 
naturally  occurring  alkali-albumin  since  it  has  many  resemblances 
to  the  artificial  alkali-albumin ;  but  for  several  reasons  it  is  desir- 
able to  consider  it  as  an  independent  body. 

Egg-albumin  like  serum-albumin  becomes  coagulated  at  a 
temperature  of  about  Ib'^  —  80*^  C,  and  though  casein  as  it  natu- 
rally exists  in  milk  is  not  coagulated  on  boiling,  when  separated 
out  in  a  special  way,  and  suspended  in  water  in  which  it  is  in- 
soluble, it  becomes  coagulated  at  about  75^  —  80*^  C. 

It  will  be  observed  that  all  these  proteids  form,  as  regards 
their  solubilities,  a  descending  series,  in  the  following  order. 
Coagulated  Proteids.  Fibrin.  Acid-albumin  with  Alkali-albu- 
min, and  Casein.  Myosin,  G-lobulins.  Serum-albumin  with  Egg- 
albumin. 

We  must  now  return  to  the  action  of  gastric  juice. 

If  a  few  shreds,  of  fibrin,  obtained  by  whipping  blood,  after 
being  thoroughly  washed  and  boiled  and  thus  by  the  boiling 
coagulated,  be  thrown  into  a  quantity  of  gastric  juice,  and  the 
mixture  be  exposed  to  a  temperature  of  from  35*^  to  40*^  C,  the 
fibrin  will  speedily,  in  some  cases  in  a  few  minutes,  be  dissolved. 
The  shreds  first  swell  up  and  become  transparent,  then  gradually 
dissolve,  and  finally  disappear  with  the  exception  of  some  granular 
debris,  the  amount  of  which,  though  generally  small,  varies  accord- 
ing to  circumstances.  If  raw,  that  is  unboiled,  uncoagulated  fibrin 
be  employed  the  same  changes  may  be  observed,  but  they  take 
place  much  more  rapidly. 

If  small  morsels  of  coagulated  albumin,  such  as  wliite  of  egg, 
be  treated  in  the  same  way,  the  same  solution  is  observed.  Tlie 
pieces  become  transparent  at  their  surfaces ;  this  is  especially  seen 
at  the  edges,  which  gradually  become  rounded  down  ;  ami  solution 
steadily  progresses  from  the  outside  of  the  piece  inwards. 

If  any  other  form  of  coagulated  albumin  {e.g.  precipitated 
acid-  or  alkali-albumin,  suspended  in  water  and  boiled)  be  treated 


324  DIGESTION   OF   PROTEIDS.  [Book  ii. 

in  the  same  way,  a  similar  solution  takes  place.  The  readiness 
with  which  the  solution  is  effected,  will  depend,  cceteris  j^cmbus, 
on  the  smallness  of  the  pieces,  or  rather  on  the  amount  of  surface 
as  compared  with  bulk,  which  is  presented  to  the  action  of  the 
juice. 

Gastric  juice  then  readily  dissolves  coagulated  proteids,  which 
otherwise  are  insoluble,  or  soluble  only,  and  that  with  difficulty, 
in  very  strong  acids. 

When  proteids,  which  are  soluble  in  water  or  in  dilute  acid, 
are  treated  with  gastric  juice,  no  visible  change  takes  place ;  but 
nevertheless,  it  is  found  on  examination  that  the  solutions  have 
undergone  a  remarkable  change,  the  nature  of  which  is  easily  seen 
by  contrasting  it  with  the  change  effected  by  dilute  acid  alone. 
If  raw  white  of  egg,  largely  diluted  with  water  and  strained,  be 
treated  with  a  sufficient  quantity  of  dilute  hydrochloric  acid,  the 
opalescence  or  turbidity  which  appeared  in  the  white  of  egg  on 
dilution  (and  which  is  due  to  the  precipitation  of  various  forms 
of  globulin  accompanying  the  egg-albumin  in  the  raw  white)  dis- 
appears, and  a  clear  mixture  results.  If  a  portion  of  the  mixture 
be  at  once  boiled,  a  large  deposit  of  coagulated  albumin  occurs. 
If,  however,  the  mixture  be  exposed  to  50*^  or  55*^  C.  for  some 
time,  the  amount  of  coagulation  which  is  produced  by  boiling  a 
specimen  becomes  less,  and,  finally,  boiling  produces  no  coagula- 
tion whatever.  By  neutralisation,  however,  the  whole  of  the 
albumin  (with  such  restrictions  as  the  presence  of  certain  neutral 
salts  may  cause)  may  be  obtained  in  the  form  of  acid-albumin, 
the  filtrate  after  neutralisation  containing  no  proteids  at  all  (or  a 
very  small  quantity).  Tbus  the  whole  of  the  albumin  present  in 
the  wbite  of  egg  may  be,  in  time,  converted,  by  the  simple  action 
of  dilute  hydrochloric  acid,  into  acid-albumin.  Serum-albumin 
similarly  treated  undergoes,  in  course  of  time,  a  similar  conversion 
into  acid-albumin,  and  we  have  already  seen  (§  56)  that  solutions 
of  myosin  or  of  any  of  the  globulins  are  with  remarkable  rapidity 
converted  into  acid-albumin.  Thus  simple  dilute  hydrochloric  of 
the  same  degree  of  acidity  as  gastric  juice,  merely  converts  these 
proteids  into  acid-albumin,  the  rapidity  of  the  change  differing 
with  the  different  proteids,  being  in  some  cases  very  slow,  and 
requiring  a  relatively  high  temperature. 

If  the  same  white  of  egg  or  serum-albumin  be  treated  with 
gastric  juice  instead  of  simple  dilute  hydrochloric  acid,  the  events 
for  some  time  see^i  the  same.  Thus  after  a  while  boiling  causes 
no  coagulation,  while  neutralisation  gives  a  considerable  precipitate 
of  a  proteid  body,  which,  being  insolulile  in  water  and  in  sodium 
chloride  solutions,  and  soluble  in  dilute  alkali  and  acids,  at  least 
closely  resembles  acid-albumin.  But  it  is  found  that  only  a 
portion  of  the  proteid  originally  present  in  the  white  of  egg  or 
serum-all lumin  can  thus  be  regained  l»y  precipitation.  Though 
the  neutralisation  be  carrieil  out  with  the  greatest  care  it  will  bo 


CiiAP.  I.]  TISSUES  AND  MECHANISMS  OF  DIGESTION.  325 

found,  on  filtering  off  the  neutralisation  precipitate,  that  the  fil- 
trate, as  shewn  on  employing  the  various  tests  for  proteid  (see 
§  15)  or  on  adding  an  adequate  quantity  of  strong  alcohol,  still 
contains  a  very  considerable  quantity  of  proteid  matter ;  and,  on 
the  whole,  the  longer  the  digestion  is  carried  on,  the  greater  is 
the  proportion  borne  by  the  proteid  remaining  in  solution  to  the 
precipitate  thrown  down  on  neutralisation ;  indeed,  in  some  cases 
at  all  events,  all  the  proteid  matter  originally  present  remains  in 
solution,  and  there  is  no  neutraKsation  precipitation  at  all,  or  at 
most  a  wholly  insignificant  one. 

§  181.  The  proteid  matter,  thus  remaining  in  solution  after 
neutralisation,  differs  from  all  the  proteids  which  we  have  hitherto 
studied  in  as  much  as,  though  existing  m  a  neutral  solution,  it  is 
not  coagulated  by  heat,  like  the  egg-albumin  or  serum-albumin 
from  which  it  has  been  produced ;  the  solution,  after  the  neutrali- 
sation precipitate  has  been  filtered  off,  remains  quite  clear  when 
boiled.  The  only  other  solutions  of  proteids  which  do  not  coagu- 
late on  boiling  are  solutions  of  acid  or  alkali-albumin  ;  but  these 
solutions  must  be  acid  or  alkali  respectively ;  the  acid-albumin  or 
alkali-albumin  is  insoluble  in  a  neutral  solution,  and  when  simply 
suspended  in  water  is  readily  coagulated  at  a  temperature  of  7b^. 
This  new  proteid  matter  of  which  we  are  speaking  is  soluble  in 
neutral  solutions,  indeed  in  distilled  water,  and  can  under  no 
circumstances  be  coagulated  by  heat. 

Upon  examination  we  find  that  the  new  proteid  matter  thus 
left  in  solution  consists  of  at  least  two  distinct  proteid  bodies. 
If  to  the  solution  neutral  ammonium  sulphate  be  added  to 
saturation,  part  of  the  proteid  matter  is  precipitated  while  part 
is  still  left  in  solution.  The  proteid  body  thus  thrown  down  is 
called  alhumose.  The  body  which  is  not  thrown  down  by  ammo- 
nium sulphate  is  called  jicptone.  Now  peptone  is  characterised 
by  being  diffusible ;  it  will  pass  through  membranes  Tlie  diffu- 
sion is  not  nearly  so  rapid  as  that  of  salts,  sugar,  and  other  simi- 
lar substances;  indeed  solutions  of  peptones  maybe  freed  from 
salts  by  dialysis.  But  it  is  very  marked  as  compared  with  that 
of  other  proteids ;  these  pass  through  membranes  with  the  great- 
est difficulty,  if  at  all.  Peptone  is  insoluble  in  alcohol,  and  may 
be  precipitated  from  its  solutions  by  the  addition  of  an  adequate 
quantity  of  this  reagent;  but  for  this  purpose  a  very  large  excess 
of  alcohol  is  needed,  otherwise  much  of  the  peptone  remains  in 
solution.  It  may  be  kept  under  alcohol  for  a  long  time  without 
undergoing  change,  whereas  other  proteids  are  more  or  less  slowly 
coagulated  by  alcohol.  A  useful  test  for  peptone  is  furnished  by 
the  fact  that  a  solution  of  peptone,  mixed  with  a  strong  solution 
of  caustic  potash,  gives  on  addition  of  a  mere  trace  of  cupric  sul- 
phate in  the  cold  a  /n'/i/^  colour,  wliereas  other  proteids  give  a 
violet  colour.  In  applying  this  test,  known  as  the  '  biuret '  test, 
however,  care  must  be  taken  not  to  add  too  much  cupric  sulphate 


326  DIGESTION   OF  PROTEIDS.  [Book  ii. 

since  in  that  case  a  violet  colour,  deepening  on  boiling,  that  is 
the  ordinary  proteid  reaction,  is  obtained.  There  are  reasons  for 
thinking  that  there  are  several  kinds  or  at  least  more  than  one 
kind  of  peptone ;  but  we  may  for  the  present  speak  of  the  sub- 
stance as  one. 

Albumose  differs  from  peptone,  not  only  in  being  precipitated 
by  ammonium  sulphate  but  also  in  being  much  less  diffusible, 
and  in  other  minor  characters.  Albumose  like  peptone  gives  the 
biuret  reaction.  We  are  able  to  distinguish  several  kinds  of 
albumose,  but  into  the  details  of  these  we  need  not  enter.  The 
amount  of  albumose  appearing  in  a  digestion  experiment,  rela- 
tive to  the  amount  of  true  peptone,  depends  on  the  activity  of 
the  juice,  and  other  circumstances.  We  may  regard  albumose 
as  a  less  complete  product  of  digestion  than  peptone.  For  a  long 
time  albumose  was  confounded  with  peptone,  and  many  of  the 
commercial  forms  of  "peptone"  consist  largely  of  albumose. 

When  fibrin,  either  raw  or  boiled,  or  any  form  of  coagulated 
proteid  is  dissolved  and  seems  to  disappear  under  the  influence 
of  gastric  juice,  the  same  products,  albumose  and  peptone,  make 
their  appearance.  The  same  bodies  result  when  myosin  or  any 
one  of  the  globulins  or  acid-albumin  or  alkali-albumin  is  subjected 
to  the  action  of  the  juice. 

Besides  albumose  other  bodies,  which  may  also  be  regarded  as 
less  complete  products  of  digestion,  make  their  appearance,  to  a 
variable  extent  under  different  circumstances  when  proteid  is 
digested  with  gastric  juice.  On  these  bodies  however,  known  as 
parapeptone  and  by  other  names,  we  need  not  dwell. 

It  is  obvious  that  the  effect  of  the  action  of  the  gastric  juice 
is  to  change  the  less  soluble  proteid  into  a  more  soluble  form,  the 
change  being  either  completed  up  to  the  stage  of  peptone,  the 
most  soluble  of  all  proteids,  or  being  left  in  part  incomplete. 
This  will  be  seen  from  the  following  tabular  arrangement  of 
proteids  according  to  their  solubilities. 


Soluble  in  distilled  water. 

Aqueous  solutions  not  coagulated  on  boiling. 

Diffusible Peptone. 

Much  less  diffusible Albumose. 

Aqueous  solutions  coagulated  on  boiling   .     Albumin. 

Insoluble  in  distilled  water. 

Eeadily  soluble  in  dilute  saline  solutions 

(NaCl  1  per  cent.) Globulins. 

Soluble  only  in  stronger  saline  solutions 

(NaCl  5  to  10  p.  c.) Myosin. 


Chap,  i.]   TISSUES  AND  MECHANISMS  OF  DIGESTION.  327 


Insoluble  in  dilute  saline  solutions. 

Eeadily  soluble  in  dilute  acid  (HCl -1  p.c.)      \   .  n    v    n 

in  the  cold i  ri 

(  Lasem. 

Soluble  with  difficulty  in  dilute  acid,  that 
is  at  high  temperature  (GO*^  C.)  and 
after  prolonged  treatment  only  .     .     .     Fibrin. 

Insoluble  in  dilute  acids,  soluble  only  in 

strong  acids Coagulated  Proteid. 

Milk  when  treated  with  gastric  juice  is  first  of  all  "  curdled." 
This  is  the  result  partly  of  the  action  of  the  free  acid  but  chieily 
of  the  special  action  of  a  particular  constituent  of  gastric  juice,  of 
which  we  shall  speak  hereafter.  The  curd  consists  of  a  particular 
proteid  matter  mixed  with  fat ;  and  this  proteid  matter  is  sub- 
sequently dissolved  with  the  same  appearance  of  peptone,  albu- 
mose  and  other  bodies  as  in  the  case  of  other  proteids.  In  fact, 
the  digestion  by  gastric  juice  of  all  the  varieties  of  proteids  con- 
sists in  the  conversion  of  the  proteid  into  peptone,  with  the  con- 
comitant appearance  of  a  certain  variable  amount  of  albumose  and 
other  bodies. 

§  182.  Circumstances  affecting  gastric  digestion.  The  solvent 
action  of  gastric  juice  on  proteids  is  modified  by  a  variety  of  cir- 
cumstances. The  nature  of  the  proteid  itself  makes  a  difference, 
though  this  is  determined  as  well  by  physical  as  by  chemical 
characters.  Hence  in  making  a  series  of  comparative  trials  the 
same  proteid  should  be  used,  and  the  form  of  proteid  most  con- 
venient for  the  purpose  is  fibrin.  If  it  be  desired  simply  to 
ascertain  whether  any  given  specimen  has  any  digestive  powers 
at  all,  it  is  best  to  use  boiled  fibrin,  since  raw  fibrin  is  eventually 
dissolved  by  dilute  hydrochloric  acid  alone,  probably  on  account 
of  some  pepsin  previously  present  in  the  blood  becoming  entangled 
with  the  fibrin  during  clotting.  But  in  estimating  quantitatively 
the  peptic  power  of  two  specimens  of  gastric  juice  under  different 
conditions,  raw  fibrin  prepared  by  Grlltzner's  method  is  the  most 
convenient. 

Portions  of  well  washed  fibi'in  are  stained  with  carmine  and  again 
washed  to  remove  the  superfluous  colouring  matter.  A  fragment  of 
this  coloured  fibrin  thrown  into  an  active  juice  on  becoming  dissolved, 
gives  up  its  colour  to  the  fluid.  Hence  if  the  same  stock  of  coloured 
fibrin  be  used  in  a  series  of  experiments,  and  the  same  bulks  of  fibrin 
and  of  fluid  be  used  in  each  case,  the  amount  of  fihrin  dissolved  may 
be  fairly  estimated  by  the  depth  of  tint  given  to  the  fluid.  Fibrin  thus 
coloured  with  carmine  may  be  preserved  in  ether. 

Since,  if  sufficient  time  be  allowed,  even  a  small  quantity  of 
gastric  juice  will  dissolve  at  least  a  very  large  if  not  an  indefinite 


328  GASTRIC   DIGESTION.  [Book  ii. 

quantity  of  fibrin,  we  are  led  to  take,  as  a  measure  of  the  activity 
of  a  specimen  of  gastric  juice,  not  tlie  quantity  of  fibrin  which  it 
will  ultimately  dissolve,  but  the  rapidity  with  which  it  dissolves 
a  given  quantity. 

The  greater  the  surface  presented  to  the  action  of  the  juice,  the 
more  rapid  the  solution  ;  hence  minute  division  and  constant  move- 
ment favour  digestion.  And  this  is  probably,  in  part  at  least,  the 
reason  why  a  fragment  of  spongy  filamentous  fibrin  is  more  readily 
dissolved  than  a  solid  clump  of  boiled  white  of  egg  of  the  same  size. 
Neutralisation  of  the  juice  wholly  arrests  digestion  ,  fibrin  may  be 
submitted  for  an  almost  indefinite  time  to  the  action  of  neutralised 
gastric  juice  without  being  digested.  If  the  neutralised  juice  be 
properly  acidified,  it  may  again  become  active  ;  when  gastric  juice 
however  has  been  made  alkaline,  and  kept  for  some  time  at  a 
temperature  of  3 5*^,  its  solvent  powers  are  not  only  suspended  but 
actually  destroyed.  Digestion  is  most  rapid  with  dilute  hydro- 
chloric acid  of  -2  p.c.  (the  acidity  of  natural  gastric  juice).  If  the 
juice  contains  much  more  or  much  less  free  acid  than  this,  its 
activity  is  distinctly  impaired.  Other  acids,  lactic,  phosphoric,  &c. 
may  be  substituted  for  hydrochloric;  but  they  are  not  so  eJEfec- 
tual,  and  the  degree  of  acidity  most  useful  varies  with  the  dif- 
ferent acids.  The  presence  of  neutral  salts,  such  as  sodium 
chloride,  in  excess  is  injurious.  The  action  of  mammalian  gastric 
juice  is  most  rapid  at  35^ — 40'^  C. ;  at  the  ordinary  temperature  it 
is  much  slower,  and  at  about  0°  C.  ceases  altogether.  The  juice 
may  be  kept  however  at  0*^  C.  for  an  indefinite  period  without 
injury  to  its  powers.  The  gastric  juice  of  cold-blooded  vertebrates 
is  relatively  more  active  at  low  temperatures  than  that  of  warm- 
blooded mammals  or  birds. 

At  temperatures  much  above  40°  or  45^^  the  action  of  the  juice 
is  impaired.  By  boiling  for  a  few  minutes  the  activity  of  the  most 
powerful  juice  is  irrevocably  destroyed.  The  presence  in  a  concen- 
trated form  of  the  products  of  digestion  hinders  the  process  of  solu- 
tion. If  a  large  quantity  of  fibrin  be  placed  in  a  small  quantity  of 
juice,  digestion  is  soon  arrested ;  on  dilution  with  the  normal  hy- 
drochloric acid  ("2  p.c),  or  if  the  mixture  be  submitted  to  dialysis 
to  remove  the  peptones  formed,  and  its  acidity  be  kept  up  to  the 
normal,  the  action  recommences.  By  removing  the  products  of 
digestion  as  fast  as  they  are  formed,  and  by  keeping  the  acidity 
up  to  the  normal,  a  given  amount  of  gastric  juice  may  be  made 
to  digest  a  very  large  quantity  of  proteid  material.  Whetlier 
the  quantity  is  really  unlimited  is  disputed ;  but  in  any  case  the 
energies  of  the  juice  are  not  rapidly  exhausted  by  the  act  of 
digestion. 

§  183.  Nature  of  the  action.  All  these  facts  go  to  shew  that 
the  digestive  action  of  gastric  juice  on  proteids,  like  that  of  saliva 
on  starch,  is  a  ferment-action  ;  in  other  words,  that  the  solvent 
action  of  gastric  juice  is  essentially  due  to  the  presence  in  it  of  a 


Chap,  i.]    TISSUES  AND  MECHANISMS  OF  DIGESTION.   329 

ferment-body.  To  this  ferment-body,  which  as  yet  has  been  only 
approximately  isolated,  the  name  of  pepsin  has  been  given.  It  is 
present  not  only  in  gastric  juice  but  also  in  the  glands  of  the 
gastric  mucous  membrane,  especially  in  certain  parts  and  under 
certain  conditions  which  we  shall  study  presently.  Concerning 
its  exact  nature  we  cannot  make  any  definite  statement ;  we  have 
no  absolute  proof  that  it  is  a  proteid,  probable  as  this  may  seem. 
We  are  as  yet  unable  to  define  it  by  any  chemical  characters.  At 
present  the  manifestation  of  peptic  powers  is  our  only  safe  test  of 
the  presence  of  pepsin. 

In  one  important  respect  pepsin,  the  ferment  of  gastric  juice, 
differs  from  ptyalin,  the  ferment  of  saliva.  Saliva  is  active  in  a 
perfectly  neutral  medium,  and  there  seems  to  be  no  special  con- 
nection between  the  ferment  and  any  alkali  or  acid.  In  gastric 
juice,  however,  there  is  a  strong  tie  between  the  acid  and  the  fer- 
ment, so  strong  that  some  writers  speak  of  pepsin  and  hydrochloric 
acid  as  forming  together  a  compound,  pepto-hydrochloric  acid. 

In  the  a])sence  of  exact  knowledge  of  the  constitution  of 
proteids,  we  cannot  state  distinctly  what  is  the  precise  nature  of 
the  change  into  peptone  ;  the  various  proteids  differ  from  each 
other  in  elementary  composition  qiiite  as  widely  as  does  peptone 
from  any  of  them.  Judging  from  the  analogy  with  the  action  of 
saliva  on  starch,  we  may  fairly  suppose  that  the  process  is  at 
bottom  one  of  hydration  ;  and  this  view  is  further  suggested  by 
the  fact  that  peptone  closely  resembling,  if  not  identical  with,  that 
obtained  by  gastric  digestion,  may  be  obtained  by  the  action  of 
strong  acids,  by  the  prolonged  action  of  dilute  acids  especially  at 
a  higli  temperature,  or  simply  by  digestion  with  super-heated 
water  in  a  Papin's  digester,  that  is  to  say  by  means  of  agents 
which,  in  other  cases,  produce  their  effects  by  bringing  about 
hydrolytic  changes. 

§  184.  All  proteids,  so  far  as  we  know,  are  converted  by  pep- 
sin into  peptone.  Concerning  the  action  of  gastric  juice  on  other 
nitrogenous  substances  more  or  less  allied  to  proteids  but  not 
truly  proteid  in  nature  our  knowledge  is  at  present  imperfect. 
Mucin,  nuclein,  and  the  chemical  basis  of  horny  tissues  are  wholly 
unaffected  by  gastric  juice.  The  gelatiniferous  tissues  are  dis- 
solved by  it ;  and  the  bundles  and  membranes  of  connective  tissue 
are  very  speedily  so  far  affected  by  it,  that  at  a  very  early  stage 
of  digestion,  the  bundles  and  elementary  fibres  of  muscle  which 
are  bound  together  by  connective  tissue  fall  asunder ;  moreover 
both  prepared  gelatine  and  the  gelatiniferous  basis  of  connective 
tissue  in  its  natural  condition,  that  is  without  being  previously 
heated  with  water,  are  by  it  changed  into  a  sulistance  so  far 
analogous  ^vith  peptone,  that  the  characteristic  property  of  gela- 
tinisation  is  entirely  lost.  Chondrin  and  the  elastic  tissues 
undercfo  a  similar  change. 

§  185.     Action  of  gastric  juice  on  milk.     It  has  long  been 


330  DIGESTION   OF   MILK.  [Book  ii. 

known  that  an  infusion  of  calves'  stomach,  called  rennet,  has  a 
remarkable  effect  in  rapidly  curdling  milk,  and  this  property  is 
made  use  of  in  the  manufacture  of  cheese.  Gastric  juice  has  a 
similar  effect ;  milk  when  subjected  to  the  action  of  gastric  juice 
is  first  curdled  and  then  digested.  If  a  few  drops  of  gastric  juice 
be  added  to  a  little  milk  in  a  test-tube,  and  the  mixture  exposed 
to  a  temperature  of  40°,  the  milk  will  curdle  into  a  complete  clot 
in  a  very  short  time.  If  the  action  be  continued  the  curd  or  clot 
will  be  ultimately  dissolved  and  digested.  Milk  contains,  besides 
a  peculiar  form,  or  peculiar  forms  of  albumin,  fats,  milk-sugar  and 
various  salines,  the  peculiar  proteid  casein.  In  natural  milk 
casein  is  present  in  solution,  and  '  curdling '  consists  essentially 
in  the  soluble  casein  being  converted  (or  more  probably  as  we 
shall  see  presently,  split  up)  into  an  insoluble  modification  of 
casein,  which  as  it  is  being  precipitated  carries  down  with  it  a 
great  deal  of  the  fat  and  so  forms  the  '  curd.'  Now  casein  is  readily 
precipitated  from  milk  upon  the  addition  of  a  small  quantity  of 
acid,  and  it  might  be  supposed  that  the  curdling  effect  of  gastric 
juice  was  due  to  its  acid  reaction.  But  this  is  not  the  case,  for 
neutralised  gastric  juice,  or  neutral  rennet,  is  equally  efficacious. 

The  curdling  action  of  rennet  is  closely  dependent  on  tempera- 
ture, being  like  the  peptic  action  of  gastric  juice  favoured  by  a  rise 
of  temperature  up  to  about  40*^.  Moreover  the  curdling  action 
is  destroyed  by  previous  boiling  of  the  juice  or  rennet.  These 
facts  suggest  that  a  ferment  is  at  the  bottom  of  the  matter ;  and 
indeed,  all  the  features  of  the  action  support  this  view.  More- 
over, as  a  matter  of  fact,  a  curdling  ferment  may  be  extracted  by- 
glycerine  and  by  the  other  methods  used  for  preparing  ferments. 
The  ferment,  however,  is  not  pepsin,  but  some  other  body ;  and 
the  two  may  be  separated  from  each  other. 

It  might  be  thought  that  the  rennet-ferment,  rennin  we  may 
call  it,  acted  by  inducing  a  fermentation  in  the  sugar  of  milk, 
giving  rise  to  lactic  acid  which  precipitated  the  casein  by  virtue 
of  its  being  an  acid.  But  this  view  is  disproved  by  the  following 
facts  which  shew  that  the  ferment  produces  its  curdling  effect  by 
acting  directly  on  the  natural  casein  itself.  Casein  may  be  pre- 
cipitated unchanged,  that  is,  capable  of  redissolving  in  water  (the 
presence  of  calcic  phosphate  being  assumed)  by  saturating  milk 
with  neutral  saline  bodies  (such  as  sodium  chloride  or  magnesium 
sulphate) ;  and  by  being  precipitated  and  redissolved  more  than 
once  may  be  detained  largely  free  from  fat  and  wholly  free  from 
milk-sugar.  Such  solutions  of  isolated  casein  freed  from  milk- 
sugar  may  be  made  to  curdle  like  natural  milk  by  the  addi- 
tion of  rennin,  shewing  that  the  milk-sugar  has  nothing  to  do 
with  the  matter.  Moreover  the  precipitate  thrown  down  from 
milk  by  dilute  acids,  lactic  acid  included,  is  itself  unaltered  or 
very  slightly  altered  casein  not  curd,  and  with  care  may  be 
so  prepared    as  to    be  redissolved  into   solutions  which   curdle 


Chap,  i.]    TISSUES  AND  MECHANISMS  OF  DIGESTION.   331 

with  rennin,  like  solutions  of  casein  prepared  by  means  of  neutral 
salts. 

When  isolated  casein  is  curdled  by  means  of  rennin  two  pro- 
teids,  it  is  stated,  make  their  appearance,  one  of  which  is  soluble 
and  allied  to  albumin,  and  another,  which  is  insoluble  and  forms 
the  curd.  Curdling,  therefore,  according  to  this  result  appears  to 
be  the  splitting  up  by  a  ferment  of  a  more  complex  body ;  and  it 
is  interesting  to  observe,  as  perhaps  throwing  light  on  the  some- 
what analogous  formation  of  fibrin,  that  this  curdling  action  will 
not  take  place  if  calcic  phosphate  be  wholly  absent  from  the  mix- 
ture. The  calcic  phosphate  appears  to  play  a  peculiar  part  in 
determining  the  insolubility  of  the  curd,  for  there  is  evidence 
that  in  the  absence  of  calcic  phosphate  the  ferment  has  power  to 
attack  the  casein  and  split  it  up,  but  that  both  products  remain 
in  solution ;  if  calcic  phosphate  be  present,  the  one,  viz.  the  curd, 
becomes  insoluble. 

Eennin  is  abundant  in  the  gastric  juice  and  in  the  gastric 
mucous  membrane  of  ruminants,  but  is  also  found  in  the  gastric 
juice  of  other  animals,  and  either  it,  or  what  we  shall  presently 
have  occasion  to  speak  of  as  the  antecedent  of  the  ferment  or 
zymogen,  is  present  also  in  the  mucous  membrane  of  the  stomach 
of  most  animals.  A  very  similar  if  not  identical  ferment  has 
also  been  found  in  many  plants. 


SEC.  2.  THE  ACT  OF  SECRETION  OF  SALIVA  AND 
GASTRIC  JUICE  AND  THE  NERVOUS  MECHANISMS 
WHICH  REGULATE  IT. 


§  186.  The  saliva  and  gastric  juice  whose  properties  we  have 
studied,  though  so  different  from  each  other,  are  both  drawn  ulti- 
mately from  one  common  source,  the  blood,  and  they  are  poured 
into  the  alimentary  canal,  not  in  a  continuous  flow,  but  intermit- 
tently as  occasion  may  demand.  The  epithelium  cells  which 
supply  them  have  their  periods  of  rest  and  of  activity,  and  the 
amount  and  quality  of  the  fluids  which  these  cells  secrete  are 
determined  by  the  needs  of  the  economy  as  the  food  passes  along 
the  canal.  We  have  now  to  consider  how  the  epithelium  cell 
manufactures  its  special  secretion  out  of  the  materials  supplied  to 
it  by  the  blood,  and  how  the  cell  is  called  into  activity  by  the 
presence  of  food,  it  may  be  as  in  the  case  of  saliva  at  some  dis- 
tance from  itself,  or  by  circumstances  which  do  not  bear  directly 
on  itself.  In  dealing  with  these  matters  in  connection  with  the 
digestive  iuices,  we  shall  have  to  enter  at  some  length  into  the 
physiology  of  secretion  m  general. 

The  question  which  presents  itself  first  is :  By  what  mechan- 
ism is  the  activity  of  the  secreting  cells  brought  into  play  ? 

While  fasting,  a  small  quantity  only  of  saliva  is  poured  into 
the  mouth;  the  buccal  cavity  is  just  moist  and  nothing  more. 
When  food  is  taken,  or  when  any  sapid  or  stimulating  substance, 
or  indeed  a  body  of  any  kind,  is  introduced  into  the  mouth,  a  flow 
is  induced  which  may  be  very  copious.  Indeed  the  quantity 
secreted  in  ordinary  life  during  24  hours  has  been  roughly  cal- 
culated at  as  much  as  from  1  to  2  litres.  An  abundant  secretion 
in  the  absence  of  food  in  the  mouth  may  be  called  forth  by  an 
emotion,  as  when  the  mouth  waters  at  the  sight  of  food,  or  by  a 
smell,  or  by  events  occurring  in  the  stomach,  as  in  some  cases  of 
nausea.  Evidently  in  these  instances  some  nervous  mechanism 
is  at  work.  In  studying  the  action  of  this  nervous  mechanism,  it 
will  be  of  advantage  to  confine  our  attention  at  first  to  the  sub- 
maxillary gland. 


Chap,  i.]    TISSUES  AND  MECHANISMS  OF  DIGESTION.  333 

§  187.  The  submaxillary  gland  is  supplied  with  two  sets  of 
nerves.  These  are  represented  in  Fig.  76,  which  is  a  very  dia- 
grammatic rendering  of  the  appearances  presented  when  the  sub- 
maxillary gland  is  prepared  for  an  experiment  in  a  dog,  the 
animal  being  placed  on  its  back  and  the  gland  exposed  from  the 
neck.  The  one  set,  and  that  the  more  important,  belongs  to 
the  chorda  Ujmpani  nerve  {ch.t").  This  is  a  small  nerve,  which 
branches  off  from  the  facial  or  seventh  cranial  nerve  in  the  Fallo- 
pian canal  before  the  nerve  issues  from  the  skull.  Whether  it 
really  belongs  to  the  facial  proper  has  been  doubted ;  in  man  the 
fibres  which  form  it  are  either  fibres  coming  not  from  the  roots  of 
the  facial  proper,  but  from  the  portio  intermedia  Wrisbergi,  or, 
according  to  some,  fibres  which  though  joining  the  facial  in  the 
Fallopian  canal  are  ultimately  derived  from  another  (the  fifth) 
cranial  nerve.  Leaving  the  facial  nerve  the  chorda  tympani 
passes  through  the  tympanic  cavity  or  drum  of  the  ear  (hence 
the  name)  and  joins  or  rather  runs  in  company  (ch.t')  with  the 
lingual  or  gustatory  branch  of  the  fifth  nerve.  Some  of  the  fibres 
run  on  with  the  lingual  right  down  to  the  tongue  (these  are  not 
shewn  in  the  figure),  but  many  leave  the  lingual  as  a  slender  nerve 
(ch.t),  which  reaching  Wharton's  duct  or  duct  of  the  submaxillary 
gland  (sm.d)  runs  along  the  duct  to  the  gland.  As  the  nerve 
courses  along  the  duct  nerve  cells  make  their  appearance  among 
the  fibres,  and  these  are  especially  abundant  just  after  the  duct 
enters  the  hilus  of  the  gland.  The  fibres  may  be  traced  into  the 
gland  for  some  distance,  but  as  we  have  said  their  ultimate  end- 
ing has  not  yet  been  definitely  made  out.  Along  its  whole  course 
up  to  the  gland,  the  fibres  of  the  chorda  are  very  fine  medullated 
fibres,  but  they  lose  their  medulla  in  the  gland. 

The  other  set  of  nerve-fibres  reaches  the  gland  along  the  small 
arteries  of  the  gland.  These  are  non-medullated  fibres  mixed 
with  a  few  medullated  fibres  and  may  be  traced  back  to  the 
superior  cervical  ganglion.  From  thence  they  may  be  traced 
still  further  back  down  the  cervical  sympathetic  to  the  spinal 
cord,  following  apparently  the  same  tract  as  the  vaso-con stricter 
fibres,  treated  of  in  §  144. 

§  188.  If  a  tube  be  placed  in  the  duct,  it  is  seen  that  when 
sapid  substances  are  placed  on  the  tongue,  or  the  tongue  is  stimu- 
lated in  any  other  way,  or  the  lingual  nerve  is  laid  bare  and  stimu- 
lated with  an  interrupted  current,  a  copious  flow  of  saliva  takes 
place.  If  the  sympatlietic  be  divided,  stimulation  of  the  tongue 
or  lingual  nerve  still  produces  a  fiow.  But  if  the  small  chorda 
nerve  be  divided,  stimulation  of  tlie  tongue  or  lingual  nerve  pro- 
duces no  flow. 

Evidently  the  flow  of  saliva  is  a  nervous  reflex  action,  the 
lingual  nerve  serving  as  the  channel  for  the  afferent  and  the 
small  chorda  nerve  for  the  efferent  impulses.  If  the  trunk  of 
the  lingual  be  divided  above  the  point  where  the  chorda  leaves 


334    NERVES  OF  THE  SUBMAXILLARY  GLAND.    [Book  ii. 

it,  as  at  n.l',  Fig.  76,  stimulation  of  the  (front  part  of)  tongue  pro- 
duces, under  ordinary  cii'cumstances,  no  flow.  This  shews  that 
the  centre  of  the  reflex  action  is  higher  up  than  the  point  of  sec- 
tion ;  it  lies  in  fact  in  the  brain. 


r.sinp.      V.  s.nu 


ch.t' 


Fig.  76. 


Diagrammatic  Representation  of  the  Submaxillary  Gland  of 
THE  Dog  with  its  Nerves  and  Blood  Vessels. 


(The  dissection  has  been  made  on  an  animal  lying  on  its  hack,  hut  since  all  the 
parts  shewn  in  the  figure  cannot  be  seen  from  any  one  poiut  of  view,  the  figure  does 
not  give  the  exact  anatomical  relations  of  the  several  structures.) 

sm.gld.  The  submaxillary  gland,  into  the  duct  [sni.d)  of  which  a  cannula  has 
been  tied.  The  sublingual  gland  and  duct  are  not  shewn,  n.l,  nV .  The  lingual 
branch  of  the  fifth  nerve,  the  part  n.l.  is  going  to  the  tongue,  ch.t.,  ch.t'.,  ch.t". 
The  chorda  tympani.  The  part  ch.t".  is  proceeding  from  the  facial  nerve ;  at  ch.t'. 
it  becomes  conjoined  with  the  lingual  n.l'.  and  afterwards  diverging  passes  as  clit.t. 
to  the  gland  along  the  duct ;  the  continuation  of  the  nerve  in  company  with  the 
lingual  n.l.  is  not  shewn,  sm.cjl.  The  submaxillary  ganglion  with  its  several  roots. 
n.car.  The  carotid  artery,  two  small  branches  of  which,  a.sm.a.  and  r.sm.p.,  pass  to 
the  anterior  and  posterior  parts  of  the  glaud.  r..s.ni.  The  anterior  and  posterior 
veins  from  the  gland,  falling  into  v.j.  the  jugular  vein,  v.sytn.  The  conjoined 
vagus  and  sympathetic  trunks,  ql.cer.s.  The  upper  cervical  ganglion,  two  branches 
of  which  forming  a  plexus  (a.f.)  over  the  facial  artery,  are  distributed  {n.sy7n.sm.) 
along  the  two  glandular  arteries  to  the  anterior  and  posterior  portions  of  the  gland. 

The  arrows  indicate  the  direction  taken  by  the  nervous  impulses  during  reflex 
stimulation  of  the  gland.  They  ascend  to  the  brain  by  the  lingual  and  descend  by 
the  chorda  tympani. 

In  the  angle  between  the  Ungual  and  the  chorda,  where  the  latter 
leaves  the  former  to  pass  to  the  gland,  lies  the  small  submaxillary  gan- 
glion (represented  diagrammatically  in  Eig.  76  sm.gl.).  This  consists 
of  small  masses  of  nerve  cells  lying  on  the  small  bundles  of  nerve-fibres 
which  spread  out  like   a  fan  from  the  lingual  and  chorda  tympani 


Chap,  i.]  TISSUES  AND  MECHANISMS  OF  DIGESTION.  335 

nerves  (ch.t.)  towards  the  ducts  of  the  submaxillary  and  sublingual 
glands.  It  has  been  much  debated  whether  this  ganglion  can  act  as  a 
centre  of  reflex  action  in  connection  with  the  submaxillar}^  gland,  but 
no  conclusive  evidence  that  it  does  so  act  has  as  yet  been  shewn;  it 
probably  belongs  in  reality  to  the  sublingual  gland. 

Stimulation  of  the  glossopharyngeal  is  even  more  effectual 
than  that  of  the  lingual.  Probably  this  indeed  is  the  chief 
afferent  nerve  in  ordinary  secretion.  Stimulation  of  the  mucous 
membrane  of  the  stomach  (as  by  food  introduced  through  a  gas- 
tric fistula)  or  of  the  vagus  may  also  produce  a  flow  of  saliva,  as 
indeed  may  stimulation  of  the  sciatic,  and  probably  of  many  other 
afferent  nerves.  All  these  cases  are  instances  of  reflex  action,  the 
cerebro-spinal  system  acting  as  a  centre.  We  may  further  define 
the  centre  as  a  part  of  the  medulla  oblongata,  apparently  not  far 
removed  from  the  vaso-motor  centre.  When  the  brain  is  removed 
down  to  the  medulla  oblongata,  that  organ  being  left  intact,  a  fiow 
of  saliva  may  still  be  obtained  by  adequate  stimulation  of  various 
afferent  nerves ;  when  the  medulla  is  destroyed  no  such  action  is 
possible.  And  a  flow  of  saliva  may  be  produced  by  direct  stimu- 
lation of  the  medulla  itself.  When  a  flow  of  saliva  is  excited  by 
ideas,  or  by  emotions,  the  nervous  processes  begin  in  the  higher 
parts  of  the  brain,  and  descend  thence  to  the  medulla  before  they 
give  rise  to  distinctly  efferent  impulses ;  and  it  would  appear  that 
these  higher  parts  of  the  brain  are  called  into  action  wlien  a  flow 
of  saliva  is  excited  by  distinct  sensations  of  taste. 

Considering  then  the  flow  of  saliva  as  a  reflex  act  the  centre 
of  which  lies  in  the  medulla  oblongata,  we  may  imagine  the 
efferent  impulses  passing  from  that  centre  to  the  gland  either  by 
the  chorda  tympani  or  by  the  sympathetic  nerve.  Although  it 
would  perhaps  be  rash  to  say  that  in  this  relation  the  sympathetic 
nei've  never  acts  as  an  efferent  channel,  as  a  matter  of  fact  we 
have  no  satisfactory  experimental  evidence  that  it  does  so ;  and 
we  may  therefore  state  that,  practically,  the  chorda  tympani  is 
the  sole  efferent  nerve.  Section  of  that  nerve,  either  where  the 
fibres  pass  from  the  lingual  nerve  and  the  submaxillary  ganglion 
to  the  gland,  or  where  it  runs  in  the  same  sheath  as  the  lingual, 
or  in  any  part  of  its  course  from  the  main  facial  trunk  to  the  lin- 
gual, puts  an  end,  as  far  as  we  know,  to  the  possibility  of  any  flow 
being  excited  by  stimuli  applied  to  the  sensory  nerves,  or  to  the 
sentient  surfaces  of  the  mouth  or  of  other  parts  of  the  body. 

The  natural  reflex  act  of  secretion  may  lie  inhibited,  like  the 
reflex  action  of  the  vaso-motor  nerves,  at  its  centre.  Thus  when, 
as  in  the  old  rice  ordeal,  fear  parches  the  mouth,  it  is  probable 
that  the  afferent  impulses  caused  by  the  presence  of  food  in  the 
mouth  cease,  through  emotional  inhibition  of  their  reflex  centre, 
to  give  rise  to  efferent  impulses. 

§  189.     In  life,  then,  the  flow  of  saliva  is  brought  about  by  the 


336  SECRETION  AND   BLOOD   SUPPLY.        [Book  ii. 

advent  to  the  gland  along  the  chorda  tympani  of  efferent  impulses, 
started  chiefly  by  reflex  actions.  The  inquiry  thus  narrows  itself 
to  the  question  :  In  what  manner  do  these  efierent  impulses  cause 
the  increase  of  flow  ? 

If  in  a  dog  a  tube  be  introduced  into  Wharton's  duct,  and  the 
chorda  be  divided,  the  flow,  if  any  be  going  on,  is  from  the  lack  of 
efferent  impulses  arrested.  On  passing  an  interrupted  current 
through  the  peripheral  portion  of  the  chorda,  a  copious  secretion 
at  once  takes  place,  and  the  saliva  begins  to  rise  rapidly  in  the 
tube ;  a  very  short  time  after  the  application  of  the  current  the 
flow  reaches  a  maximum  which  is  maintained  for  some  time,  and 
then,  if  the  current  be  long  continued,  gradually  lessens.  If  the 
current  be  applied  for  a  short  time  only,  the  secretion  may  last  for 
some  time  after  the  current  has  been  shut  off.  The  saliva  thus 
obtained  is  but  slightly  viscid,  and  under  the  microscope  a  very 
few  salivary  corpuscles,  and,  occasionally  only,  amorphous  lumps 
of  peculiar  material,  probably  mucous  in  nature,  are  seen.  If  the 
gland  itself  be  watched,  while  its  activity  is  thus  roused,  it  will  be 
seen  (as  w^e  have  already  said,  §  145)  that  its  arteries  are  dilated, 
and  its  capillaries  filled,  and  that  the  blood  flows  rapidly  through 
the  veins  in  a  full  stream  and  of  bright  arterial  hue,  frequently 
with  pulsating  movements.  If  a  veni  of  the  gland  be  opened, 
this  large  increase  of  flow,  and  the  lessening  of  the  ordinary 
deoxygenation  of  the  blood  consequent  upon  the  rapid  stream, 
will  be  still  more  evident.  It  is  clear  that  excitation  of  the 
chorda  largely  dilates  the  arteries ;  the  nerve  acts  energetically  as 
a  vaso-dilator  nerve. 

Thus  stimulation  of  the  chorda  brings  about  two  events  :  a 
dilation  of  the  blood  vessels  of  the  gland,  and  a  flow  of  saliva. 
The  question  at  once  arises.  Is  the  latter  simply  the  result  of  the 
former  or  is  the  flow  caused  by  some  direct  action  on  the  secreting 
cells,  apart  from  the  increased  blood-supply  ?  In  support  of  the 
former  view  we  might  argue  that  the  activity  of  the  epithelial 
secreting  cell,  like  that  of  any  other  form  of  protoplasm,  is 
dependent  on  blood-supply.  When  the  small  arteries  of  the  gland 
dilate,  while  the  pressure  in  the  arteries  on  the  side  towards  the 
heart  is  (as  we  have  previously  seen  when  treating  generally  of 
blood-pressure  §  102)  correspondingly  diminished,  the  pressure  on 
the  far  side  in  the  capillaries  and  veins  is  increased ;  hence  the 
capillaries  become  fuller,  and  more  blood  passes  through  them  in 
a  given  time.  From  this  we  might  infer  that  a  larger  amount  of 
nutritive  material  would  pass  away  from  the  capillaries  into  the 
surrounding  lymph-spaces,  and  so  into  the  epithelium  cells,  the 
result  of  which  would  naturally  be  to  quicken  the  processes  going 
on  in  the  cells,  and  to  stir  these  up  to  greater  activity.  But  even 
admitting  all  this  it  does  not  necessarily  follow  that  the  activity 
thus  excited  should  take  on  the  form  of  secretion.  It  is  quite 
possible  to  conceive  that  the  increased  blood-supply  should  lead 


Chap,  i.]    TISSUES  AND  MECHANISMS  OF  DIGESTION.  337 

only  to  the  accumulation  in  the  cell  of  the  constituents  of  the 
saliva,  or  of  the  raw  materials  for  their  construction,  and  not 
to  a  discharge  of  the  secretion.  A  man  works  better  for  being 
fed,  but  feeding  does  not  make  him  work  in  the  absence  of  any 
stimulus.  The  increased  blood-supply  therefore,  while  favourable 
to  active  secretion,  need  not  necessarily  bring  it  about.  Moreover, 
the  following  facts  distinctly  shew  that  it  need  not.  When  a 
cannula  is  tied  into  the  duct  and  the  chorda  is  energetically 
stimulated,  the  pressure  acquired  by  the  saliva  accumulated  in 
the  cannula  and  in  the  duct  may  exceed  for  the  time  being  the 
arterial  blood-pressure,  even  that  of  the  carotid  artery ;  that  is 
to  say,  the  pressure  of  fluid  in  the  gland  outside  the  blood  vessels 
is  greater  than  that  of  the  blood  inside  the  blood  vessels.  This 
must,  whatever  be  the  exact  mode  of  transit  of  nutritive  material 
through  the  vascular  walls,  tend  to  check  that  transit.  Again,  if 
the  head  of  an  animal  be  rapidly  cut  off,  and  the  chorda  immedi- 
ately stimulated,  a  flow  of  saliva  takes  place  far  too  copious  to  be 
accounted  for  by  the  emptying  of  the  salivary  channels  through 
any  supposed  contraction  of  their  walls.  In  this  case  secretion  is 
excited  in  the  gland  though  the  blood-supply  is  limited  to  the 
small  quantity  still  remaining  in  the  blood  vessels.  Lastly,  if  a 
small  quantity  of  atropin  be  injected  into  the  veins,  stimulation  of 
the  chorda  produces  no  secretion  of  saliva  at  all,  though  the  dilation 
of  the  blood  vessels  takes  place  as  usual ;  in  spite  of  the  greatly 
increased  blood-supply  no  secretion  at  all  takes  place.  These 
facts  prove  that  the  secretory  activity  is  not  simply  the  result 
of  vascular  changes,  but  may  be  called  forth  independently ;  they 
further  lead  us  to  suppose  that  the  chorda  contains  two  sets  of 
fibres,  one  which  we  may  call  secretory  fibres,  acting  directly  on  the 
secreting  structures  only,  and  the  other  vaso-dilator  fibres,  acting 
on  the  blood  vessels  only,  and  further  that  atropin,  while  it  has  no 
effect  on  the  latter,  paralyses  the  former  just  as  it  paralyses  the  in- 
hibitory fibres  of  the  vagus.  Hence  when  the  chorda  is  stimulated, 
there  pass  down  the  nerve,  in  addition  to  impulses  affecting  the 
blood-supply,  impulses  affecting  directly  the  secreting  cells,  and 
calling  them  into  action,  just  as  similar  impulses  call  into  action 
the  contractility  of  the  substance  of  a  muscular  fibre.  Indeed 
the  two  things,  secreting  activity  and  contracting  activity,  are 
very  parallel.* 

Since  the  chorda  acts  thus  directly  on  the  secreting  cells,  we 
should  expect  to  find  an  anatomical  connection  between  the  cells 
and  the  nerve ;  but  concerning  this  our  knowledge  is  as  yet  im- 
perfect. 

§190.  AVhen  the  cervical  sympathetic  is  stimulated,  the 
vascular  effects,  as  we  have  already  said,  §  146,  are  the  exact 
contrary  of  those  seen  when  the  chorda  is  stimulated.  The  small 
arteries  are  constricted,  and  a  small  quantity  of  dark  venous  blood 
escapes  by  the  veins.     Sometimes,  indeed,  the  flow  through  the 


338  SECRETION   OF   GASTRIC   JUICE.         [Book  ii. 

gland  is  almost  arrested.  The  sympathetic  therefore  acts  as  a 
vaso-constrictor  nerve,  and  in  this  sense  is  antagonistic  to  the 
chorda. 

As  concerns  the  flow  of  saliva  brought  about  by  stimulation  of 
the  sympathetic,  in  the  case  of  the  submaxillary  gland  of  the  dog 
the  effects  are  very  peculiar.  A  slight  flow  results,  and  the  saliva 
so  secreted  is  remarkably  viscid,  of  higher  specific  gravity,  and 
richer  in  corpuscles  and  in  the  above-mentioned  amorphous  lumps 
than  is  the  chorda  saliva.  This  action  of  the  sympathetic  is  little 
or  not  at  all  affected  by  atropin. 

In  the  submaxillary  gland  of  the  dog  then  the  contrast  between 
the  effects  of  chorda  stimulation  and  those  of  sympathetic  stimu- 
lation are  very  marked :  the  former  gives  rise  to  vascular  dilation 
with  a  copious  flow  of  fairly  limpid  saliva  poor  in  solids,  the  latter 
to  vascular  constriction  with  a  scanty  flow  of  viscid  saliva  richer 
in  solids.  And  in  other  animals  a  similar  contrast  prevails,  though 
with  minor  differences.  We  shall  return  again  presently  to  these 
different  actions  of  the  two  nerves ;  meanwhile  we  have  seen 
enough  of  the  history  of  the  submaxillary  gland  to  learn  that 
secretion  in  this  instance  is  a  reflex  action,  the  efferent  impulses 
of  which  directly  affect  the  secreting  cells,  and  that  the  vas- 
cular phenomena  may  assist,  but  are  not  the  direct  cause  of,  the 
flow. 

§  191.  We  have  dwelt  long  on  this  gland  because  it  has 
been  more  fruitfully  studied  than  any  other  But  the  nervous 
mechanisms  of  the  other  salivary  glands  are  in  their  main  features 
similar.  Thus  the  secretion  of  the  parotid  gland,  like  that  of  the 
submaxillary,  is  governed  by  two  sets  of  fibres :  one  of  cerebro- 
spinal origin,  running  along  the  auriculo-temporal  branch  of 
the  fifth  nerve  but  originating  possibly  in  the  glossopharyngeal, 
and  the  other  of  sympathetic  origin  coming  from  the  cervical 
sympathetic.  Stimulation  of  the  cerebro-spinal  fibres  produces  a 
copious  flow  of  limpid  saliva,  free  from  mucus ;  stimulation  of  the 
cervical  sympathetic  gives  rise  in  the  rabbit  to  a  secretion  also  free 
from  mucus  but  rich  in  proteids  and  of  greater  amylolytic  power 
than  the  cerebro-spinal  secretion  ;  in  the  dog  little  or  no  secretion 
is  produced,  though,  as  we  shall  see  later  on,  certain  changes  are 
brought  about  in  the  gland  itself.  In  both  animals  the  cerebro- 
spinal fibres  are  vaso-dilator,  and  the  sympathetic  fibres  vaso- 
constrictor in  action. 

§  192.  The  secretion  of  gastric  juice.  Though  a  certain  amount 
of  gastric  juice  may  sometimes  be  found  in  the  stomachs  of  fasting 
animals,  it  may  be  stated  generally  that  the  stomach,  like  the 
salivary  glands,  remains  inactive,  yielding  no  secretion,  so  long  as 
it  is  not  stimulated  by  food  or  otherwise.  The  advent  of  food  into 
the  stomach  however  at  once  causes  a  copious  flow  of  gastric  juice ; 
and  the  quantity  secreted  in  the  twenty-four  hours  is  probably  very 
considerable,  but  we  have  no  trustworthy  data  for  calculating  the 


Chap,  i.]    TISSUES  AND  MECHANISMS  OF  DIGESTION.  339 

exact  amount.  So  also  when  the  gastric  mucous  membrane  is 
stimulated  mechanically,  as  with  a  feather,  secretion  is  excited ; 
but  to  a  very  small  amount  even  when  the  whole  mterior  surface 
of  the  stomach  is  thus  repeatedly  stimulated.  The  most  efhcieut 
stimulus  is  the  natural  stimulus,  viz.  food ;  though  dilute  alkalis 
seem  to  have  unusually  powerful  stimulating  effects ;  thus  the 
swallowing  of  saliva  at  once  provokes  a  flow  of  gastric  juice. 
During  fasting  the  gastric  membrane  is  of  a  pale  grey  colour, 
somewliat  dry,  covered  with  a  thin  layer  of  mucus,  and  thrown 
into  folds ;  during  digestion  it  becomes  red,  flushed,  and  tumid, 
the  folds  disappear,  and  minute  drops  of  fluid  appearing  at  the 
mouths  of  the  glands,  speedily  run  together  into  small  stre^ams. 
When  the  secretion  is  very  active,  the  blood  flows  from  the 
capillaries  into  the  veins  in  a  rapid  stream  without  losing  its 
bright  arterial  hue.  The  secretion  of  gastric  juice  is  in  fact 
accompanied  by  vascular  dilation  in  the  same  way  as  is  the  secre- 
tion of  saliva. 

§  193.  Seeing  that,  unlike  the  case  of  the  salivary  secretion, 
food  is  brought  into  the  immediate  neighbourhood  of  the  secreting 
cells,  it  is  exceedingly  probable  that  a  great  deal  of  the  secretion 
is  the  result  of  some  direct  local  action ;  and  this  view  is  sup- 
ported by  the  fact  that  when  a  mechanical  stimulus  is  applied 
to  one  spot  of  the  gastric  membrane  the  secretion  is  limited 
to  the  neighbourhood  of  that  spot  and  is  not  excited  in  distant 
parts. 

The  stomach  is  supplied  with  nerve-fibres  from  the  two  vagi 
nerves  and  from  the  solar  plexus  of  the  splanchnic  system.  Our 
knowledge  however  of  the  action  of  the  nervous  system  upon  the 
stomach  by  means  of  these  two  sets  of  fibres  is  very  imperfect. 
There  are  many  facts  which  shew  that  the  central  nervous 
system  may  affect  the  secretion  of  gastric  juice.  On  the  other 
hand  a  secretion  of  quite  normal  gastric  juice  will  go  on  after 
both  vagi,  or  the  nerves  from  the  solar  plexus  going  to  the 
stomach  have  been  divided,  and  indeed  when  all  the  nervous 
connections  of  the  stomach  are  so  far  as  possible  severed, 

§  194.  The  contrast  presented  between  the  scanty  secretion 
resulting  from  mechanical  stimulation  and  the  copious  flow  which 
actual  food  induces  is  interesting  because  it  seems  to  shew  that 
the  secretory  activity  of  the  cells  is  heightened  by  the  absorption 
of  certain  products  derived  from  tlie  portions  of  food  first  digested. 
This  is  well  illustrated  l)y  the  following  experiment  of  Heidcnliain. 
This  observer,  adopting  the  method  employed  for  the  intestine, 
of  which  we  shall  speak  later  on,  succeeded  in  isolating  a  portion 
of  the  fundus  from  the  rest  of  the  stomacli ;  that  is  to  say,  he  cut 
out  a  portion  of  the  fundus,  sewed  together  the  cut  edges  of  the 
main  stomach,  so  as  to  form  a  smaller  but  otherwise  complete  organ, 
while  by  sutures  he  converted  the  excised  piece  of  fundus  into  a 
small  independent  stomach  opening  on  to  the  exterior  by  a  fistulous 


340  SECRETION   OF   GASTRIC   JUICE.  [Book  ii, 

orifice.  Wlien  food  was  introduced  into  the  main  stomach  secretion 
also  took  place  in  the  isolated  fundus.  This  at  first  sight  might 
seem  the  result  of  a  nervous  reflex  act ;  but  it  was  observed  that 
the  secondary  secretion  in  the  fundus  was  dependent  on  actual 
digestion  taking  place  in  the  main  stomach.  If  the  material 
introduced  into  the  main  stomach  were  indigestible  or  digested 
with  difficulty,  so  that  little  or  no  products  of  digestion  were 
formed  and  absorbed  into  the  blood,  such  ex.  gr.  as  pieces  of 
ligamentum  nuchse,  very  little  secretion  took  place  in  the  isolated 
fundus.  We  quote  this  now  as  bearing  on  the  question  of  a 
possible  nervous  mechanism  of  gastric  secretion,  but  we  shall  have 
to  return  to  it  under  another  aspect. 

The  changes  in  a  gland  constituting  the  act  of  secretion. 

§  195.  We  have  now  to  consider  what  are  the  changes  in  the 
glandular  cells  and  their  surroundings  which  cause  this  flow  of 
fluid  possessing  specific  characters  into  the  lumen  of  an  alveolus, 
and  so  into  a  duct.  It  will  be  convenient  to  begin  with  the 
pancreas. 

The  thin  extended  pancreas  of  a  rabbit  may,  by  means  of 
special  precautions,  be  spread  out  on  the  stage  of  a  microscope 
and  examined  with  even  high  powers,  while  the  animal  is  not  only 
alive  but  under  such  conditions  that  the  gland  remains  in  a  nearly 
normal  state,  capable  of  secreting  vigorously.  It  is  possible  under 
these  circumstances  to  observe  even  minutely  the  appearances 
presented  by  the  gland  when  at  rest  and  loaded,  and  to  watch 
the  changes  which  take  place  during  secretion. 

When  the  animal  has  not  been  digesting  for  some  little  time, 
the  outlines  of  the  individual  cells  lining  the  alveolus  are  very  in- 
distinct, the  lumen  is  invisible  or  very  inconspicuous,  and  each 
cell  is  crowded  with  small,  refractive  spherical  granules,  forming 
an  irregular  granular  mass  which  hides  the  nucleus  and  leaves 
only  a  very  narrow  clear  outer  zone  next  to  the  basement  mem- 
brane, or  it  may  be  hardly  any  such  zone  at  all.  Fig.  77  A.  The 
gland  is  said  to  be  '  loaded '  or  at  rest. 

The  blood-supply  moreover  is  scanty,  the  small  arteries  being 
constricted  and  the  capillaries  imperfectly  filled  with  corpuscles. 

If,  however,  the  same  pancreas  be  examined  while  it  is  in  a 
state  of  activity,  either  from  the  presence  of  food  in  the  stomach, 
or  from  the  injection  of  some  stimulating  drug,  such  as  pilocarpin, 
a  very  different  state  of  things  is  seen.  The  individual  cells 
(Fig.  77  B)  have  become  smaller  and  much  more  distinct  in 
outline,  and  the  contour  of  the  alveolus  which  previously  was  even 
is  now  wavy,  the  basement  membrane  being  indented  at  the, 
junctions  of  the  cells  ;  also  the  lumen  of  the  alveolus  is  now  wider 
and  more  conspicuous.     In  each  cell  the  granules  have  become 


Chap,  i.]   TISSUES  AND  MECHANISMS  OF  DIGESTION.  341 

much  fewer  in  number  and  as  it  were  have  retreated  to  the  inner 
margin,  so  that  the  inner  granular  zone  is  much  narrower  and  the 
outer  transparent  zone  much  broader  than  before ;  the  latter  too 
is  frequently  marked  at  its  inner  part  by  delicate  strioe  running 
into  the  inner  zone.     At  the  same  time  the  blood  vessels  are 


Fig.  77.   A  Portion  of  the  Pancreas  of  the  Rabbit.   (Kiihne  and  Sheridan  Lea). 
A  at  rest,  i5  in  a  state  of  activity. 

a  the  inner  granular  zone,  which  in  xi  is  larger,  and  more  closely  studded  with 
fine  granules,  than  in  B,  in  wliicli  the  granules  are  fewer  and  coarser. 

h  the  outer  transparent  zone,  small  in  A,  larger  in  B,  and  in  the  latter  marked 
witli  faint  strias. 

c  the  lumen,  very  obvious  in  B,  but  indistinct  in  A. 

d  an  indentation  at  the  junction  of  two  cells,  seen  in  B,  but  not  occurring  in  A. 

largely  dilated  and  the  stream  of  blood  through  the  capillaries  is 
full  and  rapid. 

With  care  the  change  from  the  one  state  of  things  to  the  other 
may  be  watched  under  the  microscope.  The  vascular  changes  can 
of  course  be  easily  appreciated,  but  the  granules  may  also  be  seen 
to  diminish  in  number.  Those  at  the  inner  margin  seem  to  be 
discharged  into  the  lumen,  and  those  nearer  the  outer  margin 
to  travel  inwards  through  the  cell-substance  towards  the  lumen, 
the  faint  strife  spoken  of  above,  apparently  at  all  events,  being  the 
marks  of  their  paths.  Obviously  during  secretion,  the  granules 
with  which  the  cell-substance  was  '  loaded  '  are  '  discharged '  from 
the  cell  into  the  lumen  of  the  alveolus.  "VVHiat  changes  these 
granules  may  undergo  during  the  discharge  w^e  shall  consider 
presently. 

Sections  of  thi  prepared  and  hardened  pancreas  of  any  animal 
tell  nearly  the  same  tale  as  that  thus  told  by  the  living  pancreas 
of  the  rabbit.  In  sections  for  instance  of  the  pancreas  of  a  dog 
whicli  has  not  been  fed,  and  therefore  has  not  been  digesting,  for 
some  hours  (24  or  30),  the  cells  are  seen  to  be  crowded  with 
granules  (which  however  are  usually  shrunken  and  irregular  owing 
to  the  influence  of  the  hardening  agent),  leaving  a  very  narrow 
outer  zone.  In  similar  sections  of  the  pancreas  of  a  dog  wliich 
has  been  recently  fed,  ?ix  hours  before  for  example,  and  in  which 


342  CHANGES  IN  PANCREATIC   CELLS.      [Book  ii. 

therefore  the  gland  has  been  for  some  time  actively  secreting,  the 
granules  are  far  less  numerous,  and  the  clear  outer  zone  accord- 
ingly much  broader  and  more  conspicuous.  With  osmic  acid  these 
granules  stain  well,  and  are  preserved  in  their  spherical  form,  so 
that  the  cell  thus  stained  maintains  much  of  the  appearance  of  a 
living  cell.  But  with  carmine,  hematoxylin  &c.  the  granules  do 
not  stain  nearly  so  readily  as  does  the  cell-substance  of  the  cells, 
so  that  a  discharged  cell  stains  more  deeply  than  does  a  loaded  cell 
because  the  staining  of  the  '  protoplasmic '  cell-substance  is  not  so 
much  obscured  by  the  unstained  granules  ;  besides  which  however 
the  actual  cell-substance  stains  probably  somewhat  more  deeply 
in  the  discharged  cell.  It  may  be  added  that  in  the  discharged 
cell  the  nucleus  is  conspicuous  and  well  formed ;  in  the  loaded  cell 
it  is  generally  in  prepared  sections,  more  or  less  irregular,  possibly 
because  in  these  it  is  less  dense  and  more  watery  than  in  the  dis- 
charged cell,  and  so  shriuks  under  the  influence  of  the  reagents 
employed. 

These  several  observations  suggest  the  conclusion  that  in  a 
gland  at  rest  the  cell  is  occupied  in  forming  by  means  of  the 
metabolism  of  its  cell-substance  and  lodguig  in  itself  (§  80) 
certain  granules  of  peculiar  substance  intended  to  be  a  part  and 
probably  an  important  part  of  the  secretion.  This  goes  on  until 
the  cell  is  more  or  less  completely  '  loaded.'  In  such  a  cell  the 
amount  of  actual  living  cell-substance  is  relatively  small,  its  place 
is  largely  occupied  by  granules,  and  it  itself  has  been  partly 
consumed  in  forming  the  granules.  During  the  act  of  secretion 
the  granules  are  discharged  to  form  part  of  the  secretion,  other 
matters  including  water,  as  we  shall  see,  making  up  the  whole 
secretion ;  and  the  cell  would  be  proportionately  reduced  in  size 
were  it  not  that  the  act  of  the  discharge  seems  to  stimulate  the  cell- 
substance  to  a  new  activity  of  growth,  so  that  new  cell-substance 
is  formed;  this  however  is  in  turn  soon  in  part  consumed  in 
order  to  form  new  granules.  And  what  is  thus  seen  with  con- 
siderable distinctness  and  ease  in  the  pancreas,  is  seen  with  more 
or  less  distinctness  in  other  glands. 

§  196.  When  we  study  an  '  albuminous,'  or  '  serous  '  salivary 
gland,  the  parotid  gland  for  instance,  in  a  living  state,  we  find 
that  the  changes  which  take  place  during  activity  are  quite 
comparable  to  those  of  the  pancreas.  During  rest  (Fig.  78  A), 
the  cells  are  large,  their  outlines  very  indistinct,  in  fact  almost 
invisible,  and  the  cell-substance  is  studded  with  granules.  Dur- 
ing activity  (Fig.  78  B),  the  cells  become  smaller,  their  outlines 
more  distinct,  and  the  granules  disappear,  especially  from  the 
outer  portions  of  each  cell.  After  prolonged  activity,  as  in 
Fig.  78  C,  the  cells  are  still  smaller  with  their  outlines  still  more 
distinct,  and  the  granules  have  disappeared  almost  entirely,  a  few 
only  being  left  at  the  extreme  inner  margin  of  each  cell,  abutting 
upon  the  conspicuous,  almost  gaping  lumen  of  the  alveolus.     And 


Chap,  i.]   TISSUES  AND  MECHANISMS  OF  DIGESTION.  343 

upon  special  examiuation  it  is  found  that  the  nuclei  are  large  and 
round.  In  fact  we  might  almost  take  the  parotid,  as  thus  studied, 
to  be  more  truly  typical  of  secretory  changes  than  even  the  pan- 
creas. For,  the  demarcation  of  an  inner  and  outer  zone  is  not 
a  necessary  feature  of  a  secreting  cell  at  rest.     What  is  essential 


Fig.  78.    Changes  ix  the  Parotid  during  Secretion.     (Langley.) 

The  figure,  wliich  is  somewhat  diagrammatic,  represents  the  microscopic  changes 
which  maV  be  observed  in  the  livimi  gland.  A.  During  rest.  The  obscure  outlines 
of  the  cells  are  introduced  to  shew  the  relative  size  of  the  cells,  they  could  not  be 
readily  seen  in  the  specimen  itself.  B.  After  moderate  stimulation.  C.  Mtev 
prolonged  stimulation.  The  nuclei  are  diagrammatic,  and  introduced  to  shew  their 
appearance  and  position. 


is  that  the  cell-substance  manufactures  material,  which  for  a 
while,  that  is  during  rest,  is  deposited  in  the  cell,  generally  ia 
the  form  of  granules  but  not  necessarily  so,  and  that  during 
activity  this  material  is  used  up,  the  disappearance  of  the  granules, 
when  these  are  visible,  being  naturally  earliest  and  most  marked 
at  the  outer  portions  of  each  cell,  and  progressing  inwards  towards 
the  lumen,  the  whole  cell  becoming  smaller  and  as  it  were 
shrunken. 

In  the  cells  of  the  parotid  gland  and  other  albumiaous  cells 
the  granules  seen  in  the  living  or  fresh  cell  differ  from  the 
granules  seen  in  the  pancreatic  cell,  inasmuch  as  they  are  easily 
dissolved  or  broken  up  by  the  action  of  alcohol,  chromic  acid,  and 
the  other  usual  hardening  reagents,  and  hence  in  hardened  speci- 
mens have  disappeared.  In  consequence,  in  sections  of  hardened 
and  pre]mred  albuminous  glands  the  differences  between  resting 
or  loaded  and  active  or  discharged  cells  are  not  so  conspicuous  as 
in  the  pancreas.  The  difference  however  even  in  hardened  speci- 
mens between  the  parotid  of  the  rabbit  at  rest,  and  that  excited 
by  stimulation  of  the  sympathetic  is  well  marked.  During  rest, 
the  cells  (Fig.  79-4)  are  pale,  transparent,  staining  with  difficulty, 
and  the  nuclei  possess  irregular  outlines  as  if  shrunken  by  the 
reagents  employed.  After  stimulation  of  the  sym]iathetic,  the 
cell-substance  becomes  turbid  (Fig.  79  B),  and  stains  much  more 
readily,  while  the  nuclei  are  no  longer  irregular  in  outline  but 
round  and  large,  with  conspicuous  nucleoli,  the  whole  cell  at  the 


344 


CHANGES   IN  ALBUMINOUS   CELLS.      [Book  ii. 


same  timo,  at  least  after  prolonged  stiraulation,  becoming  distinctly 
smaller. 

§  197.  In  a  mucous  salivary  gland  the  changes  which  take 
place  are  of  a  like  kind,  though  apparently  somewhat  more  com- 
plicated, owing  probably  to  the  peculiar  characters  of  the  mucin 
which  is  so  conspicuous  a  constituent  of  the  secretion. 


Fig.  79.  Sections  of  the  Parotid  of  the  Rabbit.  A  at  rest,  B  after  stimu- 
lation of  the  cervical  sympathetic.  Both  sections  are  from  hardened  gland.  (After 
Heidenhain.) 

If  a  piece  of  resting,  loaded  submaxillary  gland  be  teased  out, 
while  fresh  and  warm  from  the  body,  in  normal  saline  solution,  the 
cell-substance  of  the  mucous  cells  (Fig.  80  a)  is  seen  to  be  crowded 


Fig.  80.  Mucous  Cells  from  a  fresh  Submaxillart  Gland  of  Dog.   (Langley.) 

a  and  b  isolated  in  2  p.c.  salt  solution  ;  a,  from  loaded  gland,  b  from  discharged 
gland  (the  nuclei  are  usually  more  obscured  by  granules  than  is  here  represented). 

(On  teasing  out  a  fragment  of  fresh  in  2  to  5  p.c.  salt  solution,  the  cells  usually 
become  broken  up  so  that  isolated  cells  are  rarely  obtained  entire ;  isolated  cells  are 
common  if  the  gland  be  left  in  the  body  for  a  day  after  death.) 

a',  //,  treated  with  dilute  acid ;  a'  from  loaded,  b'  from  discharged  gland. 


with  granules  or  spherules  which  may  fairly  be  compared  with 
the  granules  of  the  pancreas,  though  perhaps  less  dense  and  solid 
than  these. 


Chap,  i.]    TISSUES  AND  MECHANISMS  OF  DIGESTION.  345 

If  a  piece  of  a  gland  which  has  been  secreting  for  some  time, 
and  is  therefore  a  discharged  gland,  be  examined  in  the  same  way 
(Fig.  80  b)  the  granules  are  far  less  numerous  and  largely  confined 
to  the  part  of  the  cell  nearer  the  lumen,  the  outer  part  of  the 
cell  around  the  nucleus  consisting  of  ordinary  '  protoplasmic '  cell- 
substance.  The  distinction  however  between  an  inner  '  granular 
zone '  next  to  the  lumen  and  an  outer  '  clear  zone '  next  to  the 
basement  membrane  is  less  distinct  than  in  the  pancreas,  partly 
because  the  granules  do  not  disappear  in  so  regular  a  manner  as 
in  the  pancreas  and  partly  because  the  outer  zone  of  the  mucous 
cell,  as  it  forms,  is  less  homogeneous  than  that  of  the  pancreatic 

C3ll. 

spherules '  of  the  mucous  call  are  moreover 
If  the  fresh  cell,  shewing  granules,  (either 
of  a  loaded  or  few  as  in  the  case  of  a  dis- 
charged cell)  be  irrigated  with  water  or  with  dilute  acids  or  dilute 
alkalis  the  granules  swell  up  (Fig.  80  a'  h')  into  a  transparent 
mass,  giving  the  reactions  of  mucin,  traversed  by  a  network  of 
'  protoplasmic '  cell-substance.  In  this  way  is  produced  an  ap- 
pearance very  similar  to  that  shewn  in  sections  of  mucous  glands 
hardened  and  stained  in  the  ordinary  way. 

In  the  loaded  mucous  cell  in  such  hardened  and  stained  pre- 
parations (Fig.  81  a)  there  is  seen  a  small  quantity  of  protoplasmic 


The  'granules  '  or 
of  a  peculiar  nature, 
many  as  in  the  case 


Fig.  81.     Alveoli  of  Dog's  Submaxillary  Gland  iiardenep  ix  alcohol 
AND  stained  with  CARMINE.     (Lauglev.)     (The  network  is  diagrammatic.) 

a,  from  a  loaded  glaiid. 

h,  from  a  discliarged  gland  ;  the  chorda  tympani  having  been  stimulated  at  short 
intervals  durintr  five  hours. 


cell-substance  gathered  round  the  nucleus  at  the  outer  part  of  the 
cell  next  to  the  basement  membrane;  the  rest  of  the  cell  consists 
of  a  network  of  cell-substance,  the  interstices  being  filled  with 


346 


CHANGES  IN   GASTRIC   CELLS. 


[Book  ii. 


transparent  material,  which,  unlike  the  network  itself  and  the 
mass  of  cell-substance  round  the  nucleus,  does  not  stain  with 
carmine  or  with  certain  other  dyes.  The  discharged  cell  in  simi- 
lar preparations  (Fig.  81  h)  differs  from  the  loaded  cell  in  the 
amount  of  transparent  non-staining  material  being  much  less  and 
chiefly  confined  to  the  inner  part  of  the  cell,  while  the  protoplas- 
mic cell-substance  around  the  now  large  and  well-formed  nucleus 
is  not  onl}^  both  relatively  and  absolutely,  greater  in  amount,  but 
stains  still  more  deeply  than  in  the  loaded  cell. 

It  would  appear  therefore  that  in  the  mucous  cell,  as  in  the 
pancreatic  cell,  the  cell-substance  forms  and   deposits  in  itself 


Fig.  82.    Gastric  Gland  of  Mammal  (Bat)  during  Activity.     (Langley.) 

c,  the  mouth  of  the  gland  with  its  cylindrical  cells. 

n,  the  neck,  containing  conspicuous  ovoid  cells,  Avith  their  coarse  protoplasmic 
network. 

f,  the  body  of  the  gland.  The  granules  are  seen  in  the  central  cells  to  be  limited 
to  the  inner  portions  of  each  cell,  the  round  nucleus  of  which  is  conspicuous. 

certain  material  in  the  form  of  granules.     During  secretion  these 
granules  disappear  and  presumably  form  part  of  the  secretion. 

§  198.     The  '  central '  or  '  chief  '   cells  of  the  gastric  glands 
also  exhibit  similar  changes.     In  such  an  animal  as  the  newt 


Chap,  i.]    TISSUES  AND  MECHANISMS  OF  DIGESTION.  347 

these  cells  may,  though  with  difficulty,  be  examined  in  the  living 
state.  They  are  then  found  to  be  studded  with  granules  when 
the  stomach  is  at  rest.  During  digestion  these  granules  become 
much  less  numerous  and  are  chietiy  gathered  near  the  lumen, 
leaving  in  each  cell  a  clear  outer  zone.  And  in  many  mammals 
the  same  abundance  of  granules  in  the  loaded  cell,  the  same 
paucity  of  granules  for  the  most  part  restricted  to  an  inner  zone 
in  the  discharged  cell,  may  be  demonstrated  by  the  use  of  osmic 
acid.  Fig.  82. 

When  the  stomach  is  hardened  by  alcohol  these  changes,  like 
the  similar  changes  in  an  albuminous  cell,  are  obscured  by  the 
shrinking  of  the  '  granules '"  or  by  their  swellmg  up  and  becoming 
diffused  through  the  rest  of  the  cell-substance ;  so  that  though,  in 
sections  so  prepared,  very  striking  differences  are  seen  between 
loaded  and  discharged  cells,  these  are  unlike  those  seen  in  living 
glands.  In  specimens  taken  from  an  animal  which  has  not  been 
fed  for  some  time,  the  central  cells  of  the  gastric  glands  are  pale, 
finely  granular,  and  do  not  stain  readily  with  carmine  and  other 
dyes.  During  the  early  stages  of  gastric  digestion,  the  same  cells 
are  found  somewhat  swollen,  but  turbid  and  more  coarsely  granu- 
lar ;  they  stain  much  more  readily.  At  a  later  stage  they  become 
smaller  and  shrunken,  but  are  even  more  turbid  and  granular  than 
before,  and  stain  still  more  deeply.  This  is  true,  not  only  of  the 
central  cells  in  the  cardiac  glands,  but  also  of  the  cells  of  which 
the  pyloric  glands  are  built  up.  In  the  loaded  cell  very  little 
staining  takes  place,  because  the  amount  of  living  staining  cell- 
substance  is  small  relatively  to  the  amount  of  material  with  which 
it  is  loaded  and  which  does  not  stain  readily.  In  the  cell  which 
after  great  activity  has  discharged  itself,  the  cell  is  smaller,  but 
what  remains  is  largely  living  cell-substance,  some  of  it  new,  and 
all  staining  readily.  It  would  appear  also  that  during  the  activity 
of  the  cell  some  substances,  capable  of  being  precipitated  by  alco- 
hol, make  their  appearance,  and  the  presence  of  this  material  adds 
to  the  turbid  and  granular  aspect  of  the  cell ;  possibly  also  this 
material  contributes  to  the  staining.  A  similar  material  seems  to 
make  its  appearance  in  the  cells  of  albuminous  glands. 

In  the  ovoid  or  border  cells  no  very  characteristic  clianges 
make  their  appearance.  During  digestion  they  become  larger, 
more  swollen  as  it  were,  and  in  consequence  bulge  out  the 
basement  membrane,  but  no  characteristic  disappearance  of  gran- 
ules can  be  observed.  In  the  living  state,  the  cell-substance  of 
these  ovoid  cells  appears  finely  granular,  but  in  hardened  and 
prepared  sections  has  a  coarsely  granular,  "reticulate"  look  which 
is  perhaps  less  marked  in  the  swollen  active  cells  than  in  the 
resting  cells. 

§  199.  All  these  various  secreting  cells  then,  pancreatic  cell, 
mucous  cell,  albuminous  cell,  and  central  gastric  cell,  exhibit  the 
same  series  of  events,  modified  to  a  certain  extent  in  the  several 


348  TRYPSIN  AND   TRYPSINOGEN.  [Book  ii. 

cases.  In  each  case  the  '  protoplasmic '  cell-substance  manufac- 
tures and  lodges  in  itself  material  destined  to  form  part  of  the 
juice  secreted.  In  the  fresh  cell  this  material  may  generally  be 
recognized  under  the  microscope  by  its  optical  characters  as  gran- 
ules ;  these  however  are  apt  to  become  altered  by  reagents.  But 
we  must  guard  ourselves  against  the  assumption  that  the  material 
which  can  thus  be  recognized  is  the  only  material  thus  stored  up ; 
we  may,  in  future,  by  chemical  or  other  means  be  able  to  differ- 
entiate other  parts  of  the  cell-body  as  being  also  material  similarly 
stored  up. 

During  activity,  while  the  gland  is  secreting,  this  material, 
either  unchanged  or  after  undergoing  change,  is  wholly  or  partially 
discharged  from  the  cell.  The  cell  in  consequence  of  having  thus 
got  rid  of  more  or  less  of  its  load  consists  to  a  larger  extent  of 
actual  living  cell-substance,  this  being  in  many  cases  increased  by 
rapid  new  growth,  though  the  bulk  of  the  discharged  cell  may  be 
less  than  that  of  the  loaded  cell. 

This  activity  of  growth  continues  after  the  act  of  secretion,  but 
the  discharged  cell  soon  begins  again  the  task  of  loading  itself 
with  new  secretion  material  for  the  next  act  of  secretion. 

Thus  in  most  cases  there  is,  corresponding  to  the  intermittence 
of  secretion,  an  alternation  of  discharge  and  loading ;  but  it  must 
be  borne  in  mind  that  such  an  alternation  is  not  absolutely  neces- 
sary even  in  the  case  of  intermittent  secretion.  We  can  easily 
imagine  that  the  discharge,  say,  of  'granules'  during  secretion 
should  stir  up  the  cell  to  an  increased  activity  in  forming  gran- 
ules, and  that  the  formative  activity  should  cease  when  the  secre- 
tory activity  ceased.  In  such  a  case  the  number  of  new  granules 
formed  might  always  be  equal  to  the  number  of  old  granules  used 
up,  and  the  active  cell  in  spite  of  its  discharge  would  possess  as 
many  granules,  that  is  to  say,  as  large  a  load,  as  the  cell  at  rest. 
And  in  the  central  gastric  cells  of  some  animals  it  would  appear 
that  such  a  continued  balancing  of  load  and  discharge  does  actu- 
ally take  place,  so  that  no  distinction  in  granules  can  be  observed 
between  resting  and  active  cells. 

§  200.  We  spoke  just  now  of  the  material  stored  up  in  the 
cell  and  destined  to  form  part  of  the  secretion  as  undergoing 
change  before  it  was  discharged.  In  the  mucous  cell  we  have 
seen  that  the  material  deposited  in  the  living  cell  has  at  first  the 
form  of  granules.  These  granules  however  are  easily  converted 
into  a  transparent  material  lodged  in  the  spaces  of  the  cell- 
substance,  which  material  even  if  not  exactly  identical  with  at 
least  closely  resembles  the  mucin  found  in  the  secretion ;  and  ap- 
parently, in  the  act  of  secretion  the  granules  do  undergo  some 
such  change.  In  the  case  of  some  other  glands  moreover  we 
have  chemical  as  well  as  optical  evidence  that  the  material  stored 
up  in  the  cell  is,  in  part  at  least,  not  the  actual  substance  appear- 
ing in  the  secretion  but  an  antecedent  of  that  substance. 


Chap,  i.]    TISSUES  AND  MECHANISMS  OF  DIGESTION.  349 

An  important  constituent  of  pancreatic  juice  is,  as  we  shall 
see  later  on,  a  body  called  trypsin,  a  ferment  very  similar  to 
pepsin,  acting  on  proteid  bodies  and  converting  them  into  peptone 
and  other  substances.  Though  in  many  respects  alike,  pepsin 
and  trypsin  are  quite  distinct  bodies,  and  ditler  markedly  in  this, 
that  while  an  acid  medium  is  necessary  for  the  action  of  pepsin, 
an  alkaline  medium  is  necessary  for  the  action  of  trypsin ;  and 
accordingly  the  pancreatic  juice  is  alkaline  in  contrast  to  the  acid- 
ity of  gastric  juice.  Trypsin,  can,  like  pepsin  (§  183),  be  extracted 
with  glycerine  from  substances  in  which  it  occurs ;  glycerine  ex- 
tracts of  trypsin  however  need  for  the  manifestation  of  their 
powers  the  presence  of  a  weak  alkali,  such  as  a  1  p.c.  solution  of 
sodium  carbonate. 

Now  trypsin  is  present  in  abundance  in  normal  pancreatic 
juice ;  but  a  loaded  pancreas,  one  which  is  ripe  for  secretion,  and 
which  if  excited  to  secrete  would  immediately  pour  out  a  juice 
rich  in  trypsin,  contains  no  trypsin  or  a  mere  trace  of  it ;  nay 
even  a  pancreas  which  is  engaged  in  the  act  of  secreting  contains 
in  its  actual  cells  an  insigniticant  quantity  only  of  trypsin,  as  is 
shewn  by  the  following  experiment. 

If  the  pancreas  of  an  animal,  even  of  one  in  full  digestion,  be 
treated,  while  still  loarm  from  the  hocly,  with  glycerine,  the  glyce- 
rine extract,  as  judged  of  by  its  action  on  fibrin  in  the  presence  of 
sodium  carbonate,  is  inert  or  nearly  so  as  regards  proteid  bodies. 
If,  however,  the  same  pancreas  be  kept  for  24  hours  before  being 
treated  with  glycerine,  the  glycerine  extract  readily  digests  fibrin 
and  other  proteids  in  the  presence  of  an  alkali.  If  the  pancreas, 
while  still  warm,  be  rubbed  up  in  a  mortar  for  a  few  minutes 
with  dilute  acetic  acid,  and  then  treated  with  glycerine,  the 
glycerine  extract  is  strongly  proteolytic.  If  the  glycerine  extract 
obtained  without  acid  from  the  warm  pancreas,  and  therefore  inert, 
be  diluted  largely  with  water,  and  kept  at  oo^O.  for  some  time, 
it  becomes  active.  If  treated  with  acidulated  instead  of  distilled 
water,  its  activity  is  much  sooner  developed.  If  the  inert  glyce- 
rine extract  of  warm  pancreas  be  precipitated  with  alcohol  in 
excess,  the  precipitate,  inert  as  a  proteolytic  ferment  when  fresh, 
becomes  active  when  exposed  for  some  time  in  an  aqueous  solu- 
tion, rapidly  so  when  treated  with  acidulated  water.  These  facts 
shew  that  a  pancreas  taken  fresh  from  the  body,  even  during  full 
digestion,  contains  MU  little  ready-made  ferment,  though  there  is 
present  in  it  a  body  which,  by  some  kinds  of  decomposition,  gives 
hirth  to  the  ferment.  We  may  remark  incidentally  that  though 
the  presence  of  an  alkali  is  essential  to  the  proteolytic  action  of 
the  actual  ferment,  the  formation  of  the  ferment  out  of  its  fore- 
runner is  favoured  l)y  the  presence  of  a  small  quantity  of  acid  ;  the 
acid  must  be  used  with  care,  since  the  trypsin,  once  formed,  is 
destroyed  by  acids.  To  this  body,  this  mother  of  the  ferment, 
which  has  not  at  present  been  satisfactorily  isolated,  but  which 


350        NATURE   OF  THE  ACT  OF   SECRETION.     [Book  ii. 

appears  to  be  a  complex  body,  splitting  up  into  the  ferment,  which 
as  we  have  seen  is  at  all  events  not  certainly  a  proteid  body,  and 
into  an  undeniably  proteid  body,  the  name  of  z/puogen  has  been 
applied.  But  it  is  better  to  reserve  the  term  zymogen  as  a  gene- 
ric name  for  all  such  bodies  as,  not  being  themselves  actual 
ferments,  may  by  internal  changes  give  rise  to  ferments,  for 
all  '  mothers  of  ferment '  in  fact ;  and  to  give  to  the  particu- 
lar mother  of  the  pancreatic  proteolytic  ferment,  the  name 
tryjjsinogen. 

Evidence  of  a  similar  kind  shews  that  the  gastric  glands,  both 
the  cardiac  and  the  pyloric  glands,  while  they  contain  compara- 
tively little  actual  pepsin,  contain  a  considerable  quantity  of  a 
zymogen  of  pepsin,  or  pepsinogen ;  and  there  can  be  little  doubt 
but  that  this  pepsinogen  is  lodged  in  the  central  cells  of  the 
cardiac  glands  and  in  the  somewhat  similar  cells  which  line  the 
whole  of  the  pyloric  glands. 

§  201.  The  act  of  secretion  itself.  The  above  discussion  pre- 
pares us  at  once  for  the  statement  that  the  old  view  of  secretion 
according  to  which  the  gland  picks  out,  separates,  secretes  (hence 
the  name  secretion)  and  so  filters  as  it  were  from  the  common 
store  of  the  blood  the  several  constituents  of  the  juice,  is  unten- 
able. According  to  that  view  the  specific  activity  of  any  one  gland 
was  confined  to  the  task  of  letting  certain  constituents  of  the  blood 
pass  from  the  capillaries  surrounding  the  alveolus  through  the 
cells  to  the  channels  of  the  ducts,  while  refusing  a  passage  to 
others.  We  now  know  that  certain  important  constituents  of  each 
juice,  the  pepsin  of  gastric  juice,  the  mucin  of  saliva  and  the  like 
are  formed  in  the  cell,  and  not  obtained  ready  made  from  the 
blood.  A  minute  quantity  of  pepsin  does  exist  it  is  true  in  the 
blood,  but  there  are  reasons  for  thinking  that  this  has  made  its  way 
back  into  the  blood,  either  being  absorbed  from  the  interior  of  the 
stomach  or,  as  seems  more  probable,  picked  up  directly  from  the 
gastric  glands  ;  and  so  with  some  of  the  other  constituents  of  other 
juices.  The  chief  or  specific  constituents  of  each  juice  are  formed 
in  the  cell  itself. 

But  the  juice  secreted  by  any  gland  consists  not  only  of  the 
specific  substances  such  as  mucin,  pepsin  or  other  ferment,  or  other 
bodies,  found  in  it  alorfe,  but  also  of  a  large  quantity  of  water,  and 
rf  various  other  substances,  chiefly  salines,  common  to  it,  to  other 
juices  and  to  the  blood.  And  the  question  arises.  Is  the  water, 
are  the  salts  and  other  common  substances  furnished  by  the  same 
act  as  that  which  supplies  the  specific  constituents  ? 

Certain  facts  suggest  that  they  are  not.  For  instance,  as 
mentioned  some  time  ago,  in  the  submaxillary  gland  of  the  dog, 
stimulation  of  the  chorda  tympani  produces  a  copious  flow  of 
saliva,  which  is  usually  thin  and  limpid,  while  stimulation  of  the 
cervical  sympathetic  produces  a  scanty  flow  of  thick  viscid  saliva. 
That  is  to  say,  stimulation  of  the  chorda  has  a  marked  effect  in 


Chap,  i.]   TISSUES  AND  MECHANISMS  OF  DIGESTION.  351 

promoting  the  discharge  of  water,  while  stimulation  of  the  sym- 
jDathatic  has  a  marked  effect  in  promoting  the  discharge  of  mucin. 
To  this  we  may  add  the  case  of  the  parotid  of  the  dog.  In  this 
gland  stimulation  of  a  cerebro-spinal  nerve,  the  auriculo-temporal, 
produces  a  copious  flow  of  limpid  saliva,  while  stimulation  of  the 
sympathetic  produces  itself  little  or  no  secretion  at  all ;  but  when 
the  sympathetic  and  cerebro-spinal  nerves  are  stimulated  at  the 
same  time,  the  saliva  which  Hows  is  much  richer  in  solid  and 
especially  in  organic  matter  than  when  the  cerebro-spinal  nerve 
is  stimulated  alone.  And  we  have  already  seen  that  in  this  gland 
the  microscopic  changes  following  upon  sympathetic  stimulation 
are  more  conspicuous  than  those  which  follow  upon  cerebro-spinal 
stimulation. 

These  and  other  facts  hav'e  led  to  the  conception  that  the 
act  of  secretion  consists  of  two  parts,  wliich  in  one  case  may 
coincide,  in  another  may  take  place  apart  or  in  different  propor- 
tions. On  the  one  hand,  there  is  the  discharge  of  water  carrying 
with  it  common  soluble  substances,  chiefly  salines,  derived  from 
the  blood ;  on  the  other  hand,  a  metabolic  activity  of  the  cell- 
substance  gives  rise  to  the  specific  constituents  of  the  juice.  To 
put  the  matter  broadly,  the  latter  process  produces  the  specific 
constituents,  the  former  washes  these  and  other  matters  into  the 
duct.  It  has  been  further  supposed  that  two  kinds  of  nerve  fibres 
exist :  one  governing  the  former  process  and,  in  the  case  of  the 
submaxillary  gland  for  instance,  preponderating,  though  not  to 
the  total  exclusion  of  the  other  kind,  in  the  chorda  tympani ;  the 
other  governing  the  latter  process  and  preponderating  in  the 
branches  of  the  cervical  sympathetic.  These  have  been  called 
respectively  '  secretory '  and  '  trophic '  fibres  ;  but  these  terms  are 
not  desirable.  It  may  be  here  remarked  that  even  the  former 
process  is  a  distinct  activity  of  the  gland,  and  not  a  mere  filtra- 
tion. For,  as  we  have  seen  in  the  case  of  the  salivary  glands, 
when  atropin  is  given,  not  only  do  the  specific  constituents  cease 
to  be  ejected  as  a  consequence  of  stimulation  of  the  chorda,  but 
the  discharge  of  water,  in  spite  of  the  blood  vessels  becoming 
dilatsd,  is  also  arrested :  no  saliva  at  all  leaves  the  gland.  And 
what  is  true  of  the  salivary  glands  as  regards  the  dependence  of 
the  flow  of  water  on  something  else  besides  the  mere  pressure  of 
the  blood  in  the  blood  vessels,  appears  to  hold  good  with  other 
glands  also.  The  whole  act  of  secretion  is  a  very  complicated 
one,  probably  too  complicated  to  be  described  as  consisting  merely 
of  the  two  processes  mentioned  above. 

§  202.  Throughout  the  aliove  we  have  spoken  as  if  the  secre- 
tion were  furnished  exclusively  by  the  cells  of  the  alveoli  or  se- 
creting portion  of  the  gland,  as  if  the  epitlielium  cells  lining  the 
ducts,  or  conducting  portion  of  the  gland,  contributed  nothing  to 
the  act.  In  the  gastric  glands  the  slender  cells  lining  the  mouths 
of  the  glands  (which  correspond  to  ducts)  and  covering  the  ridges 


352  THE  ACT   OF   SECRETION.  [Book  ii. 

between,  are  mucous  cells  secreting  into  the  stomach  generally  a 
small,  but  under  abnormal  conditions  a  large,  amount  of  mucus, 
which  has  its  uses  but  is  not  an  essential  part  of  the  gastric  juice. 
In  the  salivary  glands  we  can  hardly  suppose  that  the  long  stretch 
of  characteristic  columnar  epithelium  which  reaches  from  the 
alveoli  to  the  mouth  of  the  long  main  duct  serves  simply  to 
furnish  a  smooth  lining  to  the  conducting  passages ;  but  we  have 
as  yet  no  clear  indications  of  what  the  function  of  tliis  epithelium 
can  be. 

§  203.  Before  we  leave  the  mechanism  of  secretion  there  are 
one  or  more  accessory  points  which  deserve  attention. 

In  treating  just  now  of  the  gastric  glands  we  spoke  as  if  pepsin 
were  the  only  important  constituent  of  gastric  juice,  whereas,  as  we 
have  previously  seen,  the  acid  is  equally  essential.  The  formation 
of  the  free  acid  of  the  gastric  juice  is  very  obscure,  and  many 
ingenious  but  unsatisfactory  views  have  been  put  forward  to 
explain  it.  It  seems  natural  to  suppose  that  it  arises  in  some  way 
from  the  decomposition  of  sodium  chloride  drawn  from  the  blood ; 
and  this  is  supported  by  the  fact  that  when  the  secretion  of  gastric 
juice  is  actively  going  on,  the  amount  of  chlorides  leaving  the 
blood  by  the  kidney  is  proportionately  diminished ;  but  nothing 
certain  can  at  present  be  stated  as  to  the  mechanism  of  that 
decomposition. 

In  the  frog,  while  pepsin  free  from  acid  is  secreted  by  the 
glands  in  the  lower  portion  of  the  oesophagus,  an  acid  juice  is 
afforded  by  glands  in  the  stomach  itself,  which  have  accordingly 
been  called  oxyntic  (^o^vveiv  to  sharpen,  acidulate)  glands ;  but 
these  oxyntic  glands  appear  also  to  secrete  pepsin.  In  the 
mammal  the  isolated  pylorus  secretes  an  alkaline  juice ;  in  fact, 
the  appearance  of  an  acid  juice  is  limited  to  those  portions  of  the 
stomach  in  which  the  glands  contain  both  '  chief  '  or  '  central,'  and 
'  ovoid '  or  '  border '  cells  Now  from  what  has  been  previously  said 
there  can  be  no  doubt  that  the  chief  cells  do  secrete  pepsin.  On 
the  other  hand  there  is  no  evidence  whatever  of  the  formation  of 
pepsin  Ijy  the  '  border '  or  '  ovoid '  cells,  though  this  was  once 
supposed  to  be  the  case  and  these  cells  were  unfortunately 
formerly  called  '  peptic '  cells.  Hence  it  has  been  inferred  that  the 
border  cells  secrete  acid ;  but  the  argument  is  at  present  one  of 
exclusion  only,  there  being  no  direct  proof  that  these  cells  actually 
manufacture  the  acid. 

The  rennin  appears  to  be  formed  by  the  same  cells  which 
manufacture  the  pepsin,  that  is,  by  the  chief  cells  of  the  fundus 
generally  and  to  some  extent  by  the  cells  of  the  pyloric  glands. 
We  may  add  that  we  have  evidence  of  the  existence  of  a  zymogen 
of  rennin  analogous  to  the  zymogen  of  pepsin  or  of  trypsin. 

§  204.  Seeing  the  great  solvent  power  of  both  gastric  and 
pancreatic  juice,  the  question  is  naturally  suggested,  Why  does 
not   the    stomach   digest   itself  ?     After   death,  the    stomach    is 


CiiAp.  I.]  TISSUES  AND  MECHANISMS  OF  DIGESTION.  353 

frequently  found  partially  digested,  viz.  in  cases  when  death  has 
taken  place  suddenly  on  a  full  stomach.  In  an  ordinary  death, 
the  membrane  ceases  to  secrete  before  the  circulation  is  at  an  end. 
That  there  is  no  special  virtue  in  living  things  which  prevents 
their  being  digested  is  shewn  by  the  fact,  that  the  leg  of  a  living 
frog  or  the  ear  of  a  living  rabbit  introduced  into  the  stomach  of  a 
dog  through  a  gastric  fistula  is  readily  digested.  It  has  been 
suggested  that  the  blood-current  keeps  up  an  alkalinity  sufficient 
to  neutralize  the  acidity  of  the  juice  in  the  region  of  the  glands 
themselves;  but  this  will  not  explain  why  the  pancreatic  juice, 
which  is  active  in  an  alkaline  medium,  does  not  digest  the 
proteids  of  the  pancreas  itself,  or  why  the  digestive  cells  of  the 
bloodless  actinozoon  or  hydrozoon  do  not  digest  themselves.  We 
might  add,  it  does  not  explain  why  the  amoeba,  while  dissolving 
the  protoplasm  of  the  swallowed  diatom,  does  not  dissolve  its 
own  protoplasm.  We  cannot  answer  this  question  at  all  at 
present,  any  more  than  the  similar  one,  why  the  delicate  proto- 
plasm of  the  amoeba  resists  during  life  the  entrance  into  itself 
by  osmosis  of  more  water  than  it  requires  to  carry  on  its  work, 
while  a  few  moments  after  it  is  dead  water  enters  freely  by 
osmosis,  and  the  effects  of  that  entrance  become  abundantly 
evident  by  the  formation  of  bullse  and  the  breaking  up  of  the 
protoplasm. 


23 


SEC.  3.     THE  PROPERTIES  AND   CHARACTERS  OF  BILE, 
PANCREATIC   JUICE   AND    SUCCUS   ENTERICUS. 


§  205.  lu  the  living  body  the  food,  subjected  to  the  action 
first  of  the  saliva  and  then  of  the  gastric  juice,  undergoes  in  the 
stomach  changes  which  we  shall  presently  consider  in  detail,  and 
the  food  so  changed  is  passed  on  into  the  small  intestine,  where  it 
is  further  subjected  to  the  action  of  the  bile  secreted  by  the  liver, 
of  pancreatic  juice  secreted  by  the  pancreas,  and  possibly  to  some 
extent,  though  this  is  by  no  means  certain,  of  a  juice  secreted  by 
the  intestine  itself,  and  called  succus  entericus.  It  will  be  con- 
venient to  study  the  minute  structure  of  the  liver  in  connection 
with  other  functions  of  the  liver  more  important  perhaps  than  that 
of  the  secretion  of  bile,  namely  the  formation  of  glycogen,  and  other 
metabolic  events  occurring  in  the  hepatic  cells  ;  we  have  already 
studied  the  structure  of  the  pancreas ;  and  the  structure  of  the 
intestine  will  best  be  considered  by  itself.  We  therefore  turn  at 
once  to  the  properties  and  characters  of  the  above-named  juices. 

Bile. 

Though  bile,  after  secretion  in  the  lobules  of  the  liver,  is  passed 
on  along  the  hepatic  duct,  it  is  in  the  case  of  most  animals  not 
poured  at  once  into  the  duodenum  but  taken  by  the  cystic  duct  to 
the  reservoir  of  the  gall-bladder.  Here  it  remains,  until  such  time 
as  it  is  needed,  when  a  quantity  is  poured  along  the  common  bile 
duct  into  the  intestine. 

The  quality  of  bile  varies  much,  not  only  in  different  animals, 
but  in  the  same  animal  at  different  times.  It  is  moreover  affected 
by  the  length  of  the  sojourn  in  the  gall-bladder ;  bile  taken  direct 
from  the  hepatic  duct,  especially  when  secreted  rapidly,  contains 
little  or  no  mucus ;  that  taken  from  the  gall-bladder,  as  of 
slaughtered  oxen  or  sheep,  is  loaded  with  mucus.  The  colour  of 
the  bile  of  carnivorous  and  omnivorous  animals,  and  of  man,  is 
generally  a  bright  golden  red,  sometimes  a  greenish  yellow :  of 


Chap,  r.]    TISSUES  AND  MECHANISMS  OF  DIGESTION.   355 

herbivorous  animals,  a  yellowish  green,  or  a  bright  green,  or  a 
dirty  green,  according  to  circumstances,  being  much  modified  by 
retention  in  the  gall-bladder.  The  reaction  is  neutral  or  alkaline. 
Tlie  following  may  Ije  taken  as  the  average  composition  of  liuman 
bile  taken  from  the  gall-bladder,  and  therefore  containing  much 
more  mucus  as  well  as,  relatively  to  the  solids,  more  water  than 
bile  from  the  hepatic  duct. 

In  1000  part-s. 

Water 859-2 

Solids :  — 

Bile  Salts 91-4 

Fats,  &c 9-2 

Cholesterin 2*6 

Mucus  and  Pigment 29-8 

Inorganic  Salts 7'8 

140-8 

The  entire  absence  of  proteids  is  a  marked  feature  of  Ijile ;  pan- 
creatic juice,  as  we  shall  see,  contains  a  considerable  quantity, 
saliva,  as  we  have  seen,  a  small  quantity,  normal  gastric  juice 
probably  still  less  and  bile  none  at  all.  Even  the  bile  which  has 
been  retained  some  time  in  the  gall-bladder,  though  rich  in  mucus, 
contains  no  proteids. 

The  constituents  which  form,  apart  from  the  mucus,  the  great 
bulk  of  the  solids  of  bile  and  which  deserve  chief  attention,  are 
the  pigments  and  the  bile-salts;  of  these  we  shall  speak  im- 
mediately. 

With  regard  to  the  inorganic  salts  actually  present  as  such 
sodium  salts  are  conspicuous,  sodium  chloride  amounting  to  '2  or 
more  per  cent.,  sodium  phosphate  to  nearly  as  much,  the  rest 
being  earthy  phosphates  and  other  matters  in  small  quantity.  The 
presence  of  iron,  to  the  extent  of  about  -006  p.c,  is  interesting, 
since,  as  we  shall  see,  there  are  reasons  for  thinking  that  the  pig- 
ment of  bile,  itself  free  from  iron,  is  derived  from  iron-holding 
hemoglobin  ;  some,  at  least,  of  the  iron  set  free  during  the  con- 
version of  hivmoglobin  into  bile  pigment,  which  probably  takes 
place  in  the  liver,  finds  its  way  into  the  bile.  Bile  also  appears 
to  contain  a  small  quantity,  at  all  events  occasionally,  of  other 
metals,  such  as  manganese  and  copper  ;  metals  introduced  into  the 
body  are  apt  to  be  retained  in  the  liver  and  eventually  leave  it  hv 
the  bile. 

The  small  (juantity  of  fat  present  consists  in  part  <if  the  com- 
plex body  lecithin. 

The  peculiar  body  cholesterin,  which  though  fatty  looking 
(hence  the  name  'bile  fat')  is  really  an  alcohol  with  the  composi- 
tion Co,;H440,  is  conspicuous  by  its  quantity  and  constancy.  It 
forms  the  greater  part  of  most  gall-stones,  though  some  are"^  com- 
posed chiefly  of  pigment.     Insoluble  in  water  and  cold  alcohol, 


356  BILE-SALTS.  [Book  ii. 

though  soluble  in  hot  alcohol  and  readily  soluble  in  ether,  chloro- 
form &c.,  it  is  dissolved  by  the  bile-salts  in  aqueous  solution  and 
hence  is  present  in  solution  in  bile.  Its  physiological  functions 
are  obscure. 

The  ash  of  bile  consists  largely  of  soda,  derived  partly  from  the 
sodium  chloride  and  partly  from  the  bile-salts,  of  sulphates  derived 
chieriy  if  not  wholly  from  the  latter,  and  of  phosphates  partly 
ready  formed,  and  in  part  derived  from  the  lecithin. 

§  206.  Pigments  of  Bile.  The  natural  golden  red  colour  of 
normal  human  or  carnivorous  bile,  is  due  to  the  presence  of  Bili- 
rubin. This,  which  is  also  the  chief  pigmentary  constituent  of 
gall-stones,  and  occurs  largely  in  the  urine  of  jaundice,  may  be 
obtained  in  the  form  either  of  an  orange-coloured  amorphous  pow- 
der, or  of  well-formed  rhombic  tablets  and  prisms.  Insoluble  in 
water,  and  but  little  soluble  in  ether  and  alcohol,  it  is  readily 
soluble  in  chloroform,  and  in  alkaline  fluids.  Its  composition  is 
CioHigNgOs.  Treated  with  oxidizing  agents,  such  as  nitric  acid 
yellow  with  nitrous  acid,  it  displays  a  succession  of  colours  in  the 
order  of  the  spectrum.  The  yellowish  golden  red  becomes  green, 
this  a  greenish  blue,  then  blue,  next  violet,  afterwards  a  dirty 
red,  and  finally  a  pale  yellow.  This  characteristic  reaction  of  bili- 
rubin is  the  basis  of  the  so-called  Gmelin's  test  for  bile-pigments. 
Each  of  these  stages  represents  a  distinct  pigmentary  substance. 
As  alkaline  solution  of  bilirubin,  exposed  in  a  shallow  vessel  to 
the  action  of  the  air,  turns  green,  becoming  converted  into  Bili- 
verdin  (CigHigN  O4),  the  green  pigment  of  herbivorous  bile.  Bili- 
verdin  is  also  found  at  times  in  the  urine  of  jaundice,  and  is 
probably  the  body  which  gives  to  bile  which  has  been  exposed  to 
the  action  of  gastric  juice,  as  in  biliary  vomits,  its  characteristic 
green  hue.  It  is  the  lirst  stage  of  the  oxidation  of  bilirubin  in 
Gmelin's  test.  Treated  with  oxidizing  agents  biliverdin  runs 
through  the  same  series  of  colours  as  bilirubin,  with  the  exception 
of  the  initial  golden  red. 

§  207.  The  Bile-salts.  These  consist,  in  man  and  many  ani- 
mals, of  sodium  glycoclwlate  and  taurocholate,  the  proportion  of  the 
two  varying  in  different  animals.  In  man  both  the  total  quantity 
of  bile-salts  and  the  proportion  of  the  one  bile  salt  to  the  other 
seem  to  vary  a  good  deal,  but  the  glycocholate  is  said  to  be  always 
the  more  abundant.  In  ox-gall,  sodium  glycocholate  is  abundant, 
and  taurocholate  scanty.  The  bile-salts  of  the  dog,  cat,  bear,  and 
other  carnivora,  consist  exclusively  of  the  latter. 

Insoluble  in  ether  but  soluble  in  alcohol  and  in  water,  the 
aqueous  solutions  having  a  decided  alkaline  reaction,  both  salts 
may  be  obtained  by  crystallisation  in  fine  acicular  needles  They 
are  exceedingly  deliquescent.  The  solutions  of  both  acids  have  a 
dextro-rotatory  action  on  polarized  light. 

Preparation.  Bile,  mixed  Avith  animal  charcoal,  is  evaporated  to 
dryness  and  extracted  with  alcohol.     If  not  colourless,  the  alcoholic 


Chap.  I.]    TISSUES  AND  MECHANISMS  OF  DIGESTION.   357 

filtrate  must  be  further  decolourized  with  animal  charcoal,  and  the 
alcohol  distilled  off.  The  dry  residue  is  treated  with  absolute  alcohol, 
and  to  the  alcoholic  filtrate  anh3^drous  ether  is  added  as  long  as  any 
precipitate  is  formed.  On  standing  the  cloudy  jirecipitate  becomes 
transformed  into  a  crystalline  mass  at  the  bottom  of  the  vessel.  If  the 
alcohol  be  not  absolute,  the  crystals  are  very  apt  to  be  changed  into  a 
thick  syrupy  fluid.  This  mass  of  crystals  has  been  often  spoken  of  as 
bilin.  Both  salts  are  thus  precipitated,  so  that  in  such  a  bile  as  that  of 
the  ox  or  man  bilin  consists  both  of  sodium  glycocholate  and  sodium 
taurocholate.  The  two  may  be  separated  by  precipitation  from  their 
aqueous  solutions  with  sugar  of  lead,  which  throws  down  the  former 
much  more  readily  than  the  latter.  The  acids  may  be  separated  from 
their  respective  salts  by  dilute  sulphuric  acid,  or  by  the  action  of  lead- 
acetate  and  sulphydric  acid. 

On  boiling  with  dilnte  acids  (sulphuric,  hydrochloric)  or  caustic 
potash,  or  baryta  water,  glycocholic  acid  is  split  up  into  cholalic 
(cholic)  acid  and  glycin.  Taurocholic  acid  may  similarly  be  split 
up  into  cholalic  acid  and  taurin.     Thus 

glycocholic  acid  cholalic  acid  glycin 

C"  6H43NOC  +  H2O  =  C24H40O5  +  CH2 .  NH2  (CO  .  OH) 

taurocholic  acid  cholalic  acid  taurin 

C26H45NSO7 + H2O  =  C24H40O5  +  C2H4 .  NHo .  SO3H. 

Both  acids  contain  the  same  non-nitrogenous  acid,  cholalic 
acid  ;  but  this  acid  is  in  the  first  case  associated  or  conjugated  with 
the  important  nitrogenous  body  glycin,  or  amido-acetic  acid,  which 
is  a  compound  formed  from  ammonia  and  one  of  the  "  fatty  acid  " 
series,  viz.  acetic ;  and  in  the  second  case  with  taurin,  or  amido- 
isethionic  acid,  that  is  a  compound  into  which  representatives  of 
ammonia,  of  the  ethyl  group,  and  of  sulphuric  acid  enter.  The 
decomposition  of  the  bile  acids  into  cholalic  acid  and  taurin  or 
glycin  respectively  takes  place  naturally  in  the  intestine,  the  glycin 
and  taurin  being  probably  absorbed,  so  that  from  the  two  acids, 
after  they  have  served  their  purpose  in  digestion,  the  two  ammonia 
compounds  are  returned  into  the  blood.  Each  of  the  two  acids,  or 
cholalic  acid  alone,  when  treated  with  sulphuric  acid  and  cane- 
sugar,  gives  a  magnificent  purple  colour  (Pettenkofer's  test)  with  a 
characteristic  spectrum.  A  similar  colour  may  however  often  be 
produced  by  the  action  of  the  same  bodies  on  albumin,  amyl 
alcohol,  and  some  other  organic  bodies. 

§  208.  Action  of  Bile  on  Food.  In  some  animals  at  least  bile 
contains  a  ferment  capable  of  converting  starch  into  sugar ;  but  its 
action  in  this  respect  is  wholly  subordinate. 

On  proteids  bile  has  no  direct  digestive  action  whatever,  but 
being,  generally  at  least,  alkaline,  and  often  strongly  so,  tends  to 
neutralise  the  acid  contents  of  the  stomach  as  they  pass  into  the 
duodenum  and  as  we  shall  see  so  prepares  the  way  for  the  action 


358  PANCREATIC   JUICE.  [Book  ii. 

of  the  pancreatic  juice.  To  peptic  action  it  is  distinctly  antago- 
nistic ;  the  presence  of  a  sufficient  quantity  of  bile  renders  gastric 
juice  inert  towards  proteids.  Moreover  when  bile,  or  a  solution  of 
bile-salts,  is  added  to  a  fluid  containing  the  products  of  gastric  di- 
gestion, a  precipitate  takes  place,  consisting  of  parapeptone  (when 
present),  peptone,  pepsin  and  bile  salts.  The  precipitate  is  redis- 
solved  in  an  excess  of  bile  or  solution  of  bile-salts  ;  but  the  pepsin 
though  redissolved  remains  inert  towards  proteids.  This  precipi- 
tation actually  does  take  place  in  the  duodenum,  and  we  shall 
speak  of  it  again  later  on. 

With  regard  to  the  action  of  bile  on  fats,  the  following  state- 
ments may  be  made.  Bile  has  a  slight  solvent  action  on  fats,  as 
seen  in  its  use  by  painters.  It  has  by  itself  a  slight  but  only 
slight  emulsifying  power :  a  mixture  of  oil  and  bile  separate  after 
shaking  rather  less  rapidly  than  a  mixture  of  oil  and  water. 
With  fatty  acids  bile  forms  soaps.  It  is  moreover  a  solvent  of 
solid  soaps,  and  it  would  appear  that  the  emulsion  of  fats  is 
under  certain  circumstances  at  all  events  facilitated  by  the  pres- 
ence of  soaps  in  solution.  Hence  bile  is  probably  of  much  greater 
use  as  an  emulsion  agent  when  mixed  with  pancreatic  juice  than 
when  acting  by  itself  alone.  To  this  point  we  shall  return. 
Lastly,  the  passage  of  fats  through  membranes  is  assisted  by 
wetting  the  membranes  with  bile,  or  with  a  solution  of  bile-salts. 
Oil  will  pass  to  a  certain  extent  through  a  filter-paper  kept  w^et 
with  a  solution  of  bile-salts,  whereas  it  will  not  pass  or  passes 
with  extreme  difficulty  through  one  kept  constantly  wet  with 
distilled  water. 

Bile  possesses  some  antiseptic  qualities.  Out  of  the  body  its 
presence  hinders  various  putrefactive  processes ;  and  when  it  is 
prevented  from  flowing  into  the  alimentary  canal,  the  contents 
of  the  intestine  undergo  changes  different  from  those  which  take 
place  under  normal  conditions,  and  leading  to  the  appearance  of 
various  products,  especially  of  ill-smelling  gases. 

These  various  actions  of  bile  seem  to  be  dependent  on  the  bile 
salts  and  not  on  the  pigmentary  or  other  constituents. 

Pancreatic  Juice. 

§  209.  Natural  healthy  pancreatic  juice  obtained  by  means  of 
a  temporary  pancreatic  fistula  differs  from  the  digestive  juices  of 
which  we  have  already  spoken,  in  the  comparatively  large  quantity 
of  proteids  which  it  contains.  Its  composition  varies  according  to 
the  rate  of  secretion,  for,  with  the  more  rapid  flow,  the  increase  of 
total  solids  does  not  keep  pace  with  that  of  the  water,  though  the 
ash  remains  remarkably  constant. 

By  an  incision  through  the  linea  alba  the  pancreatic  duct  or  (ducts) 
can  easily  be  found  either  in  the  rabbit  or  in  the  dog,  and  a  cannula 
secured  in  it.     There  is  no  difficulty  about  a  temporary  fistula;  but 


Chap,  r.]    TISSUES  AND  MECHANISMS  OF  DIGESTION.   359 

witla  permanent  fistula;  the  secretion  is  apt  to  become  altered  in  iiuture, 
and  to  lose  many  of  its  characteristic  properties.  Some,  however,  have 
succeeded  in  obtaining  permanent  fistulre  without  any  impairment  of 
the  secretion. 

Healthy  pancreatic  juice  is  a  clear,  somewhat  viscid  fluid, 
frothing  when  shaken.  It  has  a  very  decided  alkaline  reaction, 
and  contains  few  or  no  structural  constituents. 

The  average  amount  of  solids  in  the  pancreatic  juice  (of  the 
dog)  obtained  from  a  temporary  fistula  is  about  8  to  10  p.c. ;  but 
in  even  thoroughly  active  juice  obtained  from  a  permanent  fistula, 
is  not  more  than  about  2  to  5  p.c,  S  being  inorganic  matter; 
and  this  is  probably  the  normal  amount.  The  important  con- 
stituents of  quite  fresh  juice  are  albumin,  a  peculiar  form  of 
proteid  allied  to  myosin,  giving  rise  to  a  sort  of  clotting,  a  small 
amount  of  fats  and  soaps,  and  a  comparatively  large  quantity  of 
sodium  carbonate,  to  which  the  alkaline  reaction  of  the  juice  is 
due,  and  which  seems  to  be  peculiarly  associated  with  the  proteids. 

Since,  as  we  shall  presently  see,  pancreatic  juice  contains  a 
ferment  acting  energetically  on  proteid  matters  in  an  alkaline 
medium,  it  rapidly  digests  its  own  proteid  constituents,  and,  when 
kept,  speedily  changes  in  character.  The  myosin-like  clot  is 
dissolved,  and  the  juice  soon  contains  a  peculiar  form  of  alkali- 
albumin  (precipitable  by  saturation  with  magnesium  sulphate)  as 
well  as  small  quantities  of  leucin,  tyrosin  and  peptone,  which  seem 
to  be  the  products  of  self-digestion  and  are  entirely  absent  from 
the  perfectly  fresh  juice. 

§  210.  Action  on  Food-stujfs.  On  starch,  pancreatic  juice 
acts  with  great  energy,  rapidly  converting  it  into  sugar  (chiefly 
maltose).  All  that  has  been  said  in  this  respect  concerning 
saliva  might  be  repeated  in  the  case  of  pancreatic  juice,  except 
that  the  activity  of  the  latter  is  far  greater  than  that  of  the 
fijrmer.  Pancreatic  juice  and  the  aqueous  infusion  of  the  gland 
are  always  capable  of  converting  starch  into  sugar,  whether  the 
animal  from  which  they  were  taken  be  starving  or  well  fed.  From 
the  juice,  or,  by  the  glycerine  method,  from  the  gland  itself,  an 
amylolytic  ferment  may  be  approximately  isolated. 

On  proteids  pancreatic  juice  also  exercises  a  solvent  action,  so 
far  similar  to  that  of  gastric  juice  that  by  it  proteids  are  converted 
into  peptone.  If  a  few  shreds  of  fibrin  are  thrown  into  a  small 
quantity  of  pancreatic  juice,  they  speedily  disappear,  especially  at 
a  temperature  of  ?>t)^  C,  and  the  mixture  is  found  to  contain 
peptone.  The  activity  of  the  juice  in  thus  converting  proteids 
into  peptone  is  favoured  by  increase  of  temperature  up  to  40°  or 
thereabouts,  and  hindered  by  low  temperatures ;  it  is  permanently 
destroyed  by  boiling.  The  digestive  powers  of  the  juice  in  fact 
depend,  like  those  of  gastric  juice,  on  the  presence  of  a  ferment 
which,  as  we    have    already  said,  may  be  isolated  mucli  in  tlie 


360  TRYPTIC   DIGESTION.  [Book  ir. 

same  way  as  pepsin  is  isolated,  and  to  which  the  name  trypsin  has 
been  given. 

The  appearance  of  fibrin  undergoing  pancreatic  digestion  is 
however  different  from  that  undergoing  peptic  digestion.  In  the 
former  case  the  fibrin  does  not  swell  up,  but  remains  as  opaque  as 
before,  and  appears  to  suffer  corrosion  rather  than  solution.  But 
there  is  a  still  more  important  distinction  between  pancreatic  and 
peptic  digestion  of  proteids.  Peptic  digestion  is  essentially  an 
acid  digestion ;  we  have  seen  that  the  action  only  takes  place  in 
the  presence  of  an  acid,  and  is  arrested  by  neutralisation.  Pan- 
creatic digestion,  on  the  other  hand,  may  be  regarded  as  an  alka- 
line digestion  ;  the  action  is  most  energetic  when  some  alkali  is 
present ;  and  the  activity  of  an  alkaline  juice  is  hindered  or  de- 
layed by  neutralisation  and  arrested  by  acidification  at  least  witli 
mineral  acids.  The  glycerine  extract  of  pancreas  is  under  all 
circumstances  as  inert  in  the  presence  of  free  mineral  acid  as  that 
of  the  stomach  in  the  presence  of  alkalis.  If  the  digestive  mix- 
ture be  supplied  with  sodium  carbonate  to  the  extent  of  1  p.c, 
digestion  proceeds  rapidly,  just  as  does  a  peptic  mixture  when 
acidulated  with  hydrochloric  acid  to  the  extent  of  '2  p.c.  Sodium 
carbonate  of  1  p.c.  seems  in  fact  to  play  in  tryptic  digestion  a 
part  altogether  comparable  to  that  of  hydrochloric  acid  of  '2  p.c.  in 
gastric  digestion.  And  just  as  pepsin  is  rapidly  destroyed  by 
being  heated  to  about  40*^  with  a  1  p.c.  solution  of  sodium  carbo- 
nate, so  trypsin  is  rapidly  destroyed  by  being  similarly  heated 
with  dilute  hydrochloric  acid  of  "2  p.c.  Alkaline  bile,  which 
arrests  peptic  digestion,  seems,  if  anj^thing,  favourable  to  tryptic 
digestion. 

Pancreatic  digestion  and  gastric  digestion  agree  in  that  by 
both  proteids  are  converted  into  peptones.  Naturally  in  the  alka- 
line pancreatic  digestion  no  bye  products  allied  to  acid-albumin, 
such  as  parapeptone,  make  their  appearance ;  there  are  however 
various  bye  products  on  which  we  need  not  dwell.  Albumoses 
are  not  conspicuous  in  pancreatic  digestion,  they  are  very  rapidl.y 
carried  on  to  the  further  stage  of  peptone. 

In  -one  respect  there  is  an  essential  difference  between  gastric 
and  pancreatic  digestion.  In  gastric  digestion  the  products  are 
not  carried  beyond  the  proteid  stage  ;  in  pancreatic  digestion  part  of 
the  proteid  is  changed  into  something  which  is  no  longer  proteid. 

During  the  pancreatic  digestion  of  proteids,  two  remarkable 
nitrogenous  crystalline  bodies,  leuciii  and  tyrosin  make  their  appear- 
ance. When  fibrin  (or  other  proteid)  is  submitted  to  the  action  of 
pancreatic  juice,  the  amount  of  peptone  which  can  be  recovered 
from  the  mixture  falls  far  short  of  the  original  amount  of  proteids  ; 
and  the  longer  the  digestive  action,  the  greater  up  to  a  certain  point 
is  this  apparent  loss.  If  a  pancreatic  digestion  mixture  be  freed 
from  the  bye  products  by  neutralisation  and  filtration,  the  filtrate 
yields,  when  concentrated  by  evaporation,  a  crop  of  crystals  of 


Chap,  i.]  TISSUES  AND  MECHANISMS  OF  DIGESTION.   3G1 

tyrosin.  If  these  be  removed  the  peptone  may  ])e  precipitated 
from  the  concentrated  filtrate  by  the  addition  of  a  large  excess 
of  alcohol  and  separated  by  filtration.  The  second  filtrate  upon 
being  concentrated  by  evaporation  yields  abundant  crystals  of 
leucin  and  traces  of  tyrosin.  Thus  by  the  action  of  the  pancreatic 
juice  a  considerable  amount  (jf  the  proteid,  which  is  being  di- 
gested, is  so  broken  up  as  to  give  rise  to  products  which  are  no 
longer  proteid  in  nature.  From  this  breaking  up  of  the  proteid 
there  arise  leucin,  tyrosin,  and  probably  several  other  bodies,  such 
as  fatty  acids  and  volatile  substances.  We  said  that  in  gastric 
digestion  more  than  one  kind  of  peptone  was  probably  formed, 
and  the  same  may  be  said  of  pancreatic  digestion.  We  may  now 
add  that  in  both  gastric  and  pancreatic  digestion  two  kinds  of 
peptone  are  probably  formed,  one  of  which  resists  the  action  of 
trypsin,  and  undergoes  no  furtlier  change,  but  the  other  of  which, 
whether  arising  from  gastric  or  pancreatic  digestion  undergoes 
further  change  by  the  action  of  trypsin  and  it  is  this  which 
is  the  source  of  the  leucin  and  other  bodies  of  which  we  are 
speaking. 

As  is  well  known,  leucin  and  tyrosin  are  the  bodies  which 
make  their  appearance  when  proteids  or  gelatin  are  acted  on  by 
dilute  acids,  alkalis,  or  various  oxidising  agents.  Leucin  is  a  body, 
which  in  an  impure  state  crystallizes  in  minute  round  lumps  with 
an  obscure  radiate  striation,  but  when  pure,  forms  thin  glittering 
Hat  crystals.  It  has  the  formula  C.H,,NO,,  or  C^Hjo-NHo  (C0.0?6 
and  is  amido-caproic  acid.  Xow  caproic  acid  is  one  of  the  "  fatty 
acid"  series,  so  that  leucin  maybe  regarded  as  a  compound  of 
ammonia  with  a  fatty  acid.  Tyrosin,  CgHuXOo,  on  the  other 
hand,  belongs  to  the  "  aromatic  "  series ;  it  is  a  phenyl  compound, 
and  hence  allied  to  benzoic  acid  and  hippuric  acid.  So  that  in 
pancreatic  digestion  the  large  complex  proteid  molecule  is  split 
up  into  fatty  acid  and  aromatic  molecules,  some  other  bodies 
of  less  importance  making  their  appearance  at  the  same  time. 
We  infer  that  the  proteid  molecules  are  in  some  way  built  up 
out  of  "fatty  acid"  and  "aromatic"  molecules,  together  with 
otlier  components,  and  we  sliall  later  on  eee  additional  reasons 
for  this  view. 

Among  the  supplementary  products  of  pancreatic  digestion 
may  be  mentioned  the  body  indol  (C3H7N"),  to  which  apparently 
the  strong  and  peculiarly  f;iecal  odcnir  which  sometimes  makes  its 
appearance  during  pancreatic  digestion  is  due.  Indol,  however, 
unlike  the  leucin  and  tyrosin,  is  not  a  product  of  pure  pancreatic 
digestion,  but  of  an  accompanying  decomposition  due  to  the  action 
of  organised  ferments.  A  pancreatic  digestive  mixture  soon  be- 
comes swarming  with  bacteria,  in  spite  of  ordinary  precautions, 
when  natural  juice  or  an  infusion  of  the  gland  is  used.  When 
isolated  ferment  is  used,  and  atmospheric  germs  are  excluded,  or 
when  pancreatic  digestion  is  carried  on  in  the  presence  of  salicylic 


362  TRYPTIC   DIGESTION.  [Book  ii. 

acid,  or  thymol,  which  prevent  the  devehopmeut  of  bacteria  and 
like  organisms  but  permit  the  action  of  the  trypsin,  no  odour  is 
perceived,  and  no  indol  is  produced. 

On  the  gelatiniferous  elements  of  the  tissues  in  the  condition 
in  which  they  actually  exist  in  the  tissue  previous  to  any  treat- 
ment pancreatic  juice  appears  to  have  no  solvent  action.  The 
fibrillse  and  bundles  of  fibrillse  of  ordinary  untouched  connective- 
tissue  are  not  digested  by  pancreatic  juice,  which  in  this  respect 
affords  a  striking  contrast  to  gastric  juice.  But  when  they  have 
been  previously  treated  with  acid  or  boiled  so  as  to  become  con- 
verted into  actual  gelatine,  trypsin  is  able  to  dissolve  them,  appar- 
ently clianging  them  much  in  the  same  way  as  does  pepsin. 
Trypsin  unlike  pepsin,  will  dissolve  mucin.  Like  pepsin,  it  is 
inert  towards  nucleiu,  horny  tissues,  and  the  so-called  amyloid 
matter. 

On  fats  pancreatic  juice  has  a  twofold  action.  In  the  first 
place  it  emulsifies  fats.  If  hog's  lard  be  gently  heated  until  it 
melts  and  be  then  mixed  with  pancreatic  juice  before  it  solidifies 
on  cooling,  a  creamy  emulsion,  lasting  for  almost  an  indefinite 
time,  is  formed.  So  also  when  olive  oil  is  shaken  up  with  pancre- 
atic juice,  the  separation  of  the  two  fluids  takes  place  very  slowly, 
and  a  drop  of  the  mixture  under  the  microscope  shews  tliat  the 
division  of  the  fat  is  very  minute.  An  alkaline  aqueous  infusion 
of  the  gland  has  similar  emulsifying  powers.  In  the  second  place 
pancreatic  juice  splits  up  neutral  fats  into  their  respective  acids  and 
glycerine.  Thus  palmitin  (or  tripalmitin)  (C15H31 .  CO  .  0)3 .  CsHg 
is  with  the  assumption  of  SHjO  split  up  into  three  molecules  of 
palmitic  acid  3(Ci5ll3i.  CO  .  OH)  and  one  of  glycerine  (C3H5)(OII)3; 
and  so  wath  the  other  neutral  fats.  If  perfectly  neutral  fat  be 
treated  with  pancreatic  juice,  especially  at  the  body-temperature, 
the  emulsion  which  is  formed  speedily  takes  on  an  acid  reaction, 
and  by  appropriate  means  not  only  the  corresponding  fatty  acids 
but  glycerine  may  be  obtained  from  the  mixture.  When  alkali 
is  present,  the  fatty  acids  thus  set  free  form  their  corresponding 
soaps.  Pancreatic  juice  contains  fats,  and  is  consequently  apt  after 
collection  to  have  its  alkalinity  reduced  ;  and  an  aqueous  infusion 
of  a  pancreatic  gland  (which  always  contains  a  considerable  amount 
of  fat)  very  speedily  becomes  acid. 

Thus  pancreatic  juice  is  remarkable  for  the  power  it  possesses 
of  acting  on  all  the  food-stuffs,  on  starch,  fats  and  proteids. 

The  action  on  starch,  the  action  on  proteids,  and  the  splitting 
up  of  neutral  fats  appear  to  be  due  to  the  presence  of  three  distinct 
ferments,  and  methods  have  been  suggested  for  isolating  them. 
The  emulsifying  power,  on  the  other  hand,  is  connected  with  the 
general  composition  of  the  juice  (or  of  the  aqueous  infusion  of  the 
gland),  being  probably  in  large  measure  dependent  on  the  alkali 
and  the  alkali-albumin  present.  The  proteolytic  ferment  trypsin 
as  ordinarily  prepared  seems  to  be  proteid  in  nature  and  capat^.. 


CuAF.  I.]   TISSUES  AND  MECHANISMS  OF  DIGESTION.  ii6ii 

of  giving  rise,  by  digestion,  to  peptone  ;  but  it  may  be  doubted,  as 
in  the  case  of  pepsin  and  other  ferments,  whether  the  pure  ferment 
has  yet  been  isolated.  There  are  no  means  of  distinguishing  the 
amylolytic  ferment  of  the  pancreas  from  ptyalin.  The  term  ^ja?i- 
creatin  has  been  variously  applied  to  many  different  preparations 
from  the  gland,  and  its  use  had  perhaps  better  be  avoided. 

The  action  of  pancreatic  juice,  or  of  the  infusion  or  extract  of 
the  gland,  on  starch,  is  seen  under  all  circumstances,  whether  the 
animal  be  fasting  or  not.  The  same  may  probably  be  said  of  the 
action  on  fats.  On  proteids  the  natural  juice,  when  secreted  in  a 
normal  state,  is  always  active.  The  glycerine  extract  or  aqueous 
infusion  of  the  gland,  on  the  contrary,  as  we  have  already  explained, 
§  200,  is  active  in  proportion  as  the  trypsinogen  has  been  converted 
into  trypsin. 

Succus  Entericus. 

§  211.  When,  in  a  living  animal,  a  portion  of  the  small 
intestine  is  ligatured,  so  that  the  secretions  coming  down  from 
above  cannot  enter  its  canal,  while  yet  the  blood-supply  is 
maintained  as  usual,  a  small  amount  of  secretion  collects  in  its 
interior.  This  is  spoken  of  as  the  succus  entericus,  and  is  supposed 
to  be  furnished  by  the  glands  of  Lieberklihn,  of  whicli  we  shall 
presently  speak. 

Succus  entericus  may  be  obtained  by  the  following  method,  known 
as  that  of  Thiry  modified  hj  Vella.  The  small  intestine  is  divided  in 
two  places  at  some  distance  (30  to  50  cm.)  apart.  By  fine  sutures  the 
lower  end  of  the  upper  section  is  carefully  united  \vith  the  upper  end 
of  the  lower  section,  thus  as  it  were  cutting  out  a  whole  piece  of  the 
small  intestine  from  the  alimentary  tract.  In  successful  cases,  union 
between  the  cut  surfaces  takes  place,  and  a  shortened  but  otherwise 
satisfactory  canal  is  re-established.  Of  the  isolated  piece  the  two 
ends  are  separately  brought  through  incisions  in  the  abdominal  wall 
and  their  mouths  carefully  fastened  in  such  a  manner  that  each  moutli 
of  the  piece  opens  on  to  the  exterior.  During  the  process  of  liealing 
two  fistulfe  are  thus  established,  one  leading  to  the  beginning  of  and 
the  other  to  tlie  end  of  a  short  piece  of  intestine  quite  isolated  from 
the  rest  of  the  alimentary  canal;  by  means  of  these  openings  a  small 
quantity  of  fluid  can  be  obtained. 

The  quantity  secreted  is  said  to  be  considerabl)''  increased  by  the 
administration  of  pilocarpin. 

Succus  entericus  obtained  from  the  dog  by  the  above  method 
is  a  clear  yellowish  fluid  having  a  faintly  alkaline  reaction  and 
containing  a  certain  quantity  of  mucus.  It  is  said  to  convert 
starch  into  sugar,  and  proteids  into  peptone  (the  action  being  very 
similar  to  that  of  pancreatic  juice),  to  split  up  neutral  fats,  to 
emulsify  fats  and  to  curdle  milk.  It  is  also  said  to  invert  cane- 
sugar  rapidly,  and  by  a  fermentative  action  to  convert  cane-sugar 


364  SUCCUS   ENTERICUS.  [Book  n. 

into  lactic  acid,  and  this  again  into  butyric  acid  with  the  evolution 
of  carbonic  acid  and  free  hydrogen. 

According  to  the  above  results,  succus  entericus  is  to  be  re- 
garded as  an  important  secretion  acting  on  all  kinds  of  food.  But 
even  at  the  best,  its  actions  are  slow  and  feeble.  Moreover  many 
observers  have  obtained  negative  results,  so  that  the  various  state- 
ments are  conflicting.  Besides,  we  have  no  exact  knowledge  as  to 
the  amount  to  which  such  a  secretion  takes  place  under  normal 
circumstances  in  the  living  body.  We  may  therefore  conclude 
that,  at  present  at  all  events,  we  have  no  satisfactory  reasons  for 
supposing  that  the  actual  digestion  of  food  in  the  intestine  is,  to 
any  great  extent,  aided  by  such  a  juice. 

Of  the  possible  action  of  other  secretions  of  the  alimentary 
canal,  as  of  the  caecum  and  large  intestine,  we  shall  speak  when 
we  come  to  consider  the  changes  in  the  alimentary  canal. 

§  212.  Gallstones.  Concretions,  often  of  considerable  size, 
known  as  gallstones  are  not  unfrequently  formed  in  the  gall 
bladder,  and  smaller  concretions  are  sometimes  formed  in  the  bile 
passages.  In  man  two  kinds  of  gallstones  are  common.  One  kind 
consists  almost  entirely  of  cholesterin,  sometimes  nearly  free  from 
any  admixture  with  pigment,  sometimes  more  or  less  discoloured 
with  pigment.  Gallstones  of  this  kind  have  a  crystalline  structure, 
and  when  broken  or  cut  shew  frequently  radiate  and  concentric 
markings.  The  other  kind  consists  chiefly  of  bilirubin  in  combi- 
nation with  calcium.  Gallstones  of  this  kind  are  dark  coloured 
and  amorphous.  Less  common  than  the  above  are  small  dark 
coloured  stones,  having  often  a  mulberry  shape,  consisting  not  of 
bilirubin  itself,  but  of  one  or  other  derivative  of  bilirubin.  Gall- 
stones consisting  almost  entirely  of  inorganic  salts,  calcic  carbon- 
ates and  phosphates,  are  also  occasionally  met  with.  In  the  lower 
animals,  in  oxen  for  instance,  bilirubin  gallstones  are  not  uncom- 
mon, but  cholesterin  gallstones  are  rare. 

A  gallstone  appears  always  to  contain  a  more  or  less  obvious 
'  nucleus,'  around  which  the  material  of  the  stone  has  been  de- 
posited, and  which  may  be  regarded  as  the  origin  of  the  stone ; 
the  real  cause  of  the  formation  of  the  stone  lies  however  in  certain 
changes  in  the  bile,  by  which  the  cholesterin,  or  bilirubin,  or  other 
constituent  ceases  to  remain  dissolved  in  the  bile.  But  we  cannot 
discuss  this  matter  here. 


SEC.   4.     THE   SECRETION   OF   PANCREATIC   JUICE 
AND   OF   BILE. 


§  213.  The  Secretion  of  Pancreatic  Juice.  Although  m  some 
cases,  as  that  of  the  parotid  of  the  sheep,  the  flow  of  saliva  is 
continuous  or  nearly  so,  in  most  animals,  as  in  man,  the  inter- 
mittence  of  the  secretion  is  very  nearly  absolute.  While  food  is 
in  the  mouth  saliva  Hows  freely,  but  between  meals  only  just 
sufficient  is  secreted  to  keep  the  mouth  moist,  and  probably  the 
greater  part  of  this  is  supplied  not  by  the  larger  salivary  but  by 
the  small  buccal  glands.  The  flow  of  pancreatic  juice,  on  the 
other  hand,  is  much  more  prolonged,  being  in  the  rabbit  continu- 
ous, and  in  the  dog  lasting  for  twenty  hours  after  food.  But  this 
contrast  between  the  secretion  of  saliva  and  that  of  pancreatic 
juice  is  natural,  since  the  stay  of  food  in  tlie  mouth  even  during 
a  protracted  feast  is  relatively  short,  whereas  the  time  during 
which  the  material  of  a  meal  is  able  in  some  way  or  other  to  affect 
the  pancreas  is  very  prolonged. 

The  flow  though  continuous,  or  nearly  so,  is  not  uniform.  In 
the  dog  the  flow  of  pancreatic  juice  begins  immediately  after  food 
has  been  taken,  and  rises  to  a  maximum  which  may  be  reached 
within  the  first,  or  as  in  the  case  furnishing  the  diagram  given  in 
Fig.  83  the  second  hour,  but  which  more  commonly  is  not  readied 
until  the  third  or  fourth  hour.  This  rise  is  then  followed  by  a 
fall,  after  which  there  is  a  secondary  rise,  reaching  a  second  maxi- 
mum at  a  very  variable  time  but  generally  between  the  flfth  and 
seventh  hours.  This  second  nuiximum,  liowever,  is  never  so  high 
as  the  flrst. 

The  second  rise  may  be  due  to  material  absorbed  from  the 
intestines  being  carried  in  the  circulation  to  the  pancreas  and  so 
directly  exciting  the  gland  to  activity,  much  in  the  same  way  as, 
in  the  case  of  the  stomach,  the  absorption  of  digested  material 
promotes  the  flow  of  gastric  juice,  see  §  104  ;  and  a  similar  absorp- 


366 


SECRETION   OF   PANCREATIC   JUICE.     [Book  ii. 


tion  may  coutribute  to  the  first  rise  also,  but  it  is  more  probable 
that  so  marked  and  sudden  a  rise  as  this  is  carried  out  by  some 


2  |3|4|5|6|7|8|9|lO|ll|l2il3|l4ll5,l6|  I  I  2  1  8  |  4  |  5  j  61  7|-8  i  ajlD, 


Fig.  83.     Diagram  illustrating  the  influence  of  Eood  on  the  Secretion 
OF  Pancreatic  Juice.     (N.  0.  Bernstein.) 

The  abscissa;  represent  hours  after  taking  food  ;  the  ordinates  represent  in  c.c. 
the  amount  of  secretion  in  10  min.  A  marked  rise  is  seen  at  B  immediately  after 
food  was  taken,  with  a  secondary  rise  between  the  4th  and  5th  hours  afterwards. 
Where  the  line  is  dotted  the  observation  was  interrupted.  On  food  being  again 
given  at  C,  another  rise  is  seen,  followed  in  turn  by  a  depression  and  a  secondary  rise 
at  the  5th  hour.     A  very  similar  curve  would  represent  the  secretion  of  bile. 


nervous  mechanism.  The  details  of  this  mechanism  have  how- 
ever not  as  yet  been  satisfactorily  worked  out. 

Stimulation  of  the  medulla  oblongata,  or  of  the  spinal  cord, 
will  call  forth  secretion  in  a  quiescent  pancreas,  or  increase  a 
secretion  already  going  on.  On  the  other  hand  a  secretion  already- 
going  on  may  be  arrested  by  stimulation  of  the  central  end  of  the 
vagus,  and  the  stoppage  of  the  secretion  which  has  been  observed 
as  occurring  during  and  after  vomiting  is  probably  brought  about 
in  this  way.  This  effect  however  is  not  confined  to  the  vagus, 
it  occurs  also  after  stimulation  of  other  afferent  nerves,  such 
as  the  sciatic. 

§  214.  The  Secretion  of  Bile.  The  act  of  secretion  of  bile  by 
the  liver  must  not  be  confounded  with  the  discharge  of  bile  from 
the  bile-duct  into  the  duodenum.  When  the  acid  contents  of  the 
stomach  are  poured  over  the  orifice  of  the  biliary  duct,  a  gush  of 
bile  takes  place.  Indeed,  stimulation  of  this  region  of  the  duo- 
denum with  a  dilute  acid  at  once  calls  forth  a  flow,  though 
alkaline  fluids  so  applied  have  little  or  no  effect.     When  no  such 


Chap,  i.j    TISSUES  AND  MECHANISMS  OF  DIGESTION.  367 

acid  tiuid  is  passing  into  the  duodenum  no  bile  is,  under  nurinal 
circumstances,  discharged  into  the  intestine.  The  discharge  is 
due  to  a  contraction  of  the  muscular  walls  of  the  gall-bladder 
and  ducts,  accompanied  by  a  relaxation  of  the  sphincter  of  the 
orifice ;  both  acts  are  probably  of  a  reflex  nature,  but  the  details 
of  the  mechanism  have  not  been  worked  out. 

Tlie  secretion  of  bile  on  the  other  hand,  as  shewn  by  the 
results  of  biliary  fistuLe,  is  continuous ;  it  appears  never  to  cease. 
When  no  food  is  taken  the  bile  passes  from  the  liver  along  the 
hepatic  and  then  back  along  the  cystic  duct  (the  flow  being  aided 
probably  by  peristaltic  contractions  of  the  muscular  fibres  of  the 
duct)  to  tlie  gall-bladder,  where  it  is  temporarily  stored  ;  hence  in 
starving  animals,  when  no  discharge  is  excited  by  food,  the  gall- 
bladder becomes  greatly  distended  with  bile.  But  the  secretion, 
though  continuous,  is  not  uniform.  The  rate  of  secretion  varies, 
and  is  especially  influenced  by  food ;  it  is  seen  to  rise  rapidly  after 
meals,  reaching  its  maximum,  in  dogs,  in  from  four  to  eight  hours. 
There  seems  to  be  an  immediate,  sudden  rise  when  food  is  taken, 
then  a  fall,  followed  subsequently  by  a  more  gradual  rise  up  to 
the  maximum,  and  ending  in  a  final  fall  to  the  lowest  point. 
The  curve  of  secretion,  in  fact,  resembles  that  of  the  secretion  of 
pancreatic  juice  in  having  a  double  rise ;  and  as  in  that  case  so 
in  this,  it  is  very  probable  that  the  first  rise  is  in  part  the  result 
of  nervous  action,  and  it  is  also  possible  that  nervous  influences 
intervene  in  the  second  more  lasting  rise ;  but,  as  we  shall  see 
presently,  even  nervous  influences  may  affect  the  liver  in  a  very 
indirect  manner,  and  our  knowledge  as  to  any  direct  action  of  the 
nervous  system  on  the  liver  is  at  present  very  imperfect. 

§  215.  It  must  be  remembered,  however,  that  the  liver  is  so 
peculiarly  related  to  the  other  organs  of  digestion,  and  its  vascular 
arrangements  so  special  that,  with  regard  to  it,  as  compared  with 
many  other  organs,  an  intrinsic  nervous  mechanism  must  occupy 
a  more  or  less  subordinate  position.  The  blood-supply  of  the 
pancreas  for  instance  is  dependent  chiefly  on  the  width  for  the 
time  being  of  the  pancreatic  arteries ;  it  will  be  affected  of  course 
by  the  general  arterial  pressure  and  by  any  circumstances  which 
affect  the  outflow  by  the  pancreatic  veins,  and  therefore  by  the 
condition  of  the  portal  venous  system  of  which  those  veins  form  a 
part;  but  in  the  main,  the  amount  of  blood  bathing  the  alveoli  i»f 
the  pancreas  will  depend  on  whetlier  the  pancreatic  arteries  are 
constricted  or  dilated.  The  quality  of  the  blood  reaching  tlie 
pancreas,  being  arterial  blood  drawn  direct  from  the  arterial 
foundation,  will  be  modified  only  by  such  circumstances  as  modify 
the  general  mass  of  the  blood. 

Very  different  is  the  case  of  the  liver.  The  supply  of  arterial 
blood  coming  direct  through  the  hepatic  artery  is  small  compared 
with  the  mass  pouring  through  the  vena  porta? ;  it  moreover,  as 
we  shall  see,  is  distributed  in  capillaries  among  the  small  inter- 


368  BLOOD-SUPPLY    OF   LIVER.  [Book  ii. 

lobular  branches  of  the  veua  portai  and  has  become  venous, 
indeed  merged  with  the  portal  blood,  before  it  reaches  the  actual 
lobules.  The  supply  of  blood  for  the  liver  is  mainly  that  through 
the  vena  portee ;  and  this  supply  is  not,  like  an  arterial  supply,  a 
fairly  uniform  one,  modified  chiefly  by  the  vaso-motor  events  of 
the  organ  itself,  but  is  dependent  on  what  happens  to  be  taking 
place  in  the  alimentary  canal  and  in  abdominal  organs  other  than 
the  liver  itself.  When  no  food  is  being  digested  and  the  alimentary 
canal  is  at  rest,  the  vessels  of  that  canal,  as  we  have  already  said  in 
speaking  of  the  stomach,  are  like  those  of  the  pancreas  and  salivary 
glands,  in  a  state  of  tonic  constriction ;  a  relatively  small  quantity  of 
blood  passes  through  them ;  hence  the  flow  through  the  vena  porta; 
is  relatively  inconsiderable,  and  the  pressure  in  that  vessel  is  low. 
When  digestion  is  going  on  all  the  minute  arteries  of  the  stomach, 
intestine,  spleen  and  pancreas  are  dilated,  and  general  arterial 
pressure  being  by  some  means  or  other  maintained  (see  §  172), 
a  relatively  large  quantity  of  blood  rushes  into  the  vena  porta'. 
and  the  pressure  in  that  vessel  becomes  much  increased,  though 
of  course  remaining  lower  than  the  general  arterial  pressure. 
Moreover,  during  digestion,  peristaltic  movements  of  the  muscular 
coats  of  the  alimentary  canal  are,  as  we  have  seen,  active ;  and 
these  movements,  serving  as  aids  to  the  circulation  (see  §  103), 
help  to  increase  the  portal  flow.  Further  the  spleen,  as  we 
shall  see  in  speakino-  of  that  organ,  is  in  many  animals  richly 
provided  with  plain  muscular  fibres,  and  in  such  cases  seems, 
especially  during  digestion,  to  act  as  a  muscular  pump  driving 
the  blood  onwards,  with  increased  vigour,  along  the  splenic  veins 
to  the  liver.  So  that  even  were  the  liver  not  connected  with 
the  central  nervous  system  by  a  single  nervous  tie,  the  tide 
of  blood  through  the  liver  would  ebb  and  flow  according  to  the 
absence  or  presence  of  food  in  the  alimentary  canal. 

An  increase  of  blood-supply  does  not  of  course  necessarily 
mean  an  increase  of  secretory  activity.  As  we  have  seen,  §  189, 
in  the  presence  of  atropine,  the  secretion  of  saliva  may  stand  still 
in  spite  of  dilated  blood  vessels  and  the  consequent  rush  of  blood ; 
but  we  may  safely  assert  that,  other  things  being  equal,  a  fuller 
blood-supply  is  favourable  to  activity.  Apparently  a  mere  change 
in  the  quantity  of  blood  bathing  an  alveolus  will  not  start  in  the 
cells  the  changes  which  constitute  the  act  of  secretion,  any  more 
than  an  increase  in  the  blood  bathing  a  muscular  fibre  will  neces- 
sarily set  going  a  contraction ;  but  unless  there  be  some  counter- 
acting influence  at  work,  a  fuller  and  richer  lymph  around  a  cell 
will  naturally  lead  to  the  cell  taking  up  more  material  from  the 
lymph,  and  so  will  increase  the  cell's  store  of  energy.  Hence, 
especially  in  the  hepatic  cell,  which  appears  to  be  always  at 
work,  always  undergoing  metabolism  of  such  a  kind  as  to  give 
rise  to  bile,  we  might  fairly  expect  the  greater  flow  through  the 
portal  vein  to  quicken  the  flow  through  the  bile  duct. 


Chap,  i.]    TISSUES  AND  MECHANISMS  OF  DIGESTION.  369 

And  as  a  matter  of  fact  we  do  tind  vaso-niotor  action  domi- 
nant over  the  secretion.  In  the  various  experiments  which  have 
been  made  to  ascertain  the  action  of  the  nervous  system  on  the 
secretion  of  bile,  it  has  always  been  found  that  stimulation  of 
the  medulla  oblongata,  or  of  the  spinal  cord,  or  of  the  abdominal 
splanchnic  nerves,  stops  or  at  least  checks  the  tiow  of  bile.  Now 
the  effect  of  these  stimulations  is,  as  we  have  already  seen  more 
than  once,  a  powerful  constricting  action  on  the  abdominal  blood 
vessels ;  by  such  stimulation  the  blood-supply  of  the  liver  is  ma- 
terially diminished,  and  in  consequence  the  secretory  activity  is 
slackened  or  arrested. 

But  there  is  something  besides  the  mere  quantity  of  blood  to 
be  considered  in  this  relation.  The  blood  which  passes  from  the 
alimentary  canal  at  rest  is  ordinary  venous  blood,  laden  simply 
with  carbonic  acid  and  the  ordinary  products  of  the  metabolism 
of  the  muscular  and  mucous  coats  of  the  canal.  When  digestion 
is  goinsf  on  the  portal  blood  is  laden,  as  we  shall  see,  with  some 
at  all  events  of  the  products  of  digestion,  with  sugar  probably 
and  with  various  proteid  bodies.  And  it  is  quite  possible  or  even 
probable  that  some  of  these  bodies  in  the  portal  blood  reaching 
the  hepatic  cells  stir  them  up  to  secretory  activity ;  indeed  this 
view  may  be  regarded  as  supported  by  the  facts  that  proteid 
food  increases  the  quantity  of  bile  secreted,  whereas  fatty  food, 
which  as  we  shall  see  passes,  chiefly  if  not  wholly,  not  by  the 
portal  vein  but  by  the  lymphatics-  and  which  is  probably  largely 
disposed  of  in  some  way  or  other  before  it  can  reach  the  liver,  has 
no  such  effect. 

Hence  wel"  may  infer  that  at  all  events  the  second  increase  of 
the  flow  of  bile  which  occurs  during  the  later  stages  of  digestion 
may  be  to  a  large  extent  the  direct  effect  of  blood,  laden  with 
digestive  products,  passing  from  the  stomach  and  intestines,  espe- 
cially the  latter,  to  the  liver  by  the  portal  vein,  quite  independent 
of  any  direct  nervous  action  on  the  liver  itself ;  and  indeed  it  is 
possible  that  the  first  rise  also  may  be  partly  due  to  the  increased 
flow  of  blood  from  the  stomach,  aided  by  the  absorption  from  that 
organ  of  a  certain  amount  of  digested  material.  Since,  however, 
there  is  no  evidence  of  any  decrease  in  blood-supply,  or  in  the  rate 
of  absorption,  corresponding  to  the  fall  between  the  two  rises, 
some  influences  other  than  those  which  we  are  discussing  must 
be  at  work  in  the  matter. 

§  216.  It  is  interesting  to  observe  that  the  pressure  under 
which  the  bile  is  secreted  is  relatively  low,  not  higli  like  that  of 
the  saliva  ;  it  is  much  lower  than  the  arterial  pressure  in  the  same 
animal,  whereas  in  the  case  of  saliva  (§  189)  the  pressure  is  greater 
than  the  blood-pressure  in  the  carotid  artery.  But,  in  the  case 
of  bile,  since  the  blood  which  flows  through  the  hepatic  lobules 
is,  mainly,  venous  portal  blood,  we  have  to  compare  the  pressure 
of  the  secretion  not  with   arterial  pressure  Init  with  the  venous 

'21 


370  BLOOD-SUPPLY    OF   LIVER.  [Book  ii. 

pressure  in  the  portal  system ;  and  in  the  dog  it  has  been  found 
that  while  the  pressure  of  the  bile  secreted  stood  at  about  200  mm. 
of  a  solution  of  sodium  carbonate,  that  is,  about  15  mm.  mer- 
cury, the  blood-pressure  in  a  branch  of  the  superior  mesenteric 
vein  stood  only  at  about  90  mm.  of  the  same  solution,  that  is, 
about  7  mm.  mercury.  Now  the  venous  pressure  in  the  mesen- 
teric veins  is  higher,  though  only  slightly  higher,  than  that  in  the 
portal  vein  into  which  these  pour  their  blood  (the  difference  of 
pressure  being  the  main  cause  why  the  blood  flows  from  the  one 
into  the  other),  and  is  therefore  certainly  higher  than  the  pres- 
sure in  tlie  portal  capillaries  of  the  hepatic  lobules.  So  that  what 
is  true  of  the  salivary  gland  is  also  true,  on  a  different  scale,  of 
the  liver,  viz.  that  the  pressure  exerted  by  the  secretion  is  higher 
than  the  pressure  of  the  blood  in  the  vessels  feeding  the  secreting 
cells. 

§  217.  If  the  pressure  in  the  bile  duct  be  artificiall}^  increased, 
as  by  pouring  fluid  into  the  glass  tube  or  manometer  with  which 
the  cannula  in  the  duct  is  connected,  a  resorption  of  the  secreted 
bile  takes  place ;  and  resorption  will  also  take  place  within  the 
body,  when  the  pressure  generated  by  the  act  of  secretion  itself 
reaches  and  is  maintained  at  a  sufiiciently  high  level.  Thus 
when  in  the  living  body  the  bile  duct  is  ligatured,  or  becomes 
obstructed  by  gallstones  or  otherwise,  fluid  is  accumulated  on  the 
near  side  of  the  ligature  at  a  pressure  which  goes  on  increasing 
until  resorption  of  bile  takes  place,  bile  salts  and  biliary  pigments 
are  thrown  back  upon  the  system,  and  "jaundice"  results.  It 
would  appear  that  in  these  cases  resorption  takes  place  through 
the  interlobular  bile  ducts  and  not  through  the  hematic  cells  or 
other  structures  within  the  lobules.  The  high  pressure  in  the 
ducts  does  not  lead  to  a  reversal  of  the  current  in  the  hepatic 
cells  (at  most  it  slackens  or  possibly  stops  the  current)  but  the 
bile  secreted  into  the  interlobular  ducts  escapes  from  these.  It 
further  appears  that  the  escape  is  not  into  the  blood  vessels  but 
into  the  lymphatics ;  the  bile  salts,  pigments  and  other  constitu- 
ents are  carried  into  the  thoracic  duct,  and  in  an  indirect  manner 
only  find  their  way  into  the  blood  stream. 

To  complete  the  history  of  the  secretion  of  bile  we  ought  now 
to  turn  to  the  manufacture  of  the  biliary  constituents  within  the 
cells.  But  since  the  hepatic  cells  are  also  engaged  in  labours 
other  and  more  important  perhaps  than  that  of  secreting  bile,  it 
will  be  convenient  to  defer  what  we  have  to  say  on  this  point 
until  we  come  to  speak  of  the  formation  of  glycogen  and  of  the 
general  metabolic  events  taking  place  in  the  liver. 


SEC.  5.   thp:  muscular  mechanisms  of  digestion, 


§  218.  From  its  entrance  into  the  mouth  until  such  remnant 
of  it  as  is  undigested  leaves  the  body,  the  food  is  continually 
subjected  to  movements  having  for  their  object  the  trituration  of 
the  food  as  in  mastication,  or  its  more  complete  mixture  with  the 
digestive  juices,  or  its  forward  progress  through  the  alimentary 
canal. 

Peristaltic  Movements.  The  dominant  movement  in  the  ali- 
mentary canal  is  of  the  kind  called  peristaltic,  carried  out  by 
means  of  the  circular  and  longitudinal  muscular  coats.  This  is 
seen  in  its  simplest  form  in  the  small  intestine,  is  somewhat  modi- 
fied in  other  parts  as  in  the  stomach,  and  at  the  beginning  and  end 
of  the  canal  is  replaced  or  assisted  by  complicated  movements 
carried  out  by  various  muscles. 

The  main  part  of  a  peristaltic  movement,  as  seen  in  the  small 
intestine,  is  a  wave  of  contraction  progressing  longitudinally  over 
the  circular  coat  (§  84).  A  contraction  of  the  circular  coat  takes 
place  at  some  level  or  other,  narrowing  the  intestine  at  this  level. 
From  thence,  the  circularly  disposed  bundles  contracting  in 
sequence,  the  contraction  travels  as  a  wave  downwards  or  up- 
wards or  both  downwards  and  upwards.  As  a  rule  the  wave, 
when  started  naturally,  travels  downwards  from  a  part  nearer  the 
mouth  to  a  part  nearer  the  rectum.  Thus  a  narrowing  or  con- 
stricticm  of  the  tube  travels  onwards  as  a  wave  driving  the  contents 
of  the  tube  before  it;  wlien  a  luitcher  empties  the  contents  of  the 
intestine  of  a  slaughtered  animal  by  squeezing  it  high  up  with  his 
,  hand  or  his  thumb  or  forefinger,  and  then  carrying  the  squeezing 
action  downwards  along  the  length  of  the  intestine,  he  makes  the 
1  passive  intestine  do  very  much  what  the  circular  coat  does, 
actively,  by  contraction,  in  the  living  animal. 

This  action  of  the  circular  coat  is  further  aided  l)y  a  corre- 
sponding contraction  of  the  longitudinal  coat.  When  a  length 
of  the  longitudinal  coat  is  thrown  into  contraction,  that  length  of 
the  tube  is  shortened  and  widened ;  the  effect  is  the  antagonist 
of  that  produced  by  the  contraction  of  the  circular  coat.     Hence 


372  DEGLUTITION.  [Book  ii. 

the  two  coats  must  contract  at  different  times,  otherwise  they 
would  neutralise  each  other's  action.  Most  probably  a  section  of 
the  longitudinal  coat  contracts  in  front  of  the  section  of  the  cir- 
cular coat  which  is  about  to  contract,  thus  affording  room  for  the 
contents  which  are  about  to  be  driven  on,  or  even  itself  drawing 
them  forward ;  but  a  contraction  of  the  longitudinal  coat,  even  if 
it  followed  after  that  of  the  circular  coat,  might  still  be  useful  in 
helping  to  bring  back  the  tube  to  its  normal  width. 

In  the  small  intestine  the  tube  is  hung  loosely  and  much 
twisted  so  that  many  loops  are  formed ;  the  contents  moreover  are 
largely  fluid.  Hence  the  steady  onward  movement,  such  as  is  seen 
when  more  solid  contents  pass  along  the  straight  and  somewhat 
firmly  attached  oesophagus,  is  complicated  by  movements  due  to  a 
loop  being  projected  forward  by  the  entrance  of  fluid  from  above, 
or  being  dragged  down  by  the  weight  of  its  new  contents,  or,  on 
the  other  hand,  due  to  a  loop  being  retracted  by  the  driving  on- 
ward of  its  contents  and  the  emptying  of  itself,  and  the  like.  In 
tliis  way  a  peculiar  writhing  movement  of  the  bowel  is  brought 
about,  and  the  phrase  '  peristaltic  movement '  is  generally  used  to 
denote  this  total  effect  of  the  contraction  of  the  muscular  coats  ;  it 
will  however  be  best  to  restrict  the  meaning  to  the  progressive 
contraction  of  the  circular  coat  assisted,  in  most  cases,  by  a  similar 
progressive  contraction  of  the  longitudinal  coat.  We  may  con- 
sider the  several  special  movements  of  the  different  parts  of  the 
canal. 

3fastication.  This  in  man  consists  chiefly  of  an  up  and  down 
movement  of  the  lower  jaw,  combined,  in  the  grinding  action  of 
the  molar  teeth,  with  a  certain  amount  of  lateral  and  fore-and-aft 
movement.  The  lower  jaw  is  raised  by  means  of  the  temporal, 
masseter,  and  internal  pterygoid  muscles.  The  slighter  efibrt  of 
depression  brings  into  action  chiefly  the  digastric  muscle,  though 
the  mylohyoid  and  geniohyoid  probably  share  in  the  matter. 
Contraction  of  the  external  pterygoids  pulls  forward  the  condyles, 
and  thrusts  the  lower  teeth  in  front  of  the  upper.  Contraction 
of  the  pterygoids  on  one  side  will  also  throw  the  teeth  on  to  the 
opposite  side.  The  lower  horizontally  placed  fibres  of  the  tempo- 
ral serve  to  retract  the  jaw. 

During  mastication  the  food  is  moved  to  and  fro,  and  rolled 
about  by  the  movements  of  the  tongue.  These  are  effected  by  the 
muscles  of  that  organ  governed  by  the  hypoglossal  nerve. 

The  act  of  mastication  is  a  voluntary  one,  guided,  as  are  so 
many  voluntary  acts,  not  only  by  muscular  sense  but  also  by  con- 
tact sensations.  The  motor  fibres  of  the  fifth  cranial  nerve  convey 
motor  impulses  from  the  brain  to  the  above-mentioned  muscles ; 
but  paralysis  of  the  sensory  fibres  of  the  same  nerve  renders 
mastication  difticult  by  depriving  the  will  of  the  aid  of  the  usual 
sensations. 

§  219.     Deglutition.     The  food  when  sufficiently  masticated  is. 


Chap,  i.]    TISSUES  AND  MECHANISMS  OF  DIGESTION.  873 

by  the  movements  of  the  tongue,  gathered  up  into  a  bolus  on  the 
middle  of  the  upper  surface  of  that  organ.  The  front  of  the 
tongue  being  raised  —  i)artly  by  its  intrinsic  muscles,  and  partly 
by  the  styloglossus  —  the  bolus  is  thrust  back  Ijetween  the  tongue 
and  the  palate  through  the  anterior  pillars  of  the  fauces  or  isth- 
mus faucium.  Immediately  before  it  arrives  there,  the  soft  palate 
is  raised  by  the  levator  palati,  and  so  brought  to  touch  the  poste- 
rior wall  of  the  pharynx,  whicli,  by  the  contraction  of  the  upper 
margin  of  the  superior  constrictor  of  the  pharynx,  bulges  some- 
what forward.  The  elevation  of  the  soft  palate  causes  a  distinct 
rise  of  pressure  in  the  nasal  chambers ;  this  can  be  shewn  by  in- 
troducing a  water  manometer  into  one  nostril,  and  closing  the  other 
just  previous  to  swallowing.  By  the  contraction  of  the  palato- 
pharyngeal muscles  which  lie  in  the  posterior  pillars  of  the  faucas, 
the  curved  edges  of  those  pillars  are  made  straight,  and  thus  tend 
to  meet  in  the  middle  line,  the  small  gap  between  them  being 
tilled  up  by  the  uvula.  Through  these  manceuvres,  the  entrance 
into  the  posterior  nares  is  blocked,  while  the  soft  palate  is  formed 
into  a  sloping  roof,  guiding  the  bolus  down  the  pharynx.  By 
the  contraction  of  the  stylo-pharyngeus  and  palato-pharyugeus, 
the  funnel-shaped  bag  of  the  pharynx  is  brought  up  to  meet  the 
descending  morsel,  very  miich  as  a  glove  may  be  drawn  up  over 
the  finger. 

Meanwhile  in  the  larynx,  as  shewn  by  the  laryngoscope,  the 
arytenoid  cartilages  and  vocal  cords  are  approximated,  the  latter 
being  also  raised  so  that  they  come  very  near  to  the  false  vocal 
cords  ;  and  the  cushion  at  the  base  of  the  epiglottis  covers  the  rima 
glottidis,  while  the  epiglottis  itself  is  depressed  over  the  larynx. 
The  thyroid  cartilage  is  now,  by  the  action  of  tlie  laryngeal  muscles, 
suddenly  raised  up  behind  the  hyoid  bone,  and  thus  assists  the 
epiglottis  to  cover  the  glottis.  This  movement  of  the  thyroid  can 
easily  be  felt  on  the  outside.  Thus,  both  the  entrance  into  the 
posterior  nares  and  that  into  the  larjmx  being  closed,  the  impulse 
given  to  the  bolus  by  the  tongue  can  have  no  other  effect  than  to 
propel  it  beneath  the  sloping  soft  palate,  over  the  incline  formed 
by  the  root  of  the  tongue  and  the  epiglottis.  The  palato-glossi 
or  constrictores  isthmi  faucium,  which  lie  in  the  anterior  pillars 
of  the  fauces,  by  contracting,  close  the  door  behind  the  food  which 
has  passed  them. 

When  the  bolus  of  food  is  large,  it  is  received  by  the  middle 
and  lower  constrictors  of  the  pharynx,  which,  contracting  in 
sequence  from  above  downwards,  thrust  it  into  the  oesophagus, 
along  which  it  is  driven  by  a  similar  series  of  successive  contrac- 
tions, that  is  to  say,  by  peristaltic  action.  This  comparatively 
slow  descent  of  the  food  from  the  pharynx  into  the  stomach,  may 
be  readily  seen  if  animals  with  long  necks  such  as  horses  and 
dogs  be  watched  while  swallowing.  When  however  the  morsel 
is  not  large  or  when  the  substance  swallowed  is  liquid,  the  move- 


374  DEGLUTITION.  [Book  ii. 

nient  of  the  back  part  of  the  tongue  may  be  sufficient  not  merely 
to  introduce  the  food  into  the  grasp  of  the  constrictors  of  the 
pharynx,  but  even  to  propel  it  rapidly,  to  shoot  it  in  fact,  along 
the  lax  ffisophagus  before  the  muscles  of  that  organ  have  time  to 
contract.  In  such  a  mode  of  swallowing  the  middle  and  lower 
constrictors  take  little  or  no  part  in  driving  the  food  onward, 
though  they  and  the  oesophagus  appear  to  contract  from  above 
downwards  after  the  food  has  passed  by  them,  as  if  to  complete 
the  act  and  to  ensure  that  nothing  has  been  left  behind.  Deglu- 
tition in  this  fashion  still  remains  possible  after  these  constrictors 
have  become  paralysed  by  section  of  their  motor  nerves. 

When  a  second  act  of  deglutition  succeeds  a  first  with  sufficient 
rapidity,  the  nervous  changes  which  start  the  pharyngeal  move- 
ments of  the  second  act  appear  to  inhibit  the  oesophageal  move- 
ments of  the  first  act ;  and  when  swallowing  is  repeated  rapidly 
several  times  in  succession,  the  oesophagus  remains  quiet  and  lax 
during  the  whole  time,  until  immechately  after  the  last  swallow, 
when  a  peristaltic  movement  closes  the  series. 

When  the  stethoscope  is  applied  over  the  oesophagus,  at  differ- 
ent regions,  a  sound  is  heard  during  deglutition ;  sometimes  two 
sounds  are  heard.  The  first  and  most  constant  is  coincident  with 
the  passage  of  the  bolus,  and  is  due  to  this  and  to  the  muscular 
sound  of  the  contracting  muscles.  The  later  and  less  constant 
sound  appears  to  be  caused  by  a  quantity  of  air-bubbles  with 
which  the  bolus  was  entangled,  lodged  at  the  cardiac  end  of  the 
oesophagus,  being  forced  into  the  stomach  by  the  sequent  peris- 
taltic contraction  of  the  oesophagus. 

It  will  be  seen,  from  what  has  been  said,  that  deglutition, 
though  a  continuous  act,  may  be  regarded  as  divided  into  three 
staoes.  The  first  stage  is  the  thrusting  of  the  food  through  the 
isthmus  faucium ;  this  may  be  either  of  long  or  short  duration. 
The  second  stage  is  the  passage  through  the  upper  part  of  the 
pharynx.  Here  the  food  traverses  a  region  common  both  to  the 
food  and  to  respiration,  and  in  consequence  the  movement  is  as 
rapid  as  possible.  The  third  stage  is  the  descent  through  the 
grasp  of  the  constrictors.  Here  the  food  has  passed  the  respira- 
tory orifice,  and  in  consequence  its  passage  again  becomes  compar- 
atively slow,  except  in  case  of  fluids  and  small  morsels,  when,  as 
we  have  seen,  it  may  continue  to  be  rapid.  The  passage  along 
the  oesophagus  may  perhaps  be  regarded  as  constituting  a  fourth 
stage ;  but  it  will  be  more  convenient  to  consider  the  oesophageal 
movements  by  themselves. 

The  first  stage  in  this  complicated  process  is  undoubtedly  a 
voluntary  act.  The  raising  of  the  soft  palate  and  the  approxi- 
mation of  the  posterior  pillars  may  also  be,  at  times,  voluntary, 
since  they  have  been  seen,  in  a  case  where  the  pharynx  was  laid 
bare  by  an  operation,  to  take  place  before  the  food  had  touched 
these  parts ;  but  the  movement  may  take  place  without  any  exer- 


Chap.  i.I    TISSUES  AND  MECHANISMS  OF  DIGESTION.  375 

cise  of  the  will  and  in  the  absence  of  consciousness.  Indeed  the 
second  stage  taken  as  a  whole,  though  some  of  the  earlier  com- 
ponent movements  are,  as  it  were,  on  the  borderland  between  the 
voluntary  and  involuntary  kingdoms,  must  be  regarded  as  a  reflex 
act.  The  third  and  last  stage,  whatever  be  the  exact  form  which 
it  takes,  is  undoubtedly  reflex ;  the  will  has  no  power  whatever 
over  it,  and  can  neither  originate,  stop,  nor  modify  it. 

Deglutition  in  fact  as  a  whole  is  a  reflex  act ;  it  cannot  take 
place  unless  some  stimulus  be  applied  to  the  mucous  membrane  of 
the  fauces.  When  we  voluntarily  bring  about  swallowing  move- 
ments with  the  mouth  empty,  we  supply  the  necessary  stimulus 
by  forcing  with  the  tongue  a  small  quantity  of  saliva  into  the 
fauces,  or  by  touching  the  fauces  with  the  tongue  itself. 

In  the  reflex  act  of  deglutition,  caused  in  the  ordinary  way  by 
the  food  coming  in  contact  with  the  fauces,  the  att'erent  impulses 
originated  in  the  fauces  are  carried  up  to  the  nervous  centre  by 
the  glosso-pharyngeal  nerve,  by  branches  of  the  fifth,  and  by  the 
pharyngeal  branches  of  the  superior  laryngeal  division  of  the 
vagus.  The  latter  seem  of  special  importance,  since  the  act  of 
swallowing,  quite  apart  from  the  presence  of  food  in  the  mouth, 
may  be  brought  out  by  centripetal  stimulation  of  the  superior 
laryngeal  nerve.  The  efferent  impulses  descend  the  hypoglossal 
to  the  muscles  of  the  tongue,  and  pass  down  the  glosso-pharyngeal, 
the  vagus  through  the  pharyngeal  plexus,  the  fifth,  and  the  spinal 
accessory,  to  the  muscles  of  the  fauces  and  pharynx :  their  exact 
paths  being  as  yet  not  fully  known,  and  probably  varying  in  differ- 
ent animals.  The  laryngeal  muscles  are  governed  by  the  laryngeal 
branches  of  the  vagus. 

The  centre  of  the  reflex  act  lies  in  the  medulla  oblongata. 
Deglutition  can  be  excited,  by  tickling  the  fauces,  in  an  animal 
Tendered  unconscious  by  removal  of  the  brain,  pro^dded  the 
medulla  be  left.  If  the  medulla  be  destroyed,  deglutition  is 
impossible.  The  centre  for  deglutition  lies  higher  up  than  that 
of  respiration,  so  that  in  diseases  or  injuries  involving  the  upper 
part  of  the  medulla  oblongata  the  former  act  may  be  impaired  or 
rendered  impossible  while  the  latter  remains  untouched.  It  has 
been  said  to  form  part  of  the  superior  olivary  bodies,  but  this  view 
is  based  on  anatomical  grounds  only.  We  shall  have  to  deal  with 
this  and  similar  matters  in  treating  of  the  central  nervous  system. 
It  is  probable  that,  as  is  the  case  in  so  many  other  reflex  acts,  the 
whole  movement  can  be  called  forth  by  stimuli  affecting  the  centre 
directly,  and  not  acting  on  the  usual  afferent  nerves. 

§  220.  Movements  of  the  CEsopharjus.  These  as  we  have  just 
said  are  fairly  simple.  The  circular  contraction  begun  by  the 
constrictors  of  the  pharynx  is  continued  along  the  circular  coat  of 
the  oesophagus  and  assisted  by  an  accompanying  contraction  of  the 
longitudinal  coat,  the  direction  being  always,  save  in  the  abnormal 
action  of  vomiting,  from  above  downwards. 


376  MOVEMENTS    OF   OESOPHAGUS.  [Book  n. 

It  will  be  remembered  that  the  muscular  bundles  of  the  oeso- 
phagus are  composed  of  striated  fibres  in  the  upper  part,  and  of  plain 
tinstriated  fibre-cells  in  the  lower  part,  the  transition  occupying  a 
different  level  in  different  animals.  Nevertheless,  as  far  as  the 
peristaltic  movement  is  concerned,  the  two  kinds  of  fibres  behave 
in  the  same  way  except  that  the  peristaltic  wave  if  we  may  so 
call  it  travels  more  rapidly  in  the  striated  region. 

These  peristaltic  movements  of  the  oesophagus  may,  like  those 
of  the  intestine,  be  seen  after  removal  of  the  organ  from  the  body ; 
and  indeed  may  continue  to  appear  upon  stimulation,  for  an 
unusual  length  of  time.  They  may  therefore  be  carried  out  by 
the  muscular  elements,  with  or  without  the  help  of  the  nervous 
elements  embedded  in  them,  apart  from  any  action  of  the  central 
nervous  system.  Nevertheless,  in  the  living  body,  the  movements 
of  the  oesophagus  seem  to  be  in  a  special  way  dependent  on  the 
central  nervous  system ;  the  contractions  are  not  started  and 
carried  out  by  the  walls  of  the  tube  alone  and  so  transmitted  from 
section  to  section  in  the  walls  of  the  tube  itself ;  but  afferent  im- 
pulses started  in  the  pharynx  and  passing  to  the  medulla  oblongata, 
give  rise  to  reflex  efferent  impulses  which  descend  along  nervous 
tracts  to  successive  portions  of  the  organ.  If  the  oesophagus  be 
cut  across  some  way  down,  or  if  a  portion  of  the  middle  region  be 
excised,  stimulation  of  the  pharynx  wdll  produce  a  peristaltic  con- 
traction, which  travelling  downwards  will  not  stop  at  the  cut  or 
excision  but  will  be  continued  on  into  the  lower  disconnected 
portion  by  means  of  the  central  nervous  system.  And  it  is  stated 
that  ordinary  peristaltic  contractions  of  the  lower  part  of  the 
oesophagus  can  be  readily  excited  by  stimulation  of  the  pharynx, 
but  not  by  stimuli  applied  to  its  own  mucous  membrane.  In  the 
reflex  act  which  thus  brings  about  the  peristaltic  contraction  of 
the  oesophagus  the  afferent  nerves  are  those  of  the  pharynx,  viz.  the 
superior  laryngeal  nerve  and  pharyngeal  branches  of  the  vagus, 
branches  of  the  fifth,  and  in  some  animals  at  least  branches  of  the 
glossopharyngeal,  but  chiefly  the  first ;  and  oesophageal  movements 
can  easily  be  excited  by  centripetal  stimulation  of  the  superior 
laryngeal.  The  centre  lies  in  the  medulla  oblongata,  being  a  part 
of  the  general  deglutition  centre ;  and  the  efferent  impulses  pass 
along  fibres  of  the  vagus,  reaching  the  upper  part  of  the  oesophagus 
by  the  recurrent  laryngeal  nerves  and  the  lower  part  through  the 
oesophageal  plexuses  of  the  vagus  (Fig.  84).  Section  of  the  trunk 
of  the  vagus  renders  difficult  the  passage  of  food  along  the  oeso- 
phagus, and  stimulation  of  the  peripheral  stump  causes  oesophageal 
contractions. 

The  force  of  this  movement  in  the  oesophagus  is  considerable ; 
thus  in  the  dog  a  ball  pulling  by  means  of  a  pulley  against  a  weight 
of  250  grammes  has  been  found  to  be  readily  carried  down  from 
the  pharynx  to  the  stomach. 

At  the  junction  of  the  oesophagus  with  the  stomach  the  circular 


Chap,  i.]    TISSUES  AND  MECHANISMS  OF  DIGESTION.   377 

tibres  usually  remain  iu  a  more  or  less  permanent  condition  of 
tonic  or  obscurely  rhythmic  contraction,  more  particularly  when 
the  stomach  is  full  of  food,  and  thus  serve  as  a  sphincter  to  pre- 
vent tlie  return  of  food  from  the  stomach  into  the  oesophagus. 
Upon  the  arrival  of  the  bolus  of  food  at  the  end  of  the  oesophagus, 
the  centre  for  this  sphincter  is  inhibited  and  the  orifice  is  thus 
opened  up.  Possibly  the  patency  of  the  orifice  is  still  further 
secured  by  a  contraction  of  the  longitudmal  muscular  fibres  Avhieh 
radiate  from  the  end  of  the  oesophagus  over  the  stomach. 

§  221.  Movements  of  the  Stomach.  While  the  oljject  of  the 
oesophageal  movement  is  simply  to  carry  the  swallowed  bolus  with 
all  due  speed  to  the  stomach,  and  while  the  intestinal  movement 
has,  in  like  manner,  simply  to  carry  the  intestinal  contents 
onward,  the  twisted  course  of  the  looped  path  ensuring  all  the 
mixing  of  the  constituents  of  the  contents  that  may  be  necessary, 
the  movements  of  the  stomach  have  a  double  object :  on  the  one 
hand  to  provide  an  adequate  exposure  of  the  contents  of  the 
dilated  chamber  to  the  influence  of  the  gastric  juice,  and  on  the 
other  to  propel  the  partially  digested  food,  when  ready,  into  the 
duodenum.  We  may  accordingly  distinguish  between  what  we 
may  call  the  "  churning  "  and  the  "  propulsive  "  movements  of  the 
stomach. 

When  the  stomach  is  empty  all  the  muscular  fibres  as  we  have 
said,  longitudinal,  circular  and  oblique,  fall  into  a  condition  which 
we  may  perhaps  speak  of  as  an  obscure  tonic  contraction.  The 
whole  stomach  is  small  and  contracted,  its  cavity  is  nearly  obli- 
terated, and  the  mucous  membrane,  owing  to  the  predominance 
of  the  circular  coat,  is  like  the  lining  membrane  of  an  empty  artery, 
thrown  into  longitudinal  folds.  As  more  and  more  food  enters 
the  stomach  all  the  coats  become  relaxed,  with  the  exception  of 
tlie  pyloric  sphincter,  which  remains  at  first  permanently  closed, 
and  the  less  marked  cardiac  sphincter,  which  merely  relaxes  from 
time  to  time  at  each  act  of  swallowing.  No  sooner  however  do 
the  coats  thus  become  relaxed  than  they  set  up  obscure  rhythmical 
peristaltic  contractions,  giving  rise  to  the  "  churning  "  movements. 
These  movements  have  been  described  as  of  such  a  kind  that 
the  contents  flow  in  a  main  current  from  the  cardia  along  the 
greater  curvature  to  the  pylorus,  and  back  to  the  cardia  along 
the  lesser  curvature,  subsidiary  currents  mixing  the  peripheral 
portions  of  the  contents  with  the  more  central ;  it  may  be  doubted 
however  whether  any  such  regularity  of  flow  is  marked  or  constant, 
and  it  is  not  easy  to  see  by  what  combination  and  sequence  of 
contractions  in  the  three  coats,  longitudinal,  circular  and  oblique, 
such  a  regular  flow  can  be  producetl.  But  in  any  case,  by  such 
rhythmical  contractions  the  food  and  gastric  juice  are  rolled  about 
and  mixed  together.  These  churning  movements  are  feeble  at 
first,  even  though  the  stomach  be  filled  and  distended  by  a  large 
meal  rapidly  eaten ;  they  become  more  and  more  pronounced  as 
digestion  proceeds. 


378  VOMITING.  [Book  ii. 

Before  digestion  has  proceeded  very  far  the  '  propulsive ' 
movements  begin.  These  occur  at  intervals,  and  are  repeated  at 
first  slowly  but  afterwards  more  rapidly.  Each  movement  consists 
in  a  contraction  of  the  circular  muscular  fibres  more  powerful  than 
any  taking  part  in  the  churning  movements,  and  leading  to  a  circu- 
lar constriction  which,  beginning  apparently  at  about  the  obscurely 
defined  groove  which  marks  the  beginning  of  the  antrum  pylori, 
travels  down  towards  the  pylorus,  propelling  the  food  onward. 
This  movement  is  accompanied  or  rather  preceded  by  a  relaxation 
of,  that  is  to  say  in  all  probability  an  inhibition  of  the  permanent 
contraction  of,  the  sphincter  pylori  itself,  in  order  that  the  gastric 
contents  may  pass  into  the  duodenum.  But  the  occurrence  of 
this  relaxation  is  determined  by  the  nature  of  the  gastric  con- 
tents ;  for  if  the  propulsive  movement  drives  large  undigested 
pieces  towards  the  pylorus,  the  sphincter  is  apt  to  close  again,  the 
result  of  which  is  that  the  undigested  morsels  are  carried  back 
into  the  main  body  of  the  stomach. 

The  combined  effect  then  of  the  churning  and  of  the  propulsive 
movements  is,  after  a  certain  part  of  the  meal  has  been  reduced  to 
a  thick  fluid  condition  somewhat  resembling  pea  soup  and  often 
called  chyme,  to  strain  of!  this  more  fluid  part  into  the  duodenum, 
and  to  submit  the  remaining  still  solid  pieces  to  the  further  action 
of  the  gastric  juice. 

As  digestion  proceeds,  more  and  more  material  leaves  the 
stomach,  which  is  thus  gradually  emptied,  the  last  portions  which 
are  carried  through  being  those  parts  of  the  food  which  are  least 
digestible,  and  any  wholly  indigestible  foreign  bodies  which  happen 
to  have  been  swallowed ;  the  latter  may  perhaps  never  leave  the 
stomach  at  all.  The  presence  of  food  leads  to  the  development  of 
the  movements ;  but  evidently  it  is  not  the  mere  mechanical 
repletion  of  the  organ  which  is  the  cause  of  the  movements,  since 
the  stomach  is  fullest  at  the  beginning  when  the  movements  are 
slight,  and  becomes  emptier  as  they  grow  more  forcible.  The 
one  thing  which  does  increase  p^""?^  passu  with  the  movements 
is  the  acidity,  which  is  at  a  minimum  when  the  (generally  alka- 
line) food  has  been  swallowed,  and  increases  steadily  onwards. 
It  has  not  however  been  definitely  shewn  that  the  increasing 
acidity  is  the  efficient  stimulus,  giving  rise  to  the  movements. 

The  movements  of  even  a  full  stomach  are  said  to  cease  during 
sleep.  The  nervous  mechanism  of  the  gastric  movements  had 
better  be  considered  in  connection  with  that  of  the  intestinal 
movements. 

§  222.  Vomiting.  In  a  conscious  individual  this  act  is  preceded 
by  feelings  of  nausea,  during  which  a  copious  flow  of  saliva  into  the 
mouth  takes  place.  This  being  swallowed  carries  down  with  it  a 
certain  quantity  of  air,  the  presence  of  which  in  the  stomach, 
by  assisting  in  tlie  opening  of  the  cardiac  sphincter,  subsequently 
facilitates  the  discharge  of  the  gastric  contents.     The  nausea  is 


Chap,  i.j    TISSUES  AND  MPXHANISMS  OF  DIGESTION.  ^79 

generally  succeeded  at  first  by  ineffectual  retching  in  which  a  deep 
inspiratory  effort  is  made,  so  that  the  diaphragm  is  thrust  down 
as  low  as  possible  against  the  stomach,  the  lower  ribs  being  at 
the  same  time  forcibly  drawn  in  ;  since  during  this  inspiratory 
effort  the  glottis  is  kept  closed,  no  air  can  enter  into  the  lungs  ; 
but  some  is  drawn  into  the  pharynx,  and  thence  probably  descends 
by  a  swallowing  action  into  the  stomach.  When  retching  passes 
on  to  actual  vomiting  this  mspiratory  effort  is  succeeded  by  a 
sudden  violent  expiratory  contraction  of  the  abdominal  walls,  the 
glottis  still  being  closed,  so  that  the  whole  force  of  the  effort  is 
spent,  as  we  shall  see  it  is  in  defiecatiou,  in  pressure  on  the 
abdominal  contents.  The  stomach  is  therefore  forcibly  compressed 
from  without.  At  the  same  time,  or  ratlier  immediately  before 
the  expiratory  effort,  by  a  contraction  of  its  longitudinal  fibres 
the  oesophagus  is  shortened  and  the  cardiac  orifice  of  the  stomach 
brought  close  under  tlie  diaphragm,  while  apparently  by  an 
inhibition  of  the  circular  sphincter,  aided  perhaps  by  a  contraction 
of  the  fibres  which  radiate  from  the  end  of  the  oesophagus  over 
the  stomach,  the  cardiac  orifice,  which  is  normally  closed,  is 
somewhat  suddenly  dilated.  This  dilation  opens  a  way  for  the 
contents  of  the  stomach,  which,  pressed  upon  by  the  contraction 
of  the  abdomen,  and  to  a  certain  but  probably  only  to  a  slight 
extent  by  the  contraction  of  the  gastric  walls,  are  driven  forcibl\" 
up  the  o3sophagus.  The  mouth  being  widely  open,  and  the  neck 
stretched  to  afford  as  straight  a  course  as  possible,  the  vomit  is 
ejected  from  the  body.  At  this  moment  there  is  an  additional 
expiratory  effort  which  serves  to  prevent  the  vomit  passing  into 
the  larynx.  In  most  cases  too  the  posterior  pillars  of  the  fauces 
are  approximated,  in  order  to  close  the  nasal  passage  against  the 
ascending  stream.  This  however  in  severe  vomiting  is  frequently 
ineffectual. 

Thus  in  vomiting  there  are  two  distinct  acts ;  the  dilation  of 
the  cardiac  orifice  and  the  extrinsic  pressure  of  the  abdominal 
walls  in  an  expiratory  effort.  Without  the  former  the  latter,  even 
when  distressingly  vigorous,  is  ineffectual.  Without  the  latter,  as 
in  urari  poisoning,  the  intrinsic  movements  of  the  stomach  itself 
are  rarely  sufficient  to  do  more  than  eject  gas,  and,  it  may  be,  a 
very  small  quantity  of  food  or  fluid.  Pyrosis  or  waterbrash  is 
however  probably  brought  about  by  this  intrinsic  action  of  the 
stomach. 

During  vomiting  the  pylorus  is  generally  closed,  so  tliat  but 
little  material  escapes  into  the  duodenum.  AVhen  the  gall-bladder 
is  full,  a  copious  How  of  bile  into  the  duodenum  accompanies  the 
act  of  vomiting.  Part  of  this  may  find  its  way  into  the  stomach, 
as  in  bilious  vomiting,  the  pylorus  then  having  evidently  been 
opened. 

The  nervous  mechanism  of  vomiting  is  complicated  and  in 
many  aspects  obscure.     The  efferent  impulses  which  cause  the 


380      MOVEMENTS  OF  THE  SMALL  INTESTINE.     [Book  ii. 

expiratory  effort  must  come  from  the  respiratory  centre  in  the 
medulla  ,  with  these  we  shall  deal  in  speaking  of  respiration.    The 
dilation  of  the  cardiac  orifice  is  caused,  in  part  at  least,  by  impulses 
descending  the  vagi,  since  when  these  are  cut  real  vomiting  with 
discharge  of  the  gastric  contents,  if  it  takes  place  at  all,  becomes 
difficult  through  want  of  readiness  in  the  dilation.     Such  intrinsic 
movements  of  the  stomach  as  do  take  place,  and  the  movements  of 
the  oesophagus  appear  to  be  carried  out  by  the  usual  nerves.    The 
efferent  impulses  which  cause  the  flow  of  saliva  in  the  introductory 
nausea  also  descend  along  the  usual  nerves  such  as  the  chorda 
tympani.    These  various  impulses  may  best  be  considered  as  start- 
ing from  a  vomiting  centre  in  the  medulla,  having  close  relations 
with  the  respiratory  centre.     This  centre  may  be  excited,  may  be 
thrown  into  action,  in  a  reflex  manner,  by  stimuli  applied  to  periph- 
eral nerves,  as  when  vomiting  is  induced  by  tickling  the  fauces, 
or  by  irritation  of  the  gastric  membrane,  or  by  obstruction  of  the 
intestine  due  to  ligature,  hernia,  etc.     That  the  vomiting  in  the 
last  instance  is  due  to  nervous  action,  and  not  to  any  regurgita- 
tion of  the  intestinal  contents,  is  shewn  by  the  fact  that  it  will 
take  place  when  the  intestine  is  perfectly  empty  and  may  be  pre- 
vented by  section  of  the  mesenteric  nerves.    The  vomiting  attend- 
ing renal  and  biliary  calculi  is  apparently  also  reflex  in  origin. 
Vomiting  in  fact  as  a  rule  is  a  reflex  action,  the  afferent  impulses 
passing  along  one  or  other  nerves,  but  most  frequently  along  those 
connected  with  the  alimentary  canal,  that  is  along  afferent  fibres 
running  in  the  vagus  or  in  the  splanchnic  nerves.     The  centre 
however  may  be  affected  directly,  as  probably  in  the  cases  of  some 
poisons,  and  in  some  instances  of  vomiting  from  disease  of  the 
medulla  oblongata.     Lastly,  it  may  be  throwm  into  action  by  im- 
pulses reaching  it  from  parts  of  the  brain  higher  up  than  itself,  as 
in  cases  of  vomiting  produced  by  smells,  tastes  or  emotions,  or  by 
the  recollection  of  past  events,  and  in  some  cases  of  vomiting  due 
to  cerebral  disease. 

Many  emetics,  such  as  tartar  emetic,  appear  to  act  directly  on 
the  centre,  .since,  introduced  into  the  blood,  they  will  produce  vom- 
iting after  a  bladder  has  been  substituted  for  the  whole  stomach. 
Others  again,  such  as  mustard  and  water,  act  in  a  reflex  manner 
by  irritation  of  the  gastric  mucous  membrane.  With  others, 
again,  which  cause  vomiting  by  developing  a  nauseous  taste,  the 
action  involves  parts  of  the  brain  higher  than  the  centre  itself. 

§  223.  Movements  of  the  Small  Intestine.  These,  as  we  have 
already  said,  are  the  typical  peristaltic  movements,  simple  except 
in  so  far  as  they  are  complicated  by  the  existence  of  the  pendent 
loops,  the  peculiar  oscillating  movements  of  which  appear  to  be 
produced  chiefly  by  the  longitudinal  fibres. 

The  peristaltic  movements,  as  a  rule,  take  place  from  above 
downwards,  and  a  wave  beginning  at  the  pylorus  may  be  traced  a 
long  way  down.     But  contractions  may,  and  in  all  probability 


Chap,  i.]    TISSUES  AND  MECHANISMS  OF  DIGESTION.   381 

occasionally  do,  begin  at  various  points  along  the  length  of  the 
intestine.  A  movement  started  by  artificial  stimulation  some  way 
down  the  intestine,  may  travel  not  only  downwards  but  also  up- 
wards ;  it  has  been  disputed  however,  whether  in  the  living  body 
any  natural  backward  peristaltic  movement  really  takes  place. 
In  the  living  body  the  intestines  have  periods  of  rest,  alternating 
with  periods  of  activity,  the  occurrence  of  the  periods  depending 
on  various  circumstances ;  the  intensity  of  the  movements  also 
varies  very  considerably. 

§  224.  Movements  of  the  Large  Intestine.  These  are  funda- 
mentally the  same  as  those  of  the  small  intestine,  but  distinct  in 
so  far  as  tlie  latter  cease  at  the  ileo-ctecal  valve,  at  which  spot  the 
former  normally  begin ;  they  are  simpler,  in  as  much  as  the 
l^endent  loops  are  absent,  and  not  so  vigorous,  since  relatively  to 
the  diameter  of  the  tube,  the  amount  of  muscular  fibre  is  less. 
Along  the  colon  where  the  sacculi  are  well  developed  the  move- 
ment may  perhaps  be  described  as  almost  intermittent  from 
sacculus  to  sacculus,  the  contents  of  one  sacculus  being  driven 
by  the  peristaltic  contractions  of  its  circular  fibres  into  the  next 
sacculus,  which  prepares  to  receive  them  by  a  relaxation  of  its 
circular  and  a  contraction  of  its  longitudinal  fibres 

Since  the  lips  of  the  ileo-csecal  valve  are  placed  transversely 
across  the  csecum,  not  only  does  distention  of  the  caecum,  by 
stretching  the  valve  along  the  line  of  the  lips,  bring  them  into 
apposition,  but  the  pressure  exerted  by  the  peristaltic  movement 
has  the  same  effect.  In  this  way  any  return  of  the  contents  from 
the  large  to  the  small  intestine  is  prevented. 

Arrived  at  the  sigmoid  flexure,  the  contents,  now  more  or  less 
solid  faeces,  are  supported  by  the  bladder  and  the  sacrum,  so  that 
they  do  not  press  on  the  sphincter  ani. 

§  225.  Defcccation.  This  is  a  mixed  act,  being  superficially 
the  result  of  an  effort  of  the  will,  and  yet  carried  out  by  means  of 
an  involuntary  mechanism.  Part  of  the  voluntary  effort  consists 
in  producing  a  pressure-effect,  by  means  of  the  abdominal  muscles. 
These  are  contracted  forcibly  as  in  expiration,  but  the  glottis 
being  closed  and  the  escape  of  air  from  the  lungs  prevented,  the 
whole  force  of  the  pressure  is  brought  to  bear  on  the  abdomen 
itself,  and  so  drives  the  contents  of  the  descending  colon  onward 
towards  the  rectum.  The  sigmoid  flexure  is  by  its  position  shel- 
tered from  this  pressure ;  a  body  introduced  per  anum  into  the 
empty  rectum  is  not  affected  by  even  forcible  contractions  of  the 
abclominal  walls. 

The  anus  is  guarded  by  the  sphincter  ani,  which  is  habitually 
in  a  state  of  normal  tonic  contraction,  capable  of  being  increased 
or  diminished  by  a  stimulus  applied,  either  internally  or  externally, 
to  the  anus.  The  tonic  contraction  is  in  part  at  least  due  to  the 
action  of  a  nervous  centre  situated  in  the  lumbar  spinal  cord.  If 
the  nervous  connection  of  the  sphincter  with  the  spinal  cord  be 


382  DEFECATION.  [Book  ii. 

broken,  relaxation  takes  place.  If  the  spinal  cord  be  divided 
somewhat  higher  np,  for  instance  in  the  dorsal  region,  the 
sphincter,  after  the  depressing  effect  of  the  operation,  which  may 
last  several  days,  has  passed  oft",  regains  and  subsequently  main- 
tains its  tonicity,  shewing  that  the  centre  is  not  placed  higher  up 
than  the  lumbar  region  of  the  cord.  The  increased  or  diminished 
contraction  following  on  local  stimulation  is  probably  due  to  reflex 
augmentation  or  inhibition  of  the  action  of  this  centre.  The 
centre  is  also  subject  to  influences  proceeding  from  higher  regions 
of  the  cord,  and  from  the  brain.  By  the  action  of  the  will, 
by  emotions,  or  by  other  nervous  events,  the  lumbar  sphincter 
centre  may  be  inhibited,  and  thus  the  sphincter  itself  relaxed ;  or 
augmented,  and  thus  the  sphincter  tightened.  A  second  item 
therefore  of  the  voluntary  process  in  defsecation  is  the  inhibition  of 
the  lumbar  sphincter  centre,  and  consequent  relaxation  of  the 
sphincter  muscle.  Since  the  lumbar  centre  may  remain  wholly 
efficient  when  separated  from  the  brain,  the  paralysis  of  the 
sphincter  which  occurs  in  certain  cerebral  diseases  is  probably  due 
to  inhibition  of  this  lumbar  centre,  and  not  to  paralysis  of  any 
cerebral  centre. 

Thus  a  voluntary  contraction  of  the  abdominal  walls,  accom- 
panied by  a  relaxation  of  the  sphincter,  might  press  the  contents 
of  the  descending  colon  into  the  rectum  and  out  at  the  anus. 
Since  however,  as  we  have  seen,  the  pressure  of  the  abdominal 
walls  is  warded  oft'  the  sigmoid  flexure,  such  a  mode  of  defalcation 
would  always  end  in  leaving  the  sigmoid  flexure  full.  Hence  the 
necessity  for  these  more  or  less  voluntary, acts  being  accompanied 
by  an  involuntary  augmentation  of  the  peristaltic  action  of  the 
large  intestine,  sigmoid  flexure  and  rectum. 

In  the  movements  of  the  rectum  we  can  trace  out  more 
distinctly  than  in  other  regions  of  the  alimentary  canal  the 
separate  actions  of  the  longitudinal  and  circular  fibres.  The 
former,  by  means  of  contractions  travelling  from  above  downwards, 
shorten  the  rectum,  and  since  the  anus  affords  a  more  or  less  fixed 
support  pull  the  rectvim  and  its  contents  down  ;  the  latter,  by 
means  of  contractions  travelling  from  above  downwards  but 
taking  place  somewhat  later,  narrow  the  rectum  and  so  squeeze 
the  contents  onwards  and  outwards. 

Defsecation  then  appears  to  take  place  in  the  following  man- 
ner. The  large  intestine  and  sigmoid  flexure  becoming  more  and 
more  full,  stronger  and  stronger  peristaltic  action  is  excited  in 
their  walls.  By  this  means  the  faeces  are  driven  into  the  rectum 
and  so,  by  a  continuance  of  the  movements  increasing  in  vigour, 
against  the  sphincter.  Through  a  voluntary  act,  or  sometimes  at 
least  by  a  simple  reflex  action,  the  lumbar  sphincter  centre  is 
inhibited  and  the  sphincter  relaxed.  At  the  same  time  the  con- 
traction of  the  abdominal  muscles  presses  firmly  on  the  descend- 
ing colon,  and  thus,  contractions  of  the  levator  ani  assisting,  the 
contents  of  the  rectum  are  ejected. 


Chap,  i.j    TISSUES  AND  MECHANISMS  OF  DIGESTION.   383 

It  must  however  be  remembered  that,  while  in  appealing  to 
our  own  consciousness,  the  contraction  of  the  abdominal  walls  and 
the  relaxation  of  the  sphincter  seem  purely  voluntary  efforts,  the 
whole  act  of  defsecation,  including  both  of  these  seemingly  so 
voluntary  components,  may  take  place  in  the  absence  of  conscious- 
ness, and  indeed,  in  the  case  of  the  dog  at  least,  after  the  complete 
severance  of  the  lumbar  from  the  thoracic  cord.  In  such  cases 
the  whole  act  must  be  purely  reiiex,  excited  by  the  presence  of 
fffices  in  the  rectum. 

§  226.  The  nervous  mechanisms  of  gastric  and  intestinal 
movements.  Both  the  stomach  and  intestines  when  removed 
from  the  body  and  thus  wholly  separated  from  the  central  nervous 
system  may,  by  direct  stimulation,  be  readily  excited  to  move- 
ments ;  and  indeed  in  the  absence  of  all  obvious  stimuli,  movements 
which  seem  to  be  spontaneous  may  at  times  be  observed.  The 
movements  of  which  we  are  speaking  are  orderly  movements  of  a 
peristaltic  nature,  not  mere  local  contractions  of  a  few  bundles  of 
plain  muscular  fibres.  The  alimentary  canal  therefore,  like  the 
heart,  though  to  a  less  degree,  possesses  within  itself  such  mechan- 
isms as  are  requisite  for  carrying  out  its  own  movements ;  and, 
as  in  the  case  of  the  heart,  there  is  no  adequate  evidence  that  the 
ganglia  scattered  in  its  muscular  walls,  those  namely  forming 
the  plexus  of  Auerbach,  play  any  prime  part  in  developing  these 
movements. 

On  the  other  hand,  powerful  movements  of  a  peristaltic  kind 
may  be  induced,  not  only  as  we  have  already  seen  in  the  oesoph- 
agus but  also  in  the  stomach,  in  the  small  intestine,  and  even  in 
the  large  intestine  by  stimulation  of  the  vagus  nerve. 

The  chief  and  usual  cause  of  the  movements  of  the  stomach 
and  intestines  is  the  presence  of  food  in  their  interior.  But  we 
do  not  know  definitely  the  exact  manner  in  which  the  food  pro- 
duces the  movement.  It  may  be  that  the  food,  by  stimulating  the 
mucous  membrane,  sends  up  afferent  impulses,  and  that  these 
give  rise  by  reflex  action  to  efferent  impulses  which  descend  the 
vagus  fibres  to  successive  portions  of  the  canal,  in  a  manner  simi- 
lar to  that  already  described  in  reference  to  the  oesophagus.  If 
this  be  so  the  efferent  impulses  reach  the  stomach  and  upper  part 
of  the  duodenum  by  the  terminal  portions  of  the  two  vagi.  Fig.  84, 
R.  V.  L.  v.,  and  reach  the  intestines  by  the  portion  of  the  right  or 
posterior  vagus,  Fig.  84,  R.  V'.,  which  passes  into  the  solar  plexus 
and  thence  by  the  mesenteric  nerves.  The  afferent  impulses  from 
the  stomach  travel  also  apparently  by  the  vagus ;  the  paths  of 
those  from  the  intestines  have  not  yet  been  determined. 

But  that  such  a  reflex  action  through  vagus  filires  is  not  the 
only  means  by  which  tlie  presence  of  food  brings  about  the  move- 
ments in  question,  is  shewn  by  the  fact  that  these  continue  to  be 
developed  after  section  of  both  vagus  nerves.  Probably  the  whole 
action  is  a  mixed  one  which  we  may  picture  to  ourselves  somewhat 


384 


NERVES   OF   AILMENT ARY   CANAL.       [Book  ii. 


as  follows.  The  alimentary  canal  possesses  a  power  of  spontane- 
ous movement,  feeble  it  is  true,  very  inferior  to  that  of  the  heart, 
and  very  apt  to  be  latent,  but  still  existing.     The  presence  of  food 


Ret. 


Fig.  84.     Diagram  to  illustrate  the  Nerves  of  the  Alimentary 
Canal  in  the  Dog. 


The  figure  is  for  the  sake  of  simplicity  made  as  diagrammatic  as  possible,  a7id  does 
not  represent  the  anatomical  relations. 

Oe  to  Ret. — The  alimentary  canal,  CESophagus,  stomach,  small  intestine,  large  intes- 
tine, rectum. 

LV.  Left  vagus  nerve,  ending  on  front  of  stomach,  r.l.  recurrent  laryngeal  nerve 
supplying  upper  part  of  oesophagus.  R.  V.  right  vagus,  joining  left  vagus  in 
oesophageal  plexus,  oe.  pL,  supplying  the  posterior  part  of  stomach  and  con- 
tinued as  R'.V\  to  join  the  solar  plexus,  here  represented  by  a  simile  gang  ban  and 
connected  with  the  inferior  mesenteric  ganglion  (or  plexus)  m.  gl. — a.  branches 
from  the  solar  plexus  to  stomach  and  small  intestine,  and  from  the  mesenteric 
ganglion  to  the  large  intestine. 

Spl.  maj.  Large  splanchnic  nerve  arising  from  the  thoracic  ganglia  and  rami  com- 
municantes  r.c.  belonging  to  dorsal  nerves  from  the  6th  to  the  9th  (or  10th). 

Spl.  min.  Small  splanchnic  nerve  similarly  ari.sing  from  10th  and  11th  dorsal  nerves. 
These  both  join  the  solar  plexus  and  thence  make  their  way  to  the  alimentary 
canal. 

C.r.  Nerves  from  the  ganglia  &c,  belonging  to  11th  and  12th  dorsal  and  1st  and 
2nd  lumbar  nerves,  proceeding  to  the  inferior  mesenteric  ganglia  (or  plexus) 
rn.  gl.  and  thence  by  tlie  hypogastric  nerve  n.  hyp.  and  the  hypogastric  plexus 
pi.  hyp.  to  the  circular  muscles  of  the  rectum. 

l.r.  Nerves  from  the  2nd  and  .3rd  sacral  nerves,  (S'2,  5.3  (nervi  erigentes),  proceeding 
by  the  hypogastric  plexus  to  the  longitudinal  muscles  of  the  rectum. 


€hap.  I.]  TISSUES  AND  MECHANISMS  OF  DIGESTION.    385 

in  some  way  or  other,  by  some  direct  action  quite  ajjart  from  the 
central  nervous  system,  is  able  to  increase  this  power  so  that, 
without  any  aid  from  the  central  nervous  system,  as  after  section 
of  the  vagi,  adequate  peristaltic  movements  can,  under  favourable 
circumstances,  be  carried  out.  Nevertheless  in  the  normal  course 
of  events  satisfactory  movements  are  still  further  secured  by  the 
reflex  action  through  vagus  fibres  just  described.  Thus,  in  the 
■dog,  the  act  of  swallowing  food  or  even  the  mere  smell  of  food 
has  been  observed  to  increase  the  movements  of  a  piece  of  intes- 
tine isolated  from  the  rest  of  the  alimentary  canal  but  retaining 
its  connections  with  the  central  nervous  system.  Under  this 
view  the  peristaltic  movements  produced  by  centrifugal  stimu- 
lation of  the  vagLis  in  the  neck  are  comparable  not  so  much  Avith 
•the  contraction  of  a  skeletal  muscle  when  its  motor  nerve  is  stimu- 
lated as  with  the  beats  which  may  be  called  forth  in  an  inhib- 
ited or  otherwise  quiescent  heart  by  stimulation  of  the  cardiac 
augmentor  fibres. 

Indeed  we  may  perhaps  call  the  vagus  fibres  which  pass  to  the 
stomach  and  intestines  augmentor  fibres  rather  than  motor  fibres. 
We  have  all  the  more  reason  to  do  so  since  there  exist  companion 
but  antagonistic  inhibitory  fibres.  If  while  lively  peristaltic 
action  is  going  on  in  the  bowels,  the  splanchnic  nerves  be  stimu- 
lated the  bowels  are  brought  to  rest,  often  in  a  very  abrupt  and 
marked  manner.  Inhil)itory  fibres  therefore  run  in  the  splanch- 
nic nerves.  Fig.  8-1,  Spl.  mag.  and  min..,  passing  along  them 
from  the  spinal  cord  to  the  abdominal  plexuses,  and  thence  to 
the  alimentary  canal. 

This  view  however  tliat  the  movements  of  the  alimentary 
■canal  are  of  a  spontaneous  nature,  simply  augmented  on  the 
one  hand  and  inhibited  on  the  other  by  the  central  nervous  sj's- 
tem,  can  only  be  applied  to  the  middle  regions,  to  the  stomach 
and  intestines  in  which  peristaltic  action  is  seen  in  its  simple  form. 
At  the  beginning  of  the  alimentary  canal,  at  the  mouth  and  phar- 
ynx and  also  at  the  oesophagus,  the  central  nervous  system  inter- 
venes in  a  decided  manner  ;  the  movements  of  these  parts,  as  we 
have  seen,  are  carried  out  directly  by  the  central  nei'vous  sys- 
tem. Something  similar  is  also  seen  at  the  end  of  the  canal,  at 
the  rectum  and  sigmoid  flexure.  These  parts  are  governed  on 
the  one  hand  by  fibres  reaching  them  from  the  lower  regions 
of  the  cord  by  the  sympathetic  system,  by  the  hypogastric  nerves 
and  hypogastric  plexus,  and  on  the  other  hand  by  fibres  reaching 
them  along  certain  cerebro-spinal,  namely  sacral,  nerves  (in  the 
dog  the  second  and  third  sacral  nerves)  by  the  branches  of  these 
nerves,  called  nervi  erigentes  (Fig.  84).  And  the  government  by 
these  nerves  is  one  in  which  the  movements  are  directly  carried 
out  by  means  of  the  central  nervous  system. 

Hence  this  is  the  part  of  intestinal  movement  which  fails  in 
diseases  of  the  central  nervous  system;  the  failure  leading  to 

25 


386         MOVEMENTS   OF  ALIMENTAEY   CANAL.    [Book  ii. 

obstinate  constipation  if  not  to  actual  difficulty  of  clefgecation. 
The  presence  of  faeces  in  the  sigmoid  flexure  no  longer  stirs  up 
the  reflex  mechanism  for  their  discharge ;  meanwhile  the  more 
independent  movements  of  the  higher  parts  of  the  canal  con- 
tinue to  drive  the  contents  onward ;  and  hence  the  faeces  accu- 
mulate in  the  sigmoid  flexure  and  colon  awaiting  the  delayed 
action  of  the  imperfect  reflex  mechanism. 

With  regard  to  the  exact  manner  in  which  the  presence  of 
food  acts  as  a  stimulus  it  may  be  worth  while  to  remark,  that, 
though  in  the  stomach  as  we  have  seen  mere  fulness  is  not  the 
efiicient  cause  of  the  movements,  since  these  become  more  active 
as  digestion  proceeds  and  the  bulk  of  the  contents  diminishes, 
yet  in  the  intestine  distension  of  the  bowel  up  to  certain  limits 
most  distinctly  increases  the  vigour  of  the  movements  just  as- 
distension  of  the  cardiac  cavities  within  certain  limits  improves- 
the  cardiac  stroke.  This  is  well  seen  in  obstruction  of  the 
bowels,  in  which  cases  the  bowel  distended  above  the  obstruc- 
tion is  frequently  thrown  into  violent  peristaltic  movements. 

§  227.  Next  to  the  presence  of  food  in  the  interior  of  the 
alimentary  canal,  a  deficient  oxygenation  of  the  blood  supplied 
to  the  walls  of  the  canal  or  the  sudden  cutting  off  of  the  supply 
of  blood,  may  be  regarded  as  the  most  powerful  provocatives  of 
peristaltic  action.  When  the  aorta  is  clamped  or  when  the 
respiration  is  seriously  interfered  with,  peristaltic  movements 
become  very  pronounced.  Thus,  in  death  by  asphyxia  or  suf- 
focation, an  involuntary  discharge  of  fseces,  which  is  in  part  at 
least  the  result  of  increased  peristaltic  action,  is  not  an  unfre- 
quent  result ;  and  the  marked  peristaltic  movements  which  are 
so  frequently  seen  in  an  animal  when  the  abdomen  is  laid  open 
immediately  after  death,  appear  to  be  due  to  the  cessation  of 
the  circulation  and  the  consequent  failure  in  the  supply  of  blood 
to  the  walls  of  the  alimentary  canal  and  not,  as  has  been  sug- 
gested, to  the  contact  with  air  of  the  peritoneal  surface.  Since 
it  is  blood  which  brings  oxygen  to  the  tissues,  failure  in  the 
supply  of  blood  is  tantamount  to  failure  in  the  supply  of  oxy- 
gen ;  but  the  blood  current  brings  other  things  besides  oxygen 
and  also  takes  things  away ;  and  the  failure  of  this  action  also 
probably,  as  well  as  failure  in  the  supply  of  oxygen,  provoke 
the  movements  in  question. 

The  movements  thus  produced  are  to  some  extent  the  result 
of  the  deficient  supply  of  blood  acting  directly  on  the  walls  of 
the  canal,  though  in  asphyxia  at  all  events  this  effect  may  be  in- 
creased by  the  too  venous  blood  stimulating  the  central  nervous 
system  and  thus  sending  augmentor  impulses  down  the  vagus. 

With  regard  to  the  mode  of  action  of  the  drugs  which  promote 
peristaltic  action  it  will  be  sufficient  here  to  say  that  while  some 
such  as  nicotine  appear  to  act  directly  on  the  walls  of  the  canal, 
others  such  as  strychnia  produce  their  effect  chiefly  by  acting- 
through  the  central  nervous  system. 


SEC.   6.     THE    CHANGES    WHICH    THE    FOOD    UNDER- 
GOES  IN   THE   ALIMENTARY   CANAL. 


§  228.  Having  studied  the  properties  of  the  digestive  juices 
as  exhibited  outside  the  body,  and  the  various  mechanisms  by 
means  of  which  the  food  introduced  into  the  body  is  brought 
under  the  influence  of  those  juices,  we  have  now  to  consider 
what,  as  matters  of  fact,  are  the  actual  changes  which  the  food 
does  undergo  in  passing  along  the  alimentary  canal,  what  are  the 
steps  by  which  the  contents  of  the  canal  are  gradually  converted 
into  fieces.  The  events  which  lead  to  this  conversion  are  two- 
fold. On  the  one  hand  the  digestive  juices  do  bring  about, 
inside  the  alimentary  canals  changes  which  in  the  main  are  the 
same  as  those  observed  in  laboratory  experiments  outside  the 
body  and  described  in  previous  sections,  though  the  results  are 
somewhat  modified  by  the  special  conditions  which  obtain  within 
the  body.  On  the  other  hand  absorption,  that  is  to  say,  the 
passage  from  the  interior  of  the  canal  into  the  blood  vessels  and 
lymphatics,  of  digested  material  in  company  with  water,  is  going 
on  along  the  whole  length  of  the  canal,  and  especially  in  the 
small  and  large  intestines.  It  will  be  convenient  to  confine 
ourselves  at  present  to  the  study  of  the  fii'st  class  of  events,  the 
changes  effected  in  the  canal,  merely  noting  the  disappearance  of 
this  or  that  product,  and  deferring  the  ditficult  problem  of  how 
absorption  takes  place  to  a  subsequent  and  separate  discussion. 

In  the  mouth  the  presence  of  the  food,  assisted  by  the  move- 
ments of  the  jaw,  causes,  as  we  have  seen,  a  flow  of  saliva.  By 
mastication,  and  by  the  addition  of  mucous  saliva,  the  food  is 
broken  into  small  pieces,  moistened,  and  gathered  into  a  conve- 
nient bolus  for  deglutition.  In  man  some  of  the  starch  is,  even 
during  the  short  stay  of  the  food  in  the  mouth,  converted  into 
sugar  ;  for  if  boiled  starch  free  from  sugar  be  even  momentarily 
held  in  the  mouth,  and  then  ejected  into  water  (kept  boiling 
to  destroy  the  ferment)  it  will  be  found  to  contain  a  decided 
amount  of  sugar.  In  many  animals  no  such  change  takes  place. 
The  viscid  saliva  of  the  dog  serves  almost  solely  to  assist  in 
deglutition ;  and  even  the  longer  stay  which  food  makes  in  tlie 

387 


388  CHAIs^GES   IN  THE   STOMACH.  [Book  ii. 

mouth  of  the  horse  is  insufficient  to  produce  any  marked  con- 
version of  the  starch  it  may  contain.  During  the  rapid  transit 
through  the  oesophagus  no  appreciable  change  takes  place. 

The  amount  of  absorption  of  digested  material,  or  even  of 
simple  water  from  the  mouth'  or  cesophagus,  must  always  be 
insignificant. 

The  Changes  in  the  Stomach. 

§  229.  The  arrival  of  the  food,  the  reaction  of  which  is  either 
naturally  alkaline,  or  is  made  alkaline,  or  at  least  is  reduced  in 
acidity,  by  the  addition  of  saliva,  causes  a  flow  of  gastric  juice. 
This,  already  commencing  while  the  food  is  as  yet  in  the  mouth, 
increases  as  the  food  accumulates  in  the  stomach,  and  as,  by  the 
churning  gastric  movements,  one  part  after  another  of  the  food 
is  brought  into  contact  with  the  mucous  membrane. 

The  characters  of  the  juice  appear  to  change  somewhat  as  the 
act  of  digestion  proceeds.  The  amount  of  pepsin  in  the  gastric 
contents  increases  for  some  time  after  food  is  taken,  and  prob- 
ably the  actual  secretion  increases  also.  The  acidity  of  the 
gastric  contents  is  at  first  very  feeble  ;  indeed  in  man,  in  some 
cases  at  least,  for  some  little  time  after  the  beginning  of  a  meal 
no  free  acid  is  present,  and  during  this  period  the  conversion  of 
starch  into  sugar  may  continue.  This  condition  however  is 
temporary  only ;  very  soon  the  contents  become  acid,  arresting 
the  action  of  and  ultimately  destroying  the  amylolytic  ferment ; 
and,  since  the  rate  of  the  secretion  of  acid  appears  to  be  fairly 
constant,  the  contents  of  the  stomach,  unless  fresh  alkaline  food 
be  taken,  become  more  acid  as  digestion  goes  on. 

The  gross  effect  of  gastric  digestion  is  to  break  up  and  partly 
to  dissolve  the  larger  lumps  of  masticated  food  into  a  thick 
greyish  soup-like  liquid  called  chyme.,  with  which  are  still  mixed 
in  variable  quantity  larger  and  smaller  masses  of  less  changed 
food.  This  is  the  result,  partly  of  the  solution  of  proteid  mat- 
ters, partly  of  the  solution  of  the  gelatiniferous  connective-tissue 
holding  the  proteid  elements  together.  In  a  fragment  of  meat, 
for  instance,  the  muscular  fibres,  through  the  solution  of  the 
connective-tissue  binding  them  together,  fall  asunder,  the  sarco- 
lemma  is  dissolved,  and  the  fibres  themselves  split  up  sometimes 
longitudinally  but  most  frequently  by  transverse  cleavage  into 
discs,  and  are  ultimately  more  or  less  reduced  partly  into  a 
granular  mass,  partly  to  actual  solution.  In  a  piece  of  tissue 
containing  fat,  the  connective-tissue  binding  the  fat  cells 
together  and  the  envelopes  of  the  fat  cells  are  dissolved,  so  that 
the  fat,  fluid  at  the  temperature  of  the  body,  is  set  free  from 
the  individual  cells  and  runs  together  into  larger  and  smaller 
masses.  In  vegetable  tissue  the  proteid  elements  are  in  part 
dissolved  and,  though  there  is  no  evidence  that  in  man  cellulose 


Chap,  i.]  TISSUES  AND  MECHANISMS  OF  DIGESTION.    389 

is  dissolved  in  the  stomach,  the  whole  tissue  is  softened  and  to 
a  certain  extent  disintegrated.  Milk  is  curdled  and  the  curd 
subsequently  more  or  less  dissolved. 

The  thick  soup-like  acid  chyme  consists  accordingly  partly  of 
substances  which  have  entered  into  actual  solution,  partly  of 
mere  particles  or  droplets  of  proteid,  fatty  or  other  nature  and 
partly  of  masses  small  or  great,  which  may  be  recognized  under 
the  microscope  as  more  or  less  changed  portions  of  animal  or 
vegetable  tissue.  The  amount  of  material  actually  dissolved 
is  in  most  specimens  of  chyme  exceedingly  small.  When  the 
solid  parts  are  removed  by  filtration  the  clear  filtrate  contains 
besides  salts,  pepsin  and  free  hydrochloric  acid  (the  constituents 
of  the  gastric  juice),  a  small  amount  of  sugar,  of  some  of  the 
bye  products  of  proteid  digestion,  and  of  albumose  and  peptone. 
The  sugar  is  often  absent,  and  the  amount  of  peptone  (or 
albumose)  is  always  small. 

During  gastric  digestion  the  chyme  thus  formed  is  from  time 
to  time  ejected  through  the  pylorus,  accompanied  by  even  large 
morsels  of  solid  less-digested  matter.  This  may  occur  within  a 
few  minutes  of  food  having  been  taken ;  but  the  larger  escape 
from  the  stomach  probably  does  not  in  man  begin  till  from  one 
to  two,  and  lasts  from  four  to  five  hours,  after  the  meal,  becom- 
ing more  rapid  towards  the  end,  and  such  pieces  as  are  the 
least  broken  up  by  the  gastric  juice  and  movements  being  the 
last  to  leave  the  stomach.  Water  taken  by  itself  appears  to 
be  passed  on  at  once  into  the  small  intestine. 

The  time  taken  up  in  gastric  digestion  j)robably  varies  in  the 
same  animal  not  only  with  different  articles  of  food  but  also 
with  varying  conditions  of  the  stomach  and  of  the  body  at  large. 
In  different  animals  it  varies  ver}^  considerably,  being  from  12 
to  24  hours  in  the  dog  after  a  full  meal,  while  the  stomachs  of 
rabbits  are  never  empty  but  always  remain  largely  filled  with 
food,  even  during  starvation.  In  man  the  stomach  probably 
becomes  empty  between  the  usual  meals. 

The  total  amount  of  change  which  the  food  undergoes  in  the 
stomach,  that  is  the  share  taken  by  the  stomach  in  the  Avhole 
work  of  digestion,  seems  to  vary  largely  in  different  animals, 
and  in  the  same  animal  differs  according  to  the  nature  of  the 
meal.  In  a  dog  fed  on  an  exclusively  meat  diet,  a  very  large 
part  of  the  digestion  is  said  to  be  carried  out  by  the  stomach, 
very  little  work  apparently  being  left  for  the  intestines  ;  tliat  is 
to  say,  the  larger  part  of  the  meal  is  reduced  in  the  stomach  to 
actual  solution  and  a  consideral)le  quantity  is  probably  absorbed 
directly  from  the  stomach.  In  such  cases  the  amount  of  pep- 
tone found  in  the  stomach  during  the  digestion  of  the  meal  is 
found  to  be  fairly  constant,  from  which  it  may  be  inferred  that 
the  peptone  is  absorbed  so  soon  as  it  is  formed.  There  is  also 
evidence  that  fat  may  to  a  certain  extent  undergo  in  the  stom- 


\ 


390  CHANGES  IN   THE   SMALL   INTESTINE.  [Book  ii. 

ach  changes  leading  to  emulsion,  similar  to  those  which,  as  we 
shall  see,  are  carried  out  in  the  small  intestine. 

But  such  cases  as  these  cannot  be  regarded  as  typical  cases  of 
gastric  digestion,  and  in  man,  at  all  events,  living  on  a  mixed 
diet  the  work  of  the  stomach  appears  to  be  to  a  large  extent 
preparatory  only  to  the  subsequent  labours  of  the  intestine. 
It  is  true  that  our  information  on  this  matter  is  imperfect, 
being  chiefly  drawn  from  the  study  of  cases  of  gastric  or  duo- 
denal fistula,  in  which  probably  the  order  of  things  is  not 
normal,  or  being  in  large  measure  deductions  from  experiments 
on  animals,  whose  economy  in  this  respect  must  be  largely  dif- 
ferent from  our  own ;  but  we  are  probably  safe  in  concluding 
that,  in  ourselves,  the  chief  effect  of  gastric  digestion  is  by 
means  of  the  disintegration  spoken  of  above  to  reduce  the 
lumps  of  food  to  the  more  uniform  chyme  and  so  to  facilitate 
the  changes  which  take  place  in  the  small  intestine.  During 
that  disintegration  some  of  the  proteid  in  the  meal  is  con- 
verted into  peptone ;  and  the  peptone  so  formed  is  probably 
absorbed  at  once ;  but  much  proteid  remains  unchanged  or  at 
least  is  not  converted  into  peptone,  and  the  fats  and  starches 
undergo  in  themselves  very  little  change  indeed. 

In  the  act  of  swallowing,  no  inconsiderable  quantity  of  air  is 
carried  down  into  the  stomach,  entangled  in  the  saliva,  or  in  the 
food.  This  is  returned  in  eructations.  When  the  gas  of  eruc- 
tation or  that  obtained  directly  from  the  stomach  is  examined, 
it  is  found  to  consist  chiefly  of  nitrogen  and  carbonic  acid,  the 
oxygen  of  the  atmospheric  air  having  been  largely  absorbed. 
In  most  cases  the  carbonic  acid  is  derived  by  simple  diffusion 
from  the  blood,  or  from  the  tissues  of  the  stomach,  which  sim- 
ilarly take  up  the  oxygen.  In  many  cases  of  flatulency,  however, 
it  may  arise  from  a  fermentative  decomposition  of  the  sugar 
which  has  been  taken  as  such  in  food  or  which  has  been  produced 
from  the  starch,  the  gas  being  either  formed  in  the  stomach  or 
passing  upwards  from  the  intestine  through  the  pylorus. 

The  enormous  quantity  of  gas  which  is  discharged  through 
the  mouth  in  cases  of  hysterical  flatulency,  even  on  a  perfectly 
empty  stomach,  and  which  seems  to  consist  largely  of  carbonic 
acid,  presents  difficulties  in  the  waj^^  of  explanation ;  it  is  pos- 
sible that  it  may  be  simply  diffused  from  the  blood,  but  it  is 
also  possible  that  in  many  cases  it  is  derived  from  air  which 
the  patient  has  hysterically  swallowed,  the  oxygen  having  been 
removed,  in  the  stomach,  by  absorption  and  replaced  by  carbonic 
acid. 

In  the  Small  Intestine. 

§  230.  The  semi-digested  acid  food,  or  chyme,  as  it  passes 
over  the  biliary  orifice,  causes  as  we  have  seen  (§  215)  gushes 
of  bile,  and  at  the  same  time  the  pancreatic  juice  flows  into  the 


Chap,  i.]  TISSUES  AXD  MECHANISMS  OF  DIGESTION.    391 

intestine  freely.  These  two  alkaline  fluids,  especially  the  more 
strongly  and  constantly  alkaline  pancreatic  juice,  tend  to  neu- 
tralize the  acidity  of  the  chyme,  but  the  contents  of  the  duo- 
denum do  not  become  distinctly  alkaline  until  some  distance 
from  the  pylorus  is  reached.  The  rapidity  with  which  the 
change  in  tlie  reaction  is  completed  is  not  the  same  in  all  ani- 
mals, and  in  the  same  animal  appears  to  vary  according  to  the 
nature  of  the  food,  and  various  circumstances.  In  man,  living 
on  a  mixed  diet,  the  contents  have  probably  become  distinctly 
alkaline  before  they  have  passed  far  down  the  duodenum.  On 
the  other  hand  in  dogs,  the  contents  of  the  small  intestine  have 
been  observed  to  be  acid  throughout,  and  that,  not  only  when 
fed  on  starch  and  fat,  which  might,  by  an  acid  fermentation  of 
which  we  shall  presently  speak,  give  rise  to  an  acid  reaction, 
but  even  when  fed  on  meat. 

The  conversion  of  starch  into  sugar,  which  as  we  have  seen  is 
sooner  or  later  arrested  in  the  stomach,  is  resumed  with  great 
activity  and  indeed  completed  by  the  pancreatic  juice,  possibly 
assisted  by  the  succus  entericus,  the  presence  of  bile  being  said 
to  increase  the  activity  of  the  pancreatic  amylolytic  ferment.  The 
conversion  begins  as  soon  as  the  acidity  of  the  chj^me  is  suffi- 
ciently reduced  and  continues  along  the  intestine ;  portions 
however  of  still  undigested  starch  may  be  found  in  the  large 
intestine,  and  even  at  times  in  the  fosces. 

The  pancreatic  juice,  as  we  have  seen,  emulsifies  fats,  and 
also  splits  them  into  their  respective  fatty  acids  and  glycerin. 
The  fatty  acids  thus  set  free  become  converted  by  means  of  the 
alkaline  contents  of  the  intestine  into  soaps  ;  but  to  what  extent 
saponification  thus  takes  place  is  not  exactly  known.  Undoubt- 
edly soaps  have  to  a  small  extent  been  found  both  in  jjortal 
blood  and  in  the  thoracic  duct  after  a  meal ;  but  there  is  no 
proof  that  any  large  quantity  of  fat  is  introduced  in  this  form 
into  the  circulation.  On  the  other  hand,  the  presence  of  neu- 
tral fats  in  the  lacteals,  and  to  a  slight  extent  in  portal  blood, 
is  a  conspicuous  result  of  the  digestion  of  fatty  matters ;  and 
in  all  probability  saponification  in  the  intestine  is  a  subsidiary 
process,  the  effect  of  which  is  rather  to  facilitate  the  emulsion 
of  neutral  fats  than  to  introduce  soaps  as  such  into  the  blood. 
For  the  presence  of  soluble  soaps  favours  the  emulsion  of  neu- 
tral fats.  Hence  a  rancid  fat,  i.e.  a  fat  containing  a  certain 
amount  of  free  fatty  acid,  forms  an  emulsion  with  an  alkaline 
fluid  more  readily  than  does  a  quite  neutral  fat.  A  drop  of 
rancid  oil  let  fall  on  the  surface  of  an  alkaline  fluid,  such  as  a 
solution  of  sodium  carbonate  of  suitaljle  strength,  ra})idly  forms 
a  l)road  ring  of  emulsion,  and  that  even  without  the  least  agi- 
tation. As  saponification  takes  place  at  the  junction  of  the 
oil  and  alkaline  fluid  currents  are  set  up,  by  which  globules  of 
■oil  are  detached  from  the  main  drop  and  driven  out  in  a  cen- 


392  CHANGES   IN   THE   SMALL   INTESTD^E.    [Book  ii. 

trif  iigal  direction ;  the  intensity  of  tlie  cuiTents  and  the  conse- 
quent amount  of  emulsion  depend  on  the  concentration  of  the 
alkaline  medium  and  on  the  solubility  of  the  soaps  which  are 
formed.  Now  the  bile  and  pancreatic  juice  supply  just  such 
conditions  as  the  above  for  emulsionizing  fats  :  they  both 
together  afford  an  alkaline  medium,  the  pancreatic  juice  gives 
rise  to  an  adequate  amount  of  free  fatty  acid,  and  the  bile  in 
addition  brings  into  solution  the  soaps  as  they  are  formed.  So 
that  we  may  speak  of  the  emulsion  of  fats  in  the  small  intes- 
tine as  being  carried  on  by  the  bile  and  pancreatic  juice  acting 
in  conjunction  ;  and  as  a  matter  of  fact  the  bile  and  pancreatic 
juice  do  largely  emulsify  the  contents  of  the  small  intestine^ 
so  that  the  greyish  turbid  chyme  is  changed  into  a  creamy-look- 
ing fluid,  which  has  been  sometimes  called  chyle.  It  is  advis- 
able however  to  reserve  this  name  for  the  contents  of  the  lacteals. 
Many  of  the  fats  present  in  food,  for  instance,  butter,  already 
contain  some  fatty  acids  when  eaten  ;  for  these  fats  the  initial 
action  of  the  pancreatic  juice  is  less  necessary. 

Fats  we  may  therefore  say  are  digested,  for  this  emulsifica- 
tion  is  the  main  digestion  of  fats,  by  both  bile  and  pancreatic 
juice  working  together.  Hence  if  either  bile  or  pancreatic 
juice  be  prevented  from  gaining  access  to  the  small  intestine, 
fat  is  not  digested,  is  not  absorbed,  and  appears  in  the  ffeces. 
This  is  true  at  least  of  ordinary  fat ;  milk  in  which  the  fat  is 
already  emulsified  may  be  digested  and  absorbed,  in  the  absence 
of  these  secretions. 

§  231.  We  have  seen,  §  208,  that  the  addition  of  bile  to  a 
digesting  mixture  gives  rise  to  a  precipitate.  This  is  partly  a 
coarse  fiocculent  precipitate,  consisting  of  undigested  or  par- 
tially digested  proteids  with  some  amount  of  bile  acids,  and 
partly  of  a  finer  more  granular  precipitate,  which  is  longer  in 
falling  down,  and  consists  chiefly  of  bile  acids  with  a  variable 
amount  of  peptone ;  the  latter  is  re-dissolved  on  the  further 
addition  of  bile  even  though  the  reaction  of  the  mixture  remain 
acid.  In  the  upper  part  of  the  duodenum  the  inner  surface,  if 
examined  while  digestion  is  going  on,  is  found  to  be  lined  by 
a  coloured  flocculent  and  granular  material,  which  is  probably 
a  precipitate  thus  formed ;  the  purpose  of  this  precijoitation  is 
probably  to  delay  the  passage  of  the  undigested  f)arapeptone 
along  the  duodenum.  Moreover,  apart  from  this  precipitation, 
bile  arrests  the  action  of  pepsin,  even  while  the  reaction  of  the 
mixture  still  remains  acid ;  and  as  soon  as  an  alkaline  reaction 
is  established  the  pepsin  is  apparently  destroyed  by  the  trypsin, 
so  that  with  the  flow  of  bile  and  pancreatic  juice  into  the  duo- 
denum the  processes  which  have  been  going  on  in  the  stomach 
come  to  an  end.  In  fact  it  would  seem  that  the  juices  of  the 
various  districts  of  the  alimentary  canal  are  mutually  destruc- 
tive ;  thus,  while  pepsin  in  an  acid  solution  destroys  the  active- 


Chap,  i.]   TISSUES  AND  MECHANISMS  OF  DIGESTION.    393 

constituents  of  saliva  and  of  j)ancreatic  juice  (probably  also 
those  of  the  succus  entericus),  it  is  in  its  turn  antagonized  or 
destroyed  by  the  bile  and  the  other  alkaline  juices  of  the  intes- 
tine. Hence  pancreatic  juice  introduced  through  the  mouth 
must  lose  its  powers  in  the  stomach  and  can  only  be  of  use  as 
an  alkaline  medium  containing  certain  proteid  matters.  On 
the  other  hand  if,  as  we  have  reason  to  believe,  the  contents  of 
the  stomach  as  they  issue  from  the  pylorus  still  contain  a  large 
quantity  of  undigested  proteids,  these  must  be  digested  by  the 
pancreatic  juice  (with  or  without  the  assistance  of  the  succus 
entericus),  the  action  of  which  seems  to  be  assisted  or  at  least 
not  hindered  by  bile.  And  in  dogs  fed  through  a  duodenal 
fistula,  so  that  all  gastric  digestion  is  excluded,  proteids  are 
completely  digested  and  give  rise  to  quite  normal  fteces.  To 
what  stage  the  pancreatic  digestion  is  carried,  whether  peptone 
is,  practically,  the  only  product,  or  whether  the  pancreatic  juice 
in  the  body,  as  out  of  the  body,  carries  on  its  Avork  in  the  more 
destructive  form,  whereby  the  proteid  material  subjected  to  it 
is  so  broken  down  as  to  give  rise  to  appreciable  quantities  of 
leucin  and  tyrosin,  is  at  present  not  exactly  known.  Leucin 
and  tyrosin  have  been  found  in  the  intestinal  contents,  and  may 
therefore  be  formed  during  normal  digestion,  but  whether  an 
insignificant  quantity  or  a  considerable  quantity  of  the  proteid 
material  of  food  is  thus  hurried  into  a  crystalline  form  cannot 
be  definitely  stated.  The  extent  to  which  the  action  is  carried 
is  probably  different  in  different  animals,  and  probably  varies 
also  according  to  the  nature  of  the  meal  and  the  condition  of 
the  body.  Possibly  when  a  large  and  unnecessary  quantity  of 
proteid  material  is  taken  at  a  meal  together  with  other  sub- 
stances, no  inconsiderable  amount  of  the  proteids  undergo  this 
profound  change,  and,  as  we  shall  see,  rapidly  leave  the  body  as 
urea,  without  having  been  used  by  the  tissues,  their  contribution 
to  the  energy  of  the  body  being  limited  to  the  heat  given  out 
during  the  changes  by  Avhich  they  are  converted  into  urea.  To 
this  apparently  wasteful  use  of  proteids  we  shall  return  in 
speaking  of  what  is  called  the  'luxus  consumption '  of  food. 

§  232.  In  dealing  with  the  action  of  pancreatic  juice  Ave 
drcAv  attention,  i^  210,  to  the  difference  betAveen  the  results  of 
pure  tryptic  digestion  and  those  obtained  Avhen  bacteria  or  other 
micro-organisms  Avere  alloAved  to  be  present.  We  saAV  that  indol, 
for  example,  Avas  the  product  of  the  action  of  these  organisms, 
not  of  trypsin.  Noav  indol  is  formed,  in  varying  quantity,  dur- 
ing the  digestion  Avhich  actually  takes  place  in  the  intestine, 
some  of  it  at  times  appearing  in  the  urine  as  indigo-yielding 
substance  (indican).  ^NloreoA-er  bacteria  and  other  micro-organ- 
isms are  present  in  the  intestinal  contents.  Hence  Ave  must 
regard  the  changes  taking  place  in  the  intestine  not  as  the  pure 
residts  of  the  action  of  tlu'  several  digestive  juices,  but  as  these 


394  CHANGES   IN   THE   SMALL   INTESTINE.    [Book  ii. 

results  modified  by  or  mixed  with  the  results  of  the  action  of 
micro-organisms.  We  spoke  above,  §  208,  of  bile  as  being  anti- 
septic, but  this  must  be  understood  as  meaning  not  that  the 
presence  of  bile  arrests  the  action  of  all  micro-organisms  within 
the  intestine,  but  that  it  modifies  their  action,  keeping  it  within 
certain  limits  and  along  certain  lines. 

Concerning  the  exact  nature  and  extent  of  the  changes  thus 
due  to  micro-organisms  our  knowledge  is  at  present  very  im- 
perfect. The  proteids  and  the  carbohydrates  seem  to  be  the 
food  stuffs  on  which  these  organisms  produce  their  chief  effect. 
Out  of  the  proteids  they  give  rise  not  only  to  indol  but  to  several 
other  compounds,  among  which  may  be  mentioned  phenol 
(CgHgO),  of  which  a  small  quantity  may  be  recognized  in  the 
faeces,  the  rest  being  absorbed  and  appearing  in  the  urine  in 
the  form  of  certain  phenol-compounds,  such  as  phenyl-sulphuric 
acid.  Out  of  proteids  they  may  also  form  the  peculiar  poison- 
ous bodies  called  ptomaines^  which  appear  in  the  ordinary  putre- 
faction of  proteids.  But  their  most  conspicuous  effects  are 
those  on  the  carbohydrates.  As  the  food  descends  the  intestine, 
the  presence  of  lactic  acid  becomes  more  and  more  obvious; 
indeed  in  some  cases  the  naturally  alkaline  reaction  of  the  in- 
testinal contents  may  in  the  lower  part  of  the  intestine  be 
changed  into  an  acid  one  by  the  presence  of  lactic  acid.  Now 
lactic  acid  may  be  formed  out  of  sugar  by  means  of  a  special 
organism  inducing  what  is  spoken  of  as  the  lactic  acid  fermen- 
tation. And  we  have  every  reason  to  believe  that  in  even 
normal  digestion,  a  certain  quantity  of  sugar,  either  eaten  as 
such,  or  arising  from  the  amylolytic  conversion  of  starch,  does 
not  pass  away  from  the  intestine  into  the  blood  as  sugar,  but 
undergoes  this  fermentation  into  lactic  acid.  To  what  extent 
this  change  takes  place  we  do  not  know ;  the  amount  probably 
varies  according  to  the  amount  of  carbohydrates  eaten,  the  con- 
dition of  the  alimentary  canal,  and  other  circumstances.  It 
may  be  under  certain  circumstances  simply  a  part  of  normal 
digestion ;  under  other  circumstances  it  may  be  excessive  and 
give  rise  to  troubles. 

That  fermentative  changes  may  occur  in  the  small  intestine 
is  further  indicated  by  the  facts  that  the  gas  there  present  may 
contain  free  hydrogen,  and  that  chyme  after  removal  from  the 
intestine  continues  at  the  temperature  of  the  body  to  produce 
carbonic  acid  and  hydrogen  in  equal  volumes.  This  suggests 
the  possibility  of  the  sugar  of  the  intestinal  contents  under- 
o-oing  the  butyric  acid  fermentation  during  which,  as  is  well 
known,  carbonic  anhydride  and  hydrogen  are  evolved.  By  this 
change  the  sugar  is  removed  from  the  carbohydrate  group  into 
the  fatty  acid  group ;  it  is  thus,  so  to  speak,  put  on  its  way  to 
become  fat.  We  shall  see  hereafter  that  sugar  may  be  some- 
where in  the  body  converted  into  fat  ,•  this  conversion  however 


Cjiap.  I.]   TISSUES  AND  MECHANISMS  OF  DIGESTION.    395 

takes  place  eliiefly  if  not  wholly  in  the  tissues,  and  such  change 
as  may  take  place  in  the  alimentary  canal  is  to  be  regarded  as 
suggestive  rather  than  as  important. 

The  hydrogen  thus  occurring  in  the  intestine  may  also  arise 
from  the  proteid  decompositions  spoken  of  above.  However 
arising,  it  may  act  as  a  reducing  agent,  reducing  sulphates  for 
instance,  and  thus  giving  rise  to  sulphides  and  to  sulphuretted 
hydrogen ;  as  a  reducing  agent  it  assists  in  the  formation  of 
the  fajcal  and  urinary  pigments. 

Thus  during  the  transit  of  the  food  through  the  small  intes- 
tine, by  the  action  of  the  bile  and  pancreatic  juice,  and  possibly 
to  some  extent  of  the  succus  entericus,  assisted  by  various  micro- 
organisms, the  proteids  are  largely  dissolved  and  converted  into 
peptone  and  other  products,  the  starch  is  changed  into  sugar, 
the  sugar  possibly  being  in  part  further  converted  into  lactic 
of  other  acids,  and  the  fats  are  largely  emulsified,  and  to  some 
extent  saponified.  These  products,  as  they  are  formed,  pass 
into  either  the  lacteals  or  the  portal  blood  vessels,  so  that  the 
contents  of  the  small  intestine,  by  the  time  they  reach  the  ileo- 
Ctccal  valve,  are  largely  but  by  no  means  wholly  deprived  of 
their  nutritious  constituents.  So  far  as  water  is  concerned, 
the  secretion  of  water  into  the  small  intestine  maintains  such  a 
relation  to  the  absorption  from  it  that  the  intestinal  contents  at 
the  end  of  the  ileum,  though  much  changed,  are  about  as  fluid 
as  in  the  duodenum. 

Li  the  Large  Intestine. 

§  233.  The  contents,  whether  alkaline  or  not  in  the  ileum, 
now  become  once  more  distinctly  acid.  This,  however,  is  not 
caused  by  any  acid  secretion  from  the  mucous  membrane  :  the 
reaction  of  the  intestinal  walls  in  the  large  as  in  the  small 
intestine  is  alkaline.  It  must  therefore  arise  from  acid  fermen- 
tations going  on  in  the  contents  themselves  -,  and  that  fermen- 
tations do  go  on  is  shewn  by  the  appearance  of  marsh  gas  as 
well  as  hydrogen  in  this  portion  of  the  alimentary  canal.  The 
character  and  amount  of  fermentation  probably  depend  largely 
on  the  nature  of  the  food,  and  probably  also  vary  in  different 
animals. 

Of  the  particular  changes  which  take  place  in  the  large  in- 
testine we  have  no  very  definite  knowledge ;  but  since  such 
secretions  as  are  afforded  by  the  walls  of  the  intestine  itself  do  not 
seem  to  contain  any  ferments,  we  may  conclude  that  the  changes 
which  do  take  place  are  effected  by  micro-organisms.  It  is 
exceedingly  probable  that  in  the  voluminous  crecum  of  the  her- 
bivora  a  large  amount  of  digestion  of  a  peculiar  kind  goes  on. 
We  knoAV  that  in  herbivora  a  considerable  quantity  of  cellulose 
disappears  in  passing  through  the  alimentary  canal,  and  oven  in 


396  FiECES.  [Book  ii. 

man  some  is  digested.  It  seems  probable  that  this  cellulose 
digestion  takes  place  in  the  large  intestine,  and  is  the  result  of 
fermentative  changes  carried  out  by  means  of  micro-organisms, 
marsh  gas  being  one  of  the  products  formed  at  the  same  time. 

Be  this  as  it  may,  whether  digestion,  properly  so  called,  is  all 
but  complete  at  the  ileo-csecal  valve,  or  whether  important 
changes  still  await  the  chyme  in  the  large  intestine,  one  great 
characteristic  of  the  work  done  in  the  colon  is  absorption.  By 
the  abstraction  of  all  the  soluble  constituents,  and  especially  by 
the  withdrawal  of  water,  the  liquid  chyme  becomes  as  it  ap- 
proaches the  rectum  converted  into  the  firm  solid  faeces,  and  the 
colour  shifts  from  the  bright  orange,  which  the  grey  chyme 
gradually  assumes  after  admixture  with  bile,  into  a  darker  and 
dirtier  brown. 

The  Fceces. 

§  234.  These  consist  in  the  first  place  of  the  indigestible  and 
undigested  constituents  of  the  meal :  shreds  of  elastic  tissue, 
hairs  and  other  horny  elements,  much  cellulose  and  chlorophyll 
from  vegetable,  and  some  connective-tissue  from  animal  food, 
fragments  of  disintegrated  muscular  fibre,  fat-cells,  and  not  un- 
frequently  undigested  starch-corpuscles.  The  amount  of  each 
must  of  course  vary  very  largely  according  to  the  nature  of  the 
food,  and  the  digestive  powers,  temporary  or  permanent,  of  the 
individual.  In  the  second  place,  to  these  must  be  added  sub- 
stances not  distinctly  recognizable  as  parts  of  the  food  but  de- 
rived for  the  most  part  from  the  secretions  of  the  alimentary 
canal ;  when  a  portion  of  the  intestine  is  isolated  from  the  rest 
so  that  no  food  enters  into  it,  a  quantity  of  material  accumulates- 
in  the  interior  and  this  in  the  course  of  time  assumes  a  fsecal 
appearance. 

The  f  seces  contain  Tiiucus  in  variable  amount,  sometimes  al- 
bumin, cholesterin,  butyric  and  other  fatty  acids,  lime  and  mag- 
nesia soaps,  colouring  matters,  and  inorganic  salts,  especially 
earthy  phosphates,  crystals  of  ammonio-magnesia  phosphates 
being  very  conspicuous.  The  reaction  is  generally  but  not 
always  acid.  They  also  contain  a  ferment  similar  in  its  action 
to  pepsin,  and  an  amylolytic  ferment  similar  to  that  of  saliva 
or  pancreatic  juice.  The  bile  salts  are  represented  by  a  small 
quantity  of  cholalic  acid,  or  some  product  of  that  body,  and 
sometimes  a  very  small  quantity  of  taurin.  The  glycin  and 
most  or  all  of  the  taurin  have  been  absorbed  from  the  intestine, 
and  the  cholalic  acid  has  been  partly  absorbed  and  partly  decom- 
posed. The  fact  that  the  f seces  becomes  '  clay-coloured '  when 
the  bile  is  cut  off  from  tlie  intestine  shews  that  the  bile-pigment 
is  at  least  the  mother  of  the  fsecal  pigment ;  and  a  special  pig- 
ment, which  has  been  isolated  and  called  stercobilin,  is  said  to 
be  identical  with  the  su})stance  called  urobilin,  which  may  be 


Chap,  i.]    TISSUES  AND  MECHANISMS  OF  DIGESTION.    ^97 

formed  from  bilirubin.  As  other  special  constituents  of  the 
f seces  may  be  mentioned  excretin,  a  somewhat  complex  nitrog- 
enous body,  whose  exact  chemical  nature  is  at  present  uncer- 
tain, and  skatol  (C9H9N),  a  nitrogenous  body  which  like  indol 
is  derived  from  the  decomposition  of  proteids  by  means  of 
micro-organisms,  and  which  is  the  chief  cause  of  the  faecal 
odour,  since  only  a  small  quantity  of  indol  remains  in  the  fffiC(is. 
These  odoriferous  bodies  are  derived  directly  from  the  food ;  at 
the  same  time  it  is  quite  possible  that  other  specific  odoriferous 
substances  may  be  secreted  directly  from  the  intestinal  wall, 
especially  from  that  of  the  large  intestine. 


SEC.   7.     THE  LACTEALS   AND  THE   LYMPHATIC 
SYSTEM. 


§  235.  We  have  seen  that  absorption  does,  or  at  least  may, 
take  place  from  the  stomach.  We  have  also  stated  that  a  large 
absorption,  especially  of  water,  occurs  along  the  whole  large 
intestine.  Nevertheless  it  is  during  the  transit  of  food  along 
the  small  intestine  that  the  largest  and  most  important  part  of 
the  digested  material  passes  away  from  the  canal,  partly  into  the 
lacteals,  partly  into  the  portal  vessels.  The  portal  vessels  are 
simply  parts  of  the  general  vascular  system ;  the  lacteals,  into 
which  we  may  at  once  say  the  greater  part  of  the  fat  passes,  are 
similarly  parts  of  the  general  lymphatic  system,  being  in  fact  the 
lymphatic  vessels  of  the  alimentary  canal,  and  especially  of  the 
small  intestine.  The  only  reason  for  the  special  name  of  lac- 
teals is  that,  unlike  the  lymphatic  vessels  of  other  parts  of  the 
body,  the  lymphatics  of  the  intestine  contain  at  times  a  fluid  of 
a  milky  white  appearance.  Hence  for  the  better  understand- 
ing of  absorption  by  the  lacteals  it  will  be  desirable  to  study  at 
some  length  the  whole  subject  of  the  lymphatic  system. 

The  lymphatic  vessels  may  be  said  to  begin  in  minute  pas- 
sages, possessing  special  characters,  known  as  lymph-capillaries. 
Broadly  speaking  these  lymph-capillaries  are  found,  in  the  mam- 
mal, in  all  parts  of  the  body  in  which  connective  tissue  is  found; 
and  they  have  special  connections  with  those  minute  spaces  in 
connective  tissue  which  we  have  already  more  than  once  spoken 
of  as  lymph-spaces.  These  lymph-capillaries,  which  are  fre- 
quently arranged  in  plexuses,  are  continuous  with  other  pas- 
sages also  minute  but  of  a  different  and  more  regular  structure, 
the  lymphatic  vessels  proper,  which  are  gathered  into  larger 
and  larger  vessels,  all  running  like  the  blood  vessels  in  a  bed  of 
connective  tissue,  until  at  last  all  the  lymphatic  vessels  of  the 
body  join  either  the  great  thoracic  duct  which  opens  by  a  valvu- 
lar orifice  into  the  venous  system  at  the  junction  of  the  left 
jugular  and  subclavian  veins,  or  the  small  lymphatic  trunk 
which  similarly  opens  into  the  junction  of  the  right  jugular 
and  subclavian  veins.  The  large  '  serous  cavities,  peritoneal 
and  the  like,  are  also  connected  with  the  lymphatics,  and  may  be 
regarded  as  part  of  the  lymphatic  system. 

398 


SEC.    8.       THE   NATURE  AND   MOVEMENTS   OF  LYMPH 
(INCLUDING    CHYLE). 

§  236.  We  are  thus  led  to  regard  the  multitudinous  spaces^ 
both  small  and  great,  of  connective  tissue  all  over  the  bod)% 
including  among  these  the  "serous  cavities,"  as  forming  the 
beginning  or  roots  of  the  lymphatic  system.  Into  these  spaces 
certain  parts  of  the  plasma  of  the  blood  transude  and  so  become 
lymph;  (how  far  the  epithelioid  lining  of  the  large  serous 
cavities  plays  a  distinct  part  in  regulating  the  transudation  of 
serous  fluid,  i.e.  of  lymph  into  those  cavities  we  do  not  know ;) 
from  these  spaces  the  lymph  is  continually  flowing  through  the 
lymph-capillaries  into  the  lymphatic  vessels,  and  so  by  the 
thoracic  duct  and  right  lymphatic  trunk  back  into  the  blood 
system. 

The  amount  of  lymph  occupying  the  lymph-spaces,  lymph- 
capillaries,  and  minute  lymphatic  vessels  of  any  region  varies 
from  time  to  time  according  to  circumstances.  A  hand  for 
instance  which  has  been  kept  hanging  down  for  some  time 
becomes  swollen  and  the  skin  tense ;  if  it  be  raised  the  swelling 
lessens  and  the  skin  becomes  loose ;  and  a  similar  temporary 
swelling  of  the  skin  of  the  limbs,  and  of  the  skin  generally,  is 
frequently  the  result  of  active  exercise.  Such  a  swelling  is 
partly  due  to  the  blood  vessels  being  dilated,  or  to  the  return 
flow  along  the  veins  being  retarded  so  that  the  blood  capillaries 
become  distended  with  blood,  but  is  much  more  largely  owing 
to  the  lymph-spaces  and  lymphatic  vessels  of  the  skin  and 
underlying  structures  being  unusually  filled  with  lymph.  On 
the  other  hand  the  skin  may  become  shrivelled  and  dry  from  a 
deficiency  of  lymph  in  the  lymph-spaces  and  vessels.  Under 
even  normal  circumstances  the  quantity  of  lymph  in  the  tissues 
may  vary  considerably,  and  under  abnormal  circumstances  a 
very  large  amount  of  lymph  may  greatly  distend  the  spaces  of 
the  connective  tissue  of  the  skin  and  other  structures,  giving 
rise  to  oedema  or  dropsy.  Obviously  there  are  agencies  at  work 
in  the  body  by  which  the  appearance  of  lymph  in  the  spaces  or 
its  removal  thence  along  the  lymph-channels,  or  both,  may  be 
either  increased  or  diminished. 

399 


400  CHAEACTEES    OF   LYMPH.  [Book  ii. 


The   Characters  of  Lymph. 

§  237.  As  it  slowly  flows  from  its  origin  in  the  tissues  to 
the  mouth  of  the  thoracic  duct  (we  may  for  simplicity's  sake 
omit  the  right  lymphatic  trunk)  the  lymph  is  subjected  to  the 
influence  of  the  lymphatic  glands,  and  is  possibly  affected  by 
the  walls  of  the  lymph- vessels.  Moreover  the  lymph  coming 
from  one  tissue  differs  more  or  less  in  certain  characters  from 
the  lymph  arising  in  another  tissue,  just  as  the  venous  blood  of 
one  organ  differs  from  the  venous  blood  of  another  organ ;  and 
these  differences  may  be  exaggerated  by  the  activity  of  the  one 
or  other  tissue.  Of  these  differences  by  far  the  most  striking 
is  that  between  the  lymph  coming  from  the  alimentary  canal 
during  active  digestion  and  known  as  chyle^  and  the  lymph 
•coming  from  other  parts  of  the  body.  When  digestion  is  not 
^oing  on,  and  when  consequently  no  considerable  absorption  of 
material  from  the  alimentary  canal  into  the  lacteals  is  taking 
place,  the  fluid  flowing  along  the  lacteals  is  lymph,  not  differ- 
ing from  the  lymph  of  other  regions  to  any  marked  degree. 

The  fluid  accordingly  which  flows  along  the  thoracic  duct  in 
an  animal  which  has  not  been  fed  for  some  considerable  time 
may  be  taken  as  illustrating  the  general  characters  of  lymph. 
The  contents  of  the  thoracic  duct  may  be  obtained  by  laying 
bare  the  junction  of  the  subclavian  and  jugular  (in  the  dog 
the  junction  of  the  axillary  and  jugular)  veins,  and  introducing 
a  cannula  into  the  duct  as  it  enters  into  the  venous  system  at 
that  point.     The  operation  is  not  unattended  with  difficulties. 

Lymph,  so  obtained,  is  a  clear  transparent  or  slightly  opales- 
cent fluid,  which  left  to  itself  soon  clots.  The  clotting  is  not 
so  pronounced  as  that  of  blood,  but  clotting  is  caused  as  in 
blood  by  the  appearance  of  fibrin.  The  fibrin  which  is  formed 
though  scanty,  '05  p.c,  is  identical  apparently  with  that  of 
blood,  and  as  far  as  we  know,  all  that  has  been  said  previously, 
§§  14 — 23,  concerning  the  nature  of  clotting  in  blood  applies 
equally  well  to  lymph. 

Examined  with  the  microscope  lymph  contains  a  number  of 
corpuscles,  lymph-corpuscles,  which  in  all  their  characters  so 
far  as  is  at  present  known  are  identical  with  white  blood  cor- 
puscles ;  they  vary  in  size  from  5  /a  to  15  //.,  and  the  smaller 
corpuscles  are  much  more  abundant  in  lymph  than  in  blood. 
Like  the  white  blood  corpuscles  of  blood  they  exhibit  amoeboid 
movements.  Their  number  varies  in  different  animals,  and, 
apparently,  in  the  same  animal,  according  to  circumstances ; 
on  the  whole  perhaps  it  may  be  said  that  lymph  corpuscles  are 
about  as  numerous  in  lymph  as  white  corpuscles  in  blood. 
Even  when  every  care  is  taken  to  avoid  admixture  with  blood, 
lymph,  and  especially  chyle,  not  unfrequently  contains  a  cer- 


Chap,  i.]  TISSUES  AND  MECHANISMS  OF  DIGESTION".    401 

tain  number  of  red  blood  corpuscles  ;  sometimes  these  are  suffi- 
cient to  give  the  lymph  (or  chyle)  a  reddish  tinge.  They  have 
been  observed  within  the  living  lymphatic  vessels,  even  within 
small  ones,  and  have  probably  in  some  manner  or  other  made 
their  way  from  the  blood  into  the  lymph-channels. 

§  238.  The  chemical  composition  of  Ij-mph,  even  when  taken 
in  each  case  from  the  thoracic  duct,  varies  a  good  deal.  The 
total  solids  are  much  less  than  in  blood,  amounting  in  general 
to  not  more  than  5  or  6  p.c.  Hence  the  venous  blood  of  a  vascu- 
lar area  contains  rather  more  solids  than  the  arterial  blood  of  the 
same  area,  since  the  blood  in  giving  rise  to  the  lymph  during 
its  passage  through  the  capillaries  from  the  arteries  to  the  veins 
has  parted  with  relatively  more  water  than  solid  matter. 

The  proteids  amount  on  the  average  to  about  3  or  4  p.c, 
that  is  to  say,  to  about  half  as  much  as  in  blood,  the  particular 
proteids  present  being  the  same  as  in  blood,  viz.  albumin,  para- 
globulin  and  antecedents  of  fibrin.  In  lymph,  as  distinguished 
from  chyle,  the  quantity  of  fat  is  small,  and  consists  of  the 
usual  neutral  fats  and  the  soaps  of  their  fatt}^  acids,  together 
with  lecithin ;  cholesterin  may  also  be  present.  A  certain 
amount  of  sugar  (dextrose)  appears  to  be  always  present,  and 
several  observers  have  found  an  appreciable  quantity  of  urea. 
The  ash  of  lymph  like  that  of  blood  serum  contains  a  consider- 
able quantity  of  sodium  chloride,  while  phosphates  and  potash 
are  scanty  ;  it  also  contains  iron,  apparently  in  too  great  a 
■quantity  to  be  accounted  for  by  the  few  red  corpuscles  which 
may  be  present.  From  lymph  a  certain  amount  of  gas  can  be 
extracted,  consisting  ohietly  or  almost  exclusively  of  carbonic 
acid,  with  a  small  quantity  of  nitrogen,  the  amount  of  oxygen 
present  being  exceedingly  small.  The  importance  of  this  we 
shall  see  when  we  come  to  study  respiration. 

Broadly  speaking  we  may  say  that  all  the  substances  present 
in  blood-plasma  are  present  also  in  Ijanph,  but  are  accompanied 
by  a  larger  quantity  of  water. 

§  239.  Lymph  may  also  be  obtained  from  separate  regions 
of  the  body,  as  from  the  lower  or  upper  limbs,  for  instance,  by 
introducing  a  fine  cannula  into  a  lymphatic  vessel.  In  its  gen- 
eral features  the  lympli  so  obtained  resembles  that  taken  from 
the  thoracic  duct.  Analyses  of  the  lymph  distending  the  sub- 
cutaneous connective  tissue  in  cases  of  dropsy  shew  that  this 
contains  much  less  solid  matter  than  normal  lymph  taken  from 
the  thoracic  duct  or  larger  lymphatic  vessels.  From  this  it  has 
been  inferred  that  the  lymph  normally  existing  in  the  lymph- 
spaces,  lymph-capillaries  and  minute  vessels  contains  an  excess 
of  water ;  and  indeed  it  has  been  asserted  that  the  percentage 
of  solids  increases  in  passing  from  the  smaller  to  the  larger 
vessels  ;  but  this  cannot  be  regarded  as  distinctly  proved.  The 
number  of  corpuscles  however,  as  we  have  already  said,  appears 

26 


402  CHYLE.  [Book  ii. 

to  be  increased  in  passing  through  the  lymphatic  glands.  It 
has  also  been  stated  that  the  lymph  in  the  finer  lymph- vessels 
clots  even  less  firmly  than  that  in  the  thoracic  duct.  From 
this  we  may  infer  that  some  of  the  leucocytes  in  the  adenoid 
tissue  of  the  follicles  of  a  lymphatic  gland  find  their  way  into 
the  lymph-sinus,  and  so  into  the  efferent  lymphatics,  and  that 
some  of  the  fibrin  factors  are  added  to  the  lymph,  or  at  least 
that  some  changes  favourable  to  clotting  are  brought  about. 

§  240.  We  said  that  the  large  serous  cavities  of  the  peri- 
toneum, pericardium,  &c.,  are  to  be  considered  as  parts  of  the 
lymphatic  system ;  indeed  pericardial  or  other  serous  fluid  has 
all  the  general  characters  of  lymph.  We  have  already  said, 
§  20,  that  these  fluids  when  taken  fresh  from  the  body,  clot 
(this  is,  at  least,  the  case  in  most  animals) ;  the  clot  when 
examined  microscopically  is  found  to  consist  of  colourless  cor- 
puscles like  those  of  lymph  or  of  blood  entangled  in  the  meshes 
of  fibrin.  Both  in  their  proteid  and  other  chemical  constitu- 
ents these  serous  fluids  resemble  lymph.  Analyses  of  the  ac- 
cumulations of  fluid  occasionally  occurring  in  these  cavities 
shew  that  they  contain  sometimes  less  and  sometimes  more 
solid  matter  than  ordinary  lymph.  The  aqueous  humour  of 
the  eye  contains  very  little  solid  matter;  and  the  cerebro- 
spinal fluid  is  so  peculiar  that  it  had  better  be  considered  by 
itself  in  connection  with  the  nervous  system. 

§  241.  Chyle.  In  fasting  animals  the  fluid  flowing  along 
the  lacteals,  as  may  be  seen  by  inspection  of  the  mesentery,  is 
clear  and  transparent ;  it  is  lymph,  differing,  as  we  have  said, 
in  no  essential  respects  from  the  lymph  flowing  along  other 
lymphatic  vessels.  Shortly  after  a  meal  containing  fat  (and 
every  meal  does  contain  some  fat),  the  lymph  becomes  white 
and  opaque  like  milk,  the  more  so  the  richer  the  meal  is  in  fat ; 
it  is  then  called  chyle.  Owing  to  the  relatively  large  quantity 
of  this  milky  fluid  which  for  some  time  after  a  meal  continues 
to  be  poured  into  the  thoracic  duct,  the  contents  of  that  duct 
also  become  milky,  and  are  also  called  chyle.  In  the  thoracic 
duct  the  chyle  of  the  lacteals  is  more  oi-  less  mixed  with  lymph 
from  other  lymphatic  vessels,  but  the  former  is  so  preponderat- 
ing that  the  contents  of  the  duct  may  be  taken  as  illustrating 
the  nature  of  chyle. 

Chyle  differs  from  lymph  in  one  important  respect,  and  one 
only ;  whereas  lymph  ordinarily  contains  a  small  quantity  only 
of  fat,  chyle  contains  a  very  large  amount.  The  actual  amount 
of  fat  present  in  the  chyle  of  the  thoracic  duct  varies,  as  may 
be  expected,  very  considerably,  according  to  the  nature  of  the 
meal,  the  stage  of  digestion,  and  various  circumstances.  Five 
per  cent,  is  a  very  common  amount ;  in  the  dog  it  has  been 
found  to  vary  from  2  to  15  per  cent.  The  increase  in  fat  is 
chiefly  if  not  exclusively  due  to  an  increase  in  the  neutral  fats ; 


Chap,  i.]  TISSUES  AND  MECHANISMS  OE  DIGESTION.    403 

though  whether  the  small  quantity  of  soaps  and  of  lecithin 
present  is  greater  than  in  lymph  has  not  been  distinctl}^  ascer- 
tained. Cholesterin  is  probably  present  in  greater  amount  than 
in  lymph,  since  it  probably  comes  from  the  bile  poured  into  the 
intestine  during  digestion ;  but  this  is  not  certain.  How  far 
the  nature  of  the  fat,  that  is,  the  proportion  of  the  various  kinds 
of  fat,  of  stearin,  &c.,  varies  with  the  fats  present  in  the  meal 
has  not  been  definitely  ascertained. 

The  condition  of  the  fat  in  chyle  is  peculiar.  Some  of  it 
exists,  like  the  fat  in  milk,  in  the  form  of  fat  globules  of  vari- 
ous sizes,  but  all  small.  _  A  very  considerable  quantity  however 
is  present  in  the  form  of  exceedingly  minute  spherules  or  gran- 
ules, far  smaller  than  any  globules  to  be  seen  in  milk ;  these 
exhibit  active  '  Brownian  movements.'  The  fat  present  in  this 
form  is  spoken  of  as  the  '  molecular  basis '  of  chyle,  and  is  very 
distinctive  of  chyle.  In  the  emulsified  contents  of  the  intes- 
tine, often  called  chyle,  the  fat  is  finely  divided,  and  to  a  large 
extent  into  small  globules,  but  there  is  nothing  corresponding 
to  this  molecular  basis ;  the  fat  does  not  assume  this  condition 
until  it  has  passed  out  of  the  intestine  into  the  lacteals.  Lymph 
examined  with  the  microscope  shews  besides  the  white  corpus- 
cles only  very  few  oil-globules,  and  nothing  of  this  molecular 
basis.  Just  as  in  fact  lymph  is,  broadly  speaking,  blood  minus 
its  red  corpuscles,  so  chyle  is  lymph  plus  a  very  large  quantity 
of  minutely  divided  neutral  fat. 

The  total  amount  of  lymph  or  of  chyle  which  enters  the  blood 
system  through  the  thoracic  duct,  though  it  probably  varies  con- 
siderably, is  probably  also  always  very  large.  It  has  been  cal- 
culated that  in  a  well-fed  animal  a  quantity  equal  at  least  to 
that  of  the  whole  blood  may  pass  through  the  thoracic  duct  in 
24  hours,  and  of  this  it  is  supposed  that  about  half  comes 
through  the  lacteals  from  the  alidominal  viscera,  and  therefore 
to  a  large  extent  from  food,  and  the  remainder  from  the  body 
at  large.  These  calculations  are  based  on  uncertain  data,  and 
cannot  therefore  be  taken  as  of  exact  value,  but  we  ma}^  use 
them  for  the  sake  of  an  illustration.  Thus  in  a  man  of  aver- 
age weight,  that  is,  about  70  kilos,  the  quantity  of  blood  (§  38) 
being  Jg-  of  the  body  weight  is  about  6  kilos.  The  quantity  of 
lymph  or  chyle  therefore  discharged  into  the  blood  in  an  liour 
would  be  according  to  this  calculation  a  quarter  of  a  kilo,  or 
something  less  than  a  quarter  of  a  litre ;  and  since  the  flow 
must  vary  considerably  in  the  24  hours,  would  be  therefore 
sometimes  less  and  sometimes  even  more  than  this. 

The  Movements  of  Lymph. 

§  242.  Making  every  allowance  for  the  uncertainty  of  the 
calculation  detailed  in  the  preceding  paragraph,  it  is  obvious  that 


404  MOVEMENT   OF  LYMPH.  [Book  ii. 

the  lymph  must  flow  with  a  not  inconsiderable  rapidity  (if  we 
take  about  half  the  above  estimate,  the  rate  will  be  about  1  or  2 
c.c.  per  minute)  through  the  thoracic  duct,  and  therefore  must 
also  be  continually  streaming  into  that  duct,  along  the  various 
lymphatic  channels  from  the  manifold  lymph-spaces  of  the  body. 
This  onward  progress  of  the  lymph  is  determined  by  a  variety 
of  circumstances.  In  the  first  place,  the  remarkably  wide-spread 
presence  of  valves  in  the  lymphatic  vessels  causes  every  pressure 
exerted  on  the  tissues  in  which  they  lie  to  assist  in  the  propul- 
sion forward  of  the  lymph.  Hence  all  muscular  movements 
increase  the  flow.  If  a  cannula  be  inserted  in  one  of  the  larger 
lymphatic  trunks  of  the  limb  of  a  dog,  the  discharge  of  lymph 
from  the  cannula  will  be  more  distinctly  increased  by  move- 
ments, even  passive  movements,  of  the  limb  than  by  anything 
else.  When  we  come  to  speak  of  the  entrance  of  chyle  into  the 
lacteal  radicles  of  the  villi,  we  shall  see  that  the  muscular  fibres 
of  the  villus  act  as  a  kind  of  muscular  pump,  driving  the  chyle 
past  the  valved  end  of  the  lacteal  radicle  into  the  lymphatic 
canals  below.  In  addition  to  the  presence  of  valves  along  the 
course  of  the  vessels,  the  opening  of  the  thoracic  duct  into  the 
venous  system  is  guarded  by  a  valve,  so  that  every  escape  of 
lymph  or  chyle  from  the  duct  into  the  veins  becomes  itself  a 
help  to  the  flow.  In  the  second  place,  we  have  already  seen 
that  the  blood-pressure  in  the  capillaries  and  minute  vessels  is 
considerably  greater  than  that  in  the  large  veins,  such  as  the 
jugular ;  in  fact  this  difference  of  pressure  is  the  cause  of  the 
flow  of  blood  from  the  capillaries  to  the  heart.  Now  even  as- 
suming that  the  lymph  in  the  lymphatic  spaces  outside  the  cap- 
illaries and  minute  vessels  necessarily  stands  at  a  lower  pressure 
than  the  blood  inside  the  capillaries,  on  the  ground  that  other- 
wise the  transudation  from  the  blood  into  the  tissues  would  be 
checked,  we  must  still  admit  that  the  difference  is  less,  probably 
much  less,  than  the  difference  between  the  pressure  in  the  cap- 
illaries and  that  in  the  large  venous  trunks.  So  that  the  lymph 
in  the  lymph-spaces  of  the  tissues  may  be  considered  as  standing 
at  a  higher  pressure  than  the  blood  in  the  venous  trunks,  for 
instance  in  the  jugular  vein.  That  is  to  say,  the  lymphatic  ves- 
sels as  a  whole  form  a  system  of  channels  leading  from  a  region 
of  higher  pressure,  viz.  the  lymph-spaces  of  the  tissues,  to  a 
region  of  lower  pressure,  viz.  the  interior  of  the  jugular  and 
subclavian  veins.  This  difference  of  pressure  will,  as  in  the  case 
of  the  blood  vessels,  cause  the  lymph  to  flow  onward  in  a  con- 
tinuous stream.  Further,  this  flow,  caused  by  the  lowness  of 
the  mean  venous  pressure  at  tlie  subclavian  vein,  will  be  assisted 
at  every  respiratory  movement,  since  at  every  inspiration  the 
pressure  in  the  venous  trunks  becomes,  as  we  shall  see  in  deal- 
ing with  respiration,  negative,  and  thus  lymph  will  be  sucked 
in  from  the  thoracic  duct,  while  the  increase  of  pressure  in  the 


Chap,  i.]  TISSUES  AND  MECHANISMS  OF  DIGESTION.    405 

great  veins  during  expiration  is  warded  off  from  the  duct  by  the 
valve  at  its  opening.  In  the  third  place,  the  flow  may  be  in- 
creased by  rhythmical  contractions  of  the  walls  of  the  lymphatics 
themselves,  which  are  remarkably  muscular ;  and  the  peculiar 
interlacing  of  the  muscular  fibres  above  each  valve  suggests 
that  the  walls  here  act  after  the  fashion  of  a  tiny  heart  and  by 
a  rhythmical  systole  drive  on  the  fluid,  which  by  the  action  of 
the  valve  below  collects  at  the  spot.  We  have  however  no  ex- 
perimental proof  of  this ;  for,  though  rhythmic  variations  have 
been  observed  in  the  lacteals  of  the  mesentery,  it  is  maintained 
that  these  are  simply  passive,  i.e.  caused  by  the  rhythmic  per- 
istaltic action  of  the  intestine,  each  contraction  of  the  intestine 
filling  the  lymph-channels  more  fully,  and  are  not  due  to  con- 
tractions of  the  walls  of  the  lacteal  vessels  themselves.  In  some 
of  the  lower  animals,  for  instance  in  the  frog,  the  muscular  walls 
of  the  vessels  are  developed  at  places  into  distinctly  contractile 
propulsive-organs,  spoken  of  as  lymph-hearts.  Lastly,  if  the 
very  processes  which  give  rise  to  the  appearance  of  lymph  in  the 
lymph-spaces  of  the  tissues  are,  as  we  shall  see  we  have  some 
reason  to  think,  analogous  to  the  process  of  secretion,  then 
remembering  the  pressure  which  is  developed  by  the  secretion 
of  a  secreting  gland,  such  as  a  salivary  gland  (§  189),  Ave  may 
regard  these  very  processes  as  tending  themselves  to  promote 
the  flow  of  lymph.  We  have  at  least,  under  all  circumstances, 
one  or  other  of  these  causes  at  work,  promoting  a  continual  flow 
from  the  lymphatic  roots  to  the  great  veins.  They  are  together 
sufficient  to  drive,  in  man,  the  lymph  from  the  lower  limbs  and 
trunk,  against  the  effects  of  gravity,  into  the  veins  of  the  neck. 
In  the  upper  limb,  the  influences  of  gravity  OAving  to  the  varied 
movements  of  the  limb,  are  as  often  favourable  to,  as  opposed 
to,  the  natural  floAV  of  the  lymph ;  but  as  Ave  haA^e  already  said, 
a  long-continued  unfavourable  action  of  graA^ty,  especially  in 
the  absence  of  the  aid  of  movements  in  the  skeletal  muscles,  as 
Avhen  the  arm  hangs  down  motionless  for  some  time,  leads  to 
accumulation  of  lymph  at  its  origin  in  the  lymph-spaces.  The 
strength  of  the  causes  combining  to  driA'e  on  the  lymph  is  strik- 
ingl}^  shcAvn  in  animals  Avhen  the  thoracic  duct  is  ligatured ;  in 
such  cases  a  A^ery  great  distension  of  the  lymphatic  vessels  below 
the  ligature  is  observed. 

§  243.  Although  the  phenomena  of  disease  and,  perhaps, 
general  considerations  render  it  probable  that  the  nervous  system 
governs  in  some  Avay  the  stream  of  lymph,  regulating  it  may  be 
not  only  the  floAv  along  the  definite  lymph-canals  but  also  the 
transit  of  plasma  into  the  lymph-spaces  and  the  escape  of  lymph 
thence  into  the  definite  canals,  our  knoAvledge  on  these  points  is 
very  imperfect.  We  have  as  yet  at  least  no  proof  that  the  mus- 
cular fibres  in  the  Avails  of  the  lymphatic  vessels  are  governed 
by  nerves,  or  that  tlie  lymph-spaces  are  influenced  directly  by 


406  MOVEMENT   OF  LYMPH.  [Book  ii. 

nervous  action ;  attempts  to  demonstrate  any  direct  action  of 
the  nervous  system  on  the  lymphatics  have  hitherto  failed. 

§  244.  The  passage  of  material,  namely,  of  water  containing 
certain  substances  in  solution,  from  the  interior  of  the  blood 
vessel  where  they  form  part  of  the  plasma  into  the  lymph-cap- 
illary where  they  are  called  lymph  consists  of  two  steps :  the 
passage  from  the  blood  vessel  into  the  lymph-space,  and  the 
passage  from  the  lymph-space  into  the  lymph-capillary;  for 
it  is  only  in  particular  places  that  the  lymph-capillary  imme- 
diately surrounds  the  blood  vessel.  Once  arrived  in  the  lymph- 
capillary  the  lymph  finds  an  open  path  along  the  rest  of  the 
lymphatic  system,  but  the  connection  between  the  lymph-space 
and  the  lymph-capillary  is  peculiar  and  at  least  not  a  free  and 
open  one. 

The  passage  of  material  from  the  blood  vessel  into  the  lymph- 
space  we  speak  of  as  transudation.  What  can  we  say  as  to  the 
nature  of  this  process  ?  There  are  two  known  physical  processes 
with  which  we  may  compare  it :  diffusion  through  a  membra- 
nous or  other  porous  partition,  and  filtration  through  a  smiilar 
partition.  Diffusion,  though  influenced  by  fluid  pressure,  is 
not  the  direct  result  of  fluid  pressure  but  may  on  the  contrary 
be  the  cause  of  differences  of  pressure  on  the  two  sides  of  a  par- 
tition, and  may  work  against  fluid  pressure.  When  a  strong 
solution  and  a  weak  solution  of  salt  are  separated  by  a  diffusion 
septum,  diffusion  takes  place  whether  the  columns  of  fluid  be 
at  the  same  level  on  the  two  sides  of  the  septum  or  at  different 
levels ;  and  if  the  columns  be  at  the  same  level  to  start  with, 
that  of  the  stronger  solution  soon  comes  to  exceed  the  other  in 
height,  on  account  of  the  osmotic  flow  of  water  from  the  weaker 
into  the  stronger  solution.  Filtration  on  the  other  hand  is  the 
direct  result  of  pressure ;  without  difference  of  pressure  filtra- 
tion does  not  take  place ;  and,  the  filter  remaining  of  the  same 
nature  and  in  the  same  condition,  the  amount  of  filtrate  is  de- 
pendent on  the  amount  of  pressure.  May  we  speak  of  the  pro- 
cess of  transudation  as  a  simple  process  of  diffusion  or  a  simple 
process  of  filtration,  that  is  to  say,  can  all  the  phenomena  of 
transudation  be  explained  as  simply  the  results  of  one  or  other 
of  these  physical  processes  ?  Diffusion  by  itself  will  not  ac- 
count for  the  results ;  for  the  proteids  of  the  blood-plasma  are 
indiff usible  or  very  nearly  so  and  yet  the  lymph  contains  a  con- 
siderable quantity  of  these  proteids.  We  have  no  satisfactory 
knowledge  of  the  exact  composition  of  lymph  as  it  exists  in  the 
lymph-spaces.  In  the  lymph  of  the  larger  lymph-trunks  the 
diffusible  saline  substances  are  present  in  about  the  same  pro- 
portion, and  the  indiffusible  proteids  to  about  or  less  than  half 
as  much  as  in  blood-serum ;  and  we  may  perhaps  assume  that 
the  lymph  in  the  lymph-spaces  contains  relatively  less  proteids 
but  has  otherwise  the  same  composition  as  blood-plasm.     Mere 


Chap,  i.]  TISSUES  AND  MECHANISMS  OF  DIGESTION.    407 

diffusion  would  not  give  rise  to  a  fluid  of  such  a  nature.  Can 
we  speak  of  transudation  then  as  a  filtration  ?  The  blood  is 
undoubtedly  flowing  through  the  capillaries  and  other  small  ves- 
sels under  a  certain  pressure ;  we  have  seen  (§  98)  that  the 
pressure  is  roughly  speaking  about  20  mm.  Hg.  ;  and  it  would 
be  possible  to  select  such  a  filter  or  porous  partition  as  would 
at  about  this  pressure  permit  the  passage  of  a  certain  quantity 
of  the  inorganic  and  crystalline  constituents  of  blood-plasma  to 
pass  through  in  company  with  a  relatively  smaller  quantity  of 
the  proteids  and  a  large  quantity  of  the  water,  the  red  and  white 
corpuscles  being  excluded.  Such  a  filtrate  Avould  be  more  or 
less  of  the  nature  of  lymph ;  and  so  far  we  might  be  justified 
in  speaking  of  the  transudation  of  lymph  as  a  process  of  filtra- 
tion. But  the  transit  through  the  living  wall  of  the  blood  ves- 
sel is  affected  by  circumstances  in  a  manner  so  different  from 
the  manner  in  which  the  same  circumstances  affect  the  transit 
through  an  ordinary  lifeless  filter,  that  we  gain  but  little,  and 
may  be  led  into  error  by  sj^eaking  of  the  process  as  a  filtration. 
Substances  in  solution  or  otherwise  pass  through  a  filter  when 
the  pressure  is  sufficient  to  drive  them  through  the  passages 
furnished  by  the  interstices  existing  in  the  substance  of  the 
filter.  In  the  case  of  an  ordinary  filter  the  substance  of  the  fil- 
ter is  within  limits  i3ermanent,  and  the  passages  correspond- 
ingly constant.  The  living  wall  of  a  capillary  however  is  not 
a  constant  unchanging  thing.  The  epithelioid  plates  and  other 
elements  Avhich  constitute  it  are  alive,  and  being  alive  are 
continually  undergoing  change  and  are  especially  subject  to 
change ;  moreover,  as  we  have  seen  (§§  22,  23),  the  vascular 
walls  appear  to  be  continually  acting  upon  and  being  acted 
uj)on  b}"  the  blood.  Hence  a  change  in  the  blood  tends  to 
cause  changes  in  them ;  and  these  changes  may  materially 
affect  in  one  direction  or  another  their  action  as  filters.  In  an 
ordinary  filter  increase  of  pressure  necessarily  entails  increase 
of  filtration ;  in  a  living  filter  we  should  expect  to  find  that  it 
may  or  may  not,  that  variations  of  pressure  may  according  to 
circumstances  produce  very  different  results  as  regards  the 
transudation  of  lymph,  and  that  the  latter  may  var}^  independ- 
ently of  the  former. 

Observations  seem  to  confirm  this  view.  In  the  first  place 
an  increase  of  blood-pressure  does  not  necessarily  increase  the 
transudation  of  lymph.  It  is  true  that  when  a  small  artery 
dilates,  by  which  the  pressure  in  the  still  smaller  branches  and 
capillaries  of  that  artery  is,  as  we  have  more  than  once  pointed 
out,  increased,  more  lymph  as  a  rule  appears  in  the  lymph- 
spaces  ;  indeed  it  is  one  of  the  main  purposes  of  the  widening 
of  small  arteries  to  supply  the  elements  of  the  tissue  with  more 
lymph,  that  is,  with  more  food.  But  it  does  not  therefore  fol- 
low that  under  all  circumstances  widening  of  the  artery  should 


408  TEANSUDATIOK  [Book  ii. 

increase  the  passage  of  lymph ;  something  may  occur  to  coun- 
teract the  natural  effect  of  the  increased  pressure  in  the  blood 
vessels.  An  instance  of  this  seems  to  be  afforded  by  the  case 
of  the  submaxillary  gland,  when  the  chorda  nerve  is  stimulated 
while  the  gland  is  under  the  influence  of  atropin.  As  we  have 
seen,  though  the  arteries  dilate,  no  secretion  takes  place ;  and 
we  cannot  explain  the  absence  of  a  flow  into  the  alveoli  by  sup- 
posing that  the  extra  amount  of  lymph  which  would  in  normal 
circumstances  form  part  of  the  secretion,  and  in  the  case  of  a 
fairly  copious  secretion  would  be  considerable,  now  passes  away 
by  the  lymphatics  without  reaching  the  cells  of  the  alveoli,  for 
in  such  cases  no  extra  flow  in  the  lymphatics  leading  from  the 
gland  has  been  observed,  and  there  is  no  accumulation  of  lymph 
in  the  connective  tissue  of  the  gland.  Apparently,  for  some 
reason  or  other,  in  spite  of  the  increased  pressure  in  the  blood 
vessels  more  lymph  than  usual  does  not  pass  into  the  lymph- 
spaces. 

In  the  second  place,  increase  of  pressure  does  not  always- 
produce  the  same  amount  of  transudation.  For  instance,  as  we 
shall  presently  have  occasion  to  point  out,  an  increase  of  pres- 
sure in  the  blood  vessels  produced  by  obstruction  to  the  venous 
outflow  is  much  more  efficient  in  promoting  an  increase  of  tran- 
sudation, at  all  events  an  abnormal  increase,  than  is  an  increase 
of  arterial  pressure ;  and  the  difference  between  the  two  cases 
appears  to  be  too  great  to  be  accounted  for  on  the  ground  that 
an  obstruction  to  the  venous  outflow  raises  the  pressure  within 
the  capillaries  and  small  vessels  more  readily  and  to  a  higher 
degree  than  does  the  widening  of  the  arteries.  Further  the 
same  amount  of  venous  obstruction  giving  rise  to  the  same 
amount  of  capillary  pressure  may  or  may  not  give  rise  to  exces- 
sive transudation  according  to  the  condition  of  the  blood  or 
other  circumstances.  For  instance,  though  the  obstruction 
produced  by  ligaturing  a  vein  frequently  causes  excessive  tran- 
sudation, it  does  not  always  cause  it,  and  the  femoral  vein  of  a 
dog  may  be  ligatured  without  any  excessive  transudation  tak- 
ing place  ;  yet  if,  after  the  ligature,  certain  changes  be  induced 
in  the  blood  excessive  transudation  occurs  in  the  leg,  the  vein 
of  which  has  been  ligatured  but  not  elsewhere.  Pointing  towards 
the  same  conclusion  is  the  fact  that  excessive  transudation  more 
readily  occurs  when  a  vein  is  plugged  by  a  thrombus  arising 
from  abnormal  conditions  of  the  vascular  system  than  when 
a  vein  is  simply  ligatured. 

In  the  third  place  if  we  measure  the  flow  of  lymph  along  the 
duct  (by  introducing  a  cannula  into  the  thoracic  duct  at  its 
end  near  the  great  veins),  and  we  may  take  this  as  a  measure 
of  the  transudation  going  on,  we  find  that  this  flow  may  be  very 
greatly  increased,  without  any  change  of  blood-pressure  neces- 
sarily taking  place,  by  the  introduction  into  the  blood  of  certain 


Chap,  i.]  TISSUES  AND  MECHANISMS  OF  DIGESTION.    409 

substances,  such  as  leech  extract  or  extract  of  crabs'  muscles.  The 
presence  of  these  substances  in  the  blood  appears  to  produce  such 
a  change  in  the  vascular  walls,  that  without  any  change  of  pres- 
sure, a  larger  amount  of  lymph  and  that  richer  in  solids,  espe- 
cially in  proteids,  makes  its  way  into  the  lymph-spaces,  and  so 
into  the  thoracic  duct.  The  result  has  an  obvious  resemblance 
to  the  act  of  secretion,  and  the  substances  in  question  liave 
been  called  lymjjJiagogues.  We  may  tlierefore  conclude  that  two 
things  chiefly  determine  the  amount  of  transudation :  the  pres- 
sure of  the  blood  in  the  blood  vessels,  and  the  condition  of 
the  vascular  walls  in  relation  to  the  blood,  the  latter  being  at 
least  as  important  as  the  former. 

We  said  just  now  that  we  may  take  the  flow  of  lymph  along 
the  thoracic  duct  as  a  measure  of  the  transudation  from  the 
blood  vessels.  But  this  is  not  strictly  true.  The  lymph  in 
the  lymph-spaces,  which  is  the  source  of  the  lymph  in  the 
thoracic  duct,  is  not  simply  the  result  of  the  transudation 
from  the  blood  vessels,  but  the  result  of  the  combined  action 
of  the  blood  vessels  on  the  one  hand  and  the  tissues  on  the 
other.  The  lymph  in  the  lymph-space  is  the  middleman ;  the 
tissues  take  from  and  give  to  it  in  some  such  manner  as  the  blood 
gives  to  and,  we  may  add,  takes  from  it.  We  remarked  in  §  30 
on  the  peculiar  relations  of  living  tissue  to  water,  and  there  are 
reasons  for  thinking  that  the  very  substance  of  a  cell  or  a  fibre 
(a  muscular  fibre  for  instance)  may  hold  in  itself  a  larger  quan- 
tity of  water  at  one  time  than  at  another.  The  water  thus  taken 
up  or  given  out,  and  other  substances  carried  by  that  water, 
come  from  and  go  to  the  lymph.  The  condition  of  the  tissue 
determines  by  itself  the  amount  of  lymph  in  the  lymph-spaces, 
and  thus  the  flow  of  lymph  along  the  thoracic  duct.  For 
instance,  a  certain  quantity  of  sugar  introduced  into  the  blood 
gives  rise  to  a  very  rapid  flow  of  somewhat  dilute  lymph 
along  the  thoracic  duct ;  and  similar  results  are  produced  by 
much  smaller  quantities  of  sodium  chloride  and  other  sub- 
stances. Since  the  blood  is  found  to  be,  at  the  same  time, 
more  watery,  in  spite  of  a  copious  secretion  of  urine,  we  may 
conclude  that  the  excess  of  lymph  in  the  lymph-spaces  is  drawn 
from  the  tissues. 

§  245.  Under  the  influence  of  all  these  several  actions  the 
lymph  in  the  various  lymph-spaces  of  the  body  varies  in  amount 
from  time  to  time,  but  under  normal  circumstances  never  ex- 
ceeds certain  limits.  Under  23athological  conditions  those  limits 
may  be  exceeded,  and  the  result  is  known  as  oedema  or  dropsy. 
Similar  excessive  accumulations  of  lymph  may  occur  not  in  the 
ordinary  lymph-spaces,  but  in  those  larger  lymph-spaces,  the 
serous  cavities,  any  large  excess  of  fluid  in  the  peritoneal  cav- 
ity being  known  as  ascites. 

The  possible  causes  of  oedema  are  on  the  one  hand  an  obstruc- 


410  CEDEMA.  [Book  ii. 

tion  to  the  flow  of  lymph  from  the  lymph-spaces,  and  on  the 
other  hand  an  excessive  transudation,  the  lymph  gathering  in 
the  lymph-spaces  faster  than  it  can  be  carried  away  by  a  nor- 
mal flow;  with  the  former  the  lymphatic  system  itself,  with 
the  latter  chiefly  the  vascular  system  is  concerned.  As  a  mat- 
ter of  fact,  however,  oedema  is  almost  always,  if  not  always, 
due  to  abnormal  conditions  of  the  vascular  system,  and  is  the 
result  not  of  hindered  outflow  but  of  excessive  transudation. 
Owing  to  the  numerous  anastomoses  of  the  lymph-vessels  and 
the  consequent  establishment  of  collateral  streams,  obstruction 
in  the  lymph-passages  themselves  rarely  if  ever  gives  rise  to 
oedema ;  and  it  may  be  here  remarked  that  owing  to  the  same 
free  collateral  communication  between  the  lymph-vessels  the 
labyrinthine  passages  of  the  lymphatic  glands  do  not  offer  the 
serious  obstacle  to  the  onward  flow  of  the  general  lymph-stream 
as  might  at  first  sight  be  supposed.  Nor  have  we  at  present 
any  knowledge  which  would  lead  us  to  suppose  that  any  physi- 
ological changes  in  the  walls  of  the  lymphatic  vessels  or  of  the 
lymph-capillaries,  or  in  the  lymph-spaces,  by  giving  rise  in  some 
way  to  obstacles  to  the  flow  of  lymph,  ever  lead  to  an  accumu- 
lation of  lymph  in  the  latter. 

One  kind  of  oedema  we  have  already  touched  upon  in  speak- 
ing of  the  capillary  circulation  (§  161),  viz.  the  "inflamma- 
tory" oedema.  In  this  kind  of  oedema  owing  to  changes  in 
the  vascular  walls  a  larger  amount  of  transudation  passes  into 
the  lymph-spaces,  and  that  transudation  is  richer  in  proteid 
matters,  and  contains  a  larger  amount  of  the  fibrin  factors,  or 
at  all  events  is  much  more  distinctly  coagulable  than  ordinary 
lymph,  as  well  as  crowded  with  migrating  corpuscles.  Allied 
to  this  inflammatory  oedema  is  the  increase  of  lymph,  also 
apparently  changed  somewhat  in  character,  Avhich  appears  as 
"  effusion "  in  the  serous  cavities  when  these  are  inflamed,  as 
in  pleurisy  and  peritonitis. 

One  of  the  most  common  forms  of  oedema  is  an  oedema  of 
primarily,  though  not  wholly,  mechanical  origin,  oedema  aris- 
ing from  obstruction  to  the  venous  flow ;  under  these  circum- 
stances more  lymph  passes  into  the  lymph-spaces  than  the 
lymph-vessels  are  able  to  carry.  If  the  femoral  vein  be  tied 
the  leg  may  become  oedematous,  and,  as  we  have  said,  oedema 
is  a  common  result  of  the  plugging  or  obstruction  of  veins 
through  disease  ;  the  oedema  which  is  so  common  an  accompa- 
niment of  heart  disease  involving  obstruction  to  the  return  of 
venous  blood  to  the  right  side  of  the  heart,  and  the  ascites 
which  follows  upon  hindrance  to  the  portal  flow  are  instances 
of  oedema  of  this  kind.  We  have  already  remarked  on  the 
relation  of  transudation  to  blood-pressure ;  and  in  venous  ob- 
struction the  rise  of  pressure  within  the  small  blood  vessels 
is  distinguished  from  that  due  to  arterial  dilation  by  being  ac- 


Chap,  i.]  TISSUES  AND  MECHANISMS  OF  DIGESTION.    411 

companied  with  a  want  of  adequate  renewal  of  the  blood ;  this 
probably  affects  the  epithelioid  lining  of  the  blood  vessels  in 
such  a  way  as  to  increase  the  transudation.  And  indeed,  as  is 
seen  in  cases  of  heart  disease  with  prolonged  or  repeated  venous 
obstruction,  the  oedema,  as  time  goes  on  and  the  tissues  become 
impaired,  is  more  easily  excited  and  with  greater  difficulty 
removed,  though  the  actual  amount  of  obstruction,  the  actual 
increase  of  pressure  in  the  small  vessels,  remains  the  same,  or 
at  least  is  not  proportionally  increased. 

Still  another  kind  of  cedema  is  one  due  to  changes  taking 
place  in  the  blood,  quite  apart  from  variations  of  blood-pressure. 
This  kind  of  oedema  is  seen  in  some  diseases  of  the  kidney,  in 
"  Bright's  disease  "  for  instance.  In  such  cases  the  blood  con- 
tains less  j^roteids,  and  indeed  less  solids,  is  more  watery  and 
of  lower  specific  gravity  than  is  normal.  But  the  cedema  is  not 
in  these  cases  to  be  explained  on  the  view  that  the  more  watery 
blood  passes  more  readily  through  the  capillary  walls,  for  it  may 
be  shewn  experimentally  that  the  mere  thinning  of  the  blood, 
as  by  the  injection  of  normal  saline  solution  into  the  blood  ves- 
sels, will  not  at  once  lead  to  oedema,  at  least  in  the  limbs  and 
trunk,  and  it  is  these  which  in  Bright's  disease  especially  be- 
come oedematous.  In  all  probability  the  cedema  of  Bright's 
disease  if  it  be  really  due  to  the  abnormal  character  of  the  blood, 
is  produced  by  the  abnormal  blood  so  acting  on  the  blood  vessels 
that  these  allow  a  transudation  greater  than  the  normal.  Lastly, 
calling  to  mind  what  we  said  just  now  as  to  the  relations  of 
the  tissue  to  the  lymph,  we  must  remember  that  the  cause  oi 
oedema  may  also  lie  in  changes  in  the  tissue  itself. 

But  these  are  pathological  questions  into  which  we  must  not 
enter  here.  We  have  touched  uj)on  them  because  they  illus- 
trate the  important  processes  taking  place  in  the  lymph-spaces, 
and  as  we  have  more  than  once  insisted  the  lymph  in  the  lymph- 
spaces  is  the  middleman  of  all  the  tissues,  and  hence  facts  illus- 
trating the  laws  which  govern  the  flow  of  lymph  into  and  out 
of  the  lymph-spaces  are  of  fundamental  physiological  impor- 
tance. 


SEC.  9.  ABSOEPTION  FROM  THE  ALIMENTAEY 
CANAL. 


§  246.  We  may  now  return  to  consider  the  absorption  of  the 
products  of  digestion,  that  is  to  say,  the  passage  of  these  bodies 
from  the  interior  of  the  alimentary  canal,  where  they  are  really 
outside  the  body  proper,  into  the  body  itself.  For  simplicity's 
sake  we  may  consider  digestion  in  a  broad  way  as  the  conver- 
sion of  practically  non-diffusible  proteids  and  starch  into  more 
diffusible  peptone  and  highly  diffusible  sugar,  and  as  the  emul- 
sifying, or  division  into  minute  particles,  of  fats.  We  have 
seen  reason  to  believe  that  some  of  the  sugar  may  be  changed 
into  lactic  acid  or  even  into  butyric  or  other  acids,  that  some  of 
the  proteids  are  carried  beyond  the  peptone  condition  into  leu- 
cin  and  other  bodies,  and  that  some  of  the  fat  may  be  saponified ; 
and  it  may  be  that  some  of  the  proteid  material  of  the  food 
passes  into  the  body  as  albumose  or  even  as  parapeptone,  or  in 
some  other  little  changed  condition.  But  we  may  probably 
with  safety,  for  present  purposes,  assume  that  the  greater  part 
of  the  proteid  is  absorbed  as  peptone,  that  carbohydrates  are 
mainly  absorbed  as  sugar,  and  that  the  greater  part  of  the  fat 
passes  into  the  body  as  emulsified  but  otherwise  unchanged  neu- 
tral fat ;  and  we  may  neglect  the  other  conditions  of  digested 
food  as  subsidiary,  and  as  far  as  absorption  is  concerned,  unim- 
portant. 

We  have  seen  that  two  paths  are  open  for  these  products  of 
digestion,  one  by  the  capillaries  of  the  portal  system,  the  other 
by  the  lacteals.  It  cannot  be  a  matter  of  indifference  which 
course  is  taken.  For  if  the  products  pass  by  the  lacteals  they 
fall  into  the  general  blood-current  after  having  undergone  only 
such  changes  as  they  may  experience  in  the  lymphatic  system ; 
while  if  they  pass  into  the  portal  vein  they  are  subjected  to 
certain  powerful  influences  of  the  liver  (which  we  shall  study 
in  a  future  chapter)  before  they  find  their  way  to  the  right  side 
of  the  heart.  It  has  been  possible,  in  the  dog,  so  to  connect 
the  portal  vein  with  the  inferior  vena  cava,  that  the  portal  blood 
is  diverted  into  the  latter,  and  so  is  thrown  on  the  general  cir- 

412 


Chap,  i.]  TISSUES  AND  MECHANISMS  OF  DIGESTION.    413 

dilation  without  having  passed  through  the  liver.  When  this 
is  done  very  grave  troubles  result.  We  may  therefore  consider 
first  wliicli  of  the  two  paths  is,  as  a  matter  of  fact,  taken  by  the 
several  products,  and  subsequently  study  the  mechanism  of  ab- 
sorption in  the  two  cases. 

The  Course  taken  hy  the  Several  Products  of  Digestion. 

§  247.  From  what  has  already  been  said  we  have  been  led 
to  regard  the  villi  as  the  most  active  organs  of  absorption,  and 
the  structure  of  a  villus  leads  us  further  to  conclude  that  the 
diffusible  peptones  and  sugar  pass,  together  with  the  water  in 
which  they  are  dissolved,  into  the  superficially  placed  capillary 
network  of  the  villus  and  so  into  the  portal  system,  while  the 
merely  emulsified  fat,  unable  to  traverse  the  wall  of  the  capil- 
lary, passes  on  to  the  deajD-seated  lacteal  radicle,  and  so  finds 
its  way  into  the  lymphatic  system.  And  the  results  of  obser- 
vation and  experiment,  as  far  as  they  go,  support  this  view. 

Fats.  After  a  meal  containing  fat  the  lymph  of  the  lacteals 
contains  fat,  and  is  now  called  chyle ;  and  the  richer  the  meal 
in  fat  the  more  conspicuous  is  the  fat  in  the  lymph- vessels. 
We  cannot  however  prove  that  all  the  fat  of  a  meal  absorbed 
from  the  alimentary  is  poured  by  the  thoracic  duct  into  the 
venous  system.  If  a  meal  containing  a  known  quantity  of  fat 
be  given  to  a  dog  and  the  small  quantity  of  fat  jDresent  in  the 
faces  corresponding  to  the  meal  be  subtracted  from  that  amount, 
we  can  determine  the  amount  of  fat  absorbed,  for  we  have  no 
evidence  whatever  that  any  appreciable  amount  of  fat  under- 
goes a  destructive  decomposition  in  the  alimentary  canal.  Col- 
lecting by  means  of  a  cannula  inserted  into  the  thoracic  duct 
the  whole  of  the  chyle  during  and  after  the  meal  so  long  as  it 
remains  milky,  showing  that  fat  is  being  absorbed,  we  can 
ascertain  the  quantity  of  absorbed  fat,  Avhich  would,  but  for 
the  operation,  have  passed  into  the  venous  system.  When  this 
has  been  done,  a  very  remarkable  deficit,  amounting  it  may  be 
to  40  or  50  p.c.  has  been  observed ;  that  is  to  say,  of  every  100 
parts  of  fat  which  disappear  from  the  alimentary  canal  only 
about  60  parts  find  their  way  through  the  thoracic  duct  into 
the  venous  system. 

Are  we  then  to  conclude  that  the  missing  quantity  finds  its 
way  into  the  portal  system  ?  Now  the  portal  blood  does,  dur- 
ing digestion,  contain  a  certain  quantity  of  fat ;  indeed  the 
serum  is  said  at  times  to  appear  milky  from  the  presence  of  fat. 
But  the  whole  circulating  blood  during  the  digestion  of  a  fatty 
meal  contains,  for  a  while,  the  fat  poured  into  it  by  the  thoracic 
duct ;  and  it  lias  been  ascertained  in  the  dog  that  the  blood  of 
the  portal  vein  during  digestion  contains  not  more  but  less  fat 
than  the  blood  of  the  carotid  arterv,  so  that   the   fat  which 


414  PATH   TAKEN  BY   SUGAR.  [Book  ii, 

appears  in  the  portal  blood  during  digestion  is,  for  the  most 
part  at  least,  not  fat  absorbed  by  the  capillaries  of  the  alimen- 
tary canal  but  fat  absorbed  by  the  lacteals.  Moreover,  when 
the  chyle  of  the  thoracic  duct  is  diverted  through  a  cannula, 
and  not  allowed  to  flow  into  the  blood,  the  quantity  of  fat  in 
the  portal  blood  as  in  the  blood  at  large  is  very  small  indeed. 
Lastly,  when  a  villus  of  an  intestine  in  full  digestion  of  fat  is 
treated  with  osmic  acid,  fat  cannot  be  recognized  by  the  micro- 
scope within  the  capillaries  or  other  blood  vessels,  though  it 
abounds  outside  them  in  the  substance  of  the  villus  and  in  the 
lacteal  radicle. 

We  may  probably  therefore  infer  with  safety  that  all  or  at 
least  very  nearly  all  the  fat  absorbed  from  the  intestine  takes 
the  path  of  the  lacteals.  As  to  the  deficit  mentioned  above, 
that  is  as  yet  without  explanation.  It  may  be  that  m  some 
way,  on  its  course,  in  the  lymphatic  glands,  for  instance,  the  fat 
is  taken  away  from  the  chyle,  hidden  so  to  speak  somewhere 
away  from  both  chyle  and  blood ;  but  on  this  point  we  have  no 
exact  information. 

§  248.  Water  and  Salts.  If,  in  an  animal,  the  rate  of  flow 
of  lymph  or  chyle  through  a  cannula  placed  in  the  thoracic  duct 
be  watched,  and  water  or,  to  avoid  the  injurious  effect  of  simple 
water  on  the  mucous  membrane,  normal  saline  solution  be  then 
injected  in  not  too  great  quantity  into  the  intestine,  no  marked 
increase  in  the  flow  of  chyle  through  the  cannula  is  observed. 
From  this  we  may  infer  that  the  water  of  the  intestinal  contents 
is  absorbed  not  into  the  lacteals  but  into  the  portal  system.  If 
however  a  very  large  quantity  of  the  normal  saline  solution  be 
injected  so  as  to  distend  the  intestine,  then  the  flow  of  chyle  is 
increased  to  some  extent.  It  would  appear  therefore  that 
while  under  normal  conditions  the  water  passes  from  the  intes- 
tine mainly  into  the  portal  blood,  some  of  it  may  under  circum- 
stances pass  into  the  lacteals. 

With  regard  to  the  course  taken  by  ordinary  saline  matters 
we  possess  no  detailed  information.  When  special  salts  such 
as  potassium  iodide  and  others,  easily  recognized  by  appropriate 
tests,  are  introduced  into  the  intestine,  they  may  be  speedily 
detected  both  in  the  blood  and  in  the  contents  of  the  thoracic 
duct ;  but  whether,  in  such  cases,  these  salts  find  their  way  into 
the  thoracic  duct  by  the  lacteal  radicle  of  the  villi,  or  pass  into 
the  lymph  stream  at  some  later  part  of  its  course,  we  do  not 
know.  Nor  can  we  with  regard  to  such  a  salt  as  sodium 
chloride,  state  absolutely  that  it  passes  mainly  with  the  water 
into  the  portal  blood,  though  we  may  fairly  suppose  this  to  be 
the  case. 

§  249.  Sugar.  Both  blood  and  chyle  contain,  normally,  a 
certain  small  amount  of  sugar ;  and  careful  inquiries  shew  that 
the  percentage  of  sugar  in  chyle  and  in  general  blood  is  fairly 


Chap,  i.]  TISSUES  AND  MECHANISMS  OF  DIGESTION.    415 

constant,  neither  being  to  any  marked  extent  increased  by  even 
amylaceous  meals ;  on  the  other  hand,  a  meal  containing  sugar 
or  starch  does  temporarily  increase  the  quantity  of  sugar  in  the 
portal  blood.  From  this  we  may  infer  that  such  portions  of  the 
sugar  of  the  intestinal  contents  as  are  absorbed  as  sugar  pass 
exclusively  by  the  portal  vein.  We  may  however  here  call 
attention  to  the  difficulties  attending  an  argument  of  this  kind. 
In  the  first  place  the  quantitative  determination  of  a  small 
amount  of  sugar  in  so  complex  a  fluid  as  blood  is  attended  with 
great  difficulties  and  uncertainties.  In  the  second  place  a  very 
large  quantity  of  blood  is  at  any  one  moment  streaming  through 
the  capillaries  of  the  alimentary  canal ;  and  we  may  perhaps 
speak  of  the  quantity  which  passes  through  them  during  the 
whole  period  of  digestion  as  being  enormous.  Hence  though 
each  100  c.c.  in  passing  through  the  capillaries  might  take  up  a 
quantity  of  sugar  so  small  as  to  fall  almost  within  the  limits  of 
errors  of  observation,  yet  the  whole  quantity  absorbed  during 
the  hours  of  digestion  might  be  considerable ;  or  to  put  it  in 
another  way,  an  error  of  observation,  unavoidable  with  our 
present  means  of  analysis,  on  a  sample  of  blood  taken  from  the 
portal  vessels  might  lead  to  a  wholly  unwarranted  conclusion 
that  sugar  Avas  or  was  not  being  absorbed.  Making  every 
alloAvance  however  for  these  difficulties,  the  increase  of  sugar 
which  has  been  observed  in  the  portal  blood  during  digestion 
seems  too  great  to  permit  of  any  other  conclusion  than  that 
sugar  is  really  absorbed  from  the  alimentary  canal  by  the  blood 
vessels. 

When  however  a  large  quantity  of  sugar  dissolved  in  a  large 
quantity  of  water  is  present  in  the  intestine,  the  sugar  in  the 
chyle  is  said  to  be  increased.  In  such  a  case  the  excess  of 
water,  as  stated  above,  passes  into  the  lacteals,  and  in  so  doing 
appears  to  carry  some  of  the  sugar  with  it. 

In  thus  speaking  of  the  sugar  as  passing  into  the  portal 
blood,  it  should  be  remembered,  that  while  the  greater  j)art  of 
the  sugar  of  a  meal,  that  formed  from  starch,  is  maltose,  the 
sugar  in  the  portal  blood  is  dextrose ;  either  within  the  alimen- 
tary or  more  probably  in  passing  through  the  epithelium  of 
the  wall  of  the  alimentary  canal,  the  maltose  is  changed  into 
dextrose. 

§  250.  Proteids.  The  difficulties  attending  the  experimental 
determination  of  the  path  taken  by  proteids  are  greater  even 
than  in  the  case  of  sugar ;  since  the  quantitative  determination 
of  peptone  in  portal  blood  and  chyle  respectively,  itself  a  task 
still  more  difficult  than  the  quantitative  determination  of  sugar 
in  the  same  fluids,  gives  us  no  clue.  For  we  have  evidence 
that  though  peptone  may  be  the  form,  or  chief  form,  in  which 
proteid  material  leaves  tlie  interior  of  the  alimentary,  it  is  not 
the  form  in  Avhieli  it  reaches  its  destination  be  that  destination 


416  PROTEIDS.  [Book  ii. 

tlie  portal  blood  or  the  chyle ;  during  its  transit  through  the 
epithelium  of  the  walls  of  the  alimentary  it  is  transformed  from 
peptone  back  again  into  some  or  other  of  the  natural  proteids 
of  the  blood  or  lymph.  That  peptone  is  so  changed  before  it 
€an  get  into  the  blood  is  shewn,  among  other  ways,  hy  the  fol- 
lowing observation.  If  an  artificial  circulation  of  blood  be 
kept  up  in  the  mesenteric  arteries  supplying  a  loop  of  intestine 
removed  from  the  body,  the  loop  may  be  kept  alive  for  some 
■considerable  time.  During  this  survival  a  considerable  quan- 
tity of  peptone  placed  in  the  cavity  of  the  loop  will  disappear, 
i.e.  will  be  absorbed,  but  cannot  be  recovered  from  the  blood 
which  is  being  used  for  the  artificial  circulation,  and  which 
•escapes  from  the  veins  after  traversing  the  intestinal  capillaries. 
The  disappearance  is  not  due  to  any  action  of  the  blood  itself, 
for  peptone  introduced  into  the  blood  before  it  is  driven  through 
the  mesenteric  arteries  in  the  experiment  may  be  recovered  from 
the  blood  as  it  escapes  from  the  mesenteric  veins.  That  if  it 
did  pass  into  the  chyle  it  would  undergo  a  similar  change  by 
some  action  of  the  epithelium  is  indicated  by  the  fact  that  when 
peptone  is  introduced  into  the  lymph-spaces  of  the  connective 
tissue,  its  presence  may  soon  be  recognized  in  the  lymph  of  the 
thoracic  duct,  but  that  no  peptone  can  be  detected  in  the  lymph 
of  the  thoracic  duct  when  peptone,  even  in  large  quantity,  is 
introduced  into  the  alimentary  cavity  provided  that  the  epi- 
thelium be  intact  and  healthy. 

We  are  therefore  guided  in  deciding  this  question  by  indirect 
evidence ;  and  this,  though  pointing  to  the  probability  that  the 
proteids  pass  into  the  portal  blood  and  not  into  chyle,  cannot 
be  regarded  as  conclusive.  One  argument  in  this  direction 
may  be  drawn  from  the  fact  that  when  the  portal  blood  is  ex- 
perimentally diverted  from  the  liver  into  the  vena  cava,  the 
grave  troubles  which  result  seem  to  be  chiefly  caused  by  proteid 
food. 

But,  if  this  view  be  provisionally  accepted,  it  must  be  on  the 
understanding  that  it  is  probable  only ;  and  it  may  be  that  pro- 
teids do  not  take  the  same  paths  and  are  not  absorbed  in  the  same 
condition  in  all  animals.  In  carnivorous  animals  whose  (nat- 
ural) food  contains  a  considerable  quantity  of  fat,  the  lacteals 
might  be  considered  as  preoccupied  in  the  absorption  of  fat. 
The  food  of  herbivora  on  the  other  hand  contains  a  relatively 
small  amoant  of  fat ;  and  if  in  these  animals  all  the  proteids  and 
carbohydrates  are  absorbed  by  the  blood  vessels,  there  is  com- 
paratively little  left  for  the  lacteals  to  do.  Yet  in  these  ani- 
mals the  lacteals  and  the  lymphatics  are  well  developed.  In  the 
villus  of  a  herbivorous  guinea-pig  or  rabbit,  though  the  reticular 
tissue  is  very  scanty  as  compared  with  that  present  in  the  villus 
of  a  dog,  the  lacteal  chamber  is,  relatively  to  the  diameter  of 
the  villus,  not  merely  as  large  as  but  much  larger  than  in  the 


Chap,  i.]   TISSUES  AND  MECHANISMS  OF  DIGESTION.    417 

dog.  It  is  difficult  to  suppose  that  this  wide  chamber  is  in- 
tended solely  for  the  absorption  of  the  relatively  small  amount 
of  fat  present  in  vegetable  food.  The  question  which  we  are 
discussing  is  clearly  at  present  to  be  regarded  as  by  no  means 
settled. 

The  Mechanism  of  Absorption. 

§  251.  The  Absorption  of  Fats.  We  have  now  to  consider 
the  manner  in  which  these  several  substances  pass  into  either 
the  lacteal  radicle  or  the  capillary  blood  vessel.  It  will  be  con- 
venient to  begin  with  the  absorption  of  the  fats. 

We  have  seen  reason  (§  280)  to  think  that  the  fats,  remain- 
ing chiefly  as  neutral  fats,  are  emulsified  in  the  intestine,  by 
means  of  the  bile  and  pancreatic  juice,  the  small  quantity  of 
soap  which  is  formed  probably  serving  simply  the  purpose  of 
facilitating  the  emulsification. 

The  neutral  fats  so  emulsified  pass  in  the  first  instance  into 
the  bodies  of  the  columnar  cells  of  the  villi.  It  has,  it  is  true, 
been  maintained  by  some  that  they  pass  between  the  cells  and 
not  into  them ;  but  the  evidence  is  against  this  view.  Since 
no  collections  of  fat  globules  are  seen  in  the  cubical  cells  of 
the  glands  of  Lieberkiihn  we  infer  that  these  have  nothing  to 
do  with  the  absorption  of  fat. 

How  the  fat  enters  into  the  substance  of  the  cell  we  do  not 
know.  We  may  presume  that  the  striated  border  plays  some 
part,  but  what  part  we  do  not  know.  Though  the  rods  mak- 
ing up  the  border  api^ear  able  to  move  so  far,  at  least,  as  to 
change  their  form,  we  have  no  evidence  that  the  fat  is  intro- 
duced into  the  cells  by  means  of  any  movements  of  these  rods. 
We  may  imagine  that  the  globules  pass  into  the  cell  substance 
by  help  in  some  way  of  these  rods,  through  amoeboid  move- 
ments comparable  Avith  the  ingestive  movements  of  the  body  of 
an  amoeba ;  but  we  have  no  positive  evidence  to  support  this 
view.  We  said  (§  208)  that  bile  promotes  the  passage  of  fat 
through  membranes,  possibly  by  in  some  way  promoting  a 
closer  contact  between  the  particles  of  fat  and  the  substance  of 
the  membrane ;  but  even  if  bile  has  this  effect  on  the  surface 
of  the  cells,  its  action  in  this  respect  can  be  subsidiary"  only. 
When  an  animal  is  fed  not  on  neutral  fats  but  on  fatty  acids, 
the  chyle  in  the  thoracic  duct  contains  a  large  quantity  of 
neutral  fat.  We  may  infer  that  a  synthesis  of  the  fatty  acids 
into  neutral  fats  has  in  such  a  case  somewhere  taken  place. 
And  indeed  it  has  been  urged  by  some  that  even  the  neutral 
fats  are  not  absorbed  by  the  epithelium  cells  as  merely  emulsi- 
fied fat,  but  tliat  they  are  split  up  within  the  canal,  absorbed  by 
the  cells  as  fatty  acids,  and  immediately  synthesized  again  into 
neutral  fats. 

27 


418  ABSOEPTION   OF  FAT.  [Book  ii. 

Within  tlie  columnar  cell  the  fat  may  be  seen,  both  in  fresh 
living  cells,  and  in  osmic  acid  preparations,  to  be  disposed  in 
globules  of  various  sizes,  some  large  and  some  small,  each 
globule  placed  in  a  space  of  the  protoplasmic  cell  substance.  It 
does  not  follow  that  the  fat  actually  entered  the  cell  exactly  in 
the  form  of  these  globules ;  it  may  be  that  the  fat  passes  the 
striated  border  in  very  minute  spherules  which,  reaching  the 
body  of  the  cell,  run  together  into  larger  globules  ;  but  whether 
this  is  so  or  not  we  do  not  know. 

From  the  columnar  cell  the  fat  passes  into  the  spaces  of  the 
reticular  tissue  of  the  villus.  It  has,  it  is  true,  been  contended 
that  it  passes  along  the  substance  of  the  bars  of  the  reticulum ; 
but  in  carefully  prepared  osmic  acid  specimens  of  a  villus  in 
active  digestion  of  fatty  food,  the  fat  may  be  distinctly  recog- 
nized as  largely  filling  up,  still  in  the  form  of  globules  of  vari- 
ous sizes,  the  spaces  in  the  meshes  of  the  reticulum  which  are 
not  occupied  by  the  leucocytes  or  allied  wandering  cells.  The 
bases  of  the  columnar  cells,  through  the  gaps  in  the  basement 
membrane,  directly  abut  upon  the  labyrinth  of  spaces ;  and  the 
fat  once  out  of  the  base  of  the  cell  is  free  in  the  spaces  of  this 
labyrinth.  How  it  issues  from  the  cell  we  do  not  exactly  know: 
possibly  by  a  process  analogous  to  the  excretion  of  solid  matters 
by  an  amceba. 

From  the  labyrinth  of  spaces  of  the  reticulum  of  the  villus 
the  fat  passes  into  the  cavity  of  the  lacteal  radicle ;  and  it  is 
worthy  of  note  that  in  the  passage  it  undergoes  a  change.  In 
the  interior  of  the  intestine,  in  the  substance  of  the  columnar 
cell,  and  apparently  in  the  labyrinth  of  the  reticulum  it  is 
simply  emulsified  fat  consisting  of  globules  small  and  large; 
within  the  lacteal  radicle  it  consists  partly  of  the  same  easily 
recognized  globules  but  partly  of  the  extremely  divided  '  molec- 
ular basis '  (§  241)  ;  it  is  now  no  longer  emulsified  fat  but 
chyle.  How  and  by  what  means  this  extremely  minute  division 
of  the  globular  fat  into  the  '  molecular  basis '  takes  place  we 
do  not  know ;  nor  do  we  know  the  exact  manner  in  which  the 
fat  passes  from  the  spaces  of  the  reticulum  into  the  interior  of 
the  radicle. 

We  may  here  perhaps  remark  that  the  contents  of  the  lacteal 
radicle  consist  not  exclusively  of  fat,  but  of  fat  accompanied  by 
the  proteid  and  other  substances  which  go  to  make  up  the  chyle. 
Proteid  and  other  substances  besides  fat  are  also  present  in  the 
lymph  which  occupies  in  part  the  labyrinth  of  the  body  of 
the  villus,  and  are  derived,  like  the  lymph  elsewhere,  from  the 
blood  of  adjacent  capillaries ;  at  least,  they  are  in  part  so 
derived,  though  it  may  be  not  wholly,  for  as  we  have  just  seen 
the  passage  of  proteid  material  from  the  intestine  into  the  sub- 
stance of  the  villus  past  the  capillaries,  though  not  proved,  must 
still  be  considered  as  possible. 


Chap,  i.]  TISSUES  AND  MECHANISMS  OF  DIGESTIOX,    411> 

The  spaces  of  the  reticulum  of  the  villus  are  more  or  less 
occupied  by  wandering  cells  of  which  we  spoke  under  the  gen- 
eral term  of  leucocytes.  These  do  not  all  present  the  same 
appearances  and  most  probably  are  not  of  all  the  same  kind. 
Some  of  these  leucocytes  wander  not  only  through  the  labyrinth 
of  the  reticulum  but  pass  into  the  epithelium  between  the  cells, 
and  may  project  j)rocesses  into,  or  even  make  their  way  even- 
tually into  the  interior  of  the  intestine  ;  or  following  the  re- 
verse course  may  wander  from  the  inside  of  the  canal  between 
the  epithelium  cells  into  the  body  of  the  villus ;  some  of  them 
moreover  undoubtedly  contain  fat.  Hence  the  view  has  been 
suggested  that  these  leucocytes  are  important  agents,  indeed 
the  chief  agents  in  the  absorption  of  fat.  It  has  been  supposed 
that  they,  receiving  the  globules  of  fat  into  their  cell  substance, 
in  fact  eating  the  fat  exactly  after  the  manner  of  an  amaba, 
either  while  projecting  between  the  columnar  cells,  in  which 
they  carry  their  burden  of  fat  through  the  epithelium  into  the 
villus,  or  while  wandering  in  the  labyrinth  of  the  villus,  bear  it 
away  bodily  into  the  lymphatic  system.  But  the  number  of 
leucocytes  really  containing  any  appreciable  quantity  of  fat  is 
too  small  to  account  for  the  amount  of  fat  absorbed.  Nor  is 
the  abundance  of  leucocytes  in  the  mucous  membrane  during 
the  period  of  digestion  a  sure  proof  that  they  are  concerned  in 
absorption,  but  rather  an  indication  only  that  active  changes 
of  some  kind  are  going  on,  since  after  the  administration  of  a 
saline  such  as  magnesium  sulphate,  which  produces  effects  the 
very  reverse  of  absorption,  these  leucocytes  are  present  in  unu- 
sual numbers.  Moreover  under  some  circumstances,  as  in  the 
villi  of  a  new-born  puppy  after  a  meal  of  milk,  they  are  absent 
even  when  digestion  of  fat  is  rapidly  going  on  and  the  lacteals 
are  filling  with  fat.  In  fact,  what  we  stated  above  concerning 
the  presence  of  fat  in  the  bodies  of  the  columnar  cells  shews 
that  leucocytes  can  have  little  to  do  in  transferring  fat  from 
the  interior  of  the  intestine  into  the  body  of  villus ;  and  there 
are  no  adequate  reasons  for  attributing  to  them  any  real  share 
in  the  transference  of  fat  from  the  body  of  the  \'illus  into  the 
lacteal  chamber. 

§  252.  The  lacteal  chamber  opens  at  the  base  of  the  villus 
into  the  valved  lymphatic  vessels  lying  below,  and  in  these  the 
flow  of  lymph  (chyle)  is  being  promoted  by  the  various  causes 
detailed  in  §  242.  The  pressure,  for  instance,  exerted  by  the 
peristaltic  contractions  of  the  intestine  helps  to  empty  the  lym- 
phatic vessel  into  Avhich  a  lacteal  chamber  opens  and  so  pro- 
motes the  emptying  of  the  latter.  In  addition  to  this  the  plain 
muscular  fibres  of  the  villus  supply  a  special  muscular  pump 
for  the  emptying  and  filling  of  the  lacteal  chamber.  These 
fibres  and  small  bundles  of  fibres  running  through  in  vari- 
ous   directions    and    varj-ing    in    number    and    arrangement  in 


420  ABSORPTION  OF  DIFFL'SIBLE  SUBSTANCES.  [Book  ir. 

different  animals,  take  on  the  whole  a  longitudinal  direction 
parallel  to  the  long  axis  of  the  villus.  It  has  been  supposed 
that  in  contracting  and  shortening  the  villus  they  compress  the 
lacteal  and  thus  empty  it,  and  that  when  they  relax  and  the 
villus  elongates  again,  the  emptied  chamber  fills  once  more. 
But  a  different  interpretation  of  their  action  has  been  offered 
somewhat  as  follows.  When  the  muscular  fibres  contract  they 
shorten  the  villus.  In  thus  becoming  shorter  the  body  of  the 
villus  becomes  proportionately  broader,  since  probably  no  great 
change  of  bulk  in  the  reticulum  takes  place  ;  in  this  broadening 
the  part  to  give  Avay  will  be  the  lacteal  chamber,  which  thus 
becomes  broader  and  larger.  When  the  muscular  fibres  relax, 
the  reticulum,  the  bars  of  which  have  been  put  on  the  stretch 
in  a  lateral  direction,  by  elastic  reaction  brings  back  the  villus 
to  its  former  length,  and  the  lacteal  chamber  elongates  and 
narrows.  On  this  view  the  muscular  contraction  expands  and 
so  fills,  while  the  relaxation  narrows  and  so  empties  the  lacteal 
chamber.  Whichever  view  we  adopt,  we  may  at  least  conclude 
that  contractions  and  relaxations  of  the  muscular  fibres  in  some 
way  or  other  alternately  fill  and  empty  the  lacteal  chamber,  and 
in  all  probability,  at  all  events  during  digestion,  rhythmical  con- 
tractions of  these  fibres  are  continually  going  on.  When  the 
villus  is  shortened  by  the  contraction  of  the  muscular  fibres, 
the  columnar  cells  are  compressed,  becoming  longer  and  nar- 
rower ;  when  the  muscular  fibres  relax  and  the  villus  elongates, 
the  columnar  cells  return  to  their  previous  form.  The  alter- 
natinof  changes  of  form  to  which  the  columnar  cells  are  thus 
subjected,  and  the  alternating  changes  of  pressure  taking  place 
in  the  reticulum,  may  also  serve  to  promote  the  passage  of 
material  through  the  one  and  through  the  other. 

§  253.  The  Absorption  of  Diffusible  Substances  and  of  Water. 
On  the  provisional  assumption  which  we  have  made  that  the 
proteids  are  converted  into  peptone,  we  may  consider,  for  the 
present  at  all  events,  peptone,  sugar,  and  soluble  salts  as  together 
forming  a  class  distinguished  from  fats  by  their  being  diffusible, 
some  more  so  than  others.  And  we  have  made  the  further  pro- 
visional assumption  that  these  pass  into  the  blood  vessels  and 
not  into  the  lacteals. 

The  network  of  capillary  blood  vessels  is  spread  immediately 
beneath  the  basement  membrane,  and  all  the  material  which 
enters  the  lacteal  chamber  has  to  run  the  gauntlet  of  the  meshes 
of  this  network.  During  digestion  the  capillaries  of  the  intes- 
tine are  filled  and  distended,  so  that  at  a  time  when  absorption 
is  taking  place  these  meshes  between  the  capillaries  are  unusu- 
ally narrow.  From  the  interior  of  these  capillaries,  here  as 
elsewhere,  transudation  is  taking  place ;  these  capillaries  sup- 
ply the  lymph  which  helps  to  fill  up  the  labyrinth  of  the  retic- 
ulum and  the  lacteal  chamber.     But  to  a  much  g'reater  extent 


Chap,  i.]  TISSUES  AND  MECHANISMS  OF  DIGESTION.    421 

than  elsewhere  (cf.  §  244)  this  current  of  transudation  from 
within  the  capillary  to  without  is  accompanied  by  a  reverse 
current  from  without  to  within.  The  diffusible  substances  in 
question  pass  from  the  intestine  through  the  layer  of  epithelium 
cells,  through  the  attenuated  reticular  lymph-space  between  the 
basement  membrane  and  the  capillary  wall,  and  through  the 
capillary  wall  into  the  blood  current.  Their  passage  consists 
of  two  stages  ;  that  through  the  epithelium  cells  from  the  intes- 
tine to  the  lymph-space,  and  that  from  the  lymph-space  into  the 
blood  vessels.  These  two  stages  may  be  expected  to  differ, 
seeing  that  the  structures  concerned  are  different ;  but  we  may 
at  first  consider  them  as  one,  and  speak  of  the  passage  from 
the  intestine  into  the  blood  as  a  single  event. 

In  speaking  of  these  substances  as  diffusible  we  are  using 
the  terms  in  reference  to  the  well-known  passage  of  such  sub- 
stances through  thin  membranes  or  porous  partitions.  When 
a  strong  solution  of  sugar  or  of  common  salt  is  separated  by  a 
thin  membrane  (vegetable  parchment,  dead  urinary  bladder, 
dead  intestine,  &c.)  from  a  weak  solution  of  sugar  or  of  salt, 
the  sugar  or  salt  passes  with  a  certain  rapidity  from  the  stronger 
to  the  weaker  solutioD,  and  water  passes  from  the  weaker  solu- 
tion to  the  stronger ;  if,  to  begin  with,  simple  water  be  substi- 
tuted for  the  weaker  solution  the  effect  is  at  first  still  more 
striking.  Peptone  passes  in  the  same  manner  but  as  we  have 
seen  much  more  slowly.  The  process  is  spoken  of  as  a  physi- 
cal one  since  it  is  not  accompanied,  necessarily,  by  any  chemical 
change  in  the  diffusing  substance,  nor  is  there  any  necessary 
change  in  the  membrane  or  partition.  The  rate  at  which  a 
substance  diffuses,  and  the  total  amount  of  diffusion  which  can 
take  place,  are  determined  by  certain  qualities  of  the  substance 
(which  we  may  call  physical  though  they  depend  on  the  chem- 
ical nature  of  the  substance)  in  relation  to  certain  qualities  of 
the  membrane ;  thus  two  salts  may  diffuse  through  the  same 
membrane  at  different  rates,  with  different  rates  in  the  associ- 
ated current  of  water,  the  osmotic  current  as  it  is  called,  from 
the-  weaker  to  the  stronger  solution ;  and  the  same  substance 
may  pass  at  different  rates  tlirough  different  membranes.  By 
a  number  of  observations,  in  which  various  substances  in  solu- 
tion and  several  known  membranes  or  partitions  have  been 
employed,  a  certain  number  of  "  laws  of  diffusion  "  have  been 
established. 

Now  if  by  the  statement  that  diffusible  substances  pass  by 
diffusion  into  the  blood-capillaries  of  the  intestine  we  are  led 
to  expect  that  the  passage  takes  place  exactly  according  to  the 
laws  established  by  observations  on  ordinary  membranes  we 
should  be  led  into  error  ;  for  the  disappearance  of  these  sub- 
stances from  the  interior  of  the  intestine  does  not  take  place 
according  to  the  laws  which  regulate  their  disappearance  from 


422  ABSORPTION  OF  DIFFUSIBLE  SUBSTANCES.  [Book  ii. 

one  side  of  an  ordinaiy  diffusion  septum.  This  can  be  ascer- 
tained by  introducing  solutions  of  the  substances,  of  various 
strength,  into  a  loop  of  intestine,  isolated  in  the  living  animal 
by  the  method  described  in  §  211,  and  watching  their  disap- 
pearance by  analysis  of  the  contents  of  the  loop.  For  instance, 
sodium  sulphate  passes  through  an  ordinary  diffusion  septum 
with  a  rapidity  rather  greater  than  that  of  dextrose,  whereas 
dextrose  disappears  from  the  intestine  distinctly  more  rapidly 
than  sodium  sulphate ,  peptone  which  diffuses  very  slowly 
indeed  through  an  ordinary  diffusion  septum  disappears  rap- 
idly (though  not  so  rapidly  as  dextrose)  from  the  intestine , 
and  when  the  details  of  the  disappearance  from  the  intestine  of 
weak  solutions  of  two  salts  which  diffuse  through  an  ordinary 
membrane  at  different  rates,  which  have  as  it  is  said  different 
osmotic  equivalents,  are  studied,  these  details  are  quite  differ- 
ent from  those  of  ordinary  diffusion.  The  more  the  matter  is 
studied  the  more  decidedly  apparent  becomes  the  difference  be- 
tween ordinary  diffusion  and  the  absorption  of  diffusible  sub- 
stances from  the  intestine. 

Two  opposite  processes  are  carried  on  by  the  wall  of  the 
alimentary  canal :  on  the  one  hand  material  is  transferred 
from  the  blood  stream  to  the  inside  of  the  canal  in  the  form 
of  the  several  digestible  juices,  and  on  the  other  hand  di- 
gested material  is  transferred  from  the  inside  of  the  canal  to 
the  blood  stream.  The  former  process  we  without  hesitation 
regard  as  the  work  of  the  epithelium  cells  forming  the  lining 
of  the  canal,  whether  the  cells  lie  as  in  the  gastric  and  Lieber- 
kiihn's  glands  in  the  thickness  of  the  wall  of  the  canal,  or  as  in 
the  pancreas  are  removed  to  some  distance  from  it ;  we  call  the 
process  'secretion.'  And  the  evidence  goes  increasingly  to 
show  that  the  other  process  is  also  the  work  of  epithelium  cells, 
that  the  two  processes  are  in  the  main  alike  save  that  the  cur- 
rent resulting  from  the  activity  of  the  cells  is  in  opposite  direc- 
tions in  the  two  cases.  We  might  in  fact  venture  to  speak  of 
absorption  from  the  canal  as  an  inverted  secretion.  And  we 
may  regard  as  wholly  secondary  the  fact  that  in  the  small  intes- 
tine the  cells  of  Lieberkiihn  appear  at  least  to  be  chiefly  de- 
voted to  ordinary  secretion,  and  those  of  the  villi  to  the  inverted 
secretion.  We  may  further  consider  the  conversion  of  food 
into  diffusible  substances  as  in  the  main  a  means  by  which  the 
material  of  the  food  enters  more  readily  into  the  substance  of 
the  epithelial  cell  and  so  is  placed  more  easily  within  its  grasp  ; 
and  we  have  seen  the  material,  having  thus  entered,  appears  in 
certain  cases  to  undergo  an  immediate  change,  the  maltose  being 
converted  into  dextrose,  and  the  peptone  into  some  other  pro- 
teid,  diffusil)ility  being  in  the  latter  case  lost. 

Such  a  view,  however,  of  absorption  as  a  kind  of  secretion 
must  in  the  first  instance  be  confined  to  the  first  step  of  the 


Chap,  i.]  TISSUES  AND  MECHANISMS  OF  DIGESTION.    423 

whole  process,  namely  the  transference  through  the  epithelium 
cell  from  the  inside  of  the  canal  to  the  lymph-space  of  the  villus. 
And  we  may  perhaps  be  inclined  still  to  regard  the  second  step 
the  transference  from  the  lymph-space  to  the  blood  stream  as 
more  strictly  an  act  of  diffusion.  But  the  considerations  which 
we  urged  (§  236)  in  regard  to  translation  from  the  blood  vessel 
to  the  lymph-space  may  also  be  applied  to  the  reverse  current ; 
we  may  look  upon  this  as  something  much  much  complex  than, 
and  so  different  from,  ordinary  phj^sical  diffusion. 


CHAPTER   II. 

RESPIRATION. 

§  254.  One  particular  item  of  the  body's  income,  viz. 
oxygen,  is  peculiarly  associated  with  one  particular  item  of  the 
body's  waste,  viz.  carbonic  acid,  inasmuch  as  the  means  which 
are  applied  for  the  introduction  of  the  former  are  also  used  for 
the  getting  rid  of  the  latter.  Both  are  gases,  and  the  ingress 
of  the  one  as  well  as  the  egress  of  the  other  seems  to  be  more 
directly  dependent  on  the  simple  physical  process  of  diffusion 
than  on  any  active  vital  processes  carried  on  by  means  of  tissues. 
Oxygen  passes  from  the  air  into  the  blood  mainly  by  diffusion, 
and  mainly  by  diffusion  also  from  the  blood  into  the  tissues  ; 
in  the  same  way  carbonic  acid  passes  mainly  by  diffusion  from 
the  tissues  into  the  blood,  and  from  the  blood  into  the  air. 
Whereas,  as  we  have  seen,  in  the  secretion  of  the  digestive 
juices  the  epithelium-cell  plays  an  all-important  part,  in  respira- 
tion the  entrance  of  oxygen  from  the  lungs  into  the  blood,  and 
from  the  blood  into  the  tissue,  and  the  passage  of  carbonic  acid 
in  the  contrary  direction,  appear  to  be  affected,  if  at  all,  in  a 
wholly  subordinate  manner,  by  the  behaviour  of  the  pulmonary, 
or  of  the  capillary  epithelium.  What  we  have  to  deal  with  in 
respiration  then  is  not  so  much  the  vital  activities  of  any  par- 
ticular tissue,  as  the  various  mechanisms  by  which  a  rapid 
interchange  between  the  air  and  the  blood  is  effected,  the 
means  by  which  the  blood  is  enabled  to  carry  oxygen  and  car- 
bonic acid  to  and  from  the  tissues,  and  the  manner  in  which  the 
several  tissues  take  oxygen  from  and  give  carbonic  acid  up  to 
the  blood.  We  have  reasons  for  thinking  that  oxygen  can  be 
taken  into  the  blood,  not  only  from  the  lungs,  but  also  to  a  cer- 
tain small  extent  from  the  skin,  and,  as  we  have  seen,  from  the 
alimentary  canal  also  ;  and  carbonic  acid  certainly  passes  away 
from  the  skin,  and  through  the  various  secretions,  as  well  as 
by  the  lungs.  Still  the  lungs  are  so  eminently  the  channel  of 
the  interchange  of  gases  between  the  body  and  the  air,  that  in 
dealing  at  the  present  with  respiration,  we  shall  confine  our- 
selves entirely  to  pulmonary  respiration,  leaving  the  considera- 
tion of  the  subsidiary  respiratory  processes  till  we  come  t& 
study  the  secretions  of  which  they  respectively  form  part. 

424 


SEC.  1.      THE    MECHANICS   OF  PULMOITARY  EESPIEA- 

TION. 

§  255.  The  lungs  are  placed,  in  a  state  which  is  always  one 
of  distension,  sometimes  greater,  sometimes  less,  in  the  air- 
tight thorax,  the  cavity  of  which  they,  together  with  the  heart, 
great  blood  vessels  and  other  organs,  completely  fill.  By  the 
contraction  of  certain  muscles  the  cavity  of  the  thorax  is 
enlarged.  The  lungs  must  follow  this  enlargement  and  be 
themselves  enlarged ,  otherwise  the  pleural  cavities  would  be 
enlarged,  but  this  is  impossible  so  long  as  the  thoracic  Avails 
are  intact  The  enlargement  of  the  lung  consists  chiefly  in  an 
enlargement  or  expansion  of  the  pulmonary  alveoli,  the  air  in 
which  becomes  by  the  expansion  rarefied.  That  is  to  say  the 
pressure  of  the  air  within  the  lungs  becomes  less  than  that  of 
the  air  outside  the  body ,  and  this  difference  of  pressure  causes 
a  rush  of  air  through  the  trachea  into  the  lungs  until  an  equi- 
librium of  pressure  is  established  between  the  air  inside  the 
lungs  and  that  outside.  This  constitutes  inspiration.  Upon 
the  relaxation  of  the  inspiratory  muscles  (the  muscles  whose 
contractions  have  brought  about  the  thoracic  expansion),  the 
elasticity  of  the  lungs  and  chest- walls,  aided  perhaps  to  some 
extent  by  the  contraction  of  certain  muscles,  causes  the  chest 
to  return  to  its  original  size ;  in  consequence  of  this  the  pres- 
sure within  the  lungs  now  becomes  greater  than  that  outside, 
and  thus  air  rushes  out  of  the  trachea  until  equilibrium  is  once 
more  established.  This  constitutes  expiration ;  the  inspiratory 
and  ex[)iratory  act  togetlier  form  a  respiration.  The  fresh  air 
introduced  into  the  upper  part  of  the  })ulmonary  passages  by 
the  inspiratory  movement  contains  more  oxygen  and  less  car- 
bonic acid  than  the  old  air  previously  present  in  the  lungs. 
By  diffusion  the  new  or  tidal  air,  as  it  is  frequently  called, 
gives  up  its  oxygen  to,  and  takes  carbonic  acid  from,  the  old 
or  stationary  air,  as  it  has  been  called,  and  thus  when  it  leaves 
the  chest  in  expiration  has  been  the  means  of  botli  introducing 
oxygen  into  the  chest  and  of  removing  carbonic  iicid  fi'om  it. 
In  this  way,  by  the  ebb  and  flow  of  the  tidal  air,  and  by  diffu- 
sion between  it  and  the  stationary  air,  the  whole  aii'  in  the 

425 


426  PUNCTUEE   OF   PLEUEA.  [Book  ii. 

lungs  is  being  constantly  renewed  through  the  alternate  expan- 
sion and  contraction  of  the  chest. 

§  256.  In  ordinary  respiration,  the  expansion  of  the  chest 
never  reaches  its  maximum ;  by  more  forcible  muscular  con- 
tractions, by  what  is  called  laboured  inspiration,  an  additional 
thoracic  expansion  can  be  brought  about,  leading  to  the  inrush 
of  a  certain  additional  quantity  of  air  before  equilibrium  is 
established.  This  additional  quantity  is  often  spoken  of  as 
complemental  air.  In  the  same  way,  in  ordinary  respiration, 
the  contraction  of  the  chest  never  reaches  its  maximum.  By 
calling  into  use  additional  muscles,  by  a  laboured  expiration, 
an  additional  quantity  of  air,  the  so-called  reserve  or  supple- 
mental air,  may  be  driven  out.  But  even  after  the  most  forci- 
ble expiration,  a  considerable  quantity  of  air,  the  residual  air, 
still  remains  in  the  lungs.  The  natural  condition  of  the  lungs 
in  the  chest  is  in  fact  one  of  partial  distension.  The  elastic 
pulmonary  tissue  is  always  to  a  certain  extent  on  the  stretch ; 
it  is  always,  so  to  speak,  striving  to  pull  asunder  the  pulmonary 
from  the  parietal  pleura ;  but  this  it  cannot  do,  because  the  air 
can  have  no  access  to  the  pleural  cavity.  When,  however,  the 
chest  ceases  to  be  air-tight,  when  by  a  puncture  of  the  chest- 
wall  or  diaphragm,  air  is  freely  introduced  into  the  pleural 
chamber,  the  elasticity  of  the  lungs  pulls  the  pulmonary  away 
from  the  parietal  pleura,  and  the  lungs  shrink,  driving  out  by 
the  windpipe  a  considerable  quantity  of  the  residual  air.  Even 
then,  however,  the  lungs  are  not  completely  emptied,  some  air 
still  remaining  in  them ;  this  is  probably  air  imprisoned  in  the 
infundibula  by  collapse  of  the  bronchioles,  the  walls  of  which 
are  not  rigid  but  flaccid.  If  in  a  living  animal  the  pressure  of 
the  atmosphere  continue  to  have  access  to  the  outside  of  a  lung 
the  air  thus  imprisoned  is  gradually  absorbed  and  the  lung 
becomes  solid.  The  same  result  may  occur  from  the  pressure 
of  fluid  accumulated  in  the  pleural  cavity. 

It  need  hardly  be  added  that  when  the  pleura  is  punctured, 
and  air  can  gain  free  admittance  from  the  exterior  into  the 
pleural  chamber,  since  the  resistance  to  the  entrance  of  the 
air  into  the  pleural  chamber  is  far  less  than  the  resistance  to 
the  entrance  into  the  lungs,  the  effect  of  the  respiratory  move- 
ments is  simply  to  drive  air  in  and  out  of  that  chamber,  instead 
oi  in  and  out  of  the  lung.  There  is  in  consequence  no  renewal 
of  the  air  within  the  lungs  under  those  circumstances.  If 
there  be  a  sufficient  obstacle  to  the  entrance  of  air  into  the 
pleural  chamber,  such  as  a  fold  of  tissue  blocking  up  the  open- 
ing, the  expansion  of  the  chest  may  still  lead  to  a  distension  of 
the  lungs  ;  and  in  this  way  in  some  cases  puncture  of  the  chest- 
walls  has  not  seriously  interfered  with  respiration.  The  parietal 
and  pulmonary  pleura  are,  in  normal  circumstances,  separated 
by  a  very  thin  layer  only  of  fluid,  so  that  we  may  perhaps 


CiiAP.  II.]  RESPIRATION.  41>7 

speak  of  them  as  being  in  a  state  of  '  adhesion,'  such  as  obtains 
between  two  wet  membranes  superimposed.  And  it  has  been 
suggested  that  this  adhesion,  having  to  be  overcome  before  the 
two  surfaces  can  separate,  assists  in  preventing  the  entrance  of 
air  into  the  pleural  cavity  after  puncture  of  the  thorax  ;  but 
it  has  not  been  clearly  shewn  that  this  is  really  of  importance 
in  the  matter. 

§  257.  Before  birth  the  lungs  contain  no  air  ;  they  are  in 
the  condition  called  atelectatic.  The  walls  of  the  alveoli,  the 
epithelial  lining  of  which  is  at  that  time  well  developed,  con- 
sisting of  distinctly  nucleated  cells  with  granular  cell-substance, 
are  in  contact,  the  cavity  of  the  alveolus  not  having  as  yet 
come  into  existence  ;  the  walls  of  the  bronchioles  are  similarly 
in  a  collapsed  condition,  with  their  walls  touching  ;  the  more 
rigid  bronchia,  like  the  trachea,  possess  some  amount  of  lumen 
which,  however,  is  occupied  by  fluid.  When  the  chest  expands 
with  the  first  breath  taken,  the  pressure  of  the  inspired  air  has 
to  overcome  the  "  adhesion,"  obtaining  between  the  walls  of  the 
alveoli  thus  in  contact  with  each  other  and  also  those  of  the 
bronchioles.  The  force  spent  in  thus  opening  out  and  unfold- 
ing, so  to  speak,  the  alveoli  and  bronchioles  is  considerable,  and 
in  the  expiration  succeeding  the  first  inspiration  most  of  the 
air  thus  introduced  remains,  the  force  exerted  by  the  chest  in 
returning  to  its  previous  dimensions  after  the  breathing  in,  and 
the  elastic  action  of  the  alveoli  being  insulficient  to  bring  the 
walls  of  the  alveoli  again  into  contact.  Succeeding  breaths 
unfold  the  lungs  more  and  more  until  all  the  alveoli  and  bron- 
chioles are  opened  up,  and  then  the  whole  force  of  the  expiratory 
act  is  directed  to  driving  out  the  previously  inspired  air. 

It  is  not,  however,  until  sometime  after  birth  that  the  lungs 
pass  into  that  furtlier  distended  state  of  which  we  spoke  above. 
In  a  newly-born  animal  there  is  no  negative  pressure  obtaining 
in  the  pleural  cavities,  the  lungs  when  at  rest  are  not  on  the 
stretch,  and  opening  the  thorax  does  not  lead  to  collapse  of  the 
lungs.  The  state  of  things  obtaining  later  on  is  established, 
not  at  once  but  gradually,  and  is  apparently  brought  about  by 
the  thorax  growing  more  rapidly,  and  so  liecomiiig  relatively 
more  capacious  than  the  lungs.  The  distension  of  the  lungs 
in  the  adult  may  be  familiarly  described  as  being  due  to  the 
chest  being  too  large  for  the  lungs. 

§  258.  In  man  the  pressure  exerted  by  the  elasticity  of  the 
lungs  alone  amounts  to  about  5  or  7  mm.  of  mercury.  This  is 
estimated  by  tying  a  manometer  into  the  windpipe  of  a  dead 
subject  and  observing  the  rise  of  mercury  which  takes  place 
when  the  chest-walls  are  punctured.  If  Ave  took  7-G  mm.  as 
the  pressure,  this  would  be  just  1/100  of  the  pressure  of  the 
atmosphere.  If  the  chest  be  forcibly  distended  beforehand,  a 
much  larger  rise  of  the  mercury  is  observed,  amounting,  in  the 


428  BREATHING   CAPACITY.  [Book  ii. 

case  of  a  distension  corresponding  to  a  very  forcible  inspira- 
tion, to  30  mm.  In  the  living  body  this  mechanical  elastic  force 
of  the  lungs  may  be  assisted  by  the  contraction  of  the  plain 
muscular  fibres  of  the  bronchi  ;  the  pressure,  however,  which 
can  be  exerted  by  these  probably  does  not  exceed  1  or  2  mm. 

When  a  manometer  is  introduced  into  a  lateral  opening  of 
the  windpipe  of  an  animal,  the  mercury  will  fall,  indicating  a 
negative  pressure  as  it  is  called,  during  inspiration,  and  rise, 
indicating  a  positive  pressure,  during  expiration,  both  fall  and 
rise  being  slight  and  varying  according  to  the  freedom  with 
which  the  air  passes  in  and  out  of  the  chest.  When  a  manom- 
eter is  fitted  with  air-tight  closure  into  the  mouth,  or  better, 
in  order  to  avoid  the  suction-action  of  the  mouth,  into  one 
nostril,  the  other  nostril  and  the  mouth  being  closed,  and  efforts 
of  inspiration  and  expiration  are  made,  the  mercury  falls  or 
undergoes  negative  pressure  with  inspiration,  and  rises,  or  un- 
dergoes positive  pressure  during  expiration.  It  has  been  found 
in  this  way  that  the  negative  pressure  of  a  strong  inspiratory 
effort  may  vary  from  30  to  74  mm.,  and  the  positive  pressure 
of  a  strong  expiration  from  62  to  100  mm. 

The  total  amount  of  air  which  can  be  given  out  by  the  most 
forcible  expiration  following  upon  a  most  forcible  inspiration, 
that  is,  the  sum  of  the  complemental,  tidal  and  reserve  airs, 
has  been  called  ••  the  vital  capacity  ;  '  '  extreme  differential 
capacity  '  is  a  better  phrase.  It  may  be  measured  by  a  modifi- 
cation of  a  gas-meter  called  a  Spirometer  ;  and  though  it  varies 
largely,  the  average  may  be  put  down  at  3 — 4000  c.c.  (200  to 
250  cubic  inches). 

Of  the  whole  measure  of  vital  capacity,  about  500  c.c.  (30  c. 
inch)  may  be  put  down  as  the  average  amount  of  tidal  air,  the 
remainder  being  nearly  equally  divided  between  the  comple- 
mental and  reserve  airs.  The  quantity  left  in  the  lungs  after 
the  deepest  expiration  amounts  to  about  1400  or  2000  c.c. 

Since  the  respiratory  movements  are  so  easily  affected  by  various 
circiini stances,  the  simple  fact  of  attention  being  directed  to  the 
breathing  being  sufficient  to  cause  modifications  both  of  the  rate  and 
depth  of  the  respiration,  it  becomes  very  difficult  to  fix  the  volume 
of  an  average  breath.  Thus  various  authors  have  given  figures 
varying  from  53  c.c.  to  792  c.c.  The  statement  made  above  is  the 
mean  of  observations  varying  from  177  to  698  c.c. 

§  259.  Grraphic  Records  of  Respiratory  Movements.  These 
may  be  obtained  in  many  various  ways. 

The  simplest,  readiest  and  perhaps  the  most  generally  useful 
method  is  that  of  recording  the  movements  of  the  column  of  air. 
This  may  be  effected  by  introducing  a  T  piece  into  the  trachea,  one 
cross  piece  being  left  open,  and  the  other  connected  with  a  Marey's 


<;!hap,  II.] 


EESPIRATlOJSr. 


429 


430         GRAPHIC   RECORDS   OF  RESPIRATION^.     [^ook  ii. 


Fig.  85.   Apparatus  fok  taking  Tracings  of  the  Movements  of  the 
Column  of  Air  in  Respiration. 

The  recording  apparatus  shewn  is  the  ordinary  cylinder  recording  apparatus. 
The  cylinder  A  covered  with  smoked  paper  is  by  means  of  the  friction-plate 
B  put  into  revolution  by  the  spring  clock-work  in  C  regulated  by  Foucault's  regu- 
lator D.  By  means  of  the  screw  E,  the  cylinder  can  be  raised  or  lowered,  and 
by  means  of  the  screw  F  its  speed  may  be  increased  or  diminished. 

The  tracheotomy  tube  t  fixed  in  the  trachea  of  an  animal  is  connected  by 
india-rubber  tubing  a  with  a  glass  T  piece  inserted  into  the  large  jar  G.  From 
the  other  end  of  the  X  piece  proceeds  a  second  piece  of  tubing  6,  the  end  of 
which  can  be  either  closed  or  partially  obstructed  at  pleasure  by  means  of  the 
screw  clamp  c.  From  the  jar  proceeds  a  third  piece  of  tubing  d,  connected  with 
a  Marey's  tambour  m  (see  Fig.  36),  the  lever  of  which  I  writes  on  the  recording 
surface.  When  the  tube  h  is  open  the  animal  breathes  freely  through  this,  and 
the  movements  in  the  air  of  G  and  consequently  in  the  tambour  are  slight. 
On  closing  the  clamp  c,  the  animal  breathes  only  the  air  contained  in  the  jar, 
and  the  movements  of  the  lever  of  the  tambour  become  consequently  much 
more  marked. 

Below  the  lever  is  seen  a  small  time-marker  n  connected  with  an  electro- 
magnet, the  current  through  which  coming  from  a  battery  by  the  wires  x  and  y 
is  made  and  broken  by  a  clock-work  or  metronome. 

tambour  or  with  a  receiver  which  in  turn  is  connected  with  a  tambour, 
see  Fig.  36,  and  Fig.  85.  The  movements  of  the  column  of  air  in  the 
trachea  are  transmitted  to  the  tambour,  the  consequent  expansions 
and  contractions  of  which  are  transmitted  to  the  recording  drum  by 
means  of  a  lever  resting  on  it. 

If,  a  receiver  being  used,  the  open  end  of  the  \-  be  closed,  the 
animal  breathes  into  and  out  of  the  receiver,  and  the  movements  of 
the  tambour  are  greatly  increased.  This  has  the  disadvantage  that 
the  air  in  the  receiver  soon  becomes  unfit  for  further  respiration. 
A  similar  increase  of  the  movements  of  the  lever  of  the  tambour 
may  be  obtained  by  connecting  a  piece  of  india-rubber  tubing  to  the 
open  end  of  the  \-.  By  increasing  the  length  of  this  tube,  or  slightly 
constricting  it,  the  movements  of  the  lever  may  be  increased  without 
very  seriously  interfering  with  the  breathing  of  the  animal. 

In  another  method  the  movements  of  the  chest  are  recorded. 
When  a  small  animal  such  as  a  rabbit  is  used,  the  whole  animal  may 
be  placed  in  an  air-tight  box,  breathing  being  carried  on  by  means  of 
a  tube  inserted  in  the  trachea  and  carried  through  an  air-tight  orifice 
in  the  wall  of  the  box.  By  another  orifice  and  tube  the  air  in  the  box 
is  brought  into  connection  with  a  tambour,  which  accordingly  regis- 
ters the  changes  of  pressure  in  the  air  of  the  box  produced  by  the 
movements  of  the  chest  (and  body)  and  thus  indirectly  the  raove- 
]nents  of  the  chest.  In  man  and  larger  animals  the  changes  in  the 
girth  of  the  chest  may  be  conveniently  recorded  by  means  of 
Marey's  pneumograph.  This  consists  of  a  hollow  elastic  cylinder, 
or  a  cylinder  with  elastic  ends,  the  interior  of  which  is  connected 
with  a  tambour.  By  means  of  a  strap  attached  to  each  end  of  the 
cylinder  the  instrument  can  be  buckled  round  the  chest  like  a  girdle. 
When  the  chest  expands,  the  ends  of  the  cylinder  are  pulled  out, 
and  the  air  within  the  chamber  rarefied ;  in  consequence  the  lever 
of  the  tambour  connected  with  its  interior  is  dejoressed ;  conversely, 
when  the  chest  contracts,  the  lever  is  elevated.     The  pneumatograph 


Chap,  ii.]  RESPIRATIOK  431 

of  Fick  is  somewhat  similar.  Or  changes  in  one  or  other  diameter 
of  the  chest  may  be  recorded  by  what  may  be  called  the  'callipers' 
method,  as  in  the  recording  stethometer  of  Burdon-Sanderson.  This 
consists  of  a  rectangular  framework  constructed  of  two  rigid  parallel 
bars  joined  at  right  angles  to  a  cross  piece.  The  free  ends  of  the 
bars,  the  distance  between  which  can  be  regulated  at  pleasure,  are 
armed,  the  one  with  a  tambour,  the  other  simply  with  an  ivory 
button.  The  tambour  bears  on  the  metal  plate  of  its  membrane 
(m'  Fig.  36)  a  small  ivory  button  in  place  of  the  lever.  When  it  is 
desired  to  record  the  changes  occurring  in  any  diameter  of  the  chest, 
e.g.  an  antero-posterior  diameter  from  a  point  in  the  sternum  to  a 
point  in  the  back,  the  instrument  is  made  to  encircle  the  chest  some- 
what after  the  fashion  of  a  pair  of  callipers,  the  ivory  button  at  one 
free  end  being  placed  on  the  spine  of  a  vertebra  behind  and  the 
tambour  at  the  other  on  the  sternum  in  front  in  the  line  of  the  diam- 
eter which  is  being  studied.  The  distance  between  the  free  ends  of 
the  instrument  being  carefully  adjusted  so  that  the  button  of  the 
tambour  presses  lightly  on  the  sternum,  any  variations  in  the  length 
of  the  diameter  in  question  will,  since  the  framework  of  the  tambour 
is  immobile,  give  rise  to  variations  of  pressure  within  the  tambour. 
These  variations  of  the  'receiving'  tambour  as  it  is  called  are  con- 
veyed by  a  flexible  tube  containing  air  to  a  second  or  'recording' 
tambour,  the  lever  of  which  records  the  variations  on  a  travelling 
surface.  For  the  purpose  of  measuring  the  extent  of  the  movements 
the  instrument  must  be  experimentally  graduated.  Other  forms  of 
callipers  may  of  course  be  used. 

By  still  another  method  the  variations  in  intra-thoracic  pressure^ 
by  means  of  which  the  movements  of  the  chest-walls  produce  the 
movement  of  air  in  the  lungs,  may  be  recorded.  This  may  be 
effected  by  introducing  carefully,  to  the  total  exclusion  of  air,  into 
a  pleural  cavity,  or  into  the  pericardial  cavit}^,  a  cannula  connected 
by  a  rigid  tube  with  a  manometer.  With  each  inspiration  a  nega- 
tive pressure,  or  rather  an  increase  of  the  existing  negative  pressure, 
is  produced,  the  mercury,  or  fluid,  in  the  manometer  returning  at 
each  expiration.  An  easier  method  of  recording  this  intra-thoracic 
pressure  is  to  introduce  into  the  oesophagus  an  elastic  sound  (similar 
to  the  cardiac  sound  Fig.  36)  connected  with  a  tambour.  The 
cesophagus  within  the  thorax  like  the  heart  and  great  vessels,  as 
we  shall  see,  is  affected  as  well  as  the  lungs  by  the  variations 
of  intra-thoracic  pressure  brought  about  by  the  respiratory  move- 
ments. 

In  yet  another  method  the  movements  of  the  diaphragm  which, 
as  we  shall  see,  serve  as  the  prime  agent  in  bringing  about  the 
enlargement  of  the  thoracic  cavity  are  recorded.  This  may  be  done 
by  inserting,  through  an  incision  in  the  abdominal  wall,  a  flat  elastic 
bag  between  the  diaphragm  and  abdominal  organs.  When  in  inspi- 
ration the  diaphragm  descends  it  exerts  on  the  bag  a  pressure  which, 
by  means  of  a  tube,  may  bo  communicated  to  a  tambour.  Or  a 
needle  may  be  thrust  through  the  chest-wall  so  as  to  rest  upon  or 
transfix  the  diaphragm,  and  the  head  of  the  needle  outside  the  body 
connected  by  a  thread  or  otherwise  with  a  lever ;  each  upward  and 
downward  movement  of  the  head  of  the  needle,  corresponding  to  the 


432 


RESPIRATOEY   CUEVES. 


[Book  ii. 


downward  and  upward  movements  of  the  diaphragm,  is  registered 
by  the  lever. 

Various  modifications  of  these  several  methods  have  been  adopted 
by  various  observers.  They  all,  however,  leave  much  to  be  desired. 
A  very  ingenious  method  of  registering  the  contractions  of  the  dia- 
phragm has  recently  been  introduced.  In  the  rabbit  two  slips  of 
muscular  fibres  forming  part  of  the  diaphragm,  one  on  each  side  of 
the  ensiform  cartilage,  are  so  disposed  and  possess  such  attachments 
that  one,  or  both  of  them,  may  be  isolated,  without  injury  to  either 
nerves  or  blood  vessels,  and  arranged  so  that  while  one  end  of  the 
slip  is  securely  fixed  to  the  chest-wall  as  a  fixed  point,  the  other  end 
can  by  a  thread  be  brought  to  bear  on  a  lever.  The  slip,  even  when 
thus  arranged,  appears  to  contract  rhythmically  in  complete  unison 
with  the  contractions  of  the  whole  rest  of  the  diaphragm  ;  it  serves 
so  to  speak  as  a  sample  of  the  diaphragm ;  and  hence  its  contrac- 
tions like  those  of  the  whole  diaphragm  may  be  taken  as  a  record 
of  respiratory  movements.  The  record  has  to  be  corrected  for  vari- 
ations in  the  position  of  the  fixed  point. 

§  260.  In  these  various  ways  curves  are  obtained,  which, 
while  differing  in  detail,  exhibit  the  same  general  features,  and 
more  or  less  resemble  the  curve  shewn  in  Fig.  86. 


Fig.  86.   Tracing  of  Thoracic  Respiratory  Movements  obtained  by 

MEANS    OF    MaREY'S    PNEUMOGRAPH. 

A  whole  respiratory  phase  is  comprised  between  a  and  a  ;  inspiration,  during 
which  the  lever  descends,  extending  from  a  to  6,  and  expiration  from  h  to  a. 
The  undulations  at  c  are  caused  by  the  heart's  beat. 


As  the  figure  shews,  inspiration  begins  somewhat  suddenly 
and  advances  rapidly,  being  followed  immediately  by  expira- 
tion, which  is  carried  out  at  first  rapidly,  but  afterwards  more 
and  more  slowly.  Such  pauses  as  are  seen  usually  occur 
between  the  end  of  expiration  and  the  beginning  of  inspiration. 
In  normal  breathing,  hardly  any  such  pause  exists,  but  in  cases 
where  the  respiration  becomes  infrequent,  pauses  of  considerable 
length  may  be  observed.  As  we  shall  see  in  detail  hereafter, 
the  several  parts  of  the  whole  act  vary  much,  under  various  cir- 
cumstances, in  relation  to  each  other.  Sometimes  expiration, 
sometimes    inspiration  is   prolonged;    and   either    inspiration 


IJiiAP.  II.]  RESPIRATION.  433 

or  expiration  may  be  slow  or  rapid  in  its  development.  At 
times  the  chest  may  remain  for  a  while  at  the  height  of  inspira- 
tion, thns  making  a  pause  between  inspiration  and  expiration. 

In  what  may  be  considered  as  normal  breathing,  the  respir- 
atory act  is  repeated  about  17  times  a  minute,  the  duration  of 
the  inspiration  as  compared  with  that  of  the  expiration  (and 
such  pause  as  may  exist)  being  about  as  ten  to  twelve ;  but  the 
rate  varies  very  largely ;  and  in  this  as  in  the  volume  of  each 
breath  it  is  very  difficult  to  fix  a  satisfactory  average,  the  figures 
given  varying  from  20  to  13  a  minute.  It  varies  according  to 
age  and  sex.  It  is  influenced  by  the  position  of  the  body,  being 
quicker  in  standing  than  in  lying,  and  in  lying  than  in  sitting. 
Muscular  exertion  and  emotional  conditions  affect  it  deeply. 
In  fact,  almost  every  event  which  occurs  in  the  body  ma}^  influ- 
ence it.  We  shall  have  to  consider  in  detail  hereafter  the  man- 
ner in  which  these  influences  are  brought  to  bear. 

When  the  ordinary  respiratory  movements  prove  insufficient 
to  effect  the  necessary  changes  in  the  blood,  their  rhythm  and 
character  become  changed.  Normal  respiration  gives  place  to 
laboured  respiration,  and  this  in  turn  to  dyspnoea,  which,  unless 
some  restorative  event  occurs,  terminates  in  asphyxia.  These 
abnormal  conditions  we  shall  study  more  fully  hereafter. 

The  Respiratory  Movements. 

§  261.  When  the  movements  of  the  chest  during  normal 
breathing  are  watched,  or  when  a  graphic  record  is  taken  by  one 
or  other  of  the  methods  just  described,  it  is  seen  that  during 
inspiration  an  enlargement  takes  place  in  the  antero-posterior 
diameter,  the  sternum  being  thrown  forwards,  and  at  the  same 
time  moving  upward.  The  lateral  width  of  the  chest  is  also 
increased.  The  vertical  increase  of  the  cavity  is  not  so  obvious 
from  the  outside,  though  when  the  movements  of  the  diaphragm 
are  watched  by  means  of  an  inserted  needle  or  otherwise,  it  is 
clear  that  the  upper  surface  of  that  organ  descends  at  each 
inspiration,  the  anterior  walls  of  the  abdomen  bulging  out  at 
the  same  time.  In  the  female  human  subject,  the  movement  of 
the  upper  part  of  the  chest  is  usually  conspicuous,  the  breast 
rising  and  falling  with  every  respiration  ;  while  in  the  male,  the 
movements  of  the  lower  part  of  the  chest  are  more  marked. 
In  laboured  respiration  all  parts  of  the  chest  are  alternately 
expanded  and  contracted,  the  breast  rising  and  falling  as  well 
in  the  male  as  in  the  female.  We  have  now  to  consider  these 
several  movements  in  greater  detail,  and  to  study  the  means  by 
which  they  are  carried  out. 

§  262.  Insjjiration.  There  are  two  chief  means  by  which  the 
chest  is  enlarged  in  normal  inspiration,  viz.  the  descent  of  the 
-diaphragm  and  the  elevation  of  the  ribs.     The  former  causes 

28 


434  MOVEMENTS   OF   INSPIRATION.  [Book  ii. 

that  movement  in  the  lower  part  of  the  chest  and  abdomen 
largely  characteristic  of  male  breathing,  which  is  hence  called 
diaphragmatic ;  the  latter  causes  the  movement  of  the  upper 
chest  largely  characteristic  of  female  breathing,  which  is  called 
costal.  These  two  main  factors  are  assisted  by  less  important 
and  subsidiary  events. 

Even  in  the  female  human  subject,  the  share  taken  in  respi- 
ration by  the  diaphragm  is  an  important  one,  in  the  male  the 
diaphragm  must  be  regarded  as  the  chief  respiratory  agent,  and 
in  some  animals  its  use,  for  this  purpose,  is  so  prominent  that 
the  movements  of  the  ribs  may  in  normal  breathing  be  almost 
neglected.  In  the  rabbit  for  instance,  in  normal  breathing,^ 
almost  all  the  respiratory  work  is  done  by  the  contractions  of 
the  diaphragm. 

The  descent  of  the  diaphragm  is  effected  by  means  of  the 
contraction  of  its  muscular  fibres.  When  at  rest  the  diaphragm 
presents  a  convex  surface  to  the  thorax ;  when  contracted  it 
becomes  much  flatter,  and  in  consequence  the  level  of  the  chest- 
floor  is  lowered,  the  vertical  diameter  of  the  chest  being  pro- 
portionately enlarged.  In  descending,  the  diaphragm  presses  on 
the  abdominal  viscera,  and  so  causes  a  projection  of  the  flaccid 
abdominal  walls.  From  its  attachments  to  the  sternum  and  the 
false  ribs,  the  diaphragm,  while  contracting,  naturally  tends 
to  pull  the  sternum  and  the  upper  false  ribs  downwards  and 
inwards,  and  the  lower  false  ribs  upwards  and  inwards,  towards 
the  lumbar  spine.  In  normal  breathing,  this  tendency  produces 
little  effect,  being  counteracted  by  the  accompanying  general 
costal  elevation,  and  by  certain  special  muscles  to  be  mentioned 
presently.  In  forced  inspiration,  however,  and  especially  where 
there  is  any  obstruction  to  the  entrance  of  air  into  the  lungs, 
the  lower  ribs  may  be  so  much  drawn  in  by  the  contraction  of 
the  diaphragm,  that  the  girth  of  the  trunk  at  this  point  is 
obviously  diminished. 

§  263.  The  elevation  of  the  ribs  is  a  much  more  complex 
matter  than  the  descent  of  the  diaphragm.  If  we  examine  any 
one  rib,  such  as  the  fifth,  we  find  that  while  it  moves  freely  on 
its  vertebral  articulation,  it  inclines  when  in  the  position  of  rest 
in  an  oblique  direction  from  the  spine  to  the  sternum ;  hence  it 
is  obvious  that  when  the  rib  is  raised,  its  sternal  attachment 
must  not  only  be  carried  upward,  but  also  thrown  forward. 
The  rib  may  in  fact  be  regarded  as  a  radius,  moving  on  the  ver- 
tebral articulation  as  a  centre,  and  causing  the  sternal  attach- 
ment to  describe  an  arc  of  a  circle  in  the  vertical  plane  of  the 
body ;  as  the  rib  is  carried  upwards  from  an  oblique  to  a  more 
horizontal  position,  the  sternal  attachment  must  of  necessity  be 
carried  farther  away  in  front  of  the  spine.  Since  all  the  ribs 
have  a  downward  slanting  direction,  they  must  all  tend,  when 
raised  towards  the  horizontal  position,  to  thrust  the  sternum 


Chai'.  II.]  KESFIRATIOJf.  435 

forward,  some  more  than  others  according  to  their  slope  and 
length.  The  elasticity  of  the  sternum  and  costal  cartilages, 
assisted  by  the  articulation  of  the  sternum  to  the  clavicle 
above,  permits  the  front  surface  of  the  chest  to  be  thus  thrust 
forwards  as  well  as  upwards,  when  the  ribs  are  raised.  By  this 
action,  the  antero-posterior  diameter  of  tlie  chest  is  enlarged. 

Since  the  ribs  form  arches  which  increase  in  their  sweep  as 
one  proceeds  from  the  first  downwards  as  far  at  least  as  the 
seventh,  it  is  evident  that  when  a  lower  rib  such  as  the  fifth  is 
elevated  so  as  to  occupy  or  to  approach  towards  the  position  of 
the  one  above  it,  the  chest  at  that  level  will  become  wider  from 
side  to  side,  in  proportion  as  the  fifth  arch  is  wider  than  the 
fourth.  Thus  the  elevation  of  the  rib  increases  not  only  the 
antero-posterior  but  also  the  transverse  diameter  of  the  chest. 
Further,  on  account  of  the  resistance  of  the  sternum,  the  angles 
between  the  ribs  and  their  cartilages  are,  in  the  elevation  of  the 
ribs,  somewhat  opened  out,  and  thus  also  the  transverse  as  well 
as  the  antero-posterior  diameter,  somewhat  increased.  In  more 
than  one  way,  then,  the  elevation  of  the  ribs  enlarges  the  dimen- 
sions of  the  chest. 

§  264.  The  ribs  are  raised  by  the  contraction  of  certain 
muscles.  Of  these  the  external  intercostals  are  perhaps  the 
most  important.  Even  in  the  case  where  two  ribs,  such  as  the 
fifth  and  sixth,  are  isolated  from  the  rest  of  the  thoracic  cage, 
by  section  of  the  structures  occupying  the  intercostal  spaces 
above  and  below,  the  contraction  of  the  external  intercostal 
muscle  of  the  intervening  space  raises  the  two  ribs,  thus  bring- 
ing them  towards  the  position  in  which  the  fibres  of  the  muscle 
have  the  shortest  length,  viz.  the  horizontal  one.  This  elevat- 
ing action  is,  in  the  entire  chest,  further  favoured  by  the  fact 
that  the  first  rib  is  less  moveable  than  the  second,  and  so  affords 
a  comparatively  fixed  base  for  the  action  of  the  muscles  between 
the  two,  the  second  in  turn  supporting  the  third,  and  so  on, 
while  the  scaleni  muscles  in  addition  serve  to  render  fixed,  or  to 
raise,  the  first  two  ribs.  So  that  in  normal  respiration,  the  act 
may  probably  be  described  as  beginning  by  a  contraction  of  the 
scaleni.  The  first  two  ribs  being  thus  raised  or  at  least  fixed, 
the  contraction  of  the  series  of  external  intercostal  muscles  acts 
at  a  great  advantage. 

While  the  elevating,  i.e.  inspiratory  action  of  the  external 
intercostals  is  admitted  by  nearly  all  autliors,  the  function  of 
the  internal  intercostals  has  been  much  disputed.  Some  regard 
their  action  as  wholly  inspiratory  ;  others  maintain,  what  is 
perhaps  the  more  commonly  adopted  view,  that  ^^•lnle  those 
parts  of  them  which  lie  between  the  sternal  cartilages  act  like 
the  external  intercostals  as  elevators,  i.e.  as  inspiratory  in  func- 
tion, those  parts  which  lie  between  the  osseous  ribs  act  as  depres- 
sors, i.e.  as  expiratory  in  function. 


436  MOVEMENTS    OF   THE   RIBS.  [Book  ii. 

In  the  well-known  model  consisting  of  two  rigid  bars,  rep- 
resenting the  ribs,  moving  vertically  by  means  of  their  articula- 
tions with  an  upright  representing  the  spine,  and  connected  at 
their  free  ends  by  a  piece  representing  the  sternum,  it  is  un- 
doubtedly true  that  stretched  elastic  bands  attached  to  the  bars 
in  such  a  way  as  to  represent  respectively  the  external  and  in- 
ternal intercostals,  viz.  sloping  in  the  one  case  downwards  and 
forwards  and  in  the  other  downwards  and  backwards,  do,  on 
being  left  free  to  contract,  in  the  former  case  elevate  and  in  the 
latter  depress  the  ribs.  Such  a  model  however  does  not  fairly 
represent  the  natural  conditions  of  the  ribs,  which  are  not 
straight  and  rigid,  but  peculiarly  curved  and  of  varying  elas- 
ticity, capable  moreover  of  rotation  on  their  own  axes,  and  hav- 
ing their  movements  determined  by  the  characters  of  their 
vertebral  articulations.  The  mechanical  conditions  in  fact  of 
these  muscles  are  so  complex,  that  a  deduction  of  their  actions 
from  simple  mechanical  principles,  or  from  the  direction  of  the 
fibres,  must  be  exceedingly  difficult  and  dangerous.  Actual 
experiments  on  the  cat  and  dog  tend  to  shew  that  in  these  ani- 
mals the  contraction  of  the  internal  intercostals,  along  their 
whole  length,  takes  place,  in  point  of  time,  alternately  with  that 
of  the  diaphragm,  and  thus  afford  an  argument  in  favour  of 
these  muscles  being  expiratory  in  function. 

Next  in  importance  to  the  external  intercostals  come  the 
levatores  costarum,  which,  though  small  muscles,  are  able,  from 
the  nearness  of  their  costal  insertions  to  the  fulcrum,  to  produce 
considerable  movement  of  the  sternal  ends  of  the  ribs.  The 
external  intercostals  and  the  levatores  costarum  with  the  scaleni 
may  fairly  be  said  to  be  the  elevators  of  the  ribs,  i.e.  the  chief 
muscles  of  costal  inspiration  in  normal  breathing. 

It  must  be  added  however  that  some  observers  deny  that 
either  set  of  intercostal  muscles  take  any  important  part  in  rais- 
ing the  ribs.  They  hold  that  the  chief  if  not  the  only  use  of 
these  muscles  is  by  their  contraction  to  render  the  intercostal 
spaces  firm  and  the  whole  thoracic  cage  rigid,  so  that  the  thorax 
is  moved  as  a  whole  by  the  other  muscles  mentioned,  and  the 
intercostal  spaces  do  not  give  way  during  the  respiratory  move- 
ments. 

Additional  space  in  the  transverse  diameter  is  afforded  prob- 
ably by  the  rotation  of  the  ribs  on  an  antero-posterior  axis  ;  but 
this  movement  is  quite  subsidiary  and  unimportant.  When  the 
chest  is  at  rest,  the  ribs  are  somewhat  inclined  with  their  lower 
borders  directed  inwards  as  well  as  downwards.  When  they  are 
drawn  up  by  the  action  of  the  intercostal  muscles,  their  lower 
borders  are  everted.  Thus  their  fiat  sides  are  presented  to  the 
thoracic  cavity,  which  is  thereby  slightly  increased  in  v/idth. 

§  265.  Laboured  Inspiration.  When  respiration  becomes 
laboured,  other  muscles  are  brought  into  play.     The  scaleni  are 


Chap.  II.]  RESPIRATION.  437 

strongly  contracted,  so  as  distinctly  to  raise  or  at  least  give  a 
very  ti.xed  support  to  the  first  and  second  ribs.  In  the  same 
way  the  serratus  posticus  superior,  which  descends  from  the  fixed 
spine  in  the  lower  cervical  and  upper  dorsal  regions  to  the  second, 
third,  fourth,  and  fifth  ribs,  by  its  contractions  raises  those  ribs. 
In  laboured  breathing  a  function  of  the  lower  false  ribs,  not  very 
noticeable  in  easy  breathing,  comes  into  play.  They  are  de- 
j)ressed,  retracted,  and  fixed,  thereby  giving  increased  support 
to  the  diaphragm,  and  directing  the  whole  energies  of  that  mus- 
cle to  the  vertical  enlargement  of  the  chest.  In  this  way  the 
serratus  posticus  inferior,  which  jDasses  upward  from  the  lumbar 
aponeurosis  to  the  last  four  ribs,  by  depressing  and  fixing  those 
ribs  becomes  an  adjuvant  inspiratory  muscle.  The  quadratus 
lumborum  and  lower  portions  of  the  sacro-lumbalis  may  have  a 
similar  function. 

All  these  muscles  may  come  into  action  even  in  breathing 
which,  though  deeper  than  usual,  can  hardly  perhaps  be  called 
laboured.  When,  however,  the  need  for  greater  inspiratory 
efforts  becomes  urgent,  all  the  muscles  which  can,  from  any  fixed 
point,  act  in  enlarging  the  chest,  come  into  play.  Thus  the 
arms  and  shoulder  being  fixed,  the  serratus  magnus  passing  from 
the  scapula  to  the  middle  of  the  first  eight  or  nine  ribs,  the  pec- 
toralis  minor  passing  from  the  coracoid  to  the  front  parts  of  the 
third,  fourth,  and  fifth  ribs,  the  pectoralis  major  passing  from 
the  humerus  to  the  costal  cartilages,  from  the  second  to  the  sixth, 
and  that  portion  of  the  latissimus  dorsi  which  passes  from  the 
humerus  to  the  last  three  ribs,  all  serve  to  elevate  the  ribs  and 
thus  to  enlarge  the  chest.  The  sterno-mastoid  and  other  mus- 
cles passing  from  the  neck  to  the  sternum,  are  also  called  into 
action.  In  fact,  every  muscle  which  by  its  contraction  can  either 
elevate  the  ribs  or  contribute  to  the  fixed  sujjport  of  muscles 
which  do  elevate  the  ribs,  such  as  the  trapezius,  levator  anguli 
s(;apuloB  and  rhomboidei  by  fixing  the  scapula,  may,  in  the  in- 
spiratory efforts  which  accompany  dyspnoea,  be  brought  into 
play. 

§  266.  Expiration.  In  normal  easy  breathing,  expiration  is 
in  the  main  a  simple  effect  of  elastic  reaction.  By  the  inspira- 
tory effort  the  elastic  tissue  of  the  lungs  is  put  on  the  stretch  ; 
so  long  as  the  inspiratory  muscles  continue  contracting,  the 
tissue  remains  stretched,  but  directly  those  muscles  relax,  the 
elasticity  of  the  lungs  comes  into  play  and  drives  out  a  portion 
of  the  air  contained  in  them.  Similarly  the  elastic  sternum 
and  costal  cartilages  are  by  the  elevation  of  the  ribs  put  on  the 
stretch  :  they  are  driven  into  a  jiosition  which  is  unnatural  to 
them.  When  the  intercostal  and  other  elevator  muscles  cease 
to  contract,  the  elasticity  of  the  sternum  and  costal  cartilages 
causes  them  to  return  to  their  previous  position,  thus  depressing 
the  ribs,  and  diminishing  the  dimensions  of  the  chest.     When 


438  MUSCLES   OF  EXPIRATION.  [Book  ii. 

the  diaphragm  descends,  in  pushing  down  the  abdominal  viscera, 
it  puts  the  abdominal  walls  on  the  stretch  :  and  hence,  when  at 
the  end  of  inspiration  the  diaphragm  relaxes,  the  abdominal 
walls  return  to  their  place,  and  by  pressing  on  the  abdominal 
viscera,  push  the  diaphragm  up  again  into  its  position  of  rest. 
Expiration  then  during  easy  breathing  is,  in  the  main,  simple 
elastic  reaction  ;  but  there  is  probably  some,  though  possibly  in 
most  cases,  a  very  slight,  expenditure  of  muscular  energy  to 
bring  the  chest  more  rapidly  to  its  former  condition.  This  is, 
as  we  have  seen,  supposed  by  many  to  be  afforded  by  the  inter- 
nal intercostals  acting  as  depressors  of  the  ribs.  If  these  do 
not  act  in  this  way,  we  may  suppose  that  the  elastic  return  of 
the  abdominal  walls  is  accompanied  and  assisted  by  a  contrac- 
tion of  the  abdominal  muscles.  The  triangularis  sterni,  the 
effect  of  whose  contraction  is  to  pull  down  the  costal  cartilages, 
may  also  be  regarded  as  an  expiratory  muscle. 

When  expiration  becomes  laboured,  the  abdominal  muscles 
become  important  expiratory  agents.  By  pressing  on  the  con- 
tents of  the  abdomen,  they  thrust  them  and  therefore  the  dia- 
phragm also  up  towards  the  chest,  the  vertical  diameter  of  which 
is  thereby  lessened,  while  by  pulling  down  the  sternum  and  the 
middle  and  lower  ribs  they  lessen  also  the  cavity  of  the  chest 
in  its  antero-posterior  and  transverse  diameters.  They  are,  in 
fact,  the  chief  expiratory  muscles,  though  they  are  doubtless 
assisted  by  the  serratus  posticus  inferior  and  portions  of  the 
sacro-lumbalis,  since  when  the  diaphragm  is  not  contracting, 
the  depression  of  the  lower  ribs  which  the  contraction  of  these 
muscles  causes,  serves  only  to  narrow  the  chest.  As  expiration 
becomes  more  and  more  forced,  every  muscle  in  the  body  which 
can  either  by  contracting  depress  the  ribs,  or  press  on  the  ab- 
dominal viscera,  or  afford  fixed  support  to  muscles  having  those 
actions,  is  called  into  play. 

§  267.  Facial  and  Laryngeal  Respiration.  The  thoracic 
respiratory  movements  are  accompanied  by  associated  respira- 
tory movements  of  other  parts  of  the  body,  more  particularly  of 
the  face  and  of  the  glottis. 

In  normal  healthy  respiration,  the  current  of  air  which  passes 
in  and  out  of  the  lungs,  travels,  not  through  the  mouth  but 
through  the  nose,  chiefly  through  the  lower  nasal  meatus.  The 
ingoing  air,  by  exposure  to  the  vascular  mucous  membrane  of 
the  narrow  and  winding  nasal  passages,  is  more  efficiently 
warmed  than  it  would  be  if  it  passed  through  the  mouth  ;  and 
at  the  same  time  the  mouth  is  thereby  protected  from  the  desic- 
cating effect  of  the  continual  inroad  of  comparatively  dry  air. 

During  each  inspiratory  effort  the  nostrils  are  expanded, 
probably  by  the  action  of  the  dilatores  naris,  and  thus  the 
entrance  of  air  facilitated.  The  return  to  their  previous  condi- 
tion during  expiration  is  effected  by  the  elasticity  of  the  nasal 


Chap,  ii.]  RESPIRATION.  439 

cartilages,  assisted  perhaps  by  the  compressores  naris.  This 
movement  of  the  nostrils,  perceptible  in  many  people  even  during 
tranquil  breathing,  becomes  very  obvious  in  laboured  respira- 
tion. 

When  the  mouth  is  closed,  the  soft  palate  which  is  held  some- 
what tense,  is  swayed  by  the  respiratory  current,  but  entirely 
in  a  passive  manner,  and  it  is  not  until  the  larynx  is  reached  by 
the  ingoing  air  that  any  active  movements  are  met  with.  When 
the  larynx  (the  details  of  which  we  shall  have  to  deal  with  at 
a  later  part  of  this  work)  is  examined  with  the  laryngoscope, 
it  is  frequently  seen  that,  while  during  inspiration  the  glottis 
is  widely  open,  with  each  expiration  the  arytenoid  cartilages 
approach  each  other  so  as  to  narrow  the  glottis,  the  cartilages 
of  Santorini  projecting  inwards  at  the  same  time.  Thus,  syn- 
chronous with  the  respiratory  expansion  and  contraction  of  the 
chest,  and  the  res23iratory  elevation  and  depression  of  the  ahe 
nasi,  there  is  a  rhythmic  widening  and  narrowing  of  the  glottis. 
Like  the  movements  of  the  nostril,  this  resj^iratory  action  of  the 
glottis  is  much  more  evident  in  laboured  than  in  tranquil  breath- 
ing. Indeed  in  the  latter  case  it  is  frequently  absent.  The 
manner  in  which  this  rhythmic  opening  and  narroAving  is  effected 
will  be  described  when  we  come  to  study  the  production  of  the 
voice.  Whether  there  exists  a  rhythmic  contraction  and  expan- 
sion of  the  trachea  and  bronchial  passages,  especially  the  smaller 
and  more  exclusively  muscular  ones,  effected  by  means  of  the 
plain  muscular  tissue  of  those  organs  and  synchronous  with  the 
respiratory  movements  of  the  chest,  is  uncertain. 


SEC.  2.     CHANGES    OF  THE   AIR   IN  RESPIRATION. 

§  268.  During  its  stay  in  the  lungs,  or  rather  during  its  stay 
in  the  bronchial  passages,  the  tidal  air  (by  means  of  diffusion 
chiefly)  effects  exchanges  Avith  the  stationary  air ;  in  conse- 
quence the  expired  air  differs  from  inspired  air  in  several  impor- 
tant particulars. 

The  temperature  of  expired  air  is  variable,  but  under  ordi- 
nary circumstances  is  higher  than  that  of  the  inspired  air. 
At  an  average  temperature  of  the  atmosphere,  for  instance  at 
about  20°  C,  the  temperature  of  expired  air  is,  in  the  mouth 
33 -9°,  in  the  nose  35*3°.  When  the  external  temperature  is 
low,  that  of  the  expired  air  sinks  somewhat,  but  not  to  any 
great  extent,  thus  at  -  6-3°  C.  it  is  29-8°  C.  When  the  external 
temperature  is  high,  the  expired  air  may  become  cooler  than 
the  inspired,  thus  at  41-9°  it  has  been  found  to  be  384°.  The 
expired  air  takes  its  temperature  from  that  of  the  body,  that 
is,  of  the  blood,  and  this  as  we  shall  see  later  on  while  generally- 
higher  may,  at  times,  be  lower  than  that  of  the  atmosphere. 
The  exact  temperature  of  the  expired  air  in  fact  depends  on 
the  relative  temperatures  of  the  blood  and  inspired  air,  and  on 
the  depth  and  rate  of  breathing.  The  change  in  temperature 
takes  place  not  in  the  lungs  but  in  the  upper  passages,  and 
chiefly  in  the  nose  and  pharynx. 

§  269.  The  expired  air  is  loaded  Avith  aqueous  vapour.  The 
point  of  saturation  of  any  gas,  that  is,  the  utmost  quantity  of 
water  which  any  given  volume  of  gas  can  take  up  as  aqueous 
vapour,  varies  with  its  temperature,  being  higher  with  the 
higher  temperature.  For  its  own  temperature  expired  air  is, 
according  to  most  observers,  saturated  with  aqueous  vapour. 
The  moisture,  like  the  warmth,  is  imparted  not  in  the  depths 
of  the  lung  but  in  the  upper  passages.  The  inspired  air  as  it 
passes  into  the  bronchia  is  already  saturated  with  moisture. 

§  270.  The  expired  air  contains  about  4  or  5  p.c.  less  oxy- 
gen, and  about  4  p.c.  more  carbonic  acid  than  the  inspired  air, 
the  quantity  of  nitrogen  suffering  but  little  change.     Thus 


oxygen. 

nitrogen. 

carbonic  acid. 

Inspired  air  contains  20-81 

79-15 

-04 

Expired     „         „        16-033 

79-587 

4-38 

440 

Chap,  ii.]  RESPIRATIOX.  441 

The  quantity  of  nitrogen  in  the  expired  air  is  sometimes 
found  to  be  slightly  greater  than,  as  in  the  table  above,  but  some- 
times equal  to,  and  sometimes  less  than,  that  of  the  inspired  air. 

In  a  single  breath  the  air  is  richer  in  carbonic  acid  (and 
poorer  in  oxygen)  at  the  end  than  at  the  beginning  of  the- 
breath.  Hence  the  longer  the  breath  is  held,  the  greater  the 
(artificial)  pause  between  inspiration  and  expiration,  the  higher 
the  percentage  of  carbonic  acid  in  the  expired  air.  Thus  by 
increasing  the  interval  between  two  expirations  to  100  seconds,, 
the  percentage  maybe  raised  to  7-5.  When  the  rate  of  breath- 
ing remains  the  same,  by  increasing  the  depth  of  the  breathing 
the  percentage  of  carbonic  acid  in  each  breath  is  lowered,  but 
the  total  quantity  of  carbonic  acid  expired  in  a  given  time  is 
increased.  Similarly,  when  the  depth  of  breath  remains  the 
same,  by  quickening  the  rate  the  percentage  of  carbonic  acid 
in  each  breath  is  lowered,  but  the  quantity  expired  in  a  given 
time  is  increased. 

Taking,  as  we  have  done,  the  amount  of  tidal  air  passing  in 
and  out  of  the  chest  of  an  average  man  at  500  c.c,  such  a 
person  will  expire  about  22  c.c.  of  carbonic  acid  at  each  breath  ; 
this,  reckoning  the  rate  of  breathing  at  17  a  minute,  would 
give  over  500  litres  of  carbonic  acid  for  the  day's  production. 
Actual  determinations  however  give  a  rather  smaller  total  than 
this ;  thus  in  a  series  of  experiments  of  which  we  shall  have  to 
speak  hereafter,  the  total  daily  excretion  of  carbonic  acid  in  an 
average  man  was  found  to  be  800  grms.,  i.e.  rather  more  than 
400  litres  (406),  containing  2184  grms.  carbon,  and  581-9 
grms.  oxygen,  the  oxygen  which  actually  disappeared  from  the 
inspired  air  at  the  same  time  being  about  700  grms.  This 
amount  it  should  be  said  represents,  owing  to  the  manner  in 
which  the  experiment  was  conducted,  the  gases  given  out  and 
taken  in,  not  by  the  lungs  only,  but  by  the  whole  body ;  but 
the  amount  of  carbonic  acid  given  out  by  other  channels  than 
the  lungs  is,  as  we  shall  see,  very  slight  (10  grms.  or  even 
less),  so  that  800  grms.  may  be  taken  as  the  average  production 
of  carbonic  acid  by  an  average  man.  The  quantity  however, 
both  of  oxygen  consumed  and  of  carbonic  acid  given  out,  is 
subject  to  very  wide  variations ;  thus  in  the  observations  of 
which  we  are  speaking  the  daily  quantity  of  carbonic  acid 
varied  from  686  to  1285  grms.,  and  that  of  the  oxygen  from 
594  to  1072  grms.  These  variations  and  their  causes  \\\\\  be 
discussed  when  Ave  come  to  deal  with  the  problems  of  nutrition. 

§  271.  When  the  total  quantity  of  tidal  air  given  out  at 
any  expiration  is  compared  with  that  taken  in  at  the  corre- 
sponding inspiration,  it  is  found  that,  both  being  dried  and 
measured  at  the  same  temperature  and  pressure,  the  expired 
air  is  less  in  volume  than  the  inspired  air,  the  difference 
amounting  to  about  ^^^th  or  -^^{\\  of  the  volume  of  the  latter. 


442  _  NATUEE   OF  EXPIRED   AIR.  [Book  ii. 

Hence,  when  an  animal  is  made  to  breathe  in  a  confined  space, 
the  air  is  absolutely  diminished  in  volume.  The  approximate 
equivalence  in  volume  between  inspired  and  expired  air  arises 
from  the  fact  that  the  volume  of  any  given  quantity  of  carbonic 
acid  is  equal  to  the  volume  of  the  oxygen  consumed  to  produce 
it ;  the  slight  falling  short  of  the  expired  air  is  due  to  the  cir- 
cumstance that  all  the  oxygen  inspired  does  not  reappear  in 
the  carbonic  acid  expired,  some  having  formed  within  the  body 
other  combinations. 

§  272.  Besides  carbonic  acid,  expired  air  contains  various 
substances  which  may  be  spoken  of  as  impurities,  many  of  an 
unknown  nature,  and  all  in  small  amounts.  Traces  of  ammonia 
have  been  detected  in  expired  air,  even  in  that  taken  directly 
from  the  trachea,  in  which  case  its  presence  could  not  be  due 
to  decomposing  food  lingering  in  the  mouth.  When  the  expired 
air  is  condensed  by  being  conveyed  into  a  cooled  receiver,  the 
aqueous  product  is  found  to  contain  organic  matter,  which, 
from  the  presence  of  micro-organisms,  introduced  in  the  inspired 
air,  is  very  apt  rapidly  to  putrefy.  The  organic  substances 
thus  shewn  to  be  present  in  the  expired  air  are  the  cause  in 
part  of  the  odour  of  breath.  It  is  probable  that  some  of  them 
are  of  a  poisonous  nature,  either  poisonous  in  themselves  as 
coming  direct  from  and  produced  in  some  way  or  other  in  the 
pulmonary  apparatus,  or  poisonous  as  being  the  products  of 
putrefactive  decomposition ;  for  various  animal  substances  and 
fluids  give  rise  by  decomposition  to  distinct  poisonous  products, 
known  as  ptomaines^  and  it  is  possible  that  some  of  the  constitu- 
ents of  expired  air  are  of  an  allied  nature.  In  any  case  the 
substances  present  have  a  deleterious  action,  for  an  atmosphere 
containing  simply  1  p.c.  of  carbonic  acid  (with  a  corresponding 
diminution  of  oxygen)  has  very  little  effect  on  the  animal 
economy,  whereas  an  atmosphere  in  which  the  carbonic  acid 
has  been  raised  to  1  p.c.  by  breathing,  is  highly  injurious.  In 
fact,  air  rendered  so  far  impure  by  breathing  that  the  carbonic 
acid  amounts  to  -08  p.c.  is  distinctly  unwholesome,  not  so  much 
on  account  of  the  carbonic  acid,  as  of  the  accompanying  impuri- 
ties. Since  these  impurities  are  of  unknown  nature  and  cannot 
be  estimated,  the  easily  determined  carbonic  acid  is  usually 
taken  as  an  indirect  measure  of  their  presence.  We  have  seen 
that  the  average  man  loads,  at  each  breath,  500  c.c.  of  air  with 
carbonic  acid  to  the  extent  of  4  p.c.  He  will  accordingly  at 
each  breath  load  2  litres  to  the  extent  of  1  p.c. ;  and  in  one  hour, 
if  he  breathes  17  times  a  minute,  will  load  rather  more  than 
2000  litres  to  the  same  extent.  At  the  very  least  then  a  man 
ought  to  be  supplied  with  this  quantity  of  air  hourly;  and  if  • 
the  air  is  to  be  kept  fairly  wholesome,  that  is  with  the  carbonic 
acid  reduced  considerably  below  •!  p.c,  he  should  have  even 
more  than  ten  times  as  much. 


SEC.   3.     THE   RESPIRATORY  CHANGES   IN"  THE  BLOOD. 

§  273.  While  the  air  in  passing  in  and  out  of  the  lungs  is 
thus  robbed  of  a  portion  of  its  oxygen,  and  loaded  with  a  cer- 
tain quantity  of  carbonic  acid,  the  blood  as  it  streams  along  the 
pulmonary  capillaries  undergoes  important  correlative  changes. 
As  it  leaves  the  right  ventricle  it  is  venous  blood  of  a  dark 
purple  or  maroon  colour  ;  when  it  falls  into  the  left  auricle  it  is 
arterial  blood  of  a  bright  scarlet  hue.  In  passing  through  the 
capillaries  of  the  body  from  the  left  to  the  right  side  of  the 
heart,  it  is  again  changed  from  the  arterial  to  the  venous  con- 
dition. •  We  have  to  inquire,  What  are  the  essential  differences 
between  arterial  and  venous  blood,  by  what  means  is  the  venous 
blood  changed  into  arterial  in  the  lungs,  and  the  arterial  into 
venous  in  the  rest  of  the  body,  and  what  relations  do  these 
changes  in  the  blood  bear  to  the  changes  in  the  air  which  we 
have  already  studied? 

The  facts,  that  venous  blood  at  once  becomes  arterial  in 
apjDcarance  on  being  exposed  to  or  shaken  up  with  air  or  oxy- 
gen, and  that  arterial  blood  becomes  venous  in  appearance  when 
kept  for  some  little  time  in  a  closed  vessel,  or  when  submitted 
to  a  current  of  some  indifferent  gas  such  as  nitrogen  or  hydro- 
gen, prepare  us  for  the  statement  that  the  fundamental  dif- 
ference between  venous  and  arterial  blood  is  in  the  relative 
proportion  of  the  oxygen  and  carbonic  acid  gases  contained  in 
each.  From  both,  a  certain  quantity  of  gas  can  be  extracted 
by  means  which  do  not  otherwise  materially  alter  the  constitu- 
tion of  the  blood ;  and  this  gas  when  obtained  from  arterial 
blood  is  found  to  contain  more  oxj^gen  and  less  carbonic  acid 
than  that  obtained  from  venous  blood.  This  is  the  real  differ- 
ential character  of  the  two  bloods ;  all  other  differences  are 
either,  as  we  shall  see  to  be  the  case  with  the  colour,  dependent 
on  this,  or  are  unimportant  and  fluctuating. 

If  the  quantity  of  gas  which  can  be  extracted  by  the  mer- 
curial air-pump  from  100  vols,  of  blood  be  measured  at  0°  C, 
and  a  pressure  of  7G0  mm.,  it  is  found  to  amount,  in  round 
numbers,  to  60  vols. 

443 


444 


MERCURIAL    GAS-PUAIP. 


[Book  ii. 


Fig.  87.    Diagrammatic  Ii.lustration  of  Ludwig's  Mercurial  Gas-Pump. 


A  and  B  are  two  glass  globes,  connected  by  strong  india-rubber  tubes,  a  and 
ft,  with  two  similar  glass  globes  A'  and  B'.  A  is  further  connected  by  means  of 
the  stopcock  c  with  the  receiver  C  containing  the  blood  (or  other  fluid)  to  be 
analyzed,  and  B  by  means  of  the  stopcock  d  and  the  tube  e  with  the  receiver  D 
for  receiving  the  gases.  A  and  B  are  also  connected  with  each  other  by  means 
of  the  stopcocks  /  and  g,  the  latter  being  so  arranged  that  B  also  communicates 
with  B'  by  the  passage  cj'.  A'  and  B'  being  full  of  mercury  and  the  cocks  k,  f, 
g,  and  d  being  open  but  c  and  g'  closed,  on  raising  A'  by  means  of  the  pulley  p 
the  mercury  of  A'  fills  A,  driving  out  the  air  contained  in  it,  into  B,  and  so  out 
through  e.  When  the  mercury  has  risen  above  g,  f  is  closed,  and  g'  being  opened, 
B'  is  in  turn  raised  till  B  is  completely  filled  with  mercury,  all  tlie  air  previously 
in  it  being  driven  out  through  e.  Upon  closing  d,  and  lowering  B',  the  whole  of 
the  mercury  in  B  falls  in  B',  and  a  vacuum  consequently  is  established  in  B. 
On  closing  g',  but  opening  g,  f,  and  k  and  lowering  A',  a  vacuum  is  similarly 
established  in  A  and  in  the  junction  between  A  and  B.  If  the  cock  c  be  now 
opened  the  gases  of  the  blood  in  C  escape  into  the  vacuum  of  A  and  B.  By 
raising  A',  after  the  closure  of  c,  and  opening  of  d,  the  gases  so  set  free  are 
driven  from  A  into  B,  and  by  the  raising  of  B'  from  B,  through  e  into  the 
receiver  D,  standing  over  mercury. 


Chap,  ii.]  EESPIRATIOK  445 

The  vacuum  produced  by  the  ordinary  mechanical  air-pump  is 
insufficient  to  extract  all  the  gas  from  blood.  Hence  it  becomes 
necessary  to  use  a  mercury  pump  capable  of  producing  a  large  Tor- 
ricellian vacuum.  In  the  form  of  mercurial  pump  which  bears 
Ludwig's  name  (Fig.  87)  two  large  globes  of  glass,  one  fixed  and  the 
other  moveable,  are  connected  by  a  flexible  tube ;  the  fixed  globe  is 
made  to  communicate  by  means  of  air-tight  stojDCocks  alternately 
with  a  receiver  containing  the  blood,  and  with  a  receiver  to  collect 
the  gas.  When  the  moveable  globe  tilled  with  mercury  is  raised 
above  the  fixed  one,  the  mercury  from  the  former  runs  into  and  com- 
pletely tills  the  latter,  the  air  previously  present  being  driven  out. 
After  adjusting  the  cocks,  the  moveable  globe  is  then  depressed 
thirty  incdies  below  the  fixed  one,  in  which  the  consequent  fall  of 
the  mercury  produces  an  almost  complete  vacuum.  By  turning  the 
proper  cock  this  vacuum  is  put  into  connection  Avith  the  receiver 
containing  the  blood,  which  thereupon  becomes  proportionately 
exhausted.  By  again  adjusting  the  cocks  and  once  more  elevating 
1;he  moveable  globe,  the  gas  thus  extracted  is  driven  out  of  the  fixed 
globe  into  a  receiver.  The  vacuum  is  then  once  more  established 
and  the  operation  repeated  as  long  as  gas  continues  to  be  given  oft' 
from  the  blood. 

A  modified  form  of  pump  working  on  the  same  principles  as 
that  of  Ludwig,  but  involving  the  use  of  only  one  globe  to  be  made 
vacuous  and  one  moveable  reservoir  for  mercury,  has  been  constructed 
by  Pfliiger.  It  presents  several  advantages  over  the  one  just  de- 
scribed, the  chief  being  that  (i)  non-detibrinated  blood  may  be  used 
fur  the  extraction  of  its  gases,  (ii)  the  vacuum  into  which  the  gases 
are  evolved  is  large,  (iii)  this  vacuum  is  kept  dry  by  being  con- 
nected laterally  with  a  vacuous  chamber  containing  sulphuric  acid. 
The  details  of  its  construction  are  however  complicated,  and  the 
greatest  care  is  required  in  its  use  to  avoid  breakage.  Of  later  j'ears 
a  simpliHed  form  of  pump  has  been  introduced  for  laboratory  work. 
It  was  tirst  used  by  Grehant  and  Paul  Bert,  and  is  now  frequently 
called  an  Alvergniat's  pump,  from  the  name  of  its  present  maker. 
Fig.  88  gives  a  diagrammatic  representation  of  its  construction. 

^  is  a  glass  bulb  some  five  inches  in  diameter,  blown  on  to  a 
glass  tube  a  below  and  on  to  a  vertical  tube  b  above.  The  lower 
■end  of  a  is  connected  by  a  thick-walled  india-rubber  tube  with  a 
reservoir  for  mercury  B,  which  can  be  raised  and  lowered  by  means 
-of  a  string  passing  over  a  pulley  c.  The  vertical  tube  b  is  thickened 
at  one  place,  and  into  this  thickened  portion  a  three-way  tap  cl  is 
ground.  The  upper  end  of  b  is  prolonged  (above  the  three-way  tap) 
into  a  fine  point.  This  point  passes  by  a  tight  joint  through  the 
bottom  of  a  vessel  e,  Avhich  can  be  partly  filled  with  meicur}-,  and 
over  which  a  receiver/  filled  with  mercury  for  the  collection  of  the 
gases,  ean  be  iuverte;!.  A  tube  g  fused  on  Literally  to  one  opening 
of  the  tliree-wa.y  ta]i  d  places  the  latter  in  connection  with  a  thick- 
walled  Woulfi's  bottle  C  containing  a  layer  of  strong  sulphuric  acid. 
The  second  tubulure  of  this  bottle  is  similarly  connei^ted  by  an 
elastic  tube  with  the  vessel  D,  into  which  blood  or  other  fluid  may 
be  introduced  by  means  of  the  tap  h.  All  the  moveable  joints  of 
the  apparatus  are  protected  by  india-rubber  tubes  into  which  water 


446 


MERCUEIAL   GAS-PUMP. 


[Book  ii. 


can  be  poured,  and  a  metal  casing  round  the  tap  d,  which  may  also 
be  filled  with  water,  similarly  prevents  the  possibility  of  any  leak- 
age here. 

The  pump  is  used  as  follows.     By  placing  the  tap  of  in  the  posi- 
tion shewn  in  the  figure  and  raising  B,  the  bulb  A  may  be  filled  with 


f>\ 


Fig. 


Diagram  op  Alvergniat's  Pump. 


mercury  up  to  the  top,  the  contained  air  being  expelled  through  the 
upper  end  of  6.  By  a  slight  turn  of  the  tap  all  connection  between 
A  and  either  the  tube  g'  or  the  upper  part  of  h  may  be  cut  off,  and 
on  lowering  B  a  vacuum  is  established  in  the  bulb  A  and  part  of 
the  tube  a.  A  may  now  be  connected  by  the  tap  d  with  the  tube  g, 
and  hence  with  C  and  D,  and,  h  being  closed,  a  partial  vacuum  is 
established  in  C  and  D.  By  means  of  the  tap  d  the  air  in  A  may  be 
cut  off  from  g,  and  on  raising  B  and  placing  the  plug  of  d  as  shewn  in 
the  figure  this  air  may  be  expelled  through  the  upper  end  of  h.  By 
slightly  turning  d  and  lowering  B  a  vacuum  is  again  established  in 
A,  and  as  before  a  further  portion  of  air  in  C  and  D  may  be  allowed 
to  pass  over  into  A  and  the  vacuum  in  D  and  C  increased.  In  this 
way  all  the  air  in  D  can  be  extracted,  the  final  stages  being  facili- 


Chap,  ii.]  RESPIRATIOK  447 

tated  by  the  admission  of  a  little  water  into  D,  the  last  traces  of  air 
being  driven  over  into  A  by  the  rush  of  vapour  from  the  water.  A 
known  volume  of  blood  having  been  collected  over  mercury  in  a  small 
tube  is  now  allowed  to  enter  D  through  the  tap  h  and  yields  up  its 
gases  to  the  vacuum.  A  repetition  of  the  processes  by  which  the 
air  in  D  was  originally  extracted  will  now  remove  the  gases  which 
have  been  given  off  from  the  known  volume  of  blood,  the  only  dif- 
ference being  that  now  the  tube /filled  with  mercury  is  inverted  in 
the  trough  e  over  the  upper  end  of  the  tube  h.  In  this  way  the 
gases  originally  in  D  are  not  allowed  to  escape  into  the  air,  as  was 
the  case  when  the  apparatus  was  being  originally  made  vacuous,  but 
are  collected  in /for  subsequent  analysis.  During  the  extraction 
of  the  gases  from  the  blood  the  bulb  D  is  immersed  in  a  vessel  of 
warm  water,  to  facilitate  the  exit  of  the  gases  and,  by  causing  the 
formation  of  large  quantities  of  aqueous  vapour,  to  sweep  the  gases 
rapidly  over  into  A.  The  sulphuric  acid  chamber  C  dries  the 
vacuum  before  the  admission  of  the  blood  into  D,  and  hence  makes 
it  more  perfect  and  causes  the  most  complete  and  rapid  evolution  of 
gases  from  the  blood. 

The  average  composition  of  the  gas  thus  obtained  from  each 
of  the  two  kinds  of  blood  (the  arterial  blood  being  taken  from 
a  large  artery,  and  the  venous  blood  from  the  right  side  of  the 
heart)  is,  stated  in  round  numbers,  as  follows : 

From  100  vols.  may  be  obtained 

Of  oxyscen,  of  carbonic  acid,  of  nitrogen. 

Of  Arterial  Blood,      20  vols.  40  vols.  1  to  2  vols. 

Of  Venous  Blood,       8  to  12  vols.     46  vols.  1  to  2  vols, 

all  measured  at  760  mm.  and  0°  C 

That  is  to  say,  venous  blood,  as  compared  with  arterial 
blood,  contains  8  to  12  p.c.  less  oxygen  and  6  p.c.  more 
carbonic  acid.  It  must  be  remembered,  however,  that  while 
arterial  blood  from  whatever  artery  taken  has  always  nearly 
the  same  proportion  of  gases,  or  at  all  events  the  same  amount 
of  oxygen,  the  amount  of  oxygen  in  venous  blood,  even  when 
taken  from  the  same  vein,  may  vary  a  good  deal,  still  more 
so  when  it  is  taken  from  different  veins.  The  reason  of  this 
we  shall  see  hereafter. 

It  will  be  convenient  to  consider  the  relations  of  each  of 
these  gases  separately. 

The  relations  of  Oxygen  in  the  Blood. 

§  274.  When  a  liquid  such  as  water  is  exposed  to  an 
atmosphere  containing  a  gas  such  as  oxygen,  some  of  the  oxy- 
gen Avill  be  dissolved  in  the  water,  that  is  to  say,  will  be 
absorbed  from  the  atmosphere.  The  quantity  which  is  so 
absorbed  will  depend  on  the  pressure  of  the  oxygen  in  the 


448  RELATIONS   OF   OXYGEN  IN   BLOOD.    [Book  ii. 

atmospliere  above ;  tlie  greater  tlie  pressure  of  the  oxygen,  the 
larger  the  amount  which  will  be  absorbed.  If  the  pressure  of 
the  whole  atmosphere  remain  the  same,  at  760  mm.  of  mercury 
for  instance  (the  ordinary  atmospheric  pressure),  the  pressure 
of  the  oxygen  may  be  increased  or  diminished  by  increasing  or 
diminishing  the  proportion  of  oxygen  in  the  atmosphere.  So 
that  with  an  atmosphere  remaining  at  any  given  pressure  the 
quantity  of  oxygen  absorbed  will  depend  on  the  quantity 
present  in  that  atmosphere.  If  on  the  other  hand  water, 
already  containing  a  good  deal  of  oxygen  dissolved  in  it,  be 
exposed  to  an  atmosphere  containing  little  or  no  oxygen,  the 
oxygen  will  escape  from  the  Avater  into  the  atmosphere.  The 
oxygen,  in  fact,  which  is  dissolved  in  the  water,  like  the  oxygen 
in  the  atmosphere  above,  stands  at  a  certain  pressure,  the 
amount  of  pressure  depending  on  the  quantity  dissolved ;  and 
when  water  containing  oxygen  dissolved  in  it  is  exposed  to  any 
atmosphere,  the  result,  that  is,  whether  the  oxygen  escapes  from 
the  water  into  the  atmosphere,  or  passes  from  the  atmosphere 
into  the  water,  depends  on  whether  the  pressure  of  the  oxygen 
in  the  water  is  greater  or  less  than  the  pressure  of  the  oxygen  in 
the  atmosphere.  Hence  when  water  is  exposed  to  oxygen,  the 
oxygen  either  escapes  or  is  absorbed  until  equilibrium  is  estab- 
lished between  the  pressure  of  the  oxygen  in  the  atmosphere 
above  and  the  pressure  of  the  oxygen  in  the  water  below.  This 
result  is,  as  far  as  mere  absorption  and  escape  are  concerned, 
quite  independent  of  what  other  gases  are  present  in  the  water 
or  in  the  atmosphere.  Suppose  a  half-litre  of  water  was  lying 
at  the  bottom  of  a  two-litre  flask,  and  that  the  atmosphere  in 
the  flask  above  the  water  was  one-third  oxygen ;  it  would  make 
no  difference,  as  far  as  the  absorption  of  oxygen  by  the  water 
was  concerned,  whether  the  remaining  two-thirds  of  the  atmos- 
phere was  carbonic  acid,  or  nitrogen,  or  hydrogen,  or  whether 
the  space  above  the  water  was  a  vacuum  filled  to  one-third 
with  pure  oxygen.  Hence  it  is  said  that  the  absorption  of  any 
gas  depends  on  the  partial  pressure  of  that  gas  in  the  atmos- 
phere to  which  the  liquid  is  exposed.  This  is  true  not  only  of 
oxygen  and  water,  but  of  all  gases  and  liquids  which  do  not 
enter  into  chemical  combination  with  each  other.  Different 
liquids  will  of  course  absorb  different  gases  with  differing 
readiness ;  but,  with  the  same  gas  and  the  same  liquid,  the 
amount  absorbed  will  depend  directly  on  the  partial  pressure 
of  the  gas  in  the  overlying  space.  It  should  be  added  that  the 
process  is  much  influenced  by  temperature.  Hence,  to  state 
the  matter  generally,  the  absorption  of  any  gas  by  any  liquid 
will  depend  on  the  nature  of  the  gas,  the  nature  of  the  liquid, 
the  pressure  of  the  gas,  and  the  temperature  at  which  both 
.stand. 

Now  it  might  be  supposed,  and  indeed  was  once  supposed, 


Chap,  ii.]  EESPIRATION.  449 

that  the  oxygen  in  the  blood  was  simply  dissolved  by  the  blood. 
If  this  were  so,  then  the  amount  of  oxygen  present  in  any 
given  quantity  of  blood  exposed  to  any  given  atmosphere, 
ought  to  rise  and  fall  steadily  and  regularly  as  the  jDartial 
pressure  of  oxygen  in  that  atmosphere  is  increased  or  dimin- 
ished ;  the  absorption  (or  escape)  of  oxygen  ought  to  follow 
what  is  knoAvn  as  the  Henry-Dalton  law  of  pressures.  But 
this  is  found  not  to  be  the  case.  If  we  expose  blood  containing 
little  or  no  oxygen  to  a  succession  of  atmospheres  containing 
increasing  quantities  of  oxygen,  we  find  that  at  first  there  is  a 
very  rapid  absorption  of  the  available  oxygen,  and  then  this 
somewhat  suddenly  ceases  or  becomes  very  small ;  and  if  on  the 
other  hand  we  submit  arterial  blood  to  successively  diminishing 
pressures,  we  find  that  for  a  long  time  very  little  oxygen  is 
given  off,  and  then  suddenly  the  escape  becomes  very  rapid. 
The  absorption  of  oxygen  by  blood  does  not  follow  the  general 
law  of  absorption  according  to  pressure.  The  phenomena  on 
the  other  hand  suggest  the  idea  that  the  oxygen  in  the  blood  is 
in  some  particular  combination  with  a  substance  or  some  sub- 
stances present  in  the  blood,  the  combination  being  of  such  a 
kind  that  it  holds  good  during  a  lowering  of  pressure  down  to 
a  certain  limit,  and  that  then  dissociation  readily  occurs ;  we 
may  add  that  this  limit  is  very  closely  dependent  on  tempera- 
ture. It  is,  however,  not  to  be  supposed  that  as  the  pressure 
is  lowered,  no  oxygen  whatever  is  given  off  from  the  substance 
until  a  certain  point  is  reached,  and  that  at  that  point  the  whole 
store  is  in  an  instant  dissociated,  no  more  remaining  to  be  given 
off.  The  case  is  rather  that  while  pressure  is  being  lowered 
down  to  a  certain  point,  no  appreciable  dissociation  takes  place, 
and  that  then  having  begun  it  increases  rapidly  with  each 
further  lowering  of  pressure  until  the  whole  of  the  oxj-gen  is 
given  off.  During  this  narrow  range,  between  the  first  begm- 
ning  to  give  off  oxygen  and  the  completion  of  the  giving  off, 
the  compound  of  the  oxygen  with  the  substance  or  substances 
may  be  spoken  of  as  partly,  that  is  more  or  less,  dissociated. 
What  is  the  substance  or  what  are  the  substances  with  wliich 
the  oxygen  is  thus  peculiarly  combined? 

If  serum,  free  from  red  corpuscles,  be  used  in  such  absorption 
experiments,  it  is  found  that,  as  compared  with  the  entire  blood, 
very  little  oxygen  is  absorbed,  about  as  much  as  would  be 
absorbed  by  the  same  quantity  of  water ;  and  such  as  is  absorbed 
does  follow  the  law  of  pressures.  In  natural  arterial  blood  the 
quantity  of  oxygen  which  can  be  obtained  from  serum  is  exceed- 
ingly small ;  it  does  not  amount  to  half  a  volume  in  one  hundred 
volumes  of  the  entire  blood  to  which  the  serum  belonged.  It 
is  evident  that  the  oxygen  which  is  present  in  blood  is  in  some 
way  or  other  peculiarly  connected  with  the  red  corpuscles. 
Now  the    distinguishing  feature  of  the  red  corpuscles  is  the 

29 


450  HEMOGLOBIN.  [Book  ii. 

presence  of  haemoglobin.  We  have  ah-eacly  seen  (§  24)  that 
this  constitutes  90  per  cent,  of  the  dried  red  corpuscles.  There 
can  be  a  priori  little  doubt  that  this  must  be  the  substance  with 
which  the  oxygen  is  associated  ;  and  to  the  properties  of  this 
body  we  must  therefore  direct  our  attention. 

§  275.  Hcemoglohin.  When  separated  from  the  other  con- 
stituents of  the  serum,  haemoglobin  appears  as  a  substance, 
either  amorphous  or  crystalline,  readily  soluble  in  water  (espe- 
cially in  warm  water)  and  in  serum. 

Since  haemoglobin  is  soluble  in  serum,  and  since  the  identity  of 
the  crystals  observed  occasionally  within  the  corpuscles  with  those 
obtained  in  other  ways  shews  that  the  haemoglobin  as  it  exists  in 
the  corpuscle  is  the  same  thing  as  that  which  is  artificially  prepared 
from  blood,  it  is  evident  that  some  peculiar  relationship  between 
the  stroma  and  the  haemoglobin  must,  in  natural  blood,  keep  the 
latter  from  being  dissolved  by  the  serum.  Hence  in  preparing 
haemoglobin  it  is  necessary  first  of  all  to  break  up  this  connection 
and  to  set  the  haemoglobin  free  from  the  corpuscles.  This  may  be 
done  by  the  addition  of  water,  of  ether,  of  chloroform  or  of  bile 
salts,  or  by  repeatedly  freezing  and  thawing ;  blood  so  treated  be- 
comes '  laky,'  cf .  §  24.  It  is  also  of  advantage  previously  to  remove 
the  alkaline  serum  as  much  as  possible  so  as  to  operate  only  on  the 
red  corpuscles.  The  stroma  and  haemoglobin  being  thus  separated, 
a  solution  of  haemoglobin  is  the  result.  The  alkalinity  of  the  solu- 
tion, when  present,  being  reduced  by  the  cautious  addition  of  dilute 
acetic  acid,  and  the  solvent  power  of  the  aqueous  medium  being 
diminished  by  the  addition  of  one-fourth  its  bulk  of  alcohol,  the 
mixture,  set  aside  in  a  temperature  of  0°  C.  in  order  still  further  to 
reduce  the  solubility  of  the  haemoglobin,  readily  crystallizes,  when 
the  blood  used  is  that  of  the  dog,  cat,  horse,  rat,  guinea-pig,  &c.  In 
the  case  of  the  dog  indeed  it  is  simply  sufiicient  to  add  ether  care- 
fully to  the  blood  until  it  just  becomes  'laky,'  and  then  to  let  it 
stand  in  a  cool  place ;  the  mixture  soon  becomes  a  mass  of  crystals. 
The  crystals  may  be  separated  by  filtration,  redissolved  in  water 
and  recrystallized. 

Haemoglobin  from  the  blood  of  the  rat,  guinea-pig,  squirrel, 
hedgehog,  horse,  cat,  dog,  goose,  and  some  other  animals,  crystal- 
lizes readily,  the  crystals  being  generally  slender  four-sided 
prisms,  belonging  to  the  rhombic  system,  and  often  appearing 
quite  acicular.  The  crystals  from  the  blood  of  the  guinea-pig 
are  octahedral,  but  also  belong  to  the  rhombic  system  ;  those 
of  the  squirrel  are  six-sided  plates.  The  blood  of  the  ox, 
sheep,  rabbit,  pig,  and  man,  crystallizes  with  difficulty.  Why 
these  differences  exist  is  not  known ;  but  the  crystals  obtained 
from  different  animals  differ  both  in  percentage  composition 
and  in  the  amount  of  water  of  crystallization.  In  the  dog,  the 
percentage  composition  of  the  crystals  has  been  determined  as 
C.  53-85,  H.  7-32,  N.  16-17,  O.  21-84,  S.  0-39,  Fe.  -43,  with  3 


CiiAF.  II.]  RESPIRATION.  451 

to  4  per  cent,  of  water  of  crystallization.  It  will  thus  be  seen 
that  haemoglobin  contains,  in  addition  to  the  other  elements 
usually  present  in  proteid  substances,  a  certain  amount  of  iron  ; 
that  is  to  say,  the  element  iron  is  a  distinct  part  of  the  hsemo- 
giobin  molecule :  a  fact  which  of  itself  renders  haemoglobin 
remarkable  among  the  chemical  substances  present  in  the  animal 
body. 

§  276.  The  crystals,  when  seen  in  a  sufficiently  thick  layer 
under  the  microscope,  have  the  same  bright  scarlet  colour  as 
arterial  blood  has  to  the  naked  eye ;  when  seen  in  a  mass  they 
naturally  appear  darker.  An  aqueous  solution  of  haemoglobin, 
obtained  by  dissolving  purified  crystals  in  distilled  water,  has 
also  the  same  bright  arterial  colour.  A  tolerably  dilute  solution 
placed  before  the  spectroscope  is  found  to  absorb  certain  rays 
of  light  in  a  peculiar  and  characteristic  manner.  A  portion  of 
the  red  end  of  the  spectrum  is  absorbed,  as  is  also  a  much 
larger  portion  of  the  blue  end  ;  but  what  is  most  striking  is  the 
presence  of  two  strongly  marked  absorption  bands,  lying  between 
the  solar  lines  D  and  E.  (See  Fig.  89.)  Of  these  the  one 
towards  the  red  side,  sometimes  spoken  of  as  the  band  a,  is  the 
thinnest,  but  the  most  intense,  and  in  extremely  dilute  solutions 
(Fig.  89,  1)  is  the  only  one  visible  ;  its  middle  lies  at  some 
little  distance  to  the  blue  side  of  D.  Its  position  may  be  more 
exactly  defined  by  expressing  it  in  wave-lengths.  As  is  well 
known  the  rays  of  light  which  make  up  the  spectrum  differ  in 
the  length  of  their  waves,  diminishing  from  the  red  end,  where 
the  waves  are  longest,  to  the  blue  end,  where  they  are  shortest. 
Thus  Fraunhofer's  line  D  corresponds  to  rays  having  a  wave- 
length of  589.4  millionths  of  a  millimeter.  Using  the  same 
unit,  the  centre  of  this  absorption  band  a  of  htemoglobin  corre- 
sponds to  the  wave-line  578  ;  as  may  be  seen  in  Fig.  89,  where 
however  the  numbers  of  the  divisions  of  the  scale  indicate  only 
100,000  of  a  millimeter.  The  other,  sometimes  called  /3,  much 
broader,  lies  a  little  to  the  red  side  of  E,  its  blueward  edge, 
even  in  moderately  dilute  solutions  (Fig.  89,  2)  coining  close 
up  to  that  line ;  its  centre  corresponds  to  about  wave-length 
539.  Each  band  is  thickest  in  the  middle,  and  gradually  thins 
away  at  the  edges.  These  two  absorption  bands  are  extremely 
characteristic  of  a  solution  of  haemoglobin.  Even  in  very  dilute 
solutions  both  bands  are  visible  (they  may  be  seen  in  a  thick- 
ness of  1  cm.  in  a  solution  containing  1  grm.  of  luemoglobin  in 
10  litres  of  water),  and  that  when  scarcely  any  of  the  extreme 
red  end,  and  very  little  of  the  blue  end,  is  cut  off.  They  then 
appear  not  only  faint  but  narrow.  As  the  strength  of  the  solu- 
tion is  increased,  the  bands  broaden,  and  become  more  intense  ; 
at  the  same  time  both  the  red  end,  and  still  more  the  blue  end, 
of  the  whole  spectrum,  are  encroached  upon  (Fig.  89,  3).  This 
may  go  on  until  the  two  absorption  bands  become  fused  together 


452 


HAEMOGLOBIN. 


[Book  ir. 


Fig.  89.    (After  Preyer  and  Gamgee.)     The  Spectra  of  Oxy-H^^MOGLOBiN  in 

DIFFERENT   GRADES  OF   CONCENTRATION,  OF  (REDUCED)    livTiMOGLOBIN   AND    OF 
CARBONIC-OxiDE-HiEMOGLOBIN. 

1  to  4.    Solution  of  Oxy-Hsemoglobin  containing  (1)  less  than  -01  p.c,  (2)  -09  p.c, 
(.3)  -37  p.c,  (4)  -8  p.c. 
r>.  "  "  (reduced)  Hajinoglobin  containing  about  -2  p.c. 

6.  "  "  carbonic-oxide-Haemoglobin. 

In  each  of  the  six  cases  the  layer  brought  before  the  spectroscope  was  1  cm. 
in  thickness.  The  letters  (A,  a  &c.)  indicate  Fraunhofer's  lines,  and  tlie 
figures  wave-lengths  expressed  in  100,000th  of  a  inilliraeter. 


Chap,  ii.]  RESPIRATION.  453 

into  one  broad  band  (Fig.  89,  4).  The  only  rays  of  light  which 
then  pass  through  the  haimoglobin  solution  are  those  in  the 
green  between  the  blueward  edge  of  the  united  bands  and  the 
general  absorption  which  is  now  rapidly  advancing  from  the  blue 
end,  and  those  in  the  red,  between  the  united  bands  and  the 
general  absorption  at  the  red  end.  If  the  solution  be  still 
further  increased  in  strength,  the  interval  on  the  blue  side  of 
the  united  l^ands  becomes  absorbed  also,  so  that  the  only  rays 
which  pass  through  are  the  red  rays  lying  to  the  red  side  of  D ; 
these  are  the  last  to  disappear,  and  hence  the  natural  red  colour 
of  the  solution  as  seen  by  transmitted  light.  Exactly  the  same 
appearances  are  seen  when  crystals  of  hcemoglobin  are  examined 
with  a  microspectroscope.  They  are  also  seen  when  arterial 
blood  itself  (diluted  with  saline  solutions  so  that  the  corpuscles 
remain  in  as  natural  a  condition  as  possible)  is  examined  with  the 
spectroscope,  as  well  as  when  a  drop  of  blood,  which  from  the 
necessary  exposure  to  air  is  always  arterial,  is  examined  with 
the  microspectroscope.  In  fact,  the  spectrum  of  hsemoglobin  is 
the  spectrum  of  normal  arterial  blood. 

§  277.  When  crystals  of  hsemoglobin,  prepared  in  the  way 
described  above,  are  subjected  to  the  vacuum  of  the  mercurial 
air-pump,  they  give  off  a  certain  quantity  of  oxygen,  and  at 
the  same  time  they  change  in  colour.  In  other  Avords,  the  crys- 
tals of  hsemoglobin,  over  and  above  the  oxygen  which  enters 
intimately  into  the  composition  of  the  molecule  (and  which 
alone  is  given  in  the  elementary  composition  previously  stated), 
contain  another  quantity  of  oxygen,  which  is  in  loose  combina- 
tion only,  and  which  may  be  dissociated  from  them  by  subject- 
ing them  to  a  sufficiently  low  pressure.  The  change  of  colour 
which  ensues  when  this  loosely  combined  oxj-gen  is  removed, 
is  characteristic ;  the  crystals  become  darker  and  more  of  a 
purple  hue,  and  at  the  same  time  dichroic,  so  that  while  the 
thicker  ridges  are  purple,  tlie  thin  edges  appear  greenish.  The 
quantity  of  oxygen  given  off  is  said  to  be  definite  ;  thus  1  grm. 
of  the  crystals  of  dog's  blood  gives  off  1-59  c.cm.  of  oxygen 
measured  at  760  mm.  Hg  and  0°  C.  ;  but  there  are  some  reasons 
for  thinking  that  even  in  the  same  blood  the  quantity  may  vary. 

An  ordinary  solution  of  luemoglobin,  like  the  crystals  from 
which  it  is  formed,  contains  a  definite  quantity  of  oxygen  in 
a  similarly  peculiar  loose  combination  ;  this  oxygen  it  also  gives 
up  when  subjected  in  the  air-pump  to  a  sufficiently  low  pressure, 
becoming  at  the  same  time  of  a  purplish  hue.  This  loosely 
combined  oxygen  may  also  be  removed  by  passing  a  stream  of 
hydrogen  or  other  indifferent  gas  through  the  solution ;  the 
stream  of  hydrogen  acts  like  an  oxygen-vacuum  to  the  hiBmo- 
globin  and  thus  dissociation  is  effected.  Carbonic  acid  gas  is 
unsuitable  for  this  purpose,  since,  as  we  shall  see,  being  an  acid 
it  acts  in  another  way  on  the  hemoglobin.      The  oxygen  may 


454  HEMOGLOBIN.  [Book  ii. 

also  be  removed  from  the  hsemogiobin  not  only  by  physical  but 
also  by  chemical  means,  as  by  the  use  of  reducing  agents.  Thus 
if  a  few  drops  of  ammonium  sulphide  or  of  an  alkaline  solution 
of  ferrous  sulphate,  kept  from  precipitation  by  the  presence  of 
tartaric  acid,  be  added  to  a  solution  of  heemoglobin,  or  even  to 
an  unpurified  solution  of  blood  corpuscles  such  as  is  afforded 
by  the  washings  from  a  blood  clot,  the  oxygen  in  loose  combi- 
nation with  the  hsemoglobin  is  immediately  seized  upon  by  the 
reducing  agent.  This  may  be  recognized  at  once,  by  the  char- 
acteristic change  of  colour ;  from  a  bright  scarlet  the  solution 
becomes  of  a  purplish  claret  colour,  when  seen  in  any  thickness, 
but  greenish  when  sufficiently  thin  :  the  colour  of  the  reduced 
solution  is  exactly  like  that  of  the  crystals  from  which  the 
loose  oxygen  has  been  removed  by  the  air-pump. 

Examined  by  the  spectroscope,  this  reduced  solution,  or 
solution  of  reduced  hcemoglohin^  as  we  may  now  call  it,  offers  a 
spectrum  (Fig.  89,  5)  very  different  from  that  of  the  unreduced 
solution. 

The  two  absorption  bands  have  disappeared,  and  in  their 
place  there  is  seen  a  single,  much  broader,  but  at  the  same  time 
much  fainter  band,  whose  middle  occupies  a  position  about  mid- 
way between  the  two  absorption  bands  of  the  unreduced  solu- 
tion, though  the  redward  edge  of  the  band  shades  away  rather 
farther  towards  the  red  than  does  the  other  edge  towards  the 
blue ;  its  centre  corresponds  to  about  wave-length  555.  At 
the  same  time  the  general  absorption  of  the  spectrum  is  differ- 
ent from  that  of  the  unreduced  solution ;  less  of  the  blue  end 
is  absorbed.  Even  when  the  solutions  become  tolerably  con- 
centrated, many  of  the  bluish-green  rays  to  the  blue  'side  of 
the  single  band  still  pass  through.  Hence  the  difference  in 
colour  between  hsemoglobin  which  retains  the  loosely  combined 
oxygen,^  and  haemoglobin  which  has  lost  its  oxygen  and  become 
reduced.  In  tolerably  concentrated  solutions,  or  tolerably  thick 
layers,  the  former  lets  through  the  red  and  the  orange-yellow 
rays,  the  latter  the  red  and  the  bluish-green  rays.  Accordingly, 
the  one  appears  scarlet,  the  other  purple.  In  dilute  solutions, 
or  in  a  thin  layer,  the  reduced  haemoglobin  lets  through  so 
much  of  the  green  rays  that  they  preponderate  over  the  red, 
and  the  resulting  impression  is  one  of  green.  In  the  unreduced 
hsemoglobin  or  oxyhsemoglobin,  the  potent  yellow  which  is 
blocked  out  in  the  reduced  hsemoglobin,  makes  itself  felt,  so 
that  a  very  thin  layer  of  oxyhsemoglobin,  as  in  a  single  cor- 
puscle seen  under  the  microscope,  appears  yellow  rather  than 
red. 

It  must  be  remembered  that  when  we  speak  of  reduced 

1  For  brevity's  sake  we  may  call  the  hsemoglobin  containing  oxygen  in  loose 
combination,  oxyhcemoglobin,  and  the  hsemoglobin  from  which  this  loosely  com- 
bined oxygen  has  been  removed,  reduced  haemoglobin  or  simply  htemoglolsin. 


Chap,  ii.]  EESPIRATION.  455 

haemoglobin  (or  more  briefly  haemoglobin),  with  a  jiiirple  col- 
our and  a  characteristic  onebanded  spectrum,  we  mean  hemo- 
globin which  has  lost  all  its  loosely  associated  oxygen.  If  a 
quantity  of  oxyhcemoglobin  be  exposed  to  an  insufficiently  low 
pressure,  or  to  the  action  of  an  insufficient  quantity  of  the 
reducing  action,  it  gives  up  a  part  only  of  its  oxygen ;  it  is 
only  partly  reduced.  Such  a  partly  reduced  solution  still  shews 
the  two  bands  of  oxyhcemoglobin. 

§  278.  When  the  hemoglobin  solution  (or  crystal)  which 
has  lost  its  oxygen  by  the  action  either  of  the  air-pump  or  of 
a  reducing  agent  or  by  the  passage  of  an  indifferent  gas,  is 
exposed  to  air  containing  oxygen,  an  absorption  of  oxj-gen  at 
once  takes  place.  If  sufficient  oxygen  be  present,  the  hemo- 
globin seizes  upon  sufficient  oxygen  to  obtain  its  full  complement, 
each  gramme  taking  up  in  combination  1-59  c.cm.  of  oxygen; 
if  there  be  an  insufficient  quantity  of  oxj-gen  the  hemoglobin 
still  remains  partly  reduced;  or  i3erhaps  we  ma}^  say  that  a 
part  only  of  the  hemoglobin  gets  its  allowance  while  the 
remainder  continues  reduced.  If  the  amount  of  oxygen  be 
sufficient,  the  solution  (or  crystal),  as  it  takes  up  the  oxygen, 
regains  its  bright  scarlet  colour  and  its  characteristic  absorjjtion 
spectrum,  the  single  band  being  rejDlaced  by  the  two.  Thus  if 
a  solution  of  oxyhemoglobin  in  a  test-tube,  after  being  reduced 
by  the  action  of  a  drop  or  two  of  ammonium  sulphide  solution 
and  thus  shewing  the  purple  colour  and  the  single  band,  be 
shaken  up  with  air,  the  bright  scarlet  colour  at  once  returns, 
and  when  the  fluid  is  placed  before  the  spectroscope,  it  is  seen 
that  the  single  faint  broad  band  of  the  reduced  hemoglobin 
has  wholly  disappeared,  and  that  in  its  j^lace  are  the  twosharp 
thinner  bands  of  the  oxyhemoglobin.  If  left  to  stand  in  the 
test-tube  the  quantity  of  reducing  agent  still  present  is  gener- 
ally sufficient  again  to  rob  the  hemoglobin  of  the  oxygen  thus 
newly  acquired,  and  soon  the  scarlet  hue  fades  back  again  into 
the  purple,  the  two  bands  giving  place  to  the  one.  Another 
shake  and  exposure  to  air  will  however  again  bring  back  the 
scarlet  hue  and  the  two  bands ;  and  once  more  these  may  dis- 
appear. In  fact,  a  few  drops  of  the  reducing  fluid  will  allow 
this  game  of  hemoglobin  taking  oxygen  from  the  air  and  giv- 
ing it  up  to  the  reducer  to  be  played  over  and  over  again ;  at 
each  turn  of  the  game  the  colour  shifts  from  scarlet  to  purj^le, 
and  from  pnr])le  to  scarlet,  Avhile  the  two  bauds  exchange  for 
the  one,  and  the  one  for  the  two. 

§  279.  Oolour  of  Venous  and  Arterial  Blood.  Evidently 
we  have  in  these  properties  of  hemoglobin  an  explanation  of 
at  least  one-half  of  the  great  respiratory  process,  and  they  teach 
us  tlie  meaning  of  the  change  of  colour  which  takes  place  when 
venous  blood  becomes  arterial  or  arterial  venous. 

In  venous  blood,  as  it  issues  from  the  right  ventricle,  the 


456  COLOUR   OF  BLOOD.  [Book  ii. 

oxygen  present  is  insufficient  to  satisfy  wholly  the  haemoglobin 
of  the  red  corpuscles ;  the  heemoglobin  is,  to  a  large  extent, 
reduced,  hence  the  purple  colour  of  venous  blood.  When  ordi- 
nary venous  blood,  diluted  without  access  of  oxygen,  is  brought 
before  the  spectroscope,  the  two  bands  of  oxyhaemoglobin  are 
seen.  This  is  explained  by  the  fact  that  in  partly  reduced 
haemoglobin,  which  we  may  conveniently  regard  as  a  mixture 
of  oxyhsemoglobin  and  (reduced)  heemoglobin,  the  two  sharp 
bands  of  the  former  are  always  much  more  readily  seen  than 
the  much  fainter  band  of  the  latter.  Now  in  ordinary  venous 
blood  there  is  always  some  loose  oxygen,  removable  by  dimin- 
ished pressure  or  otherwise ;  the  heemoglobin  is  only  partly 
reduced,  there  is  always  some,  indeed  a  considerable  quantity, 
of  oxyhsemoglobin  as  well  as  (reduced)  heemoglobin.  It  is  only 
under  special  circumstances,  as  for  instance  after  death  by  what 
we  shall  presently  speak  of  as  asphyxia,  that  all  the  loose  oxy- 
gen of  the  blood  disappears ;  and  then  the  two  bands  of  the 
oxyhcemoglobin  vanish  too.  If  even  only  a  small  quantity  of 
oxygen  be  present  so  distinct  are  the  two  bands  that  a  solution 
of  completely  reduced  heemoglobin  may  be  used  as  a  test  for 
the  presence  of  oxygen ;  if  oxygen  be  present  in  any  fluid  to 
which  the  reduced  heemoglobin  is  added,  the  single  band  imme- 
diately gives  way  to  the  two  bands  of  oxyheemoglobin. 

As  the  venous  blood  passes  through  the  capillaries  of  the 
lungs,  this  reduced  heemoglobin  takes  from  the  pulmonary  air 
its  complement  of  oxygen,  all  or  nearly  all  the  heemoglobin  of 
the  red  corpuscles  becomes  oxyheemoglobin,  and  the  purple 
colour  forthwith  shifts  into  scarlet.  For  careful  observations 
shew  that  the  heemoglobin  of  arterial  blood  is  saturated  or 
nearly  saturated  with  oxygen ,  it  probably  falls  short  of  com- 
plete saturation  by  about  1  vol.  of  oxygen  in  100  vols,  of  blood. 
By  increasing  the  pressure  of  the  oxygen,  an  additional  quan- 
tity may  be  driven  into  the  blood,  but  this,  after  the  heemoglobin 
has  become  completely  saturated,  is  effected  by  simple  absorp- 
tion. The  quantity  so  added  is  extremely  small  compared  with 
the  total  quantity  combined  with  the  heemoglobin. 

Passing  from  the  left  ventricle  to  the  capillaries  of  the  tis- 
sues the  oxyheemoglobin  gives  up  some  of  its  oxygen  to  the 
tissues,  becoming,  in  part,  reduced  heemoglobin,  and  the  blood 
in  consequence  becomes  once  more  venous,  with  a  purple  hue. 
Thus  the  red  corpuscles  by  virtue  of  their  heemoglobin  are  em 
phatically  oxygen-carriers.  Undergoing  no  intrinsic  change  in 
itself,  the  heemoglobin  combines  in  the  lungs  with  oxygen,  which 
it  carries  to  the  tissues ;  these,  more  greedy  of  oxygen  than 
itself,  rob  it  of  its  charge,  and  the  reduced  heemoglobin  hurries 
back  to  the  lungs  in  the  venous  blood  for  another  portion.  The 
change  from  venous  to  arterial  blood  is  then  in  part  (for  as  we 
shall  see  there  are  other  events  as  well)  a  peculiar  combination 


Chap,  ii.]  RESPIRATIOK  457 

of  hoemoglobin  with  oxygen,  while  the  change  from  arterial  to 
venous  is,  in  part  also,  a  reduction  of  oxyhsemoglobin :  and  the 
difference  of  colour  between  venous  and  arterial  blood  depends 
almost  entirely  on  the  fact  that  the  reduced  haemoglobin  of  the 
former  is  of  purple  colour,  while  the  oxyhsemoglobin  of  the  lat- 
ter is  of  a  scarlet  colour. 

There  may  be  other  causes  of  the  change  of  colour,  but  these 
are  wholly  subsidiary  and  unimportant.  When  a  corpuscle 
swells,  its  refractive  power  is  diminished,  and  in  consequence 
the  number  of  rays  which  pass  into  and  are  absorbed  by  it  are 
increased  at  the  expense  of  those  reflected  from  its  surface  ; 
anything  therefore  which  swells  the  corpuscles,  such  as  the 
addition  of  water,  tends  to  darken  blood,  and  anything,  such  as 
a  concentrated  saline  solution,  which  causes  the  corpuscles  to 
shrink,  tends  to  brighten  blood.  Carbonic  acid  has  apparently 
some  influence  in  swelling  the  corpuscles,  and  therefore  may 
aid  in  darkening  the  venous  blood. 

§  280.  We  have  spoken  of  the  combination  of  hsemoglobin 
with  oxygen  as  being  a  peculiar  one.  The  peculiarity  consists 
in  the  facts  that  the  oxygen  may  be  associated  and  dissociated, 
without  any  general  disturbance  of  the  molecule  of  hsemoglobin, 
and  that  dissociation  may  be  brought  about  very  readily.  Ha3- 
moglobin  combines  in  a  wholly  similar  manner  Avith  other  gases. 
If  carbonic  oxide  (monoxide)  be  passed  through  a  solution  of 
hsemoglobin,  a  change  of  colour  takes  place,  a  peculiar  bluish 
tinge  making  its  appearance.  At  the  same  time  the  spectrum 
is  altered  ;  two  bands  are  still  visible,  but  on  accurate  measure- 
ment it  is  seen  that  they  are  placed  more  towards  the  blue  end 
than  are  the  otherwise  similar  bands  of  oxyhfemoglobin  (see 
Fig.  89,  6)  ;  their  centres  corresponding  respectively  to  about 
wave-lengths  572  and  533,  while  those  of  oxyhemoglobin  as 
we  have  seen  correspond  to  578  and  539.  When  a  known 
quantity  of  carbonic  oxide  gas  is  sent  through  a  hsemoglobin 
solution,  it  will  be  found  on  examination  that  a  certain  amount 
of  the  gas  has  been  retained,  an  equal  volume  of  oxygen  appear 
ing  in  its  place  in  the  gas  which  issues  from  the  solution.  If 
tlie  solution  so  treated  be  crystallized,  the  crystals  Avill  have  the 
same  characteristic  colour,  and  give  the  same  absorption  spec- 
trum as  the  solution  ;  when  subjected  to  the  action  of  the  mer- 
curial pump,  they  will  give  off  a  definite  quantity  of  carbonic 
oxide,  1  grm.  of  the  crystals  yielding  1-59  c.cm.  of  the  gas.  In 
fact,  h;emoglol)in  combines  loosely  with  carbonic  oxide  just  as 
it  does  with  oxygen  ;  but  its  afliuity  with  the  former  is  greater 
than  with  the  latter.  While  carbonic  oxide  readily  turns  out 
oxygen,  oxygen  cannot  so  readily  turn  out  carbonic  oxide. 
Indeed,  carbonic  oxide  has  been  used  as  a  means  of  driving 
out  and  measuring  the  quantity  of  oxygen  present  in  any  given 
blood.      Tliis  property  of  carbonic  oxide  explains  its  poisonous 


458  H^MATIK  [Book  ir. 

nature.  When  the  gas  is  breathed,  the  reduced  and  the  unre- 
duced hsemoglobin  of  the  venous  blood  unite  with  the  carbonic 
oxide,  and  hence  the  peculiar  bright  cherry-red  colour  observ- 
able in  the  blood  and  tissues  in  cases  of  poisoning  by  this  gas. 
The  carbonic  oxide  hsemoglobin,  however,  is  of  no  use  in  res- 
piration ;  it  is  not  an  oxygen-carrier,  nay  more,  it  will  not 
readily,  though  it  does  so  slowly  and  eventually,  give  up  its 
carbonic  oxide  for  oxygen,  when  the  poisonous  gas  ceases  to 
enter  the  chest  and  is  replaced  by  pure  air.  The  organism  is 
killed  by  suffocation,  by  want  of  oxygen,  in  spite  of  the  blood 
not  assuming  any  dark  venous  colour  ;  to  adopt  a  phrase  which 
has  been  used,  the  corpuscles  are  paralyzed. 

Hiemoglobin  similarly  forms  a  compound,  having  a  charac- 
teristic spectrum,  with  nitric  oxide,  more  stable  even  than  that 
with  carbonic  oxide. 

It  has  been  supposed  by  some  that  the  oxygen  thus  associated 
with  haemoglobin  is  in  the  condition  known  as  ozone  ;  but  the 
arguments  urged  in  support  of  this  view  are  not  as  yet  con- 
clusive. 

Products  of  the  decomposition  of  Hcemoglohin. 

§  281.  Although  a  crystalline  body,  hsemoglobin  diffuses 
with  great  difficulty.  This  arises  from  the  fact  that  it  is  in 
part  a  proteid  body  ;  it  consists  of  a  colourless  proteid,  associ- 
ated with  a  coloured  substance,  which  may  be  separated  out 
from  the  hsemoglobin,  though  not  in  the  exact  condition  in 
which  it  naturally  exists  in  the  compound  ;  this  substance  when 
separated  out  appears  as  a  brownish-red  body  known  as  hcema- 
tin.  All  the  iron  belonging  to  the  hsemoglobin  is  in  reality 
attached  to  the  hsematin.  A  solution  of  hsemoglobin,  when 
heated,  coagulates,  the  exact  degree  at  which  the  coagulation 
takes  place  depending  on  the  amount  of  dilution  ;  at  the  same 
time  it  turns  brown  from  the  setting  free  of  the  hsematin.  If 
a  strong  solution  of  hsemoglobin  be  treated  with  acetic  (or 
other)  acid,  the  same  brown  colour,  from  the  appearance  of 
hsematin,  is  observed.  The  proteid  constituent  however  is  not 
coagulated,  but  by  the  action  of  the  acid  passes  into  the  state 
of  acid-albumin.  On  adding  ether  to  the  mixture,  and  shaking, 
the  hsematin  is  dissolved  in  the  supernatant  acid  ether,  which  it 
colours  a  dark  red,  and  which,  examined  with  the  spectroscope, 
is  found  to  possess  a  well-marked  spectrum,  the  spectrum  of  the 
so-called  acid  hsematin  of  Stokes  (Fig.  90,  6).  The  proteid  in 
the  water  below  the  ether  appears  in  a  coagulated  form  owing  to 
the  action  of  the  ether.  In  a  somewhat  similar  manner  alkalis 
split  up  hsemoglobin  into  a  proteid  constituent  and  hsematin. 

The  exact  nature  of  the  proteid  constituent  of  hsemoglobin 
has  not  as  yet  been  clearly  determined.  It  was  supposed  to 
be  globulin  (hence  the  name  hsematoglobulin,  contracted  into 


Chap,  ii.] 


RESPIRATION. 


469 


460  METH.EMOGLOBIN.  [Book  ii. 

hsemoglobiii),  but  though  belonging  to  the  globulin  family,  has 
characters  of  its  own  ;  it  is  possibly  a  mixture  of  two  or  more 
distinct  proteids.  It  has  been  provisionally  named  glohin  and 
is  said  to  be  free  from  ash. 

§  282.  Hgematin  when  separated  from  its  proteid  fellow,  and 
purified,  appears  as  a  dark-brown  amorphous  powder,  or  as  a 
scaly  mass  with  a  metallic  lustre,  having  the  probable  composi- 
tion of  C32,  Hg^,  N^,  Fe,  O5.  It  is  fairly  soluble  in  dilute  acid 
or  alkaline  solutions,  and  then  gives  characteristic  spectra 
(Fig.  90,  1,  2,  5). 

An  interesting  feature  in  hgematin  is  that  its  alkaline  solu- 
tion is  capable  of  being  reduced  by  reducing  agents,  the  spectrum 
changing  at  the  same  time  (Fig.  90,  3),  and  that  the  reduced 
solution  will,  like  the  hcemoglobin,  take  up  oxygen  again  on 
being  brought  into  contact  with  air  or  oxygen.  This  would 
seem  to  indicate  that  the  oxygen-holding  power  of  haemoglobin 
is  connected  exclusively  with  its  heematin  constituent. 

By  the  action  of  strong  sulphuric  acid  hsematin  may  be 
robbed  of  all  its  iron.  It  still  retains  the  feature  of  possess- 
ing colour,  the  solution  of  iron-free  hsematin  being  a  dark  rich 
brownish  red  ;  but  is  no  longer  capable  of  combining  loosely 
with  oxygen.  This  indicates  that  the  iron  is  in  some  way  asso- 
ciated with  the  peculiar  respiratory  functions  of  haemoglobin  ; 
though  it  is  obviously  an  error  to  suppose,  as  was  once  supposed, 
that  the  change  from  venous  to  arterial  blood  consists  essentially 
in  a  change  from  a  ferrous  to  a  ferric  salt. 

Though  not  crystallizable  itself,  hsematin  forms  with  hydro- 
chloric acid  a  compound,  occurring  in  minute  rhombic  crystals, 
known  as  hcemin  crystals. 

When  blood  is  left  until  it  decomposes,  the  haemoglobin  is 
very  apt  to  become  changed  into  a  peculiar  body  known  as 
methceynoglobin.,  in  the  spectrum  of  which  a  very  conspicuous 
band  is  seen  in  the  red  between  C  and  D  (see  Fig.  90,  4).  The 
same  change  may  be  brought  about  by  the  action  of  weak  acids, 
such  as  carbonic  acid,  by  ozone,  and  by  other  agents  such  as 
nitrites  and  potassium  permanganate.  When  a  stream  of  car- 
bonic acid  is  driven  through  blood  or  through  a  solution  of 
haemoglobin  the  band  in  the  red  characteristic  of  methsemoglo- 
bin  soon  makes  its  appearance.  Methtemoglobin  differs  but 
little  if  at  all  in  elementary  composition  from  haemoglobin  ;  it 
is  maintained  that  it  contains  the  same  quantity  of  oxygen  as 
oxyhsemoglobin  but  in  a  more  stable  condition,  more  intimately 
associated  with  the  molecule. 

In  conclusion,  the  condition  of  oxygen  in  the  blood  is  as 
follows.  Of  the  whole  quantity  of  oxygen  in  the  blood,  only  a 
minute  fraction  is  simply  absorbed  or  dissolved  according  to  the 
law  of  pressures  (the  Henry-Dalton  law).  The  great  mass  is 
in  a  state  of  combination  with  the  haemoglobin,  the  connection 


€hap.  II.]  RESPIRATION.  461 

being  of  such  a  kind  that  while  the  hienioglobin  readily  com- 
bines with  the  oxygen  of  the  air  to  which  it  is  exposed,  dis- 
sociation readily  occurs  at  low  pressures,  or  in  the  presence  of 
indifferent  gases,  or  by  the  action  of  substances  having  a  greater 
affinity  for  oxygen  than  has  luemoglobin  itself.  The  difference 
between  venous  and  arterial  blood,  as  far  as  oxygen  is  con- 
cerned, is  that  while  in  arterial  blood  the  hsemoglobin  holds 
nearly  its  full  complement  of  ox3-gen  and  may  be  spoken  of  as 
nearly  wholly  oxyhsemoglobin,  in  venous  blood  the  hemoglobin 
is  to  a  large  but  variable  extent,  reduced  ;  and  the  character- 
istic colours  of  venous  and  arterial  blood  are  in  the  main  due  to 
the  fact  that  the  colour  of  reduced  haemoglobin  is  purple,  while 
that  of  oxyhaemoglobin  is  scarlet. 

Tlie  relations  of  the    Carbonic  Acid  in  the  Blood. 

§  283.  The  presence  of  carbonic  acid  in  the  blood  appears 
to  be  determined  by  conditions  more  complex  in  their  nature 
and  at  present  not  so  well  understood  as  those  which  determine 
the  presence  of  oxygen.  The  carbonic  acid  is  not  simply  dis- 
solved in  the  blood  ;  its  absorption  by  blood  does  not  follow  the 
law  of  pressures.  It  exists  in  association  with  some  substance 
or  substances  in  the  blood,  and  its  escape  from  the  blood  is  a 
process  of  dissociation.  We  cannot  however  speak  of  it  as 
being  associated,  in  the  same  definite  and  clear  way  as  is  the 
oxygen,  with  the  haemoglobin  of  the  red  corpuscles. 

Several  facts  seem  to  support  the  view  that  the  carbonic 
acid  exists  associated  with  some  substance  or  substances  in  the 
plasma,  but  at  the  same  time  indicate  that  the  conditions  of  its 
association  (and  therefore  of  its  dissociation)  are  determined 
by  the  action  of  some  substance  or  substances  present  in  the 
corpuscles.  It  has  been  suggested  that  the  association  of  the 
carbonic  acid  in  the  plasma  is  with  one  or  other  of  the  proteids 
of  the  plasma ;  but  it  has  also  been  suggested  that  the  associa- 
tion is  one  with  sodium  as  sodium  bicarbonate,  and  further  that 
the  haemoglobin  of  the  corpuscles  plays  a  part  in  promoting  the 
dissociation  of  the  sodium  bicarbonate  or  even  the  carbonate, 
and  thus  keeping  up  the  carbonic  acid  of  the  entire  blood. 
Other  observers  however  maintain  that  the  plasma  does  not 
hold  this  exclusive  possession  of  the  carbonic  acid,  but  that 
a  considerable  quantity  at  least  of  this  gas  is  in  some  definite 
way  associated  Avith  the  red  corpuscles.  Further  investigations 
are  necessary  before  the  matter  can  be  said  to  liave  been  placed 
on  a  satisfactory  footing. 

The  relations  of  Nitrogen  in  the  Blood. 

§  284.  The  small  quantity  of  this  gas  which  is  present  in 
both  arterial  and  venous  blood  seems  to  exist  in  a  state  of  sim- 
ple solution. 


SEC.   4.    THE  RESPIRATORY  CHANGES  IN  THE  LUNGS. 

§  285.  The  Entrance  of  Oxygen.  We  have  already  seen 
tliat  the  blood  in  passing  through  the  lungs  takes  up  a  certain 
variable  quantity  (from  8  to  12  vols,  p.c.)  of  oxygen.  We 
have  further  seen  that  the  quantity  so  taken  up,  putting  aside 
the  insignificant  fraction  simply  absorbed,  enters  into  direct 
but  loose  combination  Avith  the  haemoglobin.  In  drawing  a 
distinction  between  the  oxygen  simply  absorbed  and  that  enter- 
ing into  combination  with  the  haemoglobin,  it  must  not  be 
understood  that  the  latter  is  wholly  independent  of  pressure. 
On  the  contrary,  all  chemical  compounds  are  in  various  degrees 
subject  to  dissociation  at  certain  pressures  and  temperatures ; 
and  the  existence  of  the  somewhat  loose  compound  of  oxygen 
and  hsemoglobin  is  dependent  on  the  partial  pressure  of  oxygen 
in  the  atmosphere  to  which  the  haemoglobin  is  exposed.  Not 
only  will  a  solution  of  haemoglobin  or  a  quantity  of  blood  either 
absorb  oxygen  and  thus  undergo  association  or  undergo  disso- 
ciation and  give  off  oxygen  according  as  the  partial  pressure 
of  oxygen  in  the  atmosphere  to  which  it  is  exposed  is  high  or 
low,  l3ut  also  the  amount  taken  up  or  given  off  will  depend  on 
the  degree  of  the  partial  pressure ;  the  hsemoglobin  as  we  have 
seen  may  be  either  partially  or  wholly  reduced.  The  law  how- 
ever according  to  which  absorption  or  escape  thus  takes  place 
is  quite  different  from  that  observed  in  the  simple  absorption 
of  oxygen  by  liquids.  The  association  or  dissociation  is  fur- 
ther especially  dependent  on  temperature,  a  high  temperature 
favouring  dissociation,  so  that  at  a  high  temperature  less  oxy- 
gen is  taken  up  than  would  be  taken  up  (or,  as  the  case  may 
be,  more  given  off  than  would  be  given  off)  at  a  lower  tempera- 
ture, the  partial  pressure  of  the  oxygen  in  the  atmosphere 
remaining  the  same. 

Moreover  in  the  blood  we  have  to  deal  not  with  hsemoglobin 
in  simple  solution,  in  which  the  molecules  are  dispersed  uni- 
formly through  the  solvent,  but  with  the  haemoglobin  segre- 
gated into  minute  isolated  masses,  bottled  up  as  it  were  in  the 
individual  corpuscles.     The  haemoglobin  of  each  corpuscle  is 

462 


Chai'.  II.]  RESPIRATIOK  463 

separated  from  its  fellows  by  a  layer,  thin  it  may  be  but  still 
a  distinct  layer,  of  colourless,  hiemoglobinless  plasma.  As  the 
corpuscle  makes  its  way  through  the  narrow  capillary  paths 
of  a  pulmonary  alveolus,  it  is  separated  from  the  air  of  the 
alveolus  by  a  thin  layer  of  plasma  as  well  as  by  the  film  of  the 
conjoined  capillary  and  alveolar  walls ;  and  a  like  layer  of 
plasma  separates  it  from  its  fellows  as  it  journeys  in  company 
with  them  through  the  wider  j^assages  of  the  arteries  and  veins. 
Through  this  layer  of  plasma,  which  containing  no  haemoglobin 
can  hold  oxygen  in  simple  solution  only,  the  oxygen  has  to 
pass  on  its  way  to  and  from  the  corpuscle  ;  and  every  coriDuscle 
may  be  considered  as  governing,  as  far  as  oxygen  is  concerned, 
a  zone  of  plasma  immediately  surrounding  itself.  The  cor- 
l^uscle  takes  its  oxygen  directly  from  this  zone  and  gives  up 
its  oxygen  directly  to  this  zone ;  and  the  ]3ressure  at  which  at 
any  moment  the  oxygen  exists  in  this  zone  will  depend  on  the 
pressure  of  oxygen  outside  the  zone,  in  the  air  of  the  pulmonary 
alveolus  for  instance,  and  on  the  smaller  or  greater  amount  of 
oxygen  associated  with  the  hfemoglobin  of  the  corpuscle. 

The  evidence,  however,  afforded  by  various  experiments, 
so  far  as  it  goes,  seems  to  shew  that  blood  absorbs  oxygen  in 
the  same  way  as  an  aqueous  solution  of  hsemogiobin  of  the 
same  concentration ;  the  zone  of  plasma  spoken  of  above  as 
surrounding  each  corpuscle  seems  to  behave  as  far  as  regards 
the  passage  of  oxygen  to  and  from  the  corpuscles  in  no  essen- 
tially different  respect  from  the  way  in  which  the  molecules  of 
water,  belonging  to  a  molecule  of  dissolved  haemoglobin,  behave 
in  regard  to  the  absorption  or  the  giving-off  of  oxygen  by  an 
aqueous  solution  of  haemoglobin. 

The  film  of  the  conjoined  capillary  and  alveolar  wall  is  a 
thin  membrane  soaked  with  lymph  and  wet ;  we  cannot  speak 
of  it  as  actually  secreting  a  liquid  secretion  into  the  alveolus, 
for  the  cavity  of  the  alveolus  is  filled  with  air  which,  though 
saturated  with  moisture,  is  air,  not  a  liquid  ;  still  enough  passes 
through  the  film  to  keep  the  film  continually  moist.  Through 
this  film  the  oxygen  has  to  make  its  way  in  order  to  gain  access 
to  the  plasma  and  so  to  the  corpuscle ;  it  makes  its  way  dis- 
solved in  the  fluid,  that  is  the  lymph,  which  keeps  the  film 
moist.  This  film  moreover  is  composed  of  living  matter,  and 
the  considerations  which  a  little  while  back  (§  253)  we  urged 
concerning  the  diffusion  through  a  living  membrane  of  solid 
substances  in  solution,  hold  good  also  for  the  diffusion  of  gases 
in  solution. 

If  now  we  ask  the  question.  Are  the  conditions  in  which 
htcmogiobin  and  oxygen  exist  in  ordinary  venous  blood  as  it 
flows  to  the  lungs,  of  such  a  kind  that  the  venous  blood  in 
passing  through  the  pulmonary  capillaries  will  find  the  partial 
pressure   of  the   oxygen    in   the    pulmonary   alveoli    sufficient 


464  THE   ENTRANCE  OF   OXYGEK  [Book  ii. 

tlirougli  the  action  of  simple  physical  causes  to  bring-  about 
the  association  of  the  additional  quantity  of  oxygen  whereby 
the  venous  is  converted  into  arterial  blood?  The  reply  is  as 
follows. 

§  286.  In  man,  as  we  have  seen,  expired  air  contains  about 
16  p.c.  of  oxygen.  The  air  in  the  pulmonary  alveoli  must 
contain  less  than  this,  since  the  expired  air  consists  of  tidal 
air  mixed  by  diffusion  with  the  stationary  air.  How  much 
less  it  contains  we  do  not  exactly  know,  but  probably  the  dif- 
ference is  not  very  great.  At  the  ordinary  atmospheric  pres- 
sure of  TOO  mm.  16  p.c.  is  equivalent  to  a  partial  pressure  of 
122  mm.  The  question  therefore  stands  thus.  Will  venous 
blood,  exposed  at  the  temperature  of  the  body  to  a  partial  pres- 
sure of  less  than  122  mm.  (less  than  16  p.c.)  of  oxygen  take 
up  sufficient  oxygen  (from  8  to  12  vols,  p.c.)  to  convert  it 
into  arterial  blood?  Numerous  experiments  have  been  made 
(chiefly  but  not  exclusively  on  the  dog)  to  determine  on  the 
one  hand  the  oxygen-pressure  of  both  arterial  and  venous  blood 
(i.e.  the  partial  pressure  of  oxygen  in  an  atmosphere  exposed 
to  which  the  arterial  blood  neither  gives  up  nor  takes  in  oxy- 
gen, and  the  same  for  venous  blood),  and  on  the  other  hand 
the  behaviour,  at  the  temperature  of  the  body  or  at  ordinary 
temperatures,  of  blood  towards  an  atmosphere  in  which  the 
partial  pressure  of  oxygen  is  made  to  vary.  Without  going 
into  detail,  we  may  state  that  these  experiments  seem  to  shew 
that  the  partial  pressure  of  oxygen  in  the  lungs  is  amply  suffi- 
cient to  bring  about,  at  the  temperature  of  the  body,  the  asso- 
ciation of  that  additional  amount  of  oxygen  by  which  venous 
blood  becomes  arterial.  When  blood  is  successively  exposed 
to  increasing  oxygen  pressures,  as  the  partial  pressure  of  oxy- 
gen is  gradually  increased,  the  curve  of  absorption  rises  at  first 
very  rapidly  but  afterwards  more  slowly ;  that  is  to  say,  the 
later  additio2is  of  oxygen  at  the  higher  pressures  are  propor- 
tionately less  than  the  earlier  ones  at  the  lower  pressures. 
And  this  is  consonant  with  what  appears  to  be  the  fact  that 
the  haemoglobin  of  arterial  blood  though  nearly  saturated  with 
oxygen,  i.e.  associated  with  almost  its  full  complement  of  oxy- 
gen, is  not  quite  saturated.  When  arterial  blood  is  thoroughly 
exposed  to  air  it  takes  up  rather  more  than  1  vol.  p.c.  of  oxy- 
gen ;  and  that  appears  to  represent  the  difference  between 
exposing  blood  to  pure  air,  such  as  enters  or  ought  to  enter 
the  mouth  in  inspiration,  and  exposing  blood  to  the  air  as  it 
exists  in  the  pulmonary  alveoli.  The  greater  relative  absorp- 
tion at  the  lower  pressures  has  a  beneficial  effect  in  as  much 
as  it  still  permits  a  considerable  quantity  of  oxygen  to  be 
absorbed  even  when  the  partial  pressure  of  oxygen  in  the  air 
in  the  lungs  is  largely  reduced,  as  in  ascending  to  great  heights. 

Similar  observations  seem  to  shew  that  arterial  blood  ceases 


Chap,  ii.]  RESPIRATION.  465 

to  take  up  oxygen  and  begins  to  give  off  oxygen,  in  other  words, 
that  dissociation  begins  to  take  place,  when  the  partial  pressure 
of  the  oxygen  in  the  atmosphere  to  which  it  is  exj^osed  sinks  to 
about  60  mm.  of  mercury,  that  is  to  say,  when  the  whole  atmos- 
pheric pressure  is  reduced  from  760  mm.  to  about  300  mm.  or 
Avhen  the  j)ercentage  of  oxygen  in  the  atmosphere  is  reduced  by 
decidedly  more  than  half.  And  this  accords  with  the  observa- 
tion that,  in  man,  when  the  oxygen  of  inspired  air  is  gradually 
diminished,  without  any  other  change  in  the  air,  symptoms  of 
dys2)n(jea  do  not  make  their  appearance  until  the  ox3-gen  sinks 
to  10  p.c.  in  the  inspired  air  and  must  therefore  be  less  than 
this  in  the  pulmonary  alveoli.  We  may  remark  that  at  ordi- 
nary altitudes,  even  taking  into  account  the  diminution  the 
oxygen  undergoes  before  it  reaches  the  pulmonary  alveoli,  the 
partial  pressure  of  the  oxygen  in  the  atmosi^here  leaves  a  Avide 
margin  of  safety.  But  at  an  altitude  of  5500  metres  (1700  feet) 
at  which  the  pressure  of  the  whole  atmosphere  stands  at  about 
the  limit  given  above  of  300  mm.,  the  partial  pressure  of  the 
oxygen  will  be  such  that  the  A^enous  blood  cannot  take  up  the 
quantity  of  oxygen  proper  to  convert  it  into  arterial  blood,  since 
at  this  limit  arterial  blood  begins  to  give  off  oxygen.  We  may 
add  that  it  is  at  this  altitude  that  breathing  becomes  especially 
difficult,  but  to  this  we  shall  return. 

§  287.  The  statements  made  so  far  refer  to  ordinary  breath- 
ing, but  the  question  may  be  asked.  What  happens  Avhen  the 
renewal  of  the  air  in  the  pulmonary  alveoli  ceases,  as  when  the 
trachea  is  obstructed  ?  In  such  a  case  the  oxygen  in  the  alveoli 
is  found  to  diminish  rapidly,  so  that  the  partial  pressure  of  oxy- 
gen in  them  soon  falls  below  the  oxygen-pressure  of  ordinary 
A-^enous  blood.  But  in  such  a  case  the  blood  is  no  longer  ordi- 
nary venous  blood  ;  instead  of  being  moderately,  it  is  largely 
and  increasingly  reduced  ;  instead  of  containing  a  comparatively 
small  amount,  it  contains  a  large  and  gradually  increasing 
amount,  of  reduced  haemoglobin.  And  as  the  reduction  con- 
tinues to  increase,  the  oxygen-pressure  of  the  venous  blood  also 
continues  to  decrease  ;  it  thus  keeps  beloAv  that  of  the  air  in  the 
lungs.  Hence  apparently  even  the  last  traces  of  oxygen  in 
the  lungs  may  be  taken  up  by  the  blood,  and  carried  away  to 
the  tissues. 

Guided  by  these  observations  then,  we  should  be  led  to  con- 
clude that  the  film  of  the  conjoined  pulmonary  and  capillary 
Avail  does  not  exert  any  influence,  by  Adrtue  of  its  being  a  living 
structure,  upon  the  entrance  of  oxygen  into  the  blood,  or  indeed 
exert  an}-  influence  at  all  even  as  a  mere  membrane  or  septum; 
the  oxygen  appears  to  pass  into  the  blood  in  the  same  Avay  that 
it  Avould  if  the  blood  Avere  freely  exposed  to  the  alveolar  air 
Avithout  any  interA^ening  partition.  NcA^ertheless  there  are  facts 
which  seem  to  throw  doubt  on  the  A^alidity  of  this  conclusion. 

30 


466  THE   EXIT   OF   CARBONIC   ACID.          [Book  ii. 

The  partial  pressure  of  the  oxygen  m  the  gas  in  the  swim-blad- 
der of  fishes  for  instance  far  exceeds  that  of  the  fishes'  blood  ; 
and  if  the  gas  be  drawn  off,  it  is  soon  replaced  by  gas  having 
the  like  high  partial  pressure  of  oxygen.  Hence  we  are  led  to 
conclude  that  oxygen  makes  its  appearance  in  the  swim-bladder 
by  a  kind  of  secretion.  And  other  facts  might  be  brought  for- 
ward, strong  enough  at  least  to  support  the  doubt,  whether  the 
purely  physical  explanation  given  above  of  the  entrance  of  oxy- 
gen into  the  blood,  adequate  as  it  at  first  sight  seems,  is  really 
the  true  one. 

§  288.  The  Exit  of  Carbonic  Acid.  In  a  similar  manner 
analogous  experiments  appear  to  support  the  view  that  the 
escape  of  carbonic  acid  from  the  blood  into  the  pulmonary  alve- 
olus is  the  result  of  ordinary  diffusion  ;  observations  seem  to 
shew  that  the  difference  obtained  between  the  pressure  of  the 
carbonic  acid  in  the  venous  blood  and  the  partial  pressure  of 
carbonic  acid  in  the  air  of  the  pulmonary  alveolus  (which  is  of 
course  greater  than  that  of  the  expired  air)  is  sufficient  to 
account  for  the  loss  of  carbonic  acid,  whereby  arterial  blood  is 
distinguished  from  venous  blood.  But  in  respect  to  this  as  in 
respect  to  the  entrance  of  oxygen,  doubts  have  been  raised,  and 
it  has  been  urged  that  the  escape  of  carbonic  acid  into  the  pul- 
monary alveoli  is  carried  out  by  some  action  of  the  walls  of  the 
alveoli  comparable  to  the  act  of  secretion. 


SEC.  5.     THE  RESPIRATORY  CHANGES  IN  THE  TISSUES. 

§  289.  In  passing  through  the  several  tissues  the  arterial 
blood  becomes  once  more  venous.  The  oxyhijemoglobin  becomes 
considerably  reduced,  and  a  quantity  of  carbonic  acid  passes 
from  the  tissues  into  the  blood.  The  amount  of  change  varies 
in  the  various  tissues,  and  in  the  same  tissue  may  vary  at  differ- 
ent times.  Thus  in  a  gland  at  rest,  as  we  have  seen,  the  venous 
blood  is  dark,  shewing  that  the  haemoglobin  is  to  a  large  extent 
in  the  reduced  condition ;  when  the  gland  is  active,  the  venous 
blood  in  its  colour,  and  in  the  extent  to  which  the  haemoglobin 
is  in  the  condition  of  oxyhsemoglobin,  resembles  closely  arterial 
blood.  The  blood  therefore  which  issues  from  a  gland  at  rest 
is  more  '  venous '  than  that  from  an  active  gland ;  though  owing 
to  the  more  rapid  flow  of  blood  Avhich,  as  we  saw  in  an  earlier 
section,  accompanies  the  activity  of  the  gland,  the  total  quan- 
tity of  oxygen  taken  up  from  and  of  carbonic  acid  discharged 
into  the  blood  from  the  gland  in  a  given  time  may  be  greater 
in  the  latter.  The  blood,  on  the  other  hand,  which  comes  from 
an  active,  i.e.  a  contracting  muscle,  is,  in  spite  of  the  more 
rapid  flow,  not  only  richer  in  carbonic  acid,  but  also,  though  not 
to  a  corresponding  amount,  poorer  in  oxygen  than  the  blood 
which  flows  from  a  muscle  at  rest. 

In  all  these  cases  the  question  which  first  comes  up  for  our 
consideration  is  this  :  Does  the  oxygen  pass  from  the  blood  into 
the  tissues,  and  does  the  oxidation  take  place  in  the  tissues,  giv- 
ing rise  to  carbonic  acid,  which  passes  in  turn  away  from  the 
tissues  into  the  blood?  or  do  certain  oxidizable  reducing  sub- 
stances pass  from  the  tissues  into  the  blood,  and  there  become 
oxidized  into  carbonic  acid  and  other  products,  so  that  the  chief 
oxidation  takes  place  in  the  blood  itself? 

There  are,  it  is  true,  reducing  oxidizable  substances  in  the 
blood,  but  tliese  are  small  in  amount,  and  the  quantity  of  car- 
bonic acid  to  which  they  give  rise  when  the  blood  containing 
them  is  agitated  with  air  or  oxygen,  is  so  small  as  scarcely  to 
exceed  the  errors  of  observation. 

We  may  add,  that  the  oxidative  power  which  the  blood 
itself  removed  from  the  body  is  able  to  exert  on  substances 

4(57 


46S  RESPIRATIOJS^   OF   THE   TISSUES.         [Book  ii. 

which  are  undoubtedly  oxidized  in  the  body  is  so  small  that  it 
may  be  neglected  in  the  present  considerations.  If  grape-sugar 
be  added  to  blood,  or  to  a  solution  of  haemoglobin,  the  mixture 
may  be  kept  for  a  long  time  at  the  temperature  of  the  body, 
without  undergoing  oxidation.  Even  within  the  body  an  even 
slight  excess  of  sugar  in  the  blood  over  a  certain  percentage 
wholly  escapes  oxidation,  and  is  discharged  unchanged. 

On  the  other  hand,  it  will  be  remembered  that  in  speaking 
of  muscle,  we  drew  attention  (§  58)  to  the  fact  that  a  frog's 
muscle  removed  from  the  body  (and  the  same  is  true  of  the 
muscles  of  other  animals)  contains  no  free  oxygen  whatever ; 
none  can  be  obtained  from  it  by  the  mercurial  air-pump.  Yet 
such  a  muscle  will  not  only  when  at  rest  go  on  producing  and 
discharging  a  certain  quantity,  but  also  when  it  contracts  evolve 
a  very  considerable  quantity,  of  carbonic  acid.  Moreover  this 
discharge  of  carbonic  acid  will  go  on  for  a  certain  time  in 
muscles  under  circumstances  in  which  it  is  impossible  for  them 
to  obtain  oxygen  from  without.  Oxygen,  it  is  true,  is  neces- 
sary for  the  life  of  the  muscle  :  when  venous  instead  of  arterial 
blood  is  sent  through  the  blood  vessels  of  a  muscle,  the  irrita- 
bility speedily  disappears,  and  unless  fresh  oxygen  be  admin- 
istered the  muscle  soon  dies.  The  muscle  may  however,  during 
the  interval  in  which  irritability  is  still  retained  after  the  sup- 
ply of  oxygen  has  been  cut  off,  continue  to  contract  vigorously. 
The  supply  of  oxygen,  though  necessary  for  the  maintenance  of 
irritability,  is  not  necessary  for  the  manifestation  of  that  irrita- 
bility, is  not  necessary  for  that  explosive  decomposition  which 
develops  a  contraction.  A  frog's  muscle  will  continue  to  con- 
tract and  to  produce  carbonic  acid  in  an  atmosphere  of  hydrogen 
or  nitrogen,  that  is,  in  the  total  absence  of  free  oxygen  both  from 
itself  and  from  the  medium  in  which  it  is  placed. 

Thus  on  the  one  hand  the  muscle  seems  to  have  the  prop- 
erty of  taking  up  and  fixing  in  some  way  or  other  the  oxygen 
to  which  it  is  exposed,  of  storing  it  up  in  its  own  substance  in 
such  a  condition  that  it  cannot  be  removed  by  simple  diminished 
pressure  (so  that  the  pressure  of  oxygen  in  the  muscular  sub- 
stance may  be  considered  as  always  nil),  and  yet  has  not  entered 
into  any  distinct  combination  which  we  can  speak  of  as  an  oxi- 
dation, but  is  still  available  for  such  a  purpose.  On  the  other 
hand  the  muscular  substance  is  always  undergoing  a  decomposi- 
tion of  such  a  kind  that  carbonic  acid  is  set  free,  sometimes,  as 
when  the  muscle  is  at  rest,  in  small,  sometimes,  as  during  a  con- 
traction, in  large  quantities.  The  oxygen  present  in  this  car- 
bonic acid,  as  an  oxidation  product,  comes  from  the  previously 
existing  store  of  which  we  have  just  spoken.  The  oxygen  taken 
in  by  the  muscle,  whatever  be  its  exact  condition  immediately 
upon  its  entrance  into  the  muscular  substance,  sooner  or  later 
enters  into    a  combination,  or  perhaps  we  should  rather  say. 


Chap,  it.]  EESPIRATION.  469 

enters  into  a  series  of  combinations.  We  have  previously 
urged  (§  30)  that  all  living  substance  may  be  regarded  as  inces- 
santly undergoing  changes  of  a  double  kind,  changes  of  build- 
ing up  and  changes  of  breaking  down.  In  the  end-products  of 
the  breaking  down,  in  the  carbonic  acid  given  out  by  muscle 
for  instance,  we  can  recognize  an  oxidation  product ;  but  we  do 
not  know  exactly  at  what  stage  or  exactly  in  what  way  the 
oxygen  is  combined  with  the  carbon.  We  may  imagine  that 
the  oxygen,  as  it  comes  from  the  blood,  is  caught  up  so  to  speak 
by,  and  disappears  in,  the  building  up  processes,  and  that 
through  those  jjrocesses  it  is  made  part  of  complex  decompos- 
able substances  whose  decomposition  ultimately  gives  rise  to 
the  carbonic  acid ;  but,  so  far  as  actual  knowledge  goes,  we 
cannot  as  yet  trace  out  the  steps  taken  by  the  oxygen  from  the 
moment  it  slips  from  the  blood  into  the  muscular  substance  to 
the  moment  when  it  issues  united  with  carbon  as  carbonic 
acid. 

But  if  the  oxygen-pressure  of  the  muscular  tissue  be  thus 
always  nil,  oxygen  will  be  always  passing  over  from  the  blood- 
corpuscles,  in  which  it  is  at  a  comparatively  high  pressure, 
through  the  plasma,  through  the  capillary  walls,  the  lymph- 
spaces  and  the  sarcolemma,  into  the  muscular  substance,  and  as 
soon  as  it  arrives  there  will  be  in  some  manner  or  other  hidden 
away,  leaving  the  oxygen-pressure  of  the  muscular  substance 
once  more  nil.  Conversely,  the  carbonic  acid  produced  by  the 
decomposition  of  the  muscular  substance  will  tend  to  raise  the 
carbonic  acid  pressure  of  the  muscle  until  it  exceeds  that  of 
the  blood ;  whereupon  carbonic  acid  will  pass  from  the  muscle 
into  the  blood,  its  place  in  the  muscular  substance  being  sup- 
plied by  freshly  generated  supplies.  There  will  always  in  fact 
be  a  stream  of  oxygen  from  the  blood  to  the  muscle  and  of  car- 
bonic acid  from  the  muscle  to  the  blood.  The  respiration  of 
the  muscle  then  does  not  consist  in  throAving  into  the  blood 
oxidizable  substances,  there  to  be  oxidized  into  carbonic  acid 
and  other  matters ;  but  it  does  consist  in  the  assumption  and 
storing  up  of  oxygen  somehow  or  other  in  its  substance,  in  the 
building  up  by  help  of  that  oxygen  of  explosive  decomposable 
substances,  and  in  the  carrying  out  of  decompositions  whereby 
carbonic  acid  and  other  matters  are  discharged  first  into  the 
substance  of  the  muscle  and  subsequently  into  the  blood. 

§  290.  Our  knowledge  of  the  respiratory  changes  in  muscle 
is  more  complete  than  in  the  case  of  any  other  tissue ;  but  we 
have  no  reason  to  suppose  that  the  phenomena  of  muscle  are 
exceptional.  On  the  contrar}-,  all  the  available  evidence  goes 
to  shew  that  in  all  tissues  the  oxidation  takes  place  in  the 
tissue,  and  not  in  the  adjoining  blood.  It  is  a  remarkable  fact, 
that  lymph,  serous  fluids,  bile,  urine,  and  milk  contain  a  mere 
trace  of  free  or  loosely  combined  oxygen,  but  a  very  consider- 


470  RESPIEATIOX   OF  THE  TISSUES.         [Book  ir. 

able  quantity  of  carbonic  acid.  And  we  may  probably  assert 
with,  safety  with  regard  to  all  the  tissues  that  in  the  tissues 
themselves,  in  the  lymph  which  bathes  their  lymph-spaces,  and 
in  the  secretions  which  some  of  them  pour  forth  free  oxygen  is 
either  wholly  absent  or  so  scanty  that  their  oxygen-pressure 
may  be  regarded  as  nil,  while  carbonic  acid  is  so  abundant  that 
the  pressure  of  carbonic  acid  in  them  may  be  regarded  as  exceed- 
ing that  of  venous  blood.  An  exception  seems  to  be  presented 
by  the  case  of  the  lymph  flowing  along  the  larger  lymphatic 
vessels,  for  in  this  the  amount  of  carbonic  acid,  while  usually 
higher  than  that  of  arterial  blood,  is  lower  than  that  of  the  gen- 
eral venous  blood ;  but  this  probably  is  due  to  the  fact  that  the 
lymph  in  its  passage  onwards  is  largely  exposed  to  arterial 
blood  in  the  connective  tissues  and  in  the  lymphatic  glands, 
where  the  production  of  carbonic  acid  is  slight  as  compared  to 
that  going  on  in  muscles.  All  the  facts  jDoint  to  the  conclu- 
sion, that  it  is  the  tissues,  and  not  the  blood,  which  become  pri- 
marily loaded  with  carbonic  acid,  the  latter  simply  receiving 
the  gas  from  the  former  by  diffusion,  except  the  (probably) 
small  quantity  which  results  from  the  metabolism  of  the  blood- 
corpuscles  ;  and  that  the  oxygen  which  passes  from  the  blood 
into  the  tissues  is  at  once  taken  up  and  placed  under  such  con- 
ditions that  it  is  no  longer  removable  by  diminished  pressure. 

It  was  shewn  long  ago  that  animals  might  continue  to 
breathe  out  carbonic  acid  in  an  atmosphere  of  nitrogen  or  hydro- 
gen ;  and  this  is  further  illustrated  by  the  experiment,  that  a 
frog  kept  at  a  low  temperature  will  live  for  several  hours,  and 
continue  to  produce  carbonic  acid,  in  an  atmosphere  absolutely 
free  from  oxygen.  The  carbonic  acid  produced  during  this 
period  was  made  by  help  of  the  oxygen  inspired  in  the  hours 
anterior  to  the  commencement  of  the  experiment.  The  oxygen 
then  absorbed  was  stowed  away  from  the  haemoglobin  into  the 
tissues,  it  was  made  use  of  to  build  up  the  explosive  compounds, 
whose  explosions  later  on  gave  rise  to  the  carbonic  acid.  Or, 
to  adopt  a  simile  which  has  been  suggested,  the  oxygen  helps 
to  wind  up  the  vital  clock ;  but  once  wound  up  the  clock  will 
go  on  for  a  period  without  further  winding.  The  frog  will 
continue  to  live,  to  move,  to  produce  carbonic  acid  for  a  while 
without  any  fresh  oxygen,  as  we  know  of  old  it  will  without 
any  fresh  food  ;  it  will  continue  to  do  so  till  the  explosive  com- 
pounds which  the  oxygen  built  up  are  exhausted  ;  it  will  go  on 
till  the  vital  clock  has  run  down. 

§  291.  To  sum  up,  then,  the  results  of  respiration  in  its 
chemical  aspects.  As  the  blood  passes  through  the  lungs,  the 
low  oxygen-pressure  of  the  venous  blood  permits  the  entrance 
of  oxygen  from  the  air  of  the  pulmonary  alveolus,  through  the 
thin  alveolar  wall,  through  the  thin  capillary  sheath,  through 
the  thin  layer  of  blood-plasma,  to  the  red  corpuscle,  and  the 


CuAP.  II.]  RESPIRATION.  471 

reduced  hsemoglobin  of  the  venous  blood  becomes  wholly,  or  all 
but  wholly,  oxyhsemoglobin.  Hurried  to  the  tissues,  the  oxy- 
gen, at  com'paratively  high  pressure  in  the  arterial  blood,  passes 
largely  into  them.  In  the  tissues,  the  oxygen-pressure  is 
always  kept  at  an  exceedingly  low  pitch,  by  the  fact  that  they, 
in  some  way  at  present  unknown  to  us,  pack  away  at  every 
moment  into  some  stable  combination  each  molecule  of  oxygen 
which  they  receive  from  the  blood.  With  its  oxyhsemoglobin 
largely  but  not  wholly  reduced,  the  blood  passes  on  as  venous 
blood.  To  what  extent  the  haemoglobin  is  reduced  will  depend 
on  the  activity  of  the  tissue  itself.  The  quantity  of  haemoglobin 
in  the  blood  is  the  measure  of  limit  of  the  oxidizing  power  of 
the  body  at  large  ;  but  within  that  limit  the  amount  of  oxidation 
is  determined  by  the  tissue,  and  by  the  tissue  alone. 

We  cannot  trace  the  oxygen  through  its  sojourn  in  the 
tissue.  We  only  know  that  sooner  or  later  it  comes  back  com- 
bined in  carbonic  acid  (and  other  matters  not  now  under  con- 
sideration). Owing  to  the  continual  production  of  carbonic 
acid,  the  pressure  of  that  gas  in  the  extravascular  elements  of 
the  tissue  is  always  higher  than  that  in  the  blood;  the  gas 
accordingly  passes  from  the  tissue  into  the  blood,  and  the  venous 
blood  passes  on  not  only  with  its  hcemoglobin  more  or  less 
reduced,  i.e.  with  its  oxygen-pressure  decreased,  but  also  with 
its  carbonic  acid  pressure  increased.  Arrived  at  the  lungs,  the 
blood  finds  the  pulmomary  air  at  a  lower  carbonic  acid  pressure 
than  itself.  The  gas  accordingly  streams  through  the  thin 
vascular  and  alveolar  walls  until  the  pressure  without  the  blood 
vessel  is  equal  to  the  pressure  within.  At  the  same  time  the 
blood  finds  in  the  air  of  the  pulmonary  alveoli  a  suppl}"  of  oxy- 
gen, more  than  adequate  to  convert,  not  entirely  but  nearly  so, 
the  reduced  hcemoglobin  back  again  to  oxy haemoglobin.  Thus 
the  air  of  the  pulmonary  alveoli,  having  given  up  oxygen  to  the 
blood  and  taken  up  carbonic  acid  from  the  blood,  having  in 
consequence  a  higher  carbonic  acid  pressure  and  a  lower  oxygen- 
pressure  than  the  tidal  air  in  the  bronchial  passages,  mixes 
rapidly  with  this  by  diffusion.  The  mixture  is  f  urtlier  assisted 
by  ascending  and  descending  currents ;  and  tlie  tidal  air  issues 
from  the  chest  at  the  breathing  out  poorer  in  oxygen  and  richer 
in  carbonic  acid  than  the  tidal  air  which  entered  at  the  breath- 
insf  in. 


SEC.    6.     THE  NERVOUS  MECHANISM  OF  RESPIRATION. 

§  292.  Breathing  is  an  involuntary  act.  Though  the  dia- 
phragm and  all  the  other  muscles  employed  in  respiration  are 
voluntary  muscles,  i.e.  muscles  which  can  be  called  into  action 
by  a  direct  effort  of  the  will,  and  though  respiration  may  be 
modified  within  very  wide  limits  by  the  will,  yet  we  habitually 
breathe  without  the  intervention  of  the  will :  the  normal  breath- 
ing may  continue,  not  only  in  the  absence  of  consciousness,  but 
even  after  the  removal  of  all  the  parts  of  the  brain  above  the 
spinal  bulb  (medulla  oblongata). 

We  have  already  seen  how  complicated  is  even  a  simple 
respiratory  act.  A  very  large  number  of  muscles  are  called 
into  play.  Many  of  these  are  very  far  apart  from  each  other, 
such  as  the  diaphragm  and  the  nasal  muscles ;  yet  they  act  in 
harmonious  sequence  in  point  of  time.  If  the  lower  intercostal 
muscles  contracted  before  the  scaleni,  or  if  the  diaphragm  con- 
tracted alternately  with  the  other  chest-muscles,  the  satisfactory 
entrance  and  exit  of  air  would  be  impossible.  These  muscles 
moreover  are  coordinated  also  in  respect  of  the  amount  of  their 
several  contractions ;  a  gentle  and  ordinary  contraction  of  the 
diaphragm  is  accompanied  by  gentle  and  ordinary  contractions 
of  the  intercostals,  and  these  are  preceded  by  gentle  and  ordi- 
nary contractions  of  the  scaleni.  A  forcible  contraction  of  the 
scaleni,  followed  by  simply  a  gentle  contraction  of  the  inter- 
costals, would  perhaps  hinder  rather  than  assist  inspiration,  and 
at  all  events  would  be  waste  of  power.  Further,  the  whole 
complex  inspiratory  effort  is  often  followed  by  a  less  marked 
but  still  complex  expiratory  action.  It  is  impossible  that  all 
these  so  carefully  coordinated  muscular  contractions  should  be 
brought  about  in  any  other  way  than  by  coordinate  nervous 
impulses  descending  along  efferent  nerves  from  a  coordinating 
nervous  centre.     By  experiment  we  find  this  to  be  the  case. 

When  in  a  rabbit  the  trunk  of  a  phrenic  nerve  is  cut,  the 
diaphragm  on  that  side  remains  motionless,  and  respiration  goes 
on  without  it.  When  both  nerves  are  cut,  the  whole  diaphragm 
remains  quiescent,  though  the  costal  respiration  becomes  exces- 
sively laboured. 

472 


Chap,  ji.]  RESPIRATIOK  473 

When  an  intercostal  nerve  is  cut,  no  active  respiratory 
movements  are  seen  in  the  intercostal  muscles  of  the  corre- 
sponding space,  and  when  the  spinal  cord  is  divided  below  the 
origin  of  the  seventh  cervical  spinal  nerve,  that  is  below  the 
exits  of  the  roots  of  the  phrenic  nerves,  costal  respiration 
ceases,  though  the  diaphragm  continues  to  act,  and  that  with 
increased  vigour.  When  the  cord  is  divided  just  below  the 
spinal  bulb,  all  thoracic  movements  cease,  but  the  respiratory 
actions  of  the  nostrils  and  glottis  still  continue.  These  how- 
ever, disappear  when  the  facial  and  recurrent  laryngeal  nerves 
are  divided.  We  have  already  stated  that  after  removal  of 
the  brain  above  the  spinal  bulb,  respiration  still  continues  very 
much  as  usual,  the  modifications  which  ensue  from  the  loss  of 
the  brain  being  unessential.  Hence,  putting  all  these  facts 
together,  it  is  clear  that  the  respiratory  movements  are,  as  we 
suggested,  brought  about  by  coordinated  impulses  which,  de- 
veloped in  the  central  nervous  system  and  starting  in  the  first 
instance  in  the  spinal  bulb,  find  their  way  along  the  several 
efferent  nerves.  The  proof  is  completed  by  the  fact  that  the 
removal  of  or  extensive  injury  to  the  spinal  bulb  alone  is,  save 
in  exceptional  cases  which  we  will  discuss  presently,  at  once 
followed  by  the  cessation  of  all  respiratory  movements,  even 
though  the  rest  of  the  nervous  system  including  every  muscle 
and  every  nerve  concerned  be  left  intact.  Nay  more,  if  only 
a  small  portion  of  the  spinal  bulb,  a  tract  whose  limits  have 
not  been  clearly  defined,  but  which  may  be  described  as  lying 
below  the  vaso-motor  centre  in  the  immediate  neighbourhood 
of  the  nuclei  of  the  vagus  nerves,  be  removed  or  injured,  respi- 
ration ceases,  and  death  at  once  ensues.  Hence  this  portion  of 
the  nervous  system  was  called  by  Flourens  the  vital  knot,  or 
ganglion  of  life,  '•  nceud  vital.^  We  shall  speak  of  it  as  the 
respiratory  centre. 

§  293.  The  nature  of  this  centre  must  be  exceedingly  com- 
plex ;  for  while  even  in  ordinary  respiration  it  gives  rise  to  a 
whole  group  of  coordinate  nervous  impulses  of  inspiration 
followed  in  due  sequence  by  a  smaller  but  still  coordinate 
group  of  expiratory  impulses  of  an  antagonistic  nature,  in 
laboured  respiration  fresh  and  larger  impulses  are  generated, 
though  still  in  coordination  with  the  normal  ones,  the  expira- 
tory events  being  especially  augmented  ;  and  in  the  cases  of 
more  extreme  dyspnoea  and  asphyxia  impulses  overflow,  so  to 
speak,  from  it  in  all  directions,  though  only  gradually  losing 
their  coordination,  until  almost  every  muscle  in  the  body  is 
thrown  into  contractions. 

We  must  not  however  conceive  of  this  centre  as  one  of  such 
a  kind  that  the  impidses  leave  it  fully  coordinated  and  equipped 
so  that  notliing  remains  for  them  but  to  travel,  unchanged, 
along-  the  several  efferent  nerve-fibres  to  their  several  muscular 


474  THE   EESPIRATOEY    CENTRE.  [Book  ii. 

destinations.  On  the  contrary  we  have  reason  to  think  that 
the  respiratory  motor  nerves,  like  other  motor  nerves,  are  con- 
nected, just  as  they  are  about  to  issue  from  the  spinal  cord, 
with  a  nervous  machinery,  in  which  nerve  cells  play  a  part  — 
a  point  which  we  shall  consider  more  fully  in  treating  of  the 
spinal  cord  ;  we  have  reason  to  think  that  the  respiratory  im- 
pulses starting  from  the  respiratory  centre  pass  into  and  are 
modified  by  secondary  spinal  nervous  mechanisms  before  they 
issue  along  the  motor  nerve-roots.  Indeed  observations  shew 
that  under  particular  conditions,  and  especially  in  young  ani- 
mals, respiratory  movements  may  be  carried  out  in  the  entire 
absence  of  the  spinal  bulb.  Thus  if  in  a  kitten  or  puppy, 
or  young  rabbit,  after  division  of  the  spinal  cord  below  the 
bulb,  artificial  respiration  be  kept  up,  and  then  pauses  be 
made  in  the  artificial  respiration,  during  these  pauses  not  only 
may  what  appear  to  be  respiratory  movements  be  induced,  in 
a  reflex  manner,  by  pinching  or  by  blowing  on  the  skin,  but, 
especially  if  the  excitability  of  the  spinal  cord  be  heightened 
by  small  doses  of  strychnia,  even  spontaneous  efforts  of  breath- 
ing may  occasionally  be  observed.  These  are  the  exceptional 
instances  mentioned  above.  We  shall  probably  not  greatly  err 
in  regarding  the  respiratory  nervous  system  as  in  many  ways 
analogous  to  the  vaso-motor  nervous  system,  with  its  head 
centre  in  the  spinal  bulb,  and  secondarj^  centres  elsewhere,  and 
in  continuing  to  speak  of  the  centre  in  the  spinal  bulb  as  being 
"the  respiratory  centre  "  while  admitting  that  it  works  through 
other  nervous  machinery  placed  lower  down  in  the  spinal  cord, 
and  that  this  subordinate  machinery  may,  in  exceptional  cases, 
carry  out,  though  inadequately,  the  work  of  the  chief  centre. 

§  294.  Admitting  then  the  existence  of  this  bulbar  respira- 
tory centre  the  question  naturally  arises.  Are  we  to  regard  its 
rhythmic  action  as  due  essentially  to  changes  taking  place  in 
itself,  or  as  due  to  afferent  nervous  impulses  or  other  stimuli 
which  affect  it  in  a  rhythmic  manner  from  without  ?  In  other 
words.  Is  the  action  of  the  centre  automatic  or  purely  reflex  ? 
We  know  that  the  centre  may  be  influenced  by  impulses  pro- 
ceeding from  without,  and  that  the  breathing  may  be  affected 
by  the  action  of  the  will,  or  by  an  emotion,  or  by  a  dash  of  cold 
water  on  the  skin,  or  in  a  hundred  other  ways  ;  but  the  fact 
that  the  action  of  the  centre  may  be  thus  modified  from  with- 
out, is  no  proof  that  the  continuance  of  its  activity  is  dependent 
on  extrinsic  causes. 

In  attempting  to  decide  this  question  we  naturally  turn  to 
the  pneumogastric  as  being  the  nerve  most  likely  to  serve  as 
the  channel  of  afferent  impulses  setting  in  action  the  respiratory 
centre.  If  both  vagus  nerves  be  divided,  respiration  still  con- 
tinues, though  in  a  modified  form.  This  proves  distinctly  that 
afferent  impulses  ascending  those  nerves  are  not  the  efficient 


Chap,  ii.]  RESPIRATION.  475 

cause  of  the  respiratory  movements.  We  have  seen  that  when 
the  spinal  cord  is  divided  beloM'  the  spinal  bulb,  the  facial  and 
laryngeal  movements  still  continue.  This  i^roves  that  the  respir- 
atory centre  is  still  in  action,  though  its  activity  is  unable  to 
manifest  itself  in  any  thoracic  movement.  But  when  the  cord 
is  thus  divided,  the  respiratory  centre  is  cut  off  from  all  sensory 
impulses,  save  those  which  may  pass  into  it  from  the  cranial 
nerves  of  sensory  function ;  and  that  these  sensory  cranial 
nerves  are  not  specially  concerned  in  developing  the  activity  of 
the  respiratory  centre  is  shewn  by  the  fact  that  the  division  of 
these  cranial  nerves  by  themselves,  when  the  bulb  and  spinal 
cord  are  left  intact,  does  not  do  away  with  the  continuance  of 
respiration.  One  cranial  nerve,  as  we  shall  see,  is  especially 
concerned  in  respiration,  viz.  the  vagus  nerve ;  but  if  after 
removal  of  the  brain  above  the  l)ulb  both  vagus  nerves  be 
divided,  respiration  still  goes  on ;  indeed  the  respiratory  im- 
pulses proceeding  from  the  centre  are,  though  in  a  peculiar 
way,  exaggerated.  Hence  though  we  cannot  put  the  matter 
to  an  experimental  test  by  dividing  every  sensory  nerve  in  the 
body,  while  leaving  the  motor  nerves  of  respiration  intact,  such 
an  operation  being  practically  impossible,  we  may  infer  that 
the  respiratory  impulses  proceeding  from  the  respiratory  centre 
are  not  simply  afferent  impulses  reaching  the  centre  along  affer- 
ent nerves  and  transformed  by  reflex  action  in  that  centre. 
They  evidently  start  de  novo  from  the  centre  itself,  however 
much  their  characters  may  be  affected  by  afferent  impulses, 
reaching  that  centre  at  the  time  of  their  being  generated.  The 
action  of  the  centre  is  automatic,  not  simply  reflex. 

§  295.  We  find,  on  inquir}^  that  the  activity  of  the  centre 
is  profoundly  influenced  by  two  classes  of  events.  These,  as 
we  might  expect,  are  on  the  one  hand  events  producing  changes 
in  the  quality  of  the  blood  distributed  to  the  spinal  bidb  through 
the  arteries,  especially  as  regards  its  gases,  that  is  to  say,  events 
modifying  the  interchange  taking  ])lace  in  the  lungs ;  and  on 
the  other  hand  nervous  impulses,  started  in  various  wa3-s  and 
reaching  the  centre  along  various  nerves  or  nervous  tracts.  It 
will  be  convenient  to  consider  the  latter  first. 

Afferent  nervous  impulses  may  affect  the  centre  in  many 
various  ways.  The  whole  act  of  breathing  or  of  taking  a 
breath  is  a  double  act  consisting  of  an  ins})iration  and  an  expira- 
tion, and  nervous  impulses  may  especially  affect  the  one  or  the 
other.  One  mode  of  breathing  may  differ  from  another  in  the 
depth  of  the  individual  breath,  in  the  volume  of  air  taken  in 
and  given  out;  and  nervous  impulses  may  increase  or  may 
diminish  the  depth  of  a  breath,  the  volume  of  air  respired. 
One  mode  of  breathing  again  differs  from  another  in  the  rapidity 
with  which  one  l)realh  succeeds  another,  that  is,  in  the  rate  of 
rhythm ;  and  ner\ous  impulses  ma}-  slow  or  may  quicken  the 


476 


INFLUENCE   OE  VAGUS   NERVES.        [Book  ii. 


rate  of  rhythm.  Then,  again,  combinations  of  effects  so  numer- 
ous and  varied  as  almost  to  baffle  description  may  result  from 
the  influence  of  various  nervous  impulses.  Emotions  may  affect 
a  single  breath  or  a  long  series  of  breaths,  may  quicken  the 
rhythm  while  making  each  breath  more  shallow  or  may  at  the 
same  time  make  each  breath  deeper,  or  may  slow  the  rhythm  in 
either  the  one  or  the  other  manner,  and  may  bear  chiefly  on 
inspiration  or  on  expiration.  Moreover  there  is  not  an  afferent 
nerve  in  the  body  which,  by  means  of  afferent  impulses  passing 
along  it,  may  not  be  the  instrument  of  influencing  the  respira- 
tory centre.  Of  all  the  automatic  centres  in  the  body  the 
respiratory  centre  is  the  one  whose  independence  is  most 
obscured  by  the  repeated  effects  of  afferent  nervous  impulses. 

Certain  afferent  nerves  however  appear  to  be  more  closely 
connected  with  it  than  others ;  and  of  these  the  most  conspicu- 
ous and  important  are  the  two  vagus  nerves,  which  we  have 
already  mentioned  in  this  connection.  Their  importance  is 
well  illustrated  by  the  following  experiments.  If  one  vagus  be 
divided  in  an  ordinary  way,  without  any  special  precautions, 


Fig.  91.   Effect  on  Respiration  of  section  of  one  Vagus. 

The  vagus  was  divided  at  the  point  marked  x.  The  curve  was  obtained  by 
means  of  a  tambour  connected  with  a  receiver  into  which  the  animal  (rabbit) 
breathed  as  shewn  in  Fig.  85,  the  lever  falling  in  inspiration  as  air  is  sucked  out 
of  the  tambour,  and  rising  in  expiration  as  the  air  returns.  Inspiration  begins 
at  a  and  ends  at  6.  Expiration  begins  at  b  and  ends  at  c.  The  lever  gradually 
falls  between  c  and  a  owing  to  the  escape  of  air  from  the  apparatus. 


the  respiration  is  either  not  materially  changed,  or  if  affected 
becomes  slower  (Fig.  91).  If  both  be  divided  (Fig.  92)  it 
becomes  very  slow,  the  pauses  between  expiration  and  inspira- 
tion being  markedly  prolonged.  The  character  of  the  respira- 
tory movement  too  is  markedly  changed ;  each  respiration  is 
fuller  and  deeper,  so  much  so  indeed  that,  according  to  some 
observers,  what  is  lost  in  rate  is  gained  in  extent,  the  amount 


Chap,  ii.] 


RESPIRATION. 


477 


of  carbonic  acid  produced  and  oxygen  consumed  in  a  given 
period  remaining  after  division  of  the  nerves  about  the  same  as 
when  these  were  intact ;  but  it  is  undesirable  to  insist  too  much 
on  the  exactness  of  this  compensation. 


Fig.  92.     Effect  on  Respiration  of  section  of  both  Vagus  nekves. 

Tlie  curve  was  obtained  in  tlie  same  way  as  Fig.  91.    The  second  vagus  nerve 
was  divided  at  x. 

When  after  division  of  both  vagus  nerves  in  the  neck,  the 
medulla  being  intact,  the  central  stump,  that  connected  with 
the  central  nervous  system,  of  one  of  them  is  stimulated  with  a 
gentle  interrupted  current,  the  effects  are  not  always  the  same ; 
one  of  two  results  may  follow  and  that  whichever  of  the  two 
nerves  be  used.  In  a  certain  number  of  cases,  and  these  may 
perhaps  be  regarded  as  the  more  typical  ones,  the  respiration, 
which  from  the  division  of  the  nerves  had  become  slow,  is 
quickened  again  (Fig.  93)  ;  and  with  care,  by  a  proper  aj^jjli- 
cation  of  the  stimulus,  tlie  normal  respiratory  rhythm  may  for 


Fig.  on.     Quickening  of  Respiration  by  gentle  stimulation  of  the 
Central  End  of  the  Vagus  trunk. 

Tlio  curve  was  obtained  in  the  same  way  as  Figs.  91,  92.     Stimulation  oi  the 
vagus  began  at  x,  and  ended  at  y. 


478 


INFLUEIvTCE   OP  VAGUS   NERVES.        [Book  ii. 


a  time  be  restored.  Upon  tlie  cessation  of  the  stimulus,  the 
slower  rhythm  returns.  If  the  current  be  increased  in  strength, 
the  rhythm  may  in  some  cases  be  so  accelerated  that  inspiration 
begms  before  the  expiration  of  the  preceding  breath  is  com- 
pleted, Fig.  94 ;  and  this  may  go  on  until  at  last  the  diaphragm 
is  brought  into  a  condition  of  prolonged  tetanus,  and  a  stand- 
still of  respiration  in  an  extreme  inspiratory  phase  is  the  result. 
On  the  other  hand  in  a  certain  number  of  cases  the  result  is  of 
an  opposite  character.     Even  though  the  respiration  be  already 


Fig.   94.     Stimulation  of  Vagus  leading  to  Inspiratory  increase. 

This  curve,  unlike  the  preceding,  -was  obtained  by  inserting  a  needle  through 
the  body  wall  so  as  to  rest  on  the  diaphragm  and  attaching  a  lever  to  the  needle  ; 
see  §  251.  The  lever  rises  with  each  contraction  of  the  diaphragm  so  that  inspira- 
tion begins  at  a  and  ends  at  b,  expiration  begins  at  b  and  ends  at  c,  the  interval 
between  c  and  a  corresponding  to  the  pause. 

Stimulation  of  the  vagus  begins  at  x.  It  will  be  seen  that  upon  stimulation 
the  inspiratory  rises  of  the  lever  begin  long  before  the  preceding  expirations  are 
complete. 

slowed  by  division  of  the  nerves,  stimulation  produces  a  still 
further  slowing,  the  pauses  between  each  expiration  and  the 
succeeding  inspiration  are  prolonged  (cf.  Fig.  95),  and  in  a 
certain  number  of  cases,  actual  standstill  is  brought  about,  but 
a  standstill  of  a  kind  the  opposite  of  the  one  just  described, 
since  the  diaphragm  which  in  that  case  was  in  prolonged  tetanus 
is,  in  this  case,  completely  relaxed,  and  remains  for  some  time 
in  the  condition  in  which  it  is  at  the  close  of  an  ordinary  breath. 
In  a  certain  number  of  cases,  and  these  are  not  uncommon,  the 
result  is  intermediate  between  the  two  above  extremes ;  the 
diaphragm  stands  still  in  a  proloiiged  contraction  in  a  position 
which  is  intermediate  between  the  height  of  inspiration  and 
expiration. 

These  results  suggest  the  conclusion  that  the  vagus  nerve  (we 
are  dealing  now  with  the  main  trunk  of  the  nerve)  contains 


Chap,  n.]  RESPIIIATIOK  479 

afferent  fibres  of  two  kinds  connected  witli  tlie  respiratory  cen- 
tre :  one  kind  augmenting  the  action  of  the  centre  somewhat  in 
the  same  way  as  the  augmentor  cardiac  fibres  augment  the  beat 
of  the  heart,  and  the  other  kind  having  an  inhibitory  effect. 
Apparently  sometimes  the  one  and  sometimes  the  other  kind  is, 
according  to  circumstances,  most  provoked  by  the  stimuUxtion, 
much  in  the  same  way  as  stimulation  of  the  vagus  in  the  frog, 
which  as  we  have  seen,  §  136,  is  the  channel  for  both  inhibitory 
and  augmentor  cardiac  impulses,  produces,  sometimes  inhibition, 
sometimes  augmentation  of  the  heart  beat.  To  affect  the  heart 
of  course  the  stimulation  of  the  vagus  must  be  centrifugal, 
directed  towards  the  periphery,  whereas  to  affect  the  respira- 
tion it  must  be  centripetal,  applied  to  the  part  of  the  nerve 
connected  with  the  brain  ;  and  while  the  usual  effect  on  the 
heart  of  ordinary  stimulation  of  the  vagus  is  inhibition,  augmen- 
tation only  occurring  in  special  cases,  the  most  common  effect 
on  respiration  is  augmentation,  though  inhibition  is  not  unfre- 
quently  seen.  When  the  experiment  is  conducted  on  an  animal 
under  the  full  influence  of  chloral  stimulation  of  the  vagus  gen- 
erally produces  inhibition  of  respiration,  probably  because  the 
chloral  renders  the  respiratory  centre  more  susceptible  to  inhibi- 
tory influence. 

§  296.  We  said  just  now  "  the  action  of  the  centre  ; "  but  the 
respiratory  centre  is  a  double  one  ;  it  gives  rise  to  insjDiratory 
and  to  expiratory  efferent  impulses,  aiid  these  are  antagonistic 
the  one  to  the  other.  If  inspiratory  and  expiratory  impulses 
issued  from  the  centre  at  the  same  time  and  in  equal  potency, 
there  could  be  no  breathing  at  all,  they  would  neutralize  each 
other's  effects  ;  and  indeed  any  amount  of  inspiratory  impulse 
is  antagonistic  to  a  simultaneous  expiratory  impulse,  and  vice 
versa.  Hence  for  the  adequate  services  of  the  respiratory  cen- 
tre we  might  expect  to  find  that  each  kind  of  afferent  impulse 
ascending  the  vagus  affected  the  centre  in  a  double  and  oppo- 
site way,  inhibiting  expiration  while  augmenting  inspiration,  or 
inhibiting  insj)iration  while  augmenting  expiration.  If  we  allow 
ourselves  to  speak  of  the  whole  respiratory  centre  as  consisting 
of  two  parts,  one  the  inspiratory  part,  or  inspiratory  centre 
concerned  in  the  issue  of  inspiratory  impulses,  and  the  other 
the  expiratory  j^art,  or  expiratory  centre  concerned  in  the  issue 
of  expiratory  impulses,  we  may  suppose  that  these  centres  are 
so  related  to  each  other  that  afferent  impulses,  reaching  the 
spinal  bulb,  which  augment  or  inhibit  the  one,  necessarily 
inhibit  or  augment  the  other.  We  need  perhaps  hardly  add 
that  of  these  two  centres  we  should  expect  to  find  the  inspira- 
tory centre  the  dominant  and  the  most  responsive  one  ;  in  nor- 
mal breathing  it  comes  almost  alone  into  obvious  use,  since  as  we 
have  seen  the  expiratory  muscles  have  then  a  very  slight  task 
only,  the  chest  being  emptied  cliiefly  by  elastic  reaction  ;  and. 


480  AUGMENTING  AND  INHIBITOEY  IMPULSES.  [Book  ii. 

speaking  generally,  breathing  in  is  the  first  consideration,  we 
breathe  out  mostly  because  we  have  already  breathed  in. 

There  are  many  facts  which  support  this  view  of  the  double 
antagonistic  action  of  afferent  respiratory  impulses.  If  the 
central  end  of  the  superior  laryngeal  branch  of  the  vagus  be 
stimulated  the  effects  are  much  more  constant  than  those  of 
stimulating  the  main  vagus  trunk.  Whether  the  main  trunk 
of  the  nerve  be  previously  severed  or  not,  the  result  of  centrip- 
etal stimulation  of  the  superior  laryngeal  branch  is  always  in 
the  direction  of  a  slowing  of  the  respiration  (Fig.  95)  ;  and 


Fig. 


95.     Slowing  of  Respiration  bt  stimulation  of  superiok  Laryn- 
geal NERVE. 


This  curve  was  obtained  in  the  same  way  as  Figs.  91,  2,  3  and  the  letters  have 
the  same  meaning  as  in  those  figures.     Stimulation  begins  at  x,  and  ends  at  y. 


this  may  by  proper  stimulation  be  carried  so  far  that  a  complete 
standstill  of  respiration  in  the  phase  of  rest  is  brought  about. 
While  the  main  trunk  of  the  vagus  contains  fibres  of  two  kinds, 
both  augmentor  and  inhibitory  of  inspiration,  the  superior 
laryngeal  branch  appears  to  contain  one  kind  only,  those  which 
inhibit  inspiration.  If  now  while  this  experiment  is  being  con- 
ducted on  a  rabbit  the  abdomen  be  watched  it  will  be  seen  that 
tlie  inhibition  of  inspiration  is  accompanied  by  a  contraction  of 
the  abdominal  muscles,  that  is  by  an  effort  at  expiration  ;  the 
stimulation  of  the  nerve  while  inhibiting  respiration  provokes, 
to  a  certain  extent,  expiration. 

§  297.  That  the  trunk  of  the  vagus  is  the  channel  of  these 
two  kinds  of  impulses,  of  a  mutually  antagonistic  character,  is 
further  shewn  by  applying  what  may  be  considered  as  natural 
stimuli  to  the  endings  of  the  nerve  in  the  lungs  ;  and  the 
results  so  obtained  have  an  especial  value  since  the  artificial 


Chap.  II.] 


EESPIEATIOK 


481 


stimulation  of  a  nerve-fibre  at  a  jjart  of  its  course  by  means  of 
an  electric  current  is  at  best  a  rough  process,  by  which  we  can- 
not hojje  to  do  more  than  approximate  to  the  results  actually 
taking-  place  in  the  living  body  when  the  nerve  is  stimulated  at 
its  endings  by  natural  stimuli  ;  and  the  approximation  is  per- 
haps less  in  the  case  of  the  exquisitely  sensitive  resi)iratory 
centre  than  in  many  other  cases. 

Jf  in  an  animal  in  which  a  careful  graphic  record  of  the 
respiratory  movements  is  being  taken,  the  trachea  be  suddenly 
closed  at  the  summit  of  an  inspiration,  the  result  is  a  jjause 
before  the  succeeding  inspiration  follows,  that  is  to  say,  a 
partial  or  temporary  inhibition  of  inspiration  ;  and  if  during 
such  an  experiment  on  a  rabbit  a  curve  be  taken  by  means  of 
the  isolated  slip  of  the  diaphragm,  §  259,  it  will  be  seen  (Fig. 
96  A)  that  the  slip  elongates  somewhat ;  that  is  to  say,  prcAd- 
ously  in  a  state  of  slight  tonic  contraction,  it  changes  in  the 
direction  of  expiration.  If  on  the  other  hand  the  trachea  be 
suddenly  closed  at  the  end  of  an  expiration  (Fig.  96  B),  when 


■J 1 1 1 1 I        I 


Fig.  96.     Effects  of  Disten'^ion  and  Coll.\pse  of  Lung.    (Head.) 

Bntli  curves  are  described  by  a  lover  attached,  as  stated  in  §  250,  to  a  slip  of 
tlie  diapliraam  of  a  rabbit.  A  contraction  of  the  diaphracm  (inspiration)  raises 
tiie  lever  ;  durin<r  relaxatit^i  of  the  diapliraam,  the  lever  falls. 

In  A,  the  trachea  is  closed  at  .r,  the  hi  iirht  of  inspiration  ;  a  pause  follows 
durinc;  which  the  lever  gradually  sinks  until  an  insjiiration  (a  very  powerful  one) 
sets  in. 

In  B,  the  trachea  is  closed  at  the  end  of  expiration,  a; ;  there  follow  powerful 
inspirations. 

31 


482    EFFECTS  OF  DISTENSION  AND  COLLAPSE.  [Book  ii. 

tlie  lungs  have  returned  to  tlieir  emptied  condition,  the  result 
is  an  increase  of  the  sequent  inspirations,  that  is  to  say,  an 
augmentation  of  inspiratory  impulses.  If  the  chest  or  if  the 
lung  only  be  gently  inflated  a  temporary  cessation  of  all 
inspiration  may  be  produced,  accompanied  sometimes  by  an 
attempt  at  expiration.  If  on  the  other  hand  air  be  sucked  out 
of  the  chest,  or  if  one  lung  be  made  to  collapse  by  puncture  of 


J \ I 1 \ L 


J ! I \ L. 


J 1 I I 1_ 


_1_J l_l I LJ L 


Fig.  97.  Effects  of  repeated  Inflations.     Positive  ventilation.    (Head.) 

The  lower  curve  is  described,  as  in  Fig.  96,  by  a  lever  attached  to  a  slip  of  the 
diaphragm.  The  upper  curve  shews  the  intiations  from  x  to  y,  which  were  made 
without  any  attempt  to  draw  the  air  out  at  each  inflation  ;  each  rise  on  this  curve 
denotes  an  inflation.  It  will  be  observed  that  as  the  inflations  are  continued  the 
respiratory  movements  of  the  diaphragm  are  gradually  "knocked  down." 

one  pleural  chamber,  a  prolonged  inspiration  is  the  frequent 
result,  the  diaphragm  being  thrown  into  a  prolonged  inspiratory 
tetanus.  If  the  lungs  are  repeatedly  inflated,  without  any 
means  being  taken  to  draw  out  the  air  after  each  inflation 
(Fig.    97),  a  procedure  which  we  may  speak  of  as  positive 


Fig.  98. 


Effects  of  repeated  Suctions  of  the  Lungs. 
ventilation.  (Head.) 


Negative 


The  curve  corresponds  exactly  to  Fig.  97,  except  that  the  lungs  are  subjected 
to  repeated  suctions  without  corresponding  inflations.  The  result  is  that  the 
inspirations  are  repeated  in  such  a  way  as  to  be  led  almost  to  an  inspiratory 
tetanus  of  the  diaphragm. 


Chap,  ii.]  RESPIEATION.  483 

ventilation,  the  result  is  that  the  inspiratory  efforts  are  dimin- 
ished, and  if  the  ventilation  is  continued  may  cease  altogether. 
If  on  the  other  hand  air  is  repeatedly  sucked  out  of  the  lungs, 
without  any  corresponding  inflations,  negative  ventilation,  the 
inspiratory  efforts  are  increased  (Fig.  98)  and  the  increase  may 
be  such  as  to  bring  the  diaphragm  to  a  state  of  tetanus.  And 
in  general,  though  several  complications  occur  which  we  cannot 
discuss  here,  the  results  of  inflation  of  the  lungs  on  the  one  hand 
and  of  suction  or  collapse  of  the  lungs  on  the  other  hand,  shew 
that  the  mere  inflation  or  perhaps  rather  the  mere  distension  of  the 
lung  tends  to  inhibit  inspiratory  and  usher  in  expiratory  impulses, 
while  collapse  of  the  lung  tends  to  inhibit  expiratory  and  to  de- 
velop inspiratory  impulses,  the  effect  on  the  inspiratory  impulses, 
as  might  be  expected  from  the  dominance  of  the  insiDiratory  por- 
tion of  the  centre  being  more  marked  than  the  effect  on  the 
expiratory  impulses.  That  the  instrument  by  which  tliese  effects 
are  produced  is  the  vagus  nerve  is  shewn  by  the  fact  that  they  are 
no  longer  distinctly  recognizable  when  both  vagus  nerves  are 
divided.  And  that  the  results  are  due  to  the  mere  mechanical 
expansion  and  collapse  of  the  lung  in  insufflation  and  collapse,  and 
not  to  any  chemical  influences  exerted  by  tlie  larger  amount  or 
smaller  amount  of  air  j)resent  in  the  lung  in  the  tAvo  cases  increas- 
ing or  diminishing  the  absorption  of  oxygen  and  escape  of  carbonic 
acid,  is  shewn  by  the  fact  that  the  results  remain  in  their  main 
features  the  same  when  some  indifferent  gas  such  as  hydrogen 
is  used  for  inflation  instead  of  air  or  oxygen.  We  infer  there- 
fore that  the  expansion  of  the  pulmonary  alveoli  in  some  way 
or  other  so  stimulates  the  endings  in  the  lung  of  the  pulmonary 
branches  of  the  vagus,  that  impulses  are  generated  which  as- 
cending the  vagus  trunk  inhibit  the  inspiratory  processes  in 
the  respiratory  centre  ;  and  that  conversely  collapse  of  the  lung 
similarly  generates  impulses  Avhich  are  augmentative  of  inspira- 
tory impulses.  And,  assuming  on  the  strength  of  analogy  the 
existence  in  the  vagus  of  two  sets  of  fibres  we  may  say  that 
expansion  stimulates  the  endings  of  the  fibres  which  iidiibit 
inspiration  and  concurrently  tend  to  augment  expiration,  Avhile 
collapse  stimulates  the  fibres  which  inhibit  expiration  and  aug- 
ment inspiration.  The  respiratory  pump  may  thus  be  looked 
upon  as  a  self-regulating  mechanism  :  the  expansion  of  the 
lungs  which  is  the  result  of  the  efferent  inspiratory  impulses 
tends  to  check  the  issue  of  these  impulses  and  to  inaugurate  the 
sequent  expiration  ;  and  the  return  of  the  lungs  in  expiration 
tends  to  set  going  the  succeeding  inspiration. 

§  298.  The  double  or  alternate  respiratory  action  of  the 
vagus  nerves  on  which  we  have  dwelt  above  may  be  taken  as 
in  a  general  way  illustrative  of  the  manner  in  which  other 
afferent  nerves  and  various  parts  of  the  cerebrum  are  enabled 
to  influence  respiration.     As  we  have  already  said,  and  indeed 


484     REGULATION  OF  EESPIRATORY  CENTRE.    [Book  ii. 

know  from  daily  experience,  of  all  the  apsycliical  nervous  cen- 
tres, tlie  respiratory  centre  is  the  one  which  is  most  frequently 
and  most  deeply  affected  by  nervous  impulses  from  various 
quarters.  Besides  the  changes  brought  about  by  the  Avill  (and 
when  we  breathe  voluntarily  we  probably  make  use  to  some 
extent  of  the  normal  nervous  machinery  of  respiration,  working 
through  this,  rather  than  sending  independent  volitional  im- 
pulses direct  to  the  diaphragm  and  other  respiratory  muscles), 
we  hnd  that  emotions  and  painful  sensations  alter  profoundly 
the  character  of  the  respiratory  movements.  And  thoug]i  these 
effects  may  be  partly  indirect  (the  emotion  modifying  the  heart- 
beat or  the  tonus  of  the  arteries,  and  so  influencing  the  flow  of 
blood  through  the  respiratory  centre),  they  are  chiefly  due  to 
the  direct  action  of  nervous  impulses  reaching  that  centre  from 
higher  parts  of  the  brain.  So  also  impulses  from  almost  every 
sentient  surface,  or  passing  along  almost  every  sensory  nerve, 
may  modify  respiration  in  one  direction  or  another.  The 
influence  in  this  way  of  stimuli  applied  to  the  skin  is  well 
known  to  all ;  but  perhaps  next  to  the  vagus  the  nerve  most 
closely  connected  with  the  respiratory  centre  is  the  fifth  nerve, 
branches  of  which  guard  the  nasal  respiratory  channels  ;  the 
slightest  stimulation  of  the  nostrils  at  once  affects  the  breathing 
and  most  frequently  arrests  it.  The  effects  of  stimuli  of  various 
strengths  brought  to  bear  on  various  nerves  are  very  varied. 
Sometimes  the  result  is  an  increase  of  inspiration  ;  and  that 
either  by  a  quickening  of  the  rhythm  or  by  an  increase  of  the 
individual  breaths  or  by  a  combination  of  the  two.  Sometimes 
the  result  is  an  inhibition  of  inspiration  accompanied  or  not  by 
an  increase  of  expiration,  and  sometimes,  as  when  the  stimula- 
tion causes  a  cough,  the  expiratory  results  may  be  out  of  all 
proportion  to  the  modifications  of  inspiration. 

§  299.  The  complicated  nature  of  the  respiratory  centre  is 
further  shewn  by  the  fact  that  it  appears  to  consist  of  two  lat- 
eral halves  which  normally  work  in  unison  and  yet  may  be  made 
to  work  independently.  If  the  spinal  bulb  be  carefully  divided 
in  the  middle  line  respiration  may  continue  to  go  on  in  quite  a 
normal  fashion.  If,  however,  one  vagus  be  then  divided,  tlie 
respiratory  movements,  both  costal  and  diaphragmatic,  on  tlie 
side  of  the  body  on  which  division  of  the  vagus  has  taken  place, 
become  slower  than  those  on  the  other  side,  so  that  the  two 
sides  are  no  longer  synchronous  ;  and  a  stimulus  confined  to 
one  vagus  affects  the  respiratory  movements  of  that  side  of  the 
body  only.  So  also  a  section  of  a  lateral  half  of  the  cord  below 
the  bulb  stops  the  respiratory  movements  on  that  side  alone. 

§  300.  Besides  these  nervous  influences,  however,  there  is 
another  circumstance  which  perhaps  above  all  others  affects  the 
respiratory  centre,  and  that  is  the  condition  of  the  blood  in 
respect  to  its  respiratory  changes  ;  the  more  venous  (less  arte- 


Chap,  ii.]  RESPIRATIOK  485 

rial)  the  blood,  the  greater  is  the  activity  of  the  respiratory- 
centre.  When  by  reason  either  of  any  hindrance  to  the  entrance 
of  air  into  the  chest,  or  other  interference  with  the  due  inter- 
cliange  between  the  blood  and  the  pulmonary  air  or  of  a  greater 
respiratory  activity  of  the  tissues,  as  during  muscular  exertion, 
the  blood  becomes  less  arterial,  more  venous,  i.e.  with  a  smaller 
charge  of  oxygen  and  more  heavily  laden  with  carbonic  acid, 
the  respiration  from  being  normal  becomes  laboured.  We  may 
speak  of  normal  breathing  as  eupnoea^  and  say  that  this,  when 
the  blood  is  insufficiently  arterialized,  passes  into  dysjjnoea.,  an 
intermediate  stage  in  which  the  respiratory  movements  are 
simi)ly  exaggerated  being  known  as  hyperpnoea.  The  modifi- 
cations of  breathing  thus  caused  by  deficient  arterialization  of 
blood  are  especially  characterized  by  an  increase  in  the  total 
energy  of  the  respiratory  impulses  generated,  and  in  this  respect 
differ  from  the  modifications  resulting  from  interference  with 
the  nervous  arrangements  such  as  those  following  upon  section 
of  the  vagus  nerves,  in  which  case  as  we  have  seen  the  rhythm 
is  much  more  profoundly  affected  than  the  amount.  In  dysp- 
noea the  breathing  is  frequently  quicker  as  well  as  deeper,  there 
is  an  increase  in  the  sum  of  efferent  respiratory  impulses,  and 
the  expiratory  impulses,  which  in  normal  respiration  are  very 
slight,  acquire  a  pronounced  importance.  As  the  blood  becomes, 
in  cases  of  obstruction,  less  and  less  arterial,  more  and  more 
venous,  the  discharge  from  the  respiratory  centre  becomes  more 
and  more  vehement,  and  instead  of  confining  itself  to  the  usual 
tracts,  and  passing  doAvn  to  the  ordinary  respiratory  muscles,  over- 
flows into  other  tracts  and  puts  into  action  other  muscles,  until 
there  is  perhaps  hardly  a  muscle  in  the  body  which  is  not  made 
to  feel  its  effects.  The  muscles  which  are  thus  more  and  more 
thrown  into  action  are  especially  those  tending  to  carry  out  or 
to  assist  expiration  ;  and  at  last,  if  no  relief  is  afforded,  the 
violent  but  still  definite  respiratory  movements  give  way  to 
general  convulsions  of  the  Avhole  body,  which  however  have,  to 
a  certain  extent,  an  expiratory  character.  With  the  onset  of 
these  convulsions  dyspntea  is  said  to  have  passed  into  asphi/xia. 
By  the  violence  of  these  convulsions  the  whole  nervous  system 
becomes  exhausted,  the  convulsions  cease  and  death  is  ushered 
in  through  a  few  infrequent  and  long-drawn  breaths  ;  but  to 
this  matter  we  shall  return.  Tlie  effect  of  venous  blood  then  is 
to  augment  all  those  natural  explosive  decompositions  of  the 
substance  of  the  central  nervous  system  which  give  rise  to  res- 
piratory impulses  ;  it  increases  their  amount,  and  also  (piiekens 
their  rhythm.  The  latter  change,  however,  is  much  less  marked 
than  the  former,  the  respiration  being  much  more  deepened  than 
hurried,  and  the  several  respiratory  acts  are  never  so  mueli  has- 
tened as  to  catch  each  other  up,  and  so  to  produce  an  inspiratory 
tetanus  like  that  resulting  from  stimulation  of  the  vaous.     On 


486      EFFECTS    OF   DEFICIENCY   OF   OXYGEN.      [Book  ii. 

the  contrary,  especially  as  exhaustion  begins  to  set  in,  the 
rhythm  becomes  slower  out  of  proportion  to  the  weakening  of 
the  individual  movements. 

§  301.  The  question  naturally  arises,  Does  this  condition  of 
the  blood  affect  the  substance  of  the  central  nervous  system,  that 
is  to  say,  of  the  respiratory  centre  in  the  spinal  bulb  (and  the 
subsidiary  spinal  nervous  mechanisms)  directly,  or  does  it  pro- 
duce its  effect  by  stimulating  the  peripheral  ends  of  afferent 
nerves  in  various  parts  of  the  body,  and,  by  the  generation  there 
of  afferent  impulses,  indirectly  modify  the  action  of  the  central 
nervous  system  ?  Without  denying  the  possibility  that  the  latter 
mode  of  action  may  help  in  the  matter,  as  regards  not  only  the 
vagus,  but  all  afferent  nerves,  the  following  facts  seem  to  shew 
that  the  main  effect  is  produced  by  the  direct  action  of  the  blood 
on  the  central  nervous  system  and  indeed  on  the  bulbary  res- 
piratory centre  itself.  If  the  spinal  cord  be  divided  below  the 
bulb,  and  both  vagi  be  cut,  want  of  proper  aeration  of  the  blood 
still  produces  an  increased  activity  of  the  respiratory  centre,  as 
shewn  by  the  increased  vigour  of  the  facial  respiratory  move- 
ments ;  in  such  a  case,  it  must  act  directly  on  the  respiratory 
centre,  for  all  afferent  paths  along  the  nerves,  except  the  few 
cranial  ones,  have  been  blocked  by  the  operation.  The  same 
direct  action  is  further  shewn  by  the  following  "  cross  circula- 
tion "  experiment.  In  two  animals  the  peripheral  portion  of  one 
carotid  of  one  animal  is  connected  by  a  tube  with  the  central  por- 
tion of  one  carotid  of  the  other  animal,  the  other  carotid  in  each 
animal  being  tied.  Hence  the  brain  and  the  brain  only  of  one 
animal  is  supplied  by  the  blood  of  the  other  animal,  the  rest  of 
its  body  being  supplied  by  its  own  blood.  If  now  respiration 
be  stopped  in  one  animal  the  other  becomes  dyspnoeic,  while  it 
in  itself  shews  no  dyspnoea  ;  it  is  the  animal  to  whose  brain 
(spinal  bulb)  alone  too  venous  blood  is  brought,  not  the  animal 
the  whole  of  whose  body  is  supplied  with  the  too  venous  blood, 
Avhich  manifests  disturbance  of  the  respiratory  centre.  Again, 
if  in  an  animal  the  supply  of  blood  be  cut  off  from  the  spinal  bulb 
by  ligature  of  the  carotid  and  intervertebral  arteries  dyspnoea 
is  produced,  though  the  operation  produces  at  first  no  change  in 
the  blood  generally,  but  simply  affects  the  respiratory  condition 
of  the  medulla  itself  by  cutting  off"  its  blood-supply,  the  imme- 
diate result  of  which  is  an  accumulation  of  carbonic  acid  and  a 
23aucity  of  available  oxygen  in  the  nervous  substance  of  that 
region.  If  the  blood  in  the  carotid  artery  in  an  animal  be 
warmed  above  the  normal,  a  dyspnoea  is  produced  which,  though 
apparently  not  quite  identical  with  the  dyspnoea  caused  by  im- 
perfect arterialization  of  the  blood,  shews  that  the  too  high 
temperature  of  the  blood  directly  affects  the  activity  of  the 
respiratory  centre.  We  may  conclude  therefore  that  the  con- 
dition of  the  blood  affects  respiration  by  acting  directly  on  the 
respiratory  centre. 


Chap,  ii.]  RESPIRATION.  487 

While  the  respiratory  centre  is  thus  being  affected  by  the 
too  venous  blood,  it  is,  until  exhaustion  begins  to  set  in,  more 
irritable,  more  easily  and  largely  affected  by  afferent  impulses 
than  in  its  normal  condition.  During  dyspnoea  a  stimulus 
which  applied  to  the  vagus  or  to  some  other  sensory  nerve 
under  normal  conditions  would  produce  little  or  no  effect,  may 
start  very  powerful  respiratory  movements. 

§  302.  Deficient  aeration  produces  two  effects  in  blood  :  it 
diminishes  the  oxygen,  and  increases  the  carbonic  acid.  Do 
both  of  these  changes  affect  the  respiratory  centre,  or  only  one, 
and  if  so,  which  ?  When  an  animal  is  made  to  breathe  an  atmos- 
phere containing  nitrogen  only,  the  exit  of  carbonic  acid  by 
diffusion  is  not  affected,  and  the  blood,  as  is  proved  by  actual 
analysis,  contains  no  excess  of  carbonic  acid.  Yet  all  the  phe- 
nomena of  dyspnoea  are  present,  and  if  the  experiment  be  con- 
tinued, convulsions  ensue  and  the  animal  dies  in  asphjrsia.  In 
this  case  the  result  can  only  be  attributed  to  the  deficiency  of 
oxygen.  On  the  other  hand,  if  an  animal  be  made  to  breathe 
an  atmosiDhere  rich  in  carbonic  acid,  but  at  the  same  time  con- 
taining abundance  of  oxygen,  though  the  breathing  becomes 
markedly  deeper  and  also  somewhat  more  frequent,  there  is  no 
culmination  in  a  convulsive  asphyxia,  even  when  the  quantity 
of  carbonic  acid  in  the  blood,  as  shewn  by  direct  analysis,  is 
very  largely  increased.  On  the  contrary,  the  increase  in  the 
respiratory  movements  may  after  a  while  pass  off,  the  animal 
becoming  unconscious,  and  appearing  to  be  suffering  rather 
from  a  narcotic  poison  than  from  simple  dyspnoea  ;  the  excess 
of  carbonic  acid  in  the  blood  appears  to  affect  other  parts  of 
the  central  nervous  system,  and  especially  portions  of  the  brain, 
more  profoundly  than  it  does  the  respiratory  centre.  It  has 
been  maintained  by  some  that  while  a  deficiency  of  oxygen 
promotes  inspiratory  movements,  an  excess  of  carbonic  acid 
stimulates  the  expiratory  movements,  the  nervous  mechanisms 
being  so  arranged  that  a  lack  of  oxygen  leads  to  an  effort  to 
get  more  of  it  and  a  too  great  load  of  carbonic  acid  to  an  effort 
to  get  rid  of  it ;  but  the  facts  are  opposed  to  the  existence  of 
any  such  teleological  adaj^tation.  It  is  obvious  however  that  a 
lack  of  oxygen  and  an  excess  of  carbonic  acid  aft'ect  the  respir- 
atory centre  in  very  different  ways,  and  that  in  ordinary  cases 
of  interference  with  the  interchange  in  the  lungs,  as  in  defi- 
cient aeration,  it  is  the  lack  of  oxygen  which  plays  the  prin- 
cipal part  in  developing  the  abnormal  respiratory  movements. 
We  may  infer  that  it  too  is  chiefly  concerned  in  reguhxting  the 
moi'e  normal  respiration,  but  cannot  as  yet  say  what  is  the 
exact  share  to  be  attributed  to  the  carbonic  acid. 

We  may  here  j^oint  out  that  it  is  not  to  be  supposed  that 
each  breatli  is  determined  by  the  condition  of  the  blood  flowing 
through  the  capillaries  of  tho  ■medulla  at  the  moment  preceding 


488  EFFECTS   OF  MUSCULAE   EXEKCISE.     [Book  ii. 

that  breath,  it  is  not  to  be  imagined  that  each  breath  is  the 
result  of  the  lack  of  oxygen  felt  immediately  before.  On  the 
contrary,  the  condition  of  blood  merely  modifies  the  natural 
automatic  action  of  the  centre. 

§  303.  There  are  reasons  for  thinking  that  conditions  of  the 
blood,  other  than  variations  in  the  amount  of  oxygen  and  car- 
bonic acid,  may  also  materially  affect  the  working  of  the  res- 
piratory centre.  It  is  a  matter  of  common  experience  that 
muscular  exertion,  especially  if  at  all  excessive,  increases  the 
respiratory  movements ;  violent  exercise  soon  puts  a  man  "  out 
of  breath."  This  increased  activity  of  the  respiratory  centre 
is  in  large  measure  at  all  events  caused  by  the  character  of  the 
blood  which  during  and  for  some  little  time  after  the  move- 
ments is  carried  to  the  spinal  bulb,  and  not  by  any  nervous 
impulses  sent  up  to  the  bulb  from  the  contracting  muscles. 
This  is  shewn  by  the  fact  that  if  in  an  animal  the  spinal  cord 
be  divided  in  the  dorsal  or  lumbar  region  and  the  hind  limbs  be 
powerfully  tetanized,  the  respiratory  movements  are  increased ; 
the  animal  pants  as  it  would  do  if  it  had  been  running.  In 
such  a  case  the  only  connection  between  the  hind  limbs  and  the 
respiratory  centre  is  through  the  blood ;  it  must  be  some  change 
in  the  blood  caused  by  the  muscular  contractions  which  affects 
the  respiratory  centre  when  the  blood  passes  from  the  hind  limbs 
to  be  distributed  by  the  heart  to  the  bulb.  Now  when  a  muscle 
contracts  its  consumption  of  oxygen  and  production  of  carbonic 
acid,  especially  the  latter  (§  60),  are  increased ;  the  blood  leav- 
ing the  muscle  is  more  venous  than  usual.  Hence  when  many 
muscles  are  contracting  powerfully  the  blood  carried  to  the 
right  side  of  the  heart  is  more  venous  than  usual;  and  we 
might  expect  that  it  is  this  unusually  venous  blood  failing  to 
be  adequately  arterialized  in  the  lungs  and  hence  reaching  the 
bulb  from  the  left  side  of  the  heart  in  a  more  venous,  less  com- 
pletely arterialized  condition  than  usual,  which  stirs  up  the 
respiratory  centre  to  increased  activity. 

On  examination  however  it  is  found  that  the  blood  leaving 
the  left  side  of  the  heart  in  such  cases,  is  not  less  arterialized  but 
if  anything  more  arterialized  than  usual.  The  increased  res- 
piratory movements  induced  by  the  changed  blood  soon  prove 
sufficient  or  even  more  than  sufficient  to  give  the  blood  the 
extra  quantity  of  oxygen  and  to  remove  the  extra  quantity  of 
carbonic  acid.  Obviously  the  blood  coming  from  the  tetanized 
muscles  affects  the  respiratory  centre  by  virtue  of  some  quality 
which,  unlike  that  due  to  the  deficiency  of  oxygen  or  excess  of 
carbonic  acid,  is  7iot  immediately  affected  by  the  passage  through 
the  lungs.  Whether  the  quality  in  question  be-  dependent  on 
an  excess  of  sarcolactic  acid,  or  on  some  other  product  or  prod- 
ucts of  muscular  metabolism,  we  do  not  as  yet  know.  But  the 
fact  that  substances  in  the  blood  may  so  affect  the  respiratory 


Chap,  ii.]  EESPIRATIOK  489 

centre  is  interesting  since  it  shews  by  how  many  safeguards  the 
working  of  the  respiratory  centre  is  carefully  adapted  to  the 
needs  of  the  economy. 

§  304.  Apnoea.  When  we  attempt  to  hold  our  breath,  we 
find  that  we  can  do  this  for  a  limited  time  only  ;  sooner  or 
later  a  breath  must  come  ;  but,  as  is  well  known,  the  time  dur- 
ing which  we  can  remain  without  breathing  may  on  occasion 
be  much  j)rolonged,  if  we  first  of  all  take  a  series  of  deep 
breaths.  The  breath  sooner  or  later  inevitably  follows  because 
at  last  the  natural  impulses  proceeding  from  the  respiratory 
centre  become  too  imperious  to  be  any  longer  held  in  check  by 
the  impulses  of  volition  passing  down  to  the  centre  from  the 
brain.  The  fact  that  a  series  of  deep  breaths,  a  thorough  ven- 
tilation of  the  lungs,  postpones  the  victory  of  the  unconscious 
centre,  shews  that  such  a  ventilation  in  some  way  delays  the 
development  of  the  natural  respiratory  impulses.  A  similar 
but  still  more  marked  delay  may  often  be  seen  in  an  animal 
under  artificial  respiration.  If  in  a  rabbit  artificial  respiration 
is  carried  on  very  vigorously  for  a  while,  and  then  suddenly 
stopped,  the  animal  does  not  immediately  begin  to  breathe. 
For  a  variable  period  no  respiratory  movements  at  all  take 
place,  and  breathing  when  it  does  begin  occurs  gently  and  nor- 
mally, only  passing  into  dyspnoea  if  the  animal  is  unable  to 
breathe  of  itself  ;  and  even  then  the  transition  is  quite  gradual. 
Evidently  during  this  period  the  respiratory  centre  is  in  a  state 
of  complete  rest,  no  explosions  are  taking  place,  no  respiratory 
impulses  are  being  generated,  and  the  quiet  transition  from  this 
condition  to  that  of  normal  respiration  shews  that  the  subse- 
quent generation  of  impulses  is  attended  by  no  great  disturb- 
ance. Not  only  is  the  centre  at  rest,  but  it  is  less  irritable 
than  the  normal  ;  impulses  along  the  vagus  or  other  nerves 
which  otherwise  would  produce  respiratory  explosions  are  now 
ineffectual.  This  state  of  things  is  known  as  that  of  apnoea^ 
the  converse  of  dyspnoea  ;  and  the  longer  pause  in  breathing 
mentioned  above  as  possible  after  unusual  ventilation  of  the 
lungs  may  be  regarded  as  a  brief  apnoea. 

Now  it  seemed  natural  to  suppose  that  such  a  state  of  rest 
of  the  respiratory  centre  was  brought  about  by  the  more  than 
necessarily  ample  supply  of  oxygen  afforded  by  the  previous 
increased  inspiratory  movements  ;  and  indeed  it  was  main- 
tained that  apnoea  was  the  result  of  too  great,  just  as  dysj^ncea 
is  the  result  of  too  little  arterialization  of  the  blood  reaching 
the  respiratory  centre.  It  was  argued  that  owing  to  the  in- 
creased vigour  of  the  artificial  respiratory  movements  the  haemo- 
globin of  the  arterial  blood,  which  in  normal  breathing  is  not 
quite  saturated  with  oxygen,  became  almost  completely  so,  and 
that  at  the  same  time  the  quantity  of  oxygen  simply  dissolved 
in  the  blood  became  largely  increased  and  its  tension  largely 


490  APNCEA.  [Book  ii. 

augmented.  But  there  are  reasons  which  render  such  a  view 
untenable.  In  the  first  place  there  is  no  direct  and  satisfactory 
proof  that  in  apncea  the  arterial  blood  is  overloaded  Avith  oxy- 
gen as  supposed  ;  indeed  during  the  course  of  apnoea  before 
it  has  come  to  an  end  the  blood  becomes  distinctly  less  arterial, 
more  venous  than  usual.  In  the  second  place  apnoea,  if  not 
entirely  impossible,  is  much  more  difficult  to  bring  about  when 
both  vagus  nerves  are  divided,  and  if  it  does  occur  after  sec- 
tion of  the  vagus  nerves  has  not  the  same  characters  as  ordinary 
apnoea.  Now,  when  artificial  respiration  is  being  carried  on 
section  of  the  vagus  nerves  can  have  no  effect  on  the  quantity 
of  oxygen  taken  up  by  the  blood  in  the  lungs.  But  the  vagus 
nerves  are  the  channel  of  impulses  affecting  the  respiratory 
centre,  and  this  relation  of  the  apnoea  to  the  vagus  nerves  sug- 
gests another  and  different  interpretation  of  apnoea.  As  we 
have  seen,  expansion  of  the  lung  by  acting  in  some  way  or 
other  on  the  pulmonary  terminations  of  the  vagus  nerve  sends 
up  along  that  nerve  impulses  which  inhibit  inspiration.  And 
it  is  argued  that  repeated  forcible  inflations  of  the  lungs  pro- 
duce apnoea  by  generating  potent  inhibitory  impulses,  which  by 
a  kind  of  summation  of  their  effects  in  the  spinal  bulb  stop 
for  a  while  the  generation  of  respiratory  impulses  in  the  respira- 
tory centre.  This  conclusion  moreover  is  strongly  suj)ported 
by  the  fact  that  an  apnoea  may  be  produced,  so  long  as  the 
vagus  nerves  are  intact,  by  forcible  artificial  respiration  with 
hydrogen  instead  of  atmospheric  air  ;  in  other  words,  the  in- 
hibitory impulses  generated  in  the  vagus  nerves  by  the  inflation 
are  sufficient  wholly  to  neutralize  the  development  of  respira- 
tory impulses  which  the  deficient  arterialization  of  the  blood 
would  otherwise  have  produced. 

§305.  Secondary  Respiratory  Rhythm.  Cheyne-Stohes  Res- 
piration. A  remarkable  abnormal  rhythm  of  respiration,  first 
observed  by  Cheyne  but  afterwards  more  fully  studied  by 
Stokes,  and  hence  called  by  their  combined  names,  occurs  in 
certain  pathological  cases.  The  respiratory  movements  grad- 
ually decrease  both  in  extent  and  rapidity  until  they  cease 
altogether,  and  a  condition  of  apncea,  lasting  it  may  be  for  sev- 
eral seconds,  ensues.  This  is  followed  by  a  feeble  respiration, 
succeeded  in  turn  by  a  somewhat  stronger  one,  and  thus  the 
respiration  returns  gradually  to  the  normal,  or  may  even  rise 
to  hyperpnoea  or  slight  dyspnoea,  after  which  it  again  declines 
in  a  similar  manner.  A  secondary  rhythm  of  respiration  is 
thus  developed,  periods  of  normal  or  slightly  dyspnoeic  respira- 
tion alternating  by  gradual  transitions  with  periods  of  apnoea. 
The  cause  of  the  phenomena  is  not  thoroughly  understood. 


SEC.   7.     THE    EFFECTS    OF   CHANGES    IN   THE   COMPO- 
SITION  AND   PRESSURE  OF   THE  AIR   BREATHED. 


§  306.  The  jDrecediiig  sections  have  shewn  us  that  the  res- 
piratory mechanism  is  arranged  to  work  satisfactorily  when 
the  lungs  are  adequately  supplied  with  air  of  the  ordinary  com- 
j)osition  of,  and  at  the  ordinar}-  pressure  of  the  atmosphere. 
We  have  further  seen  that  the  mechanism  can  adapt  itself 
within  certain  limits  to  changes  in  the  composition  and  pressure 
of  the  air  supplied.  We  may  now  consider  briefly  what  takes 
place  when  those  limits  are  overstepped.  The  most  striking 
effects  are  seen,  when,  on  account  of  occlusion  of  the  trachea, 
or  by  breathing  in  a  confined  space,  or  for  other  reasons,  a  due 
supply  of  air  not  being  obtained,  normal  respiration  gives 
place,  through  an  intermediate  phase  of  dyspnoea,  to  the  condi- 
tion known  as  asphyxia ;  this,  unless  remedial  measures  be 
taken,  rapidly  proves  fatal. 

Asphyxia.  As  soon  as  the  blood  becomes  less  arterial,  more 
venous  than  normal,  the  respiratory  movements  become  deeper 
and  at  the  same  time  more  frequent ;  both  the  inspiratory  and 
expiratory  phases  are  exaggerated,  the  su^jplementary  muscles 
spoken  of  §  265  are  brought  into  play,  and  the  rate  of  the 
rhythm  is  hurried.  These  effects,  as  we  have  seen,  are  chiefly 
to  be  ascribed  to  the  deficiency  of  oxygen  in  the  blood. 

As  tlie  blood  continues  to  become  more  and  more  venous  the 
respiratory  movements  continue  to  increase  both  in  force  and 
frequency,  a  larger  number  of  muscles  being  called  into  action 
and  that  to  an  increasing  extent.  Very  soon,  however,  it  may 
be  observed  that  the  expiratory  movements  are  becoming  more 
marked  than  the  inspiratory.  Every  muscle  which  can  in  any 
way  assist  in  exj^iration  is  in  turn  brought  into  play  ;  and  at 
last  almost  all  the  muscles  of  tlie  body  are  involved  in  the 
struggle.  The  orderly  expiratory  movements  culminate  in 
expiratory  convulsions,  the  order  and  sequence  of  which  are 
obscured  by  their  violence  and  extent.  That  these  convulsions, 
through  which  dyspnoea  merges  into  asphyxia,  are  due  to  a 
stimulation  (by  the  venous  blood)  of  the  spinal  l)ulb,  is  proved 

401 


492  ASPHYXIA.  [Book  ii. 

by  the  fact  tliat  they  fail  to  make  tlieir  appearance  when  the 
spinal  cord  has  been  previously  divided  below  the  bulb,  though 
they  still  occur  after  those  portions  of  the  brain  which  lie  above 
the  bulb  have  been  removed.  It  is  usual  to  speak  of  a  '  con- 
vulsive centre '  in  the  bulb,  the  stimulation  of  which  gives  rise 
to  these  convulsions ;  but  if  we  accept  the  existence  of  such  a 
centre  we  must  at  the  same  time  admit  that  it  is  connected  by 
the  closest  ties  with  the  normal  expiratory  division  of  the  res- 
piratory centre,  since  every  intervening  step  may  be  observed 
between  a  simple  slight  expiratory  movement  of  normal  respira- 
tion and  the  most  violent  convulsion  of  asphyxia.  An  addi- 
tional proof  that  these  convulsions  are  carried  out  by  the 
agency  of  the  bulb  is  afforded  by  the  fact  that  convulsions  of  a. 
wholly  similar  character  are  witnessed  when  the  supply  of  blood 
to  the  bulb  is  suddenly  cut  oft  by  ligaturing  the  blood  vessels 
of  the  head.  In  this  case  the  nervous  centres,  being  no  longer 
furnished  with  fresh  blood,  become  rapidly  asphyxiated  through 
lack  of  oxygen,  and  expiratory  convulsions  quite  similar  to 
those  of  ordinary  asphyxia,  and  preceded  like  them  by  a  pass- 
ing phase  of  dyspnoea,  make  their  appearance.  Similar  '  anaemic  ^ 
convulsions  are  seen  after  a  sudden  and  large  loss  of  blood  from 
the  body  at  large,  the  bulb  being  similarly  stimulated  by  the 
lack  of  arterial  blood.  In  ordinary  fainting,  which  is  loss  of 
consciousness  due  to  an  insufficient  supply  of  blood  to  the  brain^ 
the  diminution  of  blood  supply  is  not  great  enough  to  produce 
these  convulsions. 

Such  violent  efforts  speedily  exhaust  the  nervous  system  ; 
and  the  convulsions  after  being  maintained  for  a  brief  period 
suddenly  cease  and  are  followed  by  a  period  of  calm.  The  calm 
is  one  of  exhaustion  ;  the  pupils,  dilated  to  the  utmost,  are 
unaffected  by  light ;  touching  the  cornea  calls  forth  no  move- 
ment of  the  eyelids,  and  indeed  no  reflex  actions  can  anywhere 
be  produced  by  the  stimulation  of  sentient  surfaces.  All  expi- 
ratory active  movements  have  ceased ;  the  muscles  of  the  body 
are  flaccid  and  quiet ;  and  though  from  time  to  time  the  respir- 
atory centre  gathers  sufficient  energy  to  develop  respiratory 
movements,  these  resemble  those  of  quiet  normal  breathing,  in 
being,  as  far  as  muscular  actions  are  concerned,  almost  entirely 
inspiratory.  They  occur  at  long  intervals,  like  those  after  sec- 
tion of  the  vagi  ;  and  like  them  are  deep  and  slow.  The 
exhausted  respiratory  centre  takes  some  time  to  develop  an 
inspiratory  explosion  ;  but  the  impulse  when  it  is  generated  ivS- 
proportionately  strong.  It  seems  as  if  the  resistance  which  had 
in  each  case  to  be  overcome  was  considerable,  and  the  effort  in 
consequence,  when  successful,  productive  of  a  large  effect. 

Very  soon,  these  inspiratory  efforts  become  less  frequent ; 
their  rhythm  becomes  irregular ;  long  pauses,  each  one  of  which 
seems  a  final  one,  are  succeeded  by  several  somewhat  rapidly 


Chap,  ii.]  RESPIRATIOK  493 

repeated  inspirations.  The  pauses  become  longer,  and  the 
inspiratory  movements  shallower.  Each  inspiration  is  accom- 
panied by  the  contraction  of  accessory  muscles,  especially  of 
the  face,  so  that  each  breath  becomes  more  and  more  a  pro- 
longed gasp.  The  inspiratory  gasps  spread  into  a  convulsive 
stretching  of  the  whole  body  ;  and  with  extended  limbs,  and 
a  straightened  trunk,  with  the  head  thrown  back,  the  mouth 
widely  open,  the  face  drawn,  and  the  nostrils  dilated,  the  last 
breatli  is  taken  in. 

'i'lius  we  are  able  to  distinguish  three  stages  in  the  phe- 
nomena which  result  from  a  continued  deficiency  of  air :  (1)  A 
stage  of  dyspnoea,  characterized  by  an  increase  of  the  respira- 
tory movements  both  of  inspiration  and  expiration.  (2)  A  con- 
vulsive stage,  characterized  by  the  dominance  of  the  expira- 
tory efforts,  and  culminating  in  general  convulsions.  (8)  A 
stage  of  exhaustion,  in  which  lingering  and  long-drawn  inspira- 
tions gradually  die  out.  When  brought  about  by  sudden  occlu- 
sion of  the  trachea  these  events  run  through  their  course  in 
about  4  or  5  minutes  in  the  dog,  and  in  about  3  or  4  minutes 
in  the  rabbit.  The  first  stage  passes  gradually  into  the  second, 
convulsions  appearing  at  the  end  of  the  first  minute.  The 
transition  from  the  second  stage  to  the  third  is  somewhat 
abrupt,  the  convulsions  suddenly  ceasing  early  in  the  second 
minute.     The  remaining  time  is  occuj)ied  in  the  third  stage. 

The  duration  of  asphyxia  varies  not  only  in  different  animals 
but  in  the  same  animal  under  different  circumstances.  Newly 
born  and  young  animals  need  much  longer  immersion  in  Avater 
before  death  by  asphyxia  occurs  than  do  adults.  Thus  while 
in  a  full-grown  dog  recovery  from  drowning  is  unusual  after 
1|-  minutes,  a  ncAv-born  puppy  lias  been  known  to  bear  an 
immersion  of  as  much  as  50  minutes.  The  cause  of  the  differ- 
ence lies  in  the  fact  that  in  the  quite  young  or  rather  just  born 
animal  the  respiratory  changes  of  the  tissues  are  much  less 
active.  These  consume  less  oxygen,  and  the  general  store  of 
oxygen  in  the  blood  has  a  less  rapid  demand  made  upon  it. 
The  respiratory  activity  of  the  tissues  may  also  be  lessened  by 
a  deficiency  in  the  circulation ;  hence  bodies  in  a  state  of  syn- 
cope at  the  time  when  the  deprivation  of  oxygen  begins  can 
endure  the  loss  for  a  much  longer  period  than  can  bodies  in 
whicli  the  circulation  is  in  full  swing.  There  being  the  same 
store  of  oxygen  in  the  blood  in  each  case,  the  quicker  circula- 
tion must  of  necessity  bring  about  the  speedier  exhaustion  of 
the  store.  So  also  amesthetics  may  diminish  the  effects  and 
delay  the  final  results ;  large  doses  of  anivsthetics  may  prevent 
the  exaggerated  and  convulsive  movements.  In  many  cases  of 
drowning,  death  is  hastened  by  the  entrance  of  water  into  the 
lungs. 

By  training,  the  respiratory  centre  may  be  accustomed    to 


494      EFFECTS  OF  DIMINUTION  OF  PEESSUEE.  [Book  ii. 

bear  a  scanty  supply  of  oxygen  for  a  much  longer  time  than 
usual  before  dyspnoea  sets  in,  as  is  seen  in  the  case  of  divers. 

The  phenomena  of  slow  asphyxia,  where  the  supply  of  air  is 
gradually  diminished,  are  fundamentally  the  same  as  those  result- 
ing from  a  sudden  and  total  deprivation.  The  same  stages  are 
seen,  but  their  development  takes  place  more  slowly. 

§  307.  The  composition  of  the  atmosphere,  the  pressure  re- 
maining the  same,  may  be  modified  by  the  introduction  of  foreign 
gases.  To  some  of  these  the  respiratory  mechanism  is  indiffer- 
ent ;  for  instance,  hydrogen  may  be  substituted  for  nitrogen 
without  any  change  in  the  respiration,  provided  of  course  that  the 
oxygen  is  not  diminished.  Other  gases  may  produce  poisonous 
effects,  either  by  interfering  with  some  of  the  respiratory  pro- 
cesses or  in  other  ways.  Thus  carbon  monoxide,  by  combining 
with  the  hsemoglobin  of  the  red  corpuscles,  and  so  preventing 
the  corpuscles  from  acting  as  oxygen-carriers,  produces  asphyxia 
through  deficiency  of  oxygen.  Sulphurettecl  hydrogen  inter- 
feres with  the  oxygenation  of  the  blood  by  acting  as  a  reducing 
agent.  Some  gases  while  allowing  the  ordinary  respiratory 
changes  of  the  blood  to  go  on  as  usual  produce  toxic  effect  by 
acting  on  one  or  other  of  the  tissues.  Thus,  as  we  have  seen, 
an  excess  of  carbonic  acid  in  the  blood  seems  to  have  a  special 
effect  on  the  central  nervous  system  and  so  acts  as  a  narcotic 
poison.  The  peculiar  effects  of  nitrous  oxide  (laughing  gas) 
are  similarly  due  to  the  direct  action  of  the  gas  in  the  blood  on 
the  central  nervous  system.  Some  gases  are  irrespirable  and 
may  interfere  with  respiration,  even  causing  suffocation,  on 
account  of  their  causing  spasm  of  the  glottis,  and  this  is  said 
to  be,  to  a  certain  extent,  the  case  with  an  atmosphere  which  is 
wholly  or  largely  composed  of  carbonic  acid. 

§  308.  The  Effects  of  Changes  in  Atmospheric  Pressure. 
Diminution  of  pressure.  The  partial  pressure  of  the  oxygen  in 
the  inspired  air  may  be  changed,  not  only  by  altering  the  com- 
position of  the  air  entering  at  the  ordinary  atmospheric  pressure, 
but  also  by  altering  the  total  pressure  of  the  atmosphere  without 
changing  its  composition.  The  results  of  the  latter  are  however 
complicated  ;  we  have  then  to  deal  not  merely  with  the  effects 
on  the  interchange  of  gases  in  the  lungs  but  with  the  effects  on 
the  whole  organism.  AH  the  complicated  machinery  of  the  body 
is  adapted  and  arranged  to  work  under  what  we  may  call  ordi- 
nary atmospheric  pressure,  that  is  to  say,  within  the  limits  of  760 
mm.  mercury  at  the  sea  level  and  about  500  mm.,  correspond- 
ing to  an  altitude  of  6000  feet,  this  being  the  range  of  ordinary 
human  dwellings.  Any  great  increase  or  decrease  of  pressure 
beyond  these  limits  will  affect  not  only  the  exit  of  carbonic  acid 
from  and  the  entrance  of  oxygen  into  the  blood,  but,  in  varying 
degree,  all  the  physical  and  chemical  processes  of  the  body.  A 
gross  instance  of  this  is  seen  when  an  animal  is  suddenly  sub- 


CiiAP.  II.]  EESPIEATIOK  495 

jectecl  to  a  great  diminution  of  pressure,  as  when  it  is  placed  in 
the  receiver  of  an  air-pump  and  the  receiver  rapidly  exhausted. 
The  animal  is  soon  thrown  into  fatal  convulsions,  which  are  in 
part,  but  only  in  part,  due  to  the  liberation  of  gas  from  the  blood 
within  the  blood  vessels  ;  the  gas  so  set  free  mechanically  inter- 
feres with  the  circulation,  as  by  obstructing  the  play  of  the  car- 
diac valves,  or  by  plugging  the  smaller  blood  vessels,  and  thus 
helps  to  bring  the  machine  to  a  standstill.  The  free  gas  found 
in  the  vessels  upon  examination  after  death  is  said  to  be  com- 
posed chiefly  of  nitrogen,  the  carbonic  acid  and  the  oxygen, 
which  probably  were  also  set  free,  having  been  reabsorbed  before 
the  examination  was  made. 

But,  quite  apart  from  gross  effects  of  this  kind,  it  is  very 
obvious  that  the  organism  must  in  many  ways  suffer  from  a 
diminution  of  pressure.  The  complex  and  delicately  balanced 
vascular  system  is  constructed  to  work  at  the  ordinary  atmos- 
pheric pressure.  The  force  of  the  heart-beat  and  the  tonic 
contraction  of  the  small  arteries  are,  so  to  speak,  pitched  to  meet 
the  influence  exerted  on  the  outside  of  the  blood  vessels  by  the 
ordinary  pressure  of  the  atmosphere  ;  and  any  great  diminution 
of  that  pressure  must  produce  a  greater  or  less  disarrangement 
of  the  vascular  mechanism  until  it  is  counterbalanced  by  some 
compensating  changes.  And  a  little  reflection  will  supply  many 
other  instances. 

We  have  already  called  attention  (§  285)  to  the  fact  that,  the 
total  pressure  of  the  atmosphere  remaining  the  same,  the  partial 
pressure  of  the  oxygen  in  the  inspired  air  may  be  reduced  as  low 
as  about  76  mm.  (10  p.c.)  without  seriously  modifying  the 
respiration.  In  order  to  attain  this  diminution  of  the  partial 
pressure  of  the  oxygen  without  changing  the  composition  of  the 
atmosphere,  the  total  pressure  of  the  atmosphere  must  be  reduced 
to  the  limit  of  300  mm.,  corresponding  to  an  altitude  of  17,000 
feet.  Now  it  is  a  matter  of  common  experience  that  in  ascend- 
ing a  mountain  "  distress  "  is  felt  long  before  such  an  altitude 
is  reached.  The  distress  felt  on  such  occasions  is  probably  due 
not  so  much,  if  indeed  at  all  directly,  to  the  diminution  of  oxygen 
as  to  a  general  disarrangement  of  the  organism  and  perhaps  more 
particularly  of  the  vascular  system.  The  nose-bleeding  which  is 
so  frequent  an  occurrence  under  the  circumstances  shews  that 
the  minute  blood  vessels  more  directly  exposed  to  the  diminu- 
tion of  pressure  are  profoundly  affected  by  it ;  and  what  is  true 
of  them  is,  probably,  in  various  ways  and  to  different  degrees 
true  of  the  whole  vascular  system.  The  breathlessness  which  is 
so  marked  a  feature  on  these  occasions  seems  due  not  so  much 
to  the  fact  that  the  blood  which  reaches  the  respiratory  nervous 
centres  is  deficient  in  oxygen,  as  to  the  fact  that  the  troubled 
vascular  system  fails  to  deliver  to  those  centres  their  blood  in 
an  adequate  fashion. 


496        EFFECTS   OF   INCREASE   OF  PRESSURE.    [Book  ii. 

§  309.  The  Effects  of  Increase  of  Atmospheric  Pressure. 
These  are  in  many  ways  remarkable.  Up  to  a  pressure  of  sev- 
eral atmospheres  of  air,  the  only  symptoms  which  present  them- 
selves are  those  somewhat  resembling  narcotic  poisoning.  The 
animal  becomes  sleepy  and  stupid,  the  result  probably  not  so 
much  of  respiratory  changes,  as  of  the  effects  of  the  increased 
pressure  on  the  whole  organism  to  which  we  have  just  alluded. 
At  a  pressure  however  of  15  atmospheres  of  air,  or  what  amounts 
to  the  same  thing,  of  3  atmospheres  of  oxygen,  and  upwards,  a 
very  remarkable  phenomenon  presents  itself.  The  animals  die 
of  asphyxia  and  convulsions,  exactly  in  the  same  way  as  when 
oxygen  is  deficient.  Corresponding  with  this  it  is  found  that 
the  production  of  carbonic  acid  is  diminished.  That  is  to  say, 
when  the  pressure  of  the  oxygen  is  increased  beyond  a  certain 
limit,  the  oxidations  of  the  body  are  diminished,  and  with  a  still 
further  increase  of  the  oxygen  are  arrested  altogether.  The 
oxidation  of  phosphorus  is  perhaps  analogous  ;  at  a  high  pres- 
sure of  oxygen  phosphorus  will  not  burn.  Not  only  animals 
but  plants,  bacteria,  and  organized  ferments,  are  similarly  killed 
by  a  too  great  pressure  of  oxygen. 


SEC.  8.     THE    RELATIONS    OF   THE   RESPIRATORY 
SYSTEM  TO   THE   VASCULAR   AND   OTHER   SYSTEMS. 

§  310.  Many  events  in  the  body  shew  the  influence  which 
the  respiratoiy  movements  exert  on  the  circuhition.  When  the 
brain  of  a  living  mammal  is  exposed  by  the  removal  of  the  skull, 
a  rhythmic  rise  and  fall  of  the  cerebral  mass,  a  pulsation  of  the 
brain,  quite  distinct  from  the  movements  caused  by  the  pulse  in 
the  arteries  of  the  brain,  is  observed  ;  and  upon  examination  it 
will  be  found  that  these  movements  are  synchronous  with  the 
respiratory  movements,  the  brain  rising  up  during  ex2:)iration 
and  sinking  during  inspiration.  They  disappear  when  the  arte- 
ries going  to  the  brain  are  ligatured,  or  when  the  venous  sinuses 
of  the  dura  mater  are  laid  open  so  as  to  admit  of  a  free  escape 
of  the  venous  blood.  They  evidently  arise  from  the  expiratory 
movements  in  some  way  hindering  and  the  inspiratory  move- 
ments assisting  the  return  of  blood  from  the  brain.  We  have 
already  (§  98)  stated  that  during  inspiration  the  pressure  of 
])lood  in  the  great  veins  may  become  negative,  i.e.  may  sink 
below  the  pressure  of  the  atmosphere  ;  and  a  puncture  of  one 
of  these  veins  may  cause  death  by  air  being  actually  drawn  into 
the  vein  and  thus  into  the  heart  during  an  insi^iratory  move- 
ment. When  the  veins  of  an  animal  are  laid  bare  in  the  neck 
and  watched,  the  so-called  2?ulsiis  venosus  may  be  observed  in 
them,  that  is,  they  swell  up  during  expiration  and  diminisli  again 
during  inspiration.  And  indeed  a  little  consideration  will  shew 
that  the  expansion  and  contraction  of  the  chest  must  have  a 
decided  effect  on  the  flow  of  blood  tln-ough  the  thoracic  portion 
of,  and  thus  indirectly  on  that  through  the  whole  of,  the  vas- 
cular system. 

This  is  well  illustrated  by  the  effects  of  respiration  on  arte- 
rial blood-i)ressure.  We  have  seen,  while  treating  of  the  circu- 
lation, that  tlie  arterial  blood-pressure  curves  are  marketl  by 
undulations,  which,  since  their  rhytlim  is  synchronous  with  that 
of  the  respiratory  movements,  are  evidently  in  some  way  con- 
nected Avith  respiration.  Similar  undulations  may  be  observed 
in  the  pulse  tracings  taken  from  man. 
;]2  497 


498 


RESPIRATOEY   UNDULATIONS.  [Book  ii. 


Fig.  99. 


Comparison  of  Blood-Pressure  Curve  avith  Curve  of 
Intra-thoracic  Pressure.     (Dog.) 


a  is  the  blood-pressure  curve  taken  by  means  of  a  mercury  manometer ;  it 
shews  the  respiratory  undulations,  the  slower  beats  on  the  descent  being  very 
marked.  6  is  the  curve  of  intra-thoracic  pressure  obtained  by  connecting  one 
limb  of  a  manometer  with  the  pleural  cavity.  Inspiration  begins  at  z,  expiration 
at  e.  With  the  beginning  of  inspiration  {%)  the  expansion  of  the  chest  causes  a 
marked  fall  of  the  mercury  in  the  intra-thoracic  manometer ;  but  the  effect  soon 
diminishes,  since  the  lessening  of  intra-thoracic  pressure  does  not  bear  on  the 
manometer  alone  but  on  the  lungs  also  ;  and  as  the  lungs  expand  more  and  more 
the  fall  in  the  mercury  becomes  less  and  less  until  towards  the  end  of  inspiration 
the  curve  becomes  very  nearly  a  straight  line.  Conversely,  the  return  of  the 
chest  at  the  beginning  of  expiration  (e)  produces  at  first  a  marked  rise  of  the 
mercury  in  the  manometer  ;  but  this  soon  ceases  as  the  air  leaves  the  chest  and 
the  lungs  shrink,  whereupon  the  mercury  falls  slowly. 


When  these  undulations  of  the  blood-pressure  curve  are 
compared  carefully  with  the  respiratory  movements  or  with  the 
variations  of  intra-thoracic  pressure,  what  is  most  commonly 
observed  is  that  while  the  blood-pressure,  on  the  whole,  rises 
during  inspiration  and  falls  during  expiration  neither  the  rise 
nor  the  fall  is  exactly  synchronous  with  either  inspiration  or 
expiration.  Fig.  99  shews  two  tracings  from  a  dog  taken  at 
the  same  time,  one,  a,  being  the  ordinary  blood-pressure  curve 
from  the  carotid,  and  the  other,  5,  representing  the  condition  of 
the  intra-thoracic  pressure  as  obtained  by  carefully  bringing  a 
manometer  into  connection  with  the  pleural  cavity.  On  com- 
paring the  two  curves  it  is  evident  that  neither  the  rise  nor  the 
fall  of  arterial  pressure  coincides  exactly  either  with  inspiration 
or  with  expiration.  At  the  beginning  of  inspiration  (%)  the 
arterial  pressure  is  seen  to  be  falling  ;  it  soon  however  begins 
to  rise,  but  does  not  reach  the  maximum  until  some  time  after 
expiration  (e)  has  begun ;  the  fall  continues  during  the 
remainder  of  expiration,  and  passes  on  into  the  succeeding 
inspiration.  This  suggests  the  idea  that,  while  inspiration 
tends  to  increase  and  expiration  to  diminish  the  blood-pressure, 
there  are  causes  at  work  which  in  each  case  delay  the  effect. 

Extended  observations  however  shew  that  such  a  relation  as 


CiiAP.  II.]  RESPIRATIOX.  499 

that  shewn  in  tlie  figure  though  frequent  is  not  constant.  In 
fact  the  effects  of  the  respiratory  movements  on  blood-pressure 
are  found  to  vary  very  widely  according  as  the  respiration  is 
quick  or  slow,  easy  and  shallow,  or  laboured  and  deep,  and 
especially  as  the  air  enters  into  the  chest  readily  or  with  diffi- 
culty. Moreover,  respiratory  undulations  of  blood-pressure  are 
seen  not  only  with  natural  but  also  with  artificial  respiration  ; 
in  the  latter  the  mechanical  conditions  are  to  a  large  extent  the 
reverse  of  those  of  the  former,  and  might  fairly  be  expected  to 
affect  the  circulation  in  a  different  way.  The  causation  of  these 
respiratory  undulations  is  in  fact  complex.  The  respiratory  act 
affects  the  vascular  system  in  several  different  ways,  and  the 
general  effect  varies  according  as  one  or  other  influence  is  pre- 
dominant. These  several  actions  are  sufficiently  interesting 
and  important  to  deserve  discussion. 

§  311.  The  heart  and  great  blood  vessels  are,  like  the  lungs, 
placed  in  the  air-tiglit  thoracic  cavity,  and  are  subject  like  the 
lungs  to  the  pumping  action  of  the  respiratory  movements. 
Were  there  no  lungs  present  in  the  chest,  the  whole  force  of  the 
expansion  of  the  thorax  in  inspiration  would  be  directed  to 
drawing  blood  from  the  extra-thoracic  vessels  towards  the  heart, 
and  conversely  in  expiration  the  effect  of  the  return  of  the  thorax 
to  its  previous  dimensions  would  be  to  drive  the  blood  thus 
drawn  in  back  again  from  the  heart  towards  the  extra-thoracic 
vessels.  And,  even  in  the  presence  of  the  lungs,  some  of  this 
effect  is  still  felt.  The  main  purpose  and  the  main  result  of  the 
expansion  of  the  chest  in  inspiration  is  of  course  to  draw  air  into 
the  lungs  ;  by  that  expansion  the  air  in  the  pulmonary  alveoli 
is  rarified  and  brought  to  a  lower  pressure  than  that  of  the 
atmosphere  outside  the  chest ;  and  the  difference  of  pressure 
tlius  set  up  leads  to  an  inrush  of  inspired  air  until  an  equilib- 
rium of  pressure  is  established  between  the  air  in  the  lungs  and 
that  outside  the  chest.  Before  however  the  inspired  air  can 
fill  a  pulmonary  alveolus  the  elastic  walls  of  the  alveolus  have 
to  be  distended,  and  that  distension  is  effected  by  means  of  the 
pressure  which  causes  the  inspired  air  to  enter.  Part  of  the 
atmospheric  pressure  in  fact  which  causes  the  entrance  of  the  air 
into  the  lung  is  spent  in  overcoming  the  elasticity  of  the  pul- 
monary jjassages  and  cells.  So  that  while  by  the  inrush  of 
inspired  air  the  difference  of  pressure  between  the  air  inside  the 
pulmonary  alveoli  and  that  outside  the  chest,  brouglit  about  by 
the  thoracic  expansion,  is  completely  neutralized,  the  difference 
between  the  pressure  to  which  tlie  parts  lying  witliin  the  thorax 
but  outside  the  lungs  are  exposed  and  that  outside  the  chest  is 
not  so  completely  neutralized.  The  pressure  on  these  parts 
always  falls  short  of  the  pressure  of  the  atmosphere  by  the 
amount  of  pressure  necessary  to  counterbalance  the  elasticity  of 
the  pulmonary  passages  and  alveoli.     Consequently,  an}'  struct- 


500  NEGATIVE   PRESSURE  IN   THORAX.       [Book  ii. 

ure  lying  within  the  thorax  but  outside  the  lungs,  is  never,  even 
at  the  conclusion  of  an  inspiration  when  the  lungs  are  filled  with 
air,  subject  to  a  pressure  as  great  as  that  of  the  atmosphere. 
And,  since  the  fraction  of  the  atmospheric  pressure  which  is  thus 
spent  in  distending  the  lungs  increases  as  the  lungs  become  more 
and  more  stretched,  it  follows  that  the  fuller  the  inspiration  the 
greater  is  the  difference  between  the  pressure  on  structures 
within  the  thorax  but  outside  the  lungs  and  the  ordinary  pres- 
sure of  the  atmosphere.  Now  we  have  seen  that  the  pressure 
necessary  to  counterbalance  the  elasticity  of  the  lungs,  when 
they  are  completely  at  rest  (in  the  pause  between  expiration  and 
inspiration),  is  in  man  about  5  to  7  mm.  of  mercury,  and  that 
when  the  lungs  are  fully  distended,  as  at  the  end  of  a  forcible 
inspiration,  the  pressure  rises  to  as  much  as  30  mm.  of  mercury. 
Hence  at  the  height  of  a  forcible  inspiration  the  pressure  exerted 
on  the  heart  and  great  vessels  within  the  thorax  is  30  mm.  less 
than  the  ordinary  atmospheric  pressure  of  760  mm.,  and  even 
when  the  chest  is  completely  at  rest,  at  the  end  of  an  expiration, 
the  pressure  on  the  heart  and  great  vessels  is  slightly  (by  about 
5  mm.  mercury)  below  that  of  the  atmosphere.  We  may  add 
that  any  obstacle  to  the  free  ingress  of  the  inspired  air,  any  diffi- 
culty in  the  full  expansion  of  the  pulmonary  alveoli,  of  course 
increases  the  negative  pressure  to  which  the  thoracic  structures 
outside  the  lungs  are  subjected  by  the  expansion  of  the  chest. 
Hence  when  the  trachea  is  closed  a  very  large  part  of  the  tho- 
racic expansion  is  directed  to  increasing  the  negative  pressure 
around  the  heart  and  great  blood  vessels. 

During  an  inspiration  then  the  pressure  around  the  heart  and 
great  blood  vessels  becomes  considerably  less  than  that  of  the 
atmosphere  on  the  vessels  outside  the  thorax.  During  expira- 
tion this  pressure  returns  towards  that  of  the  atmosphere,  but  in 
ordinary  breathing  never  quite  reaches  it.  It  is  only  in  forcible 
expiration  that  the  pressure  on  the  thoracic  vascular  organs 
reaches  or  exceeds  that  of  the  atmosphere.  But  if  during  inspi- 
ration the  pressure  bearing  on  the  right  auricle  and  the  vena3 
cavse  becomes  less  than  the  pressure  which  is  bearing  on  the 
jugular,  subclavian,  and  other  veins  outside  the  thorax,  this 
must  result  in  an  increased  flow  from  the  latter  into  the  former. 
Hence  during  each  inspiration  a  larger  quantity  of  blood  enters 
the  right  side  of  the  heart.  This  probably  leads  to  a  stronger 
stroke  of  the  heart,  and  at  all  events  causes  a  larger  quantity  to 
be  ejected  by  the  right  ventricle  ;  this  causes  a  larger  quantity 
to  escape  from  the  left  ventricle,  and  thus  more  blood  is  thrown 
into  the  aorta,  and  the  arterial  pressure  proportionately  increased. 
Daring  expiration  the  converse  takes  place.  The  pressure  on 
the  intra-thoracic  blood  vessels  returns  to  the  normal,  the  flow 
of  blood  from  the  veins  outside  the  thorax  into  the  vense  cavse 
and  right  auricle  is  no  longer  assisted,  and  in  consequence  less 


Chap,  ii.]  RESPIRATION.  501 

blood  passes  through  the  heart  into  the  aorta,  and  arterial  pres- 
sure falls  again.  During  forced  expiration,  the  intra-thoracic 
pressure  may  be  so  great  as  to  afford  a  distinct  obstacle  to  the 
flow  from  the  veins  into  the  heart. 

The  effect  of  the  respiratory  movements  on  the  arteries  is 
naturally  different  from  that  on  the  veins.  During  inspiration 
the  diminution  of  pressure  in  the  thorax  around  the  aortic  arch 
tends  to  expand  the  aortic  arch,  and  thus  to  check  the  onward 
flow  of  blood,  and  to  diminish  the  pressure  of  blood  Avithin  the 
aorta.  During  expiration,  the  increase  of  pressure  outside  the 
aortic  arch  of  course  tends  to  increase  also  the  blood-pressure 
within  the  aorta,  acting  in  fact  just  in  the  same  way  as  if  the 
coats  of  the  aorta  themselves  contracted.  Thus  as  far  as  arterial 
blood-pressure  is  concerned  the  effects  of  the  respiratory  move- 
ments on  the  great  veins  and  great  arteries  respectively  are 
antagonistic  to  each  other;  the  effect  on  the  brains  being  to  in- 
crease arterial  pressure  during  inspiration  and  to  diminish  it 
during  expiration,  while  the  effect  on  the  arteries  is  to  diminish 
arterial  pressure  during  inspiration  and  to  increase  it  during 
expiration.  But  we  should  naturally  expect  the  effect  on  the 
thin-walled  veins  to  be  greater  than  that  on  the  stout  thick- 
walled  arteries,  so  much  so  that  the  direct  effect  on  the  arteries 
may  be  neglected.  That  is  to  say,  we  should  expect  the  blood- 
pressure  to  rise  during  inspiration  and  to  fall  during  expiration. 
This  as  we  have  seen  is  frequently  the  case,  and  indeed  when 
the  breathing  is  deep  and  laboured,  and  especially  during  violent 
and  sudden  respiratory  movements,  the  influence  in  this  direc- 
tion on  the  blood-pressure  curve  of  the  pumping  action  of  the 
chest  is  unmistakeable. 

In  attempting  however  to  estimate  the  effect  of  the  resjiira- 
tory  movements  on  blood-pressure  we  must  bear  in  mind  what 
is  taking  place  in  the  abdomen.  In  inspiration  the  descent  of 
the  diaphragm  compresses  the  abdominal  viscera,  and  so,  while 
at  the  very  first  it  drives  a  quantity  of  blood  onward  along  the 
inferior  vena  cava,  subsequently  hinders  the  upward  flow  from 
the  abdomen  and  lower  limbs;  at  the  same  time  by  compressing 
the  abdominal  aorta,  it  tends  to  raise  the  pressure  in  the  thoracic 
aorta  and  its  branches,  while  lowering  that  of  the  abdominal 
aorta  and  its  branches.  The  effect  of  easy  expiration  would  be 
the  converse  of  this;  but  in  forced  expiration  the  pressure  of 
the  contracting  abdominal  muscles  would,  as  an  inspiration,  flrst 
tend  to  drive  the  blood  onward  along  the  vena  cava  but  subse- 
quently to  hinder  the  flow  both  along  the  vena  cava  and  the 
aorta.  The  effect  of  the  abdominal  movements  therefore  is 
mixed  and  variable,  and  their  influence  on  the  blood-pressure 
in  the  femoral  artery  must  be  different  from  that  on  the  radial 
artery  or  other  branch  of  the  thoracic  aorta.  It  is  difficult  to 
predict  what  in  all  cases  the  effect  would  be;  and  the  matter 


502  EESPIEATORY   UNDULATIONS.  [Book  ii. 

cannot  be  settled  by  eliminating  the  movements  of  the  dia- 
phragm through  section  of  the  phrenic  nerves,  since  in  such  a  case 
the  whole  working  of  the  respiratory  pump  is  materially  affected. 

§  312.  In  addition  to  the  influence  thus  exerted  by  the 
thoracic  movements  on  the  great  veins  leading  to,  and  the  great 
arteries  leading  from  the  heart,  we  have  to  consider  the  be- 
haviour of  the  pulmonary  vessels  themselves  under  the  varying 
thoracic  pressure.  These,  like  the  venae  cavse  and  aorta,  tend 
to  expand  under  the  influence  of  the  inspiratory  expansion  of 
the  chest,  and  thus  to  become  fuller  of  blood,  very  much  as  they 
would  if  the  whole  lung  Avere  placed  under  a  large  cupping- 
glass.  The  first  effect  of  this  increased  filling  of  the  pulmonary 
vessels  would  be  to  retain  for  a  while  a  certain  quantity  of 
blood  in  the  lungs  and  thus  to  lessen  the  amount  falling  into 
the  left  auricle.  But  this  would  be  temporary  only;  and  the 
widening  of  the  pulmonary  vessels  would  speedily  produce  an 
exactly  contrary  effect,  namely,  an  increased  flow  through  the 
lungs  due  to  the  diminished  resistance  offered  by  the  widened 
passages.  Conversely,  the  first  effect  of  expiration  would  be 
an  increased  flow  into  the  left  auricle  due  to  the  additional 
quantity  of  blood  driven  onwards  by  the  partial  collapse  of  the 
pulmonary  vessels,  followed  by  a  more  significant  diminished 
flow  caused  by  the  greater  resistance  now  offered  by  the  nar- 
rower vascular  channels.  Thus  the  effect  of  inspiration  in  this 
way  would  be  first  to  diminish  the  flow  into  the  left  auricle  and 
so  into  the  left  ventricle,  but  afterwards,  for  the  rest  of  the 
inspiration  until  the  beginning  of  expiration,  to  increase  the 
flow  into  the  ventricle;  while  conversely  the  effect  of  expira- 
tion would  be  first,  for  a  brief  period,  to  increase  and  after- 
wards, during  the  rest  of  the  movement,  to  diminish  the  flow 
of  blood  into  the  left  ventricle.  Further,  while  this  may  be 
considered  as  the  effect  on  the  pulmonary  vessels,  large  and 
small  taken  altogether,  the  influence  both  of  the  thoracic  nega- 
tive pressure  during  inspiration,  and  the  return  in  a  positive 
direction  during  expiration,  will  bear  more  on  the  thin-walled 
pulmonary  veins  than  on  the  stouter  pulmonary  artery;  that  is 
to  say,  as  inspira,tion  becomes  established,  there  will  be  a  dimi- 
nution of  pressure  in  the  pulmonary  veins  greater  than  that  in 
the  pulmonary  artery,  and  this  will  be  an  additional  influence 
favouring  the  flow  into  the  left  ventricle;  during  expiration  a 
similar  difference  of  effect  will  be  felt  in  the  contrary  direction. 

During  the  increase  of  flow  into  the  ventricle,  the  quan- 
tity of  blood  ejected  at  each  stroke  will  increase,  and  each 
stroke  will  (§  140)  be  increased  in  vigour,  in  consequence  of 
which  the  arterial  pressure  will  rise.  Conversely,  during  the 
decrease  of  flow  into  the  ventricle,  the  arterial  pressure  will 
fall.  Hence  the  general  effect  of  the  movements  of  the  chest 
on  the  pulmonary  vessels  will  be  during  the  beginning  of  in- 


Chap,  ii.]  EESPIRATION.  503 

spiration  to  continue  the  loAvering'  of  arterial  ^jressure  which 
was  taking  place  during  expiration  but  subsequently  to  raise 
the  arterial  pressure;  and  conversely  at  the  beginning  of  ex- 
piration to  continue  the  rise  of  arterial  pressure  Avhich  was 
taking  place  during  inspiration  but  subsequently  to  lower 
arterial  pressure.  In  ordinary  breathing,  as  we  have  seen, 
what  may  be  considered  as  the  normal  relations  of  blood-pres- 
sure to  the  respiratory  movements  are  precisely  of  this  kind. 

§  313.  Effects  of  the  respiratory  movements,  however,  are 
seen  not  only  in  natural  but  also  in  artificial  respiration. 
When,  for  instance,  in  an  animal  under  urari,  artificial  is  sub- 
stituted for  natural  respiration,  undulations  of  the  blood-pres- 
sure curve,  synchronous  with  the  respiratory  movements,  are 
still  observed  (Fig.  100),  though  generally  less  in  extent  than 
those  seen  under  natural  conditions. 

Now  in  artificial  respiration,  the  mechanical  conditions  under 
which  the  thoracic  viscera  are  placed  as  regards  pressure  are 
the  exact  opposite  of  those  existing  during  natural  respiration, 
for  when  air  is  blown  into  the  trachea  to  distend  the  lungs,  the 
pressure  within  the  chest  is  increased  instead  of  diminished. 
Under  these  circumstances,  applying  the  considerations  laid 
down  in  the  preceding  paragraph  with  regard  to  natural  respi- 
ration, we  should  expect  to  find  that  while  the  first  effect  of  an 
artificial  inspiration  would  be  to  drive  an  additional  quantity 
of  blood  out  of  the  lungs  into  the  left  ventricle,  and  thus  to 
raise  arterial  pressure,  this  would  be  in  turn  followed  by  a  fall 
of  arterial  pressure  due  to  the  increased  resistance  offered  both 
to  the  passage  of  blood  through  the  lungs  and  to  the  entrance 
of  blood  through  the  vente  cavse  into  the  right  auricle.  Con- 
versely, the  effect  of  the  succeeding  expiration  would  be  an 
initial  continuance  of  the  fall  of  arterial  pressure  succeeded  by 
a  rise.  In  other  Avords,  we  should  expect  to  find  in  artificial 
respiration  effects  exactly  the  reverse  of  those  which  we  find  in 
normal  respiration ;  and  indeed  in  many  curves  of  blood-j^res- 
sure  taken  during  artificial  respiration  this  is  the  case. 

According  to  the  explanation  given  above,  the  total  effect 
of  each  respiratory  movement,  both  of  insi^iration  and  expira- 
tion, whether  natural  or  artificial,  being  the  result  of  two  factors 
acting  in  contrary  directions,  one  an  initial  one  acting  only  at 
the  mere  establishment  of  inspiration  or  expiration,  the  other 
sequent  and  acting  during  the  continuance  of  the  inspiratory 
or  expiratory  phase,  ought  to  differ  according  to  the  character 
of  the  respiratory  movement.  If,  for  instance,  the  respiration 
is  rapid,  and  each  movement  brief,  the  first  factor  will  be  more 
prominent  than  the  second ;  on  the  contrary  the  second  factor 
will  be  prominent  if  the  respiration  be  slow  and  each  phase  be 
prolonged ;  and  the  total  effects  will  differ  in  the  two  cases. 
We  should  expect  therefore  to  find,  Avliat  we  do  find,  that  both 


504  RESPIRATORY   UNDULATIONS.  [Book  ii. 

in  natural  and  in  artificial  respiration,  the  features  of  the  blood- 
pressure  curve  vary  according  as  the  breathing  is  hurried  or 
slow,  shallow  or  deep,  and  according  to  the  facility  with  which 
air  enters  the  chest.  So  much  so  indeed  is  this  the  case  that 
at  times  the  blood-pressure  curves  of  natural  and  artificial 
respiration  may  closely  resemble  each  other. 

§  314.  We  have  even  in  normal  quiet  breathing  indications 
of  another  kind  of  influence  of  respiration  on  the  beat  of  the 
heart.  One  striking  feature  of  the  respiratory  undulation  in 
the  blood-pressure  curve  of  the  dog  is  the  fact  that  the  pulse- 
rate  is  quickened  during  the  rise  of  the  undulation  and  becomes 
slower  during  the  fall ;  see  Fig.  99.  A  similar  influence  may  be 
seen  in  the  blood-pressure  curves  of  some  other  animals,  but  is 
slight  or  absent  in  others,  such  as  the  rabbit ;  it  may  be  recog- 
nized in  pulse-tracings  taken  from  man.  Now  this  influence  is 
at  once  done  away  with,  without  any  other  essential  change  in  the 
undulations,  by  section  of  both  vagus  nerves.  Evidently  the 
slower  pulse  during  the  fall  is  caused  by  a  coincident  stimulation 
of  the  cardio-inhibitory  centre  in  the  spinal  bulb,  the  quicker  pulse 
during  the  rise  being  due  to  the  fact  that,  during  that  interval, 
the  centre  is  comparatively  at  rest.  We  have  here  indications 
that,  while  the  respiratory  centre  in  the  spinal  bulb  is  at  work, 
sending  out  rhythmic  impulses  of  inspiration  and  expiration,  the 
neighbouring  cardio-inhibitory  centre  is,  as  it  were  by  sympathy, 
thrown  into  an  activity  of  such  a  kind  that  its  influence  over  the 
heart  waxes  with  each  expiration  and  wanes  with  each  inspiration. 

§  315.  Besides  the  mechanical  effects  of  the  respiratory 
movements  the  vascular  system  is  influenced  by  respiration 
through  the  changes  in  the  gases  of  the  blood.  The  many 
and  varied  changes  which  take  place  in  the  vascular  system 
when  the  blood  fails  to  be  duly  arterialized  are  well 
brought  out  by  a  study  of  asphyxia.  The  exaggerated 
respiratory  movements  and  convulsive  struggles  which  are 
characteristic  of  this  condition,  introduce  mechanical  and 
other  complications  which  it  may  be  well  in  the  first  instance 
to  eliminate  ;  this  can  readily  be  done  by  placing  the  animal 
under  urari.  If  in  an  animal  (dog)  under  urari  the  artificial 
respiration,  necessary  under  the  circumstances  for  the  due 
arterialization  of  the  blood,  be  stopped  the  blood-pressure 
curve  soon  shews  striking  changes,  cf .  Fig.  100.  The  mean  pres- 
sure after  a  brief  period,  the  length  of  which  depends  on  the 
character  of  the  previous  artificial  respiration,  begins  to  rise, 
and  continues  to  rise,  at  first  slowly,  afterwards  more  rapidly, 
until  finally  it  may  reach  the  double  or  even  more  of  its  pre- 
vious height.  On  the  curve  of  pressure  the  indications  of  the 
heart-beats  are  conspicuous.  This  is  due  on  the  one  hand  to 
the  rhythm  of  the  heart  being  slowed,  and  on  the  other  hand 
to  the  output  at  each   beat,    as  shewn  by  direct   observation 


Chap,  ii.] 


EESPIRATIOK 


506 


witli  the  cardiometer,  being  increased.  The  slowing  of  the 
rhythm  is  in  part  due  to  vagus  inhibitory  action,  the  too 
venous  blood  exciting  the  bulbar  cardio-inhibitory  centre ;  for 
the  effect  is  much  less  when  both  vagus  nerves  are  divided. 
But  as  illustrated  by  Fig.  100,  which  is  a  curve  of  blood-pressure 


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A    A 


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n. 


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.'  I  I  '  /  \.'  i' 
.'   i  (■   I  i   y    " 

Mil; 


yy 


i  "i 

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1.' 

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Fig. 100. 


Blood-Pressure  Curves  during  a  Suspension  of  Breathing. 
Traube-Hering  Curves. 


The  curves  1,  2,  3,  4,  5  are  portions  selected  from  one  long  continuous  tracing 
forming  the  record  of  a  prolonged  observation,  so  that  the  several  curves  repre- 
sent successive  stages  of  the  same  experiment.  Each  curve  is  placed  in  its 
proper  position  relative  to  tlie  base  line,  which,  to  save  space,  is  omitted  ;  and 
it  is  obvious  that,  starting  from  the  stage  represented  by  1,  the  blood-pressure 
rises  in  stages  2,  .3,  and  4,  but  falls  again  in  stage  5.  Curve  1  is  taken  from 
a  period  when  artificial  respiration  was  being  kept  up,  and  the  undulations 
visible  are  those  the  nature  of  which  has  been  discussed  ;  the  vagus  nerves 
having  been  cut  the  pulsations  on  the  ascent  and  descent  of  the  undulations  do 
not  differ.  When  the  artificial  respiration  was  suspended  these  undulations  dis- 
appeared, and  the  blood-pressure  rose  steadily  while  the  heart-beats  became 
slower.  Soon,  as  shewn  in  curve  2,  new  undulations  appeared.  A  little  later, 
the  blood-pressure  was  still  rising,  the  heart-beats  still  slower,  but  the  undula- 
tions still  more  obvious  (curve  3).  Still  later  (cui've  4),  the  pressure  was  still 
higher,  but  the  heail-beats  were  quicker,  and  the  undulations  flatter.  The  pres- 
sure then  began  to  fall  rapidly  (curve  5),  and  continued  to  fall  until  some  time 
later  artificial  respiration  was  resumed. 


during  aspliyxia  after  division  of  both  vagus  nerves,  the  effect 
is  not  wholly  done  away ;  the  slowing  is  in  part  due  to  other 
causes,  but  Avhat  these  are  is  not  very  clear. 

These  changes  in  the  heart  in  no  way  explain  the  rise  of 


506  THE   VASCULAR   SYSTEM.  [Book  ii. 

pressure  ;  that  is  obviously  due  to  a  very  great  increase  of  periph- 
eral resistance,  the  heart  contributing  to  the  result  only  so  far 
that  its  output  does  not  diminish  as  the  peripheral  resistance 
increases,  or  indeed  may  at  first,  at  least,  somewhat  increase. 
That  the  peripheral  resistance  is  due  to  a  large  vaso-constriction 
brought  about  by  the  too  venous  blood  stimulating  the  bulbar 
vaso-motor  centre  is  shewn  by  the  fact  that  the  rise  of  pressure 
is  far  less,  indeed  very  small,  if  the  cord  be  divided  below  the 
bulb ;  only  a  small  part,  at  most,  of  the  peripheral  resistance 
can  be  attributed  to  the  difficulty  which  the  blood,  on  account 
of  its  increasing  venosity,  finds  in  passing  through  the  capil- 
laries (§163). 

If  a  limb  be  placed  in  a  plethysmograph  during  this  rise  of 
pressure,  its  volume  is  found  to  increase  ;  and  the  same  is  true 
of  the  brain.  This  shews  that  the  vaso-constriction  does  not 
take  place  to  any  great  extent  in  the  skin  (or  the  muscles)  of 
the  limb  or  in  the  brain.  No  such  increase  of  volume  is  seen 
in  the  kidney  or  other  abdominal  organs.  Hence  we  may  con- 
clude that  the  vaso-constriction  is,  in  the  main,  one  of  the 
splanchnic  area  and  not  of  the  skin,  or  indeed  of  the  rest  of 
the  body. 

If  the  pressure  in  the  pulmonary  artery  be  examined  this 
is  found  to  increase,  even  out  of  proportion  to  the  increase  of 
the  systemic  pressure.  We  may  infer  that  the  peripheral  resist- 
ance in  the  lungs  is  very  largely  increased ;  and  we  may  also 
probably  infer  that  the  resistance  is  due  to  vaso-constriction, 
though  possibly  the  too  venous  blood  may  find  increased  diffi- 
culty in  traversing  the  pulmonary  capillaries. 

The  high  arterial  pressure  both  on  the  left  and  right  sides 
leads  to  great  distension  of  the  ventricles,  and  this  is  still 
further  increased  on  the  right  side  by  the  large  quantity  of 
blood  which  the  high  systemic  pressure  is  able  to  discharge 
into  the  vense  cavsB  through  the  vascular  areas  in  which  no 
A^aso-constriction  is  taking  place. 

These  then  are  the  main  features  of  the  circulation  during 
asphyxia  (under  urari) :  high  systemic  pressure  due  chiefly  to 
vaso-constriction  in  the  splanchnic  area;  high  pulmonary  pres- 
sure due  to  high  pulmonary  resistance,  working  against  an 
ample  supply  of  venous  blood  to  the  right  ventricle;  a  heart 
beating  slowly,  but  with  increased  output,  and  increasing  dis- 
tension of  both  ventricles  (leading  to  distension  of  the  auricles) 
but  greater  perhaps  on  the  right  side. 

This  state  of  things  however  lasts  for  a  certain  time  only; 
the  blood-pressure  soon  begins  to  fall,  and  falling  rapidly  soon 
becomes  very  low.  The  diminished  energy  of  the  heart-beats, 
the  output  at  the  systole  diminishing  greatly  though  the  dia- 
stolic distension  remains,  is  sufficient  to  account  for  this  fall; 
and  indeed  that  the  fall  is  not  due  to  lessening  of  the  periph- 


Chap,  ii.]  RESPIRATION".  507 

eral  resistance  by  slackening  of  the  vaso-constriction  is  shewn 
by  the  fact  that  if  the  artificial  respiration  be  resumed  while 
this  fall  is  taking  j^lace,  or  when  it  has  taken  place,  the  pres- 
sure at  once  rises  again  very  rajjidly  as  the  heart  recovers  its 
power,  shewing  that  the  vaso-constriction  is  still  at  work. 
The  diminished  energy  of  the  heart  beat  is  due  to  the  nutrition 
of  the  cardiac  tissue  suffering  under  the  increasing  venosity  of 
the  blood,  and  if  the  air  continue  to  fail  to  get  access  to 
the  blood  in  the  lungs,  the  heart  finally  ceases  to  beat.  The 
right  side,  by  virtue  of  what  appears  to  be  an  inherent  quality, 
continues  to  beat  rather  longer  than  the  left;  but  the 
pulmonary  peripheral  resistance  continuing,  the  efforts  of  the 
right  ventricle  to  eniptj^  itself  are  ineffectual;  and  at  death  it 
is  the  right  side  which  is  especially  distended. 

In  an  animal,  not  under  urari,  and  dying  by  asphyxia  in  an 
ordinary  way,  the  i:»henomena  are  in  the  main  the  same  as  those 
of  which  we  have  just  given  a  sketch;  but  as  we  have  said  the 
exaggerated  respiratory  movements,  and  especially  the  convul- 
sive struggles,  in  which  these  culminate,  introduce  complica- 
tions. Perhaps  the  most  marked  of  these  is  the  increased 
venous  inflow  to  the  right  side  of  the  heart,  of  which  these 
movements  are  the  cause,  for  as  we  have  seen  all  the  move- 
ments in  question  augment  the  flow  along  the  veins  to  the 
heart.  But  the  pulmonary  peripheral  resistance  is  a  bar  to 
the  progress  to  the  left  side,  and  hence  the  right  side  becomes 
increasingly  distended. 

During  asphyxia,  under  urari,  the  blood-pressure  curve 
shews  otlier  certain  interesting  features  deserving  of  attention. 

Upon  the  cessation  of  the  artificial  resjDiration,  the  respiratory 
undulations  of  course  cease  also,  so  that  the  blood-pressure  curve 
rises  at  first  steadily  broken  only  by  the  heart-beats  ;  yet  after 
a  while  new  undulations,  the  so-called  Traube  or  Traube-Hering 
curves,  make  their  appearance  (Fig.  lUO,  2,  8),  similar  to  the 
previous  ones,  except  that  their  curves  are  larger  and  of  a  more 
sweeping  character.  These  new  undulations,  since  they  appear 
in  the  absence  of  all  thoracic  or  pulmonar}^  movements,  passive 
or  active,  and  are  witnessed  even  when  both  vagi  are  cut,  must 
be  of  vaso-motorial  origin  ;  the  rhythmic  rise  must  be  due  to  a 
rhythmic  constriction  of  the  small  arteries ;  and  this  probably  is 
caused  by  a  rhythmic  discharge  from  vaso-motor  centres,  and 
especially  from  the  bidbar  vaso-motor  centre.  The  undula- 
tions are  maintained  so  long  as  the  blood-pressure  continues  to 
rise.  With  the  increasing  venosity  of  tlie  blood,  the  vaso-motor 
centres  become  enfeebled  and  the  undulations  disappear. 

We  may  here  incidentally  remark  that  the  occurrence  of 
long  slow  undulations  is  not  de|)endent  on  the  cessation  of  the 
respiratory  movements,  and  on  an  abnormally  venous  condition 
of  the  blood.     They  are  sometimes  (Fig.  101)  seen  in  an  animal 


508 


DEFICIENT  ARTEEIALIZATIOK, 


[Book  ii. 


whose  breatLdng  is  fairly  normal.  We  need  not  discuss  them 
any  further  now,  and  have  introduced  them  chiefly  to  illustrate 
the  fact  that  the  vaso-motor  nervous  system  is  apt  to  fall  into  a 
condition  of  rhythmic  activity. 


Fig.  101.    Blood-Pressure  Curve  of  a  Eabbit,  recorded  on  a  slowly 

MOVING    SURFACE,    TO    SHEW    TrAUBE-HeRING  CuRVES. 

(The  curve  was  described  not  by  means  of  a  mercury  manometer,  but  by  an 
instrument  similar  to  but  not  identical  with  Pick's  spring-kymograph.)  In  each 
heart-beat  the  upward  and  downward  stroke  are  very  close  together  but  may  be 
easily  distinguished  by  the  help  of  a  lens.  Tlie  undulations  of  the  next  order 
are  those  of  respiration.  The  wider  sweeps  are  the  Traube-Hering  curves,  of 
whicli  two  complete  curves  and  portions  of  two  others  are  shewn.  Each  Traube- 
Hering  curve  comprises  about  nine  respiratory  curves,  and  each  respiratory  curve 
about  the  same  number  of  heart-beats. 


§  316.  While  changes  occurring  primarily  in  the  respira- 
tory system  thus  affect  the  vascular  system,  conversely  changes 
occurring  primarily  in  the  vascular  system  affect  the  respira- 
tory system. 

Of  these  the  most  common  and  important  however  are 
changes  in  the  circulation  through  the  lungs.  In  the  normal 
organism  an  adequate  supply  of  arterial  blood  to  the  tissues 
is  secured  by  an  adequate  renewal  of  the  air  in  the  pulmonary 
alveoli  and  an  adequately  rapid  flow  of  blood  through  the  pul- 
monary capillaries.  When,  as  by  obstruction  in  the  pulmonary 
arteries,  or  by  failure  of  the  cardiac  valves,  or,  and  perhaps 
esi^ecially,  by  an  insufficient  cardiac  stroke,  the  stream  of  blood 
from  the  lungs  into  the  left  ventricle  is  lessened  either  in 
amount  or  in  rapidity,  less  oxygen  is  carried  to  the  tissues, 
including  the  nervous  tissue  of  the  spinal  bulb,  and  dyspnoea  or 
"  want  of  breath  "  follows.  When  the  circulation  through  the 
lungs  is  in  full  healthy  swing,  the  liEemoglobin  of  the  red  cor- 
puscles is  as  Ave  have  seen  saturated  or  nearly  saturated  with 
oxygen.  If  owing  to  a  slower  stream  the  red  corpuscles  tarry 
longer  in  their  passage  along  the  walls  of  the  pulmonary  alveoli 
they  cannot  thereby  take  up  a  compensating  addition  of  oxy- 
gen, indeed  it  is  doubtful  if  they  can  take  up  any  additional 
oxygen  at  all.     The  blood  falling  under  these  circumstances 


Chap,  ii.]  EESPIRATION.  509 

into  the  left  ventricle  and  sent  thence  over  the  body  is  not 
more  arterial  than  usual ;  at  the  same  time  the  amount  of  blood 
sent  out  at  each  heart  stroke  is  less,  often  much  less,  than  the 
normal ;  and  the  spinal  bulb  as  well  as  the  other  tissues  suffer 
in  consequence  from  a  deficiency  of  oxygen.  The  deficient 
supply  to  tlie  bulb  manifests  itself  in  dyspncjeic  or  at  least  in 
laboured  breathing,  which  sometimes  through  the  mechanical 
influences  discussed  above  has  the  hajDpy  result  of  imprf)ving 
tlie  pulmonary  circulation  and  so  produces  compensating  effects. 
AVhen  the  pulmonary  artery  is  suddenly  plugged  with  a  clot 
the  primary  and  urgent  symptom  is  "want  of  breath,"  though 
air  enters  freely  into  the  chest ;  and  "•  cardiac  dysjpnoea  "  is  a 
common  symptom  of  cardiac  disease. 

§  317.  Other  systems  of  the  body  are  also  related  to  the 
respiratory  system,  though  by  ties  less  striking  than  those 
which  bind  to  it  the  vascular  system.  We  have  seen  that 
deficient  arterialization  of  the  blood  stirs  up  the  muscles  of 
the  alimentary  canal  to  increased  activity,  and  we  shall  pres- 
ently see  that  the  same  condition  has  a  notable  effect  in  pro- 
moting the  perspiration ;  it  probably  has  a  similar  influence 
over  other  secretions.  On  the  other  hand,  as  we  have  seen 
§  303,  there  are  reasons  for  thinking  that  the  activity  of  the 
respiratory  centre  and  so  the  energy  of  the  whole  respiratory 
act  is  influenced  by  chemical  changes,  other  than  the  decrease 
of  oxygen  and  increase  of  carbonic  acid,  brought  about  in  the 
blood  by  the  activity  of  the  skeletal  muscles. 

The  closeness  and  the  intricacy  of  the  ties  which  thus  con- 
nect the  resj^iratory  system  with  almost  all  parts  of  the  body 
may  be  illustrated  by  considering  the  effects  of  muscular  work 
on  the  body,  and  the  conditions  Avhich,  apart  from  the  capacity 
of  the  muscles  themselves  and  of  the  motor  nervous  apparatus 
Avhich  puts  them  to  work,  determine  the  power  of  the  body 
to  do  work.  During  work,  especially  arduous  Avork,  the  mus- 
cular contractions  rob  the  blood  of  much  oxygen  and  load  it 
with  much  carbonic  acid.  This  change  in  the  blood  would 
itself  increase  the  activity  of  the  respiratory  centre  and  the 
energy  of  the  respiratory  movements,  and  might  be  suflicient 
to  secure  such  an  increase  of  these  movements  that  the  defl- 
ciency  of  oxygen  and  increase  of  carbonic  acid  should  never 
overstep  certain  limits.  But,  as  we  have  said,  apparently  other 
products  of  muscular  metabolism  act  so  potently  in  stimulating 
tlie  respiratory  centre  that  the  respiratory  movements  are  more 
tlian  suflicient  to  compensate  the  changes  in  the  gases  of  the 
blood.  The  eflicacy  of  the  augmented  respiratory  movements 
is  much  increased  by  a  concomitant  increase  in  cardiac  activity 
and  a  swifter  or  fuller  stream  of  blood  through  the  lungs ; 
indeed  unless  backed  up  by  the  cardiac  increase  the  mere 
increase  of  the  pulmonary  ventilation  might  prove  inadequate. 


510        EESPIRATION   AND   MUSCULAR   WORK.    [Book  ii. 

Hence  the  capacity  for  arduous  muscular  labour  is  deter- 
mined not  by  the  respiratory  mechanism  alone,  nor  by  the 
vascular  system  alone,  but  by  both,  and  especially  by  both 
working  together  in  harmony  and  concert.  The  increased 
ventilation  would  be  idle  unless  it  were  accompanied  by  a 
quicker  circulation,  and  the  quicker  circulation  would  simi- 
larly be  of  comparatively  little  use  unless  accompanied  by 
increased  ventilation.  To  a  bystander  the  working  of  the 
respiratory  pump  is  much  more  obvious  than  that  of  the  vascular 
system,  and  indeed  the  subject  himself  is  much  more  directly 
conscious  of  changes  in  the  former  than  of  changes  in  the  lat- 
ter. Hence  when  the  organism  ceases  to  be  able  to  meet  the 
demands  which  the  labour  is  making  upon  it,  the  subject  is 
said  to  be  "  out  of  breath,"  though  in  a  large  number  of  cases 
the  failure  lies  much  more  at  the  door  of  the  vascular  than  of 
the  respiratory  system.  And,  as  a  rule,  it  may  perhaps  be 
said  that  when  two  men  differ  in  their  capacity  for  strenuous 
work,  such  as  running  a  race,  the  difference,  though  it  is  often 
familiarly  spoken  of  as  one  of  "  wind  "  or  power  of  breathing, 
is  in  reality  not  a  difference  in  ventilating  capacity  but  a  dif- 
ference in  the  power  of  the  heart  to  keep  up  to  and  work  in 
harmony  with  the  increased  respiratory  movements. 

Thus  there  are  tAvo  main  factors  in  respiration,  the  respira- 
tory mechanism  proper,  and  the  circulation,  the  one  bringing 
the  air  to  the  blood,  and  the  other  the  blood  to  the  air.  We 
may  remind  the  reader  that  there  is  also  a  third  factor,  and  that 
one  of  great  moment,  the  amount  of  haemoglobin,  that  is,  the 
number  of  red  corpuscles,  in  the  blood.  The  amount  of  oxygen 
taken  up  from  the  lungs  depends  not  only  on  the  strokes  of  the 
respiratory  and  the  vascular  pumps  but  also  on  the  richness  of 
the  blood  in  red  corpuscles.  A  body  which  from  loss  of  blood 
or  from  disease  is  ansemic  is  thrown  out  of  breath  by  very  slight 
exertion,  not  so  much  because  the  respiratory  or  the  vascular 
pump  is  weak,  but  because,  through  lack  of  oxygen-carriers, 
with  their  best  efforts  the  combined  pumps  can  only  deliver  to 
the  tissues,  including  the  medulla,  an  inadequate  supply  of 
oxygen.  And  fat  persons,  whose  store  of  haemoglobin  in  pro- 
portion to  their  body  weight  is  always  below  par,  are  proverbi- 
ally "scant  of  breath." 


SEC.    9.      MODIFIED  RESPIRATORY  MOVEMENTS. 

§  318.  The  respiratory  mechanism  with  its  adjuncts,  in 
addition  to  its  respiratory  function,  becomes  of  service,  especially 
in  the  case  of  man,  as  a  means  of  expressing  emotions.  The 
respiratory  column  of  air,  moreover,  in  its  exit  from  the  chest, 
is  frequently  made  use  of  in  a  mechanical  way  to  expel  bodies 
from  the  upper  air-passages.  Hence  arise  a  number  of  pecul- 
iarly modihed  and  more  or  less  complicated  respiratory  move- 
ments, sighing,  coughing,  laughter,  &c.  adapted  to  secure  special 
ends  which  are  not  distinctly  respiratory.  They  are  all  essen- 
tially reflex  in  character,  the  stimulus  determining  each  move- 
ment, sometimes  affecting  a  peripheral  afferent  nerve  as  in  the 
case  of  coughing,  sometimes  working  through  the  higher  parts 
of  the  brain  as  in  laughter  and  crying,  sometimes  possibly,  as 
in  yaAvning  and  sighing,  acting  on  the  respiratory  centre  itself. 
Like  the  simple  respiratory  act,  they  may  with  more  or  less 
success  be  carried  out  by  a  direct  effort  of  the  will. 

Sighing  is  a  deep  and  long-drawn  inspiration,  chiefly  through 
the  nose,  followed  by  a  somewhat  shorter,  but  correspondingly 
large  expiration. 

Yawning  is  similarly  a  deep  inspiration,  deeper  and  longer 
continued  than  a  sigh,  drawn  through  the  widely  open  mouth, 
and  accompanied  by  a  peculiar  deiDression  of  the  lower  jaw  and 
frequently  by  an  elevation  of  the  shoulders. 

Hiccough  consists  in  a  sudden  inspiratory  contraction  of  the 
diaphragm,  in  the  course  of  which  the  glottis  suddenly  closes, 
so  that  the  further  entrance  of  air  into  the  chest  is  prevented, 
while  the  impulse  of  the  column  of  air  just  entering,  as  it 
strikes  upon  the  closed  glottis,  gives  rise  to  a  well-known 
accompanying  sound.  The  afferent  impulses  of  the  reflex  act 
are  conveyed  by  the  gastric  branches  of  the  vagus.  The  closure 
of  the  glottis  is  carried  out  by  means  of  the  inferior  laryngeal 
nerve.     See  Voice. 

In  sobbing  a  series  of  similar  convulsive  inspirations  follow 
each  other  slowly,  the  glottis  being  closed  earlier  than  in  the 
case  of  hiccough  so  that  little  or  no  air  enters  into  the  chest. 

511 


512       MODIFIED   EESPIRATOEY   MOVEMENTS.     [Book  ii. 

Coughing  consists  in  the  first  place  of  a  deep  and  long-drawn 
inspiration  by  which  the  lungs  are  well  filled  with  air.  This 
is  followed  by  a  complete  closure  of  the  glottis,  and  then  comes 
a  sudden  and  forcible  expiration,  in  the  midst  of  which  the 
glottis  suddenly  opens,  and  thus  a  blast  of  air  is  driven  through 
the  upper  respiratory  passages.  The  afferent  impulses  of  this 
reflex  act  are  in  most  cases,  as  when  a  foreign  body  is  lodged  in 
the  larynx  or  by  the  side  of  the  epiglottis,  conveyed  by  the 
superior  laryngeal  nerve  ;  but  the  movement  may  arise  from 
stimuli  applied  to  other  afferent  branches  of  the  vagus,  such  as 
those  supplying  the  bronchial  passages  and  stomach  and  the 
auricular  branch  distributed  to  the  meatus  externus.  Stimula- 
tion of  other  nerves  also,  such  as  those  of  the  skin  by  a  draught 
of  cold  air,  may  develop  a  cough. 

In  sneezing  the  movement  is  the  same,  in  so  far  that  it  con- 
sists of  a  deep  inspiration  followed  by  a  sudden  and  forcible 
expiration.  But  the  mouth,  instead  of  being  widely  open  as  in 
coughing,  is  partly,  or  at  first  even  wholly  closed,  and  the 
buccal  cavity  with  the  pharynx  is  so  disposed  that  the  blast 
of  air  in  being  driven  out  through  the  mouth  produces  the 
characteristic  sound.  If  the  obstruction,  the  sudden  removal 
of  which  initiates  the  expiratory  blast,  is  caused  by  closure  of 
the  glottis,  and  this  is  not  clear,  the  glottis  is  so  disposed  as 
not  to  give  rise  to  a  vocal  sound  as  is  the  case  in  coughing. 
Though  the  movement  is  accompanied  by  secretion  from  the 
nasal  passages,  the  outgoing  blast  appears  not  to  pass  through 
the  nose,  being  cut  off'  from  that  passage  by  elevation  and 
pressing  back  of  the  soft  palate.  The  afferent  impulses  here 
usually  come  from  the  nasal  branches  of  the  fifth.  When 
sneezing  however  is  produced  by  a  bright  light,  the  optic  nerve 
would  seem  to  be  the  afferent  nerve. 

Laughing  consists  essentially  in  an  inspiration  succeeded, 
not  by  one,  but  by  a  whole  series,  often  long  continued,  of  short 
spasmodic  expirations,  the  glottis  being  freely  open  during  the 
whole  time,  and  the  vocal  cords  being  thrown  into  character- 
istic vibrations. 

In  crying,  the  respiratory  movements  are  modified  in  the 
same  way  as  in  laughing  ;  the  rhythm  and  the  accompanying 
facial  expressions  are  however  different,  though  laughing  and 
crying  frequently  become  indistinguishable. 


CHAPTER   III. 
THE   ELIMINATION    OF   WASTE   PRODUCTS. 


§  319.  We  have  traced  the  food  from  the  alimentary  canal 
into  the  blood,  and,  did  the  state  of  our  knowledge  permit,  the 
natural  course  of  our  study  would  be  to  trace  the  food  from 
the  blood  into  the  tissues,  and  then  to  follow  the  products  of 
the  activity  of  the  tissues  back  into  the  blood  and  so  out  of  the 
body.  This  however  we  cannot  as  yet  satisfactorily  do  ;  and  it 
will  be  more  convenient  to  study  first  the  final  products  of  the 
metabolism  of  the  body,  and  the  manner  in  which  they  are 
eliminated,  and  afterwards  to  return  to  the  discussion  of  the 
intervening  steps. 

Our  food  consists  of  certain  food-stuffs,  viz.  proteids,  fats, 
and  carbohydrates,  of  various  salts,  and  of  water.  In  their 
passage  through  the  blood  and  tissues  of  the  body,  the  proteids, 
fats,  and  carbohydrates  are  converted  into  urea  (or  some  closely 
allied  body),  carbonic  acid  and  water,  the  nitrogen  of  the  urea 
being  furnished  by  the  proteids  alone.  Many  of  the  proteids 
contain  sulphur,  and  also  have  phosphorus  attached  to  them 
in  some  combination  or  other,  and  some  of  the  fats  taken  as 
food  contain  phosphorus  ;  these  elements  ultimately  midergo 
oxidation  into  phosphates  and  sulphates,  and  leave  the  bod}^  in 
that  form  in  company  with  the  other  salts. 

Broadly  speaking  then,  the  waste  products  of  the  animal 
economy  are  urea,  carbonic  acid,  salts  and  water.  These  leave 
the  body  by  one  or  other  of  three  main  channels,  the  lungs,  the 
skin,  and  the  kidney.  Some  part,  it  is  true,  leaves  the  body 
by  the  bowels,  for,  as  we  have  seen,  the  fieces  contain,  besides 
undigested  portions  of  food,  substances  which  have  been  secreted 
into  the  bowel,  and  are  therefore  waste  products ;  but  the 
amount  of  these  is  so  small  that  they  may  be  neglected. 

The  lungs  serve  as  the  channel  for  the  discharge  of  the 
greater  part  of  the  carbonic  acid,  and  a  considerable  quantity  of 
water  ;  this  discharge  we  have  just  studied.  Through  the  skin 
33  613 


514  ELIMINATION   OF  WASTE   PEODUCTS.     [Book  ii. 

there  leave  tlie  body  a  comparatively  small  quantity  of  salts,  a 
little  carbonic  acid,  and  a  variable  but  on  the  whole  large  quan- 
tity of  water. 

The  kidneys  discharge  all  or  nearly  all  the  urea  and  allied 
bodies,  the  greater  portion  of  the  salts,  and  a  large  amount  of 
water,  with  an  insignificant  quantity  of  carbonic  acid.  They 
are  especially  important  since  by  them  practically  all  the  nitro- 
genous waste  leaves  the  body,  and  to  them  we  will  turn  first. 


SEC.    1.     THE   COMPOSITION   AND   CHARACTERS   OF 

URINE. 


§  320.  These  are  so  fully  dwelt  upon  in  special  works  that 
we  may  confine  ourselves  here  to  salient  points.  The  healthy 
urine  of  man  is  a  clear  yellowish  slightly  fluorescent  fluid,  of  a 
peculiar  odour,  saline  taste,  and  acid  reaction,  having  a  mean 
specific  gravity  of  1-020,  and  generally  holding  in  suspense  a 
little  mucus.  The  mucus,  when  present,  comes  from  the  uri- 
nary passages,  as  do  also  the  occasional  epithelial  cells.  All  the 
rest  of  the  urine  may  be  considered  as  the  secretion  of  the 
kidne}^ 

The  urine  as  we  have  said  is  the  chief  channel  by  which 
solid  matters  leave  the  body,  a  small  quantity  only  passing  by 
the  skin  and  practically  none  by  the  lungs.  Hence,  neglecting 
for  the  present  the  skin,  we  may  say  that  all  the  substances 
taken  into  the  body  sooner  or  later  leave  the  body  by  the  urine, 
save  the  few  substances  which  may  be  retained  permanently 
within  the  body  and  the  substances  which  make  up  the  body  at 
the  moment  of  its  death.  We  accordingly  find  that  the  urine 
contains  a  large  number  of  substances,  the  exact  amount  of  each 
substance  present  in  a  given  quantity  of  urine  varying,  in  the 
case  of  every  substance  somewhat,  and  in  the  cases  of  many 
substances  very  largely,  from  time  to  time.  The  composition 
of  urine  is  not  only  complex  but  extremely  variable. 

Moreover  a  little  consideration  will  shew  that  the  several 
substances  present  in  urine  must  have  very  different  histories. 
Some  of  tlie  constituents  of  urine  appear  in  it  in  the  exact  form 
in  which  they  were  introduced  into  the  mouth  ;  they  have  been 
simply  absorbed  from  the  alimentary  canal  into  the  blood  and 
excreted  by  the  kidney  without  undergoing  change ;  they  are 
derived  directly  and  without  change  from  the  food. 

Others  again  are  the  products  of  changes  which  the  food  has 
undergone  in  the  body ;  and  these  changes  may  be  slight  or  may 
be  extensive,  and  may  take  place  on  the  one  hand  in  the  alimen- 
tary canal,  or  during  a  brief  transit  of  the  substance  in  the 
blood-stream,  or  even  in  the  urine  itself,  may  so  to  speak  be 

616 


516  COMPOSITION   OF   URINE.  [Book  ii. 

superficial ;  or  on  tlie  other  liand  may  take  place  in  the  very 
depths  of  the  tissues  and  be  closely  associated  with  the  very 
life  of  the  tissues.  We  shall,  however,  have  to  return  to  these 
matters  later  on,  and  may  here  briefly  consider  what  substances 
are,  normally  and  abnormally,  present  in  urine,  and  the  chief 
features  of  the  fluid  itself. 

§  321.     Besides  water,  the  constituents  of  urine  are:  — 

Nitrogenous  Crystalline  Bodies.  Neglecting  the  small  pro- 
portion of  these  bodies  which,  especially  in  the  case  of  flesh 
eaters,  are  introduced  into  the  economy  with  the  food,  as 
kreatin  and  the  like,  and  so  pass  into  the  urine  with  no  or  with 
comparatively  little  change,  we  may  on  the  whole  regard  the 
substances  of  this  class  as  the  products  of  the  changes  which 
the  proteid  matters  (and  allied  substances  such  as  gelatin  and 
the  like)  present  in  food  have  undergone  either  while  the  food 
was  simply  food,  still  in  the  alimentary  canal  for  instance,  or 
after  the  food  had  been  built  up  into  the  tissues  of  the  body. 

Of  these  by  far  the  most  important,  in  the  urine  of  man 
and  mammalia,  is  the  body  urea  (NgH^CO).  It  is  the  chief 
form  in  which,  in  these  animals,  nitrogen  leaves  the  body.  We 
shall  have  to  discuss  the  relations  and  formation  of  urea  later 
on,  but  meanwhile  we  will  simply  state  that  it  has  remarkable 
double  connections  with  two  great  groups.  On  the  one 
hand  it  is  related  to  the  ammonia  group,  and  by  hydration  is 
readily  converted  into  ammonium  carbonate  (N2H4CO  +  2H20  = 
(NH^^gCOg).  On  the  other  hand  it  is  related  to  the  great 
cyanogen  group,  ammonium  cyanate  and  urea  being  isomeric, 
and  the  former  by  simple  heating  being  converted  into  the 
latter  (NH^  .CNO  =  N2H4CO). 

Though  a  base,  forming  salts  with  acids,  such  as  nitrates, 
oxalates,  &c.  urea  occurs  in  urine  in  a  free  and  independent 
condition. 

Closely  allied  to  urea,  occurring  apparently  as  a  bye  product 
of  the  same  line  of  metabolism,  is  uric  acid  (CgH^N^Og),  which 
is  found  always  in  the  urine  of  man,  occurring  in  small  but 
variable  quantity.  In  the  urine  of  some  animals  such  as  birds 
and  reptiles  it  occurs  in  abundance,  and  indeed  in  these  replaces 
urea  as  the  chief  nitrogenous  excretion.  Uric  acid  is  a  more 
complex  body  than  urea,  one  molecule  of  uric  acid  splitting  up, 
under  the  influence  of  certain  reagents,  into  two  molecules 
of  urea  and  a  compound  of  oxalic  acid.  ,  Its  decomposition 
products  however,  under  difl^erent  reagents,  are  very  numerous 
and  complex  though  urea  occurs  among  them  frequently  and 
characteristically.  Uric  acid  may  be  synthetically  produced 
out  of  urea  and  glycin  (glycocoU). 

It  is  a  weak  dibasic  acid,  and  occurs  in  normal  human  urine, 
not  as  a  free  acid  but  as  an  acid  salt,  being  combined  with  potas- 
sium and  sodium,  and  to  a  less  extent  with  calcium  and  am- 


Chap,  hi.]     ELIMINATION    OF   WASTE   PEODUCTS.         517 

monium.  In  quite  normal  urine  these  salts  are  soluble  in  the 
urine,  even  after  the  fluid  has  cooled  down  to  the  ordinary 
temperature  of  the  air;  but  not  infrequently  the  urates,  soluble 
in  the  urine  at  the  temperature  at  which  it  leaves  the  body,  are 
precipitated  when  the  fluid  cools,  forming  the  well-known  "  de- 
posit of  urates."  On  further  standing  the  salts  are  apt  to  be 
decomposed  and  thus  to  give  rise  to  crystals  of  uric  acid. 

Besides  urea  and  uric  acid  the  urine  contains  small  but 
variable  quantities  of  more  or  less  nearly  allied  bodies  such  as 
kreatinin,  xanthin,  hypoxanthin,  and  guanin.  Concerning  these 
we  will  at  present  only  say  that  kreatinin  is  a  dehydrated  form 
of  the  body  kreatin  which  we  spoke  of  (§  69)  as  a  constituent 
of  muscles.  Kreatin  by  dehydration  is  readily  converted  into 
kreatinin,  and  kreatinin  by  hydration  into  kreatin;  kreatin 
introduced  into  the  alimentary  canal  or  into  the  blood  appears 
in  the  urine  as  kreatinin;  and  in  flesh  eaters  some  at  least  of 
the  kreatinin  of  the  urine  is  derived  directly  from  the  kreatin 
present  in  the  meat  eaten  as  food;  but  we  shall  discuss  the 
subject  of  kreatin  later  on. 

Besides  the  above,  such  bodies  as  leucin,  taurin,  cystin,  allan- 
toin  and  ammonium  oxalurate  are  occasionally  found  in  urine, 
but  cannot  be  regarded  as  constituents  of  normal  urine. 

In  the  urine  of  man  hiphuric  acid  appears  to  be  always  pres- 
ent in  small  quantities,  and  in  the  urine  of  herbivora  occurs  in 
large  quantities.  In  these  latter  it  is  derived  more  or  less 
directly,  by  changes  of  which  we  shall  have  to  speak  in  a  suc- 
ceeding chapter,  from  constituents  of  the  food  containing  bodies 
belonging  to  the  aromatic  group  (benzoic  acid  series) ;  but  the 
small  quantity  present  in  man  and  other  carnivora  appears  to 
come  from  the  metabolism  of  proteid  matter  which,  as  we  have 
already  seen,  contains  an  aromatic  constituent.  Another  mem- 
ber of  the  aromatic  group,  tyrosin,  is  occasionally  present  in  urine. 

A  special  interest  belongs  to  certain  compounds,  which  may 
be  regarded  as,  in  small  quantities,  normal  constituents  of  urine, 
but  which  may  occur  in  much  larger  quantities;  these  are  cer- 
tain phenol  compounds  such  as  phenyl-sulphuric  acid,  certain 
indigo  compounds,  the  so-called  indican,  and  others.  These 
arise  from  bodies  appearing  in  the  alimentary  canal  as  prod- 
ucts of  the  decomj^osition  of  proteids,  effected  not  by  natural 
juices,  but  by  micro-organisms,  §  §  210,  232.  Their  amount  in  the 
urine  may  be  taken  as  a  measure  of  the  extent  to  which  proteids 
are  being  changed  by  these  agents  in  the  alimentar}-  canal. 

§  322.  Inorganic  Salts.  These  for  the  most  part  exist  in 
urine  in  natural  solution,  the  composition  of  the  ash  almost  ex- 
actly corresponding  with  the  results  of  the  direct  analysis  of  the 
fluid  ;  in  this  respect  urine  contrasts  forcibly  with  blood,  the 
ash  of  which  is  largely  composed  of  inorganic  substances,  which 
previous  to  the  incineration  existed  in  peculiar  combination  with 


518  COMPOSITION   OF   UEINE  [Book  ii. 

proteid  and  other  complex  bodies.  In  the  ash  of  urine  there  is 
rather  more  sulphur  than  corresponds  to  the  sulphuric  acid 
directly  determined  ;  this  indicates  the  existence  in  urine  of 
some  sulphur -holding  complex  body.  And  there  are  traces  of 
iron,  pointing  to  some  similar  iron-holding  substance.  But 
otherwise,  all  the  substances  found  in  the  ash  exist  as  salts  in 
the  natural  fluid. 

The  chief  bases  are  sodium,  potassium,  calcium  and  mag- 
nesium in  the  form  of  chlorides,  phosphates  and  sulphates. 
The  exact  way  in  which  the  several  bases  and  acids  are  com- 
bined is  to  some  extent  a  matter  of  uncertainty ;  but  sodium 
chloride  is  certainly  present  and  in  considerable  quantity ;  it  is 
the  most  abundant  and  important  inorganic  constituent.  A 
large  portion  of  the  phosphoric  acid  seems  to  exist  as  acid 
sodium  phosphate,  the  rest  as  soluble  calcium  and  magnesium 
phosphates.  The  remaining  chief  salts,  occurring  however  in 
smaller  quantity,  are  potassium  and  sodium  sulphate,  and  cal- 
cium chloride. 

Ammonia  occurs  in  small  quantity,  alkaline  carbonates  are 
frequently  found,  traces  of  nitrates  are  at  all  events  occa- 
sionally present,  as  also  indications  of  silicates  and  of  sulpho- 
cyanates. 

The  phosphates  are  derived  partly  from  the  phosphates 
taken  as  such  in  food,  partly  from  the  phosphorus  or  phosphates 
peculiarly  associated  with  the  proteids,  and  partly  from  the 
phosphorus  of  certain  complex  fats  such  as  lecithin.  When 
urine  becomes  alkaline  (and,  as  we  shall  presently  see,  it  may 
do  so  by  changes  taking  place  in  itself)  the  calcic  and  magnesic 
phosphates  are  converted  into  basic  salts  which,  being  insolu- 
ble, are  precipitated,  the  sodium  phosphate  remaining  in  solu- 
tion. When  the  alkalinity,  as  is  frequently  the  case,  is  due  to 
ammonia,  ammonio-magnesium  phosphate  is  formed  and  is  apt 
to  appear  in  crystals.  The  sulphates  are  derived  partly  from 
the  sulphates  taken  as  such  in  food  and  partly  from  the  sulphur 
of  the  proteids.  The  carbonates,  when  occurring  in  large  quan- 
tity, generally  have  their  origin  in  the  oxidation  of  such  salts 
as  citrates,  tartrates,  &c.  The  bases  present  depend  largely  on 
the  nature  of  the  food  taken.  Thus  with  a  vegetable  diet,  the 
excess  of  the  alkalis  in  the  food  reappears  in  the  urine ;  with 
an  animal  diet,  the  earthy  bases  in  a  similar  way  come  to  the 
front. 

§  323.  Non-nitrogenous  Bodies.  These  exist  in  very  small 
quantities,  and  many  of  them  are  probably,  of  uncertain  oc- 
currence. Some  of  these  are  organic  acids,  the  most  constant 
perhaps  being  oxalic  acid ;  to  this  may  be  added  glycerin- 
phosphoric,  lactic,  formic,  acetic,  butyric  and  possibly  succinic 
acids.  Inosit  has  also  been  said  to  occur  normally.  It  has 
been  maintained  that  minute  quantities  of  sugar  (dextrose)  are 


Chap,  hi.]     ELIMINATION   OF  WASTE   PRODUCTS.        519 

invariably  present  in  even  healthy  urine  ;  this  however  has  not 
as  yet  been  placed  beyond  all  doubt.  The  nature  of  the  sub- 
stances which  give  to  urine  its  characteristic  odour  has  not  been 
made  out ;  probably  there  are  more  such  bodies  than  one. 

§  324.  Pigments.  Urine  is  always  coloured,  the  tint  vary- 
ing from  a  light  to  a  dark  yellow  with  an  admixture  of  brown. 
In  the  course  of  twenty-four  hours,  a  not  inconsiderable  quan- 
tity of  pigment  must  leave  the  body  by  the  urine ;  but  the 
nature  of  the  normal  pigment  or  pigments  of  urine  is  at  pres- 
ent obscure  and  the  subject  of  much  controversy.  The  mat- 
ter is  apparently  further  complicated  by  the  presence  in  urine 
of  what  have  been  called  '  chromogens,'  that  is  to  say,  bodies 
which  are  not  coloured  themselves  but  which  readily  give  rise 
to  pigments  upon  oxidation ;  and  it  is  probable  that  some  of 
these  '  chromogens '  of  the  urine  are  reduction  products  of  the 
respective  pigments,  the  reduction  taking  place  in  the  urine 
after  secretion,  or  during  or  even  before  secretion.  There  is 
frequently  present  in  urine,  especially  in  cases  of  fever,  a  pig- 
ment which  has  been  isolated  and  determined,  which  has  a 
characteristic  spectrum,  and  which  being  maintained  by  some 
to  be  a  derivative  of  bilirubin,  has  been  called  urobilin.  It  is 
not  this  urobilin  however  which  gives  to  urine  its  ordinary 
colour.  Some  observers,  on  the  other  hand,  maintain  that  nor- 
mal urine  does  contain  and,  in  part  at  least,  owes  its  normal 
colour  to  a  somewhat  similar  but  different  body,  which  in  con- 
sequence they  have  called  '  normal '  urobilin.  It  is  in  fact  not 
possible,  at  the  present  moment,  to  make  definite  and  satisfac- 
tory statements  as  to  whether  urine  contains  one  or  more  than 
one  normal  pigment,  as  to  its  or  their  nature,  as  to  whether 
they  are  derived  from  bile-pigment  or  directly  from  the  haematin 
of  haemoglobin  or  in  other  ways,  or  as  to  the  several  steps  by 
which  they  are  produced.  There  are  also  abnormal  colouring 
matters  present  on  occasion,  such  for  instance  as  the  peculiar 
red  colouring  matter  occurring  sometimes  in  the  urine  of  acute 
rheumatism,  which  has  been  called  uroerythrin  ;  but  our  knowl- 
edge concerning  these  is  very  imperfect. 

§  325.  Ferments  and  other  bodies.  Even  normal  urine  has 
frequently  been  found  to  contain  a  small  quantity,  hardly 
amounting  to  more  than  a  trace,  of  proteid  material,  apparently 
an  albumin  ;  but  the  normal  presence  of  even  this  small  quan- 
tity has  been  disputed.  Urine,  however,  certainly  contains 
ferment  bodies. 

When  urine  is  treated  with  many  times  its  volume  of  alco- 
hol, a  granular  or  flocculent  precipitate  is  thrown  down,  con- 
sisting chiefly  of  phosphates,  together  with  some  other  substance 
or  probably  several  other  substances,  in  very  small  quantities. 
An  aqueous  solution  of  the  precipitate,  which  may  be  freed  from 
the  phosphates,  is  both  amylolytic  and  proteolytic.     Ferments 


520  COMPOSITION   OF   URINE.  [Book  ii. 

may  also  and  more  readily  be  extracted  from  urine  by  allowing- 
slireds  of  fibrin  to  soak  in  the  urine  for  a  few  liours,  and  then 
removing  and  washing  them.  The  ferments  become  entangled 
in  the  fibrin  in  such  a  way  as  not  to  be  easily  removed  by  wash- 
ing. The  washed  shreds  will  convert  starch  into  sugar  ;  and 
when  treated  with  dilute  hydrochloric  acid  digest  themselves, 
shewing  the  presence  of  pepsin.  By  this  method  it  has  been 
ascertained  that  an  amylolytic  ferment  and  pepsin  are  present  in 
quantities  which  vary  in  the  twenty-four  hours  according  to  the 
meals.  Rennin  has  also  been  found,  and  at  times  at  least,  tryp- 
sin. From  this  it  appears  that  some  of  the  ferments  of  the  ali- 
mentary canal  escape  from  the  body  by  the  urine,  being  probably 
re-absorbed  directly  from  the  res]3ective  glands  ;  the  quantity 
moreover  which  thus  escapes  is  insignificant. 

A  small  quantity  of  gas,  about  15  vols,  p.c,  can  be  extracted 
by  the  mercurial  pump  from  urine  received  direct  from  the 
body  without  exposure  to  air.  The  gas  so  obtained  consists 
chiefly  of  carbonic  acid,  nitrogen  being  very  scanty,  and  oxygen 
occurring  in  very  small  quantities  or  being  wholly  absent.  The 
meaning  of  this  we  have  already  touched  upon  in  speaking  of 
respiration,  see  §  290. 

§  326.  The  quantities  in  which  these  multifarious  bodies, 
all  of  which  as  we  have  seen  we  may  perhaps  regard  as  con- 
stituents of  normal  urine,  are  present  in  different  specimens 
of  urine,  vary  within  very  wide  limits,  being  dependent  on  the 
nature  of  the  food  taken,  and  on  the  conditions  of  the  body. 
The  amount  not  of  water  only,  but  of  many  of  the  other  several 
constituents,  varies  widely  and  indeed  rapidly,  so  that  the  per- 
centage composition  of  urine  will  vary  from  hour  to  hour  if  not 
from  minute  to  minute.  The  causes  which  determine  these  vari- 
ations in  the  nature  and  amount  of  urine  we  shall  study  later  on. 
Meanwhile  what  may  be  called  the  average  composition  of 
human  urine  is  shewn  in  the  following  table  in  which  the  acids 
and  bases  are  put  down  separately. 


AMOUNTS   OF   THE   SEVERAL  URINARY   CONSTITUENTS  PASSED 
IN  TWENTY-FOUR   HOURS.     (After  Parkes.) 


By  an  average 
man  of  66  kilos. 

Per  1  kilo 
of  body  weight. 

Water 
Total  Solids 

1500-000  grammes 

23-0000  grammes 
1-1000 

Urea 

33-180 

•5000 

Uric  Acid 

-555 

-0084 

Hippuric  Acid 

-400 

-0060 

Kreatinin 

-910 

•0140 

Pigment,  and 
other  substances  10-000 

-1510 

Chap,  hi.]     ELIMIKATION   OF   WASTE   PEODUCTS.         521 

By  an  average  Per  1  kilo 

man  of  66  kilos.  of  body  weight. 

Total  Solids  (continued') 

Sulphuric  Acid       2-012  -0305 

Phosphoric  Acid     3-164  -0480 

Chlorine  7-000  -1260 

(8-21) 
Ammonia  '770 

Potassium  2-500 

Sodium  11-090 

Calcium  "  -260 

Magnesium  '207 

72-000 

§  327.  The  Acidity  of  Urine.  The  healthy  urine  of  man  is 
acid,  owing  to  the  presence  of  acid  sodium  phosphate,  the  ab- 
sence of  free  acid  being  shewn  by  the  fact  that  sodium  hyposul- 
phite gives  no  precipitate.  The  amount  of  acidity  is  about 
equivalent  to  2  grms.  of  oxalic  acid  in  twenty-four  hours,  but 
the  degree  of  acidity  at  any  one  time  varies  much  during  the 
day,  being  in  an  inverse  ratio  to  the  amount  of  acid  secreted  by 
the  stomach  ;  thus  it  decreases  after  food  is  taken,  and  increases 
again  as  gastric  digestion  comes  to  an  end.  It  varies  with  the 
nature  of  the  food  ;  with  a  vegetable  diet  the  excess  of  alkalis 
in  the  food,  being  secreted  by  the  urine,  leads  to  alkalinity,  or 
at  least  to  diminished  acidity,  whereas  this  effect  is  wanting 
with  an  animal  diet,  in  which  the  alkalis  are  less  abundant, 
earthy  bases  preponderating.  Hence  the  urine  of  carnivora  is 
generally  very  acid,  while  that  of  herbivora  is  alkaline.  The 
latter,  when  fasting,  are  for  the  time  being  carnivorous,  living 
entirely  on  their  own  bodies,  and  hence  their  urine  becomes 
under  these  circumstances  acid. 

The  natural  acidity  increases  for  some  time  after  the  urine 
has  been  discharged,  owing  to  the  formation  of  fresh  acid,  appar- 
ently by  some  kind  of  fermentation.  This  increase  of  acid  fre- 
quently causes  a  precipitation  of  urates,  which  the  previous 
acidity,  even  after  the  cooling  of  the  urine,  had  been  insufficient 
to  throw  down.  After  a  while  however  the  acid  reaction 
gives  way  to  alkalinity.  This  is  caused  by  a  conversion  of  the 
urea  into  ammonium  carbonate  through  the  agency  of  a  specific 
'  organized '  ferment.  This  ferment  as  a  general  rule  does 
not  make  its  appearance  except  in  urine  exposed  to  the  air ;  it 
is  only  in  unhealthy  conditions  that  the  fermentation  takes 
place  within  the  bladder,  and  in  such  cases  is  due  either  to 
micro-organisms  introduced  into  the  bladder  from  without, 
during  the  use  of  instruments  for  instance,  or  to  the  action  of 
an  unorganized  ferment,  secreted  apparently  by  the  walls  of  the 
bladder. 


522        ABNOEMAL  CONSTITUENTS  OF  URINE.     [Book  ii. 

§  328.  Abnormal  Constituents  of  Urine.  The  structural  ele- 
ments found  in  the  urine  under  various  circumstances  are  blood, 
pus  and  mucus  corpuscles,  epithelium  from  the  bladder  and 
kidney,  and  spermatozoa.  To  these  may  be  added  the  so-called 
'  casts  '  which  are  either  '  epithelial  casts,'  that  is  to  say  cylinders 
of  more  or  less  altered  epithelial  cells  shed  from  the  tubules,  or 
structureless  '  fibrinous '  casts,  which  are  cylinders  of  peculiar 
material  moulded  in  the  lumina  of  the  tubules  ;  the  exact  nature 
of  this  material  is  at  present  a  matter  of  doubt ;  it  is  not  always 
the  same  but  appears  not  to  be  fibrin. 

The  most  common  and  important  abnormal  constituents  of 
urine  are  albumin,  giving  rise  to  albuminuria,  and  sugar,  giving 
rise  to  glycosuria  or  diabetes.  The  soluble  proteids  generally 
spoken  of  as  'albumin '  in  the  urine  differ  in  different  cases.  The 
exact  determination  of  their  nature  is  a  matter  of  some  diffi- 
culty, since,  as  we  have  seen,  we  have  in  differentiating  the 
various  proteids  to  trust  largely  to  their  behaviour  as  regards 
precipitation  upon  the  addition  of  certain  saline  bodies ;  and 
the  presence  of  saline  bodies  in  the  natural  urine  introduces 
complications.  It  would  appear,  however,  that  the  proteids 
usually  present  are  serum-albumin  and  globulin ;  these  are  not 
however  as  a  rule,  if  ever,  present  in  the  same  relative  propor- 
tions as  in  blood-plasma ;  and  either  the  one  or  the  other  may 
be  present  by  itself.  A  form  of  albumose  (§  181)  called  hemi- 
albumose,  is  sometimes  found,  and  indeed  probably  very  many 
distinct  kinds  of  proteids  are  from  time  to  time  present.  If  egg- 
albumin  be  injected  into  the  blood  it  appears  in  the  urine  as 
egg-albumin,  and  peptone  similarly  injected  appears  as  peptone. 

The  sugar  which  is  found  in  the  urine  of  diabetes  is  undis- 
tinguishable  from  ordinary  dextrose  ;  but  whether  it  is  abso- 
lutely identical  with  that  body,  or  whether  the  sugar  in  all 
cases  of  diabetic  urine  is  exactly  the  same,  cannot  perhaps  as 
yet  be  regarded  as  definitely  settled. 

When  blood  is  mingled  with  urine  in  the  kidney  and  in  the 
urinary  passages  the  constituents  of  the  former  are  of  course 
added  to  those  of  the  latter  ;  and  when,  as  sometimes  happens, 
chyle  from  the  lacteals  makes  its  way  into  the  kidneys  the 
urine  contains  the  fats  and  other  constituents  of  chyle.  Fats, 
however,  may  be  present  without  the  urine  being  distinctly 
'chylous.' 

Cholesterin,  bile-acids,  bile-pigments,  and  one  or  other  of  a 
large  number  of  bodies  arising  from  a  disordered  metabolism 
of  the  body,  such  as  leucin,  tyrosin,  acetone  (in  cases  of  dia- 
betes), oxalic  acid,  taurin,  cystin  and  many  others  are  also  found 
more  or  less  frequently  ;  some  of  these  indeed  have  been  re- 
garded as  normal  constituents.  Besides  these  the  urine  serves 
as  the  chief  channel  of  elimination  for  various  bodies,  not  proper 
constituents  of  food,  which  may  happen  to  have  been  taken  into 


Chap,  hi.]     ELIMINATIOX   OF   WASTE   PEODUCTS.         523 

the  system.  Thus  various  minerals,  alkaloids,  salts,  pigmentary 
and  odoriferous  matters,  may  be  passed  unchanged.  Many  sub- 
stances thus  occasionally  taken  undergo,  however,  changes  in 
passing  through  the  body  ;  the  most  important  of  these,  since 
the  changes  which  they  undergo  throw  light  on  the  metabolic 
processes  of  the  body,  will  be  considered  in  a  succeeding  chapter. 


SEC.  2.    THE   SECRETION   OE   URINE. 

§  329.  The  kidney,  unlike  the  other  secreting  organs  which 
we  have  hitherto  studied,  consists  of  two  parts,  so  distinct  in 
structure  that  it  seems  impossible  to  resist  the  conclusion  that 
the  functions  of  the  two  parts,  are  different,  and  that  the 
mechanism  by  which  the  urine  is  secreted  is  of  a  double  kind. 
On  the  one  hand  the  tubuli  uriniferi  with  their  characteristic 
epithelium  seem  obviously  to  be  actively  secreting  structures 
comparable  to  the  secreting  alveoli  of  the  salivary  and  other 
glands.  On  the  other  hand  the  Malpighian  capsules  with  their 
glomeruli  are  organs  of  a  peculiar  nature  with  an  almost  in- 
significant epithelium,  and  their  structure  irresistibly  suggests 
that  they  act  rather  as  what  may  be  called  in  a  general  way  a 
filtering  than  as  a  truly  secreting  mechanism.  Hence  has  arisen 
the  view,  which  frequently  bears  the  name  of  Bowman  since  he 
was  the  first  to  put  it  forward,  that  certain  constituents  only 
of  the  urine  are  secreted  after  the  fashion  of  other  secreting 
glands  by  the  tubuli  uriniferi,  and  that  the  rest  of  the  con- 
stituents, including  a  great  deal  of  the  water  with  such  highly 
soluble  and  diffusible  salts  as  pre-exist  in  adequate  quantity  in 
the  blood,  are  as  it  were  filtered  off  by  the  glomeruli  of  the 
Malpighian  capsules.  We  shall  see  later  on  reason  to  doubt 
whether  we  are  justified  in  applying  the  term  '  filtration,'  which 
has  a  definite  physical  meaning,  to  the  process  by  which  water 
and  other  substances  pass  from  the  blood  vessels  of  the  glome- 
rulus into  the  lumen  of  the  tubule  ;  for  that  process  is  as  we 
shall  find  peculiar  and  complex.  But  such  a  doubt  need  not 
prevent  us  from  recognizing  that  the  whole  act  of  secretion  of 
urine  consists  of  two  parts,  one  of  which  is  much  more  closely 
dependent  on  the  flow  of  blood  through  the  kidney  than  is  the 
ordinary  process  of  secretion  such  as  has  hitherto  come  before 
us,  and  another  part  which  seems  to  bear  the  same  relation  to 
the  flow  of  blood  as  does  ordinary  secretion. 

That  the  work  of  the  kidney  is  to  an  unusual  degree  dependent 
on  the  flow  of  blood  through  it  seems  suggested  by  the  vascular 
arrangements  ;  for  these  are  extremely  favourable  to  a  full  and 

524 


Chap,  hi.]     ELIMINATION   OF   WASTE   PEODUCTS.         525 

rapid  stream  of  blood  through  the  organ.  The  short  and  rela- 
tively broad  renal  artery  comes  off  direct  from  the  abdominal 
aorta,  where  the  blood-pressure  is  extremely  high  ;  the  renal 
vein  opens  directly  into  the  vena  cava,  where  the  blood-pressure 
is  extremely  low.  Between  the  mouth  of  the  renal  artery  and 
the  mouth  of  the  renal  vein  the  difference  of  pressure  is  very 
great  indeed  ;  and  as  we  have  seen  in  treating  of  the  vascular 
system  it  is  the  difference  of  pressure  between  two  points  of 
the  vascular  tract  which  is  the  actual  cause  of  the  flow  of  blood 
from  the  one  point  to  the  other.  The  difference  of  jDressure 
indeed  which  drives  the  blood  through  the  limited  area  of  the 
kidney  is  the  same  difference  of  pressure  which  drives  the 
blood  along  the  abdominal  aorta  down  to  the  foot  and  back 
again  to  the  vena  cava. 

This  free  and  abundant  supply  of  blood  is  regulated,  is  either 
increased  or  diminished,  according  to  the  needs  of  the  moment, 
by  the  vaso-motor  system  ;  this  is  shewn  by  experimental  and 
other  results,  which  it  will  be  profitable  to  study  in  some  detail. 
Before  entering  into  these  details,  however,  it  will  be  well  to 
call  attention  to  the  fact  that  when  vaso-motor  events  modify 
the  flow  of  blood  through  an  organ  they  produce  their  effects  in 
one  direction  or  another  by  working  on  arterial  blood-pressure. 
Thus,  as  we  shall  see,  when  stimulation  or  section  of  a  nerve 
increases  the  flow  of  blood  through  the  kidney  it  does  so  by 
increasing  the  pressure  in  the  small  vessels  of  the  kidney,  includ- 
ing the  capillary  loops  of  the  glomeruli.  In  such  a  case  the 
walls  of  the  glomerular  loops,  through  which  the  passage  of 
materials  to  form  (part  of)  the  urine  takes  place,  are  subjected 
to  two  influences  ;  on  the  one  hand  to  a  fuller,  more  rapid  flow 
of  blood  past  them,  and  on  the  other  to  an  increase  of  the  pres- 
sure which  that  blood  as  it  passes  along  exerts  on  them.  We 
shall  have  subsequently  to  discuss  the  share  taken  by  these  two 
influences  in  determining  and  modifying  the  passage  of  material 
through  the  walls  of  the  glomerular  loops  ;  and  this  will  bear 
on  the  question  of  filtration  to  which  we  have  above  alluded  ; 
but  for  the  present  it  will  be  convenient  to  deal  with  the  effects 
of  variation  in  blood-pressure  apart  from  this  secondary  question. 

§  330.  The  vaso-motor  mechanisms  of  the  kidney.  It  may 
be  shewn  experimentally  that  the  kidney  is  supj3lied  with  a  vaso- 
motor mechanism  as  well  developed  perhaps  as  that  of  any  other 
part  of  the  body.  By  means  of  a  modification  of  the  plethysmo- 
graph  (Figs.  102,  103),  we  can  readily  observe  the  variations 
which  take  place  in  the  volume  of  the  kidney. 

The  instrument  consists  of  two  parts,  one  of  which  (Fig.  102), 
called  the  oneometei-,'  is  applied  to  tlie  organ  about  to  be  studied, 
while  the  other  (Fig.  103),  called  the  oncograph,  is  the  recording  part 

1  From  oncos,  bulk. 


526        FLOW   OF   BLOOD   THEOUGH   KIDNEY.      [Book  ii. 

of  the  apparatus.  Any  diminution  in  the  volume  of  the  organ  (Fig. 
102,  K),  kidney,  spleen,  etc.  as  the  case  may  be,  diminishes  the 
pressure  on  the  fluid  in  the  chamber  a ;  some  of  the  fluid  in  the 
chamber  M  (Fig.  103)  accordingly  passes  through  the  tube  K  (Fig. 
103)  and  the  tube  T  (Fig.  102)  to  the  chamber  a;  the  piston  D 
accordingly  falls  and  with  it  the  lever  H.  Similarly  an  increase  in 
the  volume  of  the  organ  causes  the  lever  to  rise. 


Fig.  102.  Renal,  Oncometer.  Seen  in  section  (semi-diagrammatic) .  K. 
Mdney,  V.  vessels  and  nerves  imbedded  in  fat,  &c.  entering  hilus  of  organ,  O.  C. 
and  I.e.  outer  and  inner  metal  capsules  screwed  together  by  the  screw  S,  and 
holding  between  them  the  edge  of  the  membrane  M  which  applies  itself  to  the 
surface  of  the  kidney,  and  forms  with  the  metal  capsule  two  chambers  a  and  B, 
one  of  which  {B)  is  closed  by  a  plug  filling  the  opening  B,  while  the  other  (a) 
communicates  by  a  tube  T  with  the  recording  instrument.  The  other  opening  O 
(which  is  closed  by  a  small  tap)  is  for  the  purpose  of  filling  the  chamber  a  with 
warm  oil,  after  the  kidney  has  been  placed  in  the  box,  the  other  chamber  B 
having  been  previously  partly  filled,  the  quantity  introduced  into  it  depending 
upon  the  size  of  the  kidney. 


The  volume  of  the  kidney  may  be  increased  by  a  swelling  of 
its  constituent  cells  and  other  structural  elements,  by  an  accumu- 
lation of  lymph  in  its  lymph-spaces,  and  by  a  distension  of  its 
blood  vessels.  Compared  with  the  third,  the  two  former  causes 
are  in  health  so  insignificant  and  problematical  that  they  may  be 
disregarded.     Further,  the  distension  of  the  blood  vessels  will 


Chap,  hi.]     ELIMINATION   OF   WASTE   PRODUCTS.         527 

in  general  depend  on  the  constriction  or  dilation  of  the  renal 
arteries  and  their  ramifications,  for  distension  due  to  venous 
obstruction  will  only  occur  in  special  cases.  Hence  variations 
in  the  volume  of  the  kidney  may  be  taken  as  a  measure  of  varia- 


FiG.  103.  Semi-diagrammatic  sectional  view  of  Oncograph.  Half  natural 
size.  K,  tube  connecting  instrument  with  oncometer.  D,  piston  floating  on  oil 
contained  in  the  cavity  M ;  the  oil  is  prevented  from  escaping  by  the  side  of  the 
piston  by  the  delicate  flexible  membrane  E,  which  does  not  interfere  with  the 
movements  of  the  piston.  H,  recording  lever  connected  with  the  piston  by  a 
needle  G  passing  through  the  guides  F,  F'.  The  screw  C  is  for  the  purpose  of 
clamping  the  edge  of  the  membrane  between  the  two  ring-shaped  surfaces  at  N, 
while  the  side  tube  L  is  for  the  purpose  of  filling  the  instrument. 

tions  in  its  vascular  supply,  increase  of  volume  indicating  dilated 
renal  vessels,  and  decrease  of  volume  indicating  constriction  of 
the  renal  vessels. 

When  by  means  of  the  instrument  just  described  a  tracing  is 
taken  of  the  volume  of  a  kidney  in  what  may  be  considered  a 
normal  condition,  some  such  result  as  that  shewn  in  Fig.  104  is 
obtained. 

The  volume  of  the  kidney  is  seen  to  be  so  delicately  respon- 
sive to  changes  in  the  mean  arterial  pressure  that  the  curve 
reproduces  almost  exactly  a  blood-pressure  curve,  shewing  not 
only  the  respiratory  undulations,  but  even  the  rise  and  fall  due 
to  the  individual  heart-beats.  With  each  rise  of  mean  arterial 
pressure  more  blood  is  driven  into  the  renal  vessels  and  the  kid- 
ney swells  :  with  each  fall  of  pressure  less  blood  enters  and  the 
kidne}^  shrinks.  On  other  tracings  taken  in  the  same  way  may 
often  be  seen  (not  shewn  in  Fig.  104)  the  wider  variations  corre- 
sponding to  the  Traube-Hering  curves ;  but  it  will  be  observed 
that  in  these  the  kidney  shrinks  with  the  rise  of  pressure  and 


528 


TLOW  OF  BLOOD  THROUGH  KIDNEY.     [Book  ii. 


swells  with  the  fall.  For  as  we  have  seen  (§  316)  the  rise  in 
the  Traube-Hering  undulation  is  due  to  an  augmentation  of 
peripheral  resistance  caused  by  the  constriction  of  minute  arte- 
ries ;  and  this  constriction  occurs  in  the  kidney  as  elsewhere ; 
the  renal  arterioles  take  their  share  in  producing  the  result, 


BLOOD         PRESSURE 


KIDNEY        CURVE 


Fig.  104.  Blood-pressure  tracing,  and  Curve  from  Renal,  Oncometer. 
Natural  size.  The  blood-pressure  abscissa  line  has  been  raised  2-75  cm.  (the 
actual  medium  blood-pressure  having  been  115  mm.  Hg.).  The  time-curve  gives 
interruptions  recurring  every  three  seconds. 

and  in  consequence  of  their  constriction  the  kidney  shrinks. 
Similarly  the  relaxation  of  the  renal  vessels  contributes  to 
bring  about  the  sequent  fall. 

§  331.  In  the  course  of  a  discussion  in  an  earlier  part  of 
this  work  (§  149)  on  the  local  and  general  effects  of  arterial 
constriction  and  dilation,  we  saw  that  the  local  blood-pressure 
in  and  flow  of  blood  through  the  capillaries  and  other  minute 
vessels  of  this  or  that  vascular  area  may  be  increased  — 

1.  By  an  increase  of  the  general  blood-pressure,  brought 
about — (a)  by  an  increased  force,  frequency,  &c.  of  the  heart's 
beat,  (5)  by  the  constriction  of  the  small  arteries  supplying 
areas  other  than  the  area  in  question. 

2.  By  a  relaxation  of  the  artery  (or  arteries)  supplying 
the  area  itself,  which,  while  diminishing  the  pressure  in  the 
artery  itself,  increases  the  pressure  in  the  capillaries  and  small 
veins  which  the  artery  supplies.  It  need  hardly  be  added  that 
this  local  relaxation  must  not  be  accompanied  by  a  too  great 
dilation  elsewhere. 

The  same  local  blood-pressure  and  flow  of  blood  may  simi- 
larly be  diminished  — 

1.  By  a  constriction  of  the  artery  of  the  area  itself  (and 
its  branches),  which,  while  increasing  the  pressure  on  the 
cardiac  side  of  the  artery,  diminishes  the  pressure  in  the  capil- 
laries and  veins  which  are  supplied  by  the  artery.  This  again 
must  not  be  accompanied  by  a  too  great  constriction  elsewhere. 


Chap,  hi.]     ELIMIXATIOX   OF   WASTE   PEODUCTS.         529 

2.  By  a  lowering  of  tlie  general  blood-pressure,  brought 
about  —  (a)  by  diminished  force,  &c.  of  the  heart's  beat,  (5) 
b}'"  a  general  dilation  of  the  small  arteries  of  the  body  at  large, 
or  by  a  dilation  of  vascular  areas  other  than  the  area  in  ques- 
tion. 

Applying  these  considerations  to  the  blood  vessels  of  the 
kidney,  we  should  expect  to  find  the  following. 

A  rise  in  general  blood-pressure,  and  that  means  a  rise  of 
pressure  in  the  abdominal  aorta  at  the  mouth  of  tlie  renal 
artery,  will  cause  a  greater  flow  of  blood  through,  and  so  an 
expansion  of  the  kidney,  provided  that  the  renal  arteries  them- 
selves are  not  unduly  constricted  at  the  same  time.  This  is 
well  shewn,  as  we  have  seen,  in  the  curve  given  above,  where 
the  increase  of  pressure  due  to  each  heart-beat,  as  well  as  that 
due  to  each  respiratory  movement,  being  of  central  origin  and 
not  due  to  arterial  constriction  and  being  unaccompanied  by 
any  compensating  constriction  of  the  renal  artery,  leads  to 
expansion  of  the  kidne}-,  that  is,  to  a  greater  flow  of  blood 
through  the  kidne}'. 

If,  however,  the  rise  of  general  blood-pressure  be  due  to 
events  which  at  the  same  time  cause  a  constriction  of  the  renal 
arteries,  the  flow  through  the  kidney  may  not  only  not  be 
increased  but  even  be  diminished ;  the  kidney  may  shrink 
instead  of  expanding.  Thus  if  dyspnoea  be  brought  about,  as 
by  stopping  artificial  respiration  during  an  experiment,  the 
kidney  at  once  shrinks;  the  too  venous  blood  stimulates 
the  vaso-motor  centre,  and  probably  also  by  direct  action  on 
the  blood  vessels  leads  to  a  general  arterial  constriction  and 
so  to  a  rise  of  blood-pressure  ;  but  the  renal  vessels  are  involved 
in  this  constriction,  so  much  so  that  their  constricted  condition 
more  than  counterbalances  the  general  rise  of  blood-pressure, 
and  less  blood  flows  through  the  renal  vessels.  So  also  when 
the  medulla  or  spinal  cord  is  directly  stimulated  by  induction 
shocks  (the  animal  being  under  urari  so  as  to  eliminate  tlie 
complications  due  to  contractions  of  the  skeletal  muscles)  the 
renal  vessels  share  so  fully  in  the  arterial  constriction  which 
results  that,  in  spite  of  the  great  rise  of  mean  pressure  which 
is  induced,  less  blood  than  normal  passes  through  the  renal 
vessels,  and  the  kidney  shrinks.  Or  if  the  abdominal  splanch- 
nic nerves  be  stimulated,  since  as  we  shall  see  these  carry 
vaso-constrictor  fibres  for  the  kidney,  in  spite  of  the  rise  of 
blood-pressure  which  follows,  the  kidney  shrinks  on  account 
of  tlie  great  constriction  of  the  renal  vessels. 

On  the  other  hand  if  a  rise  of  blood-pressure  be  for  any 
reason  not  accompanied  by  a  compensating  constriction  of  tlie 
renal  arteries,  that  rise,  whether  it  be  brought  about  by  general 
constriction  of  arteries  otlier  than  the  renal  or  by  an  increase 
of  the  cardiac  delivery,  causes  the  kidney  to  swell,  shewing 

34 


530         FLOW   OF   BLOOD   THROUGH   KIDNEY.      [Book  ii. 

a  greater  flow  of  blood.  Such  a  condition  of  things  may  be 
induced  by  section  of  the  nerves  of  the  renal  plexus,  whereby 
the  paths  of  all  vasco-constrictor  impulses  to  the  kidney  are 
blocked.  After  this  has  been  done  a  rise  of  general  pressure 
whether  by  dyspnoea,  or  by  direct  stimulation  of  the  spinal 
cord,  or  by  stimulation  of  the  abdominal  splanchnic  nerves,  leads 
to  a  greater  flow  through  the  renal  vessels  and  an  increased 
expansion  of  the  kidney. 

A  rise  of  general  blood-pressure  then  may  be  accompanied 
by  either  a  shrinking  or  a  swelling  of  the  kidney,  by  either  a 
greater  or  a  less  flow  of  blood  through  the  kidney,  according 
to  the  concomitant  condition  of  the  renal  vessels ;  or  indeed 
may  under  certain  circumstances  be  accompanied  by  no  change 
at  all  in  the  renal  circulation,  the  local  effects  exactly  counter- 
balancing the  general  ones. 

Conversely,  in  a  similar  way,  a  fall  of  blood-pressure  leads 
to  a  lesser  flow  through  the  renal  vessels  and  a  shrinking  of 
the  kidney  unless  it  be  accompanied  by  a  dilation  of  the  renal 
vessels  out  of  proportion  to  the  general  fall.  Thus  when  the 
spinal  cord  is  divided  below  the  medulla  the  fall  of  general 
blood-pressure  is,  as  we  have  seen  (§  151),  very  marked,  being 
due  to  an  abolition  for  the  time  being  of  wonted  constrictor 
impulses.  The  pressure  in  the  aorta  falls  rapidly,  and  at  the 
same  time,  owing  to  the  more  open  pathway  tlirough  the  region 
of  peripheral  resistance  in  the  body  generally,  the  pressure  in 
the  vena  cava  is  increased ;  the  difference  of  pressure  between 
the  mouth  of  the  renal  artery  in  the  aorta  and  the  mouth  of  the 
renal  vein  in  the  vena  cava  is  so  largely  reduced  that  in  spite  of 
the  concomitant  relaxed  condition  of  the  renal  vessels  themselves 
the  flow  of  blood  through  the  kidney  is  largely  diminished. 

It  will  of  course  be  understood  that,  the  general  blood- 
pressure  remaining  the  same,  the  flow  through  the  kidney  will 
at  once  be  on  the  one  hand  increased  by  dilation  and  on  the 
other  decreased  by  constriction  of  the  renal  vessels  themselves. 
The  constricted  or  dilated  condition  of  the  renal  vessels  can 
by  themselves  produce  but  little  effect  on  the  pressure  either 
in  the  aorta  or  in  the  vena  cava ;  and  the  difference  between 
the  pressure  at  the  mouth  of  the  renal  artery  and  that  at  the 
mouth  of  the  renal  vein  remaining  the  same,  the  more  open 
passages  of  the  dilated  renal  vessels  must  lead  to  a  fuller,  and 
the  narrower  passages  of  the  constricted  renal  vessels  to  a 
scantier  flow,  through  the  kidney. 

§  332.  By  means  of  the  oncometer,  watching  the  shrinking 
and  swelling  of  the  kidney  and  thus  judging  of  the  flow  of  blood 
through  it,  the  results  being  always  interpreted  with  reference 
to  the  general  blood-pressure  on  the  lines  of  the  above  discus- 
sion, the  paths  of  vaso-motor  impulses  to  the  kidney  have  been 
approximately  made  out.    Vaso-constrictor  fibres  for  the  kidney 


Chap,  hi.]     ELIMINATION    OF   WASTE   PRODUCTS.         531 

are  supplied  from  what  we  have  previously  (§  147  and  elsewhere) 
spoken  of  as  the  vaso-constrictor  region  of  the  spinal  cord.  They 
issue  from  the  spinal  cord  by  the  anterior  roots  of  a  large  number 
of  the  spinal  nerves  taking  origin  from  this  region,  and  may  be 
traced  (in  the  dog)  as  high  up  as  the  6th  thoracic,  a  few  perhaps 
even  to  the  4th  thoracic,  and  as  low  down  as  the  2nd  lumbar 
(4th  lumbar  if  only  13  nerves  be  counted  as  thoracic)  ;  but  most 
seem  to  pass  by  the  11th,  12th  and  13th  thoracic  nerves.  Passing 
through  the  corresponding  ganglia  of  the  symj^athetic  chain, 
these  fibres  reach  the  solar  plexus  and  thus  the  renal  plexus  by 
the  splanchnic  nerve ;  those  however  coming  from  some  of  the 
lower  nerves  apparently  do  not  contribute  to  the  splanchnic 
nerve,  but  take  a  separate  course.  Centiifugal  stimulation  of 
these  anterior  roots  produces  shrinking  of  the  kidney,  all  the 
more  marked  and  distinct  in  the  case  of  the  11th,  12th  and  13th 
thoracic  roots  because  the  effect  on  the  kidney  is  then  not  so  much 
masked  by  vaso-motor  effects  on  other  organs.  Stimulation  of 
the  higher  roots  also  produces  shrinking  of  the  kidney  but  less 
marked,  since  in  these  cases  the  stimulation  bears  at  the  same 
time  largely  on  vaso-constrictor  fibres  for  other  abdominal  organs, 
and  so  by  raising  the  general  blood-pressure  tends  to  neutralize 
the  local  effect  on  the  kidney.  And  even  the  veiy  decided 
shrinking  of  the  kidney  which  results  from  the  stimulation  of 
the  splanchnic  trunk  itself  is  less  than  would  take  place  if  the 
stimulation  affected  the  vessels  of  the  kidney  only. 

§  333.  There  is  also  some  evidence  gained  by  the  method 
of  slowly  repeated  rhythmical  stimulation  (§  146)  that  some  of 
the  higher  (anterior)  roots  also  contain  renal  vaso-dilator  fibres ; 
but  the  matter  is  not  at  present  beyond  dispute. 

§  334.  It  is  obvious  then  that  by  means  of  this  vaso-motor 
mechanism  the  flow  of  blood  through  the  kidney  is  governed  by 
the  central  nervous  system  in  such  a  way  that  afferent  impulses, 
started  in  this  or  that  region  or  surface,  and  passing  up  to  the 
central  nervous  system,  may  lead  either  to  constriction  or  to 
dilation  of  the  renal  vessels  ;  and  to  such  actions  of  this  kind  we 
shall  presently  return.  Meanwhile,  we  wish  to  call  attention  to 
the  fact  that  changes  in  the  flow  of  blood  through  the  kidne}-, 
as  shewn  by  changes  of  volume,  may  be  brought  about  quite 
apart  from  the  central  nervous  system.  For  instance  after  all 
the  nerves  going  to  the  kidney  have  been  severed,  the  kidney, 
as  shewn  by  the  oncometer,  swells  when  substances  such  as  urea, 
which  cause  an  increase  in  the  secretion  of  urine,  are  injected 
into  the  blood.  The  substance  reaching  the  kidney  by  the  blood 
stimulates  the  kidney  to  activity,  and  this  is  accompanied  by  a 
dilation  of  the  blood  vessels,  which,  since  the  nerves  have  been 
severed,  must  be  brought  about  by  some  local  action.  The  event 
seen  is  similar  to  the  orreater  flow  of  blood  throusfh  a  muscle 
when  it  contracts.     Cf.  §  146. 


532         FLOW   OF  BLOOD   THKOUGH   KIDNEY.     [Book  ii. 

§  335.  If,  while  the  kidney  is  in  the  oncometer,  and  the 
various  experiments  on  section  and  stimulation  of  nerves  and 
the  like  are  being  carried  on,  a  cannula  be  tied  in  the  ureter,  the 
secretion  of  urine  may  be  watched  at  the  same  time.  It  will 
then  be  seen  that  the  flow  of  urine  through  the  end  of  the 
cannula  is  not  equable,  and  does  not  either  increase  or  decrease 
in  an  even  manner.  On  the  contrary,  it  will  frequently  be 
found  that  a  sort  of  gush  of  urine  takes  place,  several  drops 
following  each  other  in  rapid  succession,  followed  by  a  cessation 
of  flow ;  and  if  the  ureter  be  watched  it  will  be  seen  that  the 
gushes  of  urine  are  synchronous  with  waves  of  peristaltic  con- 
traction sweeping  down  the  ureter.  Obviously  the  urine  collects 
to  a  certain  extent  in  the  pelvis  of  the  kidney  and  is  driven 
thence  by  muscular  action  from  time  to  time ;  to  this  point  we 
shall  return  later  on. 

Making  every  allowance,  however,  for  these  irregularities  of 
flow,  we  may  take  the  rate  of  flow  from  the  end  of  the  cannula 
as  a  measure  of  the  rate  of  secretion ;  and  it  is  found  that  as  a 
general  rule  increased  flow  of  urine  is  coincident  with  swelling 
of  the  kidney,  that  is  with  a  greater  flow  of  blood  through  it, 
and  diminished  or  arrested  How  of  urine  is  coincident  with 
shrinking  of  the  kidney,  that  is  with  a  diminished  flow  of  blood 
through  it. 

A  striking  instance  of  this  is  afforded  by  the  experiment  of 
dividing  in  the  dog  the  spinal  cord  below  the  spinal  bulb.  The 
blood-pressure  then,  as  we  know,  falls  rapidly,  owing  to  the 
removal  of  constrictor  impulses  fi'om  the  small  arteries  and  the 
great  diminution  of  peripheral  resistance  which  follows  upon 
so  many  small  arteries  becoming  dilated  ;  and  though  the  renal 
arteries  probably  share  in  the  general  relaxation  yet,  owing  to 
the  fall  of  pressure  in  the  aorta  conjoined  as  this  is  by  a  corie- 
sponding  rise  of  pressure  in  the  vena  cava,  the  flow  of  blood 
through  the  kidney  is  largely  diminished.  We  find  that  after  the 
operation  the  secretion  of  urine  is  greatly  diminished;  indeed, 
in  most  cases,  the  flow  from  the  end  of  a  cannula  is  almost 
arrested.  In  fact  we  may  almost  make  the  general  assertion  that, 
when  in  the  dog  the  blood-pressure  falls  to  about  30  mm.  Hg  or 
less,  the  secretion  of  urine  is  for  the  time  stopped.  These  and 
other  results  support  the  view  stated  above  that  the  secretion  of 
urine  is  in  quite  a  special  way  dependent  on  the  flow  of  blood 
through  the  kidney ;  and  we  may  further  conclude  that  the 
seci'etion  which  is  so  particularly  influenced  by  the  flow  of  blood 
is  that  special  kind  of  secretion,  allied  to  filtration,  which  takes 
place  through  the  glomeruli,  and  not  the  more  ordinary  kind  of 
secretion  by  means  of  the  epithelium  of  the  tubuli  uriniferi. 
But  before  we  proceed  to  discuss  how  the  increased  flow  of 
blood  increases  the  glomerular  flow  of  urine,  we  must  turn  to 
consider  the  functions  of  the  e|)ithelium  of  the  tubuli. 


Chap,  hi.]      ELIMINATION   OF   WASTE   PKODUCTS.         533 


Secretion  hy  the  Renal  Epithelium. 

§  336.  The  glomerular  mechanism  is  after  all  a  small  por- 
tion only  of  the  whole  kidney,  and  the  epithelium  over  a  large 
part  of  the  course  of  the  tubuli  uriniferi  bears  most  distinctly 
the  characters  of  an  active  secreting  epithelium.  These  facts 
would  lead  us  a  priori  to  suppose  that  the  flow  of  urine  is  in 
part  the  result  of  an  active  secretion  comparable  to  that  of  the 
salivary  or  other  glands  which  we  have  already  studied.  And 
we  have  experimental  and  other  evidence  that  such  is  the  case. 

In  the  first  place  a  flow  of  urine  may  be  artificially  excited 
even  when  the  natural  flow  has  been  arrested  by  diminution  of 
blood-pressure.  Thus  if,  when  the  urine  has  ceased  to  flow  in 
consequence  of  a  section  of  the  spinal  bulb,  certain  substances, 
such  as  urea,  urates,  sodium  acetate,  and  the  like,  be  injected 
into  the  blood,  a  more  or  less  copious  secretion  is  at  once  set 
up.  This  secretion  is,  or  at  least  may  be,  unaccompanied  by 
any  rise  of  general  blood-pressure  sufficient  to  account  for  the 
increased  secretion  as  the  mere  result  of  an  increased  flow  of 
blood.  It  is  true  (as  we  have  seen  §  334)  that  the  injection  of 
these  substances  leads  to  an  expansion  of  the  kidney,  to  a 
fuller' flow  of  blood  through  it;  but  this  is  the  effect  rather 
than  the  cause  of  the  secretory  activity.  We  may  infer  that 
the  presence  of  the  above  substances  in  the  blood  excites  the 
renal  epithelium  cells  to  an  unwonted  activity,  causing  them  to 
pour  into  the  interior  of  the  tubules  a  copious  secretion,  just  as 
the  presence  of  pilocarpin  in  the  blood  will  cause  the  salivary 
cells  to  pour  forth  their  secretion  into  the  lumen  of  their  ducts; 
and  that  this  activity  of  the  epithelium  cells  is  accompanied, 
also  as  in  the  case  of  the  submaxillary  and  other  glands,  by  a 
vascular  dilation,  which,  though  adjuvant  and  beneficial,  is  not 
the  distinct  cause  of  the  activity.  This  view  is  further  sup- 
ported by  the  following  remarkable  experiment,  which  goes  far 
to  shew  that  of  the  various  substances  which  having  found  their 
way  into  the  blood  are  thrown  out  by  the  kidney,  some  pass 
into  the  urine  through  the  glomeruli  while  others  are  distinctly 
secreted  by  the  tubuli  urinifei'i,  the  discharge  of  the  latter  being 
accompanied  by  a  general  activity  of  the  secreting  cells,  as 
shewn  by  the  flow  of  water  taking  place  at  the  same  time. 

In  the  amphibia,  the  kidney  has  a  double  vascular  supply:  it 
receives  arterial  blood  from  the  renal  artery,  but  there  is  also 
poured  into  it  venous  blood  from  another  source.  The  femoral 
vein  divides  at  the  top  of  the  thigh  into  two  branches,  one  of 
which  runs  along  the  front  of  the  abdomen  to  meet  its  fellow  in 
the  middle  line  and  form  the  anterior  abdominal  vein,  while  the 
other  passes  to  the  outer  border  of  the  kidney  and  branches  in 
the  substance  of  that  organ,  forming  the  so-called  renal  portal 


534  SECRETIOK   BY   RENAL    EPITHELIUM.     [Book  ii. 

system.  Now  the  glomeruli,  in  some  species  at  least  of  these 
animals,  are  supplied  exclusively  by  the  branches  of  the  renal 
artery,  the  renal  vena  portse  only  serving  to  form  the  capillary 
plexus  around  the  tubuli  uriniferi,  which  is  also  supplied  by  the 
efferent  vessels  of  the  glomeruli.  From  this  it  is  obvious  that 
if  the  renal  artery  be  tied,  the  blood  is  shut  off  entirely  from 
the  glomeruli ;  and  actual  observation  of  the  kidney  has,  in  the 
animals  in  question,  shewn  that  under  these  circumstances  there 
is  no  reflux  from  the  capillary  network  surrounding  the  tubules 
back  to  the  glomeruli;  thus  the  kidney  by  this  simple  operation 
is  transformed  into  an  ordinary  secreting  gland  devoid  of  any 
special  filtering  mechanism.  Such  a  kidney  may  be  used  to 
ascertain  what  substances  are  "excreted  by  the  glomeruli,  and 
what  by  the  tubules  in  some  other  part  of  their  course.  It  is 
found  that  urea  injected  into  the  blood  gives  rise  to  a  secre- 
tion of  urine  when  the  renal  arteries  are  tied ;  this  substance 
therefore  is  secreted  by  the  epithelium  of  the  tubules,  and  in 
being  so  secreted  gives  rise  at  the  same  time  to  a  flow  of  water 
through  the  cells  into  the  interior  of  the  tubules.  Sugar  and 
peptones,  on  the  other  hand,  which  injected  into  the  blood 
readily  pass  through  the  untouched  kidney  and  appear  in  the 
urine,  do  not  pass  through  a  kidney  the  renal  arteries  of  which 
have  been  tied,  even  when  a  diuretic  such  as  urea  is  given  at 
the  same  time  in  order  to  secure  a  flow  of  urine.  These  sub- 
stances therefore  are  excreted  by  the  glomeruli. 

The  validity  of  this  experiment,  which  may  be  accepted  as 
indicating  a  marked  difference  between  glomerular  secretion  on 
the  one  hand  and  epithelial  or  tubular  secretion  on  the  other, 
depends  on  the  absence  of  any  collateral  circulation  whereby  the 
glomeruli  may  be  supplied  with  blood  after  ligature  of  the  renal 
artery.  In  these  animals  anastomoses  occur  between  the  renal 
arteries  and  the  arteries  of  the  generative  organs ;  and  unless  the 
renal  artery  be  so  tied  as  to  avoid  these  collateral  communications 
the  results  of  the  experiment  are  different. 

Additional  evidence  in  favour  of  the  secretory  activity  of  the 
epithelium  cells  is  afforded  by  the  following  observation.  Into 
the  veins  of  animals  in  which  the  urinary  flow  had  been  arrested 
by  section  of  the  spinal  cord  below  the  medulla  a  quantity  of 
the  blue  colouring  material  known  as  sodium  sulphindigotate  ^ 
is  injected.  This  substance  is  rapidly  excreted  on  the  one  hand 
by  the  liver  in  the  bile,  and  on  the  other  hand  by  the  kidney. 
By  varying  the  quantity  injected,  killing  the  animals  at  appro- 
priate times  after  the  injection  of  the  material,  and  examining 
the  kidneys  microscopically  and  otherwise,  it  may  be  ascertained 
that  the  pigment  so  injected  passes  from  the  blood  into  the  renal 

1  Sometimes  called  indigo-carmine,  though  this  name  is  more  properly  applied 
to  a  crude  impure  preparation  of  potassium  sulphindigotate. 


Chap,  hi.]      ELIMINATION    OF   WASTE   PEODUGTS.         535 

epithelium,  and  from  thence  into  the  channels  of  the  tubules. 
There  being  no  stream  of  fluid  through  the  tubules,  owing  to  the 
arrest  of  urinary  flow  by  means  of  the  preliminary  operation,  the 
pigment  travels  very  little  way  down  the  interior  of  the  tubules, 
and  remains  very  much  where  it  was  cast  out  by  the  epithelium 
cells.  There  are  no  traces  whatever  of  the  pigment  having 
passed  by  the  glomeruli ;  and  the  cells  which  appear  most  dis- 
tinctly to  take  up  and  eject  it,  are  those  lining  such  portions  of 
the  tubules  Cviz.  the  first  and  second  convoluted  tubules,  zisfzacr 
tubules  and  ascending  limbs  of  the  loops  of  Henle)  as  from  their 
microscopic  features  have  been  supposed  to  be  the  actively 
secreting  portions  of  the  entire  tubules. 

The  above  observation  may  be  objected  to  on  the  ground  that 
this  colouring  matter  does  not  occur  as  a  constituent  of  the  blood 
either  in  health  or  disease,  and  especially  that  the  absence  of  any 
concomitant  discharge  of  fluid  from  the  cells  excites  suspicion 
that  the  process  observed  was  not  really  one  of  secretion ;  for 
the  injection  of  such  substances  as  urea  or  urates  into  the  blood 
does  cause  a  copious  flow  of  fluid,  and  indeed  thus  prevents  the 
microscopic  tracking  out  of  their  passage,  which  in  the  case  of 
urates  might  otherwise  be  done  much  in  the  same  way  as  with 
the  sodium  sulphindigotate.  Still  in  birds,  the  urine  of  which 
contains  little  water,  urates  may  be  detected  in  the  epithelium 
of  the  tubules  though  not  in  the  capsules.  Without  insisting 
too  much  on  the  value  of  the  sodium  sulphindigotate  experi- 
ments, they  may  be  taken  as  fairly  supporting  the  view  which 
we  are  considering.  We  may,  for  the  present,  conclude  that  the 
secretion  of  urine  does  consist  of  two  separate  and  distinct  acts : 
secretion  by  the  glomeruli,  which  we  may  for  brevity's  sake 
speak  of  as  glomerular  secretion,  and  secretion  by  the  epithelium 
of  the  tubuli,  which  we  may  speak  of  similarly  as  tubular  secre- 
tion. Both  these  forms  of  secretion,  especially  the  former  but 
to  a  certain  extent  the  latter  also,  differ  from  the  secretion  of 
such  a  gland  as  the  salivary,  and  both  deserve  some  special 
consideration. 

§  337.  Tlie  nature  of  glomerular  secretion.  We  have  seen  that 
the  expansion  of  the  kidney  which  has  for  its  accompaniment  an 
increased  flow  of  urine  is  one  brought  about  by  the  renal  artery 
and  its  various  bi-anches  becoming  dilated,  under  such  circum- 
stances that  the  difference  between  the  blood-pressure  in  the 
aorta  at  the  mouth  of  the  renal  artery  and  the  blood-pressure  at 
the  vena  cava  at  the  mouth  of  the  renal  vein  is  at  the  same  time 
increased,  or  at  all  events  is  not  diminished. 

In  dealing  with  the  vascular  system  we  saw  that  relaxation 
of  a  small  artery,  taking  place  without  any  marked  change  in 
the  general  blood-pressure  and  in  neighbouring  arteries,  leads  to 
a  fuller  and  more  rapid  stream  of  blood  through  the  capillaries 
supplied  by  the  arter}-,  and  that  at  the  same  time  the  pressure  in 


536  GLOMERULAR   SECRETION.  [Book  ii. 

the  capillaries  themselves  is  increased ;  owing  to  the  decrease  of 
peripheral  resistance  through  the  widening  of  the  artery,  the  great 
fall  of  pressure  (see  §  98)  so  characteristic  of  the  peripheral 
region  is  shifted  from  the  arterial  side  of  the  capillaries  towards 
the  venus  side  and  to  the  capillaries  themselves. 

Hence,  as  we  have  already  said,  when  the  renal  artery  dilates 
two  things  happen  in  the  loops  of  the  glomeruli :  a  fuller,  more 
rapid  stream  of  blood  passes  through  them,  and  that  blood  as  it 
flows  through  them  is  exerting  a  greater  pressure  than  before 
on  their  walls.  How  does  each  of  the  events  stand  towards  the 
secretion  of  urine  ? 

We  have  not  at  present  the  means  of  inducing  a  fuller  and 
more  rapid  flow  without  increasing  the  pressure ;  but  we  may 
easily  obtain  increase  of  pre^ssure  without  the  fuller  and  more 
rapid  flow.  If  we  hinder  or  obstruct  the  outflow  through  the 
renal,  vein  we  at  once  increase  the  pressure  in  the  glomerular 
loops  as  in  the  other  capillaries  of  the  kidney.  Now,  when  the 
blood-pressure  in  the  glomeruli  is  thus  raised  by  partial  obstruc- 
tion to  the  venous  outflow,  the  flow  of  urine  so  far  from  being 
increased  is  diminished.  Obviously  then  the  passage  of  water 
and  material  through  the  walls  of  the  glomerular  loops,  to  go  to 
form  the  urine,  is  not  the  result  of  mere  pressure,  and  cannot 
therefore  be  spoken  of  properly  as  a  process  of  filtration. 
(Cf.  §  244.)  And  we  may  here  draw  a  comparison  between 
the  passage  of  water  and  material  through  the  wall  of  a  capillary 
in  an  ordinary  situation  to  form  lymph  and  the  passage  through 
the  wall  of  the  glomerular  loop  to  form  urine  or  part  of  urine.' 
The  former  as  we  have  seen  (§  244)  appears  to  be  dependent  on 
pressure,  though  influenced  as  we  have  also  seen  in  a  very  mate- 
rial way  by  the  condition  of  the  vascular  wall ;  and  hindrance 
to  venous  outflow,  so  inefficient  in  promoting  a  flow  of  urine,  is 
as  we  have  seen  especially  favourable  to  the  transudation  of 
lymph.  Moreover,  the  substances  which  pass  through  the  capil- 
lary Avail  to  form  lymph  may  be  described  as  the  constituents 
of  the  blood  generally,  proteids  as  well  as  salts  and  other  soluble 
and  diffusible  matters.  Through  the  wall  of  the  glomerular 
loop  there  pass,  so  long  as  that  wall  is  sound  and  intact,  neither 
albumin  nor  globulin  nor  fibrin  factor,  but  only  water  accom- 
panied by  some,  and  apparently  a  selection  of  some,  of  the  soluble 
diffusible  constituents  of  the  blood ;  for,  as  we  have  said,  the 
jjresence  of  proteids  in  normal  urine  is  contested,  and,  at  most^ 
there  is  present  an  insignificant  quantity  only  (which  moreover 
may  come  from  the  tubular  epithelium).  This  difference  in  the 
material  which  passes  through  may  be  referred  to  the  differences 
in  the  nature  of  the  partition.  The  transudation  of  lymph  takes 
place  through  the  capillary  wall;  between  the  blood  on  one  side 
and  the  lymph  in  the  lymph-space  on  the  other  is  only  the  thin 
film  of  conjoined  epithelioid  plates.     But  the  corresponding  wall 


Cpiap.  III.]     ELIMINATION    OF   WASTE   PEODUCTS.         537 

of  the  glomerular  loop  is  covered  over  and  wrapped  round  so  to 
speak  by  an  adherent  layer  of  cells,  which  though  reduced  and 
thin  are  still  epithelial  cells ;  the  materials  which  go  to  form 
urine  have  to  pass  through  these  cells  as  well  as  through  the 
film  of  epithelioid  plates.  It  seems  to  be  this  layer  of  cells 
which  determines  what  shall  pass  and  what  shall  not. 

Obviously  the  passage  through  this  epithelium  is  of  a  peculiar 
nature.  The  necessary  condition  for  the  due  accomplishment  of 
the  passage  is  as  we  have  seen  a  full  and  rapid  stream  of  (ailerial) 
blood ;  the  high  pressure  which  accompanies  that  full  and  rapid 
stream,  though  probably  under  normal  circumstances  an  adju- 
vant, is  by  itself  helpless.  Thus  when  the  pressure  is  raised  by 
venous  obstruction,  in  which  case  the  high  pressure  is  accom- 
panied by  a  slow  stream  or  by  actual  arrest  of  the  flow,  even  the 
passage  of  mere  water  is  retarded.  Seeing  that  many  of  the 
constituents  of  urine  are  diffusible  substances  certainlj^  pre- 
existing in  the  blood,  inorganic  salines  for  instance,  and  seeing 
that,  if  we  may  trust  the  experiments  on  the  amjjhibian  kidney 
spoken  of  above,  diffusible  abnormal  constituents  of  blood,  such 
as  peptone  and  sugar,  pass  into  the  urine  not  by  the  tubular 
epithelium  but  by  the  glomeruli,  we  might  expect  that  diffu- 
sion, in  contrast  to  filtration  (see  §  253)  played  an  important 
part  in  the  passage  ;  and  a  full  rapid  stream  would  undoubtedly 
favour  diffusion.  But  diffusion  bj^  itself  will  not  explain  mat- 
ters. Egg-albumin  differs  very  slightly  as  regards  diffusibility 
from  serum-albumin,  and  yet  while  at  the  most  a  minute  quan- 
tity only  of  the  latter  passes  into  the  urine  in  normal  circum- 
stances, the  former  when  injected  into  the  blood  at  once  makes 
its  way  into  the  urine,  and  there  is  evidence  that  it  passes  by  the 
glomeruli.  On  the  other  hand  urea  is  an  eminently  diffusible 
body,  and  yet  if  we  can  trust  the  experiments  on  the  amphibian 
kidney,  the  main  mass  at  all  events  of  the  urea  of  the  urine 
passes  by  the  epithelium  of  the  tubules. 

The  important  part  played  by  the  epithelium  is  shewn  when 
the  epithelium  is  deranged.  If  the  renal  artery  be  temporarily 
ligatured  or  otherwise  obstructed,  so  that  the  glomeruli  are  for 
some  little  time  shut  off  from  their  blood-supply,  the  secretion  of 
urine  is  stopped ;  on  reestablishment  of  the  circulation  the 
secretion  of  urine  slowly  returns,  and  the  urine  is  then  found  to 
be  albuminous,  remaining  so  for  some  little  time.  The  serum- 
albumin  and  globulin  which  could  not  pass  through  the  intact 
epithelium,  can  pass  through  when  the  epithelium  has  been 
damaged  by  interference  with  its  nutrition.  The  appearance  of 
albumin  in  the  urine  (albuminuria)  is  a  not  infrequent  symptom 
of  kidney  disease,  and  its  presence  in  other  than  minute  quan- 
tities indicates  imperfections  in  the  glomerular  epithelium.  But 
even  under  unhealthy  conditions  that  epithelium  still  governs  to 
a  certain  extent  the  passage  of  material ;  for  the  proteids  of  the 


538  SECEETION   OF  UREA.  [Book  ii. 

blood-plasma  do  not  pass  through,  bodily  or  in  a  proportion 
which  corresponds  either  to  the  relative  proportion  in  which 
they  exist  in  the  plasma  or  to  the  relative  ease  (or  difficulty) 
with  which  they  pass  through  membranes.  Though  the  "  albu- 
min" of  albuminous  urine  frequently  consists  of  both  serum- 
albumin  and  globulin,  these  do  not  necessarily  occur  in  the 
same  proportion  as  in  blood ;  they  vary  in  urine  much  more  than 
they  do  in  blood;  and  indeed  the  one  or  the  other  may  be 
absent;  moreover  fibrin  factors  are  very  rarely  found. 

Hsemoglobinuria,  or  the  presence  of  haemoglobin  in  urine, 
may  be  brought  about  by  injecting  into  the  blood  vessels  laky 
blood,  or  some  substance  such  as  pyrogallic  acid,  which  will 
"  break  up "  the  corpuscles  of  the  blood.  Now  in  such  cases 
there  is  evidence  that  the  haemoglobin  passes  through  the  glom- 
eruli; minute  disc-like  masses  of  haemoglobin,  the  so-called 
'  menisci,'  are,  by  appropriate  methods  of  preparation,  found  in 
situ  in  the  capsules.  Such  a  passage  is  very  far  removed  from 
being  a  process  of  diffusion. 

We  may  conclude  then  that  the  passage  of  material  through 
the  glomeruli,  like  the  transudation  of  lymph  and  even  to  a 
more  marked  extent,  is  a  complex  affair  in  which  the  ordinary 
physical  processes  of  diffusion  and  filtration  may  play  their  part, 
but  are  not  masters  of  the  situation. 

§  338.  The  work  of  the  epithelium  of  the  tubules.  As  we  have 
said  the  structural  features  of  the  epithelium  cells  of  the  tubules 
seem  to  justify  the  conclusion  that  they  exercise  a  secretory 
activity  comparable  with  that  of  a  salivary  or  a  gastric  gland. 
But  their  work  is  in  many  ways  peculiar.  In  the  case  of  the 
salivary,  gastric,  and  pancreatic  glands  there  can  be  no  doubt 
that  the  specific  constituents  of  the  several  secretions,  mucin, 
pepsin,  trypsin  and  the  like,  are  manufactured  in  the  alveolar 
cells  out  of  antecedents  of  some  nature  or  other.  The  evidence, 
as  we  have  seen,  is  all  against  the  view  that  these  glands  merely 
withdraw,  secrete  in  the  old  sense  of  the  word,  from  the  blood 
these  substances  preexisting  in  the  blood.  When  the  salivary 
glands  are  extirpated  or  the  pancreas  or  the  stomach  removed 
there  is  no  accumulation  in  the  blood  of  the  specific  constituents 
of  tlie  corresponding  secretions.  So  also  when  the  liver  is  extir- 
pated there  is  no  accumulation  in  the  blood  of  either  bile  acids 
or  bile  pigment.  With  regard  to  the  kidney  in  relation  to  the 
most  important  constituent  of  urine,  namely  urea,  the  case  is 
different.  If  the  kidneys  in  a  mammal  be  extirpated,  or  if  the 
kidneys  by  disease  or  by  ligature  of  the  ureters  be  so  damaged 
as  to  be  unable  to  carry  on  their  work,  an  accumulation  takes 
place  in  blood,  not  as  was  once  thought  of  some  antecedent  of 
urea  such  as  kreatin,  but  of  urea  itself.  In  the  case  of 
birds  and  reptiles  which  excrete  not  urea  but  chiefly  uric  acid 
the  accumulation  is  one  of  uric  acid.     Obviously  in  secreting 


CuAF.  III.]     ELIMIXATIOX    OF   WASTE   PRODUCTS.         539 

urea  the  work  of  the  epithelium  of  the  tubules  is  largely  if  not 
exclusively  confinecl  to  simpl}^  picking  the  urea  out  of  the  blood 
and  pushing  it  so  to  speak  into  the  lumina  of  the  tubules.  We 
might  perhaps  say  exclusively,  for  there  is  no  evidence  that  any 
urea  at  all  is  actually  manufactured  in  the  kidney. 

How  the  urea,  which  is  in  this  peculiar  manner  taken  out  of 
the  blood,  comes  to  make  its  appearance  in  the  blood  is  a  problem 
in  which  the  kidney  is  not  concerned  and  with  which  we  shall 
deal  in  treating  of  the  metabolic  events  of  the  body  generally. 

§  339.  In  the  case  of  some  other  constituents  of  the  urine 
we  have  evidence  that  the  cells  do  something  more  than  simply 
pick  the  constituent  out  of  the  blood.  Hippuric  acid,  as  we  have 
seen,  occurs  in  small  quantity  in  the  urine  of  man,  and  in  larger 
amount  in  the  urine  of  herbivora.  Now  hippuric  acid  ma}^  be 
formed  by  the  combination,  with  dehydration,  of  benzoic  acid 
andglycin  (C7Hg02  +  C2H5N03-H20  =  C9H9N03)  ;  and  benzoic 
acid  introduced  into  the  alimentarj^  canal  or  injected  into  the 
blood,  reappears  in  large  measure  in  the  urine  as  hippuric  acid. 
Somewhere  in  the  body  the  benzoic  acid  meets  with  and  com- 
bines with  giycin.  And  we  have  experimental  proof  that  the 
combination  may  and  probably  does  take  place  in  the  kidney. 

If  a  circulation  of  blood  be  kept  up  through  the  blood  vessels 
of  the  kidney  freshly  removed  from  a  living  animal,  and  benzoic 
acid  and  giycin  be  added  to  the  blood  as  it  is  about  to  enter 
into  the  kidney,  hippuric  acid  will  be  found  in  the  blood  issuing 
from  the  kidney,  especially  if  the  same  blood  be  passed  through 
the  kidney  several  times  ;  the  blood  used  must  be  blood  contain- 
ing oxyluemoglobin,  carbonic-oxide-hsemoglobin  not  producing 
the  effect.  The  mere  mixing  with  the  blood  itself  is  insufficient; 
and  if  the  blood  be  sent  not  through  a  kidney  just  removed  from 
the  living  body  but  through  one  taken  from  a  dead  body  or  one 
which  has  been  left  to  itself  for  some  time  after  removal  from  a 
living  body,  the  sjaithesis  will  not  be  effected.  To  cany  out 
the  combination  by  means  of  the  kidne}^  which  has  been  removed 
from  the  body  the  kidney  must  retain  for  a  while  its  own  life,  it 
must  be  a  "surviving"  kidney.  Nor  is  it  absolutel}^  necessary 
to  bring  the  benzoic  acid  and  giycin  to  the  kidney  by  means  of 
a  blood-stream.  If  a  "surviving"  kidney  be  divided  rapidly 
into  small  pieces  and  the  benzoic  acid  rapidly  mixed  with  the 
pieces,  hippuric  acid  is  formed.  Nor  is  it  necessary  to  furnish 
the  giycin.  If  benzoic  acid  alone  be  used,  hippuric  acid  is 
formed  all  the  same.  Giycin,  as  we  have  previously  said,  can- 
not be  recognized  as  a  normal  constituent  of  any  of  the  tissues ; 
nevertheless,  as  we  have  seen  in  speaking  of  glycocholic  acid  in 
the  bile  and  as  Ave  shall  see  later  on,  giycin  must  make  a  momen- 
tary appearance  in  various  metabolic  processes  of  the  bod}',  being 
immediately  on  its  appearance  converted  into  something  else,  so 
that  it  never  remains  as  giycin.     It  apparently  is  formetl  in  the 


540  THE   SKIN   AND   THE   KIDNEYS.         [Book  ii. 

kidney,  and  is  thus  momentarily  available  for  the  conversion  of 
benzoic  into  hippuric  acid. 

It  seems  probable  therefore  that,  with  regard  to  this  par- 
ticular constituent  of  urine,  hippuric  acid,  the  cells  of  the 
tubules  have  the  power  of  effecting  a  combination  between  the 
benzoic  acid  brought  to  them  by  the  blood  and  the  glycin  which 
they  furnish  by  means  of  their  own  metabolism,  and  in  this  way 
produce  hippuric  acid. 

Not  only  benzoic  "acid  but  manj^  other  bodies  taken  into  the 
system  reappear  in  the  urine  combined  with  glycin,  and  in  their 
cases  also  the  combination  probably  takes  place  through  the 
activity  of  the  cells  of  the  tubules  of  the  kidney.  Moreover, 
other  changes  than  the  assumption  of  glycin,  the  various  changes 
which  many  chemical  substances  taken  into  the  system  undergo 
before  reappearing  in  the  urine,  probably  also  take  place  to  a 
large  extent  in  the  kidney,  and  are  also  carried  out  by  means  of 
the  epithelium  of  the  tubules. 

What  other  constituents  of  normal  urine  are  produced  in  this 
or  a  similar  manner  we  do  not  as  yet  definitely  know.  The 
pigment  urobilin,  which  as  we  have  seen  is  supposed  to  be  a 
derivative  from  bilirubin,  may  be  brought  ready  formed  from 
the  liver  or  may  have  the  finishing  touches  given  to  it  in  the 
kidney  itself;  and  the  other  normal  or  abnormal  urinary  pig- 
ments possibly  arise  either  directly  from  hsemoglobin  or  indirectly 
from  that  body  through  the  biliary  pigment  by  a  transformation 
taking  place  in  the  cells  of  the  tubules.  There  is  also  evidence 
in  frogs  that  acid  sodium  phosphate  is  furnished  by  the  cells  of 
the  tubules. 

In  conclusion  then  we  may  say  that  the  activity  of  the  epi- 
thelium of  the  kidney  appears  especially  modified,  as  compared 
with  other  secreting  glands,  to  meet  the  special  object  which  the 
kidney  has  to  secure.  The  purpose  of  the  kidney  is  not  to 
provide  a  fluid,  urine,  which  can  be  made  use  of  for  the  needs 
of  the  body,  but  to  cast  out  waste  matters  from  the  body.  Hence 
its  secretory  activity  is  limited  largely  to  the  mere  discharge 
of  matters  which  reach  it  preexistent  in  the  blood,  though  in 
several  cases  it  gives  the  final  shape  to  the  excreted  substance 
before  this  passes  into  the  ureter. 

§  340.  We  may  illustrate  the  preceding  discussions  by  briefly 
passing  in  review  some  of  the  more  usual  ways  in  which  the 
secretion  of  urine  is  in  ordinary  life  modified. 

In  the  preceding  section  the  composition  of  urine  was  illus- 
trated by  the  daily  output  of  the  several  constituents  rather 
than  by  a  percentage  account  of  any  specimen  of  urine,  for  the 
reason  that  the  composition  of  urine  varies  within  extremely 
wide  limits.  This  is  especially  the  case  as  regards  the  propor- 
tion of  water  to  solids.  One  urine  may  be  of  high  specific 
gravity  with  a  small  amount  of  water  relatively  to  the  solids, 


CiiAP.  III.]     ELIMINATION   OF  WASTE   PRODUCTS.         541 

while  another  may  have  so  little  colour  and  such  a  low  specific 
gravity  as  to  appear  hardly  more  than  w^ater.  The  reason  of 
these  extreme  dilferences  lies  in  the  fact  that  the  kidne}'  is  not 
only  the  channel  by  which  waste  solids  leave  the  bodj^  but  also 
an  important  outlet  for  the  discharge  of  the  stream  of  water 
which,  in  order  that  the  various  processes  of  the  body  may  be 
duly  carried  on,  is  continually  passing  through  the  system.  It 
is  frequently  of  advantage  to  the  body  to  discharge  through  the 
kidney  a  large  amount  of  water,  more  or  less  irrespective  of  the 
solid  matters  which  are  so  to  speak  washed  away  with  it ;  and 
hence  the  advantage  of  the  glomerular  mechanism  so  s^Dccially 
adapted  for  the  special  discharge  of  water. 

As  we  shall  see  presently,  to  the  skin  also  falls  the  dut}^  of 
discharging  large  quantities  of  water.  The  respiratory  organs 
also,  as  we  have  seen,  serve  for  the  discharge  of  water ;  but  the 
amount  which  the  latter  put  out  can  only  be  varied  by  the  incon- 
venient method  of  increasing  or  diminishing  the  whole  act  of 
breathing.  Hence  we  find  special  relations  between  the  skin 
and  the  kidneys  correlating  the  work  of  the  one  to  that  of  the 
other  as  regards  this  particular  work  of  the  discharge  of  water. 

When  the  body  is  exposed  to  cold  the  discharge  of  water 
from  the  skin  in  the  form  of  sweat  is  checked,  and  the  cutaneous 
vessels  are  constricted.  At  the  same  time  the  blood  vessels  of 
the  abdominal  viscera,  including  the  kidneys,  are  dilated,  but 
not  out  of  proportion  to  the  constriction  of  the  cutaneous  vessels, 
for  the  general  blood-pressure  does  not  fall  but  if  anything  rises 
somewhat.  Thus  there  is  established  just  the  state  of  things 
which  is  favourable  to  a  full  and  rapid  stream  of  blood  through 
the  renal  glomeruli ;  and  an  increased  flow  of  urine  results. 

Conversely,  when  the  body  is  exposed  to  warmth  the  skin 
perspires  freely  and  the  cutaneous  vessels  are  widely  dilated; 
and  conversely  also  the  renal  and  other  abdominal  vessels  are 
constricted,  so  that  a  slow  and  small  stream  of  blood  trickles 
through  the  glomeruli,  and  the  urine  which  is  secreted  is  scanty. 

§  341.  Even  more  important  than  its  relations  to  the  skin 
are  the  relations  of  the  kidney  to  the  water  absorbed  by  tlie 
alimentary  canal ;  this  is  especially  seen  when  large  quantities 
of  fluid  are  drunk.  The  whole  of  the  water  thus  introduced 
into  the  alimentary  canal  passes  into  the  blood,  for  in  a  healthy 
organism  no  amount  of  fluid  drunk,  unless  it  throws  the  economy 
out  of  order,  can  affect  the  amount  of  water  present  in  the 
faeces.  But  the  addition  to  tlie  blood  of  even  a  veiy  large  quan- 
tity of  fluid  does  not,  as  we  have  seen,  by  its  mere  quantity 
(§  164),  increase  the  general  blood-pressure,  and  therefore  can- 
not in  this  way  produce  what  it  undoubtedly  does  produce,  an 
increased  flow  of  urine. 

Since  a  kidney,  all  the  nerves  of  which  have  been  severed, 
dilates,  as  shewn  by  the  oncometer,  that  is  has  a  fuller  supply 


542  DIUEETICS  [Book  ii. 

of  blood  and  at  the  same  time  yields  a  fuller  flow  of  urine 
when  water  is  injected  into  the  blood,  we  may  infer  that  the 
blood  diluted  by  the  absorption  of  water  acts  directly  on  the 
kidney.  We  may  further  suppose  that  it  is  the  glomerular 
mechanism  which  is  thus  especially  increased  in  activity,  though 
it  may  be  that  the  epithelial  secretion  is  also  augmented. 

When  however  fluid  is  taken  simply  as  a  proper  accompani- 
ment of  solid  food,  the  increase  of  urine  which  results  has  prob- 
ably another  origin.  As  we  have  already  said,  and  as  we  shall 
point  out  more  fully  later  on,  the  absorption  of  proteid  material, 
which  is  a  constituent  and  generally  a  conspicuous  constituent 
of  every  meal,  leads  to  a  formation  of  urea ;  and  urea,  as  we 
have  seen  reason  to  believe,  directly  stimulates  the  epithelium 
of  the  tubules  to  secretory  activity.  And  what  seems  promi- 
nently true  of  urea  is  probably  true  of  many  other  products  of 
digestion ;  so  that  the  increased  flow  of  urine  which  follows  an 
ordinary  meal  accompanied  with  not  more  than  the  ordinary 
amount  of  fluid,  is  the  result  of  the  labours  of  the  epithelium  of 
the  tubules  as  well  as  of  the  fuller  stream  of  blood  through  the 
glomeruli. 

§  342.  What  has  just  been  said  concerning  the  influence 
on  the  kidney  of  food  and  water  may  be  applied  also  to  the 
action  of  substances  which  being  especially  efficacious  in  promot- 
ing a  flow  of  urine  when  taken  into  the  body  are  called  "  diu- 
retics." The  several  actions  of  various  diuretics  are  very  varied, 
and  it  would  be  out  of  place  to  discuss  them  fully.  We  may 
however  say  that  while  the  action  of  some  appears  simple  that 
of  others  is  complex. 

Such  agents  as  sodium  acetate  and  potassium  nitrate  appear 
to  produce  their  effect  chiefly  by  acting  directly  on  the  kidney. 
They  induce,  as  we  have  seen,  §  334,  local  vascular  dilation  and 
probably  act  by  stirring  up,  after  the  fashion  of  urea,  the  epi- 
thelium of  the  tubules  to  secretory  activity,  the  accompanying 
fuller  stream  of  blood  through  the  whole  kidney  being,  as  in  the 
case  of  the  salivary  and  other  glands,  a  useful  adjuvant,  though 
it  may  also  increase  the  glomerular  seci'etion. 

The  diuretic  effect  of  such  an  agent  as  digitalis  is  probably 
more  complex.  By  increasing  the  cardiac  stroke,  and  at  the 
same  time  constricting  many  small  vessels,  digitalis  raises  the 
general  blood-pressure  ;  but  the  tendency  of  the  increased  blood- 
pressure  to  increase  the  flow  of  urine  may  be  counterbalanced 
by  the  constriction  of  the  renal  vessels  themselves.  And  while 
it  is  a  matter  of  common  experience  that  digitalis  is  very  effec- 
tive as  a  diuretic  in  cardiac  disease,  there  is  great  doubt  whether 
it  really  acts  as  a  diuretic  in  health ;  in  cardiac  disease  it  prob- 
ably raises  the  blood-pressure  by  improving  the  cardiac  stroke 
and  not  by  constriction  of  the  blood  vessels.  But  even  in  the 
absence  of  cardiac  disease,  digitalis  has  been  found  in  particular 


CiiAP.  III.]     ELIMINATION   OF  WASTE  PRODUCTS.        543 

cases  to  act  as  a  powerful  diuretic,  and  in  these  cases  either  it 
must  act  directly  on  the  tubular  epithelium  or  its  effects  in 
constricting  the  renal  arteries  must  be  less  than  its  effects  on 
other  small  arteries  or  must  pass  off  before  the  influence  of  the 
heightened  blood-pressure  has  disappeared. 

§  343.  Quite  removed  from  the  intervention  of  chemical 
substances  in  the  blood  and  yet  most  striking  is  the  influence 
on  the  kidney  of  the  central  nervous  system.  The  potent 
influence  of  emotions  in  promoting  the  secretion  of  urine  is 
proverbial,  and  the  general  features  of  'nervous'  urine,  the  water 
increased  out  of  proportion  to  the  solid  constituents,  especially 
seen  in  the  "urina  hysterica,"  which  is  hardly  more  than  simple 
water,  often  discharged  in  enormous  quantity,  at  once  suggests 
the  view  that  impulses  originating  in  the  brain  and  passing 
down  to  the  kidney  along  the  vaso-dilator  fibres,  of  whose  exist- 
ence evidence  was  given  in  §  383,  lead  to  dilated  blood  vessels 
and  great  play  of  glomerular  activity,  without  perhaps  produc- 
ing any  other  direct  effect  on  the  economy ;  though  possibly  the 
same  emotions  by  constricting  the  cutaneous  and,  it  may  be, 
other  vessels  may  raise  the  general  blood-pressure  and  so  help 
the  dilated  renal  vessels. 


SEC.   3.     THE  DISCHAEGE   OF  UEINE. 

§  344.  The  urine,  like  the  bile,  is  secreted  continuously ; 
the  flow  may  rise  and  fall,  but,  in  health,  never  absolutely 
ceases  for  any  length  of  time.  The  cessation  of  renal  activity, 
the  so-called  suppression  of  urine,  entails  speedy  death.  The 
minute  streams  passing  continuously,  now  more  rapidly  now 
more  slowly,  along  the  collecting  and  discharging  tubules,  are 
gathered  into  the  renal  pelvis,  whence  the  fluid  is  carried  along 
the  ureters  into  the  bladder  by  pressure  and  gravity  aided 
by  the  peristaltic  contractions  of  the  muscular  walls  of  the 
ureter. 

If  in  a  living  animal  a  ureter  be  laid  bare  and  stimulated, 
mechanically  or  otherwise,  at  a  part  of  its  course,  waves  of 
peristaltic  contraction  may  be  seen  to  pass  in  both  directions 
from  the  spot  stimulated,  upwards  towards  the  kidney  and 
downwards  towards  the  bladder.  In  the  absence  of  artificial 
stimulation  spontaneous  waves  of  contraction  make  their  appear- 
ance, sometimes  repeated  with  tolerable  regularity  (about  every 
20  seconds  in  the  rabbit),  sometimes  occurring  in  groups  with 
longer  pauses  between.  These  spontaneous  contractions  inva- 
riably pass  in  one  direction,  from  the  kidney  to  the  bladder  ; 
and  their  frequency  and  vigour  seem  to  be  determined  by  the 
activity  of  the  secretion  of  urine.  But  they  are  not  directly 
called  forth  by  the  urine  either  mechanically  distending  the 
tube  or  chemically  stimulating  the  inner  surface,  for  regularly 
recurring  contractions  may  be  observed  in  a  kidney  and  ureter 
removed  from  the  body,  or  even  in  an  isolated  excised  piece 
of  the  ureter. 

The  rhythmically  repeated  contractions  arise  spontaneously 
in  the  muscular  coat  of  the  ureter  much  in  the  same  wa}^  as  the 
similar  cardiac  contractions  arise  in  the  muscular  substance  of 
the  heart;  and  it  may  here  be  mentioned,  in  support  of  what 
was  urged  in  §  154  with  regard  to  the  heart-beats  not  being 
started  by  nerve-cells,  that  rhythmically  repeated  spontaneous 
peristaltic  contractions  have  been  observed  in  isolated  pieces  of 
ureter  taken  from  the  middle  of  its  course,  in  which  no  nerve- 
cells  could  be  observed. 

544 


Chap,  hi.]     ELIMINATION   OF   WxVSTE   PRODUCTS.         545 

In  the  living  body  these  spontaneous  movements,  beats  they 
might  be  called,  are  subordinated  to  the  flow  of  urine  into  the 
pelvis ;  the  more  active  the  secretion  of  urine  the  more  fre- 
quent and  vigorous  are  the  beats  of  the  pelvis  and  ureter  ;  but 
the  exact  mechanism  by  which  the  secretion  and  the  movements 
are  maintained  in  harmony  has  not  yet  been  cleared  up. 

Micturition. 

§  345.  In  the  urinary  bladder,  the  urine  is  collected,  its 
return  into  the  ureters  being  prevented  by  the  oblique  entrance 
into  the  bladder  and  valvular  nature  of  the  orifices  of  those 
tubes  ;  and  its  discharge  from  thence  in  considerable  quantity 
is  effected  from  time  to  time  by  a  somewhat  complex  muscular 
mechanism,  of  the  nature  and  working  of  which  the  following 
is  a  brief  account. 

The  involuntary  muscular  fibres  forming  the  greater  part  of 
the  vesical  walls  are  arranged  partly  in  a  more  or  less  longitu- 
dinal, and  partly  in  a  circidar  manner.  The  bladder  after  it 
has  been  emptied  is  contracted  and  thrown  into  folds ;  as  the 
urine  gradually  collects,  the  bladder  becomes  more  and  more 
distended.  The  escape  of  the  fluid  is  in  part  prevented  by  tlie 
resistance  offered  by  the  elastic  fibres  in  the  walls  of  the  urethra 
which  help  to  keep  the  urethral  channel  closed.  But  this  is 
not  all ;  for  observation  shews  that  fluid  is  retained  within  the 
bladder  up  to  a  pressure  of  20  inches  of  water  so  long  as  the 
bladder  is  governed  by  an  intact  spinal  cord,  but  gives  way  to 
a  pressure  of  6  inches  only  when  the  lumbar  spinal  cord  is  de- 
stroyed or  the  vesical  nerves  are  severed.  This  affords  very 
strong  evidence  that  the  obstruction  at  the  neck  of  the  bladder 
to  the  exit  of  urine  depends  on  some  tonic  muscular  contraction 
maintained  by  a  reflex  or  automatic  action  of  the  lumbar  spinal 
cord.  It  has  been  maintained  that  the  circularly  disj^osed  fibres 
specially  developed  around  the  neck  of  the  bladder  are  the  sub- 
jects of  this  tonic  contraction  and  thus  the  chief  instrument  of 
the  retention  ;  hence  the  name  sphincter  vesicae.  The  continu- 
ity of  these  fibres,  however,  with  the  rest  of  the  circular  fibres 
of  the  bladder  suggests  that  they  probably  do  not  act  as  a 
spliincter,  but  that  their  use  lies  in  their  contracting  after  the 
rest  of  the  vesical  fibres,  and  thus  finishing  the  evacuation  of 
the  bladder.  The  resistance  in  question  is  supplied  by  a  tonic 
contraction  not  of  these  circular  fibres  of  the  bladder  itself  but 
of  the  muscular  fibres,  partly  plain,  partly  striated,  surrounding 
the  i)rostatic  portion  of  the  urethra,  and  constituting  the  i<p]iincter 
vesicce  externus  ov  prostatieus  or  sphincter  of  Henle.  It  is  stated 
that  artificially  excited  contractions  of  these  fibres  will  resist  a 
pressure  of  fluid  in  the  bladder. 

35 


546  MICTURITIOK  [Book  ii. 

When  the  bladder  has  become  full,  we  feel  the  need  of  making 
water,  the  sensation  being  heightened  if  not  caused  by  the  trick- 
ling of  a  few  drops  of  urine  from  the  full  bladder  into  the  ure- 
thra. We  are  then  conscious  of  an  effort ;  during  this  effort 
the  bladder  is  thrown  into  a  long-continued  contraction  of  an 
obscurely  peristaltic  nature,  the  force  of  which  is  more  than 
sufficient  to  overcome  the  resistance  offered  by  the  urethra,  and 
the  urine  issues  in  a  stream,  the  sphincter  vesicae  externus  being 
at  the  same  time  either  relaxed  after  the  fashion  of  the  sphincter 
ani,  or  at  least  overcome.  In  its  passage  along  the  urethra,  the 
exit  of  the  urine,  at  all  events  of  the  last  portions,  is  forwarded 
by  irregularly  rhythmic  contractions  of  the  bulbo-cavernosos  or 
ejaculator  urinse  muscle,  the  contractions  of  which  compress  the 
urethra ;  and  the  whole  act  is  further  assisted  by  pressure  on 
the  bladder  exerted  by  means  of  the  abdominal  muscles,  very 
much  the  same  as  in  defsecation. 

Experiments  on  cats,  dogs  and  other  animals  shew  that  con- 
tractions of  the  bladder  can  be  brought  about  by  stimulation  of 
the  anterior  roots  of  certain  lumbar  nerves  chiefly  the  third  and 
fourth,  and  of  the  first  three  sacral  nerves ;  stimulation  of  the 
anterior  roots  of  the  nerves  between  these  two  sets  does  not  give 
contractions  of  the  bladder.  The  sacral  roots  seem  to  have 
more  powerful  and  more  distinctly  unilateral  effects  than  have 
the  lumbar  roots,  and  the  movements  brought  about  have  not 
exactly  the  same  character  in  the  two  cases,  though  it  cannot 
be  said  that  the  contraction  is  in  the  former  case  strictly  longi- 
tudinal and  in  the  latter  case  circular.  The  nerve  fibres  issuing 
by  the  lumbar  nerves  pass  into  the  sympathetic  chain  and  thence 
by  the  inferior  mesenteric  ganglion  and  hypogastric  nerves  to  the 
hypogastric  plexus  ;  the  nerve  fibres  issuing  by  the  sacral  pass 
more  directly  to  the  hypogastric  plexus. 

§  346.  We  said  just  now  "when  the  bladder  has  become 
full,"  but  this  must  not  be  understood  to  mean,  "when  the 
bladder  has  received  a  certain  quantity  of  fluid."  On  the  con- 
trary, it  is  a  matter  of  common  experience  that  we  feel  the  desire 
to  make  water  sometimes  when  a  large  quantity  and  sometimes 
when  a  small  quantity  of  urine  has  accumulated  in  the  bladder. 
We  have  evidence  that  the  bladder  possesses  to  a  very  high 
degree  that  obscure  continuous  contraction  which  we  speak  of 
as  '  tone ' ;  and  further  that  the  amount  of  its  tone  is  exceed- 
ingly variable,  the  organ,  quite  independently  of  distinct  efforts 
at  micturition,  being  at  one  time  contracted  and  at  another 
flaccid  and  distended.  When  it  is  in  a  contracted  state,  a  small 
quantity  of  fluid  may  exert  the  same  effect  on  the  vesical  walls 
as  a  larger  quantity  when  the  })ladder  is  flaccid.  Hence  while 
the  determining  cause  of  the  desire  to  make  water  is  the  pressure 
of  the  urine  upon  the  vesical  walls,  the  quantity  needed  to  pro- 
duce the  necessary  fulness  is  dependent  on  the  amount  of  tonic 


Chap,  hi.]     ELIMINATION   OF   WASTE   PRODUCTS.         547 

contraction  of  the  muscular  fibres  existing  at  the  time.  And  we 
have  evidence  that  this  tone  is  regulated  by  the  nervous  system. 

§  347.  Micturition  as  sketched  above  seems  at  first  sight, 
and  especially  when  we  appeal  to  our  own  consciousness,  a  purely 
voluntary  act.  A  voluntary  effort  throws  the  muscular  fibres 
of  the  bladder  into  contractions,  an  accompanying  voluntary 
effort  lessens  the  tone  of  the  sphincter  externus,  probably  by 
inhibiting  its  centre  in  the  spinal  cord,  while  other  voluntary 
efforts  throw  the  ejaculator  and  abdominal  muscles  into  con- 
tractions, and,  the  resistance  of  the  urethra  being  thereby  over- 
come, the  exit  of  the  urine  naturally  follows. 

There  are  facts,  however,  which  prevent  the  acceptance  of 
so  simple  a  view.  In  the  first  place,  in  cases  of  urethral  ob- 
struction, where  the  bladder  cannot  be  emptied  when  it  reaches 
its  accustomed  fulness,  the  increasing  distension  sets  up  fruit- 
less but  powerful  contractions  of  the  vesical  walls,  contractions 
which  are  clearly  involuntary  in  nature,  which  wane  or  disap- 
pear, and  return  again  and  again  in  a  rhythmic  manner,  and 
which  may  be  so  strong  and  powerful  as  to  cause  great  suffering. 
It  seems  that  the  fibres  of  the  bladder,  like  all  other  muscular 
fibres,  have  their  contractions  augmented  in  proportion  as  they 
are  subjected  to  tension.  Just  as  a  previously  quiescent  ven- 
tricle of  a  frog's  heart  may  be  excited  to  a  rhythmic  beat  by 
distending  its  cavity  with  blood,  so  the  quiescent  bladder  may, 
quite  independent  of  the  will,  be  excited  by  the  distension  of 
its  cavity,  to  a  peristaltic  action  which  in  normal  cases  is  never 
carried  beyond  a  first  effort,  since  with  that  the  bladder  is 
emptied  and  the  stimulus  is  removed,  but  which  in  cases  of 
obstruction  is  enabled  clearly  to  manifest  its  rhythmic  nature. 

In  the  second  place  it  has  been  shewn  that  quite  normal 
micturition  may  take  place  in  a  dog  in  which  the  lumbar  region 
of  the  spinal  cord  has  been  completely  and  permanently  sepa- 
rated by  section  from  the  upper  dorsal  region.  In  such  a  case 
there  can  be  no  exercise  of  volition,  and  the  whole  process 
appears  as  a  reflex  action.  When  under  these  circumstances 
the  bladder  becomes  full  (and  otherwise  apparently  the  act 
fails)  any  slight  stimulus,  such  as  sponging  the  anus  or  slight 
pressure  on  the  abdominal  walls,  causes  a  complete  act  of  mic- 
turition: tlie  bladder  is  entirely  emptied,  and  the  stream  of  urine 
towards  the  end  of  the  act  undergoes  rhythmical  augmentations 
due  to  contractions  of  the  ejaculator  urinse.  These  facts  can  only 
be  interpreted  on  the  view  that  there  exists  in  the  lower  spinal 
cord  (of  the  dog)  Avhat  we  may  speak  of  as  a  micturition  centre 
capable  of  being  thrown  into  action  by  appropriate  afferent 
impulses,  the  action  of  the  centre  being  such  as  to  cause  a 
contraction  of  the  walls  of  the  bladder  and  of  the  ejaculator 
urinse,  and  at  the  same  time  to  suspend  the  tone  of  the  sphincter 
vesicJB   externus.     Clinical   experience  also  goes  to  shew  the 


548  MICTUEITIOK  [Book  ii. 

existence  of  a  similar  micturition  centre  in  man,  placed  higher 
up  in  the  cord  than  the  corresponding  '  genital '  centre  govern- 
ing the  genital  organs. 

Moreover  we  have,  in  the  case  both  of  man  and  of  other 
animals,  experimental  and  other  evidence  that  contraction  of 
the  bladder  is  frequently  brought  about  by  reflex  action.  Thus 
the  pressure  within  the  bladder  when  observed  for  any  length 
of  time  is  found  to  be  subject  to  considerable  and  manifold 
variations.  Over  and  above  passive  changes  in  pressure  due 
to  the  respiratory  movements,  through  which  the  bladder  is 
pressed  upon  at  each  descent  of  the  diaphragm,  active  contrac- 
tions, of  a  strength  inadequate  to  bring  about  micturition,  are 
from  time  to  time  observed.  These  in  some  instances  appear 
to  be  spontaneous,  or  to  be  the  result  of  emotions,  but  they 
may  be  readily  induced  in  a  reflex  manner,  by  stimulating 
various  sentient  surfaces  or  sensory  nerves.  And  common  ex- 
perience affords  many  instances  where  vesical  contractions  thus 
brought  about  in  a  reflex  manner  acquire  strength  adequate  to 
empty  the  bladder. 

Observations  of  vesical  pressure  may  be  most  conveniently  carried 
out  by  introducing  into  the  bladder  a  catheter  connected  with,  a  water 
manometer  and  a  registering  apparatus,  and  so  arranged  as  to  allow 
fluid  to  be  driven  into  or  received  from  the  bladder  at  pleasure. 

§  348.  Involuntary  micturition  obviously  of  reflex  nature 
has  frequently  been  observed  in  cases  of  paralysis  from  disease 
of  or  injury  to  the  spinal  cord ;  and  the  involuntary  micturition 
which  is  common  in  children,  as  the  result  of  irritation  of  the 
penis  and  genital  organs,  and  which  sometimes  occurs  in  the 
adult  as  the  result  of  emotions,  or  at  least  sensory  impressions, 
appears  to  be  the  result  of  reflex  action.  In  these  several  cases 
we  may  fairly  suppose  that  the  centre  in  the  spinal  cord  is 
affected  by  afferent  impulses  reaching  it  along  various  sensory 
nerves  or  descending  from  the  brain.  Hence  we  are  led  to  the 
conception  that  when  we  make  water  by  a  conscious  effort  of 
the  will,  what  occurs  is  not  a  direct  action  of  the  will  on  the 
muscular  walls  of  the  bladder,  but  that  impulses  started  by 
the  will  descend  from  the  brain  after  the  fashion  of  afferent 
impulses  and  thus  in  a  reflex  manner  throw  into  action  the 
micturition  centre  in  the  spinal  cord.  Nor  is  this  view  nega- 
tived by  the  fact  that  paralysis  of  the  bladder,  or  rather  ina- 
bility to  make  water  either  voluntarily  or  in  a  reflex  manner, 
is  a  common  symptom  of  cerebral  or  spinal  disease  or  injury. 
Putting  aside  the  cases  in  which  the  reflex  act  is  not  called 
forth  because  the  appropriate  stimulus  has  not  been  applied, 
the  failure  in  micturition  under  these  circumstances  may  be 
explained  by  supposing  that  the  shock  of  the  spinal  injury  or 


Chap,  hi.]     ELIMINATION   OF   WASTE   PKODUCTS.         549 

some  extension  of  the  disease  has  rendered  the  spinal  centre 
unable  to  act. 

The  so-called  incontinence  of  urine  in  children  is  simply  an 
easily  excited  and  frequently  repeated  reflex  micturition.  In 
cases  of  cerebral  or  spinal  disease  a  form  of  incontinence  is  fre- 
quently met  with  which  seems  to  be  of  a  different  nature.  The 
bladder  becoming  full,  but,  owing  to  a  failure  in  the  mechanism 
of  voluntary  or  reflex  micturition,  being  unable  to  empty  itself 
by  a  complete  contraction,  a  continual  dribbling  of  urine  takes 
place  through  the  urethra,  the  fulness  of  the  bladder  being  suf- 
ficient to  overcome  the  resistance  at  the  neck  of  the  urethra. 
It  is  probable,  however,  that  even  in  these  cases  the  flow  is 
partly  caused  by  obscure,  unfelt,  intrinsic  contractions  of  the 
bladder. 

§  349.  Whether,  under  normal  conditions,  the  urine  under- 
goes any  notable  change  during  its  stay  in  the  bladder  has  been 
much  debated.  Experiments  shew  that  poisonous  substances 
injected  into  the  bladder  with  all  due  care  to  avoid  any  abra- 
sion of  the  epithelium  are  absorbed  and  produce  their  usual 
effects.  It  has  also  been  stated  that  if  a  solution  of  urea  be 
injected  into  the  bladder  after  ligature  of  both  ureters,  and 
allowed  to  stay  for  some  hours,  part  of  the  urea  disappears. 
But  at  present  there  is  no  very  decided  proof  that  under  ordi- 
nary conditions  either  the  water  or  other  constituents  of  urine 
are  to  any  appreciable  extent  absorbed  by  the  bladder. 

Under  abnormal  conditions,  as  in  inflammation  or  irritation 
of  the  bladder,  the  urine  may  have  undergone  marked  changes 
during  its  stay  in  the  bladder,  one  of  the  most  common  being  a 
change  of  some  of  the  urea  into  ammonium  carbonate,  by  which 
the  urine  becomes  alkaline.  Under  abnormal  conditions  also, 
the  mucus  of  the  urine,  which  in  a  healthy  man  is  insignificant, 
though  in  some  animals,  for  instance  the  horse,  it  occurs  in  con- 
siderable quantity,  is  largely  increased  during  the  stay  in  the 
bladder.  Since  there  are  in  man  no  goblet  cells  in  the  vesical 
epithelium  (in  the  frog  they  are  present)  or  mucus  glands  in 
the  walls  of  the  bladder,  this  mucus  must  be  supplied  by  an 
abnormal  metabolism  of  the  ordinary  epithelial  cells. 


SEC.  4.    THE  NATUKE  AND  AMOUNT  OF  PERSPIEATION. 

§  350.  The  quantity  of  matter  which  leaves  the  human  body 
by  way  of  the  skin  is  very  considerable.  Thus  it  has  been  esti- 
mated that  while  '5  gram  passes  away  through  the  lungs  per 
minute,  as  much  as  "8  gram  passes  through  the  skin.  The 
amount,  however,  varies  extremely;  it  has  been  calculated, 
from  data  gained  by  enclosing  the  arm  in  a  caoutchouc  bag, 
that  the  total  amount  of  perspiration  from  the  whole  body  in 
24  hours  might  range  from  2  to  20  kilos;  but  such  a  mode  of 
calculation  is  obviously  open  to  many  sources  of  error. 

Of  the  whole  amount  thus  discharged,  part  passes  away  at 
once  as  watery  vapour  mixed  with  volatile  matters,  while  part 
may  remain  for  a  time  as  a  fluid  on  the  skin;  the  former  is  fre- 
quently spoken  of  as  insensible^  the  latter  as  sensible  perspiration 
or  sweat.  The  proportion  of  the  insensible  to  the  sensible  per- 
spiration will  depend  on  the  rapidity  of  the  secretion  in  refer- 
ence to  the  dryness,  temperature  and  amount  of  movement  of 
the  surrounding  atmosphere.  Thus,  supposing  the  rate  of 
secretion  to  remain  constant,  the  drier  and  hotter  the  air,  and 
the  more  rapidly  the  strata  of  air  in  contact  with  the  body  are 
renewed,  the  greater  is  the  amount  of  sensible  perspiration  which 
is  by  evaporation  converted  into  the  insensible  condition;  and 
conversely  when  the  air  is  cool,  moist,  and  stagnant,  a  large 
amount  of  the  total  perspiration  may  remain  on  the  skin  as  sen- 
sible sweat.  Since,  as  the  name  implies,  we  are  ourselves  aware 
of  the  sensible  perspiration  only,  it  may  and  frequently  does 
hajjpen  that  we  seem  to  ourselves  to  be  perspiring  largely,  when 
in  reality  it  is  not  so  much  the  total  perspiration  which  is  being 
increased  as  the  relative  proportion  of  the  sensible  perspiration. 
The  rate  of  secretion  may,  however,  be  so  much  increased,  that 
no  amount  of  dryness,  or  heat,  or  movement  of  the  atmosphere, 
is  sufficient  to  carry  out  the  necessary  evaporation,  and  thus  the 
sensible  perspiration  may  become  abundant  in  a  hot,  dry  air. 
And  practically  this  is  the  usual  occurrence,  since  certainly  a 
high  temperature  conduces,  as  we  shall  point  out  presently,  to 
an  increase  of  the  secretion,  and  it  is  possible  that  mere  dryness 
of  the  air  has  a  similar  effect. 

550 


Chap,  hi.]     ELIMINATION   OF  WASTE  PKODUCTS.         651 

The  amount  of  perspiration  given  off  is  affected  not  only  by 
the  condition  of  the  atmosphere,  but  also  by  the  circumstances  of 
the  body.  Thus  it  is  influenced  by  the  nature  and  quantity 
of  food  eaten,  by  the  amount  of  fluid  drunk,  by  the  character 
of  exercise  taken,  by  the  relative  activity  of  the  other  excreting 
organs,  more  particularly  of  the  kidney,  by  mental  conditions 
and  the  like.  Variations  may  also  be  induced  by  drugs  and 
by  diseased  conditions.  How  these  various  influences  produce 
their  effects  we  shall  study  immediately. 

The  fluid  perspiration,  or  sweat,  when  collected,  is  found  to 
be  a  clear  colourless  fluid  of  a  distinctly  salt  taste,  with  a  strong 
and  distinctive  odour  varying  according  to  the  part  of  the  body 
from  which  it  is  taken.  Besides  accidental  epidermic  scales,  it 
contains  no  structural  elements. 

Sweat,  as  a  whole,  is  furnished  partly  by  the  sweat-glands 
and  partly  by  the  sebaceous  glands,  for  as  we  shall  see  the  small 
amount  which  simply  transudes  tlirough  the  epidermis,  apart 
from  the  glands,  may  be  neglected.  Now  the  secretions  from 
these  two  kinds  of  glands  differ  widely  in  nature,  and  the  charac- 
ters of  the  sweat  as  a  whole  will  vary  according  to  the  relative 
proportion  of  the  two  kinds  of  secretion.  The  amount  of  secre- 
tion of  the  sebaceous  glands  appears  to  be  fairly  constant,  the 
larger  variations  of  the  total  sweat  depending  chiefly  on  the 
varying  activity  of  the  sweat-glands.  Hence  when  sweat  is 
scanty,  the  constituents  of  the  sebum  influence  largely  the  charac- 
ters of  the  sweat;  when  on  the  contrary  the  sweat  is  very  abun- 
dant, these  may  be  disregarded  and  the  sweat  may  be  considered 
as  the  product  of  the  sweat-glands. 

We  are  not  able,  at  present,  to  make  a  complete  statement 
as  to  what  bodies  occur  exclusively  in  the  sebum  and  what  in  the 
secretion  of  the  sweat-glands.  The  former  consists  very  largely 
of  fats  and  fatty  acids,  and  appears  to  contain  some  form  or 
forms  of  proteids;  but  we  have  reason  to  think  that  the  sweat- 
glands  secrete  in  small  quantity  some  forms  of  fat,  and  especially 
volatile  fatty  acids. 

When  sweat  is  scanty,  the  reaction  is  generally  acid,  but  when 
abundant,  is  alkaline;  and  when  a  portion  of  the  skin  is  well 
washed  the  sweat  which  is  collected  immediately  afterwards  is 
usually  alkaline.  From  this  we  may  infer  that  the  secretion  of 
the  sweat-glands  is  naturally  alkaline,  but  that  when  mixed 
sweat  is  acid,  the  acidity  is  due  to  fatty  (or  other)  acids  of  the 
sebum.  In  the  horse,  which  is  singular  among  hair-covered 
animals  for  its  frequent  profuse  sweating,  the  sweat  is  said  to  be 
always  alkaline,  and  to  contain  a  considerable  quantity  of  some 
form  of  proteid. 

Taking  ordinary  sweat,  such  as  may  be  obtained  by  enclos- 
ing the  arm  in  a  bag,  we  may  say  that,  in  man,  the  average 
amount  of  solids  is  from  1  to  2  p.c,  of  which  about  two-thirds 


552  COMPOSITION   OF   SWEAT.  [Book  ii. 

consist  of  organic  substances.  The  chief  normal  constituents 
are:  (1)  Sodium  chloride,  with  small  quantities  of  other  inor- 
ganic salts.  (2)  Various  acids  of  the  fatty  series,  such  as 
formic,  acetic,  butyric,  with  probably  propionic,  caproic,  and 
caprylic.  The  presence  of  these  latter  is  inferred  from  the 
odour;  it  is  probable  that  many  various  volatile  acids  are  pres- 
ent in  small  quantities.  Lactic  acid,  which  has  been  reckoned 
as  a  normal  constituent,  is  stated  not  to  be  present  in  health. 
(3)  Neutral  fats,  and  cholesterin;  these  have  been  detected  even 
in  places,  such  as  the  palms  of  the  hand,  where  sebaceous  glands 
are  absent.  (4)  The  evidence  goes  to  shew  that  neither  urea 
nor  any  ammonia  compound  exists  in  the  normal  secretion  to  any 
extent,  though  some  observers  have  found  a  considerable  quan- 
tity of  urea  (calculated  at  10  grms.  in  the  24  hours  for  the  whole 
body).  Apparently  some  small  amount  of  nitrogen  leaves  the 
body  by  the  skin  as  a  whole,  but  this  is  probably  supplied  by 
the  sebum  or  by  the  epidermis. 

In  various  forms  of  disease  the  sweat  has  been  found  to  con- 
tain, sometimes  in  considerable  quantities,  blood,  albumin,  urea 
(particularly  in  cholera),  uric  acid,  calcium  oxalate,  sugar  (in 
diabetic  patients),  lactic  acid,  indigo  (or  indigo-yielding  bodies 
giving  rise  to  '  blue  '  sweat),  bile  and  other  pigments.  Iodine 
and  potassium  iodide,  succinic,  tartaric,  and  benzoic  (partly  as 
hippuric)  acids  have  been  found  in  the  sweat  when  taken  inter- 
nally as  medicines. 

Cutaneous  Respiration. 

§  351.  A  frog,  whose  lungs  have  been  removed,  will  continue 
to  live  for  some  time ;  and  during  that  period  will  continue  not 
only  to  produce  carbonic  acid,  but  also  to  consume  oxygen. 
In  other  words,  the  frog  is  able  to  breathe  without  lungs,  respi- 
ration being  carried  on  efficiently  by  means  of  the  skin.  In 
mammals  and  in  man  this  cutaneous  respiration  is,  by  reason  of 
the  thickness  of  the  epidermis,  restricted  to  within  very  narrow 
limits;  and  indeed  it  has  been  questioned  whether  it  can  be 
spoken  of  at  all  as  a  true  respiration.  When  the  body  remains 
for  some  time  in  a  closed  chamber  to  which  the  air  passing  in 
and  out  of  the  lungs  has  no  access  (as  when  the  body  is  enclosed 
in  a  large  air-tight  bag  fitting  tightly  round  the  neck,  or  where 
a  tube  in  the  trachea  carries  air  to  and  from  the  lungs  of  an 
animal  placed  in  an  air-tight  box),  it  is  found  that  the  air  in  the 
chamber  loses  oxygen  and  gains  carbonic  acid.  The  amount  of 
carbonic  acid  which  is  thus  thrown  off  by  the  skin  of  an  average 
man  in  24  hours  amounts  to  about  10  grms.,  or  according  to 
some  observers  to  (no  more  than)  about  4  grms.,  increasing  with 
a  rise  of  temperature,  and  being  very  markedly  augmented  by 
bodily  exercise.     It  is  stated  that  the  amount  of  oxygen  con- 


Chap,  hi.]     ELIMINATION   OF   WASTE   PEODUCTS.         553 

sumed  is  about  equal  in  volume  to  that  of  the  carbonic  acid 
given  off,  but  some  observers  make  it  rather  less.  It  may  be 
doubted,  however,  whether  the  carbonic  acid  comes  direct  from 
the  blood;  it  may  come  from  decompositions  taking  place  in 
the  sweat,  of  carbonates  for  instance.  Similarly  the  oxygen 
which  disappears  may  be  simply  used  in  oxydizing  some  of  the 
constituents  of  the  sweat.  It  is  evident  that  the  loss  which  the 
body  suffers  through  the  skin  consists,  besides  a  small  quantity 
of  sodium  chloride,  chiefly  of  water. 

When  an  animal,  a  rabbit  for  instance,  is  covered  over  with 
an  impermeable  varnish  such  as  gelatin,  so  that  all  exit  or 
entrance  of  gases  or  liquids  by  the  skin  is  prevented,  death 
shortly  ensues.  This  result  cannot  be  due,  as  was  once 
thought,  to  arrest  of  cutaneous  respiration,  seeing  how  insig- 
nificant and  doubtful  is  the  gaseous  interchange  by  the  skin 
as  compared  with  that  by  the  lungs.  Nor  are  the  symptoms 
at  all  those  of  asphyxia,  but  rather  of  some  kind  of  poisoning, 
marked  by  a  very  great  fall  of  temperature,  which  however 
seems  to  be  the  result  not  of  diminished  production  of  heat, 
but  of  an  increase  of  the  discharge  of  heat  from  the  surface. 
Owing  to  the  dilated  condition  of  the  cutaneous  vessels,  caused 
by  the  application  of  varnish,  the  loss  of  heat  through  the  skin 
is  abnormally  large,  even  though  the  varnish  may  not  be  a  good 
conductor.  The  animal  may  be  restored,  or  at  all  events  its 
life  may  be  prolonged  with  abatement  of  the  symptoms,  if  the 
great  loss  of  heat  which  is  evidently  taking  place  be  prevented 
by  covering  the  body  thickly  with  cotton  wool,  or  keeping  it 
in  a  warm  atmosphere.  The  symptoms  have  not  as  yet  been 
clearly  analyzed,  but  they  seem  to  be  due  in  part  to  a  pj^rexia 
or  fever  possibly  caused  by  the  retention  within  or  reabsorp- 
tion  into  the  blood  of  some  of  the  constituents  of  the  sweat, 
or  by  the  products  of  some  abnormal  metabolism. 

§  352.  Absorption  hy  the  skin.  Although  under  normal 
circumstances  the  skin  serves  only  as  a  channel  of  loss  to  the 
body,  it  has  been  maintained  that  it  may,  under  particular  cir- 
cumstances, be  a  means  of  gain ;  and  the  little  which  we  have 
to  say  on  this  matter  may  perhaps  be  said  here.  Cases  are  on 
record  where  bodies  are  said  to  have  gained  in  weight  by 
immersion  in  a  bath,  or  by  exposure  to  a  moist  atmosphere 
during  a  given  period,  in  which  no  food  or  drink  was  taken, 
or  to  have  gained  more  than  the  Aveight  of  the  food  or  drink 
taken ;  the  gain  in  such  cases  must  have  been  due  to  the 
absorption  of  water  by  the  skin.  Direct  experiments,  how- 
ever, throw  doubt  on  these  statements,  for  they  shew  that 
under  ordinary  circumstances  such  a  gain  by  the  skin  is 
slight,  being  apparently  due  to  mere  imbibition  of  water  by 
the  upper  layers  of  the  epidermis. 

Absorption  of  various  substances  takes  place  very  readily 


554  ABSOEPTION   BY   THE   SKIN.  [Book  ii. 

by  abraded  surfaces  where  the  dermis  is  laid  bare  or  covered 
only  by  the  lowest  layers  of  epidermis,  but  it  has  been  debated 
whether  substances  in  aqueous  solution  can  be  absorbed  by  the 
skin  when  the  epidermis  is  intact,  the  evidence  on  this  point 
being  contradictory.  In  the  case  of  the  skin  of  the  frog  an 
absorption  of  water  and  of  various  soluble  substances  certainly 
takes  place.  In  the  case  of  the  sound  human  skin  there  are  no 
a  priori  reasons  why  water  carrying  substances  dissolved  in  it 
should  not  pass  inwards  through  the  corneous  as  well  as  the 
other  layers  of  the  epidermis,  the  amount  so  passing  depending, 
among  other  things,  upon  the  condition  of  the  skin ;  and  com- 
mon experience  seems  to  shew  that  it  does.  Nevertheless  the 
results  of  actual  experiment  are  conflicting.  Some  observers 
maintain  that  soluble  non-volatile  substances  are  not  absorbed, 
and  that  volatile  substances  such  as  iodine  which  may  be  de- 
tected in  the  system  after  a  bath  containing  them  are  absorbed 
not  by  the  skin  but  by  the  mucous  membrane  of  the  respiratory 
organs,  the  substance  making  its  way  to  the  latter  by  volatili- 
zation from  the  surface  of  the  bath.  Others  again  have  found 
evidence  of  absorption,  especially  with  volatile  substances,  even 
when  care  has  been  taken  to  avoid  all  errors ;  and  the  greater 
weight  may  perhaps  be  given  to  these  since  they  accord  with 
common  experience.  The  conflict  of  experimental  results,  how- 
ever, at  least  shews  that  we  do  not  fully  understand  the  condi- 
tions under  which  such  absorption  takes  place. 

There  is  moreover  evidence  that  even  solid  particles  can 
pass  through  an  intact  skin.  The  lymphatics  in  the  skin  of 
a  newborn  infant  have  been  found  crowded  with  the  particles 
of  the  peculiar  fatty  secretion  which  covers  the  skin  at  birth ; 
and  solid  particles  rubbed  into  even  the  sound  skin  may,  espe- 
cially when  applied  in  a  fatty  vehicle,  as  ex.  gr.  in  the  well- 
known  mercury-ointment,  find  their  way  into  the  underlying 
lymphatics.  The  wandering  leucocytes  which  are  at  times 
found  among  the  epidermic  cells  may  perhaps  take  part  in 
this  transport. 


SEC.   5.     THE   MECHANISM   OF  THE   SECRETION   OF 

SWEAT. 

§  353.  In  dealing  with  the  manner  in  which  various  circum- 
stances affect  the  amount  of  sweat  secreted  we  may,  as  we  have 
already  said,  consider  the  sweat  as  a  whole  to  be  supplied  by 
the  sweat-glands  alone.  For  though  it  seems  evident  that  some 
amount  of  fluid  must  pass  by  simple  transudation  through  the 
ordinary  epidermis  of  the  portions  of  skin  intervening  between 
the  mouths  of  the  glands,  yet  on  the  whole  it  is  probable  that 
the  portion  which  so  passes  is  a  small  fraction  only  of  the  total 
quantity  secreted  by  the  skin  ;  and  direct  experiment  shews  that 
even  the  simple  evaporation  of  water  is  much  greater  from  those 
parts  of  the  skin  in  which  the  glands  are  abundant  than  from 
those  in  which  they  are  scanty.  We  have  as  yet  no  evidence 
that  the  sebaceous  glands  vary  in  activity  ;  their  very  peculiar 
form  of  secretion,  if  we  may  speak  of  it  as  a  secretion,  is  not 
adapted  to  sudden  changes,  and  at  all  events  we  have  as  yet  no 
evidence  that  circumstances  rapidly  and  largely  modify  the 
amount  of  sebum  discharged  by  healthy  sebaceous  glands. 

The  secreting  activity  of  the  skin,  like  that  of  the  other 
glands,  is  usually  accompanied  and  aided  bj'  vascular  dilation. 
In  one  of  the  early  experiments  on  division  of  the  cervical  sym- 
pathetic, it  was  observed  that  in  the  case  of  the  horse,  the 
vascular  dilation  of  the  face  on  the  side  operated  on  was  ac- 
companied by  increased  perspiration.  Indeed  the  connection 
between  the  state  of  the  cutaneous  blood  vessels  and  the  amount 
of  perspiration  is  a  matter  of  daily  observation.  When  the 
vessels  of  the  skin  are  constricted,  the  secretion  of  the  skin  is 
diminished ;  when  they  are  dilated,  it  becomes  abundant.  In 
this  way,  as  we  shall  later  on  point  out,  the  temperature  of  the 
body  is  largely  regulated.  When  the  surrounding  atmospliere 
is  warm,  the  cutaneous  vessels  are  dilated,  the  amount  of  sweat 
secreted  is  increased,  and  the  consequently  augmented  evapora- 
tion tends  to  cool  down  the  body.  On  the  other  hand,  when 
the  atmosphere  is  cold,  the  cutaneous  vessels  are  constricted, 
perspiration  is  scanty,  and  less  heat  is  lost  to  the  body  by 
evaporation. 

555 


556  NERVOUS  MECHANISM  OF  SWEATING.     [Book  ii. 

The  analogy  with  the  other  secreting  organs  which  we  have 
already  studied  leads  us,  however,  to  infer  that  there  are  special 
nerves  directly  governing  the  activity  of  the  sudoriparous  glands, 
independent  of  variations  in  the  vascular  supply.  And  not  only 
is  this  view  suggested  by  many  facts,  such  as  the  profuse  per- 
spiration of  the  death  agony,  of  various  crises  of  disease,  and 
of  certain  mental  emotions,  and  the  cold  sweats  occurring  in 
phthisis  and  other  maladies,  in  all  of  which  the  skin  is  anaemic 
rather  than  hypersemic,  but  we  have  direct  experimental  evi- 
dence of  a  nervous  mechanism  of  perspiration  as  complete  as 
the  vaso-motor  mechanism. 

If  in  the  cat^  the  peripheral  stump  of  the  divided  sciatic 
nerve  be  stimulated  with  the  interrupted  current,  drops  of 
sweat  may  readily  be  observed  to  gather  on  the  hairless  sole  of 
the  foot  of  that  side.  The  sweating  is  not  due  to  any  increase 
of  blood-supply,  for  it  may  be  observed  when  the  cutaneous 
vessels  are  thrown  into  a  state  of  constriction  by  the  stimulus, 
or  even  when  the  aorta  or  crural  artery  is  clamped  previous  to 
the  stimulation,  and  indeed  may  be  obtained  by  stimulating  the 
sciatic  nerve  of  a  recently  amputated  leg.  Moreover  when 
atropin  has  been  injected,  the  stimulation  produces  no  sweat, 
though  vaso-motor  effects  follow  as  usual.  The  analogy  between 
the  sweat-glands  of  the  foot  and  such  a  gland  as  the  submax- 
illary is  in  fact  very  close,  and  we  are  justified  in  speaking  of 
the  sciatic  nerve  as  containing  secretory  fibres  distributed  to  the 
sudoriparous  glands  of  the  foot.  Similar  results  may  be  ob- 
tained with  the  nerves  of  the  fore  limb.  And  in  ourselves  a 
copious  secretion  of  sweat  may  be  induced  by  tetanizing  through 
the  skin  the  nerves  of  the  limbs  or  the  face. 

If  a  cat  in  which  the  sciatic  nerve  has  been  divided  on  one 
side  be  exposed  to  a  high  temperature  in  a  heated  chamber,  the 
limb  the  nerve  of  which  has  been  divided  remains  dry,  while 
the  feet  of  the  other  limbs  sweat  freely.  This  result  shews  that 
the  sweating  which  is  caused  by  exposure  of  the  body  to  high 
temperatures  is  brought  about  by  the  agency  of  the  central 
nervous  system,  and  not  by  a  local  action  on  the  sweat-glands ; 
for  the  foot  of  the  limb  whose  nerve  has  been  divided  is  equally 
exposed  to  the  high  temperature.  A  high  temperature  it  is  true 
increases  up  to  a  certain  limit  the  irritability  of  the  epithelium 
of  the  sweat-glands  and  predisposes  it  to  secrete,  just  as  it  pro- 
motes action  in  the  case  of  a  muscle  or  nerve  or  other  forms  of 
living  substance.     Thus  stimulation  of  the  sciatic  in  the  cat 

1  The  cat  sweats  freely  in  the  hairless  soles  of  the  feet  but  not  on  any  part 
of  the  body  covered  with  hairs.  The  dog  also  sweats  in  the  same  regions  but 
not  so  freely  as  the  cat  ;  indeed  sweating  is  often  absent,  the  ducts  being  stopped 
by  growth  of  the  corneous  epidermis.  Rabbits  and  other  rodents  appear  not  to 
sweat  at  all.  The  snout  of  the  pig  sweats  freely  ;  and  the  often  profuse  sweat- 
ing of  the  horse,  a  singular  event  among  hair-covered  animals,  is  known  to  all. 


Chap,  hi.]     ELIMmATION   OF   WASTE   PRODUCTS.         557 

produces  a  much  more  abundant  secretion  in  a  limb  exjjosed  to 
a  temperature  of  35°  or  somewhat  above,  than  in  one  which  has 
been  exposed  to  a  distinctly  lower  temperature,  and  in  a  limb 
which  has  been  placed  in  ice-cold  water  hardly  any  secretion 
at  all  can  be  gained ;  but  apparently  mere  rise  of  temperature 
without  nerve-stimulation  will  not  give  rise  to  a  secretory 
activity  of  the  glands.  The  sweating  caused  by  a  dyspnoeic 
condition  of  blood,  and  such  appears  to  be  the  sweat  of  the 
death  agony,  is  similarly  brought  about  by  the  agency  of  the 
central  nervous  system.  When  an  animal  with  the  sciatic  nerve 
divided  on  one  side  is  made  dyspnoeic,  no  sweat  aj^pears  in  the 
hind  limb  of  that  side,  though  abundance  is  seen  in  the  other 
feet. 

Sweating  may  be  brought  about  as  a  reflex  act.  Thus  when 
the  central  stump  of  the  divided  sciatic  is  stimulated  sweating 
is  induced  in  the  other  limbs,  and  in  ourselves  the  introduction 
of  pungent  substances  into  the  mouth  will  frequently  give  rise 
to  a  copious  j^erspiration  over  the  side  of  the  face.  We  are  thus 
led  to  speak  of  sweat  centres,  analogous  to  the  vaso-motor  cen- 
tres, as  existing  in  the  central  nervous  system ;  and  as  in  the 
case  of  vaso-motor  centres,  a  dispute  has  arisen  as  to  whether 
there  is  a  dominant  sweat  centre  in  the  medulla  oblongata  or 
whether  such  centres  are  more  generally  distributed  over  the 
whole  of  the  spinal  cord. 

It  does  not  at  present  appear  certain  whether  the  sweating 
caused  by  heat  is  carried  out  by  direct  action  of  the  heated  blood 
on  the  sweat  centres,  or  by  the  higher  teniiDerature  stimulating 
the  skin  and  so  sending  up  afferent  impulses  which  produce  the 
effect  in  a  reflex  manner  ;  but  in  the  case  of  dyspnoea  at  least 
Ave  may  fairly  suppose  that  the  action  of  the  venous  blood  is 
chiefly  if  not  exclusively  on  the  nerve  centres.  Some  drugs, 
such  as  pilocarpin,  which  cause  sweating,  apjDear  to  produce 
their  effect  chiefly  by  a  local  action  on  the  glands,  since  the 
action  continues  after  the  division  of  the  nerves  (though  pilo- 
carpin apparently  has  as  well  some  slight  action  on  the  nerve 
centres),  and  the  antagonistic  action  of  atropin  is  similarly 
local.  Picrotoxin  and  strychnia  appear  to  produce  their  sweat- 
ing action  chiefly  if  not  exclusively  by  acting  on  the  central 
nervous  system,  while  nicotin  seems  to  act  both  centrally  and 
peripherally. 

§  354.  In  the  cat  (in  which  animal  the  matter  has  been  most 
studied),  the  sweat  fibres  for  the  hind-foot  leave  the  spinal 
cord  by  the  anterior  roots  of  the  first  and  second  lumbar  nerves, 
but  also  to  a  less  extent  by  the  two  thoracic  nerves  above  these 
and  the  third  lumbar  nerve  below.  Passing  to  the  sympathetic 
chain,  and  running  in  it  for  a  certain  distance,  they  leave  that 
chain  by  the  grey  rami  of  the  sixth  and  seventh  lumbar  and 
first  and  second  sacral  ganglia,  thus  reaching  the  spinal  nerves 


558  COUESE   OF  SWEAT   FIBEES.  [Book  ii. 

corresponding  to  these  ganglia  and  so  the  sciatic  nerve.  Along 
their  course  the  fibres  are  connected  with  nerve-cells  in  these 
ganglia,  the  fibres  in  a  grey  ramus  starting  from  cells  in  the 
ganglion  from  which  the  ramus  run,  or  in  the  ganglion  above 
it.  In  the  same  animal,  the  sweat-fibres  for  the  fore-feet  leave 
the  spinal  cord  by  the  anterior  roots  of  the  sixth,  seventh,  and 
eighth  thoracic  nerves,  but  also,  to  a  less  extent,  by  the  nerves 
above  and  below.  Passing  into  the  sympathetic  chain,  they 
ascend  to  the  ganglion  stellatum,  with  the  nerve-cells  of  which 
alone  they  are  connected,  and  by  the  branches  of  this  ganglion 
reach  the  branchial  plexus  and  so  the  median  and  ulnar  nerves. 
The  course  of  the  sweat-fibres  in  other  animals  is  probably  very 
similar  to  the  above.  In  the  horse  the  sweat-fibres  for  the  side 
of  face  and  in  the  pig  those  for  the  snout  appear  to  run  in 
branches  of  the  fifth  nerve  and  not  in  the  facial ;  in  the  latter 
animal  at  least  some  of  these  fibres  reach  the  fifth  nerve  from 
the  cervical  sympathetic,  but  apparently  not  all. 

§  355.  The  fact  mentioned  above  that  in  the  horse,  after 
section  of  the  cervical  sympathetic  nerve  on  one  side  of  the  neck, 
profuse  sweating  is  apt  to  break  out  on  that  side  of  the  face,  has 
suggested  the  idea  that  this  nerve  conveys  inhibitory  impulses 
to  the  sweat-glands  of  the  head  and  face,  and  that  when  it  is 
divided  the  sweat-fibres  running  in  the  fifth  nerve,  having 
nothing  to  counteract  them,  set  up  sweating.  But  it  is  prob- 
ably sufficient  in  this  case  to  suppose  that  the  glands  predis- 
posed to  activity  by  the  higher  temperature  brought  about  by 
the  section  of  the  sympathetic  dilating  the  blood  vessels,  are 
more  easily  excited  by  any  stimulus  working  upon  them  through 
the  fifth  nerve.  And  though  the  idea  of  a  double  nervous 
mechanism,  augmenting  and  inhibitory,  governing  the  activity 
of  the  sweat-glands,  is  a  tempting  one,  there  are  at  present  no 
satisfactory  reasons  for  adopting  it. 


CHAPTER   IV. 
THE   METABOLIC   PROCESSES    OF   THE   BODY. 

§  356.  We  have  followed  the  food  through  its  changes  in 
the  alimentary  canal,  and  have  seen  it  enter  into  the  blood,  either 
directly  or  by  the  intermediate  channel  of  the  lacteals,  in  the 
form  of  peptone  (or  otherwise  modified  albumin),  sugar  (lactic 
acid),  and  fats,  accompanied  by  various  salts  and  water.  We 
have  further  seen  that  the  waste  products  which  leave  the  body 
are  urea,  carbonic  acid,  salts  and  water.  We  have  now  to 
attempt  to  connect  together  the  food  and  the  waste  products  ; 
to  trace  out  as  far  as  we  are  able  the  various  steps  by  which  the 
one  is  transformed  into  the  other.  There  remains  the  further 
task  to  inquire  into  the  manner  in  which  the  energy  set  free  in 
this  transformation  is  distributed  and  made  use  of. 

The  master  tissues  of  the  body  are  the  muscular  and  nervous 
tissues  ;  all  the  other  tissues  may  be  regarded  as  the  servants  of 
these.  And  we  may  fairly  presume  that,  besides  the  digestive 
and  excretory  tissues  which  we  have  already  studied,  many  parts 
of  the  body  are  engaged  either  in  further  elaborating  the  com- 
paratively raw  food  which  enters  the  blood,  in  order  that  it  may 
be  assimilated  with  the  least  possible  labour  by  the  master 
tissues,  or  in  so  modifying  the  waste  products  which  arise  from 
the  activity  of  the  master  tissues  that  they  may  be  removed  from 
the  body  as  speedily  as  possible.  There  can  be  no  doubt  that 
manifold  intermediate  changes  of  this  kind  do  take  place  in  the 
body  ;  but  our  knowledge  of  the  matter  is  at  present  very  imper- 
fect. In  a  few  instances  only  can  we  localize  these  metabolic 
actions  and  speak  of  distinct  metabolic  tissues.  In  the  majority 
of  cases  we  can  only  trace  out  or  infer  chemical  changes,  without 
being  able  to  say  more  than  that  they  do  take  place  somewhere  ; 
and  in  consequence,  perhaps  somewhat  loosely,  speak  of  them  as 
taking  place  in  the  blood. 

How  little  we  know  concerning  the  metabolism  of  the  master 
tissues  themselves  was  shewn  when  we  were  dealing  with  these 

559 


560  METABOLIC   PEOCESSES.  [Book  ii. 

tissues  in  an  earlier  part  of  tliis  work  ;  but  success  in  the  study 
of  these  can  hardly  be  expected  until  our  knowledge  is  increased 
as  regards  the  changes  which  the  blood  undergoes  before  it 
reaches  and  after  it  leaves  the  muscle  or  the  nerve.  The  fact 
that  a  large  part  of  the  absorbed  food  is  carried  through  the 
liver  before  it  is  thrown  on  the  general  circulation  leads  us  to 
suppose  that  in  this  large  organ  important  metabolic  processes 
are  carried  on  ;  and  observation  with  experiment  confirms  this 
view.  Important  as  the  secretions  of  bile  may  be  the  other 
metabolic  functions  of  the  liver  are  of  still  greater  importance. 


SEC.   1.     THE   HISTORY   OF   GLYCOGEK 

§  357.  If  the  liver  of  a  well-fed  animal  be  removed  immedi- 
ately after  death,  rapidly  divided  into  small  pieces,  thrown  into 
boiling  water,  rubbed  up  and  boiled,  a  decoction  may  be  obtained 
which  after  careful  neutralization  and  filtration  will  be  toler- 
ably free  from  proteid  matter.  Such  a  decoction  is  remarkably 
opalescent,  milky  in  fact  in  appearance,  much  more  so  than  a 
similar  decoction  from  muscle  or  other  tissue,  and  remains 
opalescent  even  after  repeated  filtration.  Treated  with  iodine, 
the  solution  turns  a  brownish  red,  port-wine  red  colour,  not 
unlike  that  given  by  dextrine  when  iodine  is  added;  the  colour 
disappears  on  warming,  but  reappears  on  cooling  provided  that 
not  too  much  proteid  matter  has  been  left  in  the  solution. 
Treated  with  Fehling's  fluid  or  other  tests  for  sugar,  the  solu- 
tion is  found  to  contain  a  small  and  variable,  but  only  a  small, 
quantity  of  sugar. 

If  the  solution  be  exposed,  preferably  in  the  warm,  to  the 
action  of  saliva  or  of  some  other  amylolytic  ferment,  or  be  boiled 
with  dilute  acid,  the  opalescence  disappears;  and  the  now  clear 
transparent  solution  gives  no  longer  the  port-wine  reaction  with 
iodine.  Tested  moreover  with  Fehling's  fluid  or  by  other  means 
it  is  now  found  to  contain  a  considerable  quantity  of  sugar. 

If  alcohol  be  added  to  the  ojDaleseent  solution  until  the 
mixture  contains  60  p.c.  of  the  alcohol  (previous  concentration 
by  evaporation  being  desirable)  a  white  amorphous  precipitate 
is  thrown  down.  This  precipitate,  removed  by  filtration,  boiled 
with  an  alcoholic  solution  of  potash  in  which  it  is  insoluble,  but 
which  dissolves  and  destroys  any  jDroteids  which  may  be  pres- 
ent, treated  with  ether  to  remove  fatty  impurities,  and  washed 
with  alcohol  may  be  obtained  in  a  pure  condition.  It  then 
appears  as  a  white  amorphous  powder,  fairly  soluble  in  water, 
but  always  giving  rise  to  a  milky  opalescent  solution  unless  an 
excess  of  alkali  be  present,  in  which  case  the  opalescence  may 
be  slight  or  absent. 

The  opalescent  solution  of  this  purified  material  gives  a 
port-wine  reaction  with  iodine,  but  no  reaction  whatever  with 
Fehling's  fluid  or  the  other  sugar  tests.  Treated  with  an 
36  561 


562  GLYCOGEIsT.  [Book  ii. 

amylolytic  ferment  or  boiled  Avitli  dilute  acid,  the  solution,  like 
the  raw  decoction  of  liver,  loses  its  opalescence  and  its  port- 
wine  reaction  with  iodine  but  now  gives  abundant  evidence  of 
the  presence  of  sugar,  dextrose,  if  boiling  with  acid  has  been 
employed,  maltose  chiefly,  if  an  amylolytic  ferment  has  been 
used.  If  quantitative  determinations  be  employed  it  will  be 
found  that  the  amount  of  sugar  obtained  is  proportionate  to  the 
amount  of  the  white  powder  acted  upon;  in  other  words  the 
substance  forming  an  opalescent  solution  is  converted  into  sugar^ 
the  solution  of  which  is  clear.  Obviously  the  substance  is  a  body 
allied  to  starch;  and  this  is  confirmed  by  its  elementary  compo- 
sition, which  is  found  to  be  CgHj^^Og  or  some  multiple  of  this. 

Hence  this  body  is  called  glycogen.  And  it  is  obvious  from 
what  has  been  stated  above,  that  the  liver  of  a  well-fed  animal 
at  the  moment  of  death  contains  a  considerable  quantity  of 
glycogen  either  in  a  free  state  or  in  such  a  condition  that  it  is 
set  free  by  subjecting  the  liver  to  the  action  of  boiling  Avater. 
We  may  add  that  it  occurs  in  the  liver  in  the  hepatic  cells,  for 
these  when  glycogen  is  present  in  the  liver  give,  when  properly 
tested  with  iodine,  the  characteristic  port-wine  reaction. 

§  358.  If  the  liver,  instead  of  being  treated  immediately 
upon  the  death  of  the  animal,  is  allowed  to  remain  in  the  body 
of  the  dead  animal  for  several  hours,  especially  in  a  warm  place, 
before  a  decoction  is  made  of  it,  the  decoction  will  be  found  to 
have  little  or  no  opalescence,  to  be  quite  or  nearly  quite  clear, 
to  give  little  or  no  port-wine  reaction  with  iodine,  but  to  con- 
tain a  very  considerable  quantity  of  sugar.  As  we  said  above,  the 
decoction  even  of  a  liver  taken  immediately  after  death  generally 
contains  some  little  sugar,  and  the  quantity  of  sugar  in  the  liver 
appears  as  a  rule  to  increase  after  death,  the  amount  of  glycogen 
diminishmg  at  the  same  time.  We  may  infer  from  this  that  the 
glycogen  present  in  the  liver  at  the  moment  of  death  is  gradually 
after  death  by  some  action  or  other  converted  into  sugar. 

The  action  is  that  of  some  agency  whose  activity  is  destroyed 
by  the  temperature  of  boiling  water;  hence  the  directions  re- 
peatedly given  above  to  throw  the  liver  into  boiling  water. 
This  naturally  suggests  the  presence  in  the  liver  of  an  amylo- 
lytic ferment.  But,  not  only  have  attempts  to  isolate  from  the 
liver  an  amylolytic  ferment  failed,  in  the  hands  of  most  observers 
at  least,  but  the  exact  nature  of  the  sugar  Avliich  appears  shews 
that  the  change  is  not  effected  by  an  ordinary  amylolytic  fer- 
ment. In  the  case  of  the  amylolytic  ferment  of  saliva,  pan- 
creatic juice,  intestmal  juice,  and  indeed  of  all  other  amylolytic 
animal  fluids,  the  sugar  into  which  starch  or  glycogen  is  con- 
verted is  maltose.  Now  the  sugar  which  appears  in  the  liver 
after  death  is  dextrose,  identical,  so  far  at  least  as  can  at  present 
be  made  out,  with  ordinary  dextrose.  We  are  led  therefore  to 
infer  that  the  change  of  glycogen  into  suger  which  appears  to 


Chap,  iv.]     METABOLIC  PROCESSES  OF  THE  BODY.        563 

go  on  after  death  is  carried  out  by  some  action  of  the  liver, 
probably  of  the  hepatic  cell  itself,  which  is  done  away  with  by 
a  temj)erature  of  100°  C,  but  which  is  not  the  action  of  a 
ferment  capable  of  being  isolated. 

§  359.  We  have  used  above  the  phrase  '  well-fed '  animal 
because  the  amount  of  glycogen  present  in  the  liver  of  an  aminal 
at  any  one  time  is  very  variable,  and  especially  dependent  on  the 
amount  and  nature  of  the  food  previously  taken.  When  all  food 
is  withheld  from  an  animal,  the  glycogen  in  the  liver  diminishes, 
rapidly  at  first,  but  more  slowly  afterwards.  Even  after  some 
days'  starvation  a  small  quantity  is  frequently  still  found;  but 
in  rabbits,  at  all  events,  the  whole  may  eventually  disappear. 

If  an  animal,  after  having  been  starved  until  its  liver  may 
be  assumed  to  be  free  or  almost  free  from  glj^cogen,  be  fed  on 
a  diet  rich  in  carbohydrates  or  on  one  consisting  exclusively  of 
carbohydrates,  the  liver  will  in  a  short  time  be  found  to  contain 
a  very  large  quantity  of  glycogen.  Obviously  the  presence  of 
carbohydrates  in  food  leads  to  an  accumulation  of  glycogen  in  the 
liver;  and  this  is  true  both  of  starch  and  of  dextrin  and  of  the 
various  forms  of  sugar,  cane,  grape  and  milk  sugar.  The  effect 
may  be  quite  a  rapid  one,  for  glycogen  has  been  found  in  the 
liver  in  considerable  quantity  within  a  few  hours  after  the  intro- 
duction of  sugar  into  the  alimentary  canal  of  a  starving  aminal. 

If  an  animal,  similarly  starved,  be  fed  on  an  exclusively  meat 
diet  a  certain  amount  of  glycogen  is  found  in  the  liver.  This 
appears  to  be  especially  the  case  with  dogs  (probably  with  other 
carnivorous  animals  also) ;  and  in  earlier  works  on  the  subject 
the  constant  presence  of  glycogen  in  the  livers  of  dogs  fed  on 
meat  was  regarded  as  an  important  indication  of  the  formation 
within  the  body  of  non-nitrogenous  from  nitrogenous  material. 
But  in  the  first  place,  the  quantity  of  glj-cogen  thus  stored  up 
in  the  liver  as  the  result  of  a  meat  diet,  is  much  less  than  that 
which  follows  upon  a  carbohydrate  diet;  and  in  the  second 
place,  ordinary  meat,  especially  horse-flesh  on  which  dogs  in 
such  experiments  are  usually  fed,  contains  in  itself  (§  59)  a 
certain  amount  either  of  glycogen  or  some  form  of  sugar. 
Moreover  when  animals  are  fed  not  on  meat  but  on  purilied 
proteid,  such  as  fibrin,  casein  or  albumin,  the  quantity  of 
glycogen  in  the  liver  becomes  still  smaller,  though  according  to 
most  observers  remaining  greater  than  during  starvation.  We 
may  infer  therefore  that  part  of  the  glycogen  which  appears  in 
the  liver  after  a  meat  diet  is  really  due  to  carbohydrate  mate- 
rials present  in  the  meat.  Part,  however,  would  appear  to  be 
the  result  of  the  actual  proteid  food;  and  we  have  similar  evi- 
dence that  gelatine  taken  as  food  leads  to  the  formation  of  some 
glycogen  in  the  liver.  But  in  this  respect  these  nitrogenous 
substances  fall  far  short  of  carbohydrate  material. 

With  regard  to  fats,  all  observers  are  agreed  that  these  lead 


564 


STOEAGE   OF   GLYCOGEN. 


[Book  ii. 


to  no  accumulation  of  glycogen  in  the  liver ;  an  animal  feci  on 
an  exclusively  fatty  diet  has  no  more  glycogen  in  its  liver  than 
a  starving  animal. 

Hence  of  the  three  great  classes  of  foocl-stuifs,  the  carbo- 
hydrates stand  out  prominently  as  the  substances  which  taken 
as  food  lead  to  an  accumulation  of  glycogen  in  the  liver.  We 
may  remark  that  the  greatest  accumulation  of  glycogen  is 
effected  not  by  a  pure  carbohydrate  diet,  but  by  a  mixed  diet 
rich  in  carbohydrates.  A  quantity  of  carbohydrate  mixed 
with  a  certain  proportion  of  proteid  gives  rise  to  a  larger 
amount  of  glycogen  in  the  liver  than  the  same  quantity  of 
carbohydrate  given  by  itself  ;  and  it  is  possible  that  the  pres- 
ence of  an  appropriate  quantity  of  fat  still  further  assists  the 
accumulation.  But  this  result  probably  depends,  in  part  at 
least,  on  the  fact  that,  though  differences  may  be  met  with  in 
different  animals,  a  mixture  of  the  several  classes  of  food-stuffs 
is  more  readily  digested  resulting  in  more  nutritive  material 
being  thrown  upon  the  blood,  than  is  a  meal  consisting  exclu- 
sively of  one  kind  of  food-stuff  alone. 

So  far  as  we  know  at  present  the  glycogen  which  thus 
appears  in  the  liver  as  the  result  of  feeding  either  with  any  of 


Fig.  105.     Section  of  Livku  of  Fkog.      (Langley.) 

The  Figure  shews  the  tubular  structure  of  the  liver.  At  (a)  a  tubule  is 
seen  in  transverse,  at  (b)  in  longitudinal  section.     I,  lumen  of  tnbiile. 

The  liver  was  that  of  a  winter  frog,  and  the  cells  shew  an  inner  zone  of 
proteid  granules  ;  the  outer  zone  was  chiefly  occupied  by  glycogen. 


Chap,  iv.]     METABOLIC  PROCESSES  OF  THE  BODY.        565 

the  various  forms  of  carbohydrates,  or  with  proteids,  or  with 
other  substances,  is  of  the  same  kind  and  presents  the  same 
characters ;  at  least  we  have  no  evidence  to  the  contrary. 

The  storing-up  of  glycogen  in  the  liver  is  also  influenced 
by  other  circumstances  than  the  taking  of  food.  For  instance 
in  the  frog  an  increase  of  glycogen  takes  place  during  the  win- 
ter months.  In  the  summer  months  the  liver  of  a  frog  will  be 
found  to  contain  very  little  glycogen.  Fig.  106  c,  unless  the 
animal  has  been  unusually  well  fed;  whereas  a  liver  examined  in 
mid  winter,  Figs.  105, 106  A,  will  be  found  to  contain  a  consider- 
able quantity,  even  though  no  food  has  been  taken  for  months. 
In  such  a  case  the  material  for  the  formation  of  the  glycogen 
in  the  liver  must  have  been  furnished  by  some  part  of  the  body 
of  the  frog,  and  could  not,  as  may  be  the  case  when  a  meal 
leads   immediately  to   an   increase   of  glycogen,    be   supplied 


Fig.  106.     Three  phases  of  the  Hepatic  Cells  of  the  Frog.     (Langley.) 

A.  Cells  rich  in  glycogen.  Taken  from  a  frog  during  winter.  The  cells 
are  large,  and  proteid  granules  are  massed  round  the  lumen,  the  homogeneous 
outer  zones  of  the  cells  being  largely  composed  of  glycogen  which  was  present 
in  considerable  abundance.  The  outer  zones  contained  numerous  fat  globules, 
shewn  as  dark  dots  ;  but  as  stated  in  the  text  these  fat  globules  vary  much. 

B.  Cells  poor  in  glycogen.  Taken  from  a  winter  frog  which  had  been  kept 
at  22°  C.  for  10  days.  The  cells  contain  very  little  glycogen  and  the  proteid 
granules  are  dispersed  throughout  the  cell.  In  a  summer  frog  well  fed  on  pro- 
teids the  cells  would  present  a  very  similar  appearance. 

C.  Starved  cells.  Taken  from  a  summer  frog  after  a  long  fast.  The  cells 
are  small  and  almost  free  from  glycogen.  The  proteid  granules  are  dispersed 
throughout  the  cell. 

All  the  specimens  were  hardened  in  1  p.c.  osmic  acid,  and  are  drawn  to  the 
same  or  nearly  to  the  same  scale. 


566  GLYCOGEN  m   HEPATIC   CELLS.         [Book  ii. 

directly  from  the  food.  It  seems  as  if  in  the  summer  the  frog 
lives  up  to  its  capital  of  hepatic  glycogen,  spending  it  as  fast 
almost  as  it  is  made,  but  that  during  the  winter  a  quantity  is 
funded  to  provide  for  the  demands  of  late  winter  and  early  spring. 

This  winter  storage  of  hepatic  glycogen  in  the  frog  seems 
closely  dependent  on  temperature.  If  a  winter  frog,  whose 
liver  is  presumably  more  or  less  loaded  with  glycogen,  be  ex- 
posed for  some  time  to  a  temperature  of  20°  or  a  little  higher, 
the  liver  will  afterwards  be  found  to  contain  little  or  no  glyco- 
gen. Fig.  106  B ;  and  conversely  if  a  summer  frog  be  exposed 
to  untimely  cold,  glycogen,  though  not  in  any  great  quantity, 
begins  to  be  stored  up  in  the  liver. 

§  360.  Before  we  attempt  to  discuss  further  how  food  and 
other  circumstances  thus  aifect  the  glycogen  in  the  liver,  it 
will  be  desirable  to  consider  certain  histological  changes  occur- 
ring in  the  hepatic  cells,  under  various  conditions.  It  will  be 
convenient  to  begin  with  the  cells  of  the  more  distinctly  tubular 
gland  of  the  frog. 

In  a  frog  which  has  not  been  subjected  to  any  special  treat- 
ment the  cell-substance  of  the  hepatic  cell  (cf .  Fig.  106  a)  will 
generally  be  found  to  contain  lodged  in  itself  three  kinds  of 
material,  the  presence  of  which,  if  not  directly  recognizable  in 
the  fresh  cell,  may  be  demonstrated  by  the  use  of  various 
reagents.  In  the  first  place,  oil  globules  of  variable  size  and  in 
variable  amount  are  scattered  throughout  the  cell ;  sometimes, 
as  we  have  already  said,  these  are  extremely  abundant ;  but 
there  is  otherwise  nothing  very  special  about  these  fat  globules 
in  the  hepatic  cell  to  demand  any  discussion  concernmg  them 
apart  from  the  general  discussion  on  the  formation  of  fat,  into 
which  we  shall  enter  later  on. 

In  the  second  place,  a  number  of  small  discrete  granules 
may  be  seen  lodged  in  the  cell-substance.  These  appear  to  be 
of  a  proteid  nature  and  are  generally  most  abundant  on  the 
inner  side  of  the  cell  near  the  lumen  of  the  bile  passage.  The 
presence  of  these  granules  is  closely  dej)endent  on  the  activity 
of  the  digestive  processes.  They  diminish  when  digestion  is 
going  on  and  accumulate  again  afterwards.  Putting  aside  cer- 
tain details  we  may  say  that  these  granules  behave  very  much 
like  the  granules  in  an  albuminous  salivary  cell,  a  pancreatic 
cell  or  a  chief  gastric  cell ;  and  we  may  probably  safely  con- 
clude that  they,  like  the  granules  in  these  cells,  are  in  some 
way  concerned  in  the  formation  of  the  secretion ;  that  is,  in 
their  case,  bile. 

In  the  third  place,  the  cell  contains  more  especially  in  its 
outer  parts,  nearer  the  blood  vessel,  away  from  the  lumen  of 
the  bile  passage,  a  variable  quantity  of  material  which  differs 
from  the  ordinary  cell-substance  in  being  hyaline  and  refractive 
and  hence  glassy  looking,  and  in  staining  port-wine  red  with 


Chap,  iv.]     METABOLIC  PROCESSES  OF  THE  BODY.       567 

iodine  instead  of  brownish  yellow  as  does  ordinaiy  cell-sub- 
stance. This  material  is,  though  with  some  little  difficulty, 
soluble  in  water,  and  by  this  means  may  be  dissolved  out  from 
the  cell.  When  this  is  done  the  places  which  it  occupied  ap- 
pear as  vacuoles  or  gaps  of  various  sizes  limited  by  bars  of  the 
cell-substance,  which  thus  takes  on  the  form  of  a  network, 
the  meshes  of  which  are  wider  and  more  conspicuous  in  the 
outer  part  of  the  cell,  in  which  the  hj'aline  material  Avas  pre- 
viously most  abundant.  In  the  inner  part  of  the  cell  where 
the  hyaline  material  was  scanty  the  cell-substance  is  more 
dense,  and  even  in  the  outer  part  a  shell  of  more  dense,  less 
reticulate  cell-substance  affords  a  definite  outline  to  the  cell. 
There  can  be  no  doubt  that  this  hyaline  material  is  either  actual 
glycogen  such  as  may  be  extracted  from  the  liver,  or,  as  seems 
more  probable  from  its  deficient  solubility,  glj^cogen  in  some  more 
or  less  loose  combination  with  some  other  bod}',  a  combination, 
however,  of  such  a  kind  that  the  iodine  reaction  makes  itself  felt. 

§  361.  The  above  may  be  taken  as  a  general  description  of 
a  cell  in  an  ordinary  condition.  The  question  now  comes  before 
us,  What  changes  are  brought  about  by  various  foods  or  by  the 
absence  of  food  ? 

If  a  frog  be  largely  fed  on  a  diet  containing  large  quantities 
of  carbohydrates,  the  liver  will  be  found  rich  in  glj-cogen  and 
the  cells  will  present  the  following  characters.  The  cell  is 
relatively  large  (cf .  Fig.  106  a)  and  as  it  were  swollen  ;  the 
cell-substance  is  largely  occupied  by  the  hyaline  material  just 
spoken  of,  especially  in  its  outer  parts,  so  that  in  sections  pre- 
pared and  mounted  iii  the  ordinary  way  in  which  the  glycogen 
has  been  dissolved  out  the  greater  part  of  the  cell  consists  of  a 
loose  open  network  of  bars  of  stained  cell-substance,  with  wide 
meshes ;  a  certain  quantity  of  more  solid,  generally  granular 
looking  cell-substance  occupies  the  part  of  the  cell  nearest  the 
lumen,  and  a  thin  shell  of  cell-substance  forms  an  envelojje  for 
the  rest  of  the  cell.  The  nucleus  is  large  and  distinct.  When 
such  a  cell  is  seen  in  a  perfectly  fresh  state,  the  hj-aline  refrac- 
tive material  (giving  the  reaction  with  iodine)  often  hides  the 
nucleus  and  the  greater  part  of  the  cell-substance  proper. 

If  on  the  other  hand  the  frog  be  fed  on  a  proteid  diet  free 
from  carbohydrates,  for  instance  on  fibrin,  the  liver  contains 
little  or  no  glycogen,  and  the  hepatic  cells  are  not  only  much 
smaller  but  present  an  appearance  very  different  from  the  above 
(cf.  Fig.  106  b).  Little  or  no  hj'aline  material  is  visible,  the 
cells  give  little  or  no  port-wine  reaction  with  iodine,  but  only 
the  usual  brown  5'ellow  proteid  reaction,  and  in  specimens 
prepared  and  mounted  in  tlie  ordinary  way  the  cell-substance 
appears  densely  granular  througliout. 

Lastl}',  if  the  frog  be  starved,  and  if  to  the  effects  of  starva- 
tion there  be  added  those  of  exposure  to  a  high  temperature 


568 


STOEAGE   OF  GLYCOGEN. 


[Book   ii. 


Fig.  107.  Section  of 
Mammallin  Liver  rich  in 
GLYCOGEN.     (Langley.) 

Osmic  acid  specimen,  gly- 
cogen not  dissolved  out. 


(25°),  by  whicli  as  we  have  seen  the  hepatic  cells  are  markedly 
affected,  the  liver  is  found  to  be  free  from  glycogen,  and  the 
hepatic  cells  to  be  extremely  small  (cf.  Fig.  106  c),  only  half 
the  size  or  even  less,  of  those  of  the  well-fed  frog,  but  otherwise 
much  like  the  cells  in  a  frog  fed  on  proteid  material. 

§  362.  In  the  mammal  changes  in  the  hepatic  cells  similar 
to  those  just  described  as  occurring  in  the  frog  have  also  been 

observed.  When  the  animal  is  fed  on 
a  diet  rich  in  carbohydrates,  and  when 
therefore  as  we  have  seen  the  liver 
abounds  in  glycogen,  the  hepatic  cells 
(Fig.  107)  are  larger  (so  large  that  they 
have  by  some  authors  been  described 
as  compressing  the  lobular  capillaries) 
and  loaded  with  the  same  refractive 
hyaline  material  staining  port-wine  red 
with  iodine.  When  this  material,  which 
is  disposed  more  centrally  in  the  cell 
than  is  the  case  in  the  frog,  is  dissolved 
out  a  coarse  open  network  of  cell-sub- 
stance is  displayed.  We  may  add  that 
in  an  animal  thus  fed  the  whole  liver 
is  very  large  and  as  it  were  swollen  ;  it  is  also  soft  and  tears 
easily. 

In  an  animal  fed  on  proteids  alone,  for  instance  on  fibrin,  the 
liver  frequently  contains  some  glycogen  and  the  hepatic  cells 
contain  a  small  quantity  of  hyaline  glycogenic  material.  As 
in  the  corresponding  case  in  the  frog,  the  cells  are  compara- 
tively small,  and  the  cell-substance  appears  finely  and  uniformly 
granular. 

In  a  starved  mammal,  the  liver  is  small,  dense  to  the  touch 
and  tough  ;  it  contains  a  trace  only  of  glycogen  or  none  at 
all;  the  cells  (Fig.  108)  are  small,  as  it  were  shrunken,  and 

the  cell-substance,  which  gives  no  port- 
wine  reaction,  or  a  mere'  trace  only, 
with  iodine,  is  still  more  finely  granu- 
lar. 

§  363.  The  microscopic  appearances 
just  described  shew,  and  indeed  general 
considerations  lead  us  to  the  same  con- 
clusion, that  the  processes  taking  place 
in  a  hepatic  cell  are  very  complex.  In 
the  first  place,  the  constituents  of  bile 
are  being  formed  and  discharged  into 
the  bile  passages  after  the  fashion  of 
an  ordinary  secreting  gland.  In  the 
second  place,  a  formation  of  glycogen  is 
also  taking  place,  and  we  shall  have  pres- 


FiG.lOS.  Section  OF  Mam- 
malian Liver,  containing 
little  or  no  glycogen. 
(Langley.) 

Osmic  acid  specimen.  The 
granules  are  not  well  pre- 
served in  some  of  the  cells. 


Chap,  iv.]     METABOLIC  PROCESSES  OF  THE  BODY.        569 

ently  to  consider  briefly  the  relations  of  the  one  process  to  the 
other.  In  the  third  phice,  as  is  especially  indicated  by  the 
somewhat  peculiar  effects  on  the  hepatic  cell  of  food  exclusively 
proteid  in  nature,  other  processes,  similar  perhaps  to  the  forma- 
tion of  glycogen  but  not  resulting  in  the  storage  of  my  carbo- 
hydrate material  and  dealing  possibly  with  proteid  substances, 
also  take  place.  Hence  the  exact  interpretation  of  all  the 
changes  which  may  be  observed  becomes  exceedingly  difficult. 

Leaving  the  processes  of  the  first  and  third  kind  whollj'  on 
one  side  for  the  present,  and  confining  our  attention  entirely  to 
the  glycogen,  it  is  obvious  that  the  hepatic  cell  manufactures  the 
glycogen  in  some  way  or  other,  and  lodges  it  in  its  own  substance 
for  the  time  very  much  in  the  way  that  a  secreting  cell  manu- 
factures and  lodges  in  itself  for  a  time  material  for  the  secretion 
which  it  is  about  to  pour  forth.  There  is  this  difference,  that  in 
the  one  case  the  material  of  the  secretion,  after  undergoing  as 
we  have  seen  more  or  less  change,  is  cast  out  into  the  lumen  of 
the  alveolus,  whereas  in  the  other  case  the  glycogen,  which  must 
undergo  change  since  it  may  be  made  to  disappear  rapidly  from 
the  hepatic  cell,  is  not  when  changed  cast  out  into  the  bile 
passages  ;  it  must  therefore  be  sent  back  again  to  the  blood. 

§  364.  We  say  "  manufactures  the  gl3^cogen  in  some  way  or 
other,"  and  we  have  now  to  inquire  what  we  know  concerning 
the  nature  and  the  several  steps  of  this  manufacture. 

We  have  already  seen  that  the  presence  of  glycogen  in  the 
liver  is  especially  favoured  by  a  carbohydrate  diet ;  and  in  our 
studies  on  digestion  we  have  seen  reason  to  think  that  a  very 
large  part  at  all  events  of  the  carbohydrate  material  of  a  meal  is 
absorbed  as  sugar  by  the  capillaries  of  the  intestine  and  carried 
as  sugar  to  the  liver  in  the  portal  blood.  Hence,  it  seems  only 
reasonable  to  conclude  that  the  glycogen  which  makes  its  appear- 
ance in  the  liver  after  an  amylaceous  meal  arises  from  a  direct 
conversion  of  the  sugar  carried  to  the  liver  by  the  portal  vein,  the 
sugar  becoming  through  some  action  of  the  hepatic  cell-substance 
dehydrated  into  glycogen,  or  animal  starch  as  it  has  been  called, 
the  process  being  a  reverse  of  that  by  which  in  the  alimentary 
canal  starch  is  hydrated  into  sugar  through  the  action  of  the 
salivary  and  pancreatic  ferments.  Vegetable  cells  can  undoubt- 
edly convert  both  starch  into  sugar  and  sugar  into  starch  ;  and 
there  are  no  a  priori  arguments  or  positive  facts  which  would 
lead  us  to  suppose  that  the  activity  of  animal  living  substance 
cannot  accomplish  the  latter  as  well  as  the  former  of  these  changes. 
We  are  quite  ignorant  it  is  true  of  the  exact  Avay  in  which  either 
the  hydration  or  the  dehydration  is  effected  by  living  substance; 
but  we  are  equally  ignorant  of  the  exact  way  in  which  an  amy- 
lolytic  ferment  effects  the  hydration  of  starch  into  sugar,  which 
it  carries  out  with  so  much  apparent  ease.  It  is  not  a  great 
assumption   to    suppose  that   the   continually   changing   living 


570  STOEAGE   OF   GLYCOGEN.  [Book   ii. 

substance,  Avliich  in  its  changes  is  continuall}"  giving  out  energy, 
lias  the  power  of  acting  on  molecules  of  starch  or  of  sugar  in 
contact  with,  or  even  only  near  to  itself,  and  so  of  hydrating 
starch  into  the  sugar  or  of  dehydrating  sugar  into  starch.  The 
latter  process  may  be  a  more  difficult  one  than  the  former,  but 
not  one  beyond  the  power  of  the  living  substance.  We  may 
fairl}^  suppose  that  a  quantity  of  sugar  in  solution  present  in  a 
vacuole,  for  instance,  of  the  hepatic  cell-substance  can  be,  by 
some  action  of  the  cell-substance,  converted  into  glycogen  in  a 
solid  form,  filling  up  the  vacuole.  Again,  as  we  have  inciden- 
tally mentioned,  sugar  injected  into  the  jugular  vein  readily 
gives  rise  to  sugar  in  the  urine ;  but  a  very  considerable  quantity 
can  be  slowly  injected  into  the  portal  vein  without  any  appearing 
in  the  urine.  This  suggests  the  idea  that  the  liver,  so  to  speak, 
catches  the  sugar  as  it  is  passing  through  the  hepatic  capillaries 
and  at  once  dehydrates  it  into  glycogen. 

Similar  considerations  may  also  be  applied  to  the  case  men- 
tioned above  of  the  appearance  of  glycogen  in  the  hepatic  cells 
of  winter  (fasting)  frogs.  We  have  reason  to  think  that  sugar 
makes  its  appearance  as  a  product  of  the  metabolism  of  various 
tissues.  The  sugar  thus  arising  finding  its  way  into  blood 
may  be  made  use  of  at  once  elsewhere,  converted  speedily  for 
instance  into  carbonic  acid  and  so  got  rid  of.  But  we  can 
readily  imagine  that  under  certain  circumstances,  as,  for  in- 
stance when  the  activities  *of  the  animal  were  lessened  by  a 
low  temperature,  it  was  not  so  made  use  of  and  remained  in 
the  blood.  If  so  it  would  in  the  course  of  the  circulation  be 
carried  to  the  liver,  and  might  be  at  once  taken  up  by  the 
hepatic  cells  and  converted  into  glycogen ;  and  these  might  be 
so  active  that  the  blood  was  never  at  any  time  allowed  to  remain 
loaded  with  sugar  to  such  an  extent  as  to  permit  a  loss  through 
the  urine. 

§  365.  Upon  such  a  view,  the  carbohydrate  taken  as  food 
would  be  converted  into  glycogen  by  the  agency  of  the  hepatic 
cell,  without  at  any  time  becoming  an  integral  part  of  the  liv- 
ing substance  of  the  cell.  Such  a  view  may  be  the  true  one  ; 
but  it  is  open  for  us  to  look  at  the  matter  in  another  light. 
We  may  push  still  further  the  analogy  between  the  glycogen 
of  the  hepatic  cell  and  the  material  with  which  a  secreting  cell 
is  loaded.  In  dealing  with  secretion  we  saw  reasons  for  re- 
garding such  a  body  as  mucin  to  be  a  product  of  the  metabo- 
lism of  the  cell-substance  of  the  mucous  cell  ,  and  we  may 
similarly  regard  glycogen,  or  sugar  readily  convertible  into 
glycogen,  or  at  least  some  or  other  carbohydrate  material,  as 
a  normal  product  of  the  metabolism  of  the  hepatic  cell.  We 
may  thus  conceive  of  the  hei^atic  cells  as  being  continually  en- 
gaged in  giving  rise  to  carbohydrate  material  in  the  form  either 
of  sugar  or  of  some  other  body ;   and  we  may  suppose  that  under 


Chap,  iv.]    METABOLIC  PROCESSES  OF  THE  BODY.         571 

certain  circumstances,  as  in  the  absence  of  adequate  food,  the 
carbohydrate  material  thus  formed  is  at  once  discharged  into 
the  blood  of  the  hepatic  vein  for  the  general  use  of  the  body, 
but  that  under  other  circumstances,  as  when  an  amylaceous 
meal  has  been  taken,  the  immediate  wants  of  the  economy  being 
covered  by  the  carbohydrates  of  the  meal,  the  carbohydrate 
products  of  the  hepatic  metabolism  are  stored  up  as  glycogen. 
Under  such  a  ^dew  the  sugar  of  the  meal  is  used  up  somewhere 
in  the  body,  and  the  glycogen  to  the  storage  of  which  in  the 
liver  it  gives  rise  comes  direct  from  the  hepatic  substance.  And 
a  similar  explanation  may  be  given  of  the  storing-uj)  of  glyco- 
gen in  the  liver  under  such  circumstances  as  those  of  the  winter 
frog  previously  mentioned. 

We  do  not  possess  at  present  ex23erimental  or  other  evidence 
of  so  clear  a  kind  as  to  enable  us  to  decide  dogmatically  be- 
tween these  two  views.  It  may  be  that  both  views  are  true, 
or  rather  that  the  true  conception  embraces  both  views.  It 
may  be  that  the  normal  metabolism  of  the  hepatic  cell  does  pro- 
duce a  certain  amount  of  carbohydrate  material ;  but  if  so  the 
probability  is  that  the  exact  form  in  which  that  carbohydrate 
appears  in  the  first  instance  in  the  laboratory  of  the  cell  is  not 
that  of  glycogen  but  of  sugar  of  some  kind  or  other,  and  that 
the  conversion  into  glycogen  is  a  subsidiary  act  for  the  purpose 
of  retaining  the  carbohydrate  material  in  the  grasp  of  the  cell. 
If  this  be  the  case,  then  until  it  has  been  shewn  that  there  is 
something  peculiar  about  the  sugar  thus  produced  by  the  cell 
itself,  by  virtue  of  which  it  alone  can  be  converted  by  the  cell 
into  glycogen,  Ave  may  fairly  infer  that  the  cell  might  also 
convert  into  glycogen  sugar  passing  into  the  interstices  of  the 
cell-substance  from  the  portal  capillaries. 

§  366.  We  may  now  turn  to  another  question,  the  answer 
of  which  is  in  a  measure  dependent  on  the  one  which  we  have 
just  discussed.  What  is  the  use  and  purpose  of  this  hepatic 
glycogen  ?  What  ultimately  becomes  of  the  glycogen  thus  for 
a  while  stored  up  in  the  liver? 

One  view  which  has  been  put  forward  is  as  follows.  We 
have  evidence,  as  we  shall  presently  learn,  that  a  great  deal  of 
the  fat  of  the  body  is  not  taken  as  such  in  the  food,  but  is 
constructed  anew  in  the  bod}'  out  of  other  substances.  Both 
carbohydrates  and  proteids,  taken  in  excess  or  under  certain 
circumstances,  lead  to  an  accumulation  of  fat ;  and  we  liave 
reason  to  believe  that  carbohydrates  on  the  one  hand  and  the 
carbon-holding  portions  of  various  proteids  on  the  other,  may 
by  some  process  or  other  be  converted  into  fat.  And  it  has 
been  suggested  that  the  glycogen  in  the  liver  is  a  phase  of  a 
constructive  fatty  metabolism,  that  it  is  material  on  its  way  to 
become  fat.  There  is,  however,  no  positive  evidence  in  favour 
of  this  view. 


572  GLYCOGEN   IN   THE   MUSCLES.  [Book   it. 

Another  view,  which  has  already  been  suggested  while  we 
were  dealing  with  the  manner  of  formation  of  glycogen,  makes 
use  of  the  formation  of  fat  for  the  purposes  of  analogy  only. 
Seeing  that  adipose  tissue  serves  as  a  storehouse  of  fat  which 
is  not  wanted  by  the  body  at  the  moment  but  may  be  wanted 
presently,  the  question  readily  presents  itself,  May  not  the 
hepatic  glycogen  have  an  analogous  function  ?  May  we  not 
regard  the  presence  of  glycogen  in  the  liver  as  in  large  measure 
due  to  the  fact  that  it  is  deposited  there  simply  as  a  store  of 
carbohydrate  material,  being  accumulated  whenever  amyla- 
ceous material  is  abundant  in  the  alimentary  canal,  ^.nd  being 
converted  into  sugar  and  so  drawn  upon  by  the  body  at  large 
to  meet  the  general  demands  for  carbohydrate  material  during 
the  intervals  when  food  is  not  being  taken  ?  And  we  can 
accept  this  view  without  being  able  to  say  definitely  what  be- 
comes of  the  sugar  thus  thrown  into  the  hepatic  blood.  It  was 
formerly  believed  that  this  sugar  underwent  an  immediate  and 
direct  oxidation  as  it  was  circulating  in  the  blood,  but  we  have 
already  dwelt  (§  290)  on  the  objections  to  such  a  ^dew.  It  is 
sufficient  for  us  at  the  present  to  admit  that  the  sugar  is  made 
use  of  in  some  way  or  other. 

Now,  many  considerations  lead  us  to  believe  that  a  certain 
average  composition  is  necessary  for  that  great  internal  medium 
the  blood,  in  order  that  the  several  tissues  may  thrive  upon  it 
to  the  best  advantage,  one  element  of  that  composition  being 
a  certain  percentage  of  sugar.  It  would  appear  that  some  at 
least  if  not  all  of  the  tissues  are  continually  drawing  upon  the 
blood  for  sugar,  and  that  hence  a  certain  supply  must  be  kept 
up  to  meet  this  demand.  On  the  other  hand  an  excess  of 
sugar  in  the  blood  itself  would  be  injurious  to  the  tissues. 
And  as  a  matter  of  fact  we  find  that  the  quantity  of  sugar  in 
blood  is  small  but  constant ;  it  remains  about  the  same  when 
food  is  being  taken  as  in  the  intervals  between  meals.  If  sugar 
be  injected  into  the  jugular  vein  in  too  large  quantities  or  too 
rapidly,  a  certain  quantity  appears  in  the  urine,  indicating  an 
effort  of  the  system  to  throw  off  the  excess  and  so  bring  back 
the  blood  to  its  average  condition.  The  maintenance  of  such 
a  constant  percentage  of  sugar  would  obviously  be  provided 
for  or  at  least  largely  assisted  by  the  liver  acting  as  a  structure 
where  the  sugar  might  at  once  and  without  much  labour  be 
packed  away  in  the  form  of  the  less  soluble  glycogen,  at  those 
times  when,  as  during  an  amylaceous  meal,  sugar  is  rapidly 
passing  into  the  blood,  and  there  is  a  danger  of  the  blood 
becoming  loaded  with  far  more  sugar  than  is  needed  for  the 
time  being ;  and  it  may  be  incidentally  noted  that  a  larger 
quantity  of  sugar  may  be  injected  into  the  portal  than  into  tlie 
jugular  vein  without  any  reappearing  in  the  urine,  apparently 
because  a  large  portion  of  it  is  in  such  a  case  retained  in  the 


Chap,  iv.]     METABOLIC  PEOCESSES  OF  THE  BODY.        573 

liver  as  glycogen.  At  those  times,  on  the  other  hand,  when 
we  may  suppose  that  sugar  ceases  to  pass  into  the  blood  from 
the  alimentary  canal,  the  average  percentage  in  the  blood  is 
maintained  by  the  glycogen  previously  stored  up  becoming 
reconverted  into  sugar,  and  being  slowly  discharged  into  the 
hepatic  blood. 

Moreover,  this  view,  that  the  glycogen  of  the  liver  is  a 
reserve  fund  of  carbohydrate  material,  is  strongly  supported 
by  the  analogy  of  the  migration  of  starch  in  the  vegetable 
kingdom.  We  know  that  the  starch  of  the  leaves  of  a  jDlant, 
whether  itself  having  previousl}^  passed  through  a  glucose  stage 
or  not,  is  normally  converted  into  sugar,  and  carried  down  to 
the  roots  or  other  parts,  where  it  frequently  becomes  once  more 
changed  back  again  into  starch. 

§  367.  Glycogen  is  found  in  other  parts  of  the  body  than 
the  liver,  and  a  study  of  the  facts  relating  to  the  presence  of 
glycogen  in  other  tissues  will  help  us  to  a  true  conception  of 
the  purpose  of  the  hepatic  glycogen.  Next  to  the  liver,  the 
skeletal  muscles  are  perhaps  the  most  conspicuous  glycogen 
holders.  So  frequently  is  glycogen  found  in  muscle  that  it 
may  be  regarded  as  an  ordinary  though  not  an  invariable  con- 
stituent of  that  tissue ;  indeed  it  may  almost  be  considered  as 
a  constituent  of  all  contractile  tissues.  The  quantity  varies 
very  largely  both  in  the  different  muscles  of  the  same  animal 
and  corresponding  muscles  of  different  animals.  It  disappears, 
according  to  some  observers,  readily  upon  starvation,  even 
before  the  hepatic  glycogen  is  exhausted ;  but  all  observers  are 
not  agreed  on  this  point,  and  in  some  muscles,  at  least,  it 
appears  to  be  retained  for  a  very  long  time.  It  is  said  to  be 
increased  in  quantity  when  the  nerve  of  the  muscle  is  divided, 
and  the  muscle  thus  brought  into  a  state  of  quiescence.  On 
the  other  hand  it  diminishes  or  even  disapj)ears,  being  appar- 
ently converted  into  dextrose,  when  the  muscle  enters  into 
rigor  mortis.  Some  observers  have  found  that  it  diminishes 
during  tetanus,  and  maintain  that  it,  after  conversion  into  dex- 
trose, is  used  up  in  the  act  of  contraction,  forming  through  its 
oxidation  the  immediate  supply  of  the  energy  set  free  in  the 
contraction.  But  even  granting  that  the  glycogen  in  a  muscle 
may  be  diminished  during  prolonged  labour,  it  cannot  be 
admitted  that  the  oxidation  or  other  chemical  change  of  gly- 
cogen is  a  necessarj^  part  of  the  ordinary  metabolism  of  a  mus- 
cular contraction,  since  many  muscles  wholly  free  from  glycogen 
are  perfectly  Avell  able  to  carr}-^  on  long-continued  contractions. 

What  is  probably  the  use  of  glycogen  in  muscle  is  sug- 
gested b}^  the  fact  that  undeveloped  embryonic  muscles 
are  peculiarly  rich  in  glycogen.  In  a  young  embryo,  at  the 
time  Avhen  the  muscular  substance,  though  undergoing  stria- 
tion,  is  still  largely  '  protoplasmic '  in  nature,  the  quantity  of 


574  GLYCOGEN  IN   THE  MUSCLES.  [Book  ii. 

glycogen  present  is  enormous ;  it  frequently  amounts  to  40 
p.c.  of  the  dry  material.  At  this  period  the  hepatic  cells  are 
immature  and  very  little  glycogen  is  present  in  them.  Later 
on,  as  the  muscles  become  more  wholly  striated,  the  glycogen 
largely  disappears  from  the  muscle,  and  very  soon  afterwards 
begins  to  be  stored  up  in  the  liver.  The  meaning  of  this  can 
hardly  be  mistaken.  The  glycogen  in  the  immature  muscle  is 
a  store  of  carbohydrate  material,  laid  down  on  the  spot,  and 
ready  at  once  to  be  used  in  what  we  may  probably  call  the 
fierce  metabolic  struggle  by  which  the  simple  protoplasmic  cell- 
substance  of  the  rudiment  of  the  muscular  fibre  is  transformed 
into  the  highly  differentiated  striated  contractile  substance. 
And  we  shall  probably  not  err  in  considering  the  glycogen  of 
the  mature  muscle  to  hold  a  similar  position  ;  it  is  carbohydrate 
material  stored  up  on  the  spot,  a  local  branch  so  to  speak  of 
the  great  carbohydrate  bank.  It  is  destined  to  become  part  of 
the  contractile  substance,  and  as  such  will  contribute  to  the 
energy  set  free  in  a  muscular  contraction ;  but  its  energy  is 
only  available  in  this  way  after  it  has  undergone  the  necessary 
metabolism  and  become  part  of  muscular  substance ;  it  cannot 
be  fired  off  in  a  contraction  while  it  lies  as  raw  glycogen  in  the 
interstices  of  the  muscular  fibre. 

§  368.  Glycogen  may  also  be  found  in  considerable  quan- 
tity in  the  placenta.  Here,  as  we  shall  see  in  a  later  part  of 
this  work,  it  is  laid  down  in  epithelial  cells  which  lie  on  the 
boundary  between  the  maternal  and  the  foetal  tissues.  And 
here  too  there  can  be  little  doubt  that  it  serves  as  a  store  of 
carbohydrate  material  for  the  nourishment  of  the  foetus. 

It  has  also  been  found  in  leucocytes',  and  in  cartilage  cor- 
puscles, especially  in  those  large  rapidly  growing  and  rapidly 
multiplying  cartilage  corpuscles  which  lie  in  the  outer  zone  of 
endochondral  ossification,  and  in  other  situations.  In  cases  of 
diabetes,  where  the  body  is  overloaded  with  carbohydrate  mate- 
rial, it  has  been  found  in  considerable  quantity  in  the  testis,  in 
the  brain  and  elsewhere.  Its  occurrence  in  these  situations,  and 
under  these  circumstances,  may  be  regarded  as  additional  evi- 
dence of  the  truth  of  the  view  which  we  have  expounded  above 
that  the  main  purpose  of  the  deposition  of  glycogen  is  to  afford 
a  store,  either  general  or  local,  of  carbohydrate  material,  which 
can  be  packed  away  without  much  trouble  so  long  as  it  remains 
glycogen,  but  which  can  be  drawn  upon  as  a  source  of  soluble 
circulating  sugar  whenever  the  needs  of  this  or  that  tissue  demand 
it.  It  thus  forms  a  very  complete  analogue  to  the  vegetable 
starch,  and  fitly  earns  the  name  of  animal  starch. 

We  have  some  reasons  for  thinking  that  there  are  several 
varieties  of  glycogen,  and  that  the  glycogen  which  exists  in 
muscle  is  not  quite  identical  with  that  which  occurs  in  the  liver. 
Indeed  there  seem  to  be  intermediate  stages  between  glycogen 


Chap,  iv.]      METABOLIC  PEOCESSES  OF  THE  BODY.       575 

and  starch  or  dextrin.  The  physiological  value  of  these  differ- 
ences has  not  yet  however  been  clearly  determined,  and,  with 
this  caution,  we  may  continue  to  speak  of  glycogen  as  a  single 
substance. 

Diabetes. 

§  369.  Natural  diabetes  is  a  disease  characterized  by  the 
appearance  of  a  large  quantity  of  sugar  in  the  urine.  This  is 
due,  as  we  have  already  said,  to  the  presence  of  an  abnormal 
quantity  of  sugar  in  the  blood.  The  system  can  only  dispose 
(either  by  oxidation,  or  as  seems  more  probable  in  other  ways) 
of  a  certain  quantity  of  sugar  in  a  certain  time.  Sugar  injected 
into  the  jugular  vein  reappears  in  the  urine  whenever  the  injec- 
tion becomes  so  rapid  that  the  percentage  of  sugar  in  the  blood 
reaches  a  certain  (low)  limit.  Sugar  in  the  urine  means  an 
excess  of  sugar  in  the  blood.  Into  the  pathology  of  the  various 
forms  of  this  disease  it  is  impossible  to  enter  here ;  but  a 
temporary  diabetes,  the  appearance  for  a  while  of  a  large 
quantity  of  sugar  m  the  urine,  may  be  artificially  produced  in 
animals  in. several  ways. 

If  the  spinal  bulb  of  a  well-fed  rabbit  be  punctured 
in  the  region  which  we  have  previously  described  (§  154)  as 
that  of  the  vaso-motor  centre  (the  area  marked  out  as  the  "dia- 
betic area  "  agreeing  very  closely  with  that  defined  as  the  vaso- 
motor area),  though  the  animal  need  not  necessarily  be  in  any 
other  way  obviously  affected  by  the  operation,  its  urine  will  be 
found,  in  an  hour  or  two,  or  even  less,  to  be  increased  in  amount 
and  to  contain  a  considerable  quantit}-  of  sugar.  A  little  later 
the  quantity  of  sugar  will  have  reached  a  maximum,  after  which 
it  declines,  and  in  a  day  or  two,  or  even  less,  the  urine  will  be 
again  perfectly  normal.  The  better  fed  the  animal,  or,  more 
exactly,  the  richer  in  glycogen  the  liver,  at  the  time  of  the  oper- 
ation, the  greater  the  amount  of  sugar.  If  the  animal  be  pre- 
viously starved  so  that  the  liver  contains  little  or  no  glycogen, 
the  urine  will  after  the  operation  contain  little  or  no  sugar.  It 
is  clear  that  the  urinary  sugar  of  this  form  of  artificial  diabetes 
comes  from  the  glycogen  of  the  liver.  The  puncture  of  the 
bulb  causes  such  a  change  in  the  liver  that  the  previously 
stored-up  glycogen  disappears,  and  the  blood  becomes  loaded 
with  sugar,  much  if  not  all  of  Avhicli  passes  away  by  the  urine. 
In  the  absence  of  any  proof  to  the  contrary,  we  may  assume 
that  in  this  form  of  artificial  diabetes  the  glycogen  previously 
present  in  the  liver  becomes  converted  into  sugar,  just  as  we 
know  that  it  does  become  so  converted  by  post-mortem  changes. 
The  glycogenic  function  of  the  liver  is  therefore  subject  to  the 
influence  of  the  nervous  system,  and  in  particular  to  the  influ- 
ence of  a  region  of  the  cerebro- spinal  centre  which  we  already 


576  DIABETES.  [Book  ii. 

know  as  the  vaso-motor  centre,  or  at  least  of  a  part  of  that 
region. 

§  370.  With  regard  to  the  exact  nature  of  the  influence 
started  by  the  puncture  of  the  spinal  bulb,  and  the  path  by  which 
that  influence  reaches  the  liver,  our  information  is  at  present 
very  imperfect.  One  thing  seems  clear,  viz.  that  the  influence 
in  question  is  not  carried  down  by  the  main  vagus  trunks  ;  for 
not  only  has  the  section  of  both  these  nerves  in  the  neck  no 
marked  effect  in  the  way  of  producing  diabetes ;  but  the 
'  diabetic  puncture '  of  the  spinal  bulb  is  as  efficient  after 
division  of  both  vagus  nerves  as  before.  The  influence  appears 
to  reach  the  lines  by  way  of  the  sympathetic  system ;  but  no 
authoritative  statement  as  to  the  exact  path  can  as  yet  be  made. 
As  to  the  nature  of  the  influence  we  can  perhaps  at  present 
only  say  that  most  probably  the  normal  actions  of  the  hepatic 
cells  are  in  some  wa}^  directly  interfered  with,  for  we  have  no 
satisfactory  evidence  that  vaso-motor  changes,  such  as  dilation 
of  the  hepatic  artery,  and  consequent  increase  of  the  supply  of 
arterial  blood  relatively  to  the  supply  of  venous  blood  by  the 
portal  vein,  bring  about  the  result  in  question. 

§  371.  A  temporary  diabetes  may  be  brought  about  by  the 
administration  of  the  substance  phloridzin.  This  however  is  a 
glucoside,  and  part  of  the  sugar  which  appears  in  the  urine, 
after  a  dose  of  it,  may  come  direct  from  the  drug  itself ;  but 
the  quantity  of  sugar  discharged  is  too  great  to  be  accounted 
for  in  this  way,  and  similar  diabetic  effects  are  produced  by  the 
administration  of  phloretin,  a  derivate  of  pliloridzin,  not  a  glu- 
coside, and  not  giving  rise  to  sugar  by  its  own  decomposition. 
The  sugar  which  appears  in  the  urine  after  a  dose  of  either  of 
these  substances  seems  to  come  in  part  at  least  from  the  hepatic 
store  of  glycogen  when  that  is  present ;  but  the  drug  will  give 
rise  to  sugar  in  the  urine  of  starving  animals,  from  whose  livers 
(and  other  tissues)  glycogen  is  presumably  absent. 

Artificial  diabetes  is  also  a  prominent  symptom  of  urari 
poisoning.  This  is  not  due  to  the  artificial  respiration,  which 
is  had  recourse  to  in  order  to  keep  the  urarized  animals  alive  ; 
because,  though  disturbance  of  the  respiratory  functions  suffi- 
cient to  interfere  with  the  hepatic  circulation  may  produce 
sugar  in  the  urine,  artificial  respiration  may  with  care  be 
carried  on  without  any  sugar  making  its  appearance.  More- 
over, urari  causes  diabetes  in  frogs,  although  in  these  animals 
respiration  can  be  satisfactorily  carried  on  without  any  pul- 
monary respiratory  movements.  The  exact  way  in  which  this 
form  of  diabetes  is  brought  about  has  not  yet  been  clearly  made 
out. 

A  very  similar  diabetes  is  seen  in  carbonic  oxide  poisoning ; 
and  is  one  of  the  results  of  a  sufficient  dose  of  morphia,  or 
amylnitrite  and  of  some  other  drugs. 


Chap,  iv.]     METABOLIC  PROCESSES  OF  THE  BODY.       577 

A  diabetes  of  a  j)ermaneiit  character,  much  more  closely 
resembling  the  disease  as  occurring  naturally,  may  be  brought 
about  in  the  following  remarkable  manner.  If  in  a  dog  (and 
the  same  result  may  be  obtained  in  many  other  animals)  the 
whole  of  the  pancreas  be  removed,  sugar  makes  its  appearance 
in  the  urine,  and  the  animal  soon  becomes  emaciated,  with  all 
the  symptoms  of  ordinary  diabetes.  The  gland  must  be 
removed ;  mere  ligature  or  blocking  of  the  duct  does  not 
produce  the  effect.  And  the  whole  gland  must  be  removed ; 
if  only  a  small  portion  be  left,  the  symptoms  do  not  appear 
or  are  slight  and  temj)orary.  Moreover,  it  has  been  found  pos- 
sible to  transplant  a  portion  of  the  gland,  removing  it  from 
its  normal  surroundings  and  grafting  it  in  some  other  situa- 
tion. In  such  a  case  the  whole  of  the  rest  of  the  gland  may 
be  removed  without  causing  diabetes ;  but  the  symptoms  imme- 
diately appear  if  the  transplanted  ]3ortion  be  subsequently 
removed.  We  may  infer  that  the  pancreas,  besides  secreting 
pancreatic  juice,  produces  some  effect  on  the  blood  circulating 
through  it,  probably  discharges  into  the  blood  some  substance, 
and  that  this  effect,  this  substance,  has  to  do  with  the  regula- 
tion of  the  sugar  in  the  blood.  So  long  as  even  a  small  portion 
of  the  gland  is  left,  adequate  effect  is  produced,  and  sugar  does 
not  accumulate  in  the  blood  ;  but  if  the  whole  gland  is  wanting, 
then  in  consequence  of  the  lack  of  the  normal  effect,  sugar  does 
accumulate  in  the  blood  and  the  condition  of  diabetes  is  set  up. 
How  this  result  comes  about,  whether  by  reason  of  a  failure 
to  get  rid  of  the  sugar  which  is  normally  produced  or  by  an 
abnormal  production  of  sugar,  has  not  yet  been  clearly  made 
out.  The  salivary  glands,  in  many  respects  so  like  the  pan- 
creas, have  no  such  action. 

The  diabetes  thus  set  up  by  extirpation  of  the  pancreas  has 
further  the  following  resemblance  to  ordinary  diabetes.  In 
mild  forms  of  the  natural  disease,  sugar  only  makes  its  appear- 
ance in  the  urine  when  carbohydrate  food  is  taken  ;  but  in  severer 
forms  a  large  quantity  of  sugar  may  be  present  in  the  urine 
even  though  no  carbohydrate  food  at  all  be  taken.  The  sugar 
in  such  a  case  probably  comes  from  the  splitting  up  of  proteid 
matter,  and  this  view  is  supported  by  the  fact  that  a  certain 
relation  may  be  observed  between  the  sugar  and  the  urea 
secreted  in  the  urine.  So  also  after  extirpation  of  the  jjan- 
creas,  especially  if  some  of  the  pancreas  be  left  behind,  a  mild 
effect  may  be  produced,  in  which  sugar  appears  in  the  urine 
only  after  carbohydrate  food.  On  the  other  hand  severer 
forms  are  also  met  with  in  which  sugar  passes  away  by  the 
urine,  though  carbohydrates  be  rigidl}^  excluded  from  the  food. 

As  a  sort  of  converse  to  diabetes  we  ma}'  mention  that  the 
administration  of  arsenic  in  sulhcient  doses  or  for  an  adequate 
time  prevents  an  accumulation  of  glycogen  in  the  liver  and 

37 


578  DIABETES.  [Book  ii. 

apparently  in  the  body  generally,  whatever  be  the  diet  used. 
The  presence  of  the  metal  in  the  hepatic  cell  seems  to  prevent 
the  cell-substance  from  manufacturing  glycogen  either  from 
carbohydrate  material  brought  to  it,  or  out  of  its  own  sub- 
stance. As  another  kind  of  converse  we  may  also  state  that 
the  administration  of  glycerine,  especially  through  the  alimen- 
tary canal,  diminishes  the  effect  of  the  diabetic  puncture,  or  of 
morphia  or  other  poisoning,  in  hurrying  on  the  hepatic  store 
of  glycogen  into  sugar,  and  thus  diminishes  the  sugar  in  the 
urine ;  the  presence  of  the  glycerine  in  the  hepatic  cell  appears 
to  be  in  some  way  a  hindrance  to  the  conversion  of  the  glyco- 
gen into  sugar.  Now  glycerine  injected  into  the  alimentary 
canal  of  a  normal  animal  leads  to  an  increase  of  glycogen  in 
the  liver  ;  and  the  view  very  naturally  suggests  itself  that  this 
increase  arising  from  the  glycerine  is  to  be  explained  by  the 
glycerine  inhibiting  in  some  way  a  normal  conversion  of  the 
glycogen  store  into  sugar  which  is  continually  going  on,  and 
thus  increasing  for  the  time  that  store. 


SEC.  2.     THE   SPLEEN. 

§  372.  The  Movements  of  the  Spleen.  A  salient  structural 
feature  of  tlie  spleen  is,  that  many  of  the  minute  arteries  open 
out  into  the  labyrinths  of  the  coarse  reticulum  which  occupy 
the  irregular  chambers  marked  off  by  the  trabecuhe;  blood 
passes  bodily  into  the  spaces  between  the  branched  cells  of  the 
reticulum.  The  amount  of  blood  which  thus  travels  slowly 
through  or  even  for  a  while  tarries  in  the  meshes  of  the  retic- 
ulum, forming  the  so-called  "spleen-pulp,"  as  compared  with 
the  amount  which  traverses  the  spleen  in  the  ordinary  way 
confined  to  the  closed  channels  of  the  capillaries,  varies  from 
time  to  time  according  to  the  condition  of  the  organ.  For  the 
spleen  is  subject  to  changes  leading  to  considerable  variations 
in  its  volume. 

After  a  meal  the  spleen  increases  in  size,  reaching  its  maxi- 
mum about  five  houi's  after  the  taking  of  food;  it  remains 
swollen  for  some  time,  and  then  returns  to  its  normal  bulk. 
In  certain  diseases,  such  as  in  the  pyrexia  attendant  on  certain 
fevers  or  inflammations,  and  more  especially  in  ague,  a  somewhat 
similar  temporary  enlargement  takes  place.  In  prolonged  ague 
a  permanent  hypertrophy  of  the  spleen,  the  so-called  ague-cake, 
occurs. 

The  turgescence  of  the  spleen  seems  to  be  due  to  a  relaxation 
both  of  the  small  arteries  and  of  the  muscular  tissue  of  the  cap- 
sule and  of  the  trabecuhe;  to  be,  in  fact,  a  vascular  dilation 
accompanied  by  a  local  inhibition  of  the  tonic  contraction  of 
the  other  plain  muscular  fibres  entering  into  the  structure  of 
the  organ,  the  latter,  at  all  events  in  some  animals,  being  prob- 
ably the  more  important  of  the  two.  And  the  condition  of  the 
spleen,  like  that  of  other  vascular  areas,  appears  to  be  regulated 
by  the  central  nervous  system,  the  digestive  turgescence  being 
fairly  comparable  to  the  flushed  condition  of  the  pancreas  and 
of  the  gastric  membrane  during  their  phases  of  activity. 

The  application  of  the  plethysmographic  method  to  the 
spleen,  carried  out  in  the  way  which  we  described  in  speaking 
of  the  kidney  (§  330),  enables  us  to  study  more  exactly  the 
variations  in  volume  which  the  organ  undergoes. 

579 


580  MOVEMENTS   OF   THE   SPLEEN.  [Book  ii. 

A  '  spleen  curve '  (Fig.  109)  taken  in  the  same  way  as  a 
'  kidney  curve '  does  not,  in  tlie  dog  at  all  events,  shew  varia- 
tions in  the  volume  of  the  spleen  corresponding  with  the  pulse 
waves.  The  kidney  curve,  as  we  have  seen  (§  330),  gives 
clear  indications  of  each  heart-beat,  but  the  spleen  curve  shews, 
besides  the  larger  waves  of  which  we  shall  speak  directly,  only 
undulations  due  to  the  respiratory  movements;  and  these,  always 
very  slight,  are  sometimes  not  visible.  In  other  words,  the 
spleen  does  not  expand  with  the  increase  of  blood-pressure 
occurring  in  the  splenic  arteries  after  each  heart-beat.  More- 
over when  the  supply  of  blood  to  the  spleen  is  wholly  and 
suddenly  cut  off,  as  by  clamping  the  aorta,  the  spleen  curve 
sinks  very  slowly,  shewing  that  the  spleen  is  diminishing  in 
volume  not  suddenly  but  very  slowly.  The  pathway  of  the 
blood  through  the  splenic  reticulum  is  peculiar;  and  increase 
or  decrease  in  the  volume  of  the  spleen  means  more  or  less 
blood  held  in  the  spleen  pulp,  not  necessarily  a  greater  or  less 
flow  of  blood  through  the  organ. 


1 1 1 1 1 1 J 1 1 J  in  n 


Fig.  109.     Normal  Spleen  Curve  from  Dog.     (Roy.) 

The  upper  curve  is  the  spleen  curve  shewing  the  rhythmic  contractions  and 
expansions ;  the  smaller  waves  are  due  to  the  respiratory  movements.  The 
lower  curve  is  the  blood-pressure  curve,  and  the  point  a  of  the  spleen  curve 
corresponds  in  time  to  the  point  b  of  the  blood-pressure  curve.  The  marks  on 
the  time  curve  below  indicate  seconds. 

Of  special  interest  are  the  large  slow  variations  of  volume 
which,  besides  the  respiratory  undulations,  the  spleen  curve 
usually  shews,  as  seen  in  the  figure.  Rhythmic  contractions 
and  expansions,  though  not  always  present,  frequently  make 
their  appearance,  each  contraction  with  its  fellow  expansion 
lasting  in  the  cat  and  dog  about  a  minute,  and  recurring  with 
great  regularity  for  a  long  time;  and  besides  these  the  volume 
varies  widely  from  time  to  time.  There  can  be  little  doubt 
but  that  the  rhythmic  variations  in  volume  are  due  in  these 
animals  to  rhythmic  contractions,  with  intervening  relaxations, 


Chap,  iv.]     METABOLIC  PROCESSES  OF  THE  BODY.       581 

of  the  muscular  trabeculse  and  capsule;  the  slower  variations 
are  also  probably  due  to  the  same  cause.  In  many  animals  the 
contractility  of  the  splenic  tissue  is  shewn  by  the  white  lines  of 
constriction  which  appear  when  the  electrodes  of  an  induction 
machine  in  action  are  drawn  over  its  surface;  and  similar  lines 
may  be  produced  by  mechanical  stimulation  with  the  point  of  a 
needle.  So  that  the  spleen  in  these  animals  may  be  considered 
as  a  muscular  organ,  now  expanding  to  receive  a  larger  quan- 
tity of  blood  and  now  contracting  to  drive  the  blood  on  to  the 
liver.  When  the  muscular  elements  are  scanty  in  or  absent 
from  the  capsule  and  trabeculije,  the  expansion  and  contraction 
of  the  whole  organ  must  depend  alone  or  chiefly  on  variations 
in  the  width  of  the  supplying  arteries.  We  have  evidence 
moreover  that  the  muscular  activity  of  the  sj)leen,  whether  of 
the  muscular  capsule  and  trabeculse  and  arteries  combined  or 
of  the  latter  alone,  is  under  the  dominion  of  the  nervous  sys- 
tem. A  rapid  contraction  of  the  spleen  may  be  brought  about 
in  a  direct  manner  by  stimulation  of  the  splanchnic  or  vagus 
nerves,  or  in  a  reflex  manner  by  stimulation  of  the  central  end 
of  a  sensory  nerve;  it  may  also  be  caused  by  stimulation  of  the 
medulla  oblongata  with  a  galvanic  current  or  by  means  of  as- 
phyxia. Though  the  matter  has  not  yet  been  fully  worked 
out,  we  have  already  sufficiently  clear  indications  that  the  flow 
of  blood  through  the  spleen  is,  through  the  agency  of  the 
nervous  system,  varied  to  meet  changing  needs.  At  one  time 
a  small  quantity  of  blood  is  passing  through  or  is  being  held 
by  the  organ,  and  the  metabolic  changes  which  it  undergoes  in 
the  transit  are  comparatively  slight.  At  another  time  a  larger 
quantity  of  blood  enters  the  organ,  and  is  let  loose,  so  to  speak, 
into  the  splenic  pulp,  there  to  undergo  more  profound  changes, 
and  afterwards  to  be  ejected  by  the  rhythmic  contractions  of 
the  muscular  trabeculse. 

It  is  further  obvious  that  these  changes  going  on  in  the 
spleen  must  have  an  important  influence  on  the  changes  going 
on  in  the  liver;  it  cannot  be  of  indifference  to  the  latter  organ 
whether  a  relatively  small  quantity  of  blood,  relatively  little 
changed,  reaches  it  from  the  spleen,  or  whether  it  receives  a 
relatively  large  quantit}^  of  blood,  j)rofoundly  altered  by  the 
changes  which  it  has  undergone  in  the  spleen  pulp.  Some  of 
the  changes  taking  place  in  the  spleen  are  histological  in 
nature. 

§  373.  When  the  so-called  spleen  pulp  is  examined  under 
the  microscope,  it  is  found  to  consist,  besides  the  branched  cells 
and  fibres  constituting  the  reticulum,  of  cells  which  may  be 
described  as  partly  red  corpuscles  and  partly  white  corpuscles 
or  leucocytes.  We  spoke  of  the  meshes  of  the  reticulum  as 
being  filled  with  blood;  but  it  is  obvious  that  the  corpuscles  of 
the  blood  must  move  less  readily  through  the  labyrinth  than 


682  MOVEMENTS   OF  THE  SPLEEN.  [Book  ii. 

does  the  fluid  plasma,  and  tliat  hence  a  concentration  of  the 
corpuscles  as  compared  with  the  plasma  must  take  place  in  the 
meshes.  The  contents  of  the  meshes  cannot,  properly  speaking, 
be  called  blood,  but  are  rather  aggregations  of  corpuscles  with 
a  relatively  small  quantity  of  fluid. 

The  white  corpuscles  or  leucocytes  are  of  various  kinds. 
Some  are  small,  like  the  leucocytes  of  a  lymphatic  gland,  the 
cell-substance  being  scanty  relatively  to  the  nucleus.  Others 
are  indistinguishable  from  the  kinds  of  white  corpuscles  pres- 
ent in  the  blood.  Others  again  are  large,  twice  as  large  as  an 
ordinary  white  corpuscle  or  even  larger  than  this,  possess  more 
than  one  nucleus,  and  contain  in  their  cell-substance  numerous 
refractive,  pale  yellow  or  colourless  granules.  Some  of  these 
larger  forms,  which  like  the  others  exhibit  amceboid  movements, 
and  are  often  irregular  in  form,  are  characterized  by  the  pres- 
ence in  their  cell-substance  of  red  corpuscles,  sometimes  in 
almost  a  natural  condition,  sometimes  more  or  less  irregular  in 
shape  with  their  red  hsemoglobin  changing  into  the  browner 
hsematin,  and  sometimes  disintegrated  into  a  mass  of  brown 
granules.  The  fluid  or  plasma  in  which  these  cells  float  also 
contains  besides  normal  red  corpuscles  a  certain  number  of  red 
corpuscles  in  various  stages  of  change,  as  well  as  pigment 
granules  which  appear  to  be  derived  from  hsemoglobin.  Ob- 
viously a  certain  number  of  red  corpuscles  do  undergo  change 
in  the  spleen,  but  whether  the  change  is  mainly  effected  in  the 
cell-substance  of  the  cells  just  mentioned,  or  takes  place  in  the 
plasma,  the  products  of  disintegration  being  subsequently  taken 
up,  in  amceboid  fashion,  by  the  cells  in  question,  is  not  as  yet 
clear.  Besides  the  above,  in  the  spleen  of  young  animals, 
nucleated  cells  with  hsemoglobin  holding  cell-substance,  hse- 
matoblasts  (see  §  27),  have  been  described;  these  are  said  to 
appear  also  in  the  spleen  of  adults  after  very  great  loss  of  blood. 

§  374.  The  Chemical  Constituents  of  the  Spleen.  Besides 
the  chemical  bodies  which  one  would  expect  to  find  in  a  vas- 
cular, muscular  organ  full  of  blood,  the  spleen  contains  bodies, 
lodged  apparently  in  the  spleen  pulp,  which  give  it  special 
chemical  characters.  One  of  the  most  important  of  these  is  a 
special  proteid  of  the  nature  of  alkali-albumin,  holding  iron  in 
some  way  peculiarly  associated  with  it.  The  occurrence  of  this 
ferruginous  proteid,  accompanied  as  it  is  by  several  peculiar 
but  at  present  little  understood  pigments,  rich  in  carbon,  which 
are  partly  present  in  the  cells  spoken  of  above  and  partly  de- 
posited in  the  branched  cells  of  the  reticulum,  appears  to  be 
connected  with  the  changes  undergone  by  the  hsemoglobin 
which  we  shall  presently  discuss.  The  inorganic  salts  of  the 
spleen,  or  at  least  those  of  its  ash,  are  remarkable  for  the 
large  amount  of  both  soda  and  phosphates,  and  the  small 
amount  of  potash  and  chlorides  which  they  contain,  thus  dif- 


€hap.  IV.]     METABOLIC  PEOCESSES  OF  THE  BODY.        583 

feriiig  from  those  of  blood-corpuscles  on  the  one  hand,  and 
from  those  of  blood-serum  on  the  other.  But  perhaps  the  most 
striking  feature  of  the  spleen-pulp  is  its  richness  in  the  so-called 
extractives.  Of  these  the  most  common  and  plentiful  are  suc- 
cinic, formic,  acetic,  butyric  and  lactic  acids,  inosit,  leucin, 
xanthin,  hypoxanthin,  and  uric  acid.  Tyrosin  apparently  is 
not  present  in  the  perfectly  fresh  spleen,  though  leucin  is:  both 
are  found  when  decomposition  has  set  in.  The  constant  pres- 
ence of  uric  acid  is  remarkable,  especially  since  it  has  been 
found  even  in  the  spleen  of  animals,  such  as  the  herbivora, 
whose  urine  contains  none. 

The  richness  of  the  spleen  in  these  extractives  is  an  indica- 
tion of  the  importance  of  the  metabolic  events  with  which  the 
organ  has  to  do;  but  it  will  be  more  profitable  to  discuss  what 
goes  on  in  the  spleen  in  connection  with  the  metabolic  changes 
in  other  parts  of  the  body,  in  the  liver  for  instance,  than  to 
attempt  to  lay  down  any  so-called  '  functions  '  of  the  spleen. 
When  we  confine  our  attention  to  the  spleen  itself  we  learn 
very  little;  thus  the  whole  organ  may  be  successfully  removed 
without  any  very  obvious  changes  in  the  economy  resulting. 
We  may  return  therefore  to  the  discussion  of  the  formation  of 
the  bilirubin  of  bile,  and  of  the  changes  undergone  by  haemo- 
globin, with  which  as  we  shall  see  the  spleen  is  connected,  and 
which  moreover  has  to  do  with  the  formation  of  other  pig- 
ments. 


SEC.   3.     THE  FOEMATION   OF   THE   CONSTITUENTS 

OF  BILE. 


§  375.  Bile  Pigments.  Aiter  extirpation  of  the  liver  no 
accumulation  of  bile  pigment  or  bile  salts  takes  place  in  the 
blood.  This  is  well  shewn  in  frogs,  which  survive  the  operation 
for  some  considerable  time ;  but  the  same  results  have  been 
obtained  in  birds  (geese  and  ducks).  There  can  be  no  doubt 
therefore  that  these  substances  are  formed  in  the  liver  and  not 
simply  withdrawn  from  the  blood  bj  the  liver  in  some  such  way 
as  we  have  seen  reason  to  think  urea  is  withdrawn  from  the 
blood  by  the  kidney. 

When  the  plasma  of  circulating  blood  is  made  to  contain 
haemoglobin  detached  from  the  corpuscles,  bile  pigment  fre- 
quently makes  its  appearance  in  the  urine.  The  presence  of  free 
haemoglobin  may  be  secured  by  injecting  into  the  veins  a  solu- 
tion of  haemoglobin  or  blood  made  '  laky '  by  freezing  and  thaw- 
ing or  by  the  addition  of  a  small  quantity  of  bile  salts,  or  by 
simply  injecting  into  the  veins  a  quantity  of  distilled  water  or 
a  small  quantity  of  ether  or  chloroform  or  of  bile  salts,  all  of 
which  tend  to  'break  up'  red  corpuscles  and  set  free  haemoglobin. 
A  similar  result  occurs  in  poisoning  by  certain  drugs,  such  as 
toluylendiamine.  Under  these  circumstances  not  only  does  bile 
pigment,  bilirubin,  make  its  appearance  in  the  urine,  but  the 
quantity  of  bilirubin  secreted  by  the  liver  is  increased.  Obvi- 
ously the  presence  of  dissolved  haemoglobin  in  the  plasma  of 
the  blood,  and,  presumably  more  especially  of  the  blood  reaching 
the  liver  by  the  portal  vein,  leads  to  an  increased  formation  of 
bilirubin,  which  takes  place  in  such  a  manner  that  the  whole 
of  the  bilirubin  so  formed  does  not  pass  into  the  bile  but  part  is 
retained  in  or  thrown  back  into  the  circulation  and  appears  in 
the  urine. 

We  have  already  mentioned  the  chemical  connection  between 
haemoglobin  and  bilirubin.  Haemoglobin,  after  the  detachment 
of  its  proteid  component  becomes  haematin  (C32H32N4Fe04).  By 
treatment  with  sulphuric  acid  or  otherwise  (§  282),  haematin 
may  be  deprived  of  its  iron  ;  and  this  iron-free  haematin  (some- 

584 


Chap,  iv.]      METABOLIC  PROCESSES  OE  THE  BODY.       585 

times  called  haematoporphyrin)  is  said  to  have  the  composition 
Cg^Hg^N^Or,  differing  from  bilirubin  only  in  its  oxvgen  and 
hydrogen  (Cg^Hg^N^Og  +  2H2O  -  O  =  C32HggN40e).i  Moreover 
in  old  blood  clots  in  the  body  the  litemoglobin  of  the  clot  becomes 
in  time  transformed  into  an  iron-free  body  which  has  been  called 
hsematoidin,  but  which  both  in  composition  and  in  reactions 
appears  to  be  identical  with  bilirubin. 

These  several  facts  lead  us  to  the  conclusion  that  the  biliru- 
bin of  the  bile  is  simply  some  of  the  hsemoglobin  of  the  blood 
transformed  by  the  throwing  off  of  its  proteid  and  its  iron  com- 
ponents. It  is  natural  to  suppose  that  the  transformation  takes 
place  in  and  is  effected  by  the  agency  of  the  hepatic  cells ;  and 
this  view  is  supported  by  the  fact  that  the  hepatic  cells  are 
characterized  by  containing  certain  peculiar  iron  compounds. 
When  all  the  blood  is  carefully  washed  out  of  the  liver  by  injec- 
tion through  the  blood  vessels,  by  which  means  the  remaining 
bile  is  got  rid  of  at  the  same  time,  the  hepatic  substance  is  found 
to  contain  a  small  quantity  of  iron,  sufficient  to  give  the  cells 
a  diffused  dark  colour  when  treated  with  ammonium  sulphide ; 
the  exact  amount  appears  to  vary  largely,  but  the  causes  of  the 
variation  have  not  been  determined.  That  this  iron  is  in  organic 
combination  is  indicated  by  the  fact  that  with  potassium  ferro- 
cyanide  and  sulphocyanide  the  blue  or  red  reaction  is  not 
observed  until  after  treatment  with  hydrochloric  acid.  Appar- 
ently there  are  several  such  compounds,  of  a  proteid  or  of  a 
nuclein  (§  29)  nature,  from  some  of  which  the  iron  is  more 
easily  removed  than  others,  and  these  compounds  appear  to  be 
present  in  both  the  cell-substance  and  the  nucleus.  It  will  be 
remembered  (§  205)  that  bile  contains  a  distinct  quantity  of 
iron,  which  probably  has  its  origin  in  the  iron  thus  set  free  from 
haemoglobin  and  retained  in  the  hepatic  cell;  but  it  does  not 
follow  that  all  the  iron  thus  set  free  makes  its  way  into  the 
bile ;  and  indeed  the  quantity  of  iron  discharged  in  the  bile  in 
24  hours  is  much  smaller  than  the  quantity  calculated  to  be  set 
free  in  the  formation  out  of  hfemoglobin  of  the  quantity  of  bili- 
rubin discharged  during  the  same  period.  Apparently  the  iron 
compounds  of  the  hepatic  cell  have  some  other  work  than  the 
simple  discharge  of  iron  into  the  bile. 

§  376.  We  may  assume  then  that  the  hepatic  cell  has  the 
power  of  splitting  up  the  h;Tenioglobin  brought  to  it,  and  of 
discharging  part  as  bilirubin  while  it  retains  for  a  time  the  iron 
component  in  some  organic  combination.  But  are  we  justified 
in  assuming  that  the  whole  work  is  done  by  the  hepatic  cells  ? 
Are  we  to  conclude  that  bilirubin  is  manufactured  by  some 
act  of  the  hepatic  cells  Avhich  includes  not  only  the  conversion 
of  haemoglobin  into  l)ilirul)in,  l)ut  also   the   extraction   of  the 

'  Doubling  the  formula  for  bilirubin  given  in  §  206. 


686  FOUMATION   OF  BILIEUBIN.  [Book  ii. 

hemoglobin  from  the  red  corpuscles  as  these  are  streaming 
slowly  through  the  lobular  hepatic  capillaries  in  close  contact 
with  the  hepatic  cells?  Now,  as  far  as  we  know  at  present, 
haemoglobin  can  only  be  set  free  by  means  of  a  disintegration 
of  the  corpuscles ;  we  have  no  instances  of  a  corpuscle  parting 
with  some  of  its  haemoglobin  and  proceeding  on  its  way  other- 
wise unchanged ;  and  we  have  no  histological  evidence  of  any 
disintegration  of  red  corpuscles  in  the  liver  corresponding  to 
the  formation  of  bile.  Nor  can  we  draw  any  conclusion  from  the 
results  of  a  comparative  enumeration  of  red  corpuscles  in  the 
portal  and  hepatic  blood,  for  these  are  too  insecure  to  rest  any 
conclusion  upon.  On  the  other  hand,  as  we  have  just  seen, 
the  presence  in  the  plasma  of  the  blood  of  haemoglobin  in  a  free 
condition  is  peculiarly  potent  in  exciting  the  formation  of  bili- 
rubin. The  evidence  therefore  is  very  strong  for  the  view  that, 
as  far  as  the  formation  of  the  greater  part  at  least  of  the  biliru- 
bin is  concerned,  the  action  of  the  hepatic  cell  is  limited  to 
converting  into  bilirubin  the  free  haemoglobin  offered  to  it  by 
the  portal  blood. 

By  what  means,  under  normal  conditions,  is  the  presence  of 
that  free  haemoglobin  secured?  We  have  seen  reason  (§  373) 
to  conclude  from  histological  appearances  that  a  certain  number 
of  red  corpuscles  undergo  change  in  the  spleen  pulp;  and  it 
seems  natural  to  infer  that  one  duty  of  the  spleen  is  to  set  free 
haemoglobin  from  the  corpuscles  and  thus,  through  the  splenic 
veins  and  so  the  portal  vein,  to  supply  the  liver  with  material 
for  bilirubin.  But  this  cannot  be  the  only  source,  since  the 
secretion  of  bile  continues  after  extirpation  of  the  spleen. 
There  must  therefore  be  other  regions  of  the  body  in  which  a 
similar  change  of  red  corpuscles  is  going  on ;  it  has  been  sug- 
gested that  the  red  marrow  of  bones  is  one  of  these  ;  but  further 
information  on  these  points  is  needed. 

We  may  then  go  so  far  as  to  say  that  the  bilirubin  of  the 
bile  is  derived  from  the  liaemoglobin  of  the  blood,  and  that  the 
later  stages  of  the  transformation,  including  the  discharge  of 
the  iron  of  the  haematin  component,  take  place  in  and  by  means 
of  the  hepatic  cell ;  but  much  beyond  this  is  at  present  uncer- 
tain. It  must  be  remembered  too  that,  though  after  extirpation 
of  the  liver  no  accumulation  of  bilirubin  takes  place,  shewing 
that  the  bilirubin  is  formed  by  the  liver  and  not  elsewliere  ;  yet 
the  whole  change  from  red  corpuscle  to  bilirubin  may  occasion- 
ally take  place  quite  apart  from  the  liver,  as  shewn  by  the 
presence  of  hfematoidin  in  old  blood-clots. 

§  377.  The  formation  of  the  bile  acids.  About  this  we  know 
still  less.  Taking  glycocholic  and  taurocholic  acids  as  the  typi- 
cal bile  acids,  recognizing  (§  207)  that  these  arise  from  the  union 
of  cholalic  acid  with  glycin  and  taurin  respectively,  and  remem- 
bering that  taurin  is  found  in  several  tissues,  and  that  glycin 


Chap,  iv.]     METABOLIC  PROCESSES  OF  THE  BODY.       587 

(see  §  339)  though  not  an  actual  constituent  of  any  of  the 
tissues  must  certainly  arise  in  tissue  metabolism,  we  may  con- 
clude that  tlie  chief  work  in  this  respect  of  the  hepatic  cell  is 
to  provide  the  cholalic  acid,  and  to  effect  the  combination  with 
glycin  and  taurin,  though  possibl}^  some  amount  of  either  one 
or  the  other  of  these  bodies  ma}^  be  furnished  by  the  hepatic 
substance  itself.  As  to  how  cholalic  acid  arises  out  of  the 
metabolism  of  the  hepatic  cell  we  know  no  more  than  we  do 
about  the  formation  of  kreatin  in  muscle  or  of  pepsin  in  a  gastric 
cell.  We  are  equally  ignorant  about  the  origin  of  glycin  and 
taurin,  and  cannot  explain  why  in  one  animal  glycocholic,  and 
in  another  taurocholic  acid  is  prominent  in  the  bile,  though  the 
two  bodies,  as  shewn  especially  by  the  presence  of  sulphur  in 
the  taurin,  are  widely  different.  It  has  been  observed  that  the 
presence  of  bile  in  the  intestine  seems  to  excite  the  liver  to 
increased  biliary  action  ;  since  the  bile  acids  are  rapidly  changed 
in  the  intestine  and  the  cholalic  acid  speedily  altered,  it  seems 
probable  that  the  increased  biliary  activity  is  due  to  the  absorp- 
tion of  the  glycin  and  taurin  respectively.  From  which  we  may 
conclude  that  the  presence  of  these  bodies  stirs  up  the  hejjatic 
cell  to  an  increased  formation  of  cholalic  acid. 

§  378.  As  a  general  rule  the  formation  of  bile  acids  runs 
parallel  with  the  formation  of  bile  pigment,  an  increase  or  de- 
crease of  bile  meaning  an  increase  or  decrease  of  both  constitu- 
ents. But  there  are  some  facts  which  seem  to  shew  that  the 
two  actions  may  be  dissociated. 

The  condition  or  symptom  known  as  '  jaundice  '  is  essentially 
an  excess  of  bilirubin  in  the  blood,  whereby  the  tissues  such  as 
the  skin,  and  the  fluids  such  as  the  urine  are  coloured  with  the 
yellow  pigment.  In  most  of  the  maladies  of  which  jaundice  is 
a  symptom,  there  is  evidence  of  an  obstruction  to  the  flow  of 
bile  through  the  bile  passages;  and  the  presence  of  bile  in  the 
blood  and  hence  in  the  tissues  at  large  is  in  such  cases  due  to 
the  fact  that  the  bile  after  secretion  by  the  hepatic  cells  is  reab- 
sorbed from  the  bile  ducts,  see  §  217.  But  in  certain  cases 
where  jaundice  is  a  prominent  syniptom,  no  evidence  of  any 
obstruction  whatever  to  the  flow  of  bile  can  be  obtained.  This 
is  the  case  in  the  jaundice  of  yellow  fever  and  of  a  peculiar 
allied  malady  known  as  'acute  yellow  atrophy  of  the  liver.' 
Now  in  these  cases  there  is  no  evidence  of  an  accumulation  in 
the  blood  or  elsewhere  of  bile  acids  though  there  is  of  bile  pig- 
ment. And  in  the  obscure  malad}-  known  as  simple  or  idiopathic 
jaundice,  in  which  though  the  anatomical  conditions  are  unknown 
there  is  at  least  no  sign  of  obstruction,  the  urine  though  loaded 
with  bile  pigment  is  said  to  contain  no  bile  acids. 

§  379.  The  question  may  be  asked,  Is  the  secretion  of  bile 
independent  of  or  in  some  way  or  other  connected  with  the 
glycogenic  activity  of  the  cells  ?     To  this  we  cannot  at  present 


588  JAUNDICE.  [Book  ii. 

give  a  definite  answer.  In  some  of  the  invertebrata  the  cells  in 
the  organ,  called  a  liver,  which  manufacture  glycogen,  are  dis- 
tinct from  those  which  secrete  bile  or  other  digestive  juices; 
and  it  might  be  inferred  that  in  the  vertebrate  the  two  actions 
though  taking  place,  as  they  certainly  do,  in  the  same  cell,  take 
place  apart  and  distinct.  There  are  facts  which  seem  to  indicate 
that  the  two  are  intimately  connected;  bat  we  have  as  yet  no 
exact  knowledge  concerning  the  matter.  It  has  been  urged 
that  the  portal  blood  is  chiefly  concerned  with  the  formation  of 
glycogen,  and  the  blood  of  the  hepatic  artery  with  the  secretion 
of  bile ,  but  there  is  no  adequate  support  of  this  view.  It  must 
be  remembered  moreover  that,  in  addition  to  the  formation  of 
glycogen  and  the  secretion  of  bile,  other  metabolic  events,  espe- 
cially affecting  proteid  or  at  least  nitrogenous  constituents  of 
the  body,  are  also  taking  place  ;  and  to  these  we  must  now  turn. 


SEC.    4.     ON    UKEA    AND    ON    NITROGENOUS   METABO- 
LISM IN   GENERAL. 


§  380.  We  have  seen  that  nitrogenous  proteid  material  in 
some  form  or  other  enters  into  the  composition  of  all  the  tissues 
of  the  body,  and  we  have  further  seen  that  it  is  so  conspicuously 
and  constantly  present  wherever  living  substances  are  manifest- 
ing vital  energies  as  to  justify  the  conclusion  that  the  changes 
which  it  undergoes  are  in  some  way  essential  to  the  manifesta- 
tion of  those  energies.  We  have  seen,  it  is  true,  reason  to  think 
that  in  some  tissues  at  least,  in  muscle  for  instance,  a  large  part 
of  the  energy  set  free  during  activity  preexisted  as  latent  energy 
and  had  its  immediate  source  not  in  proteid  (nitrogenous)  but  in 
some  other  constituents  of  muscle  ;  and  indeed,  as  we  shall  see 
later  on,  the  greater  part  of  the  whole  energy  of  the  body  must 
be  regarded  as  the  energy  of  carbon  compounds  and  not  of 
nitrogen  compounds ;  but  this  is  quite  consistent  with  the  view 
that  proteid  material  in  some  way  or  other  essentially  intervenes 
in,  we  may  perhaps  go  so  far  as  to  say  directs,  the  changes  by 
which  in  the  body  energy  is  set  free  in  the  peculiar  way  which 
we  speak  of  as  living. 

We  have  seen  that  at  all  events  the  greater  part  of  the  pro- 
teid material  of  the  food  enters  the  blood  as  proteid  material 
either  as  peptone  or  in  some  other  form,  and  is  carried  as  proteid 
material  to  the  tissues. 

We  have  seen  that  the  nitrogen  of  proteid  material  leaves 
the  body  so  largely  in  the  form  of  urea,  that  the  other  nitro- 
genous excretions  may  for  the  time  be  left  out  of  consideration. 

And  lastly  we  have  seen  reason  to  think  that  this  urea  which 
leaves  the  body  in  urine  is  brought  to  the  kidnej's  as  urea  in 
the  blood,  the  kidneys  themselves  apparently  having  no  special 
power  of  forming  urea  out  of  something  which  is  not  lU'ea,  but 
only  contributing  to  the  general  stock  of  urea  by  virtue  of  their 
own  proteid  metabolism.  We  have  now  to  study  the  little  we 
know  concerning  the  steps  by  which  the  proteid  material  of  the 
food  and  of  the  body  is  converted  into  this  urea  of  the  blood 
which  is  the  source  of  the  urea  of  the  urine. 

589 


590  UREA   AND   KREATIN.  [Book  ii. 

§  381.  In  tlie  first  place  we  may  take  it  for  granted  that 
tlie  urea  carried  to  the  kidney  in  the  blood  had  an  antecedent 
in  something  which  was  not  urea.  We  can  hardly  suppose  that 
the  proteid  constituent  of  living  substance,  when  in  the  course 
of  its  metabolism  it  ceases  to  be  proteid,  breaks  up  at  once  into 
urea  and  into  non-nitrogenous  bodies.  All  we  have  learnt  goes 
to  shew  that  what  we  call  metabolism  is  not  a  single  abrupt 
change,  but  consists  essentially  in  a  series  of  changes  ;  and  we 
may  safely  conclude  that  proteid  material  in  becoming  urea 
passes  through  phases  in  which  the  nitrogen  exists  in  chemical 
combinations  distinct  from  proteid  material  on  the  one  hand  and 
urea  on  the  other. 

In  the  second  place  it  is  extremely  probable  that  the  series 
of  changes  by  which  proteid  material  becomes  urea  is  not  the 
same  in  all  the  tissues  and  on  all  occasions.  We  should  natu- 
rally expect  to  find  the  proteid  material  following  different  lines 
of  metabolism  in  different  places  or  under  different  circum- 
stances, the  different  lines  all  converging  to  the  same  body  urea, 
because  for  some  reasons  or  other  urea  appears  to  be,  in  the 
main,  the  most  convenient  form  in  which  the  nitrogen  can  leave 
the  blood  and  the  body. 

We  should  accordingly  expect  to  find,  on  the  one  hand, 
various  nitrogenous  bodies  resulting  from  proteid  metabolism 
in  various  parts  of  the  body,  and,  on  the  other  hand,  arrange- 
ments by  means  of  which  these  various  bodies  were  reduced  to 
the  common  form  urea,  preparatory  to  their  discharge  from  the 
body  by  the  kidney.  And  actual  observation  as  far  as  it  goes 
supports  this  view,  though  our  knowledge  of  the  whole  matter  is 
very  imperfect. 

§  382.  We  may  turn  our  attention  first  to  the  metabolism  of 
the  skeletal  muscles,  since  these  represent,  as  far  as  mere  quan- 
tity is  concerned,  by  far  the  greater  part  of  the  proteid  capital 
of  the  body.  We  may  safely  infer  that  they  furnish  a  large  part 
of  the  urea  of  the  urine  ;  though  undoubtedly  a  small  mass  of 
tissue  might  by  reason  of  its  more  rapid  metabolism  work  over 
a  greater  quantity  of  proteid  material  than  a  much  larger  mass 
with  a  slower  metabolism ;  yet  we  have  no  reason  to  think  that 
the  proteid  metabolism  of  skeletal  muscle,  obscure  though  it  is 
in  its  nature,  is  so  slow  as  to  neutralize  the  probable  effect  of  the 
great  bulk  of  muscle  existing  in  the  body. 

In  dealing  with  the  chemistry  of  muscle  (§  59)  we  saw  that 
urea  was  conspicuous  by  its  absence  from  the  extract  of  muscle, 
whereas  a  very  appreciable  quantity  of  kreatin  was  invariably 
present,  and  indeed  was  the  prominent  nitrogenous  crystalline 
constituent  of  that  extract.  It  seems  difficult  to  resist  the  con- 
clusion that  kreatin  is  the  main  normal  nitrogenous  product  of 
the  metabolism  of  skeletal  muscles.  If  we  accept  this  view,  then 
upon  the  fact  of  the  presence  of  kreatin  in,  and  the  absence  of 


Chap,  iv.]     METABOLIC  PROCESSES  OF  THE  BODY.        591 

urea  from,  the  muscle  itself,  we  may  base  the  conclusion  that 
while  the  muscle  produces  kreatin  as  an  antecedent  of  urea,  the 
kreatin  so  produced  is  converted  into  urea  in  some  part  of  the 
body  other  than  the  muscle  itself.  Kreatin  as  we  have  already 
seen  may  be  easily  split  up,  and  we  may  probaljly  with  safety 
assume  is  split  up  somewhere  in  the  body,  into  urea  and  sarcosin. 
But  sarcosin  does  not  appear  in  the  urine  as  such ;  hence  the  con- 
version of  kreatin  into  (part  of)  the  urea  of  the  urine  entails  as 
Avell  the  further  conversion  of  sarcosin  into  urea.  Now  sarcosin 
as  we  have  seen  is  methyl-glycin  ;  we  may  regard  it  for  our 
present  purposes  as  simple  glycin,  and  hence  the  total  conver- 
sion of  kreatin  into  urea  entails  the  conversion  of  glycin  into 
urea.  This  however  does  not  offer  any  additional  difficult}^ 
since  we  know  from  direct  observation  that  glycin  introduced 
into  the  alimentary  canal  does  not  reappear  as  such  in  the  urine 
but  produces  a  corresponding  increase  in  the  urea  of  the  urine  ; 
from  which  we  infer  that  glycin  absorbed  from  the  alimentary 
canal  is  somewhere  in  the  body  converted  into  urea.  We  shall 
speak  of  this  conversion  later  on,  and  shall  then  see  that,  so  far 
as  urea  is  concerned,  glycin  (amido-acetic  acid)  and  sarcosin 
(methyl-glycin,  methyl-amido-acetic  acid)  undergo  the  same 
change,  the  amide  moiety  in  each  case  being  converted  into 
urea,  while  the  non-nitrogenous  moiety  is  oxidized  and  thrown 
off.  Meanwhile  we  may  state  the  conclusion  at  which  we  have 
provisionally  arrived,  namely  that  the  nitrogenous  metabolism 
of  muscle  probably  gives  rise  to  kreatin,  which  in  some  part  of 
the  body  other  than  muscle  is  probably  split  up  into  urea,  ready 
for  excretion,  and  into  sarcosin  Avhich  also,  somewhere  in  the 
body,  is  further  converted  into  urea.  And  bearing  in  mind  the 
large  mass  of  the  skeletal  muscles,  we  may  further  conclude  that 
a  large  portion  of  the  urea  leaving  the  body  by  the  urine  is 
formed  in  this  way. 

§  383.  We  must  not  however  leave  this  statement  without 
referring  to  a  difticulty.  Kreatinin  as  we  have  seen  is  so  fre- 
quently found  in  urine  as  to  be  regarded  as  a  normal  constit- 
uent, at  all  events  of  human  urine  ;  and  kreatinin  is  as  we  have 
seen  the  urinary  form  so  to  speak  of  kreatin  ;  the  one  body 
easily  changes  into  the  other  by  the  assumption  or  removal  of 
HgO.  This  suggests  the  question,  Is  not  the  kreatinin  of  urine 
the  representative  of  the  kreatin  of  the  muscles,  which  is  thus 
excreted  directly  without  undergoing  the  change  into  urea  just 
discussed?  In  answer  to  this  we  may  sa}"  in  the  first  j)lace  that 
the  quantity  of  kreatinin  in  urine,  though  variable,  is  small ;  we 
may  put  the  average  at  about  1  grm.  in  24  hours.  Now  muscle 
contains  from  -2  to  -4  p.c.  of  kreatin ;  and  this,  taking  the  total 
muscle  of  the  body  (to  say  nothing  of  other  sources  of  kreatin 
which  we  shall  mention  presently)  at  about  30  kilos  would  give 
60  to  120  grms.  kreatin  as  present  in  the  muscles  of  the  body  at 


592  FOEMATION   OF   UKEA.  [Book  ii. 

any  one  moment.  We  can  liardly  suppose  that  the  metabolism 
of  muscle  is  so  slow  as  out  of  this  stock  only  to  provide  the  1 
grm.  of  kreatinin  in  24  hours.  Moreover  the  kreatinin  in  urine 
vanishes  during  starvation,  is  very  markedly  increased  by  a  diet 
of  flesh  which  contains  kreatin,  and  is  not  increased  either  by 
muscular  exercise  (which  however  would  only  indirectly  affect 
the  nitrogenous  metabolism  of  muscle)  or  by  such  conditions, 
fever  for  instance,  as  notably  increase  the  urea  of  urine  by  in- 
creasing the  nitrogenous  metabolism  of  muscle.  We  infer  there- 
fore that  the  normal  presence  of  kreatinin  in  urine  is  due  to  the 
direct  administration  of  kreatin  present  in  a  (normal)  flesh  diet 
and  has  nothing  to  do  with  the  muscular  metabolism  of  the 
individual  who  is  secreting  the  kreatinin  in  his  urine. 

The  fact  however  that  the  kreatin  present  in  the  muscle  of 
the  food  and  absorbed  from  the  alimentary  canal  does  not 
undergo  a  change  into  urea  but  is  excreted  as  kreatinin,  that 
is  virtually  as  kreatin,  warns  us  to  be  careful  in  adopting  the 
conclasion  arrived  at  above  that  the  kreatin  produced  by  mus- 
cular metabolism  in  the  living  body  is  a  conspicuous  antecedent 
of  the  urea  of  the  urine.  It  is  difficult  to  see  why  kreatin  pass- 
ing into  the  blood  of  the  capillaries  of  the  muscle  should  be 
changed  into  urea  while  that  which  passes  into  the  capillaries 
of  the  portal  system  is  not ;  for  reasons  which  will  be  apparent 
presently  we  should  rather  expect  that  the  latter  being  more 
directly  exposed  to  the  influence  of  the  liver  would  be  more 
readily  and  more  completely  converted  than  the  former.  In- 
deed the  question  forces  itself  upon  us,  Is  kreatin  after  all  the 
natural  main  product  of  the  nitrogenous  metabolism  of  muscle  ? 
Is  it  possible  that  in  the  normal  metabolism  of  the  living  mus- 
cle the  nitrogen  leaves  the  muscular  substance  and  passes  into 
the  blood  in  another  form,  as  some  substance  not  kreatin,  and 
that  it  is  as  the  muscle  dies  that  kreatin  is  formed,  just  as  the 
solid  myosin  is  unknown  to  the  living  fibre  but  makes  its  ap- 
pearance in  a  dying  one  ?  We  have  no  positive  evidence  how- 
ever that  this  is  so,  and  meanwhile  may  continue  to  suppose 
that  kreatin  is  formed,  and  that  in  consequeiice  kreatin  is  a  con- 
spicuous antecedent  of  the  urea  of  the  urine ;  but  we  must  not 
regard  this  as  proved. 

§  384.  Our  knowledge  of  the  metabolism  of  the  nervous 
tissues  is,  as  we  have  seen,  very  imperfect  (§  67),  but  the  pres- 
ence of  kreatin  in  the  central  nervous  system  leads  us  to  infer 
that  the  nitrogenous  metabolism  of  the  living  substance  of  nerve 
cells  and  of  the  axis  cylinder  of  nerve  fibres,  is  in  its  broad 
features  identical  with  that  of  muscle  substance.  The  mass 
however  of  the  nerve  cells  and  axis  cylinders  of  the  body,  all 
put  together,  is  small  compared  with  the  mass  of  skeletal  mus- 
cle ;  moreover,  the  energy  set  free  by  the  metabolism  of  a  mass 
of  nervous  matter  tliough  'higher '  in  quality  is  less  in  quantity 


Chap,  iv.]     METABOLIC  PROCESSES  OF  THE  BODY.       593 

than  that  set  free  by  the  metabolism  of  an  equal  mass  of  muscle, 
or  in  other  words  its  metabolism  is  less  rapid.  Hence  we  may 
probably  consider  the  metabolism  of  the  nervous  system  as  a 
mere  addition  to  that  of  the  muscular  system,  at  least  as  regards 
the  point  on  which  we  are  now  dwelling.  The  amount  of  nitro- 
genous metabolism  taking  place  in  connective  tissue,  cartilage, 
bone,  and  the  skin  is  probably  still  less,  and  for  our  present  pur- 
poses needs  no  special  discussion. 

§  385.  The  nitrogenous  metabolism  of  the  glands  however, 
more  particularly  that  of  the  liver,  does  deserve  special  con- 
sideration ;  and  we  may  at  once  turn  to  a  quite  different  aspect 
of  the  question  in  hand. 

When  the  rate  of  discharge  of  urea  from  the  body  is  observed 
during  a  period  of  some  length,  especiall}^  under  varied  circum- 
stances, the  direct  effect  of  nitrogenous  food  becomes  most 
striking.  We  have  alread}^  said,  and  shall  again  return  to  the 
point,  that  muscular  contraction  does  not  directly  increase  the 
output  of  urea  ;  the  discharge  of  urea  for  instance  is  not  neces- 
sarily increased  by  even  great  bodily  labour.  The  introduction 
however  of  even  a  small  quantity  of  proteid  material  into  the  ali- 
mentary canal  at  once  increases  the  urea  of  the  urine  ;  and  in 
the  curve  of  the  discharge  of  urea  in  the  twenty-four  hours  each 
meal  is  followed  by  a  conspicuous  rise.  The  absorption  of  pro- 
teid material  from  the  alimentary  canal  is  followed  by  an  imme- 
diate proportionate  increase  in  the  quantity  of  urea  which  is 
secreted  by  the  kidneys,  and  that  as  we  have  seen  means  an. 
increase  in  the  urea  brought  to  the  kidney  by  the  renal  artery. 
What  is  the  origin  of  this  additional  urea  ? 

Two  views  present  themselves.  On  the  one  hand  since  some 
portion  of  the  proteid  material  of  every  meal,  at  all  events  of 
every  necessary  meal,  goes  to  repair  the  proteid  waste  continu- 
ally going  on  in  the  parts  of  the  body  where  proteid  metabolism 
is  taking  place,  we  may  suppose  that  the  presence  of  an  extra 
quantity  of  proteid  material  thrown  upon  the  blood  from  the 
food  acts  as  a  stimulus  to  the  tissues,  to  the  muscles  for  instance 
as  well  as  others,  stirs  them  up  to  increased  nitrogenous  metabo- 
lism and  thus  produces  an  increase  of  energy,  chiefly  if  not 
exclusively  in  the  form  of  heat,  accompanied  by  an  increase  of 
the  antecedents  of  urea  and  so  of  urea.  In  other  words  the 
increase  of  urea  in  question  is  the  result  of  an  increase  in  the 
general  nitrogenous  metabolism  of  the  body. 

On  the  other  hand  we  may  suppose  that  in  order  to  prevent 
the  whole  body  being  encumbered  with  it,  this  excess  of  proteid 
food  material  is,  in  some  special  jiart  of  the  body,  split  up  into 
a  nitrogenous  and  a  non-nitrogenous  moiety,  and  that,  while  the 
latter  is  stored  up  as  fat  or  glycogen,  the  former  is  at  once  con- 
verted into  urea  and  got  rid  of.  We  have  already  (§  210)  seen 
that  a  step  in  this  direction  may  take  place  while  the  food  is  as 

38 


594  SYNTHESIS   OF   UREA.  [Book  ii. 

yet  in  the  alimentary  canal ;  we  have  seen  that  pancreatic  juice 
may  carry  part  of  the  proteids  on  which  it  acts  beyond  the  stage 
of  albumose  and  peptone,  and  reduce  that  part  into  leucin,  tyro- 
sin,  and  other  bodies.  We  do  not  know,  as  we  have  already 
said,  to  what  extent  this  more  profound  digestion  by  pancreatic 
juice  does  actually  take  place  in  the  living  body ;  it  may  under 
certain  circumstances  take  place  to  a  very  slight  extent  and 
under  others  to  a  considerable  extent.  But  in  any  case  it  illus- 
trates the  way  in  which  a  somewhat  similar  disruption  of  proteid 
material,  a  disruption  which  may  be  broadly  described  as  a  split- 
ting up  of  the  proteid  into  a  nitrogenous  and  a  non-nitrogenous 
moiety,  may  take  place  somewhere  in  the  body  and  so  lead  to 
the  sudden  formation  of  some  antecedent  of  urea.  The  ante- 
cedent may  be  leucin  or  may  be  some  other  body  or  bodies. 

In  support  of  this  view  may  be  urged  the  fact  that  such 
bodies  as  leucin,  glycin,  asparagin  and  many  others  when  intro- 
duced into  the  alimentary  canal  are  transformed  into  urea. 
When  these  bodies  are  administered  in  not  too  great  quantities 
they  do  not  reappear  in  the  urine  but  the  urea  is  proportion- 
ately increased. 

§  386.  We  have  seen  reason  to  think  that  proteids  of  a 
meal  are  absorbed  not  by  the  lacteals  but  by  the  portal  blood 
vessels,  and  such  bodies  as  leucin  probably  take  the  same  course. 
This  being  so,  all  these  bodies  pass  through  the  liver  and  are 
subjected  to  such  influences  as  may  be  exerted  by  the  hepatic 
cells.  Now  we  have  no  positive  evidence  that  the  liver  does  or 
can  exert  such  an  action  on  proteid  material  itself  as  to  sepa- 
rate a  relatively  simple  nitrogen  compound  from  the  remaining 
constituents,  leaving  these  to  form  a  body  rich  in  carbon;  we 
have  no  positive  proof  that  the  increase  of  proteid  metabolism 
just  spoken  of  as  leading  to  an  increase  of  urea  takes  place  in 
the  liver  rather  than  in  the  tissues  at  large;  we  may  perhaps 
suspect  that  it  is  so  but  we  have  no  convincing  demonstration. 
We  have  however  a  convergence  of  evidence  that  the  last  stage 
of  the  process,  namely  the  conversion  into  urea  of  some  or  other 
product  of  proteid  metabolism  which  though  allied  to  is  not 
exactly  urea  does  occur  in  the  liver.  In  the  first  place,  a  large 
quantity  of  urea  seems  to  be  present  in  the  liver  of  mammals; 
in  this  respect  the  liver  presents  a  strong  contrast  to  the  mus- 
cles; in  the  liver  of  birds  the  urea  is  represented  by  urates. 
In  the  second  place,  in  certain  cases  of  a  form  of  disease  of  the 
liver  known  as  acute  yellow  atrophy  in  which  the  hepatic  cells 
are  so  changed  that  their  functional  activity  is  largely  dimin- 
ished, the  urea  of  the  urine  not  only  undergoes  a  very  marked 
decrease  but  appears  to  be  replaced  to  a  very  large  extent  by 
leucin.  This  fact  suggests  that  leucin  (and  not  for  instance 
kreatin)  is  the  chief  immediate  product  of  the  nitrogenous 
metabolism  of  the  body,  and  that  the  leucin  thus  produced  is 


Chap,  iv.]      METABOLIC  PROCESSES  OF  THE  BODY.       595 

in  a  normal  state  of  tilings  converted  into  urea  by  the  liver. 
And  in  this  connection  it  may  be  remarked  that  not  only  is  leu- 
cin  found  in  nearly  all  the  tissues  after  death,  esj)ecially  in  the 
glandular  tissues,  but  also  appears  with  striking  readiness  in 
almost  all  decompositions  of  proteids,  and  is  moreover  a  product 
of  decomposition  of  gelatiniferous  substances.  Without  going 
however  so  far  as  to  conclude  that  leucin  is  the  chief  antecedent 
of  urea,  we  may  take  the  above  observation  as  indicating  that 
the  normal  liver  has,  in  some  way  or  other,  the  power  of  con- 
verting leucin  into  urea.  If  this  be  so  then  we  may  also  vent- 
ure to  suppose  that  Avhen  such  bodies  as  leucin,  glycin,  &c., 
introduced  into  the  alimentary  canal  appear  in  the  urine  as 
urea  the  transformation  has  taken  place  in  the  liver.  The  body 
tyrosin  which  so  often  accompanies  leucin,  belonging  as  it  does 
to  the  aromatic  series,  stands  on  a  different  footing  from  leucin 
and  the  like. 

§  387.  The  transformation  however  of  leucin  into  urea  raises 
a  new  point  of  view.  Leucin,  as  we  know,  is  amido-caproic 
acid;  and,  with  our  present  chemical  knowledge,  we  can  con- 
ceive of  no  other  way  in  which  leucin  can  be  converted  into 
urea  than  by  the  complete  reduction  of  the  former  to  the  am- 
monia condition  (the  caproic  acid  residue  being  either  elabo- 
rated into  a  fat  or  oxidized  into  carbonic  acid)  and  by  a 
reconstruction  of  the  latter  out  of  the  ammonia  so  formed. 
We  have  a  somewhat  parallel  case  in  glycin,  which  is  amido- 
acetic  acid;  here  too  a  reconstruction  of  urea  out  of  an  am- 
monia phase  must  take  place.  Moreover  when  ammonium 
chloride  is  given  to  a  dog  a  very  large  portion  reapj^ears  as 
urea,  i.e.  there  is  an  increase  in  the  urea  of  the  urine  corre- 
sponding to  a  large  portion  of  the  nitrogen  contained  in  the 
ammonium  chloride.  And  in  the  case  of  other  animals  also, 
indeed  of  man  himself,  there  is  evidence  that  somewhere  in  the 
body  ammonia  may  be  converted  into  urea.  Hence  in  all  these 
cases  where  ammonia  or  ammonia  compounds  are  changed  into 
urea  the  last  step  at  all  events  is  one  of  synthesis;  and  this 
suggests  the.  possibility  that  in  the  ordinary  proteid  metabolism 
also,  the  downward  katabolic  series  of  changes  may  finish  off 
with  a  synthetic  effort,  the  last  stage  of  the  former  being  the 
appearance  of  an  ammonia  compound  which  is  subsequently 
reconstructed  into  urea. 

This  synthesis,  like  the  transformation  of  leucin  and  other 
bodies,  probably  takes  place  in  the  liver;  and  in  support  of 
this  view  we  have  a  certain  amount  of  experimental  evidence. 
Birds  may  be  kept  alive  after  total  extirpation  of  the  liver  for 
a  longer  time  than  can  mammals;  and  when  in  geese  the  liver 
is  removed  the  uric  acid  (representing  in  these  animals  the  urea 
of  the  mammal)  is  largely  decreased,  while  the  ammonia  of  the 
urine  is  largely  increased.     After  the  removal  of  the  liver  also. 


596  URIC   ACID.  [Book  ii. 

leucin,  glycin,  and  other  amides  or  amido-acids  administered 
by  the  alimentary  canal  no  longer  increase  the  uric  acid  of  the 
urine,  as  they  do  in  the  intact  animal.  In  these  animals,  the 
synthesis  of  ammonia  compounds  into  uric  acid,  which  is  par- 
allel to  the  synthesis  into  urea  occurring  in  the  mammal,  seems 
to  take  place  in  the  liver,  and  we  may  infer  is  in  some  way  or 
other  effected  by  the  hepatic  cells. 

As  to  the  exact  way  in  which  ammonia  either  as  such  or  in 
form  of  an  amide  or  amido-acid  changes  into  urea  we  have  no 
certain  knowledge.  Ammonium  carbonate,  we  know,  is  readily 
formed  out  of  urea  by  simple  hydration,  and  we  may  imagine 
that  the  living  organism  can  carry  out  the  reverse  process  and 
dehydrate  ammonium  carbonate  into  urea.  There  is,  however, 
a  certain  amount  of  evidence  that  not  ammonium  carbonate  but 
ammonium  carbamate  is  the  immediate  antecedent  of  urea ;  and 
indeed,  out  of  the  body,  by  electrolyzing  a  solution  of  ammo- 
nium carbamate  with  alternating  currents,  a  certain  amount  of 
urea  may  be  artificially  produced.  But  this  is  a  matter  too 
obscure  to  be  discussed  here. 

§  388.  Uric  Acid.  This,  like  urea,  is  a  normal  constituent 
of  human  urine,  and,  like  urea,  has  been  found  in  the  blood, 
in  the  liver  and  in  the  spleen ;  it  is  a  conspicuous  constituent 
of  an  extract  of  the  latter  organ.  In  various  diseases  the  quan- 
tity in  the  urine  is  increased  ;  and  at  times,  as  in  gout,  uric 
acid  accumulates  in  the  blood,  and  a  deposit  of  urates  takes 
place  in  the  tissues.  In  some  animals,  such  as  birds  and  most 
reptiles,  uric  acid  takes  the  place  of  urea.  Since  by  oxidation 
a  molecule  of  uric  acid  can  be  split  up  into  two  molecules  of 
urea,  and  a  molecule  of  some  carbon  acid,  uric  acid  is  com- 
monly spoken  of  as  a  less  oxidized  product  of  proteid  metabo- 
lism than  urea.  But  there  is  no  evidence  whatever  to  shew 
that  the  former  is  a  necessary  antecedent  of  the  latter ;  on  the 
contrary,  all  the  facts  known  go  to  shew  that  the  appearance 
of  uric  acid  is  the  result  of  a  metabolism  slightly  diverging 
from  that  leading  to  urea  ;  indeed  it  is  probable  that  the 
divergence  occurs  towards  the  end  of  the  series  of  changes,  for 
urea  given  by  the  mouth  to  birds  appears  in  the  urine  as  uric 
acid,  and,  conversely,  uric  acid  given  to  mammals  appears  in 
the  urine  as  urea.  We  have  no  evidence  to  prove  that  the 
cause  of  the  divergence  lies  in  an  insulificient  supply  of  oxygen 
to  the  organism  at  large  ;  on  the  contrary,  uric  acid  occurs  in 
the  rapidly  breathing  birds  as  well  as  in  the  more  torpid  rep- 
tiles. Nor  can  the  fact  that  in  the  frog  again  urea  replaces 
uric  acid  be  explained  by  reference  to  that  animal  having  so 
large  a  cutaneous  in  addition  to  its  pulmonary  respiration. 
The  final  causes  of  the  divergence  are  to  be  sought  rather  in 
the  fact  that  urea  is  the  form  adapted  to  a  fluid,  and  uric  acid 
to  a  more  solid  excrement.     Nor  is  there  in  man  or  the  mam- 


Chap,  iv.]     METABOLIC  PROCESSES  OF  THE  BODY.        597 

mal  any  satisfactory  physiological  or  clinical  evidence  that  an 
increase  of  uric  acid  is  the  result  of  deficient  oxidation.  The 
absolute  amount  of  uric  acid  discharged  by  man  and  its  pro- 
portion to  the  urea  passed  at  the  same  time  varies  a  good  deal. 
There  is  no  positive  evidence  that  the  quantity  excreted  is 
necessarily  increased  by  nitrogenous  diet,  unless  some  disorder 
supervenes  ;  indeed  it  is  asserted  that  both  absolutely  and  rela- 
tively to  the  urea  the  quantity  excreted  is  greater  upon  a  mixed 
diet  than  upon  a  highly  proteid  one.  Alkalis  in  the  food  seem 
undoubtedly  to  diminish  it,  and  alcohol,  at  least  in  excess,  to 
increase  it. 

So  far  from  considering  uric  acid  as  a  less  oxidized  antece- 
dent of  urea  we  ought  perhaps  rather  to  regard  its  appearance 
as  a  result  of  a  synthesis  in  which  urea  or  some  allied  body 
takes  part.  As  we  have  said  uric  acid  may  be  formed  syn- 
thetically by  heating  together  urea  and  glycin  ;  and  it  has 
more  recently  been  similarly  prepared  from  various  allied 
bodies.  As  to  where  or  how  such  a  synthesis  is  effected  in  the 
living  body,  we  know  little  or  nothing  for  certain,  and  can  only 
make  conjectures.  The  constant  presence  of  uric  acid  in  the 
spleen  however,  and  the  frequently  noted  connection  between 
a  rise  and  fall  of  uric  acid  in  the  urine  and  variations  in  the 
volume  and  therefore  presumably  in  the  activity  of  the  spleen, 
suggest  that  the  change  may  be  brought  about  in  this  organ  ; 
but  it  must  be  remembered  that  in  birds  and  reptiles  the  for- 
mation of  uric  acid  seems  to  be  effected  in  the  same  oro'ans  as 
that  of  urea  and  in  an  analogous  manner  ;  and  the  arguments 
which  we  have  used  concerning  the  formation  of  urea  in  the 
liver  of  mammals  may  be  applied  to  the  formation  of  uric  acid 
in  the  livers  of  birds  and  reptiles.  It  is  more  probable  there- 
fore that  in  the  mammal  the  turn  to  uric  acid  rather  than  urea 
is  given  in  the  liver,  the  spleen  however  possibly  playing  its 
part  also  in  the  matter. 

§  389.  Of  the  meaning  of  the  appearance  in  the  tissues  of 
such  bodies  as  xanthin,  hypoxanthin,  guanin  and  the  like,  and 
of  the  exact  nature  of  the  metabolism  which  gives  rise  to  them 
or  wliich  they  themselves  undergo,  we  know  little  or  nothing. 
The  presence  of  these  several  bodies  may  be  taken  as  illustrat- 
ing the  complex  and  varied  nature  of  proteid  metabolism  to 
which  we  referred  above.  Urea  is  the  chief  end-product  of 
proteid  metabolism,  but  that  end  is  probably  reached  in  several 
ways  ;  so  that  probably  a  very  large  number  of  nitrogenous 
chemical  substances  make  a  momentary  appearance  in  the  body. 
Some  of  these  fail  to  become  urea,  and  either  without  or  after 
further  change  make  their  appearance  in  the  urine.  But  we  do 
not  know  whether  their  appearance  is  accidental,  the  result  of 
imperfect  chemical  machinery ;  or  whether  they,  though  small 
in  quantity,  serve  some  special  ends  in  the  economy.     Perhaps 


598  NITEOGENOUS   METABOLISM.  [Book  ii. 

sometimes  or  with  some  of  them  it  is  the  one  case,  at  other 
times  or  with  others  it  is  the  other  case. 

When  proteid  material  undergoes  outside  the  body,  either 
by  the  action  of  trypsin  or  as  the  result  of  decomposition  or 
under  the  influence  of  chemical  agents,  that  change  by  which 
it  is  converted  into  leucin,  the  leucin,  which  appears  in  some 
considerable  quantities,  is  accompanied  by  tyrosin,  which  ap- 
pears in  smaller  quantities,  as  well  as  by  other  bodies.  The 
almost  constant  appearance  of  tja^osin  as  a  result  of  the  decom- 
position of  proteid  material  leads  one,  as  we  have  previously 
said,  to  the  conception  that  some  representative  of  the  aromatic 
series  enters  into  the  constitution  of  proteid  substance  ;  and  it 
is  possible  that  the  hippuric  acid  of  flesh-eating  animals  derives 
its  benzoic  acid  constituent  from  this  aromatic  radicle  of  pro- 
teid matter.  Tyrosin  itself  does  not  appear  in  the  body  as  a 
normal  product  of  proteid  metabolism,  and  we  are  therefore 
led  to  infer  that  in  proteid  metabolism  the  aromatic  radicle 
takes  on  some  other  form.  Whether  as  in  tyrosin  the  aromatic 
(phenyl)  nucleus  is  associated  with  an  ammonia  representative 
or  no,  we  do  not  know.  But  if  it  is  then,  since  neither  tyrosin 
nor  any  similar  body  is  a  constituent  of  normal  urine,  the 
ammonia  constituent  is  somewhere  dissociated  from  the  phenyl 
one ;  and  while  the  former  contributes  to  the  stock  of  urea,  the 
latter  is  either  discharged  by  the  urine  as  hippuric  acid,  having 
as  we  have  seen  effected  in  the  kidney  a  neAV  association  with 
the  ammonia  representative  glycin,  or  leaves  the  body  as  one 
or  other  of  the  urinary  phenyl  compounds,  or  possibly  may  be 
oxidized  somewhere  into  carbonic  acid  and  water.  Our  knowl- 
edge on  this  point  is  limited,  but  we  have  ventured  to  refer  to 
the  i^oint  since  it  further  illustrates  the  complexity  of  proteid 
metabolism. 

§  390.  In  speaking  of  urea  (§  321)  we  alluded  to  its  rela- 
tions to  the  cyanogen  compounds.  Bearing  in  mind  the  pecu- 
liarly large  amount  of  energy  set  free  as  heat  during  the  iso- 
meric transformation  of  many  cyanogen  compounds,  as  well 
as  the  large  store  of  potential  energy  existing  in  cyanogen 
itself,  the  heat  of  combustion  of  which  is  very  large,  and  con- 
trasting these  properties  Avith  those  of  ammonia  and  the  am- 
monia compounds,  we  cannot  help  being  tempted  towards  the 
view  that  in  the  actual  living  structure  the  nitrogen  exists  in 
the  form  of  cyanogen  compounds,  and  that  in  the  passage  to 
dead  nitrogenous  waste,  during  which  energy  is  set  free,  the 
cyanogen  compound  changes  to  the  amide  or  other  ammonia 
representative.  And  there  are  several  facts  which  lend  sup- 
port to  such  a  view,  such  as  the  presence  of  sulphocyanates  in 
saliva  and  urine,  which  we  may  look  upon  as  a  sort  of  leakage 
of  cyanogen  factors,  the  artificial  production  of  kreatinin  out 
of  cyanamide  and  sarcosin,  and  other  facts.     But  the  matter. 


Chap,  iv.]     METABOLIC  PROCESSES  OF  THE  BODY.        599 

though  it  deserves  to  be  borne  in  mind,  is  too  obscure  to  be 
dwelt  on  here. 

§  391.  We  may  now  briefly  sum  up  the  varied  discussions 
which  have  occupied  us  in  the  present  section. 

Urea  is  the  main  end-product  of  proteid  metabolism.  Un- 
like hippuric  acid  and  some  other  constituents  of  urine,  urea  is 
simply  excreted  by  the  kidneys,  being  brought  to  them  in  the 
blood,  they  apparently,  beyond  the  simple  act  of  excretion, 
doing  no  more  than  merely  contributing  to  the  stock  of  urea 
in  so  far  as  they  are  masses  of  proteid  material  undergoing  pro- 
teid metabolism  as  part  of  their  general  life.  What  are  the 
immediate  antecedents  of  urea  we  do  not  clearly  know  ;  but  it 
is  probable  that  they  are  not  one  but  several  and  indeed  possi- 
bly many.  We  have  reason  to  think  that  urea  may  be  formed 
out  of  amides  or  amido-acids,  or  out  of  ammonia  itself  by  a 
synthetic  process  ;  and  we  have  indications  that  this  syjithesis 
is  effected  in  the  liver  by  the  agenc}'  of  the  hepatic  cells.  But 
we  do  not  know  whether  this  synthesis  bears  only  on  particular 
nitrogen-jholding  substances  of  food  or  of  the  body,  or  whether 
it  comes  into  play  in  the  normal  metabolism  of  proteid  mate- 
rial. If  the  kreatin  which  is  so  conspicuous  a  constituent  of 
muscular  and  nervous  structures  is  a  stage  in  the  direct  line  to 
urea,  then  the  synthesis  would  affect  only  the  sarcosin  which 
the  kreatin  in  becoming  urea  sets  free.  But  we  have  seen  that 
it  is  by  no  means  clear  that  kreatin  is  such  a  stage. 

The  evidence  as  far  as  it  goes  tends  to  shew  that  the  meta- 
bolism of  proteid  is  very  complex  and  varied,  that  a  large 
number  of  nitrogen-holding  substances  make  a  momentar}-  ap- 
pearance in  the  body,  taking  origin  at  this  or  that  step  in  the 
downward  stairs  of  katabolic  metabolism  and  changing  into 
something  else  at  the  next  step,  and  that  the  presence  in  vari- 
ous parts  of  the  body  and  even  in  the  urine,  in  small  quantities, 
of  so  many  varied  nitrogenous  crystalline  substances,  forming 
a  large  part  of  what  are  known  as  extractives,  has  to  do  with 
this  varied  metabolism.  Possibly  the  transformations  by  which 
nitrogen  thus  passes  downwards  take  place  to  a  certain  extent 
in  such  organs  as  tlie  liver  and  the  spleen  which  are  remarka- 
bly rich  in  these  extractives. 


SEC.  5.     ON    SOME    STRUCTURES    AND    PROCESSES   OF 
OBSCURE  NATURE. 

§  392.  The  Thyroid  Body.  Certain  structures  which, 
though  tliey  differ  in  many  ways,  we  may  conveniently  treat  of 
together,  such  as  the  thyroid  and  pituitary  bodies,  the  supra- 
renal capsules,  and  the  thymus,  appear  to  play  not  unimportant 
parts  in  the  metabolic  processes  of  the  body. 

In  regard  to  the  thyroid  we  have  clinical  and  experimental 
evidence  pointing  distinctly  in  this  direction.  In  certain  animals- 
(such  as  monkeys  and  dogs)  the  removal  of  the  thyroid  gives 
rise  to  various  symptoms  of  disorder.  Among  the  earlier  of 
these  are  muscular  tremors,  spasms  or  even  tetanic  convulsions, 
accompanied  or  succeeded  by  irregularity  or  failure  of  voluntary 
movements,  all  indicating  mischief  in  the  central  nervous 
system,  in  which  indeed  histological  changes  may  be  detected. 
Subsequently  there  ensue  other  varied  symptoms  which  may  be 
described  under  the  general  term  of  those  of  disordered  nutri- 
tion, and  which  eventually  end  in  death.  In  order  to  obtain 
these  results  the  whole  of  the  thyroid  gland,  including  the 
small  so-called  accessory  thyroids,  when  these  are  present,  must 
be  removed;  if  a  part  only  of  the  body  be  left  behind  the 
symptoms  do  not  appear,  or  are  slight  and  transient.  Mere 
injury  either  to  the  thyroid  body  itself,  or  to  the  surrounding 
nervous  and  other  structures,  is  insufficient  to  produce  the 
characteristic  results.  Moreover,  if  the  thyroid,  after  the 
removal  from  its  natural  position  and  attachment,  be  inserted  as 
a  whole  or  in  part  in  some  other  part  of  the  body,  so  as  to  live 
and  thus  be  "grafted,"  the  symptoms  do  not  appear.  The 
story  in  fact  is  very  similar  to  that  of  the  pancreas  in  relation  to 
sugar  in  the  blood  (see  §  374).  And  we  may  infer  that  in  these 
animals  the  blood  in  passing  through  the  thyroid  undergoes 
some  special  change,  some  thing  or  things  being  taken  away 
from  it  or  added  to  it,  by  which  it  is  fitted  for  the  nutrition  of 
the  rest  of  or  at  least  of  other  parts  of  the  body.  We  may  add 
that  in  other  animals,  herbivora  for  instance,  these  symptoms 
are  not  so  easily  produced.     The  reason  may  be  the  greater 

600 


Chap,  iv.]     METABOLIC  PROCESSES  OF  THE  BODY.        601 

difficulty  in  ensuring  the  removal  of  the  whole  gland;  but  this 
is  not  wholly  clear. 

The  view  that  the  thyroid  effects  some  change  on  the  blood 
passing  through  it  is  further  confirmed  by  clinical  experience. 
The  disease  known  as  myxoedema,  characterized  by  disordered 
nutrition,  notably  of  the  skin  and  of  the  nervous  system,  but 
also  of  other  parts  of  the  body,  is  closely  associated  with  morbid 
changes  of  the  thyroid  body,  and  thus  is  allied  to  goitre  and 
cretinism.  The  symptoms  in  many  respects  resemble  those 
produced  in  animals  by  removal  of  the  thyroid.  Now  in  such 
cases  the  symptoms  are  in  a  most  remarkable  way  lessened  or 
even  removed  by  the  systematic  subcutaneous  injection  of  the 
extract  of  the  fresh  thyroid  body  of  an  animal,  or  even  by 
the  extract  being  taken  regularly  by  the  mouth.  The  small 
quantity  of  substance  thus  introduced  into  the  blood  is  sufficient 
to  modify  the  altered  nutrition  of  the  body,  and  to  bring  it  back 
to  its  normal  condition.  The  inference  is  that  under  normal 
conditions  the  thyroid  gives  up  in  some  way  to  the  blood  the 
substance  or  substances  which  in  the  above  instance  are  arti- 
ficially administered  in  the  thyroid  extract,  and  the  presence  of 
which  is  in  some  way  essential  to  the  normal  nutrition  of  the 
body. 

What  that  substance  is  or  those  substances  are,  and  how  they 
act,  we  are  not  yet  in  a  position  to  say.  The  characteristic 
presence  in  the  alveoli  of  the  thyroid  of  mucin,  or  of  a  substance, 
the  so-called  "  colloid  "  having  at  least  a  superficial  resemblance 
to  mucin,  and  the  fact  that  in  myxoedema  a  mucin-like  body  is 
in  excess  in  the  tissues,  hence  the  name,  have  led  to  speculations 
as  to  the  connection  of  the  thyroid  and  mucin.  But  our  knowl- 
edge is  not  at  present  such  as  to  justify  any  definite  statement. 

§  393.  The  Pituitai-y  Body.  The  lower,  posterior,  lobe  of 
this  organ  in  many  respects  resembles  the  thyroid  body  (the 
upper,  anterior,  lobe  is  of  quite  distinct  nature,  being  really 
a  part  of  the  central  nervous  system),  but  concerning  the  proc- 
esses which  take  place  in  this  lobe  and  the  purposes  of  the 
organ  as  a  whole  we  know  absolutely  nothing. 

§  394.  The  Suprarenal  Bodies.  These  differ  wholly  in 
structure  from  the  thyroid  body.  The  two  parts  of  which 
the  body  consists,  cortex  and  medulla,  are  not,  like  the  cortex 
and  medulla  of  a  lymphatic  gland,  different  arrangements  of 
the  same  material,  but  are  of  essentially  different  nature  and 
indeed  are  of  different  origin.  The  medulla  is  derived  from, 
is  a  modification  of,  sympathetic  ganglia,  while  the  cortex  is 
derived  from  masses  of  mesoblastic  cells  surrounding  the  great 
blood  vessels;  and  in  some  animals  the  two  form  wholly  separate 
bodies. 

Some  of  the  histological  features  of  the  suprarenal  bodies, 
namely  the  groups  of  cells  and  their  abundant  blood  supply. 


602  THE   THYMUS.  [Book  ii. 

suggest  on  the  one  hand  that  important  metabolic  processes 
take  place  in  them,  some  of  which  are  probably  connected  with 
the  history  of  the  pigments  of  the  body  at  large.  On  the  other 
hand  the  unusually  large  nerve  supply,  and  the  derivation  of 
part  of  the  body  from  the  sympathetic  ganglia,  suggest  pecu- 
liar nervous  connections.  And  the  organ  has  often  served  as 
a  starting  point  for  speculations  in  these  two  directions;  but 
our  exact  knowledge  concerning  them  is  very  limited.  By 
experiment  Ave  learn  that  removal  of  the  whole  of  both  supra- 
renal bodies  entails  speedy  death,  the  symptoms  having  a  general 
resemblance  to  those  due  to  the  removal  of  the  thyroid.  The 
removal  of  one  suprarenal  alone  is  inadequate  to  produce  this 
result;  and  the  symptoms  following  removal  of  both  may  be  at 
least  mitigated  by  injection  of  an  extract  prepared  from  the 
organ.  This  suggests  a  function  of  the  suprarenals  analogous 
to  that  of  the  thyroid.  Injection  of  the  extract  of  suprarenals 
in  adequate  doses  also  produces  distinct  physiological  effects, 
notably  constriction  of  the  blood  vessels  and  inhibition  of  the 
heart. 

One  fact,  gained  by  clinical  experience,  pointing  in  the 
same  direction  is  of  great  interest.  Disease  of  the  suprarenal 
bodies,  apparently  tubercular  in  nature  and  beginning  in  the 
medulla,  is  so  often  associated  with  a  change  in  the  colour  of, 
with  an  increase  of  the  pigment  of  the  skin,  'bronzed  skin,' 
'Addison's  disease,'  that  some  connection  between  the  two 
must  exist;  but  the  several  links  of  the  chain  are  as  yet 
unknown.  It  is  tempting  to  associate  the  increase  of  pigment 
in  the  bronzed  skin  with  the  fact  that  the  suprarenal  body  con- 
tains some  substance  or  substances,  possessing  striking  colour 
reactions,  giving  a  dark  blue  or  dark  green  colour  with  ferric 
chloride,  and  a  carmine  red  tint  with  various  oxidizing  agents; 
but  we  have  no  exact  knowledge  at  present. 

§  395.  The  Thymus.  This,  again,  is  essentially  a  lymphatic 
structure,  and  indeed  might  be  regarded  as  a  part  of  the  lym- 
phatic system. 

From  the  thymus  there  may  be  extracted  by  means  of  saline 
solution  a  form  of  a  peculiar  proteid,  a  so-called  nucleo-albumin 
which,  like  the  corresponding  bodies  from  lymphatic  glands  or 
from  leucocytes,  seems  to  have  some  special  relations  to  the  for- 
mation of  fibrin.  Thus,  as  has  already  been  said  (§  22),  a  solu- 
tion of  this  body  from  the  thymus,  injected  into  the  veins,  will 
give  rise  to  extensive  intravascular  clotting. 

The  thymus,  like  the  other  bodies  on  which  we  are  now 
dwelling,  is  also  rich  in  extractives.  Thus  xanthin,  hypoxan- 
thin,  leucin,  lactic,  succinic  and  other  acids  have  been  found 
in  it. 

But  of  what  really  takes  x^lace  in  the  body  we  have  no  exact 
knowledge.     Since  the  thymus  is  best  developed  before  birth. 


Chap,  iv.]      METABOLIC  PROCESSES  OF  THE  BODY.       603 

disappearing  after  birth  at  a  rate  which  varies  much  in  differ- 
ent individuals  and  still  more  in  different  kinds  of  animals, 
and  being  eventually  replaced  by  fat  and  connective  tissue,  it 
is  obvious  that  its  chief  functions  are  in  some  way  associated 
with  events  taking  place  before  birth  or  in  early  life. 


SEC.    6.     THE   HISTORY   OF  FAT.     ADIPOSE   TISSUE. 

§  396.  Globules  of  fat  of  various  sizes  make  their  appear- 
ance in  the  very  elements  of  most  of  the  tissues,  in  muscular 
fibres,  in  epithelial  cells,  in  nerve  cells,  in  leucocytes,  and  so  on  ; 
and  the  medulla  of  mediillated  nerves  consists  largely  of  a 
peculiar  fatty  material.  Besides  this,  certain  cells  of  connec- 
tive tissue  at  various  times,  and  in  various  places,  become  so 
loaded  with  fat  that  groups  of  the  cells  become  practically 
masses  of  fat.  Connective  tissue  thus  loaded  with  fat  is  called 
adipose  tissue ;  and  masses  of  adipose  tissue  of  all  manner  of 
sizes  and  of  shapes  adapted  to  the  several  situations  are  found 
in  various  parts  of  the  body.  Many  of  the  internal  organs, 
more  especially  the  kidneys,  are  wrapped  in  adipose  tissue ;  but 
the  largest  deposit  is  one  lying  in  the  subcutaneous  connective 
tissue,  sometimes  called  the  "panniculus  adiposus  ; "  and  a 
'  fat '  body  is  distinguished  from  a  '  lean '  body  chiefly,  though 
by  no  means  exclusively,  by  the  amount  of  subcutaneous  adi- 
pose tissue. 

Of  all  the  tissues  of  the  body  adipose  tissue  is  the  most 
fluctuating  in  bulk  ;  within  a  very  short  space  of  time  a  large 
amount  of  adipose  tissue  may  disappear,  and  within  a  very  short 
space  of  time  the  quantity  present  in  a  body  may  be  several 
times  multiplied.  When  too  much  or  too  little  food  is  given  it 
is  the  subcutaneous  adipose  tissue  which  first  and  most  rapidly 
increases  or  decreases  in  bulk. 

§  397.  A  fat-cell  is  a  cell,  belonging  to  connective  tissue, 
in  the  cell-substance  of  which  fat  has  been  collected  to  such  an 
extent  that  the  cell,  which  increases  largely  in  bulk  during  the 
process,  is  almost  wholly  transformed  into  a  large  vacuole  filled 
with  fat,  the  cell -substance  being  reduced  to  a  thin  envelope  of 
the  vacuole,  thickened  at  one  part  where  the  nucleus,  thrust  on 
one  side  by  the  gathering  fat,  is  placed.  Adipose  tissue  is  a 
collection  of  such  fat-cells  held  together  by  a  meagre  quantity 
of  vascular  connective  tissue. 

By  studying  the  development  of  adipose  tissue  in  the  embryo 
or  elsewhere,  we  may  trace  out  the  steps  of  the  formation  of  the 

604 


Chap,  iv.]     METABOLIC  PEOCESSES  OF  THE  BODY.       605 

fat-cells.  In  the  embryo,  in  a  situation  where  adipose  tissue  is 
about  to  be  formed,  the  connective  tissue  is  seen  to  contain  a 
number  of  small  nucleated  cells,  rounded  or  somewhat  irregular 
in  form,  the  cell-substance  of  which  at  first  presents  no  special 
characters,  and  contains  not  more  than  what  may  be  called  the 
ordinary  amount  of  fat  globules  or  spherules.  Very  soon  how- 
ever these  minute  drops  or  specks  increase  in  number,  the  cell- 
substance  at  the  same  time  increasing  in  bulk  Avhile  remaining 
round  or  becoming  more  distinctly  so,  and  the  smaller  drops  run 
together  into  larger  ones.  This  goes  on  ;  the  fat  increasing  in 
quantity  coalesces  more  and  more,  and  the  cell,  as  a  whole, 
becomes  larger  and  larger,  the  cell-substance  at  first  keeping 
up  in  bulk  with  the  increasing  fat,  but  subsequently  ceasing  to 
increase,  being  apparently  used  up  in  the  formation  of  the  fat. 
Thus  the  original  small  '  protoplasmic '  cell  is  at  last  transformed 
into  the  larger  fat-cell,  all  the  fat  having  run  together  into  a 
vesicle  the  envelope  of  which,  thickened  on  one  side  to  carry 
the  nucleus,  is  furnished  by  the  remnant  of  the  cell-substance. 
In  some  cases,  the  nucleus  instead  of  being  pushed  early  on  one 
side,  remains  central  though  the  collection  of  fat  has  become 
considerable  ;  it  is  however  eventually  displaced.  The  whole 
process  appears  very  similar  to  the  deposition  of  mucin  in  the 
cells  of  a  mucous  gland,  §  197  ;  and  we  may  by  analogy  infer 
that  the  fat-cell  becomes  a  fat-cell  by  the  cell  manufacturing  fat 
in  some  way  or  other,  and  depositing  the  fat  so  formed  in  the 
interstices  of  its  substance.  The  most  striking  superficial  dis- 
tinctions seem  to  be  that  in  the  mucous  cell  the  granules  or 
spherules  remain  discrete  within  the  cell,  being  separated  by 
bars  of  cell-substance,  whereas  in  the  fat-cell  the  globules,  as  they 
form,  run  together  until  at  last  they  unite  into  a  single  mass;  and 
further  that  while  in  the  mucous  cell,  even  when  most  heavily 
loaded,  a  relatively  large  amount  of  active  cell-substance  still 
remains,  in  the  fat-cell  a  mere  remnant  is  left  and  that  chiefly 
surrounding  the  displaced  nucleus. 

The  fat  in  the  interior  of  bones  forming  the  yellow  marrow 
appears  to  have  the  same  general  structure  and  to  be  formed  in 
the  same  way  as  the  rest  of  the  adipose  tissue. 

§  398.  The  fat  thus  deposited  in  a  fat-cell  sooner  or  later 
disappears.  It  is  not  ejected  bodily  into  the  surrounding 
lymph-spaces  of  the  connective  tissue,  but  passes  away  grad- 
ually either  into  the  lymphatics  or  into  the  blood  stream  by 
some  processes  not  as  yet  fully  understood.  During  the  disap- 
pearance of  the  fat  the  cell  behaves  in  one  of  two  different 
ways.  On  the  one  hand,  as  the  fat  gradually  disappears,  little 
by  little,  the  rounded  distended  vesicle  gradually  lessening 
assumes  the  characters  of  a  connective  tissue  corpuscle,  even 
of  a  branched  one.  On  the  other  hand,  especially  when  the 
disappearance  is  rapid  and  total,  the  space  previously  occupied 


606  THE  POEMATION   OF  FAT.  [Book  ii. 

by  fat  becomes  filled  with  a  clear  fluid  resembling  lymph,  the 
fat  vesicle  being  transformed  into  a  lymph  vesicle.  This  con- 
dition however  is  temporary  only,  the  lymph  is  subsequently 
absorbed  and  the  vesicle  shrinks.  Or  the  cell-substance  may 
shrink  round  the  lessening  fat,  but  in  doing  so  deposits  on  its 
outside  a  mucous  substance.  At  times,  the  emptying  of  the 
cell,  whether  by  the  one  method  or  the  other,  is  followed  by  a 
rejuvenescence  of  the  cell,  the  nucleus  by  division  gives  rise  to 
several  nuclei,  and  the  cell  divides  into  new  cells,  each  of  which 
may,  under  appropriate  conditions,  develope  again  into  a  fat-cell. 

§  399.  The  fat  thus  lodged  in  adipose  tissue  varies  some- 
what in  composition  in  various  animals,  but  is  chiefly  composed 
of  olein,  palmitin  and  stearin  in  varying  proportions,  with  small 
quantities  of  the  glycerine  compounds  of  such  fatty  acids  as 
butyric,  capronic,  caprylic,  &c.,  together  with  a  little  lecithin  and 
cholesterin.  The  '  fat '  of  one  animal,  that  is  the  fat  thus  con- 
tained in  adipose  tissue,  differs  from  the  fat  of  another  animal 
partly  by  the  presence  of  more  or  less  of  one  or  more  of  these 
less  abundant  fats,  but  chiefly  by  the  proportion  in  which  the 
three  main  fats,  olein,  palmitin,  and  stearin,  are  respectively 
present  in  the  mixed  fat.  The  melting  points  of  these  three 
fats  being  different,  the  melting  point  of  the  fat  of  the  body 
will  differ  according  to  the  relative  proportions  in  which  the 
three  are  present.  Thus  the  subcutaneous  fat  of  man  melts  at 
from  15°  to  22°  or  higher,  the  fat  round  the  kidney  being  firmer 
and  not  melting  until  25°;  the  fat  of  the  dog  melts  at  about 
22°,  that  of  the  goose  at  about  25°,  of  the  ox  at  about  40°,  and 
of  the  sheep  at  50°,  the  less  resistant  fat  of  the  man  and  dog 
containing  relatively  more  olein  than  that  of  the  ox  or  of  the 
sheep. 

§  400.  When  we  come  to  consider  the  question.  By  what 
processes  does  the  fat  make  its  appearance  in  the  fat-cell  ?  we 
are  brought  face  to  face  with  much  the  same  kind  of  problem 
as  that  which  occupied  us  in  dealing  with  glycogen.  On  the 
one  hand  we  may  suppose  that  the  fat  is  brought  to  the  fat-cell 
as  fat  and  is  in  some  way  taken  up  by  the  cell  and  deposited  in 
the  cell-substance  with  little  or  no  change.  On  the  other 
hand,  we  may  suppose  that  the  fat  is  manufactured  by  the  fat- 
cell  in  son^e  such  way  as  mucin  or  pepsin  is  manufactured  by  a 
mucous  or  a  gastric  cell,  out  of  and  by  means  of  its  cell- 
substance,  and  that  the  process  of  fattening,  or  of  producing 
fat  in  fat-cells,  consists  essentially  in  feeding  and  so  building 
up  the  cell-substance  which  subsequently  breaks  down  into  fat, 
and  does  not  consist  merely  in  bringing  fat  within  reach  of  the 
cell.  Which  of  these  views  is  the  true  one,  or  how  far  are  both 
these  operations  carried  on  in  the  animal  body  ? 

In  support  of  the  latter  view  it  may  be  urged  that,  not  only 
the  more  complex  living  substance,  but,  as  we  have  more  than 


Chap,  iv.]      METABOLIC  PROCESSES  OF  THE  BODY.       GU7 

once  urged,  the  simpler  proteicl  constituent  of  living  substance 
obviously  contains  what  we  may  call  a  fatty  radicle,  so  that  we 
might  expect  fat  to  be  formed  out  of  its  metabolism.  And  as 
a  matter  of  fact  not  only  in  adipose  tissue,  but  in  every  part  of 
the  body,  living  substance  is  continuously  giving  rise  to  and 
temporarily  depositing  in  itself  some  amount  of  fat,  and  in 
what  is  known  as  fatty  degeneration  there  seems  to  be  evi- 
dence of  the  formation  of  fat  out  of  proteid  material. 

On  the  other  hand,  we  have  traced  the  fats  taken  as  food, 
and  found  that  they  pass  with  comparatively  little  change  from 
the  alimentary  canal,  chiefly  througii  the  intermediate  passage 
of  the  lacteals,  into  the  blood,  from  which  they  rapidly  dis- 
appear after  a  meal.  We  might  infer  from  this  that  an  excess 
of  fat  thus  entering  the  blood  would  naturally  be  disposed  of 
by  being  simply  stored  up  in  the  available  adipose  tissue  with- 
out any  further  change ;  we  can  imagine  that  the  fat,  not 
immediately  wanted  by  the  economy,  passes  in  some  way  from 
the  blood  to  the  connective  tissue  (the  white  blood  corpuscles 
which  appear  loaded  with  fat  after  a  meal  possibly  acting  as  inter- 
mediaries), and  that  the  connective  tissue  corpuscles  swallow  the 
fat  brought  to  them  after  the  fashion  of  an  amoeba,  not  digesting 
it  but  simply  keeping  it  in  store  until  it  was  wanted  elsewhere. 

What  do  experiments  teach  on  this  matter  ? 

In  the  first  place,  it  is  evident  that  in  an  animal  fattened 
on  ordinary  fattening  food,  only  a  small  fraction  of  the  fat 
stored  up  in  the  body  can  possibly  come  direct  from  the  fat 
of  the  food.  Long  ago,  in  opposition  to  the  views  of  Dumas 
and  his  school,  who  taught  that  all  construction  of  organic 
material,  that  all  actual  manufacture  of  living  substance  or 
even  of  its  organic  constituents,  was  confined  to  vegetables 
and  unknown  in  animals,  Liebig  shewed  that  the  butter  pres- 
ent in  the  milk  of  a  cow  was  much  greater  than  could  be 
accounted  for  by  the  scanty  fat  present  in  the  grass  or  other 
fodder  she  consumed.  He  also  urged,  as  an  argument  in  the 
same  direction,  that  the  wax  produced  by  bees,  which  though 
having  a  different  composition  from  fat  may  be  used  as  an 
analogy,  is  out  of  all  proportion  to  the  wax  or  allied  bodies 
contained  in  their  food,  consisting  as  this  does  chiefly  of  sugar. 
And  it  has  since  been  shewn  in  many  ways  that,  in  fattening 
animals,  the  fat  accumulated  in  the  body  cannot  be  accounted 
for  by  the  fat  which  has  been  taken  in  the  food.  It  has  been 
proved  by  direct  analysis.  Thus  of  two  young  pigs,  as  much 
alike  as  possible,  of  the  same  litter,  one  was  killed  and  analyzed, 
the  amount  of  fat  in  the  body  being  among  other  things  deter- 
mined. The  other  was  fattened  for  a  certain  length  of  time 
on  food  whose  composition  was  known,  and  then  killed  and 
analyzed.  It  was  found  that  for  every  100  parts  of  fat  in  the 
food  472  parts  of  fat  were  stored  up  in  the  body  during  the 


608  THE   FOEMATION   OF  FAT.  [Book  ii. 

fattening  period.  It  is  clear  that  fat  may  be  formed  in  the  body 
out  of  something  which  is  not  fat. 

§  401.  There  are  two  possible  sources  of  this  manufactured 
fat.  The  carbohydrates  of  the  food  form  one  source.  In  treat- 
ing of  digestion  (§  232),  we  referred  to  the  possibility  of  car- 
bohydrates during  digestion  in  the  alimentary  canal  becoming 
by  fermentation  converted  into  butyric  acid ;  and  we  suggested 
that  higher  and  more  complex  members  of  the  same  fatty  acid 
series  might  be  obtained  out  of  carbohydrates  by  somewhat 
analogous  changes,  carried  on  however  not  in  the  alimentary 
canal  by  means  of  foreign  organized  ferments,  but  in  the  tis- 
sues through  the  activity  of  the  tissues  themselves.  We  can- 
not as  yet  trace  out  the  steps  nor  can  we  definitely  point  to 
any  particular  tissues  other  than  the  fat-cells  themselves  as 
the  seats  of  any  such  changes ;  though  it  is  possible  that  the 
fat  may  be  manufactured  in  this  or  that  tissue  and  subse- 
quently transferred,  for  storage,  to  the  fat-cells.  But  there 
can  be  no  doubt  that  carbohydrate  material  does  in  some  way 
or  other  give  rise  to  fat.  A  carbohydrate  diet  is  the  kind  of 
diet  most  efficacious  in  producing  an  accumulation  of  fat  in  the 
body :  sugar  or  starch,  in  some  form  or  other,  is  always  a  large 
constituent  of  ordinary  fattening  foods. 

Another  source  of  fat  is  to  be  found  in  the  proteids.  We 
have  seen  that  the  urea  of  the  urine  practically  represents  the 
whole  of  the  nitrogen  which  passes  through  the  body.  Now 
in  any  given  quantity  of  urea  the  amount  of  carbon  is  far  less 
than  that  found  in  the  quantity  of  proteid  containing  the  same 
amount  of  nitrogen.  Thus  the  percentage  composition  of  the 
two  being  respectively, 

Sulphur. 

143 

100  grms.  of  urea  contain  about  as  much  nitrogen  as  300  grms. 
of  proteid ;  but  the  300  grms.  of  proteid  contain  139  grms. 
(159  —  20)  more  carbon  than  do  the  100  grms.  urea.  Hence 
the  300  grms.  of  proteid  in  passing  through  the  body  and  giv- 
ing rise  to  100  grms.  of  urea,  would  leave  behind  139  grms.  of 
carbon,  in  some  combination  or  other ;  and  this  surplus  of  car- 
bon, if  the  needs  of  the  economy  did  not  demand  that  it  should 
be  immediately  converted  into  carbonic  acid  and  thrown  off 
from  the  body,  might  be  deposited  somewhere  in  the  form  of 
fat.  It  has  been  calculated  that  in  this  way  100  grms.  of  pro- 
teid food  might  furnish  42  grms.  of  fat.  We  have  already 
seen,  in  treating  of  the  action  of  the  pancreatic  juice  (§  210), 
that  there  is  evidence  of  a  fatty  element  (viz.  leucin,  which  is 
amido-caproic  acid,  and  so  belongs  to  the  fatty  acid  series)  being 
thrown  off  from  the  complex  proteid  compound  in  the  very  pro- 


Carbon. 

Hydrogen. 

Oxygen. 

Nitrogen, 

20-00 

6-6Q 

26-67 

46-67 

id     53 

7-30 

23-04 

15-53 

Chap,  iv.]      METABOLIC  PROCESSES  OF  THE  BODY.       609 

cess  of  digestion  ;  and  though,  as  w^e  have  said,  we  have  no  proof 
that  this  action  of  pancreatic  juice  takes  place  largely  in  the 
normal  body,  its  value  as  an  example  is  none  the  less  imjjortant. 

Some  observers  have  pushed  this  view  of  the  production  of 
fat  out  of  proteids  so  far  as  to  insist  that  all  the  fat  formed  in 
the  body  arises  in  this  way  out  of  proteid  material,  and  that 
when  carbohydrate  food  gives  rise  to  the  formation  of  fat  it 
does  so  by  shielding  from  oxidation  the  carbon  moiet}-  of  the 
proteid  food  taken  at  the  same  time  and  thus  permitting  it  to 
be  stored  up  as  fat.  The  carbohydrate  itself,  they  argue,  never 
becomes  fat  but  its  presence  allows  fat  to  be  formed  out  of  pro- 
teid material.  This  view  has  obviously  a  very  important  eco- 
nomical bearing,  since,  if  it  were  true,  it  would  be  useless  to 
increase  the  carbohydrate  material  of  food  for  the  purpose  of  fat- 
tening, unless  a  sufficient  proj)ortion  of  proteid  material  be  given 
at  the  same  time.     It  has  however  been  proved  to  be  untenable. 

§  402.  It  is  clear  then  that  a  construction  of  fat  does  occur 
in  the  body  somewhere.  What  limits  can  we  i^lace  on  the  de- 
gree to  which  this  construction  is  carried?  When  the  food 
contains  sufficient  actual  fat  to  account  for  the  fat  stored  up  in 
the  body,  does  any  construction  of  fat  take  place?  In  the  first 
place  we  find  that  when  the  food  contains  abnormal  fats  such 
as  are  not  present  in  the  body,  spermaceti  for  instance,  or  eru- 
cin  (from  rape-seed  oil),  these  fats  are  not  to  be  found,  or  are 
found  in  very  small  quantity  only,  in  the  fat  which  is  stored 
up  in  the  body  as  a  consequence  of  a  large  supply  of  that  food. 
lu  the  second  place  we  may  call  to  mind  the  statement  previ- 
ously made,  that  the  composition  of  fat  varies  in  different 
animals.  The  fat  of  a  man  differs  from  the  fat  of  a  dog,  even 
if  both  feed  on  exactly  the  same  food,  fatty  or  otherwise. 
Were  the  fat  which  is  taken  as  food  stored  up  as  adipose  tissue 
directly  and  without  change,  recourse  being  had  to  other  sources 
of  food  for  the  construction  of  fat  only  in  cases  where  the  fat 
in  the  food  was  deficient,  we  should  expect  to  find  that  the 
nature  of  the  fat  of  the  body  would  vary  greatly  with  the  food. 
So  far  from  this  being  the  case,  direct  experiment  shews  that 
the  fat  of  the  dog  is,  as  far  as  composition  is  concerned,  very 
largely  independent  of  the  food,  that  the  normal  constituents 
of  fat  make  their  appearance  very  much  as  usual  and  in  very 
much  their  appropriate  proportion,  though  their  proportion  in 
the  food  may  largely  vary,  and  though  some  of  them  may  be 
wholly  absent.  Thus  in  one  experiment  the  fat  of  the  body 
contained  considerable  quantities  of  stearin  after  a  diet  free 
from  stearin,  and  in  another  preserved  the  normal  amount  of 
olein  after  a  diet  free  from  olein.  This  shews  that  the  con- 
structive power  of  the  economy  is,  as  regards  fat,  very  great  ; 
indeed  it  is  even  possible  that  all  the  fat  stored  up  in  the  body 
is  fat  formed  anew. 

.39 


SEC.  7.     THE   MAMMARY   GLAND. 

§  403.  Since  milk  is  a  secretion,  and  indeed  an  excretion, 
the  mammary  gland  ought  not  to  be  classed  as  a  metabolic  tis- 
sue, in  the  limited  meaning  we  are  now  attaching  to  those 
words.  Yet  the  metabolic  phenomena  giving  rise  to  the  secre- 
tion of  milk  are  so  marked  and  distinct,  have  so  many  analogies 
with  the  purely  metabolic  events  which  take  place  in  adipose 
tissue,  and  so  strikingly  illustrate  metabolic  events  in  general, 
that  it  will  be  more  convenient  to  consider  the  matter  here, 
rather  than  in  any  other  connection. 

The  mammary  gland,  formed  like  a  sweat  gland,  of  which 
it  may  be  considered  an  extreme  development,  by  an  ingrowth 
of  the  Malpighian  layer  of  the  epidermis,  is  a  compound  race- 
mose gland,  constructed  after  the  general  plan  of  such  a  gland 
and  thus  composed  of  branching  ducts  ending  in  secreting  al- 
veoli. 

§  404.  The  appearances  presented  by  the  alveoli  differ 
widely  according  as  the  gland  is  one  which  is  being  used  for 
suckling  or  is  one  in  a  resting  or  dormant  condition,  that  is  to 
say  before  any  pregnancy  at  all  has  taken  place  or  in  the  inter- 
val between  two  suckling  periods.  In  the  suckling  gland  each 
alveolus  consists  of  a  basement  membrane,  presenting  the  usual 
characters,  lined  with  a  single  layer  of  cells  leaving  a  wide 
lumen  ;  but  the  appearances  presented  by  the  cells  differ  from 
time  to  time  according  to  circumstances  and  are  not  the  same 
in  all  the  alveoli  at  the  same  time.  We  may  however  distin- 
guish two  conditions  which,  since  they  seem  to  correspond  to 
the  loaded  and  discharged  conditions  of  an  ordinary  gland,  we 
may  call  the  loaded  and  the  discharged  phase  respectively,  con- 
ditions intermediate  between  the  two  being  met  with. 

In  the  discharged  phase  the  alveolus  is  lined  by  a  layer  of 
low  cubical  or  even  flattened  cells,  so  that  the  relatively  large 
area  of  the  alveolus  is  almost  wholly  occupied  by  the  lumen  in 
which  some  of  the  constituents  of  the  milk  may  still  be  retained. 
Each  cell  consists  of  granular  cell-substance  in  which  is  placed 
a  rounded  or  oval  nucleus.     Sometimes  the  free  edge  of  the 

610 


Chap,  iv.]      METABOLIC  PROCESSES  OF  THE  BODY.       611 

cell  is  jagged  and  uneven  as  if  a  portion  of  the  free  border  had 
been  torn  away. 

In  a  fully  loaded  phase  the  appearances  are  very  different. 
The  alveolus  is  now  lined  with  a  layer  of  tall  columnar  cells 
projecting  unevenly  into  the  lumen,  the  outline  of  which  is 
correspondingly  irregular  and  the  area  of  which  is  much  re- 
duced. While  the  broader  base  of  each  cell  rests  on  the  base- 
ment membrane,  the  other  end,  conical  or  irregular,  stretches 
towards  the  centre  of  the  lumen.  Instead  of  one  nucleus,  two 
or  even  more  are  now  present,  one  well  formed  and  normal  be- 
ing placed  nearer  the  base,  and  the  others,  often  shewing  signs 
of  breaking  up  or  degeneration,  nearer  the  free  end.  Some- 
times constrictions  are  seen  whereby  the  free  peripheral  portion 
of  the  cell,  including  one  or  more  of  the  nuclei,  is  apparently 
being  separated  from  the  basal  portion  in  which  the  remaining 
nucleus  is  lodged ;  and  occasionally  portions  or  fragments  of 
cells,  nucleated  or  nucleusless,  may  be  seen  lying  in  the  cavity 
of  the  alveolus.  In  the  cell-substance,  especially  towards  the 
free  border  of  the  cell,  are  numerous  oil  globules  of  various  sizes 
as  well  as  granules  or  particles  of  other  nature ;  some  of  the 
larger  oil  globules  may  be  seen  projecting  from  the  surface  as 
if  about  to  be  extruded  from  the  cell ;  and  in  the  cavity  of  the 
alveolus  oil  globules  with  a  thinner  or  thicker  coating  of  cell- 
substance  are  frequently  present. 

Between  such  a  fully  loaded  phase,  and  a  completely  dis- 
charged phase,  various  intermediate  conditions  may  be  observed, 
the  cells  being  of  greater  or  less  height,  containing  one  nucleus 
only  or  more  than  one,  the  cell-substance  occupied  with  few  or 
with  many  oil  globules  and  other  granules,  and  the  free  border 
more  or  less  jagged. 

§  405.  The  dormant  resting  mammary  gland,  that  for  in- 
stance of  an  animal  which  has  never  been  pregnant,  is  much 
smaller  than  a  suckling  gland,  owing  to  the  alveoli  being  both 
smaller  and  less  numerous.  Each  alveolus  moreover  is  not  a 
cavity  lined  with  a  single  layer  of  epithelium,  but  a  solid  cylin- 
der or  mass  of  comparatively  small,  rounded  or  polyhedral  cells. 
So  long  as  pregnancy  does  not  occur  the  growth  of  these  is 
exceedingly  slow,  and  the  products  of  such  metabolism  as  goes 
on  in  them  are  carried  away  by  the  blood,  so  that  under  normal 
circumstances  no  secretion  takes  place. 

When  pregnancy  occurs  rapid  growth  of  the  mamma  takes 
place,  numerous  new  alveoli  being  formed  by  budding,  but  all 
for  a  time  remaining  solid  cylinders  of  cells.  At  the  approach 
of  the  birth  of  the  offspring,  the  central  cells  undergo  metabolic 
changes,  especially  a  fatty  transformation,  and  either  before  or 
after  birth  are  cast  off,  leaving  a  single  layer  to  line  tlie  alveoli 
and  to  carry  on  the  work  of  secretion  as  described  above.  It 
is  generally  supposed  that  these  shed  cells  su|)ply  the  so-called 


612  THE   NATUEE    OF   MILK.  [Book    ii. 

'  colostrum  corpuscles '  characteristic  of  the  first  milk,  of  which 
we  shall  speak  presently. 

At  the  end  of  lactation  an  absorption  of  some  of  the  alveoli 
takes  place ;  and  in  old  age  still  further  absorption  goes  on 
Avith  great  diminution  of  the  lumina. 

§  406.  In  the  lymphatic  spaces  of  the  connective  tissue 
which  joins  together  the  lobules  of  various  sizes,  surrounds  the 
lobules  and  runs  in  between  the  projecting  blind  ends  of  the 
alveoli  within  the  lobules  leucocytes  are  numerous,  and  some 
of  these  may  make  their  way  through  the  basement  membrane 
and  between  the  secreting  cells  into  the  cavities  of  the  alveoli 
and  so  appear  in  the  milk. 

§  407.  The  nature  of  milk.  Human  milk  has  a  specific 
gravity  of  from  1-028  to  1-034,  and  when  quite  fresh  possesses 
a  slightly  alkaline  reaction.  It  speedily  becomes  acid ;  and 
cow's  milk,  even  when  quite  fresh,  is  sometimes  slightly  acid, 
the  change  of  reaction  taking  place  during  the  stagnation  of 
the  milk  in  the  mammary  ducts. 

The  constituents  of  milk  are: 

1.  Proteids^  viz.  casein,  and  an  albumin,  agreeing  in  its 
general  features  with  ordinary  serum-albumin,  but  which,  since 
it  is  said  to  differ  somewhat  in  its  solubilities  and  rotatory 
power  from  serum-albumin,  has  been  called  lactalhumin.  The 
casein,  as  we  have  seen,  §  185,  undergoes  through  the  action  of 
rennin  a  change  whereby  insoluble  casein  (tyrein)  makes  its 
appearance  and  the  milk  is  curdled.  Casein  may  however  be 
precipitated  in  an  unchanged  form  by  saturating  milk  with 
neutral  salts,  or  by  the  careful  addition  of  acetic  acid  to  diluted 
milk,  or  by  first  adding  to  the  diluted  milk  a  slight  quantity 
of  acetic  acid  and  then  passing  through  it  a  stream  of  carbonic 
acid.  In  the  filtrate  the  presence  of  the  lactalbumin,  which 
occurs  in  small  and  variable  quantities,  may  be  shewn  by  coagu- 
lation with  heat,  or  by  precipitation  with  potassium  ferro- 
cyanide,  &c.  In  the  process  of  curdlmg  the  casein,  as  stated 
in  §  185,  appears  to  be  not  simply  changed  into  tyrein  but  to 
be  split  up  into  tyrein  and  into  another  proteid,  which  unlike 
the  lactalbumin  is  not  coagulated  by  heat  and  which  appears  to 
be  allied  to  albumose.  This  or  a  similar  albumose-like  body  has 
also  been  found  in  small  quantities  even  in  milk  which  has  not 
curdled;  it  has  been  called  lactoprotein.  The  lactalbumin, 
though  coagulated  by  heat  when  isolated,  is  not  so  coagulated 
as  it  exists  in  the  natural  milk,  the  alkalinity  of  the  milk,  which 
is  increased  by  boiling,  preventing  this.  Similarly  casein, 
though  coagulated  by  heat  when  simply  suspended  in  water 
after  being  precipitated,  is  not  coagulated  by  heat  when  it  exists 
in  a  natural  condition  in  milk;  in  these  respects  casein  behaves 
like  alkali-albumin,  which  it  resembles  in  other  features  also. 
Hence  milk  when  boiled  does  not  coagulate  as  a  whole,  though 


Chap,  iv.]      METABOLIC  PROCESSES  OF  THE  BODY.       613 

in  the  superficial  layers  exposed  to  the  air  changes  take  place 
by  which  a  film  or  skin,  derived  chietly  from  the  albumin  but 
partly  from  the  casein,  appears  on  the  surface;  if  this  be  re- 
moved a  fresh  portion  undergoes  the  same  change. 

2.  Fats.  These  are,  in  the  main,  palmitin,  stearin,  and 
olein;  but  other  fats,  supplied  by  butyric  and  other  fatty  acids 
in  combination  with  glycerine,  accompany  the  above  in  small 
quantities.  In  this  respect  the  fat  of  milk  resembles  that  of 
adipose  tissue.  Lecithin  and  cholesterin  are  also  present  in 
very  small  quantity,  as  well  as  a  yellow  colouring  matter. 
The  fat  present  in  milk  differs  in  different  animals  as  to  the 
relative  proportion  of  olein,  palmitin  and  stearin,  and  as  to  the 
kinds  and  relative  amount  of  the  other  scantier  fats. 

The  mixture  of  these  fats,  fluid  at  ordinary  temperatures,  is 
present  in  natural  ;nilk  in  the  form  of  globules  of  various  sizes 
but  for  the  most  part  exceedingly  small  (in  man  from  2jx  to 
5//,).  Milk  is  in  fact  a  typical  emulsion,  and  it  is  the  presence 
of  the  casein  in  the  milk  which  brings  about  the  emulsion. 

On  standing  a  great  deal  of  the  fat  collects  on  the  top  of 
the  milk  in  the  form  of  cream,  but  in  this,  as  in  the  butter 
which  is  formed  from  it,  the  globules  are  still  discrete,  so  long 
at  least  as  the  butter  is  '  fresh. '  By  the  use  of  a  centrifugal 
machine  nearly  the  whole  of  the  fat  may  be  separated  from  the 
plasma. 

3.  Milk  sugar  or  lactose.  This  is  very  apt  to  undergo  fer- 
mentation into  lactic  acid,  through  the  agency  of  an  organized 
ferment;  the  milk  thus  becomes  sour,  and  the  casein  is  precipi- 
tated in  a  fiocculent  form  when  the  acid  is  produced  in  sufficient 
quantity.  Since  the  change  will  take  place  even  when  ever}'^ 
care  is  taken  to  exclude  germs  from  the  atmosphere  ha^dng 
access  to  the  milk,  the  organized  ferments  must  be  present  in 
the  milk  in  the  ducts  of  the  gland. 

4.  Salts.  Though  traces  of  urea  and  kreatinin  have  been 
noted  by  some  observers,  the  extractives  of  milk,  beyond  the 
lecithin  and  cholesterin  already  mentioned,  are  insignificant. 
The  salts  are  of  more  importance;  these  are  chiefiy  calcic 
phosphate,  of  whose  function  in  the  process  of  curdling  we 
spoke  in  §  185,  and  potassic  and  sodic  chlorides,  with  a  small 
quantity  of  magnesic  phosphate.  Sulphates  appear  to  be 
absent.  A  small  quantity  of  an  iron  salt  is  present,  and  traces 
of  sulpho-cyanide  have  been  observed.  Besides  the  phosphorus 
in  the  actual  form  of  phosphates,  milk  contains  a  further  con- 
siderable quantity  of  pliosphorus  in  the  proteids  and  in  the 
nuclein,  as  well  as  some  sulphur  in  the  former.  The  inorganic 
constituents  of  milk  may,  broadly  speaking,  be  said  to  differ 
distinctly  from  those  of  blood,  and  to  much  more  nearly  re- 
semble those  of  the  entire  body. 

The  composition  of  milk  in  the  same  animal  varies  widely 


614  COMPOSITION  OF  MILK.  [Book  ii. 

from  time  to  time,  and  besides  undergoes  marked  changes  dur- 
ing the  period  of  lactation.  The  relative  general  composition 
of  human  milk  and  that  of  the  cow,  the  mare,  and  the  bitch 
may  perhaps  be  shewn  by  the  following  table :  —  but  it  is  diffi- 
cult to  draw  an  average  since  the  individual  analyses  given 
differ  so  much;  the  figures  given  for  casein  and  fat  in  the  milk 
of  the  bitch  may  be  unusually  high. 

Average  Composition  of  Milk  iii  Different  Animals. 


Woman. 

Cow. 

Mare. 

Bitch. 

Casein  &c.     2 

4 

2-5 

10 

Fats               2-75 

4 

2 

10 

Sugar             5 

4-4 

5 

3-5 

Salts                 -25 

•6 

•5 

•5 

Total  Solids 

10 

13 

10 

24 

Water 

90 

8T 

90 

76 

The  quantity  of  milk  secreted  by  a  woman  in  twenty  hours 
at  the  height  of  lactation  has  been  calculated  at  700  to  800  cc. 
A  good  milch  cow  will  yield  about  10  litres  of  milk  per  diem. 

§  408.  Colostrum.  This  is  the  name  given  to  the  milk 
secreted  at  the  beginning  of  a  period  of  lactation,  just  before 
and  for  some  days  after  parturition.  This  milk  dilf ers  from  the 
subsequent  milk  in  microscopical  characters  and  in  chemical 
composition. 

When  ordinary  milk  is  examined  under  the  microscope  hardly 
anything  is  seen  besides  the  fat  globules  except  a  very  few  imper- 
fect cells  or  portions  of  cells  consisting  of  cell-substance  more  or 
less  loaded  with  fat  and  containing  sometimes  a  more  or  less 
altered  nucleus.  A  few  minute  granules,  thought  by  some  to 
be  particles  of  suspended  casein  or  nuclein,  are  however  also 
visible. 

Colostrum  on  the  other  hand  contains  a  large  number  of 
cells  or  corpuscles,  which  have  been  called  '  colostrum  corpus- 
cles.' Some  of  these  closely  resemble  leucocytes,  others  are 
either  cells  of  about  the  same  size,  round  or  irregular,  and  pos- 
sessing a  nucleus,  often  misshapen,  or  are  merely  portions  of 
cell-substance  without  a  nucleus.  In  all  of  them  the  cell-sub- 
stance may  be  loaded  with  fat  globules  or  may  be  fairly  free  from 
fat.  Some  of  these  cells  appear  to  be  undergoing  disintegration ; 
some  may  at  a  favourable  temperature  exhibit  slow  amoeboid 
movements,  and  must  then  at  least  be  regarded  as  living. 

Colostrum  also  differs  from  ordinary  milk  in  containing  not 
only  a  large  quantity  of  albumin  (lactalbumin)  but  also  a  decided 
amount  of  globulin.  In  consequence  of  this  colostrum  differs 
from  milk  inasmuch  as  it  is  distinctly  coagulated  by  heat. 

As  stated  above,  during  the  rapid  growth  l^y  which  the  gland 


Chap,  iv.]     METABOLIC  PROCESSES  OF  THE  BODY.       615 

is  enlarged  preparatory  to  lactation,  the  alveoli  are  at  first  solid 
masses  of  cells  with  little  or  no  Imnen,  and  a  lumen  is  established 
subsequently  by  the  discharge  of  the  central  cells.  It  is  usu- 
ally supposed  that  the  cells  so  discharged,  some  undergoing 
much,  others  comparatively  little  change,  supply  the  colostrum 
corpuscles  just  spoken  of,  and  at  the  same  time  furnish  the 
globulin  and  excess  of  albumin  also  characteristic  of  colostrum. 
But  this  is  not  certain.  The  alveoli  at  this  time  contain  pecul- 
iar cells  resembling  colostrum  corjDuscles  except  that  they  are 
free  from  fat ;  and  it  is  suggested  that  these  being  discharged 
and  taking  up  fat  in  amoeboid  fashion  become  colostrum  cor- 
puscles. Some  regard  the  colostrum  corpuscles  as  simply  leu- 
cocytes which  have  similarly  taken  up  fat. 

§  409.  The  mammary  gland  is  present  both  in  the  female 
and  the  male  child  at  birth ;  and  in  both  sexes  at  and  for  a  few 
days  after  birth  is  thrown,  in  common  with  all  the  other  secret- 
ing glands,  into  secretory  activity,  and  a  small  quantity  of  milk, 
the  "  witches'  milk "  so  called  by  the  Germans,  is  discharged 
from  the  nipple.  This  milk  resembles  in  all  essential  features 
the  milk  of  lactation.  In  both  sexes  this  initial  activity  soon 
passes  off,  the  gland  in  the  female  further  developing  at  puberty, 
but  in  the  male  remaining,  save  in  exceptional  cases,  in  its  infan- 
tile condition  or  somewhat  retrograding. 

§  410.  Tlie  secretion  of  milk.  From  what  has  been  already 
said  it  is  obvious  that  the  secretion  of  milk,  while  resembling 
the  secretion  of  the  other  secreting  glands  Avhich  we  have  studied 
in  being  essentially  an  activity  of  the  epithelium  cells  lining  the 
alveoli,  nevertheless  presents  certain  interesting  features  special 
to  itself.  If  the  account  given  in  §  404  be  a  true  one,  morpho- 
logical changes  in  the  cells  are  more  prominent  than  in  the  case 
of  other  glands ;  and  we  may  interpret  the  appearances  there 
related  somewhat  as  follows.  When  the  discharged  gland  with 
its  low  epithelium  begins  the  work  of  loading,  the  cells  distinctly 
'grow.'  Their  cell-substance  increases  in  bulk,  and  elongating 
projects  into  the  lumen  of  the  alveolus.  At  the  same  time  the 
nucleus  divides  as  if  the  cell  were  about  to  give  birth  to  new 
cells ;  but  at  first  at  all  events  no  division  of  the  cell-substance 
takes  place,  and  the  new  nuclei  lie  imbedded  in  a  common  cell 
body.  The  cell-substance  meanwhile  puts  on  secretory  activ- 
ity ;  it  deposits  in  itself  material  to  form  milk.  Tlie  deposit 
of  fat  is  conspicuous  and  easily  recognized,  but  we  may  fairly 
infer  that  the  other  less  easily  distinguished  proteid  and  carbo- 
hydrate materials  are  deposited  in  the  cell-substance  in  a  similar 
fashion.  Tlien  follows  the  ejection  of  the  prepared  material ; 
and  this  may  take  place  in  one  of  two  ways.  The  oil  globules 
of  fat  may  be  protruded  from  the  cell-substance  much  in  the 
same  way  that  an  am(eba  extrudes  its  excrement,  and  possibly 
other  constituents  of  milk  may  be  ejected  by  a  similar  method. 


616  THE    SECRETION    OF  MILK.  [Book  ii. 

But  besides  this,  the  deferred  cell  division  now  takes  place  in 
a  somewhat  imperfect  fashion,  so  that  portions  of  the  old  cell 
carrying  nuclei  with  them  come  asunder  from  the  rest  of  the 
cell  in  which  a  nucleus  is  left,  and  lie  loose  in  the  lumen  of 
the  alveolus ;  portions  of  cell-substance  free  from  nuclei  appear 
also  to  be  cast  off.  Here,  in  the  lumen  of  the  alveolus,  they 
rapidly  undergo  change ;  the  cell-substance  is  altered  and  dis- 
solved, and  its  load  of  prepared  material,  probably  undergoing 
in  the  act  some  further  change,  is  set  free,  the  nuclei  also  under- 
going change  and  becoming  ultimately  broken  up.  Hence  the 
constituents  of  milk  are  provided  for,  not  only  as  in  other  glands 
by  the  material  with  which  the  cell  loads  itself  and  subsequently 
discharges  into  the  lumen  of  the  alveolus,  but  also  by  the  actual 
substance  of  part  of  the  cell  itself.  The  characteristic  nuclein 
of  the  milk  has  thus  its  origin  in  all  probability  in  the  shed 
nuclei  of  the  secreting  cells,  and  we  may  perhaps  infer  that  the 
still  more  characteristic  casein  exists  in  milk  in  the  form  of 
casein  and  not  of  some  other  proteid  in  consequence  of  this 
intervention  of  the  actual  cell-substance  in  the  formation  of  the 
milk. 

The  secretion  of  milk  differs  from  such  a  secretion  as  that 
of  saliva,  and  approaches  the  formation  of  sebum  inasmuch 
as  the  transformed  cell-substance  is  shed  bodily  to  form  part  of 
the  milk.  We  say  form  part  of  the  milk  because  this  gross  mode 
of  secretion  is  accompanied  by  the  more  ordinary  mode.  The 
cells  are  at  the  same  time  in  the  more  ordinary  way  discharging 
into  the  lumen  water  holding  saline  and  other  constituents  in 
solution.  And  the  peculiar  features  of  milk,  as  we  shall  see 
presently,  correspond  to  this  double  mode  of  secretion.  Per- 
haps however  we  ought  not  to  call  it  a  double  mode,  for  the 
one  method  really  passes  insensibly  into  the  other.  The  dis- 
charge of  sodium  chloride  in  solution  from  every  kind  of  gland, 
of  mucin  from  a  mucous  gland,  of  oil  globules  with  a  proteid 
envelope  from  a  mammary  gland,  and  lastly  of  nucleated  loaded 
cell-substance  from  the  mammary  gland,  present  so  many  dif- 
ferent phases  of  the  same  act  of  secretion. 

§  411.  The  secretion  of  milk  then  would  appear  to  illus- 
trate, even  more  fully  and  clearly  than  do  other  glands,  the 
truth  on  which  we  have  so  often  insisted,  that  a  secretion  is 
eminently  the  result  of  the  metabolic  activity  of  the  secreting 
cell.  The  blood  is  the  ultimate  source  of  milk,  but  it  becomes 
milk  only  through  the  activity  of  the  cell,  and  that  activity 
consists  largely  in  a  metabolic  manufacture  by  the  cell  and  in 
the  cell  of  the  common  things  brought  by  the  blood  into  the 
special  things  present  in  the  milk.  Experimental  results  tell 
the  same  tale.  Thus  the  quantity  of  fat  present  in  milk  is 
largely  and  directly  increased  by  proteid,  but  not  increased, 
on  the  contrary  diminished,  by  fatty  food.     This  effect  on  the 


Chap,  iv.]      METABOLIC  PROCESSES  OF  THE  BODY.       617 

mammary  gland  in  particular  is  in  accordance  with  what  we 
shall  presently  learn  to  be  the  general  effect  on  the  body  of 
proteid  in  contrast  to  that  of  fatty  food  ;  proteid  food  seems  to 
increase  the  general  metabolic  activity  of  the  body  while  fatty 
food  tends  to  lessen  it.  Moreover  the  proteid  food  seems 
actually  to  furnish  the  fat ;  and  we  have  already  suggested  a 
manner  in  which  proteids  may  give  rise  to  fat.  That  the  fat 
of  the  milk  need  not  necessarily  come  from  the  fat  of  the  food 
is  shewn  by  the  following  experiment.  A  bitch  fed  on  meat 
for  a  given  period  gave  off"  more  fat  in  her  milk  than  she  could 
possibly  have  taken  in  her  food  ;  and  this  moreover  took  place 
while  she  was  gaining  in  weight  and  'laying  on  fat,'  so  that 
she  could  not  have  supplied  the  mammary  gland  with  fat  by 
simply  transferring  fat  from  the  store  previously  existing  in 
the  adipose  tissue  of  her  body ;  she  apparently  obtained  the 
fat  ultimately  from  the  proteids  of  her  food.  And  the  histo- 
logical facts  given  above  favour  the  view  that  the  formation  of 
fat  out  of  proteids  in  such  cases  takes  place  in  the  cells  of  the 
alveoli.  The  experimental  then  as  well  as  the  histological  evi- 
dence goes  to  shew  that  the  fat  of  milk  is  formed  in  the  cell 
and  by  the  cell,  and  is  not  simply  gathered  out  of  the  blood. 

The  casein  in  a  similar  way  seems  to  be  formed  by  the  action 
of  the  cell.  It  cannot  be  gathered  out  of  the  blood  since  the 
blood  contains  no  real  casein ;  it  must  be  formed  in  the  gland. 
Some  observers  have  maintained  that  when  milk  is  kept  at  35°, 
the  casein  is  increased  through  some  ferment  action  taking 
place  in  the  milk  itself ;  but  this  seems  not  to  be  the  case,  and 
the  formation  of  casein  must  be  regarded  as  the  result  of  the 
action  of  the  cell.  Even  the  albumin  present  appears  to  be 
not  the  ordinary  serum-albumin  simply  passed  from  the  blood 
through  the  cell  into  the  lumen  of  the  alveolus,  but  the  slightly 
different  lactalbumin.  We  may  perhaps  regard  the  albumin  as 
less  difficult  to  manufacture  than  the  casein ;  and  we  may  ex- 
plain the  fact  that  relatively  to  the  albumin  the  casein  is  less  at 
the  very  beginning  and  especially  toward  the  end  of  lactation, 
by  supposing  that  the  cell  has  in  the  first  case  not  got  into  full 
working  order  and  in  the  second  case  is  waning  in  power.  The 
peptone-like  body  in  milk  though  small  in  quantity  is  a  further 
indication  of  the  proteid  metabolism  taking  place  in  the  cell. 

That  the  milk-sugar,  lactose,  also  is  formed  in  and  by  the 
cell,  is  indicated  by  the  facts  that  it  is  found  in  no  other  part 
of  the  body,  and  that  its  presence  in  milk  is  not  dependent  on 
carbohydrate  food,  for  it  is  maintained  in  abundance  in  the 
milk  of  carnivora  when  these  are  fed  exclusively  on  meat,  as 
free  as  possible  from  any  kind  of  sugar  or  glycogen.  A  glyco- 
gen-like  body  has  moreover  been  described  as  existing  in  the 
cells,  and  it  is  suggested  that  this  body  is  the  antecedent  of 
the  lactose.     We  thus  have  evidence  in  the  mammary  gland 


618  THE    SECEETIOX   OF   MILK.  [Book  ii. 

of  the  formation,  by  the  metabolic  activity  of  the  secreting  cell, 
of  the  representatives  of  the  three  great  classes  of  food-stuffs, 
proteicls,  fats,  and  carbohydrates. 

§  412.  That  both  the  secretion  and  ejection  of  milk  are 
under  the  control  of  the  nervous  system  is  shewn  by  common 
experience,  but  the  exact  nervous  mechanism  has  not  yet  been 
fully  worked  out.  While  erection  of  the  nipple  ceases  when 
the  spinal  nerves  which  supply  the  breast  are  divided,  the  secre- 
tion continues,  and  is  not  arrested  even  when  the  sympathetic 
as  well  as  the  spinal  nerves  are  cut. 


CHAPTER   V. 
NUTRITION. 
SEC.   1.     THE   STATISTICS   OF  NUTRITION.   . 

§  413.  The  preceding  chapter  has  shewn  us  how  wholly 
impossible  it  is  at  present  to  master  the  metabolic  phenomena  of 
the  body  by  attempting  to  trace  out  forwards  or  backwards  the 
several  changes  undergone  by  the  individual  constituents  of  the 
food,  the  body,  or  the  waste  products.  Another  method  is 
however  open  to  us,  the  statistical  method.  We  may  ascertain 
the  total  income  and  the  total  expenditure  of  the  body  during 
a  given  period,  and  by  comparing  the  two  may  be  able  to  draw 
conclusions  concerning  the  changes  which  must  have  taken  place 
in  the  body  while  the  income  was  being  converted  into  the 
output.  Many  researches  have  been  carried  out  by  this  method  ; 
but  valuable  as  are  the  results  which  have  been  thereby  gained, 
they  must  be  received  with  caution,  since  in  this  method  of 
inquiry  a  small  error  in  the  data  may,  in  the  process  of  calcula- 
tion and  inference,  lead  to  most  wrong  conclusions.  The  great 
use  of  such  inquiries  is  to  suggest  ideas,  but  the  views  to  which 
they  give  rise  need  to  be  verified  in  other  ways  before  they  can 
acquire  real  worth. 

Composition  of  the  Aniinal  Body.  The  first  datum  we  require 
is  a  knowledge  of  the  composition  of  the  body,  as  far  as  the 
relative  proportion  of  the  various  tissues  is  concerned.  In  the 
human  body  the  proportions  by  weight  of  the  chief  tissues,  in 
the  fresh  state,  are  probably  somewhat  as  follows : 


Adult  Man. 

Newborn  Baby, 

Skeleton 

15-9  p.c. 

17-7  p.c. 

Muscles 

-il-S   „ 

22-9   „ 

Thoracic  viscera 

1-7   „ 

3-0   „ 

Abdominal  viscera 

7-2   „ 

11-5   „ 

Fat 
Skin 

18-2   „ 
6-9   „ 

20-0    „ 

Brain 

1-9   „ 

15-8   „ 

620  STARVATIOK  [Book  ii. 

An  analysis  of  a  cat  has  given  the  following  result: 

Muscles  and  tendons  45*0  p.c. 

Bones  14-7 

Skin  12-0 

Mesentery  and  adipose  tissue  3-8 

Liver  4-8 

Blood  (escaping  at  death)  6-0 

Other  organs  and  tissues  13-7 

One  point  of  importance  to  be  noticed  in  these  analyses  is 
that  the  skeletal  muscles  form  nearly  half  the  body ;  we  have 
already  seen  (§  38)  that  about  a  quarter  of  the  total  blood  in 
the  body  is  contained  in  them,  and  have  already  (§  382)  insisted 
that  a*  large  part  of  the  metabolism  of  the  body  is  carried  on  in 
the  muscles.  Next  to  the  muscles  we  must  place  the  liver,  for 
though  far  less  in  bulk  than  them,  it  is  subject  to  a  very  active 
metabolism ;  this  is  suggested  by  the  fact  that  it  alone  may 
hold  about  a  quarter  of  the  whole  blood,  and  is  also  indicated 
by  the  numerous  facts  brought  before  us  in  the  preceding 
chapter. 

§  414.  The  Starving  Body.  Before  attempting  to  study  the 
influence  of  food,  it  will  be  useful  to  ascertain  what  changes 
occur  in  a  body  when  all  food  is  withheld.  A  cat  of  known 
weight  was  starved  for  13  days.  At  the  beginning  of  the  period 
the  body  was  presumed  to  have  the  composition  given  above ; 
at  the  close  of  the  period  a  direct  analysis  of  the  body  was  made. 
From  this  it  appeared  that  during  the  hunger  period  the  cat 
had  lost  734  grammes  of  solid  material,  of  which  248*8  were  fat 
and  118*2  muscle,  the  remainder  being  derived  from  the  other 
tissues.  The  percentages  of  dry  solid  matter  lost  by  the  more 
important  tissues  during  the  period  were  as  follows : 


Adipose  tissue 

97*0  p.c 

Spleen 

63*1    „ 

Liver 

56*6   „ 

Muscles 

30*2   „ 

Blood 

17*6   „ 

Brain  and  spinal 

cord 

0-0   „ 

Thus  the  loss  during  starvation  fell  most  heavily  on  the  fat, 
indeed  nearly  the  whole  of  this  disappeared.  Next  to  the  fat, 
the  glandular  organs,  the  tissues  which  we  have  seen  to  be  emi- 
nently metabolic,  suffered  most.  Then  come  the  muscles,  that 
is  to  say,  the  skeletal  muscles,  for  the  loss  in  the  heart  was  very 
trifling ;  obviously  this  organ,  on  account  of  its  importance  in 
carrying  on  the  work  of  the  economy,  was  spared  as  much  as 
possible :  it  was  in  fact  fed  on  the  rest  of  the  body.     The  same 


Chap,  v.]  NUTKITION.  621 

remark  applies  to  the  brain  and  spinal  cord;  in  order  that  life 
might  be  prolonged  as  much  as  possible,  these  important  organs 
were  nourished  by  material  drawn  from  less  noble  organs  and 
tissues.  The  blood  suffered  proportionally  to  the  general  body- 
waste,  becoming  gradually  less  in  bulk  but  retaining  the  same 
specific  gravity ;  of  the  total  dry  proteid  constituents  of  the 
body  17-3  p.c.  was  lost,  which  agrees  very  closely  with  the 
17'6  p.c.  dry  material  (almost  wholly  proteid)  lost  by  the  blood. 
It  is  worthy  of  remark  that  the  tissues  in  general  became  more 
watery  than  in  health.  Similar  observations  on  other  animals 
have  led  to  similar  results,  the  chief  discordance  being  that  in 
some  cases  the  bones  have  suffered  considerable  loss,  in  others 
comparatively  little.  We  might  be  inclined  to  infer  from  these 
data  the  conclusions  that  metabolism  is  most  active  in  the  adi- 
pose tissue,  next  in  such  metabolic  tissues  as  the  hepatic  cells 
and  spleen-pulp,  then  in  the  muscles,  and  so  on ;  but  we  have 
no  warrant  for  these  conclusions.  Because  the  loss  of  cardiac 
and  nervous  tissue  was  so  small,  we  must  not  therefore  infer 
that  their  metabolism  was  feeble ;  they  may  have  undergone 
rapid  metabolism,  and  yet  have  been  preserved  from  loss  of  sub- 
stance by  their  drawing  upon  other  tissues  for  their  material. 
The  great  loss  of  adipose  tissue  is  obviously  to  be  explained 
by  the  fact  that  that  tissue  is  essentially  a  storehouse  of  mate- 
rial, and  the  similarly  great  though  less  loss  in  the  spleen  and 
liver  indicates,  as  indeed  the  facts  recorded  in  the  previous 
chapter  suggest,  that  these  organs  too  serve  in  part  as  store- 
houses. 

During  this  starvation  period,  the  urine  contained  in  the  form 
of  urea  (and  that  practically  represents  all  the  nitrogen  of  the 
urine)  27*7  grammes  of  nitrogen.  Now  the  amount  of  muscle 
which  was  lost  during  the  period  contained  about  16-2  of  nitrogen. 
Thus,  more  than  half  the  nitrogen  of  the  output  during  the 
starvation  period  must  have  come  ultimately  from  the  metabolism 
of  muscular  tissue.  This  fact  we  have  already  used  in  discuss- 
ing the  history  of  urea  and  shall  have  occasion  to  make  further 
use  of  it  hereafter.  The  amount  of  urea  excreted  per  diem  has 
been  observed  in  some  cases  to  fall  very  rapidly  during  the  first 
day  or  two  of  starvation,  and  then  to  diminish  gradually,  though 
often  shewing  considerable  irregularities.  In  other  cases  no 
such  large  initial  fall  has  been  observed.  It  is  most  marked  in 
animals  which  have  been  well  fed  before  the  beginning  of  the 
starvation,  especially  in  those  which  have  had  a  rich  nitrogenous 
diet;  and  the  discharge  in  these  cases  of  an  extra  quantity  of 
urea  in  the  first  day  or  two  is  obviously  connected  with  that 
immediate  effect  of  food  on  the  excretion  of  urea  to  which  we 
have  already  (§  385)  referred  and  to  which  we  shall  have 
to  return  in  speaking  of  what  is  known  as  "luxus-consump- 
tion." 


622  INCOME   AND   OUTPUT.  [Book  ii. 

Comparison  of  Income  and   Output  of  Material. 

§  415.  Method.  We  have  now  to  inquire  how  the  elements 
of  food  are  distributed  in  the  excreta,  in  order  that,  from  the 
manner  of  the  distribution,  we  may  infer  the  nature  of  the 
intermediate  stages  which  take  place  within  the  body.  By 
comparing  the  ingesta  with  the  excreta,  we  shall  learn  what 
elements  have  been  retained  in  the  body,  and  what  elements 
appear  in  the  excreta  which  were  not  present  in  the  food  ;  from 
these  we  may  infer  the  changes  which  the  body  has  undergone 
through  the  influence  of  the  food. 

In  the  first  place,  the  real  income  must  be  distinguished 
from  the  apparent  one  by  the  subtraction  of  the  faeces.  We 
have  seen  that  by  far  the  greater  part  of  the  faeces  is  undigested 
matter,  i.e.  food  which,  though  placed  in  the  alimentary  canal, 
has  not  really  entered  into  the  body.  The  share  in  the  fseces 
taken  up  by  matter  which  has  been  excreted  from  the  blood 
into  the  alimentary  canal,  is  so  small  that  it  may  be  neglected ; 
certainly  with  regard  to  nitrogen,  the  whole  quantity  of  this 
element,  which  is  present  in  the  fseces,  may  be  regarded  as 
indicating  simply  undigested  nitrogenous  matter. 

The  income,  thus  corrected,  will  consist  of  so  much  nitrogen, 
carbon,  hydrogen,  oxygen,  sulphur,  phosphorus,  saline  matters, 
and  water,  contained  in  the  proteids,  fats,  carbohydrates,  salts, 
and  water  of  the  food,  together  with  the  oxygen  absorbed  by 
the  lungs,  skin,  and  alimentary  canal.  The  output  may  be 
regarded  as  consisting  of  (1)  the  respiratory  products  of  the 
lungs,  skin,  and  alimentary  canal,  consisting  chiefly  of  carbonic 
acid  and  water,  with  small  quantities  of  hydrogen  and  car- 
buretted  hydrogen,  these  two  latter  coming  exclusively  from 
the  alimentary  canal ;  (2)  of  perspiration,  consisting  chiefly  of 
water  and  salts,  for  the  dubious  excretion  (see  §  350)  of  urea  by 
the  skin  may  be  neglected,  and  the  other  organic  constituents 
of  sweat  amount  to  very  little ;  and  (3)  of  the  urine,  which  is 
assumed  to  contain  all  the  nitrogen  really  excreted  by  the  body, 
besides  a  large  quantitj'-  of  saline  matters  and  of  Avater.  Where 
great  accuracy  is  required  the  total  nitrogen  of  the  urine  ought 
to  be  determined ;  it  is  maintained,  however,  that  no  errors  of 
serious  importance  arise  when  the  urea  alone,  as  determined 
by  Liebig's  method  (which  was  largely  used  in  the  researches 
forming  the  basis  of  the  present  discussion),  is  taken  as  the 
measure  of  the  total  quantity  of  nitrogen  in  the  urine,  since,  in 
this  method,  other  nitrogenous  bodies  besides  urea  are  precipi- 
tated, and  so  contribute  to  the  quantitative  result.  It  has  been 
and  indeed  still  is  debated  wliether  the  body  may  not  suffer 
loss  of  nitrogen  by  other  channels  than  by  the  urine  and  fneces, 
whether  nitrogen  may  not  leave  the  body  hj  the  skin  or  indeed 
in  a  gaseous  state  by  the  lungs.     The  balance  of  the  conflicting 


Chap,  v.]  NUTRITION".  623 

evidence  seems  however  in  favour  of  the  view  that  no  such  loss 
takes  place.  It  would  appear  that  though  nitrogen,  the  pivot, 
so  to  speak,  of  the  chemical  changes  of  living  beings,  forms  so 
large  a  portion  of  the  atmosphere  and  moreover  is  physically 
diffused  through  the  bodies  of  both  plants  and  animals,  free 
nitrogen  is  of  no  chemical  use  to  either  of  them.  It  enters  into 
and  remains  in  their  bodies  as  an  inert  substance,  and  the  nitrogen 
which  leaves  a  plant  or  animal,  in  a  gaseous  state,  is  simply  a 
part  of  the  same  inert  supply  and  does  not  come  from  the  break- 
ing up  of  the  nitrogenous  substances  of  the  body  or  of  the  food. 

Of  these  elements  of  the  income  and  output,  the  nitrogen, 
the  carbon,  and  the  free  oxygen  of  resj)iration  are  by  far  the 
most  important.  Since  water  is  of  use  to  the  body  for  merely 
mechanical  purposes,  and  not  solely  as  food  in  the  strict  sense 
of  the  word,  the  hydrogen  element  becomes  a  dubious  one, 
the  sulphur  of  the  proteids  and  the  phosphorus  of  the  fats  are 
insignificant  in  amount ;  while  the  saline  matters  stand  on  a 
wholly  different  footing  from  the  other  parts  of  food,  inasmuch 
as  they  are  not  sources  of  energy,  and  pass  through  the  body 
with  comparatively  little  change.  The  body- weight  must  of 
course  be  carefully  ascertained  at  the  beginning  and  at  the  end 
of  the  period,  correction  being  made  where  possible  for  the  faeces. 

It  will  be  seen  that  the  labour  of  such  inquiries  is  con- 
siderable. The  urine,  which  must  be  carefully  kept  separate 
from  the  fseces,  requires  daily  measurement  and  analysis.  Any 
loss  by  the  skin,  either  in  the  form  of  sweat,  or,  in  the  case  of 
woolly  animals,  of  hair,  must  be  estimated  or  accounted  for. 
The  food  of  the  period  must  be  as  far  as  possible  uniform  in 
character,  in  order  that  the  analyses  of  specimens  may  serve 
faithfully  for  calculations  involving  the  whole  quantity  of  food 
taken ;  and  this  is  especially  the  case  when  the  diet  is  a  meat 
one,  since  portions  of  meat  differ  so  much  from  each  other. 
But  the  greatest  difficulty  of  all  lies  in  the  estimation  of  the 
carbonic  acid  produced  and  the  oxygen  consumed.  In  some 
of  the  earlier  researches  this  factor  was  neglected  and  the  varia- 
tions occurring  were  simply  guessed  at,  through  which  very 
serious  errors  were  introduced.  No  comparison  of  income  and 
output  can  be  considered  satisfactory  unless  at  least  the  carbonic 
acid  produced  be  directly  measured  by  means  of  a  respiration 
chamber.  And  in  order  that  the  comparison  should  be  really 
complete,  the  water  given  off  by  the  skin  and  lungs  must  be 
directly  measured  also  ;  but  this  seems  to  be  more  difficult  than 
the  determination  of  the  carbonic  acid. 

In  the  plan  originally  adopted  by  Regnault  and  Reiset  and  fol- 
lowed by  some  other  observers,  the  animal  experimented  on  is 
allowed  to  breathe  a  limited  and  measured  atmosphere.  The  car- 
bonic acid,  as  fast  as  it  is  formed,  is  fixed  and  removed  by  a  strong 


624  INCOME   OUTPUT.  [Book  ii. 

solution  of  caustic  potash,  and  the  normal  percentage  of  oxygen  in 
the  atmosphere  is  maintained  by  a  supply  of  this  gas  from  a  gas- 
holder. In  this  way  both  the  oxygen  consumed  and  the  carbonic 
acid  produced  are  directly  determined,  while  the  continual  supply 
of  fresh  oxygen  prevents  any  evil  effects  due  to  breathing  a  confined 
portion  of  air.  In  order  however  to  avoid  all  possible  errors  arising 
from  a  too  restricted  atmosphere  a  different  method  has  been  adopted 
by  Pettenkofer  and  Voit.  Their  apparatus  consists  essentially  of 
a  large  chamber,  capable  of  holding  a  man  comfortably.  By  means 
of  a  steam-engine  a  current  of  pure  air,  measured  by  a  gasometer, 
is  drawn  through  the  chamber.  Measured  portions  of  the  outgoing 
air  are  from  time  to  time  withdrawn  and  analyzed;  and  from  the 
data  afforded  by  these  analyses,  the  amounts  of  carbonic  acid  (and 
other  gases)  and  of  water  given  off  by  the  occupant  of  the  chamber 
during  a  given  time  are  determined.  The  oxygen  consumed  is  not 
determined  directly ;  but  if  the  total  amounts  of  carbonic  acid  and 
of  water  given  out  by  the  lungs  and  skin  are  ascertained  and  the 
amount  of  urine  and  faeces  known,  the  quantity  of  oxygen  consumed 
may  be  arrived  at  by  a  simple  calculation.  Por  evidently  the  differ- 
ence between  the  terminal  weight  plus  all  the  egesta  and  the  initial 
weight  plus  all  the  ingesta  can  be  nothing  else  than  the  weight  of 
the  oxygen  absorbed  during  the  period.  This  method  in  turn  how- 
ever is  also  open  to  objections,  since  minute  errors  in  the  analyses 
of  the  small  samples  of  air  employed  for  the  determinations  attain 
considerable  dimensions  when  these  are  multiplied  so  as  to  give  the 
changes  in  the  whole  mass  of  air  passed  through  the  apparatus.  It 
seems  moreover  undesirable  to  leave  the  quantity  used  of  so  impor- 
tant an  element  as  oxygen  to  be  determined  by  indirect  calculations. 

Let  us  imagine,  then,  an  experiment  of  this  kind  to  have 
been  completely  carried  out,  that  the  animal's  initial  and  ter- 
minal weights  have  been  accurately  determined,  the  composi- 
tion of  the  food  satisfactorily  known  to  consist  of  so  much 
proteid,  fat,  carbohydrates,  salts,  and  water,  and  to  contain 
so  much  nitrogen  and  carbon,  the  weight  of  the  fseces  and  the 
nitrogen  they  contain  ascertained,  the  nitrogen  of  the  urine 
determined,  the  carbonic  acid  and  water  given  off  by  the  whole 
body  carefully  measured,  and  the  amount  of  oxygen  absorbed 
calculated  —  what  interpretation  can  be  placed  on  the  results  ? 

Let  us  suppose  that  the  animal  has  gained  w  in  weight 
during  the  period.  Of  what  does  w  consist  ?  Is  it  fat  or  pro- 
teid material  which  has  been  laid  on,  or  simply  water  which 
has  been  retained,  or  some  of  one  and  some  of  the  other  ?  Let 
us  further  suppose  that  the  nitrogen  of  the  urine  passed  during 
the  period  is  less,  say  by  x  grammes,  than  the  nitrogen  in  the 
food  taken,  after  deduction  of  course  of  the  nitrogen  in  the 
faeces.  This  means  that  x  grammes  of  nitrogen  have  been 
retained  in  the  body;  and  we  may  with  reason  infer  that  they 
have  been  retained  in  the  form  of  proteid  material.  We  may 
even  go  farther  and  say  that  they  are  retained  in  the  form  of 


Cjiap.  v.]  nutrition.  625 

flesh,  i.e.  of  muscle.  In  this  inference  we  are  going  somewhat 
beyond  our  tether,  for  the  nitrogen  might  be  stored  up  as  some 
proteid  constituent  of  the  hepatic  cells  or  of  some  other  tissue; 
indeed  it  might  be  for  the  while  retained  in  the  form  of  some 
nitrogenous  crystalline  body.  But  this  last  event  is  unlikely; 
and  if  we  use  the  word  '  flesh '  to  mean  nitrogen  (proteid) 
holding  living  substance  of  any  kind,  we  may  without  fear  of 
any  great  error  reckon  the  deflciency  of  x  grammes  nitrogen 
as  indicating  the  storing  up  of  a  grammes  flesh.  There  still 
remain  w  —  a  grammes  of  increase  to  be  accounted  for.  Let 
us  suppose  that  the  total  carbon  of  the  egesta  has  been  found 
to  be  y  grammes  less  than  that  of  the  ingesta;  in  other  words, 
that  y  grammes  of  carbon  have  been  stored  up.  Some  carbon 
has  been  stored  up  in  the  flesh  with  the  nitrogen  just  consid- 
ered; this  we  must  deduct  from  ?/,  and  we  shall  then  have 
«/'  grammes  of  carbon  to  account  for.  Now  there  are  only  two 
principal  forms  in  which  carbon  can  be  stored  up  in  the  body: 
as  glycogen  or  as  fat.  The  former  is  even  in  most  favourable 
cases  inconsiderable,  and  we  therefore  cannot  err  greatly  if  we 
consider  the  retention  of  y'  grammes  carbon  as  indicating  the 
laying  on  of  h  grammes  fat.  If  a  +  5  are  found  equal  to  w, 
then  the  whole  change  in  the  economy  is  known;  if  tv  —  (a  +  H) 
leaves  a  residue  <?,  we  infer  that  in  addition  to  the  laying  on 
of  flesh  and  fat  some  water  has  been  retained  in  the  sjstem. 
li  w  —(a  -\-h^  gives  a  negative  quantity,  then  water  must  have 
been  given  off  at  the  same  time  that  flesh  and  fat  were  laid  on. 
In  a  similar  way  the  nature  of  a  loss  of  weight  can  be  ascer- 
tained, whether  of  flesh,  or  fat,  or  of  water,  and  to  what  extent 
of  each.  The  careful  comparison,  the  debtor  and  creditor 
account  of  income  and  output,  enables  us,  with  the  cautions 
rendered  necessary  by  the  assumptions  just  now  mentioned,  to 
infer  the  nature  and  extent  of  the  bodily  changes.  The  results 
thus  gained  ought  of  course,  if  an  account  is  kept  of  the  water 
taken  in  and  given  out,  to  agree  with  the  amount  of  oxygen 
consumed,  and  also  to  tally  Avitli  the  conclusions  arrived  at 
concerning  the  retention  or  the  reverse  of  water. 

Having  thus  studied  the  method  and  seen  its  weakness  as 
well  as  its  strength,  we  may  briefly  review  the  results  which 
have  been  ol)tained  by  its  means. 

§  416.  Nitrogenous  3fetabolism.  When  a  meal  of  lean  meat, 
as  free  as  possible  from  fat,  is  given  to  a  dog,  which  has  pre- 
viously been  deprived  of  food  for  some  time,  and  whose  body 
therefore  is  greatly  deflcient  in  flesh,  it  might  be  expected  that 
the  larger  part  of  the  food  would  be  at  once  stored  up  to  supply 
pressing  deficiencies,  and  that  only  the  smaller  part  would  be 
immediately  worked  oft"  as  urea  corresponding  to  the  nitroge- 
nous metabolism  going  on  in  the  body  at  the  time,  increased 
somewhat  by  the  labour  thrown  on  the  economy  by  the  very 

40 


626  NITROGENOUS   METABOLISM.  [Book  ii. 

presence  of  the  food.  This  however  is  not  the  case  as  far  as  the 
nitrogen  of  the  meal  is  concerned;  the  larger  portion  passes  off  as 
urea  at  once,  and  only  a  comparatively  small  quantity  is  retained. 
If  the  diet  be  continued,  and  we  are  supposing  the  meals  given  to 
be  large  ones,  the  proportion  of  the  nitrogen  which  is  given  off 
in  the  form  of  urea  goes  on  increasing  until  at  last  a  condition 
is  established  in  which  the  nitrogen  of  the  egesta  exactly  equals 
that  of  the  ingesta.  This  condition,  which  is  spoken  of  as 
"  nitrogenous  equilibrium  "  is  attained  in  dogs  with  an  exclu- 
sively meat  diet  only  when  large  quantities  of  food  are  given, 
and  is  not  easily  maintained  for  any  length  of  time.  The  exact 
quantity  of  meat  required  to  attain  nitrogenous  equilibrium 
varies  with  the  previous  condition  of  the  dog;  equilibrium  is 
frequently  attained  when  1500  or  1800  grms.  of  meat  are  given 
daily. 

Thus  the  most  striking  effect  of  a  purely  nitrogenous  diet  is 
largely  to  increase  the  nitrogenous  metabolism  of  the  body  ;  and 
we  shall  see  later  on  that  it  increases  the  metabolism  not  only 
of  the  nitrogenous  but  also  of  the  other  constituents  of  the  body. 

The  establishment  of  nitrogenous  equilibrium  does  not  mean 
that  a  body-equilibrium  is  established,  that  the  body-weight 
neither  increases  nor  diminishes.  On  the  contrary,  when  the 
meal  necessary  to  balance  the  nitrogen  is  a  large  one,  the  body 
though  it  is  neither  gaining  nor  losing  nitrogen  may  gain  in 
total  weight ;  and  the  increase  is  proved  by  calculation  from 
the  income  and  output,  and  indeed  by  actual  examination  of  the 
body,  to  be  due  to  the  laying  on  of  fat.  The  amount  so  stored 
up  may  be  far  greater  than  can  possibly  be  accounted  for  by  any 
fat  still  adhering  to  the  meat  given  as  food.  We  are  therefore 
driven  to  the  conclusion  that  the  proteid  food  is  split  into  a  urea 
moiety  and  a  fatty  moiety,  that  the  urea  moiety  is  at  once  dis- 
charged, and  that  such  of  the  fatty  moiety  as  is  not  made  use  of 
directly  by  the  body  is  stored  up  as  adipose  tissue.  And  this 
disruption  of  the  proteid,  as  we  have  already  (§  385)  suggested, 
explains  at  the  same  time  why  the  meat  diet  so  largely  and 
immediately  increases  the  urea  of  the  egesta. 

This  characteristic  effect  of  proteid  food  to  increase  the 
metabolism  of  the  body  is  shewn  on  other  animals  besides  the 
dog,  and  not  only  by  means  of  calculations  of  what  is  supposed 
to  take  place  in  the  body,  but  also  by  direct  analysis.  Thus  the 
analysis  of  the  body  of  a  pig,  which  had  been  fed  on  a  known 
diet,  compared  with  the  analysis  with  that  of  another  pig  of  the 
same  litter,  killed  at  the  time  when  the  first  was  put  on  the  fixed 
diet,  gave  as  a  result  that  of  the  dry  nitrogenous  material  of  the 
food  only  about  7  p.c.  was  laid  up  as  dry  proteid  material  during 
the  fattening  period,  though  the  amount  of  proteid  food  was  low. 
This  contrasts  strongly  with  the  amount  of  fat  stored  up  during 
the  same  period  (nee  §  400).     Similar  observations  carried  out 


Chap,  v.]  NUTKITIOK  627 

on  sheep  shewed  that  in  these  animals  the  storing  up  of  nitro- 
genous material  was  even  less,  only  about  4  p.c.  of  that  given 
in  the  food. 

Every  quantity  of  proteid  material  taken  into  the  alimen- 
tary canal  thus  appears  to  affect  proteid  metabolism  in  two  ways. 
On  the  one  hand  it  excites  a  rapid  proteid  metabolism  giving 
rise  to  an  immediate,  and  generally  large,  increase  of  urea;  on 
the  other  hand,  it  serves  to  maintain  the  more  regular  normal 
proteid  metabolism  continually  taking  place  in  the  body,  and  so 
contributes  to  the  normal  regular  discharge  of  urea.  It  seems 
very  natural  to  suppose  that  the  proteid  which  plays  the  first  of 
these  two  parts  is  not  really  built  up  into  the  tissues,  does  not  be- 
come actual  living  substance,  but  undergoes  the  changes  which 
give  rise  to  urea  outside  the  actual  living  substance  in  the  blood 
or  elsewhere  ;  and  we  have  seen  that  under  the  influence  of  the 
pancreatic  juice  some  of  the  proteid  food  may  undergo  the 
greater  part  of  such  a  change  while  it  is  as  yet  within  the  ali- 
mentary canal.  Hence  has  arisen  the  very  natural  distinction 
to  which  we  have  already  alluded  between  "  tissue  proteids  "  or 
"  morphotic  proteids  "  which  are  actuall}^  built  up  into  the  living 
substance  of  the  tissues  and  give  rise  to  urea  through  the  metab- 
olism of  living  substance,  and  "  circulating  proteids  "  or  "  float- 
ing proteids  "  which  do  not  at  any  period  of  their  career  within 
the  body  become  an  integral  part  of  the  living  substance  and  by 
their  metabolism  set  free  energy  not  in  the  way  of  vital  mani- 
festations but  in  the  form  of  heat  only.  We  shall  later  on  con- 
sider what  is  the  exact  meaning  which  we  ought  to  attach  to  the 
words  "  becoming  part  of  the  living  substance ; "  and  hence 
shall  defer  until  then  any  discussion  of  the  appropriateness  of 
these  phrases  and  of  the  validity  of  the  distinction  which  they 
formulate. 

It  was  once  thought,  as  we  shall  presently  see  erroneously, 
that  the  exclusive  purpose  of  proteid  food  was  to  supply  the 
proteid  tissues,  and  that  all  the  energy  set  free  in  the  body  in 
vital  manifestations,  such  as  movement  and  the  like  as  distin- 
guislied  from  heat,  had  its  origin  in  proteid  metabolism,  the 
metabolism  of  fats  and  carbohydrates  giving  rise  to  heat  only. 
Hence  when  it  first  became  known  that  a  certain  proportion  of 
proteid  food  apparently  underwent  a  metabolism  giving  rise  to 
heat  only,  without  becoming  part  of  the  tissues,  this  seemed  to 
be  a  wasteful  expenditure  of  precious  material ;  and  the  metab- 
olism of  this  portion  of  proteid  food  was  accordingly  spoken  of 
as  a  ''  luxus-consumption."  a  wasteful  consumption. 

§  417.  The  Effects  of  Fatty  and  of  Carbohydrate  Food.  Un- 
like those  of  j)roteid  food,  the  effects  of  fats  and  carbohydrates 
cannot  be  studied  alone.  When  an  animal  is  fed  simply  on 
non-nitrogenous  food,  death  soon  takes  place ;  the  food  rapidly 
ceases  to  be  digested,  and  starvation  ensues.     We  can  there- 


628  GELATIN   AS   FOOD.  [Book  ii. 

fore  only  study  tlie  nutritive  effects  of  these  substances  when 
they  are  taken  together  with  proteid  material. 

When  a  small  quantity  of  fat  is  taken,  in  company  with  a 
fixed  moderate  quantity  of  proteid  material,  the  whole  of  the 
carbon  of  the  food  reappears  in  the  egesta.  No  fat  is  stored 
up ;  some  even  of  the  previously  existing  fat  of  the  body  may 
be  consumed.  As  the  fat  of  the  meal  is  increased,  a  point  is 
soon  reached  at  which  carbon  is  retained  in  the  body  as  fat. 
So  also  with  starch  or  sugar ;  when  the  quantity  of  this  is 
small,  there  is  no  retention  of  carbon ;  as  soon  however  as  it 
is  increased  beyond  a  certain  limit,  carbon  is  stored  up  in  the 
form  of  fat  or,  to  a  smaller  extent,  as  glycogen.  Fats  and  car- 
bohydrates therefore  differ  markedly  from  proteid  food  in  that 
they  are  not  so  distinctly  provocative  of  metabolism.  This  is 
exceedingly  well  shewn  in  the  results  obtained  on  the  pig  pre- 
viously mentioned.  It  was  found  that  472  units  of  fat  were 
laid  on  for  every  100  units  of  fat  taken  as  such  in  the  food 
(which  consisting  of  barley-meal,  &c.  contained  a  very  small 
amount  of  actual  fat),  while  for  every  100  units  of  the  total 
dry  non-nitrogenous  food  including  fat,  starch,  cellulose,  &c., 
no  less  than  21  units  were  retained  in  the  body  in  the  form  of 
fat.  No  clearer  proof  than  this  could  be  afforded  that  fat  is 
formed  in  the  body  out  of  something  which  is  not  fat.  In 
§  401  we  have  already  discussed  this  formation  of  fat  out  of 
carbohydrates. 

As  one  might  imagine,  the  presence  of  fat  or  carbohydrates 
in  the  food  is  found  to  decrease  the  amount  of  proteid  material 
necessary  to  establish  nitrogenous  equilibrium.  For  instance, 
with  a  diet  of  800  grms.  meat  and  160  grms.  fat,  the  nitrogen 
in  the  egesta  became  equal  to  that  in  the  ingesta  in  a  dog,  in 
whose  case  1800  grms.  meat  had  to  be  given  to  produce  the 
same  result  in  the  absence  of  fats  or  carbohydrates. 

On  the  other  hand,  it  was  found  that,  with  a  fixed  quantity 
of  fatty  or  carbohydrate  food,  an  increase  of  the  accompanying 
proteid  led  not  to  a  storing  up  of  the  surplus  carbon  contained 
in  the  extra  quantity  of  proteid,  but  to  an  increase  in  the  con- 
sumption of  carbon.  Proteid  food  increases  not  only  proteid 
but  also  non-nitrogenous  metabolism.  This  explains  how  an 
excess  of  proteid  food  may,  by  the  increase  of  general  metabo- 
lism, actually  reduce  the  fat  of  the  body. 

We  have  at  present  no  exact  information  concerning  the 
nutritive  differences  between  fats  and  carbohydrates,  beyond 
the  fact  that  in  the  final  combustion  of  the  two,  while  carbo- 
hydrates require  sufficient  oxygen  to  combine  with  their  carbon 
only,  there  being  already  sufficient  oxygen  in  the  carbohydrate 
itself  to  form  water  with  the  hydrogen  present,  fats  require  in 
addition  oxygen  to  combine  with  some  of  their  hydrogen, 
rience  in  herbivora,  living  largely  on  carbohydrates,  a  larger 


Chap,  v.]  NUTRITION.  629 

portion  of  the  oxygen  consumed  reappears  in  the  carbonic  acid 
of  tJie  egesta  than  in  carnivora,  in  which  animals,  living  chiefly 
on  proteids  and  fats,  more  of  it  leaves  the  body  combined  with 
hydrogen  to  form  water.  This  relation  of  the  oxygen  to  the 
carbonic  acid  is  often  expressed  as  the  quotient  of  the  volume 
of   the  carbonic  acid  expired  divided  by  the  volume  of   the 

oxygen  consumed,  the  'respiratory  quotient,'  -^,  which  is  in 

herbivora  about  -9  and  in  carnivora  about  -6  or  -7.  When  a 
herbivorous  animal  starves,  it  feeds  on  its  own  fat,  and  under 
these  circumstances  the  respiratory  quotient  falls  to  the  car- 
nivorous standard  ;  and  indeed  many  circumstances  affect  this 
respiratory  quotient.  The  carbohydrates  are  notably  more 
digestible  than  the  fats,  but  on  the  other  hand  the  fats  contain 
more  potential  energy  in  a  given  weight.  As  to  the  nutritive 
difference  between  starch  and  sugar,  we  know  nothing  very 
definite  ;  it  has  been  thought  however  that  cane-sugar  is  rather 
more  fattening  than  starch. 

§  418.  The  Effects  of  G-elathi  as  Food.  It  is  a  matter  of 
common  experience  that  gelatin  will  not  supply  the  place  of 
proteids  as  a  constituent  of  food.  Animals  fed  on  gelatin 
together  with  fat  or  carbohydrates  die  very  much  in  the  same 
way  as  when  they  are  fed  on  non-nitrogenous  material  alone. 
Nevertheless  it  would  appear,  as  might  be  expected,  that  the 
presence  of  gelatin  in  food  is  not  without  effect.  Thus  nitro- 
genous equilibrium  is  established  at  a  lower  level  of  real  proteid 
food  when  gelatin  is  added.  In  a  dog,  moreover,  fed  on  a  diet 
of  gelatin  and  fat,  the  excess  of  nitrogen  in  the  excreta  over 
that  in  the  ingesta  is  less  than  when  the  same  dog  is  fed  on  a 
diet  of  fat  alone  ;  that  is  to  say,  the  gelatin  has  sheltered  from 
metabolism  some  proteid  constituents  of  the  body;  and  the 
consumption  of  fat  seems  also  to  be  lessened  by  the  presence  of 
gelatin.  These  facts  become  intelligible  if  we  suppose  that 
gelatin  is  rapidly  split  up  into  a  urea  and  a  fat  moiet}^  in  the 
same  way  that  we  have  seen  a  certain  quantity  of  proteid  ma- 
terial to  be.  It  is  this  direct  destructive  metabolism  of  proteid 
matter  which  gelatin  can  take  up;  it  seems  however  unable  to 
imitate  the  other  function  of  proteid  matter,  and  to  take  part 
in  the  formation  of  living  substance ;  or  in  the  phraseology  of  a 
preceding  paragraph  (§  416),  it  can  take  the  place  of  circulat- 
ing but  not  of  tissue  proteid.  What  is  the  cause  of  this  differ- 
ence, we  cannot  at  present  say. 

§  419.  Peptone  as  Food.  Since  proteids  are  at  least  largely, 
as  we  have  seen  (§  250),  converted  into  and  absorbed  as  pep- 
tone, and  since  as  we  have  also  seen  the  peptone  appears  during 
the  very  act  of  absorption  to  be  reconverted  into  some  other 
form  of  proteid  matter,  possibly  serum-albumin,  it  might  seem 
natural  to  suppose  tliat  peptone  given  as  food  would  as  far  as 


630  SALTS   AS   FOOD.  [Book  ii. 

metabolism  is  concerned  play  the  same  part  as  other  proteids. 
Nevertheless,  some  observers  have  maintained  with  regard  to 
both  peptones  and  the  allied  albumoses  that,  like  gelatin,  these 
bodies  ' '  can  take  the  place  of  circulating  but  not  of  tissue 
proteid."  On  the  whole,  however,  the  evidence  goes  to  shew 
that  animals  can  '  lay  on  flesh '  when  the  proteid  in  their  food 
consists  entirely  of  peptone  or  albumose.  A  difficulty,  apper- 
taining to  digestion,  prevents  any  large  substitution  of  peptone 
for  ordinary  proteids,  since  as  might  be  expected  diarrhoea  is 
apt  to  be  set  up. 

§  420.  TJie  Effects  of  Salts  as  Food.  All  food  contains, 
besides  the  substances  possessing  potential  energy,  which  we 
have  just  studied,  certain  saline  matters,  organic  and  inorganic, 
having  in  themselves  little  or  no  such  potential  energy,  but 
yet  either  absolutely  necessar}^  or  highly  beneficial  to  the  body. 
These  must  have  important  functions  in  directing  the  metabo- 
lism of  the  body  :  the  striking  distribution  of  them  in  the  tissues, 
the  preponderance  of  sodium  and  chlorides  in  blood-serum  and 
of  potassium  and  phosphates  in  the  red  corpuscles  for  instance, 
must  have  some  meaning ;  but  at  present  we  are  in  the  dark 
concerning  it.  The  element  phosphorus  seems  no  less  im- 
portant from  a  biological  point  of  view  than  carbon  or  nitro- 
gen ;  it  is  as  absolutely  essential  for  the  growth  of  a  lowly 
being  like  Penicillium  as  for  man  himself.  We  find  it  proba- 
bly playing  an  important  part  as  the  conspicuous  constituent  of 
lecithin  and  other  complex  fats  belonging  to  the  nervous  sys- 
tem, we  find  it  prominent  in  the  peculiar  body  nuclein,  we  find 
it  peculiarly  associated  with  the  proteids ;  but  we  cannot  ex- 
plain its  rdle.  The  element  sulphur,  again,  is  only  second  to 
phosphorus,  and  we  find  it  as  a  constituent  of  nearly  all 
proteids ;  but  we  cannot  foretell  the  exact  changes  which 
would  take  place  in  the  economy  if  all  the  sulphur  of  the  food 
were  withdrawn.  In  the  keratin  of  the  epidermis  and  its  ap- 
pendages, hairs,  &c.,  it  is  probably  undergoing  excretion,  though 
its  presence  in  this  body  may  have  to  do  with  the  peculiar 
physical  characters  of  corneous  epithelium. 

We  know  that  the  various  saline  matters  are  essential  to 
health,  that  when  they  are  not  present  in  proper  proportions 
nutrition  is  affected.  Dogs  fed  on  food,  freed  as  much  as  pos- 
sible from  all  saline  matters,  but  otherwise  abundant,  with  a 
proper  proportion  of  the  food-stuffs,  soon  exhibit  symptoms 
shewing  that  the  metabolism  of  their  tissues,  especially  of  their 
central  nervous  system,  is  going  wrong ;  they  suffer  from 
weakness,  soon  amounting  to  paralysis,  and  are  often  carried 
off  by  convulsions.  And  more  or  less  similar  derangements  of 
nutrition  follow  the  absence  or  a  deficiency  of  individual  salts. 
During  starvation  these  various  salts  continue  to  be  discharged 
from  the  body ;  in  some  way  or  other  they  are  carried  along 


Chap,  v.]  NUTRITION^.  631 

in  tlie  metabolic  stream,  and  their  presence  is  in  some  way 
essential  to  the  various  metabolic  processes ;  hence  they  need 
to  be  always  present  in  daily  food.  In  what  way  it  is  that 
they  thus  direct  metabolism  we  do  not  know ;  we  are  aware 
that  the  properties  and  reactions  of  various  proteid  substances 
are  closely  dependent  on  the  presence  of  certain  salts ;  but 
beyond  this  we  know  very  little.  The  inorganic  salts  are 
those,  the  nutritive  value  of  which  has  been  chiefly  studied  by 
experiment ;  but  we  have  reason  to  believe  that  the  organic 
salts,  or  extractives,  which  are  present  in  greater  or  less  quan- 
tity in  all  food  of  both  vegetable  and  animal  origin,  are  no  less 
essential  to  the  proper  metabolic  activities  of  the  body.  The 
undoubted  connection  of  scurvy  with  the  lack  of  fresh  vegeta- 
ble food,  other  conditions  helping,  may  perhaps  turn  in  j^art 
on  this,  for  the  evidence  that  the  disease  is  due  to  the  defi- 
ciency of  potash  alone  is  not  conclusive. 

Lastly,  water  has  an  effect  on  metabolism,  as  shewn,  among 
other  things,  by  the  fact  that  when  the  water  of  a  diet  is 
increased,  the  urea  is  increased  to  an  extent  beyond  that  which 
can  be  explained  by  the  increase  of  fluid  increasing  the  facilities 
of  mere  excretion. 


SEC.   2.     THE  ENERGY   OF   THE   BODY. 

The  Income  of  Energy. 

§  421.  Broadly  speaking,  the  animal  body  is  a  machine  for 
converting  potential  into  actual  energy.  The  potential  energy 
is  supplied  by  food  ;  this  the  metabolism  of  the  body  converts 
into  the  actual  energy  of  heat  and  mechanical  labour.  We  have 
in  the  present  section  to  study  what  is  known  of  the  laws  of 
this  conversion,  and  of  the  distribution  of  the  energy  set  free. 

Neglecting  all  subsidiary  and  unimportant  sources  of  energy, 
we  may  say  that  the  income  of  animal  energy  consists  in  the 
oxidation  of  food  into  its  waste  products,  viz.  the  oxidation  of 
proteids,  fats  and  carbohydrates  into  urea,  carbonic  acid  and 
water.  A  principle  laid  down  by  the  chemist  teaches  that  the 
potential  energy  of  any  body,  considered  in  relation  to  any 
chemical  change  which  it  may  undergo,  is  the  same  when  the 
final  result  is  the  same,  whether  that  result  be  gained  at  one 
leap  or  by  a  series  of  steps ;  that,  for  instance,  the  energy  set 
free  by  the  oxidation  of  1  grm.  of  fat  into  carbonic  acid  and 
water  is  the  same,  whatever  the  changes  forwards  or  backwards 
which  the  fat  undergoes  before  it  finally  reaches  the  stage  of 
carbonic  acid  and  water;  and  similarly,  that  the  energy  available 
for  the  body  in  1  grm.  of  dry  proteid  is  the  energy  given  out 
by  the  complete  combustion  of  that  1  grm.,  less  the  energy 
given  out  by  the  complete  combustion  of  that  quantity  of  urea 
to  which  the  1  grm.  of  proteid  gives  rise  in  the  body.  Taking 
this  as  our  guide  we  can  readily  calculate  the  amount  of  poten- 
tial energy  contained  in  an  average  24  hours'  diet,  and  thus 
obtain  the  average  daily  income  of  energy.  For  the  potential 
energy  of  most  of  the  substances  used  as  food  has  been  deter- 
mined by  direct  calorimetric  observations  ;  and  the  several 
determinations,  though  they  vary  somewhat,  agree  sufficiently 
closely  to  serve  as  data  for  the  calculations  in  question. 

The  total  combustion  of  the  following  substances  has  given 
for  one  gramme  of  each  substance  the  following  results  expressed 
in  calories,  that  is  in  gramme-degree  units  of  heat. 

632 


Chap,  v.]  NUTRITION.  633 

Meat,  free  from  fat,  5103,  and  5324.  Fibrin  5511.  Egg- 
albumin  5579.  Thus,  taking  round  numbers  we  may  say  that 
1  grm.  of  proteid  material  contains  5000  or  5500  calories  of 
potential  energy,  according  as  we  use  the  lower  or  higher 
determinations . 

Fat  of  beef  or  mutton  9069,  9365,  9423.  Butter  7267  or 
9192.  Again  in  round  numbers  we  may  say  that  1  grm.  of  fat 
contains  about  9000  calories. 

Arrowroot  (nearly  pure  starch)  3912.  Starch  4123.  Cel- 
lulose 4146.  Dextrose  3692.  Cane  Sugar  3866.  Here  again, 
taking  round  numbers,  we  shall  not  be  far  wrong  in  saying 
that  the  potential  energy  of  1  grm.  of  carbohydrate  material  is 
about  4000  calories. 

The  combustion  of  1  grm.  of  urea  sets  free  an  amount  of 
energy  which  has  been  determined  by  one  observer  at  2206,  by 
another  as  2465  calories.  We  have  seen  (§  401)  that  1  grm. 
of  proteid  gives  rise  in  the  body  to  ^  grm.  urea.  Hence,  to 
obtain  the  energy  of  1  grm.  proteid  material  available  for  the 
economy,  we  must  deduct  from  its  total  potential  energy,  one 
third  the  potential  energy  of  1  grm.  urea,  that  is,  in  round 
numbers  700  or  800  calories.  This  will  give  us  5000  —  700,  or 
5500  —  800,  that  is  4300  or  4700  calories,  according  as  we  take 
the  lower  or  higher  data ;  or  we  may  take  as  a  mean  4500 
calories.     The  data  then  so  far  are  as  follows, 

1  grm.  proteid  4500  calories. 

1  grm.  fat  9000 

1  grm.  carbohydrate  4000        ,, 

The  average  diet  of  an  average  man,  that  is  the  average 
amount  of  each  food-stuff  respectively  taken  daily,  may  be 
determined  experimentally  or  statistically.  Thus  a  man  may 
determine  by  a  series  of  trials  the  diet  on  which,  while  neither 
losing  nor  gaining  weight  and  maintaining  '  nitrogenous  equi- 
librium,' §  416,  he  enjoys  good  health.  Or  an  average  may  be 
struck  of  a  large  number  of  diets  used  by  various  people.  We 
shall  have  something  to  say  of  this  latter  statistical  method 
when  we  come  to  speak  of  diet.  For  the  present  purpose  we 
may  use  one  arrived  at  experimentally  which  we  will  speak 
of  as  Ranke's  diet,  since  it  was  determined  by  a  physiologist  of 
that  name  from  observations  on  himself.  It  was  composed 
of  100  grm.  proteid,  100  grm.  fat,  240  grm.  carbohydrate. 
Such  a  diet  would  give 

100  grm.  proteid  (4500)  450,000  calories. 

100  grm.  fat  (9000)  900,000 

240  grm.  carbohydrate  (4000)  960,000 

2,810,000 


634  THE   POTENTIAL   ENEEGY   OF   FOOD.     [Book  ii. 

If  we  translate  the  units  of  heat  into  units  of  work,  the 
2,310,000  gramme-degree,  or  2,310  kilogramme-degree  calories 
will  give  us  about  980,000,  or,  in  round  numbers,  somewhere 
about  one  million  kilogramme-meters. 

We  may,  in  passing,  call  attention  to  the  fact  that  the  pro- 
teids  supply  a  relatively  small  part  of  the  total  energy,  and 
that  the  share  contributed  by  the  large  mass  of  carbohydrates 
is  not  much  greater  than  that  belonging  to  the  much  smaller 
quantity  of  fat.  In  the  average  diet  obtained  by  the  statistical 
method,  in  which  the  data  are  largely  drawn  from  public  insti- 
tutions, the  (cheaper)  carbohydrates  are  still  further  increased 
at  the  expense  of  the  (dearer)  fats,  a  change  which  may  tend 
to  reduce  somewhat  the  total  energy ;  but  this  does  not  mate- 
rially affect  the  broad  result  just  given. 


The  Expenditure. 

§  422.  There  are  two  ways  only  in  which  energy  is  set 
free  from  the  body  :  mechanical  labour  and  heat.  The  body 
loses  energy  in  producing  muscular  work,  as  in  locomotion  and 
in  other  kinds  of  labour,  in  the  movements  of  the  air  in  respira- 
tion and  speech,  and,  though  to  a  hardly  recognizable  extent, 
in  the  movements  of  the  air  or  contiguous  bodies  by  the  pulsa- 
tions of  the  vascular  system.  The  body  loses  energy  in  the 
form  of  heat  by  conduction  and  radiation,  by  respiration  and 
perspiration,  and  by  the  warming  of  the  urine  and  faeces.  All 
the  internal  work  of  the  body,  all  the  mechanical  labour  of  the 
internal  muscular  mechanisms  with  their  accompanying  fric- 
tion, all  the  molecular  labour  of  the  nervous  and  other  tissues, 
is  converted  into  heat  before  it  leaves  the  body.  The  most 
intense  mental  action,  unaccompanied  by  any  muscular  mani- 
festations, the  most  energetic  action  of  the  heart  or  of  the 
bowels,  with  the  slight  exceptions  mentioned  above,  the  busiest 
activity  of  the  secreting  or  metabolic  tissues,  all  these  end  sim- 
ply in  augmenting  the  expenditure  in  the  form  of  heat. 

A  normal  daily  expenditure  in  the  way  of  mechanical  labour 
can  be  easily  determined  by  observation.  Whether  the  work 
take  on  the  form  of  walking,  or  of  driving  a  machine,  or  of 
any  kind  of  muscular  toil,  a  good  day's  work  may  be  put  down 
at  about  150,000  kilogramme-meters. 

The  normal  daily  expenditure  in  the  way  of  heat  cannot  be 
so  readily  determined.  Direct  calorimetric  observations  on 
living  structures  are  in  all  cases  attended  with  many  difficul- 
ties and  subject  to  many  sources  of  error.  These  are  very 
great  when  the  observations  are  made  on  the  whole  body,  even 
in  the  case  of  small  animals  ;  and  observations  made  by  placing 
a  part  only  of  the  body,  an  arm  or  leg  for  example,  in  the  calo- 


CiiAi'.  v.]  NUTRITION.  636 

rimeter,  and  from  the  data  thus  gained  calculating  the  heat 
produced  by  the  whole  body,  are  subject  to  additional  sources 
of  error.  Improved  methods,  however,  especially  of  recent 
years,  have  so  far  eliminated  many  sources  of  error  that  the 
results  obtained  by  observations  on  the  whole  body  may  be 
received  with  increasing  confidence. 

The  calorimeters  usually  employed  in  chemical  operations,  in 
measuring  for  instance  the  heat  given  out  in  chemical  changes,  are 
unsuitable  for  experiments  on  living  animals.  Such  are  the  mer- 
cury-calorimeter, in  which  the  chemical  action  to  be  studied  is  made 
to  take  place  in  the  midst  of  a  mass  of  mercury,  from  the  consequent 
expansion  of  which  through  the  heat  taken  up  the  amount  of  heat 
given  out  is  calculated,  or  the  ice-calorimeter  in  which  in  a  similar 
way  the  heat  given  out  is  calculated  from  the  amount  of  ice  melted. 
The  latter  has  been  used  for  physiological  purposes,  but  an  animal 
surrounded  by  ice  is  under  such  abnormal  conditions  that  the  results 
are  of  little  value.  The  methods  usually  adopted  by  physiologists 
are  as  follows. 

In  one  method,  the  water-calorimeter,  the  animal  is  placed  in  a 
metal  chamber  surrounded  by  a  jacket  filled  with  water.  The  heat 
given  out  by  the  animal  warms  the  water  in  the  jacket,  and  the 
amount  given  out  is  calculated  upon  the  increase  of  the  temperature 
of  the  water.  By  supplying  the  animal  with  air  through  a  long 
spiral  tube  passing  through  the  water-jacket,  the  heat  given  out  in 
the  expired  air  is  prevented  from  being  lost. 

This  method  may  be  employed  in  a  simpler  form,  when  the  heat 
given  out  by  a  part  of  the  body,  the  arm  or  leg  for  instance,  is  all 
that  has  to  be  determined.  The  part  is  then  merely  placed  in  a 
bath  of  water,  from  the  changes  of  temperature  of  which  the  amount 
given  out  is  calculated.  And  this  modification  of  the  method  may 
with  due  precautions  be  employed  for  the  whole  body. 

In  Rosenthal's  calorimeter  the  chamber  in  which  the  body  or 
part  of  the  body  is  placed  is  surrounded  by,  not  a  water-jacket,  but 
an  air-jacket,  which  tlu;s  serves  as  an  air-calorimeter.  The  instru- 
ment consists  essentially  of  three  concentric  copper  cylinders ;  the 
inner  one  contains  the  animal  (or  other  source  of  heat)  ;  the  outer 
one  serves  merely  as  a  casing  to  protect  those  inside  from  changes 
of  temperature  due  to  currents  of  air  and  the  like ;  and  the  middle 
one  encloses  an  air  space  between  itself  and  the  inner  one.  There 
are  special  arrangements  for  closing  the  cylinders  after  the  intro- 
duction of  the  animal,  and  for  supplying  the  animal  with  air  for 
breathing  purposes.  With  the  air-jacket,  or  space  between  the  inner 
and  middle  cylinders,  are  connected  a  manometer  and  a  thermome- 
ter. When  an  animal  (or  other  source  of  heat)  is  placed  in  the 
inner  cylinder,  the  temperature  and  the  ]iressure  of  the  air  in  the 
air-jacket  are  increased;  and  from  the  amounts  of  increase  measured 
by  the  thermometer  and  the  manometer  the  amount  of  heat  given 
out  from  the  animal  is  calculated. 

The  calorimeters  of  D'Arsonval  and  Rubner  are  constructed  on 
very  similar  principles. 


636  UEEA   AND   MUSCULAR   WORK.  [Book  n. 

Various  attempts  have  been  made  to  ascertain  the  amount 
of  heat  given  out  by  the  body  in  an  indirect  manner,  as  for 
instance  by  calculating  the  heat  given  out  by  the  oxidation  of 
the  food.  As  trustworthy  as  any  is  the  plan  of  simply  sub- 
tracting the  normal  daily  mechanical  expenditure  from  the 
normal  daily  income.  Thus,  150,000  k.-m.  subtracted  from 
one  million  k.-m.  gives  850,000  k.-m.  as  the  daily  expenditure 
in  the  form  of  heat ;  i.e.  between  one-fifth  and  one-sixth  of  the 
total  income  is  expended  as  mechanical  labour,  the  remaining 
four-fifths  or  five-sixths  leaving  the  body  in  the  form  of  heat. 
The  results  given  by  direct  calorimetric  observations  and  by 
other  calculations  give  somewhat  higher  figures  than  these ; 
and  indeed  these  may  probably  be  taken  as  under  rather  than 
over  the  true  amount.  In  any  case  they  are  to  be  regarded  as 
furnishing  hardly  more  than  a  rough  average  estimate  for  a 
man  of  average  build  and  weight,  taking  an  average  amount 
of  average  food  and  doing  an  average  amount  of  work. 

§  423.  The  Energy  of  Mechanical  Work.  We  have  already 
in  treating  of  muscle  and  elsewhere  partly  discussed  this  sub- 
ject, but  may  here  say  the  rest  that  has  to  be  said. 

The  older  writers,  even  after  it  had  been  proved  that  the 
animal  body  was  constructive  so  far  as  the  formation  of  fat  was 
concerned,  still  held  to  the  distinction  between  nitrogenous  or 
plastic  and  non-nitrogenous  or  respiratory  food.  Put  broadly, 
this  view  was  that  all  the  nitrogenous  food  went  to  build  up  the 
proteid  tissues,  the  muscular  flesh  and  the  like,  and  that  the 
nitrogenous  egesta  arose  solely  from  the  functional  metabolism 
of  these  tissues,  while  the  non-nitrogenous  food  was  used  with 
equal  exclusiveness  for  respiratory  or  calorific  purposes,  being 
either  directly  oxidized  in  the  blood  or,  if  present  in  excess, 
stored  up  as  fatty  tissue.  According  to  this  view  the  two  classes 
of  income  corresponded  exactly  to  the  two  forms  of  expenditure. 
We  have  already  urged  several  objections  against  this  view. 
We  have  seen  that  in  the  blood  itself  very  little  oxidation  takes 
place,  that  it  is  the  active  tissue,  and  not  the  passive  blood- 
plasma,  which  is  the  seat  of  oxidation.  We  have  further  seen 
that  proteid  food  may  undoubtedly  be,  in  the  above  sense, 
respiratory  and  incidentally  give  rise  to  the  storing-up  of  fat. 
One  division  of  the  view  is  thereby  overthrown.  We  have  now 
to  inquire  whether  the  other  division  holds  good,  whether 
muscle  and  the  other  proteid  tissues  are  fed  exclusively  on  the 
proteid  material  of  food,  and  whether  muscular  energy  comes 
exclusively  from  the  metabolism  of  the  proteid  constituents  of 
muscle.  We  have  already  seen  (§  60)  that  when  the  muscle 
itself  is  examined,  we  find  no  proof  of  nitrogenous  waste,  but, 
on  the  other  hand,  clear  evidence  of  the  production  of  non- 
nitrogenous  bodies,  such  as  carbonic  acid.  And  when  we  ask 
the  question.  Does  muscular  exercise  proportionately  increase 


CiiAp.  v.]  NUTRITION.  637 

the  urea  given  off  by  the  body  as  a  whole  ?  for  this,  according 
to  the  theory  in  question  it  certainly  ought  to  do,  the  evidence 
we  can  obtain,  though  somewhat  varying,  gives  on  the  whole  a 
decidedly  negative  answer. 

In  the  majority  of  observations  no  marked  change  at  all  in 
the  amount  was  met  with ;  indeed  in  some  cases  there  was  a  dis- 
tinct decrease,  followed  by  an  increase  on  the  following  days. 
Some  observers  however  found  a  very  marked  increase,  and  this 
was  especially  the  case  when  the  subject  under  observation  took 
a  large  amount  of  food  and  performed  very  severe  labour.  On 
the  whole  the  various  results  obtained  by  different  observers 
justify  the  conclusion  that  exercise  by  itself,  even  when  severe. 
does  not  necessarily  increase  the  amount  of  urea  excreted,  but 
that  conditions  may  obtain  in  which  such  an  increase  undeniably 
occurs.  We  may  draw  the  further  conclusion  that  experiments 
of  this  kind  do  not  supply  the  right  method  for  determining  the 
point  at  issue.  It  must  be  remembered  that  it  is  not  the  muscles 
alone  which  feel  the  influence  of  the  labour  ;  the  circulation  and 
indeed  the  whole  body  are  affected  by  it.  If  we  suppose  a  large 
part  or  even  only  some  part  of  the  urea  to  come  from  other  than 
muscular  metabolism,  from  changes  in  the  hepatic  cells  for 
instance,  we  should  expect  that  these  changes,  and  with  them 
the  amount  of  urea  discharged,  woidd  be  influenced  by  labour, 
especially  by  severe  labour. 

In  no  case  has  a  direct  relation  between  the  amount  of  labour 
and  amount  of  urea  been  observed.  More  than  this,  the  follow- 
ing experience  lands  us  in  an  absurdity  if  we  suppose  the  whole 
energy  of  muscular  work  to  arise  from  proteid  metabolism. 
Two  observers  performed  a  certain  amount  of  work  (an  ascent 
of  a  mountain)  on  a  non-nitrogenous  diet,  and  estimated  the 
amount  of  urea  passed  during  the  period.  Assuming  the  urea 
to  represent  the  oxidation  of  so  much  proteid  matter,  Avhich 
oxidation  represented  in  turn  so  much  energy  set  free,  they 
found  that  whereas  the  actual  work  done  amounted  to  129-026 
and  148-656  kilogram. -kilometers,  for  each  observer  respec- 
tively, the  total  energy  available  from  proteid  metabolism  dur- 
ing the  period  was  in  the  case  of  the  first  68-69,  and  of  the 
second  68-376  kilogram. -kilometers.  That  is  to  say,  the  energy 
set  free  by  the  proteid  metabolism  of  the  muscles  engaged  in 
the  work  was  far  less  than  the  amount  necessary  to  accomplish 
the  work  actually  done,  to  say  nothing  of  its  having  to  provide 
as  well  for  the  movements  of  respiration  and  circulation.  Their 
muscular  energy  therefore  must  have  had  other  sources  than 
proteid  metabolism. 

That  on  the  contrary  muscular  exercise  at  once  and  largely 
increases  the  production  of  carbonic  acid  is  be3'^ond  all  doubt. 
One  hour's  hard  labour  will  increase  fivefold  tlie  quantity  of 
carbonic  acid  given  off  witliin  tlie  liour.      And  \n  an  experiment 


638  SOURCES   OF   HEAT.  [Book  ii. 

directed  to  this  point  it  was  found  that  a  man  in  24  hours  con- 
sumed 95-4:  grms.  oxygen  and  produced  1284  grms.  carbonic  acid 
when  doing  work,  as  against  708  grms.  oxygen  consumed  and 
911  grms.  carbonic  acid  produced  when  remaining  at  rest,  the 
quantity  of  urea  secreted  being  in  the  first  case  37  grms.,  in  the 
second  37-2  grms. 

It  is  evident  that  the  conclusions  arrived  at  by  the  statistical 
method  entirely  corroborate  those  gained  by  an  examination  of 
muscle  itself,  viz.  that  during  muscular  contraction  the  explosive 
decomposition  which  takes  place  bears  chiefly,  if  not  exclusively, 
on  the  non-nitrogenous  constituents  of  the  muscle,  and  that  it 
is  the  non-nitrogenous  products  which  alone  escape  from  the 
muscle  and  from  the  body,  any  nitrogenous  products  which 
result  being  retained  within  the  muscle,  or  at  least  within  the 
body.  We  must  therefore  reject  the  second  as  well  as  the  first 
division  of  the  views  under  discussion;  not  only  is  the  muscle 
not  fed  exclusively  on  proteid  material,  but  also  its  energy  does 
not  arise  from  an  exclusively  proteid  metabolism. 


Animal  Heat. 

§  424.  The  Sources  and  Distrihution  of  Heat.  We  have 
already  seen  that  the  conception  of  the  non-nitrogenous  por- 
tions of  food  being  solely  calorifacient  or  respiratory  proves  to 
be  unfounded  when  we  attempt  to  trace  the  history  of  the  food 
on  its  way  through  the  body.  The  same  view  is  still  more 
strikingly  shewn  to  be  inadequate  when  we  study  the  manner  in 
which  the  heat  of  the  body  is  produced.  We  may  indeed  at  once 
affirm  that  the  heat  of  the  body  is  generated  by  the  chemical 
changes,  which  we  may  speak  of  generally  as  those  of  oxidation, 
undergone  not  by  any  particular  substances,  but  by  the  tissues 
at  large.  Wherever  metabolism  is  going  on,  or  to  be  more 
exact  wherever  destructive  metabolism,  katabolism,  is  going  on, 
heat  is  being  set  free.  In  growth  and  in  repair,  in  the  deposi- 
tion of  new  material,  in  the  transformation  of  lifeless  pabulum 
into  living  tissue,  in  the  constructive  metabolism,  the  anabolism 
of  the  body,  and  in  the  smaller  synthetic  processes  of  which  we 
spoke  in  dealing  with  urea  (§  387),  heat  is  undoubtedly  to  a 
certain  extent  being  absorbed  and  rendered  latent:  the  energy 
of  the  construction  may  be,  in  part  at  least,  supplied  by  the 
heat  present.  But  all  this,  and  more  than  this,  viz.  the  heat 
present  in  a  potential  form  in  the  substances  themselves  so 
built  up  into  the  tissue,  is  lost  to  the  tissue  during  its  destruc- 
tive metabolism ;  so  that  the  whole  metabolism,  the  whole  cycle 
of  changes  from  the  lifeless  pabulum  through  the  living  tissue 
back  to  the  lifeless  products  of  vital  action,  is  eminently  a 
source  of  heat. 


Chap,  v.]  NUTRITION.  639 

Of  all  the  tissues  of  the  body  the  muscles,  not  only  from 
their  bulk,  forming  as  they  do  so  large  a  portion  of  the  whole 
frame,  but  also  from  the  characters  of  their  metabolism,  must 
be  regarded  as  the  chief  sources  of  heat. 

In  treating  (§  62)  of  the  thermal  changes  in  muscle  we 
have  seen  that  in  the  total  energy  expended  in  a  muscular  con- 
traction, the  ratio  of  that  whicli  appears  as  heat  to  that  which 
appears  as  external  work  is  variable.  If  we  assume  that  the 
energy  appearing  as  work  done  in  a  muscular  contraction  is 
on  the  average  about  one-tenth  of  the  total  energy  expended, 
the  rest  going  out  as  heat,  then,  upon  the  calculation  that  the 
total  external  work  of  the  body  is  about  one-fifth  of  the  total 
energy  set  free  in  the  body,  it  is  clear  that  the  heat  given  out 
by  the  muscles,  even  if  we  consider  only  the  heat  given  out 
when  they  are  contracting,  must  form  a  very  large  part  of  the 
total  heat  given  out  by  the  body.  And  even  if,  as  recent 
researches  indicate,  the  muscular  machine  works  more  eco- 
nomically than  we  have  hitherto  supposed,  the  amount  of  heat 
given  out  by  the  skeletal  muscles  must  still  remain  very  large. 
Moreover  to  the  skeletal  muscle  we  must  add  the  heart  which, 
never  resting,  does  in  the  twenty-four  hours  as  we  have  seen, 
§  120,  no  inconsiderable  amount  of  work,  and  must  give  rise 
to  no  inconsiderable  amount  of  heat.  But  the  skeletal  muscles, 
though  frequently,  are  not  continually  contracting;  they  have 
periods,  at  times  long  periods,  of  rest;  and  during  these  periods 
of  rest,  metabolism,  of  a  subdued  kind  it  is  true,  but  still  a 
metabolism  involving  an  expenditure  of  energy,  is  going  on. 
This  quiescent  metabolism  must  also  give  rise  to  a  certain  amount 
of  heat;  and  if  we  add  this  amount,  which  in  the  present  state  of 
our  knowledge  we  cannot  exactly  gauge,  to  that  given  out  during 
the  movements  of  the  bod}^  it  is  very  clear,  even  in  the  absence 
of  exact  data,  that  the  metabolism  of  the  muscles  must  supply 
a  very  large  proportion  of  the  total  heat  of  the  body.  They 
are  par  excellence  the  thermogenic  tissues. 

Next  to  the  muscles  in  importance  come  the  various  secret- 
ing glands.  In  these  the  secreting  elements,  at  the  periods  of 
secretion  at  all  events,  are  in  a  state  of  metabolic  activity, 
which  activity  as  elsewhere  will  naturally  give  rise  to  heat. 
In  the  case  of  the  salivary  gland  a  rise  of  temperature  has  been 
actually  observed  ;  but  objections  have  been  brought  against 
the  observation.  Of  all  these  various  glands,  the  liver  deserves 
special  attention  on  account  of  its  size  and  large  supply  of  blood, 
and  because  it  appears  to  be  continually  at  work.  If  there  be 
any  truth  in  the  views  urged  in  the  preceding  chapter  touching 
the  large  and  varied  metabolic  Mcn-k  of  the  liver,  we  must  con- 
clude that  a  very  large  amount  of  heat  is  set  free  in  this  organ; 
and  that  holds  good  even  if  we  make  a  large  allowance  for  the 
various  synthetic  anabolic  processes  which  may  take  place  and 


640     THE  CONSTANT  BODILY  TEMPERATURE.     [Book  ii. 

by  wliicli  heat  would  be  absorbed  and  made  latent.  We  find 
indeed  that  the  blood  in  the  hepatic  vein  is  the  warmest  in  the 
body.  Thus  in  the  dog  a  temperature  of  40-73°  C.  has  been 
observed  in  the  hepatic  vein,  while  that  of  the  vena  cava  in- 
ferior was  38-35°  to  39-58,  and  that  of  the  right  heart  37-7. 
The  fact  that  the  blood  of  the  hepatic  vein  is  warmer  than  that 
of  either  the  portal  vein  or  the  aorta,  shews  that  the  increased 
temperature  is  not  due  simply  to  the  liver  being  far  removed 
from  the  surface  of  the  body. 

The  brain  too  may  be  regarded  as  a  source  of  heat,  since  its 
temperature  is  higher  than  that  of  the  arterial  blood  with  which 
it  is  supplied ;  though  from  the  smaller  quantity  of  blood 
passing  through  its  vessels  as  well  as  from  the  changes  in  it 
being  less  massive,  it  cannot  in  this  respect  compare  with  either 
the  liver  or  the  muscles  as  a  source  of  heat  to  the  body. 

The  blood  itself  cannot  be  regarded  as  a  source  of  any  con- 
siderable amount  of  heat,  since,  as  we  have  so  frequently  urged, 
the  oxidations  or  other  metabolic  changes  taking  place  in  it 
are  comparatively  slight.  The  heat  evolved  by  the  indifferent 
tissues  such  as  iDone,  cartilage  and  connective  tissue,  may  be 
passed  over  as  insignificant ;  and  we  cannot  even  regard  the 
adipose  tissue  as  a  seat  of  the  production  of  heat,  since  the  fat 
of  the  fat-cells  is  in  all  probability  not  oxidized  in  situ  but 
simpl}^  carried  away  from  its  place  of  storage  to  the  tissue  which 
stands  in  need  of  it,  and  it  is  in  the  tissue  that  it  undergoes 
the  metabolism  by  which  its  latent  energy  is  set  free.  Some 
amount  of  heat  is  also  produced  by  the  changes  which  the  food 
undergoes  in  the  alimentary  canal  before  it  really  enters  the 
body. 

Hence,  taking  a  survey  of  the  whole  body,  we  may  conclude 
that  since  metabolism  is  going  on  to  a  greater  or  less  extent 
everywhere,  heat  is  everywhere  being  generated ;  but  that, 
looked  at  from  a  quantitative  point  of  view,  the  muscles  and 
the  glandular  organs  must  be  regarded  as  the  main  sources 
of  the  heat  of  the  body,  the  muscles  being  the  more  important  of 
the  two. 

§  425.  But  heat,  while  being  thus  continually  produced,  is 
as  continually  being  lost,  by  the  skin,  the  lungs,  the  urine  and 
the  faeces.  The  blood  passing  from  one  part  of  the  body  to 
the  other,  and  carrying  warmth  from  the  tissues  where  heat  is 
being  rapidly  generated,  to  the  tissues  or  organs  where  heat 
is  being  lost  by  radiation,  conduction  or  evaporation,  tends  to 
equalize  the  temperature  of  the  various  parts,  and  thus  main- 
tains a  "constant  bodily  temperature." 

When  the  production  of  heat  is  not  great  as  compared  with 
the  loss  there  is  no  great  accumulation  of  heat  within  the  body, 
the  temperature  of  which  consequently  is  but  slightly  raised 
above  that  of  surroxinding  objects.      Thus  the  temperature  of 


Chap,  v.]  NUTRITION.  641 

the  frog,  for  instance,  is  rarely  more  than  -04°  to  '05°  above 
that  of  the  atmosphere,  though  in  the  breeding  season  the 
difference  may  amount  to  1°.  Such  animals,  and  they  comprise 
all  classes  except  birds  and  mammals,  are  spoken  of  as  cold- 
blooded ;  they  have  been  also  called  poikilothermic,  that  is,  of 
varied  temperature.  Exceptions  among  them  are  not  uncom- 
mon. Some  fish,  such  as  the  tunny,  are  warmer  than  the  water 
in  which  they  live,  and  in  a  species  of  Python  (P.  hivittatus)  a 
difference  of  as  much  as  12°  has  been  observed.  In  a  beehive 
the  temperature  may  rise  at  times  to  as  much  as  40°.  In  the 
so-called  warm-blooded  animals,  birds  and  mammals,  the  loss 
and  production  of  heat  are  so  balanced  that  the  temperature  of 
the  body  remains  constant  at,  in  round  numbers,  35°  or  40°, 
whatever  be  the  temperature  of  the  air ;  hence  these  have 
been  called  homoiothermic,  of  constant  temperature.  The 
temperature  of  man  is  about  37° ;  in  some  birds  it  is  as  high 
as  44°  (Hirundo)  and  in  the  wolf  it  is  said  to  be  as  low  as 
35-24°. 

This  temperature  is  with  slight  variations  maintained 
throughout  life.  After  death  the  generation  of  heat  rajiidly 
diminishes,  and  the  body  speedily  becomes  cold ;  but  for  some 
short  time  immediately  following  upon  systemic  death,  a  rise 
of  temperature  may  be  observed,  due  to  the  fact  that,  while  the 
metabolism  of  the  tissues  is  still  going  on,  the  loss  of  heat  is 
somewhat  checked  by  the  cessation  of  the  circulation.  The 
onset  of  pronounced  rigor  mortis  causes  a  marked  accession  of 
heat,  and  when  occurring  after  certain  diseases  may  give  rise 
to  a  very  considerable  elevation  of  temperature. 

This  mean  bodily  temperature  of  warm-blooded  animals  is, 
during  health,  maintained,  with  small  variations  of  which  we 
shall  presently  speak,  within  a  very  narrow  margin,  a  rise  or 
indeed  a  fall  of  much  more  than  a  degree  above  or  below  the 
limit  given  above  being  indicative  of  some  failure  in  the  organ- 
ism, or  of  some  unusual  influence  being  at  work.  It  is  evident, 
therefore,  that  the  mechanisms  which  coordinate  the  loss  with 
the  production  of  heat  must  be  exceedingly  sensitive.  It  is 
obvious,  moreover,  that  these  mechanisms  may  act  when  the 
bodily  temperature  is  tending  to  rise,  by  eitlier  checking  the 
production  or  by  augmenting  the  loss  of  heat ;  conversely  when 
the  bodily  temperature  is  tending  to  fall,  they  may  act  by  either 
increasing  the  production  or  by  diminishing  the  loss  of  heat. 
As  the  regulation  of  temperature  by  variations  in  the  loss  of 
heat  is  better  known  than  regulation  by  variations  in  produc- 
tion, it  will  be  best  to  consider  this  first. 

§  426.  Ref/ulation  by  variations  in  loss.  Heat  is  lost  to  the 
body  by  the  warming  of  the  f;eces  and  of  the  urine,  by  the 
warming  of  the  expired  air,  by  the  evaporation  of  the  water 
of  respiration,  by  conduction  and  radiation  from  the  skin,  and 

41 


642  REGULATION   OF  LOSS   OF  HEAT.        [Book  ii. 

by  the  evaporation  of  the  water  of  perspiration.  It  has  been 
calculated  that  the  relative  amounts  of  the  loss  by  these  several 
channels  are  as  follows :  In  warming  the  fseces  and  urine  about 
3,  or  according  to  others  6  per  cent.  By  respiration  about  20, 
or  according  to  others  about  9  only  per  cent.,  leaving  77,  or 
alternatively  85,  per  cent,  for  conduction  and  radiation  and 
evaporation  by  the  skin. 

The  two  chief  means  of  loss  then,  which  are  at  all  susceptible 
of  any  great  amount  of  Variation,  and  which  can  be  used  to 
regulate  the  temperature  of  the  body,  are  the  skin  and  the  lungs. 

The  more  air  passes  in  and  out  of  the  lungs  in  a  given  time, 
the  greater  will  be  the  loss  in  warming  the  expired  air,  and  in 
evaporating  the  water  of  respiration.  In  such  animals  as  the 
dog,  which  do  not  perspire  freely  by  the  skin,  respiration  is  a 
most  important  means  of  regulating  the  temperature ;  and  in 
the  dog  a  very  close  connection  may  be  observed  between  the 
production  of  heat  and  respiratory  activity.  The  changes 
which  give  rise  to  this  loss  take  place  before  the  inspired  air 
reaches  the  pulmonary  alveoli ;  both  the  warming  and  the 
evaporation  are  effected  in  the  nasal  and  pharyngeal,  and  to 
some  extent  in  the  bronchial  passages.  Some  observers  have 
maintained  that  the  left  side  of  the  heart  is  warmer  than  the 
right,  and  hence  have  argued  that  chemical  changes  leading  to 
a  considerable  development  of  heat  take  place  in  the  pulmonary 
capillaries.  It  would  appear  however  that  the  right  ventricle, 
owing  to  its  lying  nearer  to  the  liver,  the  high  temperature  of 
which  has  already  been  mentioned,  is  in  reality  rather  hotter 
than  the  left.  And  indeed  we  have  no  satisfactory  evidence  of 
any  large  amount  of  heat  being  produced  by  any  pulmonary 
metabolism. 

The  great  regulator  however  is  undoubtedly  the  skin ;  and 
this  has  a  more  or  less  double  action.  In  the  first  place  it  reg- 
ulates the  loss  of  heat  by  means  of  the  vaso-motor  mechanism. 
The  more  blood  passes  through  the  skin  the  greater  will  be  the 
loss  of  heat  by  conduction,  radiation,  and  evaporation.  Hence, 
any  action  of  the  vaso-motor  mechanism  which,  by  causing  dila- 
tion of  the  cutaneous  vascular  areas,  leads  to  a  larger  flow  of 
blood  through  the  skin,  will  tend  to  cool  the  body ;  and  con- 
versely, any  vaso-motor  action  which,  by  constricting  the  cuta- 
neous vascular  areas,  or  by  dilating  the  splanchnic  vascular 
areas,  causes  a  smaller  flow  through  the  skin,  and  a  larger 
flow  of  blood  through  the  abdominal  viscera,  will  tend  to  heat 
the  body.  In  the  second  place,  besides  this,  the  special  nerves 
of  perspiration  will  act  directly  as  regulators  of  temperature, 
increasing  the  loss  of  heat  when  they  promote,  and  lessening 
the  loss  when  they  cease  to  promote,  the  secretion  of  the  skin. 
The  working  of  this  heat-regulating  mechanism  is  well  seen  in 
the  case  of  exercise.     Since  every  muscular  contraction  gives 


Chap,  v.]  NUTRITIOK  643 

rise  to  heat,  exercise  must  increase  for  the  time  being  the  pro- 
duction of  heat  ;  yet  the  bodily  temperature  rarely  rises  so 
much  as  a  degree  centigrade,  if  at  all.  By  exercise  the  respi- 
ration is  quickened,  and  the  loss  of  heat  by  the  lungs  increased. 
The  circulation  of  blood  is  also  quickened,  and  the  cutaneous 
vascular  areas  becoming  dilated,  a  larger  amount  of  blood 
passes  through  the  skin.  Added  to  this,  the  skin  perspires 
freely.  Thus  a  large  amount  of  heat  is  lost  to  the  body,  suffi- 
cient to  neutralize  the  addition  caused  by  the  muscular  con- 
traction, the  increase  which  the  more  rapid  flow  of  blood 
through  the  abdominal  organs  might  tend  to  bring  about 
being  more  than  sufficiently  counteracted  by  their  smaller 
supply  for  the  time.  The  sense  of  warmth  which  is  felt 
during  exercise  in  consequence  of  the  flashing  of  the  skin,  is 
in  itself  a  token  that  a  regulative  cooling  is  being  carried  on. 
In  a  similar  way  the  application  of  external  cold  or  heat  defeats 
its  own  ends,  either  partially  or  completely.  Under  the  influ- 
ence of  external  cold  the  cutaneous  vessels  are  constricted,  and 
the  splanchnic  vascular  areas  dilated,  so  that  the  blood  is  with- 
drawn from  the  colder  and  cooling  regions  to  the  hotter  and 
heat-producing  organs.  This  vascular  change  may  be  used  to 
explain  the  fact  that  stripping  naked  in  a  cold  atmosphere 
often  gives  rise  to  a  distinct  increase  in  the  mean  temperature 
of  the  blood,  as  indicated  by  a  thermometer  placed  in  the 
mouth,  though  j)ossibly  the  effect  may  be  partly  due  to  an 
actual  increase  of  the  production  of  heat.  Under  the  influence 
of  external  warmth,  on  the  other  hand,  the  cutaneous  vessels 
are  dilated,  a  rapid  discharge  of  heat  takes  place ;  and  if  the 
circumstances  be  such  that  the  body  can  perspire  freely,  and 
the  perspiration  be  readily  evaporated,  the  temperature  of  the 
body  may  remain  very  near  to  the  normal,  even  in  an  excessively 
hot  atmosphere.  Thus,  more  than  a  century  ago,  two  observers 
were  able  to  remain  with  impunity  in  a  chamber  heated  even  to 
127°  (260°  Fahr.),  and  with  ease  in  one  so  hot,  that  it  became 
painful  for  them  to  touch  the  metal  buttons  of  their  clothing. 
It  is  unnecessary  to  give  any  more  examples  of  this  regulation 
of  temperature  by  variations  in  the  loss  of  heat ;  they  all  readily 
explain  themselves. 

§  427.  Tlie  production  of  heat^  its  variations  and  regulation. 
As  we  have  already  said  the  exact  determination  of  the  amount 
of  heat  produced  in  the  living  body  is  attended  with  great 
difficulties ;  still  certain  conclusions  have  been  arrived  at  based 
partly  on  direct  calorimetric  observations,  the  nu)re  recent  ones 
with  improved  calorimeters  being  especially  valuable,  and  partly 
on  what  seem  to  be  trustworthy  deductions  from  observed  chem- 
ical changes. 

The  rate  of  production  of  heat  in  a  living  body  is  deter- 
mined by  a  variety  of  circumstances.     In  the  first  place  what 


644     REGULATION  OF  PRODUCTION  OF  HEAT.    [Book  ii. 

may  be  called  the  general  rate  of  metabolism,  and  so  of  the 
production  of  heat,  varies  in  different  kinds  of  animals.  Of 
two  animals  of  the  same  bulk  and  weight  placed  under  the 
same  circumstances  one  '  lives  faster '  than  the  other,  metabolizes 
its  living  substance  more  rapidly,  and  so  produces  heat  more 
rapidly.  Thus  direct  calorimetric  observations,  as  far  as  they 
at  present  go,  shew  that  a  man  on  the  average  produces  more 
heat,  per  kilo,  per  hour,  than  does  a  dog,  and  a  dog  more  than 
a  rabbit.  Probably  every  species  has  what  may  be  called  its 
specific  coefficient,  and  every  individual  his  personal  coefficient 
of  heat-production,  the  coeificient  being  the  expression  of  the 
inborn  qualities  proper  to  the  living  substance  of  the  species 
and  of  the  individual. 

A  larger  living  body  will  naturally  produce  more  heat  than 
a  smaller  living  body  of  the  same  nature,  since  the  larger  body 
possesses  so  to  speak  a  greater  number  of  heat-producing  units. 
But  this  is  neutralized  by  an  opposing  tendency.  The  smaller 
body,  having  relatively  to  its  bulk  a  larger  amount  of  surface, 
loses  heat  at  a  more  rapid  rate  than  does  the  larger  body ;  and 
therefore,  to  maintain  the  balance  between  loss  and  production, 
so  as  to  secure  the  same  constant  bodily  temperature  (and  as  we 
have  just  seen  the  bodily  temperature  of  warm-blooded  animals 
is  remarkably  uniform),  it  must  produce  heat,  per  unit  of  its 
body,  at  a  more  rapid  rate.  As  a  rule  the  greater  loss  of  heat 
owing  to  the  relatively  greater  surface  is  so  marked  that  of  two 
animals  having  the  same  constant  bodily  temperature,  of  two 
species  of  mammals,  or  of  two  individuals  of  the  same  race,  we 
should  expect  the  smaller  one  to  produce  a  relatively  larger 
amount  of  heat.  And  direct  calorimetric  observations  shew 
that  this  is  so.  The  struggle  for  existence  has  raised  what  we 
have  just  called  the  specific  or  personal  coefficient  of  the  smaller 
animal. 

From  what  we  have  seen  concerning  the  immediate  effects 
of  a  meal,  we  should  be  inclined  to  expect  that  food  would  tem- 
porarily increase  the  production  of  heat ;  and  not  only  is  this 
view  confirmed  by  common  experience  and  by  our  own  sensa- 
tions, but  direct  calorimetric  observations  afford  experimental 
proof  of  its  truth.  In  the  dog  it  has  been  found  that  the  rate 
of  production  increases  after  a  meal,  reaching  its  maximum  from 
the  6th  to  the  9th  hour,  and  then  declining  to  a  level  which  may 
be  regarded  as  that  secured  by  the  general  metabolism  of  the 
body,  and  which  appears  to  be  maintained  with  remarkable  con- 
stancy until  after  long  starvation  the  economy  begins  to  break 
down. 

Labour,  muscular  work,  has  a  powerful  influence  in  increas- 
ing the  production  of  heat.  As  we  have  seen,  of  the  total  heat 
produced  in  the  body,  a  certain  portion  must  always  be  attrib- 
uted to  muscular  contractions  whicli  even  in  the  most  quiet  body 


Chap,  v.]  NUTRITION.  645 

are  always  going  on;  in  an  ordinary  active  body  a  considerable 
quantity  of  heat  must  be  thus  generated.  Hence  the  more 
active  the  body  the  greater  tlie  production  of  heat.  As  we 
stated  before,  in  a  contraction  the  proportion  of  the  energy  set 
free  to  do  work  to  that  set  free  as  heat  appears  to  vary  under 
different  circumstances  ;  and  the  increase  of  heat  due  to  labour 
probably  varies  in  a  corresponding  way.  The  details  of  this 
relation  have  yet  to  be  worked  out,  but  we  may  at  least  conclude 
that,  when  a  man  pushes  his  dailj^  labour  beyond  the  150,000 
k.m.,  the  additional  energy  thus  leaving  liis  body  as  work  done 
is  not  taken  out  of  the  850,000  k.m.  given  in  §  422  as  the  aver- 
age daily  output  of  heat,  but  the  total  setting  free  of  energy 
and  the  total  production  of  heat  is  at  the  same  time  in- 
creased. 

§  428.  The  production  of  heat  thus  determined  by  these 
several  influences,  some  of  which  are  themselves  regulated  by 
the  nervous  system,  is  further  regulated  in  a  remarkable  manner. 
For  it  is  not  solely  by  variations  in  the  loss  of  heat  that  the  con- 
stant temperature  of  the  warm-blooded  animal  is  maintained. 
Variations  in  the  amount  of  heat  actually  generated  in  the  body 
constitute  an  important  factor  not  only  in  the  maintenance  of 
the  normal  temperature,  but  also  in  the  production  of  the  abnor- 
mally high  or  low  temperatures  of  various  diseases.  Many  con- 
siderations have  long  led  physiologists  to  suspect  the  existence 
of  a  nervous  mechanism  by  which  afferent  impulses  arising  in 
the  skin  or  elsewhere  might  through  the  central  nervous  system 
originate  efferent  impulses  whose  effect  would  be  to  increase  or 
to  diminish  the  metabolism  of  the  muscles  or  other  organs,  and 
thus  to  increase  or  diminish  the  amount  of  heat  generated  for 
the  time  being  in  the  body.  And  Ave  have  experimental  evi- 
dence that  such  metabolic  or  thermogenic  nervous  mechanism, 
comparable  in  many  respects  to  the  vaso-motor  meclianism  or 
to  the  various  secreting  nervous  mechanisms,  does  really 
exist. 

The  warm-blooded  animal  is  distinguished  from  the  cold- 
blooded animal  by  the  fact  that  when  it  is  exposed  to  cold  or 
heat,  it  does  not  like  the  latter  become  colder  or  hotter,  as  the 
case  may  be,  but,  within  certain  limits,  maintains  its  normal  tem- 
perature. If  the  maintenance  of  the  temperature  of  tlie  warm- 
blooded animal  during  exposure  to  cold  is  assisted  by  an  increased 
production  of  heat  and  is  not  due  simply  to  a  diminished  loss,  we 
ought  to  find  evidence  of  an  increased  metabolism  during  that 
exposure.  We  ought  to  find  under  these  circumstances  an  in- 
creased production  of  carbonic  acid,  and  an  increased  consump- 
tion of  oxygen,  since  it  is  to  these  products,  rather  than  to  the 
nitrogenous  factors,  on  the  peculiarities  of  which  as  uncertain 
signs  of  metabolism  we  have  already  insisted,  we  must  look  for 
indications  of  the  rise  or  fall  of  metabolic  activity. 


646     EEGULATIOX  OF  PRODUCTION  OF  HEAT.     [Book  ii. 

Taking  then  tlie  consumption  of  oxygen,  and  the  production 
of  carbonic  acid,  as  a  measure  of  metabolic  activity  and  so  of 
heat-production,  it  has  been  shewn  that  a  marked  contrast  in 
this  respect  exists  between  cold-blooded  and  warm-blooded  ani- 
mals exposed  to  changes  of  temperature.  In  the  cold-blooded 
animal,  cold  diminishes  and  heat  increases  the  metabolic  activ- 
ity of  the  body  ;  as  the  temperature  to  which  the  animal  is 
subjected  rises  or  falls,  so  the  consumption  of  oxygen  and  pro- 
duction of  carbonic  acid  is  increased  or  lessened.  The  body  of 
a  cold-blooded  animal  behaves  in  this  respect  like  a  mixture  of 
dead  substances  in  a  chemist's  retort  :  heat  promotes  and  cold 
retards  chemical  action  in  both  cases.  Very  different  is  the 
beha^dour  of  a  warm-blooded  animal.  In  this  case,  within  a 
lower  and  a  higher  limit,  cold  increases  and  heat  diminishes  the 
bodily  metabolism,  as  shewn  by  the  increased  or  diminished 
consumption  of  oxygen  and  production  of  carbonic  acid  as  the 
temperature  falls  or  rises.  In  these  animals  there  is  obviously 
a  mechanism  of  some  kind,  counteracting  and  indeed  overcom- 
ing those  more  direct  effects  which  alone  obtain  in  cold-blooded 
animals.  And  that  this  mechanism  is  of  a  nervous  nature,  is 
indicated  by  the  following  facts. 

When  a  warm-blooded  animal  is  poisoned  by  urari,  the  tem- 
perature falls  and  the  metabolism,  measured  by  the  consumption 
of  oxygen  and  the  production  of  carbonic  acid,  sinks  also;  and 
that  the  latter  is  the  cause,  not  the  effect,  of  the  former  is 
shewn  by  the  fact  that  the  metabolism  continues  to  fall  though 
loss  of  heat  be  prevented  by  surrounding  the  animal  with  wrap- 
pings of  cotton  wool.  In  such  a  urarized  animal,  exposure  to 
higher  temperatures  augments  and  exposure  to  lower  tempera- 
tures diminishes  metabolism;  the  urarized  warm-blooded  ani- 
mal in  fact  behaves  like  a  cold-blooded  animal.  Similar  but 
perhaps  not  such  striking  or  so  constant  results  are  gained  by 
division  of  the  spinal  bulb.  After  this  operation  the  tempera- 
ture of  the  body  sinks,  and  the  fall,  though  partly  due  to 
increased  loss  of  heat  by  the  skin,  caused  by  the  dilated  condi- 
tion of  the  cutaneous  vessels,  is  also  accompanied  by  diminished 
metabolism  and  is  therefore  in  part  due  to  diminished  produc- 
tion of  heat.  And  when  an  animal  is  in  this  condition,  expo- 
sure to  higher  temperatures  increases  and  exposure  to  lower 
temperatures  diminishes  the  bodily  metabolism.  We  can  best 
explain  these  results  by  supposing  that,  under  normal  condi- 
tions, the  muscles,  which  as  we  have  seen  contribute  so  largely 
to  the  total  heat  of  the  body,  are  j)laced,  by  means  of  their 
motor  nerves  and  the  central  nervous  system,  in  some  special 
connection  with  the  skin,  so  that  a  lowering  of  the  temperature 
of  the  skin  leads  to  an  increase,  while  a  heightening  of  the 
temperature  of  the  skin  leads  to  a  decrease,  of  the  muscular 
metabolism.     Further,  the   centre  of   this   thermotaxic  reflex 


Cjiap.  V.J  NUTRITION.  C47 

meclianism  appears  to  be  placed  somewhere  in  the  nervous 
system  above  the  spinal  cord.  When  urari  is  given,  the  reflex 
chain  is  broken  at  its  muscular  end  ;  when  the  spinal  cord  is 
divided  the  break  is  nearer  the  centre. 

We  may  add  that  the  muscular  metabolism  which  thus  helps 
to  regulate  temperature  need  not  involve  visible  muscular  con- 
tractions. At  the  same  time  the  heat  given  out  by  the  muscles 
will  be  temporarily  increased  at  every  contraction  which  ma}' 
occur.  Thus,  the  shivering  which  follows  exposure  to  cold 
distinctly  helps  to  warm  the  body;  indeed  some  observers  have 
been  led  to  think  that,  in  man,  tliis  visible  effect  of  cold  plan's 
a  more  important  part  in  his  heat  regulation  than  the  invisible 
actions  which  we  have  just  described.  We  may  also  add  that 
the  regulative  nervous  mechanism  may  apparently  be  overborne 
by  an  exposure  to  too  great  heat  or  cold.  When  for  instance 
the  cold  to  which  the  animal  is  exposed  becomes  excessive,  the 
reaction  of  the  thermotaxic  nervous  system  is  powerless  against 
the  direct  action  on  the  tissues  of  the  depressing  influences, 
and  the  metabolism,  together  with  the  temperature,  sinks. 

§  429.  In  a  number  of  experiments  it  has  been  shewn  that 
injuries  to,  such  as  those  caused  by  puncture  or  galvanic  cauter}-, 
or  electrical  stimulation  of  limited  portions  of  the  more  cen- 
tral portions  of  the  brain  may  give  rise  to  a  great  increase  of 
the  temperature  of  the  body  without  producing  any  other 
marked  symptom.  The  increase  is  shewn,  by  the  increase  of 
metabolism,  increased  production  of  carbonic  acid  and  increased 
consumption  of  oxygen,  as  Avell  as  by  direct  calorimetric  obser- 
vations, to  be  due  to  an  increased  production  of  heat.  This 
naturally  suggests  that  the  portions  of  the  brain  in  question 
contain  the  hypothetical  heat  centre  just  mentioned,  the  lesion 
on  stimulation  exciting  the  centre  to  activity  by  direct  action 
on  it,  instead  of  in  the  usual  reflex  manner.  The  matter  has  not 
however  as  yet  been  clearly  worked  out;  and  indeed  observers 
are  not  agreed  as  to  the  exact  parts  of  the  brain  injury  to  whieli, 
or  stimulation  of  which,  produces  the  effect. 

§  430.  By  regulative  mechanisms  of  the  kind  just  dis- 
cussed the  temperature  of  the  warm-blooded  animal  is  main- 
tained within  very  narrow  limits.  In  ordinary  health  the 
temperature  of  man  varies  between  36°  and  38°,  the  narrower 
limits  being  36*25°  and  37-5°,  when  the  thermometer  is  placed 
in  the  axilla.  In  the  mouth  the  reading  of  the  thermometer 
is  somewhat  (-25°  to  1*5°)  higher;  in  the  rectum  it  is  still 
liigher  (about  '9°)  than  in  the  mouth.  The  temperature  of 
infants  and  children  is  slightly  higher  and  much  more  sus- 
ceptible of  variation  than  that  of  adults,  and  after  40  years  of 
age  the  average  maximum  temperature  (of  health)  is  somewliat 
lower  than  before  that  epocli.  A  diurnal  variation,  indepen- 
dent of  food  or  other  circumstances,  has  been  observed,  the 


648  PYREXIA.  [Book  ii. 

maximum  ranging  from  9  a.m.  to  6  p.m.  and  the  minimum 
from  11  P.M.  to  3  a.m.  Meals  cause  sometimes  a  slight  eleva- 
tion, sometimes  a  slight  depression,  the  direction  of  the  influ- 
ence depending  on  the  nature  of  the  food :  alcohol  seems 
always  to  produce  a  fall.  Exercise  and  variations  of  external 
temperature,  within  ordinary  limits,  cause  very  slight  change, 
on  account  of  the  compensating  influences  which  have  been 
discussed  above.  The  rise  from  even  active  exercise  does  not 
amount  to  1°  ;  when  labour  is  carried  to  exhaustion  a  depres- 
sion of  temperature  may  be  observed.  In  travelling  from  very 
cold  to  very  hot  regions  a  variation  of  less  than  a  degree  occurs, 
and  the  temperature  of  inhabitants  of  the  tropics  is  practically 
the  same  as  of  those  dwelling  in  arctic  regions. 

§  431.  Many  of  the  maladies  of  the  body  are  characterized 
by  an  increase  of  the  bodily  temperature  known  as  "fever"  or 
"pyrexia,"  the  thermometer  very  frequently  rising  to  39°  or 
40°,  not  unfrequently  to  41°,  and  at  times  reaching  43°  or  even 
44° ;  but  these  higher  temperatures  cannot  long  be  borne  with- 
out the  organism  failing.  And  as  we  have  said,  any  increase 
in  man  of  the  bodily  temperature  beyond  38°,  or  even  beyond 
37*5°,  indicates  some  disturbance.  In  most  cases  the  rise  of 
temperature  has  a  definite  objective  cause,  some  local  inflamma- 
tion or  suppuration,  or,  as  in  specific  fevers,  the  presence  in 
the  economy  of  some  "materies  morbi,"  of  the  nature  of  an 
organized  germ  or  of  some  other  nature.  We  cannot  here  dis- 
cuss the  connection  between  the  local  inflammation  or  the  spe- 
cific poison  and  the  high  temperature,  but  we  have  increasing 
evidence  that  the  high  temperature  of  fever  is  due,  not  merely 
to  a  diminution  of  the  loss  of  heat,  though  this  may  be  a  factor, 
but  also,  and  indeed  chiefly,  to  an  increased  production  of  heat. 
In  fever,  the  production  of  carbonic  acid,  and  the  consumption 
of  oxygen,  that  is  to  say,  the  metabolic  changes  of  the  tissues, 
are  increased.  The  urea  also  is  increased,  and  that  in  such  a 
way  as  to  confirm  the  view  already  expressed  that  much  of  the 
heat  comes  from  such  a  metabolism  of  the  skeletal  muscles  as, 
unlike  an  ordinary  contraction,  directly  involves  the  nitroge- 
nous elements.  The  inordinate  metabolism  of  the  body  at 
large  thus  characteristic  of  fever  is  shewn  by  the  wasting 
which  it  entails.  Calorimetric  observations  also  shew  in  a 
direct  manner  that  the  production  of  heat  is  increased.  Of 
course  mere  increased  production  alone  would  be  insufficient 
to  raise  the  temperature  of  the  body,  for  it  might  be  met,  up 
to  a  very  high  limit,  by  a  compensating  increase  of  loss  of  heat ; 
but  in  fever  this  compensation  is  wanting,  and  it  is  perhaps 
this  absence  of  due  regulation  which  is  most  characteristic  of 
the  febrile  condition. 

In  some  maladies  the  bodily  temperature  falls  distinctly 
below  the  normal  average,  reaching  for  instance  35°  or  even 


Chap,  v.]  NUTRITION.  649 

lower.  In  such  cases  there  can  be  little  doubt  that  tlie  uni- 
dition  is  due  to  diminished  metabolism  and  diminished  heat 
production. 

One  of  the  most  marked  phenomena  of  starvation  is  the 
fall  of  temperature,  which  becomes  very  rapid  during  the  last 
days  of  life.  The  lowered  metabolism  diminishes  the  produc- 
tion of  heat,  and  the  lowered  temperature  in  turn  still  further 
diminishes  the  metabolism.  Indeed  the  low  temperature  is  a 
powerful  factor  in  bringing  about  death,  for  life  may  be  much 
prolonged  by  wrapping  a  starving  animal  in  some  bad  con- 
ductor so  as  to  economize  the  bodily  heat. 

§  432.  Effects  of  G-reat  Heat.  As  we  said  above,  the  regu- 
lative heat  mechanism  is  unable  to  withstand  the  strain  of  too 
great  an  external  heat  or  too  prolonged  an  exposure  to  a  great 
but  less  degree  of  heat.  The  temperature  of  the  body  then 
rises  above  the  normal  ;  and  it  has  been  observed  that  the 
temperature  is  more  easily  raised  by  warmth  than  depressed 
by  cold,  at  least  when  neither  are  very  intense.  When  either 
in  this  way  by  external  warmth  or  through  pyrexia  the  tem- 
perature of  the  body  is  raised  some  6°  or  7°  above  the  normal, 
to  45^^  or  thereabouts,  death  speedily  ensues.  The  chain  of 
events  thus  leading  to  death  has  not  been  as  yet  clearly  made 
out,  and  most  likely  the  events  do  not  take  exactly  the  same 
course  in  all  cases ;  but  we  shall  probably  not  go  far  wrong 
in  attributing  death  to  the  fact  that  the  high  temperature 
hurries  on  the  metabolism  of  the  several  tissues,  of  some  more 
than  others,  at  such  a  spendthrift  rate  that  their  capital  is  soon 
exhausted.  We  have  seen,  §  301,  that  too  warm  blood  pro- 
duces dyspnoea,  and  soon  exhausts  the  metabolic  capital  of  the 
respiratory  centre.  Too  warm  blood  similarly  hurries  on  the 
beats  of  the  heart :  an  explosion  of  the  contractile  substance  is 
each  time  prematurely  brought  on  before  a  sufficient  quantity 
of  explosive  substance  is  accumulated,  each  stroke  becomes 
more  and  more  feeble  as  the  rate  is  quickened,  the  beats  be- 
come irregular,  and  finally  cease.  Either  of  these  two  events 
alone  and  certainly  both  together  are  enough  to  bring  the 
working  of  the  bodily  mechanism  to  an  end ;  but  other  tissues 
beside  the  heart  and  the  respiratory  centre  are  suffering  in  the 
same  way,  notably  the  rest  of  the  central  nervous  system. 
This  too  is  being  Imrried  on  unduly  in  its  inner  changes,  so 
that  not  only  consciousness  is  lost  and  other  objective  manifes- 
tations of  nervous  action  go  wrong  or  fail,  but  that  regulative 
grasp  of  the  central  nervous  system  on  the  tissues  of  the  body 
at  large  is  loosened,  and  tumult  takes  the  place  of  order. 
Whether  this  or  that  sign  of  disorder  comes  to  the  front, 
whether  for  instance  convulsions  take  place,  would  appear  to 
depend  upon  the  exact  turn  taken  by  tlie  abnormal  events.  In 
heat-stroke,  more  connnoiilv  known  as  sun-stroke,  the  essential 


650  EFFECTS  OF  GREAT  HEAT  AND  COLD.  [Book  ii. 

condition  of  whicli  seems  to  be  a  rapid  rise  of  tlie  temperature 
of  the  body  owing  to  a  sudden  failure  of  the  thermotaxic  mech- 
anism, the  symptoms  varj^.  Sometimes  the  heart  suddenly 
gives  way,  at  other  times  the  respiratory  centre  seems  to  be 
more  directly  affected ;  sometimes  convulsions  make  their  ap- 
pearance, but  more  commonly  death  takes  place  through  a 
comatose  condition  of  the  brain,  an  initial  phase  of  excitement 
of  the  central  nervous  system  being  not  unfrequently  witnessed. 

Mammalian  muscle,  it  will  be  remembered,  §  79,  becomes 
rigid  at  about  50°;  but  death  probably  always  occurs  before 
that  higher  temperature  is  reached  by  the  blood,  so  that  a  sud- 
den rigor  mortis  from  heat  (rigor  caloris)  cannot  be  regarded 
as  a  factor  in  death  from  exposure  to  too  great  heat. 

§  433.  Effects  of  Crreat  Void.  The  effects  of  a  too  great 
lowering  of  the  temperature  of  body,  which  is  generally  the 
result  of  too  great  external  cold  and  rarely  if  ever  arises  from 
internal  causes  lowering  the  metabolism  and  thus  the  produc- 
tion of  heat,  are  in  their  origin  the  reverse  of  those  of  a  too 
high  temperature.  The  metabolism  of  the  tissues  is  lowered; 
and  not  only  are  the  katabolic  changes  which  lead  to  the  setting 
free  of  energy  thus  affected,  but  the  anabolic  changes  also  share 
in  the  depression.  The  "living  substance"  falls  to  pieces  less 
readily,  but  is  also  made  w^  less  readily;  and  could  this  slack- 
ening of  metabolism  be  carried  on  in  the  several  tissues  at  a 
rate  proportionate  to  the  rate  at  which  each  tissue  lives,  life 
might  thus  be  brought  to  a  peaceful  end  by  gradual  arrest  of 
the  life  of  each  part  of  the  whole  body.  And  indeed  in  some 
cases,  where  the  lowering  of  the  temperature  takes  place  gradu- 
ally, something  like  this  does  occur  even  in  warm-blooded  ani- 
mals. The  diminished  metabolism  tells  first  and  chiefly  on  the 
central  nervous  system,  especially  on  the  brain  and  more  particu- 
larly on  those  parts  of  that  organ  which  are  concerned  in  con- 
sciousness. The  intrinsic  lowering  of  the  cerebral  metabolism 
is  further  assisted  by  a  slowing  of  the  heart-beat  and  of  the 
breath,  drowsiness  is  succeeded  by  a  condition  very  like  to,  if 
not  identical  with  that  known  as  sleep,  which  we  shall  study 
later  on,  but  by  a  sleep  which  insensibly  passes  into  the  sleep 
of  death.  In  some  cases,  however,  especially  those  in  which 
the  lowering  of  the  temperature  is  sudden  and  rapid,  disorders 
of  the  nervous  system  intervene,  and  convulsions  like  those  of 
asphyxia  are  produced. 


SEC.   3.     ON   NUTRITION   IN   GENERAL. 

§  434.  It  may  now  be  profitable  to  take  a  brief  survey  of 
tlie  various  conclusions  at  which  we  have  arrived  concerning 
the  problems  of  nutrition. 

We  have  seen  that  the  several  tissues,  using  lymph  as  a 
medium,  live  upon  the  blood,  taking  up  from  the  blood  the  mate- 
rials for,  and  returning  to  the  blood  the  products  of,  their  meta- 
bolism. The  blood  itself  we  have  also  seen  to  be  replenished 
with  food  from  the  alimentary  canal  and  with  oxygen  from  the 
lungs,  and  to  be  freed  from  waste  products  by  means  of  the 
excretory  organs.  In  this  double  action  the  raw  material  of 
the  food  on  the  one  hand  undergoes,  between  its  being  placed 
in  the  mouth  and  its  taking  part  in  the  metabolism  of  the  tissue 
which  ultimately  uses  it,  many  intermediate  changes  carried 
on  in  various  parts  of  the  body,  and  the  waste  products  simi- 
larly undergo  intermediate  changes  between  leaving  the  tissue 
and  appearing  in  the  urine,  the  sweat  or  the  expired  air. 

We  have  further  seen  reason  to  think  that  the  metabolic 
events  of  the  body  take  place  in  the  main  in  the  tissues,  not  in 
the  blood  stream  on  its  way  between  the  heart  and  the  tissues. 
Changes,  proper  to  the  blood  itself,  take  place  in  the  blood; 
the  corpuscles,  red  and  white,  with  the  plasma  undergo  like 
the  rest  of  the  body,  their  proper  metabolic  cycles,  and  in  this 
sense  blood  may  be  called  a  tissue  if  there  is  any  advantage  in 
using  the  phrase ;  but,  apart  from  these  intrinsic  blood  changes, 
as  far  as  we  can  see  at  present,  the  metabolism  undergone  dur- 
ing their  transit  along  the  blood  channels,  by  the  substances 
which  are  merely  carried  in  the  blood  from  place  to  place,  is 
an  insignificant  part  of  the  total  metabolism  of  the  body. 

By  metabolism  of  a  tissue  we  understand  the  total  chemical 
changes  taking  place  in  the  tissue ;  and  we  divide  these  changes 
into  tliose  which  either  directly  or  indirectly  are  concerned  in 
the  building  up  (anabolic)  and  those  whicli  are  in  like  manner 
concerned  in  the  breaking  down  (katabolic)  of  the  living  sub- 
stance.    We  shall   explain   presently  ^^'hat   we  mean  by  the 

651 


652  THE  NUTRITION   OF  MUSCLE.  [Book  ii. 

words  '  directly '  and  '  indirectly '  used  in  this  connection. 
And  we  may  here  repeat  the  caution  (§  30)  that  though  for 
convenience  sake  we  use  the  phrase  '  living  substance, '  what 
is  really  meant  by  the  words  is  not  a  thing  or  body  of  a  par- 
ticular chemical  composition  but  matter  undergoing  a  series  of 
changes. 

435.  We  know  more  about  the  chemical  changes  of  muscle 
than  perhaps  of  any  other  tissue,  though  this  even  at  the  most 
is  not  much,  and  we  may  perhaps  take  the  nutrition  of  muscle 
as  a  tj'pe  of  nutrition  in  general.  The  muscle  in  a  normal  state 
of  things  lives  ultimately  on  the  proteids,  fats,  carbohydrates, 
salts  and  water  of  the  food,  and  on  the  oxygen  of  the  inspired 
air,  but  lives  directly  on  the  blood  which  brings  these  things 
to  it. 

Concerning  the  relation  of  proteids  to  muscle,  we  know 
little  more  than  was  stated  in  §  140  in  speaking  of  the  heart. 
We  can  do  no  more  than  infer,  and  that  doubtfully,  that  serum- 
albumin  is  the  form  in  which  the  muscle  takes  up  proteids. 
Concerning  carbohydrates  we  have  apparently  a  definite  fact. 
Dextrose  is,  as  we  have  repeatedly  said,  always  present  in 
the  blood  in  small  quantity,  and  appears  to  be  the  only  carbo- 
hydrate constituent  of  blood-plasma.  Experiments  carried  out 
on  a  large  animal,  such  as  the  horse  or  cow,  have  shewn  that 
the  venous  blood  coming  from  a  muscle  contains  less  dextrose 
than  the  arterial  blood  going  to  the  muscle,  and  that  the  dif- 
ference is  much  increased  by  throwing  the  muscle  into  contrac- 
tion. From  this  we  may  provisionally  conclude  that  dextrose 
is  an  essential  part  of  the  food  of  the  muscle. 

Concerning  fats  we  have  little  or  no  knowledge,  but  we 
may  perhaps  infer  that  the  body  has  power  to  transform  fats 
into  carbohydrates  as  it  has  the  power  to  transform  carbohy- 
drates into  fats,  and  that  the  carbon  whether  of  the  fat  or  of 
the  carbohydrates  of  food  is  presented  to  the  muscle  in  the 
form  of  carbohydrate,  namely  of  dextrose.  But  we  have  no 
distinct  proof  of  this. 

The  various  salts  brought  to  the  muscle  by  the  plasma, 
though  they  supply  no  energy,  are  as  essential  to  the  life  of 
muscle  as  the  energy-holding  proteid  or  carbon  compound;  and 
experiments  made  with  regard  to  some  of  them,  calcic  salts  for 
instance,  shew  that  their  presence  or  absence  materially  affects 
the  maintenance  or  restoration  of  irritability.  Some  of  these 
probably  play  the  part  only  of  securing  by  their  presence 
favourable  conditions  for  the  due  metabolic  processes,  some- 
what after  the  v/ay  in  which  the  presence  of  a  calcic  salt  deter- 
mines the  clotting  of  blood  and  the  curdling  of  milk;  but  some 
we  probably  ought  to  regard  as  actually  entering  into  the  pro- 
cesses themselves.  Of  these  matters  however  we  know  very 
little. 


Chap,  t.]  NUTEITIOX.  653 

§  436.  The  products  of  muscurar  metabolism  pass  into  the 
lymph  bathing  the  tibre  and  so,  either  by  a  direct  path  into  the 
capillaries  or  by  a  more  circuitous  course  through  the  general 
lymphatic  system,  into  the  blood.  The  fate  of  the  carbonic 
acid  we  have  fully  treated  of  in  dealing  with  respiration;  the 
little  we  know  concerning  the  nitrogenous  product  or  products 
has  been  stated  in  dealing  with  urea;  the  third  recognized 
product  is  lactic  acid,  sarcolactic  acid.  Did  any  considerable 
amount  of  oxidation  take  place  in  the  blood  stream  while  the 
blood  is  flowing  along  the  larger  channels,  subject  only  to  the 
influence  of  the  vascular  walls,  we  might  fairly  expect  that 
the  lactic  acid  discharged  from  the  muscles  would  be  subjected 
to  oxidizmg  influences  while  still  within  the  blood  stream  of 
the  larger  channels.  We  have  however  no  satisfactory  evi- 
dence of  any  lactic  acid  being  oxidized  in  this  way.  On  the 
contrary,  there  is  a  certain  amount  of  experimental  and  other 
evidence  that  lactic  acid  present  in  the  blood  is  somehow  or 
other  disposed  of  by  the  liver;  and  that  if  the  liver  fails  to  do 
its  duty  lactic  acid  may  apjDear  in  the  urine. 

§  437.  We  may  here  ask  the  question.  What  is  the  relation 
of  these  various  metabolic  processes  to  the  structural  elements 
of  the  tissue  ?  When  we  say  that  the  muscular  fibre  is  continu- 
ally undergoing  metabolism  do  we  mean  that  ever}^  jot  and  tittle 
of  the  fibre  is  undergoing  change  and  that  at  the  same  rate? 
We  can  hardly  suppose  this.  It  seems  unlikely,  for  instance, 
that  the  metabolism  of  the  fibrillar  substance  is  identical  with 
that  of  the  interfibrillar  substance,  whatever  be  the  view  we 
take  as  to  the  properties  or  meaning  of  the  two  substances. 
We  should  thus  be  led  to  regard  the  metabolic  events  occurrino' 
in  muscle  as  falling  into  two  classes  at  least;  those  taking  place 
in  the  living  more  permanent  framework,  and  those  bearing  on 
the  formation  and  destruction  of  the  contractile  substance 
lodged  in  that  living  framework.  These  of  course  are  at  pres- 
ent matters  of  speculation;  but  on  the  whole  the  evidence  we 
can  gather  tends  and  perhaps  increasingly  tends  to  shew  that 
in  muscle  there  does  exist  such  a  framework  of  what  we  may 
call  more  distinctly  living  substance  which  rules  the  histological 
features  of  the  fibre,  and  whose  metabolism  though  high  in 
quality  does  not  give  rise  to  massive  discharges  of  energy,  and 
that  the  interstices  so  to  speak  of  this  framework  are  occupied 
by  various  kinds  of  material  related  in  different  degrees  to  the 
framework  and  therefore  deserving  to  be  spoken  of  as  more  or 
less  living,  the  chief  part  of  the  energy  set  free  by  muscle  com- 
ing directly  from  the  metabolism  of  some  or  other  of  this  mate- 
rial. And  the  same  view  may  be  extended  to  other  tissues. 
Both  tlie  framework  and  the  intercalated  material  undergo 
metabolism,  and  have,  in  different  degrees,  their  anabolic  and 
katabolic  changes;  both  are  concerned  in  the  life  of  tlie  living 


654      INFLUEIsTCES  DETERMINING  NUTRITION.    [Book  ii. 

substance,  but  one  more  directly  than  the  other,  and  this  is  what 
was  meant  by  the  terms  '  directly '  and  '  indirectly, '  used  in  §  434. 

§  438.  Whether  the  chief  product  of  the  metabolism  of  any 
tissue  be  a  proteid  substance,  or  a  fat,  or  a  carbohydrate,  proteid 
substance  is  the  pivot  so  to  speak  of  the  metabolism,  and  nitro- 
genous bodies  alway  appear  as  the  products  of  metabolism. 
This  is  strikingly  seen  in  the  nutrition  of  plants  where,  as  far 
as  mere  bulk  or  weight  is  concerned,  the  active  metabolizing- 
tissue  is  insignificant  compared  with  the  mass  of  products  of 
metabolism  heaped  up  in  the  form  of  starch  or  cellulose  or  some 
allied  carbohydrate.  The  protoplasm  of  a  vegetable  cell  soon 
becomes  a  mere  film  bearing  a  heavy  burden  of  heaped  up 
metabolic  products  and  eventually  disappears  ;  and  of  that  film 
only  a  part  corresponds  to  what  we  spoke  of  above  as  the  living- 
framework  of  the  muscle.  Yet  that  scanty  proteid-built  frame- 
work is  more  or  less  directly  concerned  in  the  production  of  the 
carbohydrate  material  and  the  various  conversions  which  that 
material  undergoes.  Proteid,  nitrogen,  changes  are  entangled 
with  the  carbon  changes  ;  and  since  the  products  of  metabolism 
in  the  plant  are  not  as  in  the  animal  cast  out  of  the  organism, 
but  for  the  most  part  heaped  up  within  it,  we  find  the  plant 
storing  up  in  parts,  where  if  they  serve  no  useful  purpose  they 
at  least  do  not  harm,  nitrogenous  products  of  metabolism,  such 
as  those  known  as  vegetable  alkaloids,  many  of  which  by  their 
amide  nature  betray  their  kinship  to  the  animal  nitrogenous 
product  urea. 

§  439.  In  the  preceding  chapters  of  this  work  we  have  had 
abundant  evidence  that  the  metabolism  of  the  tissues  is  subject 
to  the  government  of  the  central  nervous  system ;  the  contrac- 
tion of  a  muscle,  the  secretory  activity  of  a  gland,  the  increased 
or  diminished  production  of  heat  all  afford  instances  of  nervous 
impulses  affecting  metabolism.  In  most  of  these  instances  the 
changes  induced  fall  within  the  downward,  katabolic,  phase 
and  have  a  downward  character ;  thus  when  a  muscle  contracts, 
the  result  is  a  conversion  of  more  complex  bodies  into  simpler 
bodies ;  and  the  same  as  far  as  we  can  see  is  true  of  most  other 
cases.  But  it  is  open  for  us  to  suppose  that  nervous  impulses 
might  affect  the  upward,  anabolic,  phase  and  have  a  constructive 
influence. 

At  all  events  we  are  not  justified  in  assuming  that  a  nervous 
impulse  can  only  produce  disruptive  katabolic  changes  such  as 
are  seen  in  muscular  contraction  or  in  secretion.  The  effects 
of  stimulating  a  nerve  going  to  a  muscle  or  a  salivary  gland 
are  striking  and  obvious  and  the  behaviour  of  a  muscle  or  a 
gland  as  far  as  contraction  and  secretion  are  concerned  is,  within 
certain  limits,  under  experimental  control.  But  there  are  cer- 
tain phenomena,  seen  chiefly  in  the  course  of  disease,  and  lying, 
to  a  very  small  extent  only,  within  the  control  of  experiment, 


Chap,  v.]  >^UTKITI0N.  655 

which  seem  to  shew  that  the  central  nervous  system  governs 
the  metabolic  changes,  the  nutrition,  not  only  of  muscle  and 
gland,  but  of  various  other  tissues  in  a  deeper  and  more  general 
way  than  that  of  simply  promoting  (or  hindering)  contraction 
or  secretion.  Thus  as  we  have  seen  (§  78)  when  the  connection 
between  a  muscle  and  the  central  nervous  system  is  severed, 
the  muscle  eventually  wastes  and  loses  its  vitality;  when  all 
the  nerves  going  to  the  sub-maxillary  gland  are  severed,  the 
gland  instead  of  being  as  in  the  normal  condition  intermittently 
active  and  quiescent,  pours  forth  a  continuous  "  paralj'tic  " 
secretion  and  eventually  degenerates  and  wastes.  When  in  a 
rabbit  the  fifth  nerve  is  divided  in  the  skull  the  loss  of  sensation 
in  those  parts  of  the  face  of  which  it  is  the  sensory  nerve  is 
followed  by  nutritive  changes.  Very  soon,  within  twenty-four 
hours,  the  cornea  becomes  cloudy ;  and  this  is  the  precursor  of 
an  inflammation  which  may  involve  the  whole  eye  and  end  in 
its  total  disorganization.  At  the  same  time  the  nasal  chambers 
of  the  side  operated  on  are  inflamed,  and  very  frequently  ulcers 
make  their  appearance  on  the  lips  and  gums.  And  similar 
results  have  been  seen  in  other  animals  including  man.  If  the 
operation  be  conducted  in  a  young  animal,  which  subsequently 
lives  to  maturity,  the  head  may  become  bilaterally  unsymmet- 
rical,  as  shewn  especially  by  the  skull.  Again  division  of  both 
vagus  nerves  is  ver}-  apt  to  be  followed  by  inflammation  of 
both  lungs,  by  fatty  degeneration  of  the  heart,  and  so  by  death. 
In  several  of  these  instances  the  effect  is  a  mixed  one  and 
the  problem  complicated.  Thus,  in  the  case  of  division  of  the 
fifth  nerve,  seeing  how  delicate  a  structure  the  eye  is,  and  how 
carefully  it  is  protected  by  the  mechanisms  of  the  eyelids  and 
tears,  it  seems  reasonable  to  suppose  that  the  inflammation  in 
question  might  simply  be  the  result  of  the  irritation  caused  by 
dust  and  contact  with  foreign  bodies,  to  which  the  e3"e,  no 
longer  guided  and  protected  by  sensations,  these  being  destroyed 
by  the  section  of  the  nerve,  became  suljject.  In  the  same  way 
the  ulcers  on  the  lips  and  gums  might  be  explained  as  injuries 
inflicted  by  the  teeth  on  those  structures  in  their  insensitive 
condition.  And  some  observers  maintain  that  tlie  inflammation 
of  the  eye  may  be  greatly  lessened  or  altogether  prevented  if 
the  organ  be  carefully  covered  up  and  in  all  possible  ways  pro- 
tected from  the  irritating  influences  of  foreign  bodies.  Other 
observers  however  have  failed  to  prevent  the  inflammation  in 
spite  of  ever}^  care.  So  also  the  inflammation  of  the  lungs 
following  upon  division  of  both  vagus  nerves  seems  to  be  due 
not  to  any  direct  nutritive  action  of  the  pulmonary  branches  of 
the  vagus  on  the  pulmonary  tissue,  but  to  food  accumulating 
in  the  pharynx  owing  to  the  paralysis  of  the  oesophagus  and 
larynx,  and  then  passing  into  the  air  passages  and  so  setting 
up  inflammation.     Death  in  these  cases  is  moreover  often  the 


656      INFLUENCE  OF  NEKVES  ON  NUTEITION.     [Book  ti. 

simple  result  of  inanition  caused  by  the  paralysis  of  the  oesoph- 
agus allowing  no  food  to  reach  the  stomach.  The  phenomena 
of  the  paralytic  secretion  of  saliva  are  also  of  a  complicated 
nature. 

But  even  without  insisting  on  such  instances  as  the  above, 
various  other  phenomena  of  disease  seem  to  indicate  such  an 
influence  of  the  nervous  system  on  nutrition  as  we  are  discuss- 
ing. As  examples  we  might  mention  the  rapid  and  peculiar 
degeneration  of  and  loss  of  contractility  in  the  skeletal  muscles 
in  certain  affections  of  the  spinal  cord,  the  changes  in  the 
muscles  being  more  rapid  and  profound  than  in  the  nerves  ;  the 
phenomena  of  bed-sores,  especially  the  so-called  acute  bed-sores 
of  cerebral  apoplexy;  some  at  least  of  the  cases  of  vesical  affec- 
tions attendant  on  spinal  or  cerebral  diseases  or  injuries ;  the 
more  rapid  atrophy  and  loss  of  contractility  in  muscles  which 
follow  upon  contusions  of  nerves  as  com]3ared  with  the  effects 
of  simple  section  of  nerves ;  the  occurrence  of  certain  eruptions, 
such  as  lichen,  zona,  ecthyma,  &c.,  in  various  spinal  or  cerebral 
diseases,  and  indeed  the  general  phenemona,  and  especially  the 
topography  of  the  eruption,  of  a  large  number  of  cutaneous 
diseases.  Lastly  but  not  least  we  might  quote  the  general  pro- 
cess of  inflammation.  These  are  examples  of  disordered  nutri- 
tion. To  them  we  might  add  as  instances  of  altered  but  yet 
orderly  nutrition  the  remarkable  connections  observed  between 
changes  in  the  form  of  the  lingers  and  growth  of  the  nails  and 
hairs,  and  certain  internal  maladies,  such  for  instance  as  the 
'  clubbed  fingers  '  of  phthisical  and  other  patients,  and  the  like. 
We  might  also  call  attention  to  the  influence  of  light  on  the 
nutrition  of  animals.  The  experience  of  blind  people  and  blind 
animals  indicates  some  special  connection  between  visual  sensa- 
tions and  the  nutrition  of  the  skin;  and  this  can  hardly  be  other 
than  a  nervous  connection.  The  eft'ects  of  prolonged  darkness 
on  nutrition  in  general  and  the  experimental  results  which  shew 
that  the  total  metabolism  of  the  body  is  influenced  by  light, 
also  suggest  some  nervous  action.  The  influence  of  cold  again 
in  determining  the  growth  of  hair  points  in  the  same  direction. 

Making  every  allowance  for  the  intervention  as  factors  in 
the  production  of  the  phenomena  quoted  above  of  such  common 
actions  of  the  nervous  system  as  are  already  well  known  to  us, 
such  as  vaso-motor  changes,  making  every  allowance  for  the 
consequences  of  the  failure  or  bluntness  of  sensation  and  the 
absence  of  those  beneficial  after  results  of  muscular  activity 
which  we  pointed  out  in  §  81,  recognizing  moreover  that  changes 
in  one  organ  may  affect  the  condition  of  other  distant  organs 
by  changes  induced  in  the  composition  or  qualities  of  the  blood, 
there  still  remains  a  residue  which  seems  distinctly  to  point  to 
the  conclusion  that  the  influence  of  the  nervous  system  is  not 
limited  to  such  changes  of  the  muscles  as  belong  to  tlie  produc- 


Chap,  v.]  NUTKITIOK  657 

tion  of  contractions  or  the  generation  of  heat,  but  bears  on  the 
whole  nutrition  of  the  muscle.  Similar  considerations  lead  us 
also  to  conclude  that  the  influence  of  the  nervous  system  bears 
on  the  whole  nutrition  of  the  glands,  of  the  blood  vessels,  of  the 
skin  and  the  connective  tissues  in  general,  in  fact  of  nearly  the 
whole  body. 


42 


SEC.  4.     ON  DIET. 

§  440.  An  ordinary  man  living  an  ordinary  life  will  need 
for  the  maintenance  of  vigorous  health  a  certain  amount  of 
food  of  a  certain  kind ;  this  we  may  take  as  a  normal  diet. 

Presuming  that  the  experience  of  man  has  led  him  to  adopt 
what  is  good  for  him,  we  may  ascertain  approximately  the 
normal  diet  by  means  of  the  statistical  method,  by  examining 
the  nature  and  amount  of  the  daily  food  of  a  very  large  number 
of  individuals.  The  most  valuable  data  for  this  purpose  are 
those  gained  by  inquiries  among  persons  who  choose  their  own 
food ;  the  results  gained  from  the  diets  used  in  prisons  or  other 
institutions,  or  among  bodies  of  men  such  as  the  army,  though 
more  readily  arrived  at,  are  open  to  the  objection  that  the  diets 
in  question  are  determined  in  part  by  the  theoretical  opinions 
of  those  whose  duty  it  is  to  fix  the  diet.  Putting  together  the 
various  statistical  results  thus  obtained,  and  selecting  the  quan- 
tities which  seem  to  be  most  commonly  used  rather  than  at- 
tempting to  strike  a  strict  average  or  take  a  strict  mean,  we 
find  that  in  an  ordinary  diet  for  the  twenty-four  hours  the 
several  food-stuffs  are 

Proteids  from    100  to  130  grms. 
Fats  „        40  „     80      „ 

Carbohydrates  450  „  550      „ 

to  these  we  must  add 

Salts         30  grms. 
Water  2800     „ 

The  total  (available)  potential  energy  of  the  lower  estimate  is 
2610,  of  the  higher  3505  (kilogramme-degree)  calories,  calcu- 
lated, in  round  numbers,  on  the  data  of  §  421.  With  such  a 
statistical  diet  we  may  compare  an  experimental  diet,  that  is  to 
say  a  diet  arrived  at  through  a  series  of  trials  on  an  individual 
man  whose  body  might  be  taken  to  be  an  average  one,  that  diet 
being  considered  a  normal  one  in  which  the  body,  maintaining 
vigorous  health,  neither  gained  nor  lost  in  weight,  and  remained 
moreover  in  nitrogenous  equilibrium  with  the  nitrogen  of  the 


CiiAP.  v.]  NUTRITION.  659 

egesta  equal  to  that  of  the  ingesta.  To  make  sure  that  under 
such  a  diet  the  body  was  remaining  of  the  same  composition 
there  ought  to  be  evidence  of  a  carbon  equilibrium  also,  other- 
wise during  the  period  of  the  experiment  fat  might  be  being 
replaced  by  water  (see  §  415);  but  this  is  unlikely,  and  we 
may  therefore  accept  the  method  as  a  fair  one.  It  has  given 
in  the  hands  of  two  different  observers  the  following  somewhat 
different  results,  the  diet  A  being  that  already  quoted  in  §  421 : 

A  B 

Proteids               100  grms.  118 

Fats                     100      „  56 

Carbohydrates    240       „  500 

Salts                       25       „  — 

Water                2600       „  — 

The  total  (available)  potential  energy  is  respectively  2310,  and 
3035  calories. 

On  the  whole  the  diets  gained  by  the  two  methods  agree 
very  largely.  To  put  down  a  single  column  of  figui-es  as  "  the 
normal  diet "  would  be  to  affect  a  vain  and  delusive  accuracy. 
If  we  desire,  for  theoretical  purposes,  to  select  some  one  set  of 
figures  rather  than  others,  we  might  be  influenced  by  the  con- 
siderations that  the  lower  amount  of  proteids  in  the  experi- 
mental diet  was  nearer  the  mark  than  the  higher  amount  of 
some  of  the  statistical  diets,  and  further  that,  where  cost  is  not 
of  moment,  the  substitution  of  fat  for  an  excess  of  carbohydrates 
is  desirable.  We  should  be  thus  led  to  take  the  experimental 
diet  A  as  on  the  whole  the  best  or  most  '  normal '  one,  and  that 
is  the  one  which  we  employed  in  the  calculations  of  §  421.  It 
will  be  observed  that  the  potential  energy  of  this  diet  is  less 
than  that  of  any  of  the  others,  and,  as  we  said  while  then  speak- 
ing of  it,  may  be  considered  low ;  but  there  was  no  evidence 
that  it  was  insufficient.  Still  it  miist  be  remembered  that 
neither  it  nor  any  of  the  others  is  to  be  regarded  as  distinctly 
proved  to  be  the  real  normal  diet.  Against  the  experimental 
diet  we  may  urge  that  the  number  of  experiments  have  been 
few  and  conducted  on  a  few  individuals  only  at  most,  and  that 
a  larger  number  of  experiments,  with  a  variety  of  combinations 
of  different  amounts  of  the  several  food-stuffs,  might  lead  to  a 
different  result ;  that  for  instance  with  certain  amounts  of  fats 
and  carbohydrates,  the  amount  of  proteid  needed  to  maintain 
healthy  bodily  equilibrium,  including  nitrogenous  equilibrium, 
might  be  reduced  much  below  the  100  grammes,  especiall}"  if 
particular  kinds  of  proteids,  fat  or  carbohydrates  were  used, 
and  especial  attention  (see  §  420)  were  paid  to  the  salts. 
And  indeed  a  considerable  number  of  observations  have  been 
made  tending  to  shew  that  a  man  of  average  size  and  weight 


660  THE   NOEMAL   DIET.  [Book  ii. 

may  continue  in  nitrogenous  equilibrium  and  in  good  health 
with  a  daily  ration  of  much  less  than  100  grm.  proteid,  with 
as  little  as  40  grm.  for  example.  To  this  we  shall  have  to 
refer  in  speaking  of  a  vegetable  diet.  Against  the  statistical 
diet  on  the  other  hand  we  may  urge  that  instinct  is  not  an 
unerring  guide,  and  that  the  choice  of  a  diet  is  determined  by 
many  other  circumstances  than  the  physiological  value  of  the 
food. 

.  §  441.  Taking  however  some  such  diet  as  the  above  to  be 
the  approximately  true  normal  diet,  we  may  call  attention  to 
the  fact  that  the  normal  diet  is  made  up  of  each  of  the  three 
great  food-stuffs,  carbohydrates  being  in  excess.  We  may  here 
remark  incidently  that  the  diets  of  both  the  carnivora  and 
herbivora  agree  with  that  of  omnivora  in  containing  all  three 
food-stuffs :  they  differ  from  each  other  as  to  the  relative  pro- 
portions only.  As  we  have  seen,  the  body  may  be  maintained 
m  equilibrium  on  proteid  food  alone ;  but  an  exclusively  pro- 
teid diet  is  not  only  bought  dearly  in  the  market,  but  also  paid 
for  dearly  within  the  economy  ;  we  are  of  course  now  speaking 
of  man.  To  obtain  the  necessary  carbon  out  of  the  carbon 
moiety  of  proteid  unnecessary  labour  is  thrown  on  the  economy, 
and  the  system  tends  to  become  blocked  with  the  amides  and 
other  nitrogenous  waste  arising  out  of  the  nitrogen  moiety 
simply  thrown  off  to  secure  the  carbon. 

Fats  and  carbohydrates  are  much  more  akin  to  each  other 
than  is  either  to  proteid ;  and  if  on  the  one  hand,  as  (§  435) 
seems  possible  or  even  probable,  the  fat  of  the  food  and  of  the 
body  is  converted  into  sugar  either  on  its  way  to  become  built 
up  into  the  tissue  or  in  the  course  of  the  changes  taking  place 
outside  the  real  living  framework  of  the  tissue  by  which  it  is 
reduced  to  carbonic  acid,  and  that  on  the  other  hand  carbohy- 
drates can  furnish  the  fat  whose  presence  in  the  body  is  neces- 
sary, we  might  expect  that  carbohydrate  alone  without  fat 
might,  with  proteid,  form  a  normal  diet.  But  on  this  point 
experience  is  probably  to  be  trusted  ;  and  we  may  infer  that 
in  every  normal  diet  some  fat  at  least  must  be  added  to  the 
starches  and  the  sugars. 

The  advantage  of  this  mixture  is  probably  felt  while  the 
food  is  as  yet  within  the  alimentary  canal.  What  we  have 
learnt  concerning  digestion  leads  us  to  regard  it  as  a  compli- 
cated process,  and  we  cannot  readily  imagine  that  the  proteo- 
lytic, amylolytie  and  adipolytic  changes  run  their  several 
courses,  especially  in  the  small  and  large  intestine,  apart  from 
and  irrespective  of  each  other.  We  are  rather  led  to  suppose 
that  the  accompaniment  of  one  set  of  changes,  in  some  indirect 
manner,  favours  the  others  ;  and  it  is  for  that  reason  probably 
that  we  take  our  food-stuffs  not  separately  but  mixed  in  the 
same  meal,  often  on  the  same  plate  and  even  in  the  same  mouth- 


CHAf.  v.]  NUTRITION.  6G1 

ful.  But  apart  from  this  the  two  food-stuffs,  fats  and  carbohy- 
drates, must  play  different  parts  in  the  economy,  so  that  the 
one  cannot  be  wholly  substituted  for  the  other  ;  and  though, 
beyond  the  fact  that  tlie  one  seems  to  be  a  source  of  energy  and 
the  other  not,  we  do  not  as  yet  know  the  true  physiological 
function  of  the  hydrogen  of  the  fat  as  compared  with  that  of 
the  differently  disposed  hydrogen  of  the  carbohj^drate,  we  may 
perhaps  infer  that  the  difference  of  use  within  the  body  of  the 
two  kinds  of  food-stuffs  bears  not  so  much  on  their  ultimate 
consumption  to  supply  energy  as  on  the  various  complicated 
processes  which  they  undergo  and  arrangements  in  which  they 
take  part  before  the  end  of  their  woi-k  is  reached.  We  have 
had  a  hint  that  the  carbohydrate  more  rajjidly  supplies  the  heat- 
givmg  metabolism  than  does  the  fat  ;  and  this  suggests  an 
advantage  to  the  economy  in  receiving  daily  a  certain  portion 
of  the  more  tardy  material,  while  at  the  same  time  it  may  be 
taken  to  mean  that  the  fat  before  it  is  used  to  give  rise  to 
energy  has  first  to  be  converted  into  sugar,  and  so  takes  more 
time  in  its  work. 

The  main  carbohydrate  of  every  diet  is  starch,  and  as  far  as 
we  can  learn  at  present,  the  starch  which  is  so  large  a  part  of 
the  cereals  and  vegetables  consumed  by  man  is  the  same  body 
in  all  of  them  ;  for  the  use  of  such  bodies  as  inulin  is  so  insig- 
nificant that  it  may  be  neglected.  Man  however  consumes  no 
inconsiderable  quantity  of  sugar,  chiefly  cane  sugar.  Since  the 
starch  of  a  meal  does  not  become  available  for  the  economy 
until  it  has  been  converted  into  sugar,  we  might  be  inclined  to 
infer  that  it  was  a  matter  of  indifference  whether  the  carbo- 
hydrate of  a  diet  were  supplied  as  starch  or  as  sugar.  Our 
knowledge  of  sugars  and  of  their  fate  in  the  economy  is  too 
im]3erfect  for  us  to  be  able  to  state  the  effects  on  the  body  of 
digested  starch  as  compared  with  those  of  cane  sugar  or  milk 
sugar ;  but  that  these  are  or  may  be  different  is  shewn  by  the 
experience  of  medical  practice.  In  many  cases  the  total  effect 
on  the  body  of  a  diet  from  which  cane  sugar  is  as  much  as 
possible  eliminated,  though  starch  be  allowed,  is  very  different 
from  that  of  one  of  which  cane  sugar  forms  an  appreciable  part. 

Concerning  cellulose,  which  in  herbivora  appears  certainly 
to  serve  as  a  source  of  energy  and  to  be  a  real  food-stuff",  our 
knowledge  will  not  allow  us  to  decide  whether  it  has  any 
special  uses  of  its  own,  or  whether  the  body  is  simply  led  to 
utilize  and  make  the  best  of  what  is  a  necessary  accompaniment 
of  the  starch  of  vegetable  food. 

Concerning  the  salts  present  in  a  diet  we  need  only  repeat 
what  was  said  in  §  420  that  these,  though  affording  of  them- 
selves little  or  no  energy,  are  as  essential  a  part  of  a  diet  as  the 
energy  giving  food-stuffs,  in  as  much  as  they  in  some  way  or 
other  direct  metabolism  and  tlie  distribution  of  enerofy.     And 


662  STAECH  A^D   SUGAR.  [Book  ii. 

this  is  true  not  only  of  the  morganic  salines  such  as  chlorides 
and  phosphates  but  also  of  the  so-called  extractives.  As  we 
have  seen,  the  presence  of  these  bodies,  both  the  simpler  inor- 
ganic and  the  more  complex  organic  salts,  in  the  blood  or  in  the 
extravascular  juices  or  lymph  of  the  tissues  is  essential  to  or 
directs  or  modifies  the  metabolic  activity  of  the  several  tissues. 
The  beneficial  effects,  as  components  of  special  diets,  of  such 
things  as  beef-tea  and  meat-extract,  which  consist  chiefly  of 
salts  and  extractives  with  a  very  small  quantity  of  albumose  or 
other  forms  of  proteid,  and  the  effects  either  beneficial  or  dele- 
terious of  drugs  both  turn  in  common  upon  their  taking  a  part 
of  some  kind  or  other  in,  it  may  be  upon  their  interference  with 
metabolic  processes.  The  salts  and  extractives  of  a  diet  may 
be  looked  upon  as  necessary  daily  medicines,  and  a  medicine  as 
a  more  or  less  extraordinary  variation  in  these  elements  of  a 
diet. 

Alcohol,  to  the  use  of  which  as  a  component  of  an  ordinary 
diet  special  interest  for  various  reasons  attaches,  comes  in  this 
class.  For  though  observations  shew  that  the  greater  part  of 
a  moderate  dose  of  alcohol  is  oxidized  within  the  body  and  so 
serves  as  a  source  of  energy,  man  has  recourse  to  alcohol  not 
for  the  minute  quantity  of  energy  which  is  supplied  by  itself, 
but  for  its  powerful  influence  on  the  distribution  of  the  energy 
furnished  by  other  things.  That  influence  is  a  very  complex 
one  and  cannot  be  fully  discussed  here.  We  may  add  that  the 
physiological  action  of  alcoholic  drinks  is  still  further  compli- 
cated by  the  fact  that  most  such  drinks  contain  besides  ethylic 
alcohol,  various  other  allied  substances,  Avhose  action  is  even 
more  potent  than  that  of  the  ethylic  alcohol  itself,  and  whose 
presence  very  markedly  determines  the  total  effect  of  the  drink. 
Such  articles  of  diet  as  tea  and  coffee  stand  upon  very  mucli 
the  same  footing  as  alcohol. 

The  quantity  of  fluid  which  a  man  drinks  or  should  drink 
daily,  or  more  correctly  the  quantity  of  water  which  he  should 
daily  add  to  the  dry  solids  of  his  diet,  must  vary  widely  accord- 
ing to  circumstance.  It  will  differ  according  as  he  is  persj)ir- 
ing  greatly  or  not,  according  to  the  nature  of  the  dry  solids 
of  the  diet,  whether  largely  carbohydrate  or  not,  and  so  on. 
A  lower  limit,  below  which  excretion  is  impeded,  and  a  higher 
limit,  above  which  digestion  and  metabolism  are  injuriously 
affected,  probably  exist;  but  we  have  as  yet  no  adequate  data 
which  will  enable  us  to  fix  either  of  them. 

§  442.  In  the  selection  of  articles  of  food  to  supply  the 
food-stuffs  and  other  constituents  of  a  normal  diet,  regard 
must  of  course  be  had  in  the  first  place  to  the  amount  of  poten- 
tial energy  present  in  the  material.  The  articles  chosen  for 
the  daily  fare  must  contain  between  them  so  much  proteid,  fat, 
and  carbohydrate  representing  so  much  available  energy.     But 


Ohap.  v.]  NUTEITIOK  663 

it  is  no  less  important  to  secure  that  the  energy  potential  in 
the  material  should  be  really  available  for  the  economy.  The 
material  must  have  such  qualities  that  it  is  digested  within  the 
alimentary  canal,  and  further  that  its  digestion  and  absorption 
do  not  give  rise  to  trouble  either  in  the  alimentary  canal  or  in 
that  secondary  digestion  carried  on  by  means  of  the  various 
metabolic  events  which  we  have  discussed  in  preceding  sec- 
tions. A  really  nutritious  substance  is  one  which  not  only 
contains  in  itself  an  adequate  supply  of  energy,  but  is  of  such 
a  nature  that  its  energy  can  be  appropriated  by  the  economy 
with  ease  or  at  least  with  as  little  trouble  as  possible.  We 
have  approximate  data  for  determining  how  far  an  estimate  of 
the  relative  usefulness  of  various  articles  of  food  must  be  cor- 
rected by  allowing  for  the  proportion  of  each  which  after  an 
ordinary  meal  merely  passes  through  the  alimentary  canal  and 
the  energy  of  which  is  not  in  any  way  available  for  the  body's 
use.  Thus  a  number  of  observations  carried  out  on  healthy 
individuals  gave  in  the  case  of  the  following  articles  of  food, 
the  following  figures  as  the  percentage,  reckoned  in  each  case 
on  dry  material,  which  could  be  recovered  from  the  fieces,  and 
was  therefore  not  digested  and  not  used  by  the  body:  —  Meat 
5  p.c,  Eggs  5  p.c,  Milk  9  p.c.  Bread  (white)  4  p.c.  Black 
Bread  15  p.c.  Rice  4  p.c,  Maccaroni  4  p.c,  Maize  7  p.c, 
Peas  9  p.c,  Potatoes  11  p.c.  It  must  however  be  remembered 
that  the  actual  correction  to  be  made  in  any  case  will  dej)end 
on  the  mode  of  cooking  of  the  material,  on  the  character  of 
the  meal  of  whicli  it  forms  part  and  on  the  individual  capabili- 
ties of  the  consumer,  the  latter  too  varying  under  different 
circumstances. 

The  above  refers  to  what  may  be  called  rough  digestibility, 
but  besides  this  there  are  other  circumstances  to  be  considered. 
The  same  food-stuff  in  two  articles  of  food,  though  actually 
digested,  that  is  to  say  taken  up  by  the  alimentar}^  canal,  may, 
even  while  still  within  the  alimentary  canal,  undergo  changes 
in  the  one  case  differing  from  those  in  the  other.  A  proteid 
may  for  instance  in  one  case  tend  to  be  entirely  converted  into 
peptone,  or  to  break  up  into  leucin,  &c.,  or  in  other  cases  to 
undergo  other  changes;  and  a  carbohydrate  may  in  one  case 
be  absorbed  as  sugar,  and  in  another  give  rise  to  lactic  acid. 
Indeed,  when  we  speak  of  the  digestibility  or  the  indigestibility 
of  this  or  that  article  of  food,  we  do  not  in  many  cases  so  much 
mean  the  relative  amount  of  the  substance  taken  up  in  some 
way  or  other  by  the  alimentary  canal,  as  the  characters  advan- 
tageous or  otherwise  of  the  changes  which  it  undergoes  in  being 
so  taken  up. 

Hence  the  purely  chemical  statement  of  the  amount  of 
potential  energy  present  in  an  article  of  food  is  no  safe  guide 
of  the  physiological  value  of  the  substance.     A  chunk  of  cheese 


664  IMPOHTANCE   OF  DIGESTIBILITY.      [Book  ii. 

stands  very  high  on,  generally  at  the  top  of,  a  table  of  the 
nutritive  value  of  articles  of  food  drawn  up  on  exclusively 
chemical  principles,  according  to  the  units  of  energy  present 
in  a  unit  of  the  material;  but  it  is  very  low  down  in  a  corre- 
sponding physiological  table.  And  similarly  a  dish  of  old  peas 
has  a  very  different  physiological  function  from  a  plate  of  fresh 
meat  even  when  both  contain  the  same  amount  of  nitrogen. 

In  thus  correcting  for  digestion  the  nutritive  value  of  a 
diet  it  must  also  be  borne  in  mind  that  the  alimentary  canal, 
while  chiefly  a  receptive  organ,  is  also  to  some  extent,  §  234, 
an  excretory  organ:  a  free  passage  through  the  canal  is  needed 
not  only  for  carrying  off  undigested  matter  but  also  for  getting 
rid  of  excreted  matter ;  and  the  presence  of  the  former,  up  to 
certain  limits,  assists  the  discharge  of  the  latter.  Were  it  pos- 
sible to  prepare  a  diet  every  jot  and  tittle  of  which  could  be 
digested  and  absorbed,  the  use  of  such  a  diet  would  probably 
bring  about  disorder  in  the  economy,  through  the  absence  of  a 
sufficiently  rapid  discharge  of  the  matters  excreted  into  the  ali- 
mentary canal.  Hence  cellulose  and  like  substances  even  when 
unutilized  through  absorption,  are  not  without  their  use,  and 
experience  shews  that  digestion  may  be  promoted  by  eating- 
undigestible  things. 

§  443.  The  several  food-stuffs  of  a  diet  may  be  draAvn  from 
the  animal  or  from  the  vegetable  kingdom.  Vegetable  proteids 
appear  to  undergo  the  same  changes  in  the  alimentary  canal  as 
do  animal  proteids,  and  the  main  effects  on  the  body  of  proteids 
from  the  two  sources  seem  to  be  the  same.  Our  knowledge 
at  present  however  is  too  imperfect  to  enable  us  to  decide 
whether  the  functions  of  the  two  are  exactly  the  same,  whether 
the  body  behaves  exactly  the  same  upon  a  diet  in  which  the 
proteids  are  exclusively  of  vegetable  origin,  as  upon  a  diet  in 
which,  otherwise  the  same,  the  proteids  are  partly  of  animal 
origin  also.  Nor  have  we  much  better  knowledge  of  the  rela- 
tive nutritive  value  of  vegetable  and  animal  fats.  And  as  we 
have  already  said,  we  possess  little  or  no  exact  knowledge  as 
to  the  part  played  by  those  extractives  in  respect  to  the  amount 
and  nature  of  which  animal  food  strikingly  differs  from  vege- 
table food.  In  attempting  therefore  a  judgment  from  a  purely 
physiological  point  of  view  as  to  the  value  of  an  exclusively 
vegetarian  diet  compared  with  a  diet  of  both  animal  and  vege- 
table origin,  we  can  do  little  more  at  present  than  inquire 
whether  the  former  supplies  the  several  food-stuffs  in  adequate 
quantity,  in  proper  proportion,  and  in  such  a  form  as  to  be 
economically  utilized  by  the  body. 

The  careful  examination  during  three  separate  periods  of 
several  days  each  of  the  ingesta  and  egesta  of  a  man,  28  years 
old,  weighing  57  kilos,  who  had  for  three  years  lived  on  an 
exclusively  vegetable  diet,  viz.  bread,  fruit  and  oil,  gave  the 
foUowinsr  results. 


Chap,  v.]  NUTEITION.  665 

The  daily  diet  consisted  on  the  average  of  719  grm.  solid 
matter  and  1084  grm.  water.     It  contained 

Proteids  54  grm.  containing  8-4  N. 

Fats  22    „ 

Carbohydrates  557     „  (about  I  sugar  and  i-  starch) 

(Cellulose)  16    „ 

The  daily  fseces  weighed,  when  fresh,  333  grm.  containing 
75  grm.  solid  matter,  and  were  therefore  both  bulky  and 
watery.  There  were  present  in  the  fseces  fat  7  grm.,  starch 
17  grm.  and  cellulose  9  grm.  shewing  that  30  p.c.  of  the  fat, 
6  p.c.  of  the  starch  and  56  p.c.  of  the  cellulose  had  not  been 
utilized  by  the  body.  The  subject  had  really  lived  on  fat 
15  grm.,_carbohydrates  540  grm.  (and  cellulose  7  grm.).  The 
faeces  contained  no  less  than  3-46  nitrogen.  If  we  reckon  the 
whole  of  this  as  proteid,  this  would  give  22  grm.  of  undigested 
proteid,  so  that  there  had  been  a  waste  of  41  p.c.  of  the  pro- 
teids, leaving  only  32  grm.  available  for  real  use  in  the  body; 
and  indeed  a  very  small  portion  only  of  this  nitrogen  can  be 
regarded  as  really  discharged  from  the  body  itself.  The  total 
solids  of  the  faeces  must  be  reckoned  as  partly  excreta  but 
chiefly  undigested  food.  If  we  regard  the  75  grm.  of  solid 
faeces  as  entirely  undigested  food,  the  whole  solid  food  avail- 
able for  the  body  must  be  reduced  from  701  grm.  to  644  grm. 

The  urine  of  the  day  contained  5-33  grm.  nitrogen;  tliis 
added  to  the  3*46  grm.  nitrogen  in  the  faeces  gives  8-79  grm. 
nitrogen  in  the  total  egesta  as  compared  with  the  8-4  grm. 
nitrogen  of  the  food,  indicating  a  slight  loss  of  nitrogenous 
material  from  the  body;  but  if  we  suppose  that  all  the  nitro- 
gen in  the  faeces  was  not  in  the  form  of  undigested  food  we 
may  neglect  this ;  and  indeed  the  subject  of  the  observation 
was  in  apparently  good  health  and  stationary  weight. 

Compared  with  either  of  the  normal  diets  given  in  §  440 
the  above  diet  is  striking  for  the  low  amount  of  proteids  and 
of  fats  and  the  relative  excess  of  carbohydrates.  But  though 
such  a  diet  may  be  taken  as  perhaps  fairly  typical  of  the  daily 
food  of  a  rigid  vegetarian,  a  much  more  richly  proteid  diet 
may  be  obtained  from  sources  still  strictly  vegetable.  Thus 
the  diet,  entirely  vegetable  in  nature,  of  an  average  Japanese 
labourer  of  about  the  same  weight  as  the  individual  whose  data 
we  have  just  given  has  been  estimated  to  consist  of  Proteids 
102  grm.,  Fat  17  grm.,  Carbohydrates  578  grm.  And  the  diet 
of  a  Roumanian  peasant,  living  chiefly  on  beans  and  maize  with 
the  addition  of  fat  of  some  kind,  lias  been  calculated  to  furnish 
no  less  than  Proteids  182  grm..  Fat  93  grm..  Carbohydrates 
968  grm. ;  but  the  real  nutritive  value  of  such  a  diet  must  need 
very  large  correction  indeed,     (^f.  §  442. 


666  VEGETABLE  DIET.  [Book  ii. 

The  examination  of  the  diet  of  an  incli\ddual  living  with  a 
fah'  nitrogenous  equilibrium  and  apparently  good  health  on  a 
modified  vegetable  diet,  that  is  to  say  one  which  included  milk 
and  eggs,  gave  the  following:  Proteids  74  grm.,  Fat  58  grm., 
Carbohydrates  490  grm.,  a  diet  which  differs  from  the  normal 
diet  almost  solely  in  the  lesser  amount  of  proteids,  one  third  of 
which  by  the  bye  was  supplied  by  the  animal  material,  eggs  and 
milk.  In  another  instance,  nitrogenous  equilibrium  and  fairly 
good  health  were  secured,  for  some  weeks  at  all  events,  on  a 
vegetable  diet  yielding  Proteids  about  100  grm.,  Pats  70  grm., 
Carbohydrates  400  grm. ;  but  in  this  nearly  the  whole  of  the 
fat  was  furnished  by  the  animal  product  butter,  and  Liebig's 
extract  was  freely  used. 

Confining  ourselves  however  to  the  more  strictly  vegetarian 
diet,  we  may  conclude  in  the  first  place  that,  unless  the  daily 
food  be  very  large  in  amount,  the  proteid  element  of  such  a  diet 
falls  considerably  below  the  100  or  more  grm.  given  in  the 
normal  diet.  But  we  cannot  authoritatively  say  that  such  a 
reduction  is  necessarily  an  evil;  for  as  we  stated  above,  §  440, 
our  knowledge  will  not  at  present  permit  us  to  make  an  authori- 
tative exact  statement  as  to  the  extent  to  which  the  proteid  may 
be  reduced  without  disadvantage  to  the  body  when  accompanied 
by  adequate  provision  of  the  other  elements  of  food;  and  this 
statement  holds  good  whether  the  body  be  undertaking  a  small 
or  large  amount  of  labour.  A  second  feature  of  such  a  diet  is 
the  marked  reduction  of  the  fat  and  its  replacement  by  carbo- 
hydrates. Although  here  again  we  cannot  make  a  distinctly 
authoritative  statement,  the  evidence  which  we  possess  bears 
clearly  in  the  direction  that  such  a  reduction  is  a  marked  dis- 
advantage. A  third  and  very  characteristic  feature  of  the 
strictly  vegetarian  diet  is  the  relatively  large  amount  of  undi- 
gested food  lost  to  the  body  and  discharged  as  fteces.  Even 
when  the  diet  is  scanty,  so  that  the  proteid  element  is  low,  the 
amount  of  foeces  relatively  to  the  total  food  is  high;  and  when 
a  more  normal  proteid  contribution  is  secured  by  ample  meals 
the  faeces  become  exceedingly  voluminous.  Indeed  when,  leav- 
ing man,  we  compare  the  herbivorous  with  the  carnivorous 
mammal,  we  find  that  the  former  is  almost  as  clearly  distin- 
guished from  the  latter  by  its  frequent  and  abundant  fseces  as 
by  the  anatomical  features  of  its  organization.  We  have  already 
urged  that,  since  the  feeces  serve  as  a  means  of  excretion  of  the 
real  waste  products  of  metabolism,  a  certain  amount  of  vehicle 
to  carry  these  away  is  of  advantage  or  even  necessary;  but  there 
are  no  facts  at  present  known  to  us,  which  shew  that  the  larger 
intestinal  current  of  the  purely  vegetable  diet  effects  any  such 
good  as  can  compensate  for  the  obvious  waste  of  labour  incurred 
in  its  transport  and  management,  to  say  nothing  of  the  oppor- 
tunities of  mischief  offered  by  a  mass  of  material  more  subject 


€hap.  v.]  >^UTRITI0K  667 

to  the  dominion  of  foreign  organisms  than  even  to  that  of  the 
body  itself,  though  these  opportunities  are  less  than  with  a  cor- 
responding mass  of  animal  origin.  With  respect  to  these  three 
features  then,  the  strictly  vegetarian  diet  seems,  on  physiologi- 
cal grounds,  inferior  to  one  of  a  mixed  nature.  There  are  as  we 
said  other  aspects,  still  of  a  strictly  physiological  kind,  to  be 
considered,  such  as  the  relative  digestibility  of  vegetable  articles 
of  food,  the  relative  metabolic  value  of  the  food-stulfs  of  vege- 
table origin,  and  the  influence  of  animal  extractives;  but  any 
fuller  discussion  of  these  points  would  be  out  of  place  here. 

§  444.  We  have  treated  the  diet  discussed  above  as  a  normal 
diet,  suitable  for  man  under  ordinary  or  general  circumstances. 
Ought  such  a  diet  to  be  modified  for  the  various  exigences  of 
life  such  as  fabour,  age,  climate,  and  the  like? 

We  shall  discuss  the  influence  of  age  in  the  concluding  por- 
tions of  this  work. 

We  may  be  inclined  at  first  sight  to  assume  that  the  total 
amount  of  the  diet  should  vary  with  the  weight,  that  is  the  size 
of  the  individual;  and  indeed  in  discussions  on  nutrition,  state- 
ments concerning  metabolism  and  amount  of  food  are  often 
given  in  terms  of  per  kilo  of  body  weight.  In  a  broad  sense  it 
may  be  true  that  a  small  man  needs  less  food  than  a  large  one ; 
but  it  must  be  remembered  that,  as  we  saw  in  speakmg  of  animal 
heat,  the  smaller  organism,  having  the  relatively  larger  surface, 
carries  on  a  more  rapid  metabolism  per  unit  of  body  weight, 
and  so  needs  relatively  more  food.  And  moreover  the  influence 
of  size  is  probably  far  less  than  the  influence  exerted  by  the 
inborn  individual  characters  of  the  organism,  giving  rise  to 
what  we  may  call  the  personal  equation  of  metabolism.  The 
smaller  metabolism  of  woman,  leading  to  the  use  of  a  scantier 
diet,  as  compared  with  that  of  man,  is  to  be  regarded  in  this 
light  rather  than  with  reference  to  the  average  lesser  weight  of 
woman.  The  relative  metabolism  of  the  two  sexes  may  be  illus- 
trated by  the  case  of  an  active  man  and  his  wife,  both  of  about 
the  same  age  and  weight,  the  man  being  rather  the  heavier  and 
the  woman  rather  the  older,  who,  in  carrjang  out  together  an 
experiment  on  the  relative  values  of  vegetable  and  animal  food, 
both  lived  for  some  time  on  the  same  kind  of  diet,  and  found 
that  nutritive  equilibrium  was,  in  the  one  case  and  in  the  otlier, 
maintained  when 

Pi'oteids. 
The  man  consumed  daily  about  100 
The  wife         „  „         ,,        60 

The  most  striking  difference  is  in  the  proteids. 
§  445.     With   regard   to    climate   the    chief    considerations 
attach  to  temperature.     When  the  body  is  exposed  to  a  low 
temperature  the  general  metabolism  of  the  body  is  increased 


Fats. 

Carbohydrates. 

70 

400 

67 

340 

668  MODIFICATION   OF   DIET.  [Book  ii. 

owing  to  a  regulative  action  of  the  nervous  system,  §  428.  We 
might  infer  from  this  that  more  food  is  necessary  in  cold  cli- 
mates ;  and,  since  the  increase  in  the  metabolism  appears  to  man- 
ifest itself  chiefly  in  a  greater  discharge  of  carbonic  acid  and 
therefore  to  be  especially  a  carbon  metabolism,  we  might  infer 
that  the  carbon  elements  of  food  should  be  especially  increased. 
When  the  body  is  exposed  to  high  temperatures  the  same  reflex 
mechanism  tends  to  lower  the  metabolism ;  but  the  effects  in 
this  direction  are  much  less  clear  than  those  of  cold,  and  soon 
reach  their  limits ;  the  bodily  temperature  is  maintained  con- 
stant under  the  influence  of  surrounding  warmth  not  so  much 
by  diminished  production  as  by  increased  loss.  We  may  infer 
from  this  that  in  warm  climates  not  less  but  if  anything  rather 
more  food  than  in  temperate  climates  is  necessary  in  order  to 
supply  the  perspiration  needed  for  the  greater  evaporation  and 
discharge  of  heat  by  the  skin. 

In  both  cold  and  warm  climates  however  man  trusts  much 
more  to  variations  in  his  clothings  and  immediate  surroundings 
to  protect  him  against  cold  or  to  guard  him  from  heat  than  to 
any  marked  variations  in  his  normal  diet.  In  the  former  he 
may  perhaps  be  expected  to  eat  somewhat  more,  since,  in  spite 
of  wrappings,  his  skin  still  feels  in  part  the  cold,  and  thus  the 
nervous  mechanism  for  the  increase  of  metabolism  is  to  a  certain 
extent  set  to  work.  And  since  the  metabolism  thus  increased 
appears  to  affect  especially  the  carbon  of  the  body,  he  may  fur- 
ther be  expected  to  increase  the  fats  rather  than  the  carbohy- 
drates of  his  food  seeing  that  the  former  supply  him  with  the 
most  energy  for  their  weight.  But  it  is  very  doubtful  whether 
what  he  might  thus  be  expected  to  gain  over  a  corresponding 
increase  in  carbohydrates  is  not  more  than  counterbalanced  by 
the  increased  labour  of  digestion  ;  and  the  habits  of  the  dwellers 
in  arctic  climates  cannot  safely  be  taken  as  guides  in  this  mat- 
ter, for  their  reputed  love  of  fat  is  probably  the  result  of  that 
being  their  most  available  form  of  carbon.  Indeed  the  evidence 
that  the  increase  of  metabolism  provoked  by  cold  bears  exclu- 
sively on  carbon  constituents  is  so  uncertain  that  it  may  be 
doubted  whether  any  change  in  the  normal  diet,  beyond  some 
increase  in  the  whole,  should  be  made  to  meet  a  cold  climate. 
Similar  reasons  would  lead  one  to  infer  that  man  in  the  warmer 
climate  would  maintain  on  the  whole  the  same  normal  diet,  the 
only  change  perhaps  being  to  increase  it  slightly,  possibly 
throwing  the  increase  chiefly  on  the  carbohydrates  with  the 
special  view  of  furthering  perspiration. 

§  446.  A  special  diet  for  the  purpose  of  fattening,  that  is 
to  say  for  the  accumulation  of  adipose  tissue  out  of  proportion 
to  the  rest  of  the  body,  is  not  needed  in  the  case  of  man.  The 
power  to  store  up  fat  in  adipose  tissue  is  much  more  dependent 
on  certain  inborn  qualities  of  the  organism  which  we  cannot  at 


Chap,  v.]  NUTRITION.  669 

present  define  than  on  the  kind  of  food ;  of  two  bodies  living 
on  the  same  diet,  and  under  the  same  circumstances,  one  will 
become  fat  while  the  other  will  remain  lean  ;  and  it  is  an  object 
of  the  agriculturalist  to  develope  by  breeding  and  selection  a 
"constitution  "  which  will  store  up  the  most  fat  on  the  cheap- 
est diet.  In  fattening  animals,  the  chief  care,  when  the  selec- 
tion of  the  kind  of  animal  has  been  made,  is  to  provide  adequate 
carbohydrate  food,  which  as  we  have  seen  is  the  chief  fattener  ; 
and  the  object  of  the  farmer  in  rearing  stock  for  the  butcher  is 
mainly  to  convert  cheap  vegetable  carbohydrate  into  dear  ani- 
mal fat.  Further  aids  in  fattening  may  be  found  in  providing 
repose  for  the  body  of  such  a  kind  that,  while  sufiicient  energy 
is  expended  to'secure  adequate  digestion  and  absorption  of  food, 
all  causes  leading  to  an  increase  of  metabolism,  hj  which  energy 
is  set  free  and  leaves  the  body,  are  avoided  as  much  as  possible. 

To  avoid  fat  rather  than  to  increase  it  is  often  an  object  of 
human  care.  This  may  be  effected  by  diminishing  fats  and 
carbohydrates,  but  also,  in  a  very  marked  manner,  by  relatively 
increasing  the  proteids.  Proteid  food  as  we  have  seen  augments 
the  whole  metabolism  of  the  body,  hurrying  on  the  destruction 
not  only  of  proteid  but  of  carbon  food ;  and  a  tendency  to  cor- 
pulency may  be  counteracted  by  a  diet  in  which  fats  and  carbo- 
hydrates are  much  restricted,  and  proteids  are  largely  increased. 
When,  as  in  what  is  known  as  the  Banting  method,  the  diet  is 
almost  exclusively  proteid,  the  nitrogenous  overwork  entails 
dangers  on  organisms  which  do  not  possess  the  power  of  ridding 
themselves  freel}'  of  the  large  amount  of  nitrogenous  waste 
which  such  a  diet  produces.  A  less  severe  method  in  which 
the  fats  and  carbohydrates  are  diminished  only,  not  entirely 
done  away  with,  and  the  proteids  only  moderately  increased, 
is  less  open  to  objection;  and  such  a  diet,  assisted  by  other 
hygienic  conditions,  has  proved  successful. 

An  increase  of  daily  food,  largely  proteid  in  nature,  given 
under  circumstances,  such  as  a  large  amount  of  passive  exercise 
and  skin  stimulation,  known  as  "  massage,'  which  will  not  only 
favour  digestion  but  also  promote  metabolism  in  general,  may 
be  given,  with  favourable  results.  In  this  way,  an  enormous 
metabolism  ma}'  be  excited,  and  yet  so  carried  on  that  the  body 
gains  both  in  flesh  and  in  fat.  Thus,  in  one  case,  the  patient 
with  an  initial  weight  of  45  kilos,  and  a  daily  nitrogenous 
metabolism,  calculated  as  28  grm.  proteid,  reached  in  the  course 
of  about  50  days  a  weight  of  60  kilos,  the  daily  nitrogenous 
metabolism  being  raised  on  one  occasion  to  182  grm.  proteid, 
with  an  average  on  the  whole  period  of  150  grm.  During  the 
treatment  no  less  than  8420  grm.  of  proteid  were  taken  as 
food. 

§  447.  With  regard  to  labour,  since  as  we  have  seen  the 
energy  expended  as  work  done  is  not  taken  out  of  and  away 


670  FOOD   AND    LABOUR.  [Book  ii. 

from  the  amount  set  free  as  heat,  the  two  forms  of  energy  be- 
ing so  related  that  an  increase  of  work  clone  is  accompanied  by 
a  o-reater  or  less  increase  of  heat  set  free,  it  is  obvious  that  a 
man  who  is  doing  a  hard  day's  muscular  work  needs  a  larger 
income  of  energy  for  the  day  than  does  an  idle  man.  What  we 
have  learnt  concerning  muscular  metabolism  further  shews  us 
that  the  additional  energy  needed  is  not  necessarily  to  be  sup- 
plied by  an  increase  in  the  proteid  components  of  the  diet ;  the 
energy  of  muscular  contraction  does  not  come  as  was  once 
thought  from  proteid  metabolism  (§  423).  The  fact  that  it  is 
the  carbon  metabolism  which  is  augmented  in  muscular  work 
may  suggest  that  the  extra  food  for  extra  work  should  be 
exclusively  carbon  compounds  ;  and  if,  as  we  have  seen  to  be 
probable,  the  carbohydrates  are  more  readily  and  directly  avail- 
able for  the  functional  metabolism  of  muscle  than  are  the  fats, 
we  might  be  further  led  to  recommend  an  increase  in  carbo- 
hydrates to  form  a  diet  especially  suited  for  labour.  This  view 
seems  directly  supported  by  the  experimental  result  that  even 
a  small  quantity  of  sugar  taken  by  the  mouth  has  an  immediate 
favourable  effect  on  the  power  of  the  muscles.  But  several 
considerations  have  to  be  taken  into  account  in  this  matter. 
A  muscle  is  not  a  machine  within  the  body  which  can  be  loaded 
and  fired  off  irrespective  of  the  rest  of  the  body.  In  the  per- 
formance of  muscular  labour,  the  condition  of  the  muscle,  the 
amount  of  energy  available  in  the  muscle  itself,  is  of  course  of 
prime  importance  ;  but,  and  this  perhaps  especially  holds  good 
in  severe  labour,  of  great  importance  also,  we  might  almost  say 
of  no  less  importance,  is  as  we  have  urged  (§  317)  the  power 
of  the  body  as  a  whole  to  avail  itself  of  the  energy  latent  in 
the  muscle.  The  power  of  doing  work  hangs  not  on  the  muscle 
alone,  but  on  the  heart,  the  lungs,  the  nervous  system  and 
indeed  on  the  whole  body.  It  is  very  doubtful  whether  we 
ever,  even  in  supreme  efforts,  draw  upon  more  than  a  portion 
of  the  capital  of  energy  lodged  in  the  muscle  itself ;  fatigue  is 
far  more  a  nervous  than  a  muscular  condition,  and  even  the 
distinctly  muscular  fatigue  is  as  we  have  seen  (§81)  partly  at 
least  the  result  of  the  accumulation  of  products  and  not  alone 
the  using  up  of  available  energy.  In  choosing  a  diet  for  mus- 
cular labour  we  must  have  in  view  not  the  muscle  itself  but 
the  whole  organism.  And  though  it  is  possible  that  future 
research  may  suggest  minor  changes  in  the  various  components 
of  a  normal  diet  such  as  would  lessen  the  strain  during  labour 
on  this  or  that  part  of  the  body,  on  the  muscles  as  well  as  on 
other  organs,  our  present  knowledge  would  rather  lead  us  to 
conclude  that  what  is  good  for  the  organism  in  comparative 
rest  is  good  also  for  the  organism  in  arduous  work,  that  the 
diet,  normal  for  the  former  condition,  would  need  for  the  latter 
a  limited  total  increase  but  no  striking  change  in  its  composi- 


Chap,  v.]  NUTEITION. 


671 


tion.  In  preparing  the  body  for  some  coming  arduous  labour 
in  "training"  as  it  is  called,  an  increase  of  proteid  food,  for 
the  purpose  of  hurrying  on  the  general  metabolism  of  the  body 
and  thus  of  making  '  new  flesh '  and  renovating  the  body,  so  to 
speak,  in  view  of  the  strain  to  be  put  upon  it,  may  perhaps 
suggest  Itself  ;  but  even  this  is  doubtful. 

The  principles  of  such  a  conclusion  with  regard  to  muscular 
work  may  be  applied  with  still  greater  confidence  to  nervous 
or  mental  work.  The  actual  expenditure  of  energy  in  nervous 
work  IS  relatively  small,  but  the  indirect  influence  on  the 
economy  IS  very  great.  The  closeness  and  intricacies  of  the 
ties  which  bind  ali  parts  of  the  body  together  is  very  clearly 
shewn  by  the  well-known  tendency  of  so-called  brain  work  to 
derange  the  digestive  and  metabolic  activities  of  the  body  • 
and  if  there  be  any  diet  especially  suited  for  intellectual  labour 
It  is  one  directed  not  m  any  way  towards  the  brain,  but  entirely 
towards  lightening  the  labours  of  and  smoothing  the  way  for 
such  parts  of  the  body  as  the  stomach  and  the  liver 


BOOK  III. 


THE   CENTRAL  NERVOUS   SYSTEM  AND   ITS 
INSTRUMENTS. 


43 


CHAPTER   I. 
THE   SPINAL   CORD. 

SEC.   1.    ON  SOME  FEATUEES  OF  THE  SPINAL  NEEVES. 

§  448.  We  have  called  the  muscular  and  nervous  tissues  the 
master  tissues  of  the  body ;  but  a  special  part  of  the  nervous 
system,  that  which  we  know  as  the  central  nervous  system,  the 
brain  and  spinal  cord,  is  supreme  among  the  nervous  tissues 
and  is  master  of  the  skeletal  muscles  as  well  as  of  the  rest  of 
the  body.  We  have  already  (Book  i.  Chap,  m.)  touched  on 
some  of  the  general  features  of  the  nervous  system,  and  have 
now  to  study  in  detail  the  working  of  the  brain  and  spinal  cord. 
We  have  to  inquire  what  we  know  concerning  the  laws  which 
regulate  the  discharge  of  efferent  impulses  from  the  brain  or 
from  the  cord,  and  to  learn  how  that  discharge  is  determined 
on  the  one  hand  by  intrinsic  changes  originating,  apparently,  in 
the  substance  of  the  brain  or  of  the  cord,  and  on  the  other  hand 
by  the  nature  and  amount  of  the  afferent  impulses  which  reach 
them  along  afferent  nerves. 

As  we  shall  see,  the  study  of  the  spinal  cord  cannot  be  wholly 
separated  from  that  of  the  brain,  the  two  being  very  closely 
related.  Nevertheless  it  will  be  of  advantage  to  deal  with  the 
spinal  cord  by  itself  as  far  as  we  can.  The  medulla  oblongata 
or  spinal  bulb^  we  shall  consider  as  part  of  the  brain.  But 
before  we  speak  of  the  spinal  cord  itself,  it  will  be  desirable  to 
say  a  few  words  concerning  the  spinal  nerves,  that  is  to  say  the 
nerves  which  issue  from  the  spinal  cord. 

We  have  already  seen  (§  88)  that  each  of  the  spinal  nerves 
arises  by  two  roots,  an  anterior  root  attached  to  the  ventral  or 

1  The  term  medulla  oblongata  is  not  only  long,  but  presents  difficulties, 
since  the  word  medulla  is  now  rarely  used  to  denote  the  whole  spinal  cord  (me- 
dulla spinalis)  but  is  generally  used  to  denote  the  peculiar  coat  of  a  nerve  fibre, 
the  white  substance  of  Schwann.  In  using  instead  the  word  bulb  or  if  neces- 
sary, spinal  bulb  there  is  little  fear  of  confusion  with  any  other  kind  of  bulb. 
The  adjective  is  in  not  uncommon  use,  in  such  phrases  as  '  bulbar  paralysis.' 

675 


676  SPINAL   NEEVES.  [Book  in. 

anterior  surface,  and  a  posterior  root  attached  to  the  dorsal  or 
posterior  surface  of  the  cord.  We  have  further  seen  that  the 
latter  bears  a  ganglion,  a  'ganglion  of  the  posterior  root'  or 
'  spinal  ganglion.'  We  stated  at  the  same  time  that  while  the 
trunk  of  a  spinal  nerve  contained  both  efferent  and  afferent 
fibres,  the  efferent  fibres  were  gathered  up  into  the  anterior 
root  and  the  afferent  fibres  into  the  posterior  root ;  but  we  gave 
no  proof  of  this  statement. 

§  449.  Before  we  proceed  to  do  so,  it  will  be  as  well  to  say 
a  few  words  on  the  terms  'efferent'  and  'afferent.'  By  efferent 
nerve  fibres  we  mean  nerve  fibres  which  in  the  body  usually 
carry  impulses  from  the  central  nervous  system  to  peripheral 
organs.  Most  efferent  nerve  fibres  carry  impulses  to  muscles, 
striated  or  plain,  and  the  impulses  passing  along  them  give  rise 
to  movements ;  hence  they  are  frequently  spoken  of  as  '  motor ' 
fibres.  But  all  efferent  fibres  do  not  end  in  or  carry  impulses 
to  muscular  fibres;  we  have  seen  for  instance  that  some  efferent 
fibres  are  secretory.  Moreover  all  the  nerve  fibres  going  to 
muscular  fibres  do  not  serve  to  produce  movement ;  some  of 
them,  as  in  the  case  of  certain  vagus  fibres  going  to  the  heart, 
are  inhibitory  and  may  serve  to  stop  movement. 

By  'afferent'  nerve  fibres  we  mean  nerve  fibres  which  in  the 
body  usually  carry  impulses  from  peripheral  organs  to  the  cen- 
tral nervous  system.  A  very  common  effect  of  the  arrival  at 
the  central  nervous  system  of  impulses  passing  along  afferent 
fibres  is  that  change  in  consciousness  which  we  call  a  'sensa- 
tion'; hence  afferent  fibres  or  impulses  are  often  called  'sensory' 
fibres  or  impulses.  But  as  we  have  already  in  part  seen,  and  as 
we  shall  shortly  see  in  greater  detail,  the  central  nervous  system 
may  be  affected  by  afferent  impulses,  and  that  in  several  ways, 
quite  apart  from  the  development  of  any  such  change  of  con- 
sciousness as  may  be  fairly  called  a  sensation.  We  shall  see 
reason  for  thinking  that  afferent  impulses  reaching  the  spinal 
cord,  and  indeed  other  parts  of  the  central  nervous  system,  may 
modify  reflex  or  automatic  or  other  activity  without  necessarily 
giving  rise  to  a  "sensation."  Hence  it  is  advisable  to  reserve 
the  terms  'efferent'  and  'afferent'  as  more  general  modes  of 
expression  than  '  motor  '  or  '  sensory.' 

We  have  seen  in  treating  of  muscle  and  nerve,  that  the 
changes  produced  in  the  muscle  serve  as  our  best  guide  for 
determining  the  changes  taking  place  in  a  motor  nerve ;  when 
a  motor  nerve  is  separated  from  its  muscle  (§  67)  the  only 
change  which  we  can  appreciate  in  it  is  an  electrical  change. 
Similarly  in  the  case  of  an  afferent  nerve,  the  central  system  is 
our  chief  teacher;  in  a  bundle  of  afferent  fibres  isolated  from 
the  central  nervous  system,  in  a  posterior  root  of  a  spinal  nerve 
for  instance,  the  only  change  which  we  can  appreciate  is  an 
electrical  change.     To  learn  the  characters  of  afferent  impulses 


Chap,  i.]  THE   SPINAL   CORD.  677 

we  must  employ  the  central  nervous  system.  But  in  this  we 
meet  with  difficulties.  In  studying  the  phenomena  of  motor 
nerves  we  are  greatly  assisted  by  two  facts.  First,  the  muscular 
contraction  by  which  we  judge  of  what  is  going  on  in  tlie  nerve 
is  a  comparatively  simple  thing,  one  contraction  differing  from 
another  only  by  such  features  as  extent  or  amount,  duration, 
frequency  of  repetition- and  the  like,  and  all  such  differences 
are  capable  of  exact  measurement.  Secondly,  when  we  apply  a 
stimulus  directly  to  the  nerve  itself,  the  effects  differ  in  degree 
only  from  those  which  result  when  the  nerve  is  set  in  action  by 
natural  stimuli,  such  as  the  will.  When  we  come,  on  the  other 
hand,  to  investigate  the  phenomena  of  afferent  nerves,  our 
labours  are  for  the  time  rendered  heavier.,  but  in  the  end  more 
fruitful,  by  the  following  circumstances :  —  First,  when  we 
judge  of  what  is  going  on  in  an  afferent  nerve  by  the  effects 
which  stimulation  of  the  nerve  produces  in  some  central  ner- 
vous organ,  in  the  way  of  exciting  or  modifying  reflex  action, 
or  modifying  automatic  action,  or  affecting  consciousness,  we 
are  met  on  the  very  threshold  of  every  inquiry  b}^  the  difficulty 
of  clearly  distinguishing  the  events  which  belong  exclusively 
to  the  afferent  nerve  from  those  which  belong  to  the  central 
organ.  Secondly,  the  effects  of  applying  a  stimulus  to  the 
peripheral  end-organ  of  an  afferent  nerve  are  very  different  from 
those  of  applying  the  same  stimulus  directly  to  the  nerve-trunk. 
This  may  be  shewn  by  the  simple  experience  of  comparing  the 
sensation  caused  by  bringing  any  sharp  body  into  contact  with 
a  nerve  laid  bare  in  a  wound  with  that  caused  by  contact  of  an 
intact  skin  with  the  same  body.  These  and  like  differences 
reveal  to  us  a  complexity  of  impulses,  of  which  the  phenomena 
of  motor  nerves  gave  us  hardly  a  hint. 

We  shall  further  see  in  detail  later  on  that  our  consciousness 
may  be  affected  in  many  different  waj^s  by  afferent  impulses ; 
we  must  distinguish  not  only  sensory  from  other  afferent  im- 
pulses, but  also  different  kinds  of  sensory  impulses  from  each 
other.  Certain  afferent  nerves  are  spoken  of  as  nerves  of 
special  sense,  and  the  nature  of  the  afferent  impulses  passing 
along  these  special  nerves  together  Avith  the  modifications  of 
consciousness  caused  by  arrival  of  these  impulses  at  the  central 
nervous  system  constitute  by  themselves  a  complex  and  diihcult 
branch  of  study.  In  some  of  the  problems  connected  with  the 
central  nervous  system  we  shall  have  to  appeal  to  the  results 
of  a  study  of  these  special  senses ;  but,  on  the  other  hand,  a 
knowledge  of  the  central  nervous  system  is  necessary  to  a 
proper  understanding  of  the  special  senses ;  and  on  the  whole 
it  will  be  more  convenient  to  study  the  former  before  the  latter. 

§  450.  The  proof  that  the  afferent  and  efferent  fibres 
which  are  both  present  in  the  trunk  of  a  spinal  nerve  are 
parted  at  the  roots,  the  efferent  fibres  running  exclusively  in 


678  SPINAL  NERVES.  [Book  hi. 

the  ventral  or  anterior  root  and  the  afferent  fibres  exclusively 
in  the  dorsal  or  posterior  root,  is  as  follows. 

When  the  anterior  root  is  divided,  the  muscles  supplied  by 
the  nerve  cease  to  be  thrown  into  contractions  either  by  the 
will,  or  by  reflex  action,  while  the  structures  to  which  the 
nerve  is  distributed  retain  their  sensibility.  During  the  sec- 
tion of  the  root,  or  when  the  proximal  stump,  that  connected 
with  the  spinal  cord,  is  stimulated,  no  sensory  effects  are  pro- 
duced. When  the  distal  stump  is  stimulated,  the  muscles  sup- 
plied by  the  nerve  are  thrown  into  contractions.  When  the 
posterior  root  is  divided,  the  muscles  supplied  by  the  nerve 
continue  to  be  thrown  into  action  by  an  exercise  of  the  will  or 
as  part  of  a  reflex  action,  but  the  structures  to  which  the  nerve 
is  distributed  lose  the  sensibility  which  they  previously  pos- 
sessed. During  the  section  of  the  root,  and  when  the  proximal 
stump  is  stimulated,  sensory  effects  are  produced.  When  the 
distal  stump  is  stimulated  no  movements  are  called  forth. 
These  facts  demonstrate  that  sensory  impulses  pass  exclusively 
by  the  posterior  root  from  the  peripheral  to  the  central  organs, 
and  that  motor  impulses  pass  exclusively  by  the  anterior  root 
from  the  central  to  the  peripheral  organs  ;  and  so  far  as  our 
knowledge  goes  the  same  holds  good  not  only  for  sensory  and 
motor  but  also  for  afferent  and  efferent  impulses. 

An  exception  must  be  made  to  the  above  general  statement, 
on  account  of  the  so-called  "  recurrent  sensibility "  which  is 
witnessed  in  conscious  mammals,  under  certain  circumstances. 
It  sometimes  happens  that  when  the  distal  stump  of  the  divided 
anterior  root  is  stimulated,  signs  of  pain  are  witnessed.  These 
are  not  caused  by  the  concurrent  muscular  contractions  or 
cramp  which  the  stimulation  occasions,  for  they  persist  after 
the  whole  trunk  of  the  nerve  has  been  divided  some  little  way 
below  the  union  of  the  roots  above  the  origins  of  the  muscular 
branches,  so  that  no  contractions  take  place.  They  disappear 
when  the  posterior  root  is  subsequently  divided,  and  they  are 
not  seen  if  the  mixed  nerve-trunk  be  divided  close  to  the  union 
of  the  roots.  The  phenomena  are  probably  due  to  the  fact, 
that  bundles  of  sensory  fibres  of  the  posterior  root  after  run- 
ning a  short  distance  down  the  mixed  trunk  turn  back  and  run 
upwards  in  the  anterior  root,  (being  distributed  probably  to 
the  pia  mater,)  and  by  this  recurrent  course  give  rise  to  the 
recurrent  sensibility. 

§  451.  Concerning  the  ganglion  on  the  posterior  root,  we 
have  already  said  that  we  have  no  evidence  either  that  it  can 
act  as  a  centre  of  reflex  action,  or  that  it  can  spontaneously 
give  origin  to  efferent  impulses  and  thus  act  as  an  automatic 
centre,  as  can  the  central  nervous  system  itself.  The  bodies 
of  the  nerve-cells  behave  somewhat  differently  from  the  axis- 
cylinders  at  some  distance  from  the  cells,  though,  as  we  have 


Chap,  i.]  THE   SPINAL   CORD.  679 

seen,  these  are  in  reality  iDrocesses  of  the  nerve-cells  ;  thus  the 
nerve-cells  in  the  ganglion  appear  to  be  more  sensitive  to  cer- 
tain poisons  than  are  the  nerve-fibres  of  the  nerve-trunk.  But 
beyond  this,  our  knowledge  concerning  the  function  of  the 
ganglion  is  almost  limited  to  the  fact  that  it  is  in  some  way 
intimately  connected  with- the  nutrition  of  the  nerve.  As  we 
have  already  (§  78)  said,  when  a  mixed  nerve-trunk  is  divided 
the  peripheral  portion  degenerates  from  the  point  of  section 
downwards  towards  the  periphery.  The  central  portion  does 
not  so  degenerate,  and  if  the  length  of  nerve  removed  be  not 
too  great,  the  central  portion  may  grow  downwards  along  the 
course  of  the  degenerating  peripheral  portion,  and  thus  regen- 
erate the  nerve.  This  degeneration  is  observed  when  the 
mixed  trunk  is  divided  in  any  part  of  its  course  from  the 
periphery  to  close  up  to  the  ganglion.  When  the  posterior 
root  is  divided  between  the  ganglion  and  the  spinal  cord,  the 
portion  attached  to  the  spinal  cord  degenerates,  but  that 
attached  to  the  ganglion  remains  intact.  When  the  anterior 
root  is  divided,  the  proximal  portion  in  connection  with  the 
spinal  cord  remains  intact,  but  the  distal  portion  between  the 
section  and  the  junction  with  the  other  root  degenerates  ;  and 
in  the  mixed  nerve-trunk  many  degenerated  fibres  are  seen, 
which,  if  they  be  carefully  traced  out,  are  found  to  be  motor 
(efferent)  fibres.  If  the  posterior  root  be  divided  carefully 
between  the  ganglion  and  the  junction  with  the  anterior  root, 
the  small  portion  of  the  posterior  root  left  attached  to  the 
peripheral  side  of  the  ganglion  above  the  section  remains 
intact,  as  does  also  the  rest  of  the  root  from  the  ganglion 
to  the  spinal  cord,  but  in  the  mixed  nerve-trunk  are  seen 
numerous  degenerated  fibres,  which  when  examined  are  found 
to  have  the  distribution  of  sensory  (afferent)  fibres.  Lastly, 
if  the  posterior  ganglion  be  excised,  the  whole  posterior  root 
degenerates,  as  do  also  the  sensory  (afferent)  fibres  of  the 
mixed  nerve-trunk.  Putting  all  these  facts  together,  it  would 
seem  that  the  growth  of  the  efferent  and  afferent  fibres  takes 
place  in  opposite  directions,  and  starts  from  different  nutritive 
or  '  trophic '  centres.  The  afferent  fibres  grow  away  from  the 
ganglion  either  towards  the  periphery,  or  towards  the  spinal 
cord.  The  efferent  fibres  grow  outwards  from  the  spinal  cord 
towards  the  periphery.  This  difference  in  their  mode  of  nutri- 
tion is  frequently  of  great  help  in  investigating  the  relative 
distribution  of  efferent  and  afferent  fibres.  When  a  posterior 
root  is  cut  beyond  the  ganglion,  or  the  ganglion  excised,  all  the 
afferent  nerves  degenerate,  and  in  the  mixed  nerve  branches 
these  afferent  fibres,  by  their  altered  condition,  can  readily 
be  traced.  Conversely,  when  the  anterior  roots  are  cut,  the 
efferent  fibres  alone  degenerate,  and  can  be  similarly  recognized 
in  a  mixed  nerve  tract.      When  the  anterior  root  is  divided 


680  SPINAL  NERVES.  [Book  in. 

some  few  fibres  in  it  do  not,  like  the  rest,  degenerate,  and 
when  the  posterior  root  is  divided,  a  few  fibres  in  the  anterior 
root  are  seen  to  degenerate  like  those  of  the  posterior  root ; 
these  appear  to  be  the  fibres  which  give  to  the  anterior  root  its 
"recurrent  sensibility."  In  the  case  of  certain  spinal  nerves 
at  all  events,  it  has  also  been  ascertained  that  when  the  pos- 
terior root  is  divided,  while  most  of  the  fibres  in  the  part  of 
the  root  thus  cut  off  from  the  ganglion  but  left  attached  to  the 
cord  degenerate,  some  few  do  not.  These  few  appear  to  have 
their  trophic  centre  not  in  the  ganglion,  but  in  some  part  of 
the  spinal  cord  itself ;  we  shall  refer  to  these  later  on. 

This  method  of  distinguishing  nerve  fibres  by  the  features 
of  their  degeneration,  called  the  "  degeneration  method,"  or 
sometimes  from  the  name  of  the  physiologist  who  introduced 
it,  the  "Wallerian  method,"  has  proved  of  great  utility.  Thus 
in  the  vagus  nerve  which  is  composed  not  only  of  fibres  which 
spring  from  the  real  vagus  root  but  also  of  fibres  proceeding 
from  the  spinal  accessory  roots,  the  two  may  be  distinguished 
by  section  of  the  vagus  and  spinal  accessory  roots  respectively. 
We  shall  presently  see  that  this  method  may  be  applied  to  the 
differentiation  of  tracts  of  fibres  in  the  brain  and  spinal  cord. 


SEC.   2.     THE   STRUCTURE   OF   THE   SPIXAL   COED. 

§  452.  Before  we  proceed  to  discuss  the  functions  of  the 
spinal  cord  it  may  be  as  well  to  call  to  mind  some  of  the  main 
features  of  its  minute  structure.  (Figs.  110,  111,  112.)  The  greater 
number  of  the  fibres  of  the  anterior  root  are  the  clothed  axis-cyl- 
inder processes  of  cells  lying  in  the  anterior  horn  of  the  same  side, 
placed  not  far  from  the  origin  of  the  root  and  belonging  to  the 
segment  of  the  cord  indicated  by  the  root.  The  rest  of  the 
fibres  of  the  root  are  the  clothed  axis-cylinder  processes  of  cells 
lying  in  other  parts  of  the  grey  matter,  in  the  anterior  horn  of 
the  opj)Osite  side  and  elsewhere.  In  the  case  of  all  the  fibres 
of  the  root,  the  axis  cylinder  is  the  continuation  of  a  process  of 
a  cell  lying  in  the  grey  matter  not  far  from  the  origin  of  the 
root.  We  may  infer  that  changes  started  in  these  cells  issue 
as  efferent  impulses  along  the  fibres  of  the  root;  those  started 
in  the  cells  of  anterior  horn  are  motor  impulses  passing  to  the 
skeletal  muscles,  the  smaller  number  started  in  other  cells  are 
probably  efferent  impulses  destined  for  other  structures,  vaso- 
constrictor impulses  and  the  like. 

The  fibres  of  the  posterior  root,  coming  from  and  dependent 
for  their  nutrition  on  the  ganglion  of  the  root  (we  may  neglect 
the  few  exceptional  fibres  of  a  dilferent  nature  and  having 
different  relations),  passing  for  the  most  part  into  the  posterior 
column  so  as  to  form  the  "  root-zone  "  lying  close  to  the  median 
side  of  the  posterior  horn,  take  the  following  course.  Each 
fibre,  soon  after  its  entrance  into  the  cord,  bifurcates,  one 
division  running  forwards  towards  the  head,  the  other  back- 
wards towards  tlie  hind  extremity.  Each  division  gives  off 
"collateral"  fibres,  and  the  end  of  each  collateral  as  well  as  the 
final  termination  of  each  division  is  furnished  by  an  arborescence 
which  embraces  and  is  in  contact  with  but  not  in  continuity 
with,  the  body  of,  and  generally  with  the  branches  of  the  body  of 
some  nerve-cell  in  the  grey  matter.  But  the  course  of  the  fibres 
is  not  the  same  in  all  cases.  Many  of  the  fibres  (and  appar- 
ently all  the  backward  travelling  divisions  of  all  the  fibres)  are 
thus  by  means  of  the  collaterals  or  the  main  terminations  brouglit 

681 


682  STRUCTUEE   OF   SPINAL   COED.        [Book  in. 

P.F 


A.F 

Fig.  110.   A  Transverse  Dorsoventral  Section  of  the  Spinal  Cord  (Human) 
AT  THE  Level  of  the  Sixth  Thoracic  (Dorsal)  Nerve.     (Sherrington. )i 

Magnified  15  times.     One  lateral  half  only  is  shewn.     The  large  conspicuous 
nerve-cells  (drawn  from  actual  specimens)  are  shaded  black  to  render  their  rela- 
tive size,  shape  and  position  more  obvious ;  the  outline  of  the  grey  matter  has 
been  made  thick  and  dark  in  order  to  render  it  conspicuous. 
A.F.  anterior  fissure.     P.F.  posterior  fissure,     c.c.  central  canal,     cg.s.  central 
gelatinous  substance.  A.r.  anterior  root,  P.r.  lateral  (or  intermediate)  bundle, 
P.r'.  median  bundle  of  posterior  root  of  spinal  nerve,  p',  p''  fibres  of  poste- 
rior root  passing  p\  indirectly  through  the  substance  of  Rolando,  p" ,  directly 
into  grey  matter,      a.g.o.  anterior  grey  commissure,    p.g.c.  posterior  grey 
commissure,     a.c.  anterior  white  commissure,      ant.  col.  anterior  column. 
lat.  col.  lateral  column,    post.  col.  posterior  column,     s.g.  the  substance  of 
Rolando,     s.  septum  marking  out  the  external  posterior  column  or  column 
of  Burdach,  e.p.,  from  the  median  posterior  column  or  column  of  Goll,  m.p. 
1.    cells  of  the  anterior  horn.     3.  posterior  vesicular  column  or  vesicular  cylin- 
der, or  column  of  Clarke  ;  the  area  of  the  cylinder  is  defined  by  a  dotted  line. 
4.  cells  of  the  intermedio-lateral  tract  or  lateral  horn.     6.  cells  of  the  poste- 
rior horn.     7.  cells  of  the  anterior  cervix,     y.  a  tract  of  fibres  passing  from 
the  vesicular  cylinder  to  the  lateral  column. 
1  For  this  and  many  succeeding  figures  I  am  deeply  indebted  to  my  friend  and 
former  pupil  Dr.  Sherrington  who  has  kindly  prepared  the  figures  for  me  from 
his  original  drawings. 


Chap,  i.] 


THE   SPINAL   CORD. 


683 


Fig.  111.    Transverse  Dorsoventral  Section  of  Spinal  Cord  (Human)  at 

THE  Level  of  the  Sixth  Cervical  Nerve.     (Sherrington.) 
This  is  drawn  on  the  same  scale  as  Fie:.  110,  that  is  magnified  15  times. 
r.f.l.  lateral  reticular  formation,     r.f.p.  posterior  reticular  formation,    p'.  fine 
fibres  of  lateral  bundle  of  the  posterior  root ;  p'  ,p"'  fibres  of  median  bundle 


684  THE  NERVE-CELLS   OF  THE   COED.     [Book  in. 

of  posterior  root,  entering  grey  matter  from  external  posterior  column. 
X.  grey  matter  of  posterior  horn.  Sp.  a.  bundles  of  fibres  belonging  to  the 
spinal  accessory  nerve  ;  in  the  lateral  reticular  formation  they  are  seen  cut 
transversely,  b.  is  a  natural  septum  of  connective  tissue  marking  out  the 
cerebellar  tract  C.T.  fi'om  the  crossed  pyramidal  tract  G.P.T.  z.  s.  zona 
spongiosa.  2  a,  /3,  y,  lateral  cells  of  the  anterior  horn.  5.  Cells  in  the  region 
of  the  lateral  reticular  formation.  The  other  letters  of  reference  are  the 
same  as  in  Fig.  110. 

into  connection  with  cells  not  far  from  the  entrance  of  the  root. 
The  cells  with  which  the  fibres  are  thus  connected  are  situated 
in  various  parts  of  the  grey  matter,  on  one  or  the  other  side  of 
the  cord.  Among  these,  we  have  reason  to  believe,  are  the  cells 
of  the  anterior  horn  giving  off  axis-cylinder  processes  to  ante- 
rior roots,  and  in  this  way  a  direct  chain  between  afferent  and 
efferent  fibres  seems  to  be  established.  Others  of  these  cells 
give  off  axis-cylinder  processes,  which  run  upwards,  headwards, 
forming  strands  in  the  white  matter  of  the  cord  and  end  in 
parts,  lying  in  many  cases  at  least  in  the  brain  above  the  cord. 
Such  cells  form  relays  in  the  transmission  of  afferent  impulses; 
the  posterior  vesicular  cylinder,  the  column  of  Clarke,  is  a 
group  of  such  cells. 

In  a  certain  number  of  cases  however  the  (anterior  divisions 
of  the)  fibres  do  not  make  such  speedy  connections,  but  run 
upwards  in  the  median  posterior  column,  column  of  GoU,  which 
indeed  they  for  the  most  part  form.  Hence  when  a  posterior 
root  is  cut,  degenerating  fibres  are  found  in  the  median  poste- 
rior column  above  the  entrance  of  the  root,  and  may  be  traced 
as  an  'ascending'  degeneration  right  up  to  the  spinal  bulb. 
The  fibres  coming  from  successive  roots  take  up  definite  posi- 
tions in  the  column  as  is  illustrated  in  Fig.  114.  The  contri- 
bution to  the  column  furnished  by  each  root  diminishes  however 
upwards,  as  shewn  by  the  diminishing  area  of  degeneration; 
that  is  to  say,  some  of  the  fibres  of  the  posterior  root  after 
ruianing  in  the  median  posterior  column  for  a  greater  or  less 
distance,  and  the  distance  may  be  considerable,  eventually  turn 
aside,  and  enter  the  grey  matter  of  the  cord;  here  they  end  in 
connection  with  nerve  cells.  The  contribution  to  the  column, 
though  thus  diminished,  maybe  traced  to  the  spinal  bulb;  here 
the  remaining  fibres  of  the  root  end  in  connection  with  the  cells 
of  the  gracile  nucleus. 

Thus  the  fibres  forming  the  posterior  root  of  each  spinal 
nerve  have  a  wide  and  manifold  grip  upon  the  central  nervous 
system.  Some  stretch  right  beyond  the  spinal  cord  and  lay 
hold  of  the  spinal  bulb ;  and  these,  it  will  be  remarked,  keep 
entirely  to  the  side  on  which  they  enter.  Others,  running  for 
variable  distance  in  the  posterior  median  column  of  the  same 
side,  lay  hold  of  the  grey  matter,  it  may  be  far  above  the 
entrance  of  the  root,  while  yet  others  lay  hold  of  the  grey  mat- 
ter soon  after  their  entrance  into  the  cord,  and  that  either  above 


Chap,  i.] 


THE   SPINAL   CORD. 


685 


or  below  the  level  of  the  entrance,  on  the  same  side  or  on  the 
opposite  side.  In  other  words,  an  afferent  impulse  passing 
along  the  posterior  root  may,  according  to  the  course  which  it 


Fig.  112.    Transverse  Dorsoventral  Section  of  the  Spinal  Cord  (Htman) 
AT  THE  Level  of  the  Third  Lumhar  Nerve.     (Sherrington.) 

This  is  drawn  to  the  same  scale  as  Fig.  110,  and  in  the  same  way  except 
that  the  outline  of  the  grey  matter  is  not  e.xaggerated.  Pr'.  median,  Pr.  inter- 
mediate, Pr".  lateral  bundles  of  posterior  roots.  The  region  comprised  under 
m.t.  is  the  marginal  zone  or  Lissauer's  zone.  The  other  letters  of  reference  are 
the  same  as  in  110  and  111. 

The  three  figures  110,  111,  112  are  intended  to  illustrate  the  main  differential 
features  of  the  thoracic,  cervical,  and  lumbar  cord. 


686 


THE   TEACTS    OF   WHITE   MATTEE.     [Book  hi. 


takes,  produce  effects  almost  immediately  above  or  below,  on 
the  same  or  on  the  other  side,  or  may  have  to  travel  some  length 
upwards,  on  the  same  side,  before  it  produces  its  effects,  or  may 
produce  no  effect  on  the  spinal  cord  itself,  but  spend  its  whole 
strength  on  the  spinal  bulb. 

The  method  of  degeneration,  confirmed  by  the  method  of 
development,  has  shewn  that  certain  fibres,  starting  as  axis- 
cylinder  processes  of  certain  cells  in  the  region  of  the  cerebral 
cortex  which  we  shall  speak  of  as  the  motor  area,  finding  their 
way  to  the  spinal  bulb  through  the  crus  cerebri,  form  the  ante- 
rior pyramids  of  the  bulb,  cross  at  the  decussation  of  the  pyra- 
mids to  the  lateral  column  of  the  cord  and  there  form  a  definite 
strand,  the  '•'■  crossed  pyramidal  tract^''  (Figs.  113,  114).     This 


Tig.  113.    Diagram  to  illustrate  the  general  arrangement  of  the  sev- 
eral Tracts  of  White  Matter  in  the  Spinal  Cord.     (Sherrington.) 

The  section  is  taken  at  the  level  of  the  fifth  cervical  nerve.  The  relations  of 
the  tracts  in  different  regions  of  the  cord  are  shewn  in  Pig.  114. 

The  ascending  tracts,  tracts  of  ascending  degeneration,  are  shaded  with  dots, 
the  descending  tracts,  tracts  of  descending  degeneration,  are  shaded  with  lines  ; 
the  shading  is  in  each  case  put  on  one  side  of  the  cord  only,  the  reference  letters 
being  placed  on  the  other  side. 

cr.P.  crossed  pyramidal  tract,  or  more  shortly  pyramidal  tract.  d.P.  direct  pyra- 
midal tract,  shaded  on  the  side  opposite  to  that  on  which  cr.P.  is  shaded, 
in  order  to  indicate  the  difference  of  the  two  as  to  crossing.  C.b.  cerebellar 
tract,  s.lr.  and  c.r.  together  indicate  the  median  posterior  tract  or  tract  of 
fibres  of  the  posterior  roots,  c.r.  representing,  as  is  explained  more  fully  in  the 
text,  the  cervical  and  s.lr.  the  sacral,  lumbar  and  dorsal  roots,  asc.a.l.  the 
antero-lateral  ascending  tract,  desc.l.  the  antero-lateral  descending  tract. 
The  area,  not  shaded,  marked  x,  is  the  small  descending  tract  or  rather 
patch  mentioned  in  the  text  as  observed,  in  certain  regions  of  the  cord,  in 
the  external  posterior  column  rz.  The  small  area  at  the  tip  of  the  posterior 
horn,  marked  L,  is  the  posterior  marginal  zone  or  Lissauer's  zone. 

tract  lying  in  the  dorsal  region  of  the  lateral  column,  lateral  to 
the  posterior  horn,  and  marked  out  by  a  '  descending '  degen- 
eration, may  be  traced  down  the  whole  length  of  the  cord, 
diminishing  as  it  goes.  It  diminishes  because  fibres  succes- 
sively leave  it  to  make  connections,  also  of  contact  merely  not 


Chap,  i.] 


THE   SPINAL   CORD. 


687 


of  continuity,  with  the  cells  of  the  anterior  horn  of  the  same 
side  which  give  off  axis-cylinder  processes  to  form  fibres  of  an 
anterior  root.  By  means  of  it,  impulses  leaving  the  cells  of  the 
cortex  of  one  side  of  the  brain  and  crossing  over  in  the  spinal 
bulb  are  able  to  give  rise  to  efferent  impulses  in  the  spinal 
nerves  of  the  opposite  side.  Some  of  the  fibres  starting  from 
like  cells  in  the  cortex  do  not  take  exactly  this  course ;  they  do 
not  cross  over  at  the  decussation  of  the  pyramids,  but  continue 
along  the  same  side  of  the  cord,  forming  in  the  median  part  of 
the  anterior  column,  the  direct  pyramid  tract  (Figs.  113,  114). 


asc.a.L 


688    THE  FEATUEES  OF  DIFFEEENT  EEGIONS.  [Book  in. 


sr.  It.  dr. 


Sac 


Fig.  114.    Diagram  illustrating  some  of  the  features  of  the  Spinal  Cord 
AT  DIFFERENT  LEVELS.     (Sherrington.) 

All  the  figures  are  drawn  to  scale,  and  represent  the  cord  magnified  four 
times.  They  shew  the  differences  at  different  levels  in  the  shape  and  size  of 
the  cord,  in  the  outline  of  the  grey  matter,  and  in  the  relative  position  of  the 
anterior  and  posterior  fissures,  and  also  shew  the  variations  at  different  levels 
of  the  several  '  tracts  '  of  the  white  matter. 

C2  at  the  level  of  the  second  cervical  nerve,  C5  of  the  fifth  cervical,  Cs  of  the 
eighth  cervical.  D2  of  the  second  thoracic,  D5  of  the  fifth  thoracic,  Li  of 
the  first  lumbar,  L5  of  the  fifth  lumbar,  and  Sac.  of  the  second  sacral  nerve. 
The  shading  of  the  tracts  is  the  same  as  in  Fig.  113  ;  but  in  the  median  posterior 
column  of  Do  the  areas  of  fibres  coming  from  the  sacral  nerves  s.r.,  and  lum- 
bar nerves  l.r.  are  distinguished  from  the  area,  d.r.  of  fibres  belonging  to 
the  thoracic  nerves.  In  C  8,  no  distinction  is  made  between  any  of  these 
sets  of  fibres  ;  in  L  5  only  fibres  of  sacral  nerves  are  represented  ;  in  Li  Dg 
D5,  the  more  dorsal  small  portion  corresponds  to  sacral  fibres  and  the  next 
to  lumbar,  or  lumbar  thoracic  nerves. 

This  tract,  very  variable  in  different  animals,  is  only  found  in 
the  upper  part  of  the  cord ;  it  diminishes  rapidly  and  soon  dis- 


Chap,  i.]  THE   SPINAL   CORD.  C89 

appears,  the  fibres  forming  it,  crossing  over  in  the  anterior  com- 
missure, i)ass  to  tiie  nerve-cells  of  the  anterior  horn  of  the 
opposite  side  with  whicli,  like  the  brother-fibres  of  the  crossed 
pyramidal  tract,  they  make  connections  b}^  contact. 

A  conspicuous  tract  in  the  white  matter  of  the  lateral  column 
is  the  cerebellar  tract  (Figs.  113, 114)  marked  out  by  a  descend- 
ing degeneration.  This,  starting  in  the  lower  parts  of  the  cord, 
and  as  a  whole  increasing  as  it  goes,  may  be  traced  through  the 
restiform  bod}-  of  the  same  side  to  the  cerebellum.  We  have 
reason  to  believe  that  the  cells,  the  axis-cylinder  processes  of 
cells  which  supply  tlie  fibres  composing  the  tract,  are  those  form- 
ing the  posterior  vesicular  cylinder.  The  tract,  we  may  con- 
clude, carries  to  the  cerebellum  afferent  impulses  furnished  by 
the  posterior  roots,  but  modified,  we  may  presume,  at  the  relay  in 
the  posterior  vesicular  cylinder. 

Other  tracts  have  also  been  made  out  in  the  white  matter  of 
the  cord,  but  these  are  not  so  conspicuous  as  the  above.  It  is 
more  important  to  remember  that  a  large  number  of  the  fibres, 
both  of  the  lateral  and  anterior  (ventral)  columns  are  axis- 
cylinder  processes  of  cells  of  one  part  of  the  cord,  and  end  by 
arborescences  making  contact  with  cells  in  another  part  of  the 
cord.  Some  of  these  fibres  run  their  whole  course  on  the  same 
side  of  the  cord ;  others  cross  over  to  the  opposite  side ;  these 
may  be  regarded  as  commissural  fibres,  longitudinal  and  trans- 
verse. Lastly,  some  of  the  cells  in  the  grey  matter  are  such 
that  all  their  processes  end  as  they  begin,  within  the  grey 
matter,  and  do  not  contribute  at  all  to  the  white  matter. 

§  453.  TJie  Special  Features  of  the  several  regions  of  the 
Spinal  Cord.  The  cord  begins  below  in  the  slender  filament 
called  the  filum  tertninale,  which  Ij'ing  in  the  vertebral  canal,  in 
the  midst  of  the  mass  of  nerve  roots  called  the  cauda  equina^ 
rapidly  enlarges  at  about  the  level  of  the  first  lumbar  vertebra 
into  the  conns  medullaris.  This  may  be  regarded  as  the  begin- 
ning of  the  lower  portion  of  a  fusiform  enlargement  of  the  cord 
known  as  the  lumhar  swelling.,  which  reaches  as  liigh  as  about 
the  attachment  of  the  roots  of  the  twelfth  or  eleventh  thoracic 
nerve  at  the  level  of  the  eighth  thoracic  vertebra,  the  broadest 
part  of  the  swelling  being  about  opposite  the  third  lumbar  nerve. 
Above  the  lumbar  swelling,  through  the  thoracic  region  the 
somewhat  narrowed  cord  retains  about  the  same  diameter  until 
it  reaches  the  level  of  the  first  or  second  thoracic  nerve  opposite 
the  seventh  cervical  vertebra  where  a  second  fusiform  enlarge- 
ment, the  cervical  sivelling,  broader  and  longer  than  the  lumbar 
swelling,  begins.  The  broadest  part  of  the  cervical  swelling  is 
about  opposite  to  the  fifth  or  sixth  cervical  nerve  ;  from  tlience 
the  diameter  of  the  cord  becomes  gradually  somewhat  less  until 
it  begins  to  expand  into  the  bulb,  but  even  in  the  liighest  part 
is  greater  than  in  the  thoracic  region.     The  sectional  area  of 

44 


690    THE  FEATURES  OF  DIFFERENT  REGIONS.  [Book  in. 

the  cord  increases  therefore  from  below  upwards,  but  not  regu- 
larly, the  irregularity  being  due  to  the  lumbar  and  cervical 
swellings. 

The  extremity  of  the  filum  terminale  is  said  to  consist  en- 
tirely of  neuroglia  closely  invested  by  the  membranes,  even  the 
central  canal  being  absent.  A  little  higher  up  the  central  canal 
begins,  and  nerve-cells  with  nerve-fibres  make  their  appearance 
in  the  neuroglia  ;  thus  a  kind  of  grey  matter  covered  by  a  thin 
superficial  layer  of  white  matter  is  established.  We  have  else- 
where referred  to  the  peculiar  features  of  the  lower  end  of  the 
conns ;  but  higher  up  the  canal  becomes  central  and  small,  the 
posterior  columns  are  developed,  and  the  grey  matter  contains 
more  nervous  elements  and  relatively  less  neuroglia,  becomes  in 
fact  ordinary  grey  matter.  From  thence  onward  to  very  near 
the  junction  with  the  bulb,  where  transitional  features  begin  to 
come  in,  the  spinal  cord  may  be  said  to  have  the  general  structure 
previously  described. 

The  sectional  area  of  the  white  matter  increases  in  absolute 
size  and  on  the  whole  in  a  steady  manner  from  below  upwards. 


V    IV  III    II    I    V    IV  III   II     I    X.l  XI    X    IX  VIII VII  VI    V    IV   III    II    I   VIII VII  VI  V    IV  m    11     I 


Fig.   115.     Diagram  shewing  the  united  sectional  areas  of  the  Spinal 
Nerves,  proceeding  from  below  upwards. 

In  this  as  in  the  succeeding  figures  116 — 17,  — 18,  — 19,  — 20,  all  of  whicli  refer 
to  man,  the  left-hand  side  represents  the  bottom  of  the  cord  a.nd  the  right-hand  the 
top  of  the  cord,  the  numerals  indicating  successfully  the  sacral,  lumbar,  thoracic 
and  cervical  nerves.     The  several  figures  are  not  drawn  to  the  same  scale. 

In  other  words,  in  a  section  at  any  level,  the  number  of  longi- 
tudinal fibres  forming  the  white  matter  is  greater  than  the 
number  at  a  lower  level,  and  less  than  the  number  at  a  higher 
level  ;  for  any  difference  which  may  exist  in  the  diameter  of  the 
individual  fibres  is  insufficient  to  explain  the  differences  in  the 
total  sectional  area  of  the  white  matter.  If  we  were  to  measure 
in  man  the  sectional  area  of  each  of  the  spinal  nerves  as  it  joins 
the  cord,  and  to  add  them  together,  passing  along  the  cord  from 
below  upwards  the  results  put  in  the  form  of  a  curve  would  give 
us  some  such  figure  as  that  shewn  in  Fig.  115  ;  the  area  gained 
by  adding  together  the  sectional  areas  of  the  nerves  increases 
in  a  fairly  steady  manner  from  below  upwards.  The  curve  of 
the  sectional  area  of  the  white  matter  of  the  cord  taken  from 
below  upwards  would  be  very  similar,  but  if  anything  more 


Chap,  i.] 


THE   SPINAL   COED. 


691 


regular.  It  must  be  understood  ho^Yever  that  the  dimensions  of 
tlie  areas  would  not  be  the  same  in  the  two  cases.  The  sectional 
area  of  the  white  matter  at  the  top  of  the  cervical  region,  though 
greater  than  anywhere  lower  down,  is  far  less  than  the  united 
sectional  area  of  all  the  nerves  below  that  level.  The  white 
matter  is  not  formed  by  all  the  fibres  from  the  nerves  which 
join  the  spinal  cord  continuing  to  run  along  the  cord  up  to  the 
brain  ;  as  we  have  seen,  some  at  least  of  the  fibres  end  in  the 
grey  matter.  Nevertheless  the  white  matter  in  passing  up 
the  cord  appears  to  receive  a  permanent  addition  at  the  entrance 
of  each  nerve.  We  may  infer  that  each  nerve  has  a  representa- 
tive of  itself  starting  from  the  level  of  its  entrance  and  running 
up  to  some  part  of  the  brain. 

§  454.  The  grey  matter  in  contrast  to  the  white  matter 
shews  great  variations  in  area  along  the  length  of  the  cord  (Fig. 
116).     From  the  entrance  of  the  coccygeal  nerve  upwards  the 


V    IV    III     II     I     V     IV    III    II     I    XII    XI    X    IX  VIII  VII  VI    V    IV    III     II     I    VIU  VII  VI    V    iV    III     II      I 

Fin.  llfi.    Diagram  shewixg  the  takiations  in  the  sectional  akea  of  the 

GREY    MATTER    OF    THE    SpINAL    CoRD,    ALONG    ITS    LENGTH. 

area  increases  very  rapidly,  reaching  a  maximum  at  about  the 
level  of  the  5th  lumbar  nerve.  It  then  rapidly  decreases  to  about 
tlie  level  of  the  11th  thoracic  nerve,  maintains  about  the  same 
dimensions  all  through  the  thoracic  region,  and  begins  to  increase 
again  at  about  the  level  of  the  2nd  thoracic  nerve.  Its  second 
maximum  is  reached  at  about  the  level  of  the  5th  or  6th  cervical 


y  IV  ui   II    I   V  IV  III   II    I  XII  XI  X  IX  VIII  vii  vi  v  iv  iii  ii    i  uiii  vii  vi  v  iv  iii   m    i 


Fig.  117.  Diagram  shewing  the  relative  sectional  areas  of  the  Spinal 
Nerves,  as  they  join  the  Spinal  Cord. 


nerve,  after  which  the  area  again  becomes  smaller,  remaining 
however  at  the  upper  cervical  region  much  larger  than  in  the 
thoracic  region. 

The  meaning  of  these  variations  becomes  clear  when  we  turn 
to  Fig.  117,  which  shews  in  a  similar  diagrammatic  manner  the 


692    THE  FEATUEES  OE  DIFEEREKT  EEGIONS.  [Book  hi. 

sectional  areas  of  the  several  spinal  nerves.  It  will  be  observed 
that  the  increase  and  decrease  of  the  sectional  area  of  the  grey 
matter  follow  very  closely  the  increase  and  decrease  of  the  quan- 
tity of  nerve,  that  is  to  say,  neglecting  differences  in  the  diam- 
eter of  the  fibres,  in  the  number  of  nerve-fibres  passing  into  the 
cord.  The  sectional  areas  of  the  1st  and  2nd  sacral,  4th  and  5th 
lumbar  nerves  are  very  large,  and  opposite  to  these  the  sectional 
area  of  the  grey  matter  of  the  cord  is  very  large  also  ;  the  en- 
largement of  grey  matter  which  is  the  essential  cause  of  the 
lumbar  swelling  is  correlated  to  the  large  number  of  fibres  which 
enter  and  leave  the  cord  at  this  region  to  supply  chiefly  the  lower 
limbs.  Similarly  the  enlargement  of  grey  matter  which  is  the 
essential  cause  of  the  cervical  swelling  is  correlated  to  the  large 
number  of  fibres  which  enter  and  leave  this  region  of  the  cord 
to  supply  chiefly  the  upper  limbs.  In  the  thoracic  region,  where 
the  number  of  fibres  entering  and  leaving  the  cord  is  relatively 
less,  the  sectional  area  of  the  grey  matter  is  also  less.  Since 
the  attachments  of  the  several  spinal  nerves  are  not  exactly 
equidistant  from  each  other  along  the  length  of  the  cord,  the 
sectional  area  is  not  an  exact  measure  of  bulk  ;  the  total  bulk 
of  grey  matter  for  instance  belonging  to  two  nerves  which  enter 
the  cord  close  together  is  less  than  that  of  two  nerves  giving 
rise  to  the  same  sectional  area  of  grey  matter  as  the  former  two 
but  entering  the  cord  far  apart  from  each  other.  Still  the  error 
which  may  be  introduced  by  taking  sectional  area  to  mean  bulk 
is,  for  the  present  purposes  at  all  events,  so  small  that  we  may 
permit  ourselves  to  say  that  in  the  successive  regions  of  the 
spinal  cord  the  bulk  of  grey  matter  in  any  segment  is  greater 
or  less  according  to  the  size  of  the  nerve  (or  pair  of  nerves,  right 
and  left)  belonging  to  that  segment. 

From  this  anatomical  fact  we  appear  justified  in  drawing 
the  conclusion  that  at  all  events  a  great  deal  of  the  grey  mat- 
ter of  the  spinal  cord  may  be  considered  as  furnishing  a  ner- 
vous mechanism,  with  which  the  efferent  fibres  of  each  spinal 
nerve  just  before  they  leave  the  cord,  and  the  afferent  fibres 
soon  after  they  join  the  cord  are  more  immediately  connected. 
It  may  be  that  the  whole  of  the  grey  matter  is  thus  directly 
connected  with  and  thus  rises  and  falls  with  the  fibres  of  the 
nerves ;  or  it  may  be  that  there  is  a  sort  of  core  of  grey  mat- 
ter, which  maintains  a  uniform  bulk  along  the  whole  length 
of  the  cord  and  serves  as  a  basis  which  is  here  more  and  there 
less  swollen  by  the  addition  of  the  grey  matter  more  immedi- 
ately connected  with  the  fibres  of  the  nerves.  This  question 
the  method  which  we  are  now  using  cannot  settle. 

§  455.  Owing  to  these  different  rates  of  increase  of  the 
grey  and  white  matter  respectively  along  the  length  of  the 
cord,  we  find  that  in  sections  of  the  cord  taken  at  different 
levels  the  appearances  presented  vary  in  a  very  distinct  man- 


Chap,  i.]  THE   SPINAL   CORD.  G93 

ner.  This  is  strikingly  shewn  by  comparing  Figs.  110,  111  and 
112.  At  the  level  of  the  third  lumbar  nerve  (Fig.  112)  the  grey 
matter  is  very  large,  reaching,  as  we  have  seen,  its  maximal 
sectional  area  at  about  this  point,  so  that  although  the  area  of 
white  matter  is  not  very  great  the  whole  area  of  the  cord  is 
considerable. 

At  the  level  of  the  sixth  thoracic  nerve  (Fig,  110),  in  spite 
of  the  white  matter  having  very  decidedly  increased,  the  grey 
matter  has  shrunk  to  such  very  small  dimensions,  that  the 
total  sectional  area  of  the  cord  has  markedly  diminished. 

At  the  level  of  the  sixth  cervical  (Fig.  Ill)  the  grey  matter 
has  again  increased,  reaching  here  as  we  have  seen  its  second 
maximum ,-  the  white  matter  has  also  further  increased,  and 
that  indeed  very  considerably,  so  that  the  total  area  of  the 
cord  is  much  greater  than  in  any  of  the  lower  regions. 

Further  details  of  the  varying  size  of  the  white  matter  and 
of  the  grey  matter  at  different  levels  are  also  shewn  in  the 
series  given  in  Fig.  114.  In  these,  combined  with  the  three 
figures  just  referred  to,  it  will  be  observed  that  tlie  serial  in- 
crease and  decrease  of  the  grey  matter  does  not  affect  all  parts 
of  the  grey  matter  alike,  so  that  the  outline  of  the  grey  mat- 
ter changes  very  markedly  in  passing  from  below  uj^wards. 
In  the  coccygeal  region  each  lateral  half  is  a  somewhat 
irregular  oval,  and  in  the  sacral  region.  Fig.  114,  Sac^ 
the  differentiation  into  anterior  and  posterior  horns  is  still 
very  indistinct.  In  the  lumbar  region  the  two  horns  are 
sharply  marked  out,  though  both  the  posterior  and  anterior 
horns  are  broad  and  more  or  less  quadrate.  In  the  thoracic 
region  the  decrease  of  grey  matter  has  affected  both  horns,  so 
that  both  are  pointed  and  slender,  while  the  junction  between 
them  has  not  undergone  so  much  diminution,  so  tliat  what  has 
been  called  the  lateral  horn  is  relatively  conspicuous.  In  the 
cervical  region  the  returning  increase  bears  much  more  on  the 
anterior  horn  which  again  becomes  large  and  broad,  than  on 
the  posterior  horn  which  still  remains  slender  and  pointed. 
Taking  the  form  of  tlie  grey  matter  in  the  thoracic  region  as 
the  more  typical  form  of  the  grey  matter  we  may  say  that 
while  the  increase  on  the  lumbar  swelling  bears  equally  on 
the  anterior  and  posterior  horns,  that  in  the  cervical  region 
bears  chiefly  on  the  anterior  horns. 

§  456.  The  white  matter  as  we  have  seen  increases  in  sec- 
tional area  with  considerable  regularity  from  below  upwards. 
If  instead  of  a  diagram  of  the  increase  of  the  whole  white  mat- 
ter, we  construct  in  a  similar  way  diagrams  of  the  anterior, 
posterior  and  lateral  columns  respectively  we  find  that  while 
the  sectional  area  of  the  lateral  column  (Fig.  118)  increases 
with  some  considerable  regularity  from  below  upwards,  though 
not  so  regularly  as  does  the  whole  area  of  white  matter,  both 


694    THE  FEATURES  OF  DIFFERENT  REGIONS.  [Book  hi. 

the  anterior  (Fig.  119)  and  the  posterior  (Fig.  120)  columns 
agree  to  a  certain  extent  with  the  grey  matter  in  shewing  a 
decided  increase  in  both  tlie  lumbar  and  the  cervical  swellings. 
We  may,  provisionally  at  least,  infer  from  this  that,  while  con- 
siderable portions  of  both  the  anterior  and  the  posterior  col- 
umns are  like  the  adjoining  grey  matter  in  some  way  or  other 
concerned  in  the  exit  and  entrance  of  efferent  and  afferent 
fibres,  the  larger  portion  of  the  lateral  column  is  concerned 
in  the  transmission  of  impulses  to  and  fro,  between  the  local 
mechanisms   below^,  immediately  connected   with   the   several 


V    !«/    Ill     II     I     V    IV    III     II      I    XII  XI    X    IX  VIII  VII  VI     V    IV    III     II      I    VIII   Vll  VI     V     IV    III    II     I 


Fig.   118.     Diagram   shewing   the   variations   in   the   sectional   area  of 

THE    LATERAL    COLUMNS    OF    THE    SpINAL    CoRD,    ALONG    ITS    LENGTH. 


15 

10 

6 
Oi. 


(     V  1/  III   II    I    V   IV  III    II    I  XII  XI   X  IX  via  VII  vi  v   iv  in   ii    i  via  vii  vi   v  iv  iii   u    i 
ITiG.  119.     Diagram   shewing   the   variations   in   the   sectional   area   of 

THE    ANTERIOR    COLUMNS    OF    THE    SpINAL    CoRD,    ALONG    ITS    LENGTH. 


V    IV  III    II      1     V    IV    III     11      I    XII  XI   X    IX  VIII  Vll  VI    V    IV 


11     1    VIII  Vll  VI    V    IV    III    II     1 


Fig.  120.     Diagram   shewing   the   variations   in   the   sectional   area   of 

THE    POSTERIOR    COLUMNS    OF    THE    SpINAL    CoRD,    ALONG    ITS    LENGTH. 


spinal  nerves,  and  the  brain  above.  This  conclusion  seems 
incidentally  confirmed,  (though  these  diagrams  must  not  be 
strained  to  carry  detailed  inferences,)  by  the  sudden  increase 
of  the  lateral  column  above  the  lumbar  swelling,  as  if  the  large 
mass  of  nervous  mechanism  for  the  lower  limbs  concentrated 
in  this  region  demanded  a  sudden  increase  in  the  number  of 
fibres  connecting  it  with  the  brain  above. 

This  more  or  less  continuous  increase  of  the  lateral  column 
partly  explains  the  change  of  form  in  the  general  outline  of 


Chap,  i.]  THE   SPINAL   COED.  695 

the  transverse  section  of  the  cord  which  is  observed  in  passing 
upwards  from  the  lower  to  the  higher  regions.  In  the  coccy- 
geal, sacral  and  lumbar  regions  the  outline,  though  varying 
somewhat  chiefly  owing  to  the  disposition  of  the  grey  matter, 
is  on  the  whole  circular.  In  the  thoracic  region  especially  in 
the  uj^per  part  the  increase  of  the  lateral  columns  increases  the 
side  to  side  diameter  so  much  that  the  section  becomes  oval, 
and  in  the  cervical  region  this  increase  of  the  side  to  side 
diameter  out  of  proportion  to  the  dorso-ventral  diameter  is 
very  marked.  The  actual  outline  of  the  whole  transverse 
section  is  however  determined  also  to  a  certain  extent  by  the 
changes  of  form  of  the  grey  matter. 

The  cord  moreover  undergoes  along  its  length  a  change 
which  is  not  very  clearly  indicated  in  the  diagrams  Figs.  119, 
120.  By  comparing  the  series  of  transverse  sections  given  in 
Fig.  114  it  will  be  seen  that  the  relative  position  of  the  central 
canal  shifts  along  the  length  of  the  cord.  In  the  sacral  and 
lumbar  regions  the  central  canal  is  nearly  at  the  centre  of  the 
circle  of  outline,  and  the  posterior  and  anterior  fissures  are 
nearly  of  equal  depth.  Even  in  the  upper  lumbar  region,  and 
still  more  in  the  thoracic  region,  the  position  of  the  central 
canal  is  shifted  nearer  to  the  ventral  surface  so  that  the  poste- 
rior fissure  becomes  relatively  longer,  deeper,  than  the  anterior. 
This  shifting  goes  on  through  the  cervical  region  up  to  about 
the  level  of  the  2nd  cervical  nerve,  where  it  is  arrested  by  the 
beginning  of  the  changes  through  which  the  spinal  cord  is 
transformed  into  the  far  more  complicated  bulb. 

This  lengthening  of  the  posterior  fissure  indicates  an  in- 
crease in  the  dorso-ventral  diameter  of  the  posterior  columns, 
and  this,  not  being  accompanied  by  a  compensating  diminution 
of  the  side  to  side  diameter,  shews  in  turn  that  the  posterior 
columns  undergo  an  increase  in  passing  upwards.  From  this 
we  may  add  to  the  provisional  conclusion  just  arrived  at  Avith 
regard  to  the  lateral  columns,  the  further  conclusion  that  some 
part  of  the  posterior  columns  also  is  concerned  in  transmitting 
impulses,  in  a  more  or  less  direct  manner,  between  the  various 
regions  of  the  cord  below  and  the  brain  above.  The  anterior 
columns  do  not  increase  in  the  same  marked  manner,  though 
over  and  above  the  increase  due  to  the  lumbar  and  cervical 
swellings,  a  continued  increase  may  be  observed  especially  in 
tlie  upper  cervical  region ;  it  is  in  this  upper  region  that  the 
direct  pyramidal  tract  is  best  developed. 


SEC.   3.     THE  EEELEX  ACTIOi^S   OF  THE 
SPINAL   COED. 

§  457.  In  the  preceding  portions  of  this  work  we  have 
repeatedly  seen  that  though  we  can  learn  much  concerning  the 
working  of  an  organ,  or  tissue  or  part  of  the  body  by  studying 
its  behaviour  when  isolated  from  the  rest  of  the  bodj^  all  the 
conclusions  thus  gained  have  to  be  checked  by  a  study  of  the 
behaviour  of  the  same  organ  or  part,  while  it  is  still  an  integral 
part  of  the  intact  body.  All  the  several  organs  and  tissues 
are  so  bound  together  by  various  ties,  that  the  actions  of  each 
depend  on  the  actions  of  the  rest ;  and  to  say  that  the  life  of 
each  part  is  a  function  of  the  life  of  the  whole,  is  no  less  true 
than  to  say  that  the  life  of  the  whole  is  a  function  of  the  life 
of  each  part.  This  is  esiDecially  borne  in  upon  us,  when  we 
come  to  study  the  actions  of  the  central  nervous  system.  We 
may,  on  anatomical  grounds,  separate  the  spinal  cord  from  the 
brain ;  but  when  we  come  to  consider  the  respective  functions 
of  the  two,  we  are  brought  face  to  face  with  the  fact  that  in 
actual  life  a  large  part  of  the  work  of  the  brain  is  carried  out 
by  means  of  the  spinal  cord,  and  conversely  the  spinal  cord 
does  its  work  habitually  under  the  influence  of,  if  not  at  the 
direct  bidding  of  the  brain.  We  may  gain  certain  conclusions 
by  studying  the  behaviour  of  the  spinal  cord  isolated  from  the 
brain,  or  of  parts  of  the  spinal  cord  isolated  from  each  other ; 
but  we  must  be  even  more  cautious  than  when  we  were  dealing 
with  other  parts  of  the  body,  and  must  greatly  hesitate  to  take 
it  for  granted  that  the  work  which  we  can  make  the  spinal 
cord  or  a  part  of  the  spinal  cord  do,  when  isolated  from  the 
brain,  is  the  work  which  is  actually  done  in  the  intact  body 
when  the  brain  and  spinal  cord  form  an  unbroken  whole. 
Moreover  this  caution  becomes  increasingly  necessary,  when  in 
our  studies  we  pass  from  the  simpler  nervous  system  of  one 
animal  to  the  more  complex  nervous  system  of  another  ;  for  it 
is  by  the  complexity  of  their  central  nervous  systems  more 
than  by  anything  else,  that  the  'highest'  animals  are  differ- 
entiated from  those  '  below '  them.  When  we  compare  a  rabbit, 
a  dog,  a  monkey  and  a  man,  tlie  differences  in  the  vascular, 

696 


Chap,  i.]  THE   SPINAL   COED.  697 

digestive  and  respiratory  systems  of  the  four,  striking  as  they 
may  appear,  sink  into  insignificance  compared  with  the  dif- 
ferences exhibited  by  their  respective  central  nervous  systems. 
We  need  caution  when  from  the  results  of  experiments  on  dogs 
or  rabbits,  we  draw  conclusions  as  to  the  digestion  or  circula- 
tion of  man,  but  we  need  far  greater  caution  when  from  the 
behaviour  of  the  isolated  sj^inal  cord  of  one  of  these  animals 
we  infer  the  behaviour  of  the  intact  sj^inal  cord  of  man. 

A  further  difficulty  meets  us  when  an  experimental  investi- 
gation entails  operative  interference  with  the  central  nervous 
system,  llemoval  or  section  of,  or  other  injur}^  to  parts  of  the 
brain  or  spinal  cord  is  very  apt  to  give  rise  in  varying  degree 
to  what  is  known  as  'shock.'  The  cutting  or  tearing  or  other 
lesion  of  any  considerable  mass  of  nervous  substance  affects  the 
activity,  not  only  of  the  structures  immediately  injured,  but  of 
other,  it  may  be  far  distant,  structures.  The  nature  of  '  sliock ' 
is  not  as  yet  thoroughly  understood,  but  may  perhaps,  in  part 
at  all  events,  be  explained  by  regarding  the  lesion  as  a  very 
powerful  stimulus,  which,  partly  by  way  of  inhibition  but  still 
more  by  way  of  exhaustion,  depresses  or  suspends  for  a  while 
normal  functions,  and  thus  gives  rise  to  temporary  diminution 
or  loss  of  consciousness,  or  volition,  of  reflex  movements  and 
other  nervous  actions.  Thus  a  section  through  the  spinal  cord, 
even  when  made  with  the  sharpest  instrument  and  with  the 
utmost  skill,  so  as  to  avoid  all  bruising  as  much  as  possible, 
may  for  a  while  suspend  all  reflex  activity  of  the  cord,  or  indeed 
all  the  obvious  activities  of  the  whole  central  nervous  system. 
AVe  may  add  that  such  a  '  shock '  of  the  central  nervous  system 
ma}^  also  be  produced  by  sudden  lesions  not  bearing  directly 
on  the  central  nervous  system,  as  for  instance  by  extensive 
injury  to  a  limb. 

i\Ioreover  in  many  cases  in  which  the  effects  of  experimental 
interference  have  been  watched  for  some  considerable  time, 
days,  months  or  years  after  the  operation,  it  has  been  observed, 
on  the  one  hand,  that  j^henomena  which  are  conspicuous  in  the 
early  period  may  eventuall}'  disappear,  and,  on  the  other  hand, 
that  activities  which  are  at  first  absent  may  later  on  make  their 
appearance  ;  movements  for  instance  which  are  at  first  frequent 
after  a  while  die  away,  and  conversely,  movements  which  at 
first  seemed  impossible  are  later  on  easily  achieved.  We  have 
to  distinguish  or  to  attempt  to  distinguish  between  the  tem- 
porar}"  and  the  lasting  effects  of  the  operation,  including  among 
the  former  not  only  those  of  ordinary  'shock,'  but  others  of 
slower  development  or  longer  duration.  In  many  instances 
where  a  part  of  the  central  nervous  system  is  by  section  or 
otherwise  suddenly  separated  from  the  rest,  the  plienomena 
suggest  tliat  the  separated  part  is  at  first  profoundly  infiuenced 
as  to  its  activities  by  the  withdrawal  of  various  influences  which 


698  EEFLEX  ACTIONS.  [Book  iii. 

previously  were  being  exerted  upon  it  by  the  rest  of  the  system, 
but  hiter  on  accommodates  itself  to  its  new  conditions,  and 
learns,  so  to  speak,  to  act  without  the  help  of  those  influences. 
And  indeed  it  is  possible  that  some  of  the  effects  of  even  imme- 
diate '  shock '  may  be  due,  not,  as  suggested  above,  to  the  action 
of  an  inhibitory  or  exhausting  stimulus,  but  to  the  sudden 
cessation  of  habitual  influences. 

Still,  in  spite  of  all  these  difficulties,  it  is  possible  not  only 
to  ascertain  the  working  of  an  isolated  portion  of  the  central 
nervous  system,  but  even  to  infer  from  the  results  some  con- 
clusions as  to  the  share  taken  by  that  portion  in  the  working 
of  the  entire  and  intact  system.  There  can  be  no  doubt,  for 
instance,  that  the  spinal  cord  can,  quite  apart  from  the  brain, 
carry  out  various  reflex  actions,  and  that  moreover  it  does 
carry  out  actions  of  this  kind  when  in  the  intact  organism  it 
is  working  in  concert  with  the  brain.  Indeed  the  carrjdng  out 
of  various  reflex  actions  seems  to  be  one  of  the  most  important 
functions  of  the  spinal  cord,  so  much  so  that,  though  the  brain 
or,  at  least,  parts  of  the  brain  can  also  and  do  develope  reflex 
actions,  the  spinal  cord  offers  the  best  field  for  the  study  of 
these  actions.  We  have  already  (§  90)  touched  on  the  general 
features  of  reflex  actions,  and  elsewhere  have  incidentally  dwelt 
on  particular  instances ;  we  may  therefore  confine  ourselves 
now  to  certain  points  of  special  interest. 

§  458.  Many  of  the  features  of  reflex  movements  are  best 
studied  in  the  frog  and  other  cold-blooded  animals,  since  in 
these  the  actions  of  the  cord  are  less  dependent  on,  and  hence 
less  obscured  by  the  working  of,  the  other  parts  of  the  central 
nervous  system.  In  these  animals  moreover  the  shock,  which 
as  we  have  said  follows  upon  division  of  the  spinal  cord,  and 
for  a  while  suspends  reflex  activity,  soon  passes  away ;  within 
a  very  short  time  after  the  bulb  for  instance  has  been  divided 
the  most  complicated  reflex  movements  can  be  carried  on  by 
the  frog's  spinal  cord  when  the  appropriate  stimuli  are  applied. 
In  the  frog  reflex  actions  may  be  carried  out  by  a  very  small 
portion  of  the  cord,  by  a  single  segment  corresponding  to  a 
pair  of  nerves,  isolated  by  two  transverse  sections  from  the 
parts  above  and  below.  Stimulation  of  the  sensory  fibres  of 
the  nerve  belonging  to  the  segment  will  give  rise  to  contrac- 
tions in  the  muscles  supplied  by  the  motor  fibres  of  the  nerve. 
The  movements  thus  evoked  are  naturally  simple  in  character. 
When  a  larger  portion  of  the  cord  is  experimented  upon,  and 
especially  when  the  whole  cord  is  employed,  there  are  brought 
to  light  what  are  perhaps  the  most  notable  features  of  reflex 
movements,  their  complexity  and  purposeful  character ;  a  few 
afferent  impulses  developed  in  a  few  afferent  fibres  may  lead 
to  many  muscles  being  thrown  into  contractions,  the  time, 
order  and  viefour  of  the  several  contractions  being  so  related 


Chap,  i.]  THE   SPINAL   CORD.  699 

to  each  other,  being  so  "  coordinated  "  that  the  movement,  the 
result  of  the  contractions,  secures  some  end,  either  apparent  or 
reaL  Thus  when  tlie  afferent  filjres  distributed  to  a  small,  it 
may  be  a  very  small,  area  of  the  skin  of  the  flank  are  stimulated 
by  placing  on  the  area  a  small  piece  of  pajjer  soaked  in  dilute 
acid,  the  hind  leg  of  the  same  side,  by  a  series  of  flexions  and 
extensions,  repeatedly  sweeps  over  the  spot  with  the  foot,  the 
purpose  of  the  movement,  namely,  to  brush  off  the  cause  of 
irritation,  being  obvious.  For  the  carrjang  out  of  any  reflex 
movement,  simple  or  complex,  some  portion  of  the  grey  matter 
of  the  cord  must  intervene  between  the  afferent  and  efferent 
fibres.  In  the  simpler  movement,  spoken  of  above,  as  carried 
out  by  a  single  segment  of  the  cord,  we  may  perhaps  suppose 
that  the  arborescent  terminations  of  the  afferent  fibres  impinging 
on  the  nerve-cells  giving  off  the  efferent  fibres,  supply  all  the 
nervous  mechanism  which  is  needed.  In  the  more  complex 
and  more  ordinary  movements,  the  mechanism  must  be  propor- 
tionately more  complex  ;  and  we  may  perhaps  conclude  that  in 
these  other  cells  of  the  grey  matter  intervene  between  the  ter- 
mination of  the  afferent  fibre,  and  the  cell  whose  axis-cylinder 
process  is  the  efl'erent  fibre.  In  any  case  we  may  venture  to 
speak  of  a  nervous  mechanism  within  the  cord  as  the  means  by 
which  a  reflex  movement  is  carried  out. 

The  exact  working  of  such  a  mechanism  in  each  particular 
case,  and  so  the  character  of  the  movement  carried  out,  is  deter- 
mined by  various  causes.  It  is  in  part  determined  by  the  charac- 
ters of  the  afferent  impulses.  Simple  nervous  impulses  generated 
by  the  direct  stimulation  of  afferent  nerve  fibres  generally  evoke 
as  reflex  movements  merely  irregular  spasms  in  a  few  muscles ; 
whereas  the  more  complicated  differentiated  sensory  impulses 
generated  by  the  application  of  the  stimulus  to  the  skin,  readily 
give  rise  to  large  and  purposeful  movements.  It  is  easier  to 
produce  a  complex  reflex  action  by  a  slight  pressure  on  or  other 
stimulation  of  the  skin  than  by  even  strong  induction-shocks 
applied  directly  to  a  nerve  trunk.  If,  in  a  brainless  frog,  the 
area  of  skin  supplied  by  one  of  the  dorsal  cutaneous  nerves  be 
separated  by  section  from  the  rest  of  tlie  skin  of  the  back,  the 
nerve  being  left  attached  to  the  piece  of  skin  and  carefully 
protected  from  injur}^  it  will  be  found  that  slight  stimuli  aj^plied 
to  the  surface  of  the  piece  of  skin  easily  evoke  reflex  actions, 
whereas  the  trunk  of  the  nerve  may  be  stimulated  with  even 
strong  currents  without  producing  anything  more  than  irregidar 
movements. 

§  459.  The  character  of  the  movement  forming  part  of  a 
reflex  action  is  also  influenced  by  the  intensity  of  the  stimulus. 
A  slight  stimulus,  such  as  gentle  contact  of  the  skin  with  some 
body,  will  produce  one  kind  of  movement ;  and  a  strong  stim- 
ulus, such  as  a  sharp  prick  applied  to  the  same  spot  of  skin,  will 


700  EEFLEX   ACTIONS.  [Book  iii. 

call  fortli  quite  a  different  movement.  Wlien  a  decapitated 
snake  or  newt  is  suspended  and  tlie  skin  of  the  tail  lightly 
touched  with  the  finger,  the  tail  bends  towards  the  finger ;  when 
the  skin  is  pricked  or  burnt,  the  tail  is  turned  away  from  the 
offending  object.  And  so  in  many  other  instances.  It  must  be 
remembered  of  course  that  a  difference  in  the  intensity  of  the 
stimulus  entails  a  difference  in  the  characters  of  the  afferent  im- 
pulses ;  gentle  contact  gives  rise  to  what  we  call  a  sensation  of 
touch,  while  a  sharp  prick  gives  rise  to  pain,  consciousness  being 
differently  affected  in  the  two  cases  because  the  afferent  impulses 
are  different.  Hence  the  instances  in  question  are  in  reality 
fuller  illustrations  of  the  dependence  of  the  characters  of  a  reflex 
movement  on  the  characters  of  the  afferent  impulses. 

§  460.  Further,  the  movement,  forming  part  of  a  reflex 
action,  varies  in  character  according  to  the  particular  part  of 
the  body  to  which  the  stimulus  is  applied.  The  reflex  actions 
developed  by  stimulation  of  the  internal  viscera  are  different 
from  those  excited  by  stimulation  of  the  skin.  We  have  reason 
to  think  that  the  contraction  of  or  other  changes  in  a  skeletal 
muscle  may  produce,  by  reflex  action,  contractions  of  other  mus- 
cles ;  and  such  reflex  actions  also  differ  from  those  started  by 
stimulation  of  the  skin.  In  reflex  actions  started  by  applying 
a  stimulus  to  the  skin  the  movements  vary  largely  according  to 
the  particular  area  of  the  skin  which  is  affected.  Thus,  pinch- 
ing the  folds  of  skin  surrounding  the  anus  of  the  frog  produces 
different  effects  from  those  witnessed  when  the  flank  or  the  toe 
is  pinched ;  and,  speaking  generally,  the  stimulation  of  a  par- 
ticular spot  calls  forth  particular  movements.  In  the  case  of 
the  simpler  reflex  movements,  it  appears  to  be  a  general  rule 
that  a  movement  started  by  the  stimulation  of  a  sensory  surface 
or  region  on  one  side  of  the  body,  is  developed  on  the  same  side 
of  the  body,  and  if  it  spreads  to  the  other  side,  still  remains  most 
intense  on  the  same  side  ;  the  movement  on  the  other  side  more- 
over is  symmetrical  with  that  on  the  same  side. 

§  46i.  From  these  and  similar  phenomena  it  is  obvious  that 
when  we  allow  ourselves  to  speak  of  the  grey  matter  of  the  cord 
as  supplying  central  mechanisms  for  carrying  out  reflex  move- 
ments we  must  not  regard  these  several  mechanisms  as  rigidly 
separate  from  each  other  or  indeed  as  anatomically  distinct. 
On  the  contrary  without  any  anatomical  change,  as  the  result 
simply  of  physiological  changes,  the  afferent  impulses  reaching 
the  cord  along  a  set  of  afferent  fibres,  and  it  may  be  any  set,  will 
under  certain  conditions  give  rise  to  efferent  impulses,  not  re- 
stricted to  a  definite  set  of  efferent  fibres  and  so  leading  to  a 
definite  purposeful  movement,  but  overflowing  along  almost  all 
the  efferent  fibres  leaving  the  cord,  and  bringing  about  con- 
tractions of  almost  all  the  skeletal  muscles.  Thus  when  a  frog 
is  poisoned  with  strychnia,  a  mere  touch  of  the  skin  at  almost 


Chap,  i.]  THE   SPINAL   CORD.  701 

any  point  will  Ijring  out  tetanic  convulsions  of  the  whole  body. 
In  such  a  case  the  strychnia  has  produced  no  anatomical  change, 
it  has  only  modified  the  molecular  condition  of  the  suljstance  of 
the  cord  ;  and  that  lias  led  to  the  apparent  disappearance  of  all 
the  central  mechanisms  directing  the  paths  of  impulses ;  these 
now  seem  to  travel  in  all  directions  all  over  the  cord.  In  other 
words  the  mechanisms  of  which  we  are  speaking  are  mapped  out, 
not  by  anatomical  relations,  but  by  the  physiological  condition 
of  the  cord.  Hence  we  cannot  always  predict  exactly  the  nature 
of  the  movement  Avhich  will  result  from  the  stimulation  of  any 
particular  spot,  because  the  result  will  vary  according  to  the 
condition  of  the  spinal  cord,  especially  in  relation  to  the  strength 
and  character  of  the  stimulus.  Indeed,  under  a  change  of  cir- 
cumstances a  movement  quite  different  from  the  normal  one  may 
make  its  appearance.  Thus  when  a  drop  of  acid  is  placed  on 
the  right  flank  of  a  brainless  frog,  the  right  foot  is  almost  inva- 
riably used  to  rub  off  the  acid ;  in  this  there  appears  nothing 
more  than  a  mere  'mechanical'  reflex  action.  If  however  the 
right  leg  be  cut  off,  or  the  right  foot  be  otherwise  hindered  from 
rubbing  off  the  acid,  the  left  foot  is,  under  the  exceptional  cir- 
cumstances, used  for  the  purpose.  This  at  first  sight  looks  like 
an  intelligent  choice.  A  choice  it  evidently  is  ;  and  were  there 
many  instances  of  choice,  and  were  there  any  evidence  of  a  vari- 
able automatism,  like  that  which  we  call  '  volition,'  being  mani- 
fested by  the  spinal  cord  of  the  frog,  we  should  be  justified  in 
supposing  that  the  choice  was  determined  by  an  intelligence. 
But,  as  we  shall  have  occasion  later  on  to  point  out,  a  frog,  de- 
prived of  its  brain  so  that  the  spinal  cord  only  is  left,  makes  no 
spontaneous  movements  at  all.  Such  an  entire  absence  of  spon- 
taneity is  wliolly  inconsistent  with  the  possession  of  intelligence. 
Then  again  the  above  experiment,  if  not  the  only  instance,  is  at 
all  events  by  far  the  most  striking  instance  of  choice  on  the  part 
of  a  brainless  frog.  We  are  therefore  led  to  conclude  that  the 
phenomena  must  be  explained  in  some  other  way  than  b}^  being 
referred  to  the  working  of  an  intelligence.  Moreover  this  con- 
clusion is  supported  by  the  behaviour  of  other  animals.  Thus 
similar  vicarious  reflex  movements  may  be  Avitnessed  in  mam- 
mals, though  not  perhaps  to  such  a  striking  extent  as  in  frogs. 
In  dogs,  in  which  partial  removal  of  the  cerebral  hemispheres 
has  apparently  heightened  the  reflex  excitability  of  the  spinal 
cord,  the  remarkable  scratching  movements  by  means  of  the  hind 
leg  which  are  called  forth  by  stimidating  a  particular  spot  on 
the  loins  or  side  of  the  body,  are  executed  by  the  leg  of  the 
opposite  side,  if  the  leg  of  the  same  side  be  gently  held.  In  this 
case  the  vicarious  movements  are  ineffectual,  the  leg  not  being, 
as  in  the  case  of  the  frog,  crossed  over  so  as  to  bear  on  the  spot 
stimulated,  but  simply  made  to  scratch  the  corresponding  but 
unstimulated  spot  on  its  own  side,  and  cannot  be  considered  as 


702  REFLEX   ACTIONS.  [Book  hi. 

betokening  intelligence.  Again  the  '  mechanical '  nature  of  re- 
flex actions  is  well  illnstratecl  by  the  behaviour  of  a  decapitated 
snake.  When  the  bodj'  of  the  animal  in  this  condition  is  brought 
into  contact  at  several  places  at  once  with  an  arm  or  a  stick, 
complex  reflex  movements  are  excited,  the  obvious  purpose  as 
well  as  effect  of  which  is  to  twine  the  body  round  the  object. 
A  decapitated  snake  will  however  with  equal  and  fatal  readiness 
tAvine  itself  round  a  red-hot  bar  of  iron,  which  is  made  to  touch 
its  skin  in  several  places  at  the  same  time. 

§  462.  The  explanation  of  the  above-quoted  instance  of 
apparent  choice  on  the  part  of  the  frog's  spinal  cord  is  to  be 
sought  for  in  the  modiiications  of  the  activity  of  the  central 
mechanism,  resulting  from  afferent  impulses  other  than  those 
which  supply  the  exciting  cause  of  the  reflex  movement.  The 
existence  and  importance  of  these  other  afferent  impulses  will 
be  seen  if  we  bear  in  mind  that  most  reflex  movements  are,  as 
we  have  said,  '  coordinated ; '  that  is  to  say  not  only  are  many 
distinct  muscles  brought  into  play  but  certain  relations  are 
maintained  between  the  amount,  duration  and  exact  time  of 
occurrence  of  the  contraction  of  each  muscle  and  those  of  the 
contractions  of  its  fellow  muscles  sharing  in  the  movement. 
In  the  absence  of  such  coordination  the  movement  would  be- 
come irregular  and  inefl:ectual.  We  shall  have  occasion  later 
on  in  dealing  mth  voluntary  movements  to  point  out  that  the 
coordination  and  hence  the  due  accomplishment  of  a  voluntary 
movement  is  dependent  on  certain  afferent  impulses  passing  up 
from  the  contracting  muscles  to  the  central  nervous  system, 
and  guiding  the  discharge  of  the  efferent  impulses  which  call 
forth  the  contractions.  When  these  afferent  impulses  affect 
consciousness  we  speak  of  them  as  constitvting  a  '  muscular 
sense;'  it  is,  as  we  shall  see,  by  the  'muscular  sense'  that  we 
become  aware  of  and  can  appreciate  the  condition  of  our  mus- 
cles. But  we  have  reason  to  think  that  the  afferent  impulses 
which  constitute  the  basis  of  the  muscular  sense,  whatever  be 
their  exact  nature,  in  order  to  play  their  part  in  bringing 
about  the  coordination  of  a  voluntary  movement  need  not 
pass  right  up  to  the  brain  and  develope  a  distinct  muscular 
'  sense,'  but  may  produce  their  effect  by  working  on  the  ner- 
vous mechanisms  of  the  spinal  cord  with  which  the  motor  fibres 
carrying  out  the  movement  are  connected.  In  other  words,  the 
coordination  of  a  voluntary  movement  may  take  place  in  the 
part  of  the  spinal  cord  which  carries  out  the  movement. 

But  if  the  spinal  cord  possesses  mechanisms  for  carrying  out 
coordinated  movements,  which  in  the  case  of  voluntary  move- 
ments are  discharged  by  nervous  impulses  descending  from  the 
brain,  we  may  infer  that  in  reflex  actions  the  same  mechanisms 
are  brought  into  action  though  they  are  discharged  by  afferent 
impulses  coming  along  afferent  nerves  instead  of  by  impulses 


Chap,  i.]  THE   SPINAL    CORD.  703 

descending  from  the  brain.  The  movements  of  reflex  origin, 
in  all  their  features  except  their  exciting  cause,  appear  identi- 
cal Avith  voluntary  movements;  the  two  can  only  be  distin- 
guished from  each  other  by  a  knowledge  of  the  exciting  cause. 
And  it  seems  unreasonable  to  suppose  that  the  spinal  cord 
should  possess  two  sets  of  mechanisms  in  all  respects  identical 
save  that  the  one  is  discharged  by  volitional  impulses  from 
the  brain  and  the  other  by  afferent  impulses  from  afferent 
nerves.  We  are  led  therefore  to  the  conclusion  that  in  a  reflex 
action  two  kinds  of  afferent  impulses  are  concerned:  the  ordi- 
nary afferent  impulses  which  discharge  the  nervous  mechanism 
within  the  cord  and  so  provoke  the  movement,  and  the  afferent 
impulses  which  connect  that  nervous  mechanism  with  the  mus- 
cles about  to  be  called  into  play,  and  which  take  part  in  tl^ 
coordination  of  the  movement  provoked.  The  nature  of  tli 
latter  afferent  impulses  is  at  present  obscure;  but  if  we  q-d 
as  we  seem  compelled  to  do,  that  the  character  of  a  ^e; 
action  is  determined  by  them  as  well  as  by  the  afferent 
pulses  which  actually  discharge  the  mechanism,  the  way 
opened  for  an  explanation  of  the  fact  that  when,  as  in  the  case 
of  the  frog  in  question,  the  usual  set  of  muscles  cannot  be  em- 
ployed by  the  nervous  mechanism,  recourse  is  had  to  another  set. 

§  463.  Lastly,  the  characters  of  a  reflex  movement  are,  as 
Ave  need  hardly  say,  dependent  on  the  intrinsic  condition  of  the 
cord.  The  action  of  strj'chnia  just  alluded  to  is  an  instance 
of  an  augmentation  of  reflex  action.  CouA^ersely,  by  A^arious 
influences  of  a  depressing  character,  as  by  A-arious  aniesthetics 
or  other  poisons,  reflex  action  may  be  lessened  or  prevented. 
So  also,  A^arious  diseases  may  so  affect  the  spinal  cord  as  to 
produce  on  the  one  hand  increased  reflex  excitability  so  that  a 
mere  touch  may  produce  a  Adolent  moA-ement,  and  on  the  other 
hand  diminished  reflex  excitability  so  that  it  becomes  difficult 
or  impossible  to  call  forth  a  reflex  action. 

In  the  mammal  the  study  of  reflex  action  is  rendered  some- 
what difficult  by  the  effects  of  shock  to  AA^hich  Ave  referred 
aboA'e.  For  days  even  after  division  of  the  spinal  cord  the 
parts  of  the  body  supplied  by  nerves  springing  from  the  cord 
below  tlie  section  may  exhibit  A'ery  feeble  reactions  only.  In 
the  dog,  for  instance,  after  division  of  the  spinal  cord  in  the 
loAver  thoracic  region,  the  hind  limbs  hang  flaccid  and  motion- 
less, and  pinching  the  hind  foot  evokes  as  a  response  either 
slight  irregular  moA'ements  or  none  at  all.  Indeed  Avere  our 
observations  limited  to  this  period  we  might  infer  that  the 
reflex  actions  of  the  spinal  cord  in  the  mammal  AA'ere  but  feeble 
and  insigniiieant.  If  hoAvever  the  animal  be  kept  alive  for  a 
longer  period,  for  Aveeks  or  better  still  for  months,  though  no 
union  or  regeneration  of  the  spinal  cord  takes  place,  reflex 
moA^ements  of  a  powerful,  A'aried  and  complex  character  mani- 


704  REFLEX  ACTIONS.  [Book  in. 

fest  themselves  in  the  hind  limbs  and  hinder  parts  of  the  body ; 
a  very  feeble  stimnlus  applied  to  the  skin  of  these  regions 
promptly  gives  rise  to  extensive  and  yet  coordinate  movements. 
Indeed  the  more  the  matter  is  studied,  the  stronger  is  the  evi- 
dence that  the  reflex  movements  carried  out  by  isolated  por- 
tions of  the  spinal  cord  of  the  mammal  are  hardly  less  definite, 
complete  and  purposeful,  than  those  witnessed  in  the  frog. 
And  the  main  points  on  which  we  have  dwelt  above  in  relation 
to  the  frog  hold  good  for  the  mammal.  It  is  worthy  of  atten- 
tion, as  bearing  out  the  remarks  made  above  on  the  great 
differentiation  of  the  central  nervous  system  in  the  higher 
animals,  that  the  reflex  phenomena  in  mammals  vary  very 
much  not  only  in  different  species  but  also  in  different  indi- 
viduals and  in  the  same  individual  under  different  circum- 
stances. Race,  age,  and  previous  training,  seem  to  have  a 
marked  effect  in  determining  the  extent  and  character  of  the 
reflex  actions  which  the  spinal  cord  is  capable  of  carrying  out ; 
and  these  seem  also  to  be  largely  influenced  by  passing  circum- 
stances, such  as  whether  food  has  been  recently  taken  or  no. 
It  has  been  asserted  that  the  isolated  spinal  cord  of  the  rabbit, 
which  has  been  the  subject  of  so  many  experiments,  is,  as  com- 
pared with  that  of  the  dog  and  many  other  mammals,  singu- 
larly deficient  in  the  power  of  carrying  out  complex  reflex 
movements. 

§  464.  When  we  come  to  study  the  reflex  actions  of  man 
we  should  at  first  perhaps  be  inclined  to  infer  that,  since  in 
him  the  spinal  cord  is  so  largely  used  as  the  instrument  of  the 
brain,  the  independent  reflex  actions  of  the  cord,  at  least  such 
as  affect  skeletal  muscles,  are  in  him  of  much  less  importance 
than  they  appear  to  be  in  animals  ;  and  experience  seems  to 
support  this  view.  But  it  must  be  remembered  that  in  his 
case,  as  we  have  already  stated  (§  458),  we  lack  the  guidance 
of  experimental  results  ;  we  are  obliged  to  trust  to  the  entan- 
gled phenomena  of  disease  or  to  a  study  of  the  behaviour  of 
the  cord  Avhile  it  is  still  a  part  of  an  intact  nervous  system  ; 
and  each  of  these  methods  presents  difficulties  of  its  own.  The 
movements,  which  in  the  intact  human  body  we  can  recognize 
as  indubitable  reflex  actions,  are  as  a  rule  simple  and  unimpor- 
tant. Tliey  are,  in  by  far  the  greater  number  of  instances, 
occasioned  by  stimulation  of  the  skin  or  of  the  mucous  mem- 
brane, for  the  most  part  involve  a  few  muscles  only,  and  rarely 
indicate  any  very  complex  coordination.  The  flexion,  followed 
by  extension,  of  the  leg  which  is  called  forth  by  tickling  the 
sole  of  the  foot,  may  perhaps  be  regarded  as  the  type  of  these 
movements.  A  very  common  form  of  reflex  action  is  that  in 
which  a  muscle  or  grouj)  of  muscles  is  thrown  into  contraction 
by  stimulation  of  the  overlying  or  neighbouring  skin,  as  when 
the  abdominal  muscles  contract  upon  stroking  the  skin  of  the 


Chap,  i.]  THE   SPII^AL   CORD.  705 

abdomen  or  the  testicle  is  retracted  upon  stroking  the  inside 
of  the  thigh.  A  special  form  of  reflex  action,  or  at  least  an 
action  maintained  by  many  to  be  a  reflex  action,  is  called  forth 
by  sharply  striking  certain  tendons.  It  is  well  known  for 
instance  that  when  the  leg  is  placed  in  an  easy  position,  rest- 
ing for  instance  on  the  other  leg,  a  sharp  blow  on  the  patellar 
tendon  will  cause  a  sudden  jerk  forward  of  the  leg,  brought 
about  by  a  contraction  of  part  of  the  quadriceps  femoris  ;  it  is 
necessary  or  at  least  desirable  for  a  good  development  of  the 
jerk  that  the  tendon  (and  muscle)  should  be  somewhat  on  the 
stretch.  Similarly  the  muscles  of  the  calf  may  be  thrown  into 
action  by  tapj^ing  the  tendo  Achillis  put  somewhat  on  the 
stretch  by  flexion  of  the  foot ;  and  in  some  cases  tlie  same 
muscles  may  be  made  to  execute  a  series  of  regular  rhythmic 
contractions,  called  '  clonic '  contractions,  b}'  suddenl}'  pressing 
back  the  sole  of  the  foot  so  as  to  put  them  on  tlie  stretch.  The 
"knee-jerk,"  as  the  sudden  extension  of  the  leg  when  the 
patella  is  struck  is  familiarly  called,  has  acquired  great  interest 
in  medical  practice,  and  has  been  carefully  studied  not  only  in 
man  but  in  various  animals.  It  has  been  contended  by  some 
that  the  act  is  not  truly  a  reflex  one,  but  the  evidence  on  the 
whole  goes  to  shew  that  it  is.  From  various  experiments  we 
learn  that  in  it  as  in  an  ordinary  reflex  movement  there  are 
three  factors,  namel}^  an  afferent  limb,  a  spinal  centre  and  an 
efferent  limb  ;  if  any  one  of  these  three  factors  fails,  the  whole 
act  fails.  The  eft'erent  limb  is  furnished  by  fibres  in  the 
anterior  crural  nerve  which  reaching  that  nerve  by  the  anterior 
roots  of  (in  man)  the  3d  and  4th  lumbar  nerves,  and  passing 
to  the  vastus  internus  muscle  and  adjoining  portions  of  the 
crureus  muscle,  these  being  the  parts  of  the  great  quadriceps 
concerned.  The  spinal  centre  lies  in  the  lumbar  portion  of  the 
cord,  in  what  in  man  corresponds  to  the  3d  and  4th  lumbar 
sesfments.  The  afferent  limb  is  furnished  bv  fibres  startino-  in 
the  patellar  tendon,  running  in  the  anterior  crural  nerve,  and 
reaching  the  cord  by  the  posterior  root  of  (in  man)  the  4th 
lumbar  nerve.  It  is  further  worthy  of  notice  as  an  illustration 
of  what  we  were  urging  a  little  while  back  that  the  vigour  of 
the  knee-jerk  is  influenced  by  the  antagonistic  hamstring  mus- 
cles, and  that  not  merely  in  a  mechanical  way  but  by  nervous 
action.  The  knee-jerk  is  reinforced  by  cutting  the  nerves  sup- 
plying the  hamstring  muscles,  or  by  simple  division  of  certain 
posterior  roots,  fibres  of  which  take  origin  in  those  muscles  or 
their  tendons  ;  it  is  depressed  by  central  stimulation  of  those 
nerves,  or  by  simply  stretching  the  hamstring  muscles.  It 
would  api)ear  that  these  flexor  muscles  antagonistic  to  the 
extensor  muscles,  send  up  to  the  spinal  cord,  according  to  their 
condition,  afl^erent  impulses  which  influence  the  spinal  centre 
for  the  knee-jerk.     The  activity  of  the  same  centre  may  also  be 

45 


706  EEFLEX  ACTIONS.  [Book  hi. 

influenced,  in  tlie  way  of  augmentation  or  depression,  by  ner- 
vous impulses  reaching  it  from  other  parts  of  the  nervous 
system. 

When  we  turn  to  the  teaching  of  disease,  we  again  find  that 
reflex  movements  carried  out  by  the  cord  or  by  parts  of  the 
cord  are,  on  the  whole,  scanty  and  simple. 

In  some  stages  of  certain  diseases  of  the  spinal  cord  exten- 
sive reflex  movements  it  is  true  are  witnessed  ;  but  these  are 
not  purposeful  coordinated  movements,  such  as  have  been 
described  above  as  occurring  in  frogs  and  mammals  after  ex- 
perimental interference,  but  rather  mere  exaggerations  of  the 
simpler  reflex  movements  witnessed  when  the  nervous  system 
is  intact.  In  cases  of  paraplegia  (such  being  the  term  gener- 
ally used  when  disease  or  injury  has  cut  off  the  cord,  generally 
the  lower  part  of  the  cord,  from  the  brain  so  that  the  will  can- 
not bring  about  movements  in,  and  the  mind  derives  no  sensa- 
tions from,  the  parts  below  the  lesion,  the  legs  for  instance), 
it  sometimes  happens  that  contact  with  the  bedclothes,  or 
other  external  objects,  sets  up  from  time  to  time  rhythmically 
repeated  movements,  the  legs  being  alternately  drawn  up  and 
thrust  out  again.  And  an  exaggeration  of  the  '  knee-jerk '  or 
other  '  tendon  reflexes '  is  a  very  common  symptom  in  certain 
spinal  diseases.  It  is  rarely  if  ever  that  reflex  movements  of  a 
really  complicated  character  are  observed.  Moreover  clinical 
experience  shews  that  in  man,  when  a  portion  of  the  cord  is 
isolated,  reflex  actions  carried  out  by  means  of  that  portion  so 
far  from  being  exaggerated  are  much  more  commonly  exceed- 
ing feeble  or  absent  altogether.  In  the  cases  in  which  the 
physiological  continuity  of  the  lower  with  the  upper  part  of 
the  cord  has  been  broken  by  disease,  by  some  growth  invading 
the  nervous  structures  or  by  some  changes  of  the  nervous 
structures  themselves,  we  may  attempt  to  explain  the  absence 
from  the  lower  part  of  coordinate  reflex  activity,  such  as  is 
seen  in  the  lower  animals,  as  due  to  the  disease  not  only  affect- 
ing the  powers  of  the  actually  diseased  part,  but  influencing 
the  whole  cord  below,  and  either  by  inhibition,  of  which  we 
shall  speak  presently,  or  in  some  other  way  depressing  its  func- 
tions. But  the  same  absence  of  complex  reflex  movements  is 
also  often  observed  in  cases  in  which  the  cord  has  been  severed 
by  accident,  and  indeed,  though  accidental  injuries  to  the 
human  cord  generally  produce  more  profound  and  extensive 
mischief  than  that  which  results  in  animals  from  skilful  experi- 
mental interference,  clinical  experience  tends,  on  the  whole,  to 
support  the  view  that  in  man  the  more  complete  subordination 
of  the  spinal  cord  to  the  brain  has  led  to  the  dying  out  of  tlie 
complex  reflex  actions  which  are  so  conspicuous  in  the  lower  an- 
imals. This  however  cannot  be  regarded  as  distinctly  proved. 
§  465.     We   have    dwelt   above    chiefly   on   reflex   actions. 


Chap,  i.]  THE    SPIKAL   COED.  707 

in  which  the  efferent  impulses  cause  contractions  of  skeletal 
muscles  since  these  are  undoubtedly  the  most  common  and 
the  most  prominent  forms  of  reflex  action ;  but  it  must  not  be 
forgotten  that  the  efferent  impulses  of  reflex  origin  may  pro- 
duce contractions  of  other  muscles,  as  well  as  other  effects, 
such  as  secretion  for  instance.  On  several  of  these  Ave  have 
dwelt,  from  time  to  time  in  previous  parts  of  this  work,  and  it 
will  be  unnecessary  to  repeat  them  here.  But  it  may  be  worth 
while  to  point  out  that  the  spinal  cord  by  serving  as  a  reflex 
centre  for  innumerable  ties  which  correlate  the  nutritive  or 
metabolic  activities  of  the  several  tissues  to  events  taking  place 
in  other  parts  of  the  body,  plays  a  conspicuous  part  in  securing 
the  welfare  of  the  whole  body.  In  dealing  (§  439)  with  the 
general  problems  of  nutrition,  we  stated  that  an  orderly  nutri- 
tion appears  to  be  in  some  way  dependent  on  nervous  influ- 
ences. Many  of  these  nervous  influences  appear  to  issue  from 
the  spinal  cord,  either  as  parts  of  a  reflex  act,  or  as  the  out- 
come of  some  automatic  processes.  In  man,  extensive  injuries 
to  the  spinal  cord  are  followed  by  bed  sores  and  other  results 
of  impaired  nutrition ;  and  indeed  death  is  generally  brought 
about  in  this  way,  in  cases  of  paraplegia  caused  by  accidental 
crushing  or  severance  of  the  cord. 

§  466.  Inhibition  of  Reflex  Action.  The  reflex  actions  of 
the  spinal  cord,  like  other  nervous  actions,  may  be  totally  or 
partially  inhibited,  that  is  to  say  may  be  arrested  or  hindered 
in  their  development  by  impulses  reaching  the  centre  wliile  it 
is  already  in  action.  Thus  if  the  body  of  a  decapitated  snake 
be  allowed  to  hang  down,  slow  rhythmic  pendulous  move- 
ments, which  appear  to  be  reflex  in  nature,  soon  make  their 
appearance,  and  these  may  be  for  a  while  arrested  by  slight 
stimulation,  as  by  gently  stroking  the  tail.  We  have  already 
seen  that  the  action  of  such  nervous  centres  as  the  respiratory 
and  vaso-motor  centres,  which  frequently  at  all  events  is  of 
a  reflex  nature,  may  be  either  inhibited  or  augmented  by  affer- 
ent impulses.  The  micturition  centre  in  the  mammal,  which  is 
also  largely  a  reflex  centre,  may  be  easily  inhibited  by  impulses 
passing  downward  to  the  lumbar  cord  from  the  brain,  or 
upward  along  the  sciatic  nerves.  In  the  case  of  dogs,  whose 
spinal  cord  has  been  divided  in  the  thoracic  region,  micturition 
set  up  as  a  reflex  act  by  simple  pressure  on  the  abdomen  or  by 
sponging  the  anus,  is  at  once  stopped  by  sharply  pinching  the 
skin  of  the  leg.  And  it  is  a  matter  of  common  experience  that 
in  man  micturition  may  be  suddenly  checked  by  an  emotion  or 
other  cerebral  cA^ent.  The  erection  centre  in  the  lumbar  cord, 
also  in  large  measure  a  reflex  centre,  is  similarly  susceptible  of 
being  inhibited  by  impulses  reaching  it  from  various  sources. 
And  indeed  many  similar  instances  of  the  inhibition  of  reflex 
movements  might  readily  be  quoted. 


708  INHIBITION   OF   EEFLEX  ACTIONS.     [Book  in. 

Several  apparent  instances  of  the  inhibition  of  reflex  acts 
are  not  really  such :  in  these  cases  all  the  nervous  processes  of 
the  act  may  take  place  in  their  entirety  and  yet  fail  to  produce 
their  effect  on  account  of  a  failure  in  the  muscular  part  of  the 
act.  Thus  when  we  ourselves  by  an  effort  of  the  will  stop 
the  reflex  movements  which  otherwise  would  be  produced  by 
tickling  the  soles  of  the  feet,  we  achieve  this  to  a  large  extent 
by  throwing  voluntarily  into  action  certain  muscles,  the  con- 
tractions of  which  antagonize  the  action  of  the  muscles  engaged 
in  carrying  out  the  reflex  movements.  But  it  may  be  doubted 
even  in  these  cases,  whether  inhibition  is  always  or  wholly  to 
be  explained  in  this  way ;  and  certainly  in  very  many  instances 
of  reflex  inhibition,  no  such  muscular  antagonism  is  present, 
and  the  reflex  act  is  checked  at  its  nervous  centre. 

When  the  brain  of  a  frog  is  removed,  and  the  effects  of 
shock  have  passed  away,  reflex  actions  are  developed  much 
more  readily  and  to  a  much  greater  degree  than  in  the  entire 
animal,  and  in  mammals  also  reflex  excitability  has  been  ob- 
served to  be  increased  by  removal  of  the  cerebral  hemispheres. 
This  suggests  the  idea  that  in  the  intact  nervous  system  the 
brain  is  habitually  exerting  some  influence  on  the  spinal  cord 
tending  to  prevent  the  normal  development  of  the  spinal  reflex 
actions.  And  we  learn  by  experiment  that  stimulation  of 
certain  parts  of  the  brain  has  a  remarkable  effect  on  reflex 
action.  If  a  frog,  from  which  the  cerebral  hemispheres  have 
been  removed  (the  optic  lobes,  bulb  and  spinal  cord  being  left 
intact),  be  suspended  by  the  jaw,  and  the  toes  of  the  pen- 
dent legs  be  from  time  to  time  dipped  into  very  dilute  sulphuric 
acid,  a  certain  average  time  will  be  found  to  elapse  between 
the  dipping  of  the  toe  and  the  resulting  withdrawal  of  the 
foot.  If,  however,  the  optic  lobes  or  optic  thalami  be  stimu- 
lated, as  by  putting  a  crystal  of  sodium  chloride  on  them,  it 
will  be  found  on  repeating  the  experiment  while  these  struct- 
ures are  still  under  the  influence  of  the  stimulation,  that  the 
time  intervening  between  the  action  of  the  acid  on  the  toe  and 
the  withdrawal  of  the  foot  is  very  much  prolonged.  That  is 
to  say,  the  stimulation  of  the  optic  lobes  has  caused  impulses 
to  descend  to  the  cord,  which  have  there  so  interfered  with  the 
nervous  processes  engaged  in  carrying  out  reflex  actions  as 
greatly  to  retard  the  generation  of  efferent  impulses,  or  in 
other  words,  has  inhibited  the  reflex  action  of  the  cord.  And 
similar  results  may  be  obtained  in  mammals  by  stimulating 
certain  parts  of  the  corpora  quadrigemina,  which  bodies  are 
homologous  to  the  optic  lobes  of  frogs.  From  this  it  has  been 
inferred  that  there  is  present  in  this  part  of  the  brain  a  special 
mechanism  for  inhibiting  the  reflex  actions  of  the  spinal  cord, 
the  impulses  descending  from  this  mechanism  to  the  various 
centres  of  reflex  action  being  of  a  specific  inhibitory  nature. 


Chap,  i.]  THE   SPIRAL   CORD.  709 

But,  as  we  have  already  seen,  impulses  of  an  ordinary  kind, 
passing  along  ordinary  sensory  nerves,  may  inhibit  reflex 
action.  We  have  quoted  instances  where  a  slight  stimu- 
lus, as  in  the  pendulous  movements  of  the  snake,  and  where 
a  stronger  stimulus  as  in  the  case  of  the  micturition  of  the 
dog,  may  produce  an  inhibitory  result  ;  we  may  add  that  in 
the  frog  adequately  strong  stimuli  applied  to  any  afferent 
nerve  will  inhibit,  i.e.  will  retard  or  even  wholly  prevent 
reflex  action.  If  the  toes  of  one  leg  are  dipped  into  dilute 
sulphuric  acid  at  a  time  when  the  sciatic  of  the  other  leg  is 
being  powerfully  stimulated  with  an  interrupted  current  the 
period  of  incubation  of  the  reflex  act  will  be  found  to  be  much 
prolonged,  and  in  some  cases  the  reflex  withdrawal  of  the  foot 
will  not  take  place  at  all.  And  this  holds  good,  not  only  in 
the  complete  absence  of  the  optic  lobes  and  bulb,  but  also 
when  only  a  portion  of  the  spinal  cord,  sufficient  to  carry  out 
the  reflex  action  in  the  usual  way,  is  left.  There  can  be  no 
question  here  of  any  specific  inhibitory  centres,  such  as  have 
been  supposed  to  exist  in  the  optic  lobes.  But  if  it  is  clear  that 
inhibition  of  reflex  action  may  be  brought  about  by  impulses 
which  are  not  in  themselves  of  a  specific  inhibitory  nature, 
we  may  hesitate  to  accept  the  view  that  a  special  inhibitory 
mechanism  in  the  sense  of  one  giving  rise  to  nothing  but  in- 
hibitory impulses  is  present  in  the  optic  lobes  of  frogs,  and 
after  removal  of  the  brain  that  the  ^xaltation  of  reflex  actions 
which  is  manifest  is  due  to  the  withdrawal  of  such  a  specific 
inhil^itory  mechanism. 

§  467.  The  Time  required  for  Reflex  Actions.  When  one 
eyelid  is  stimulated  Avith  a  sharp  electrical  shock,  both  eyelids 
blink.  Hence,  if  the  length  of  time  intervening  between  the 
stimulation  of  the  right  eyelid  and  the  movement  of  the  left 
eyelid  be  measured,  this  will  give  the  total  time  required  for 
the  various  processes  which  make  up  a  reflex  action.  It  has 
been  found  to  be  from  -0662  to  -0578  sec.  Deducting  from 
these  figures  the  time  required  for  tlie  passage  of  afferent  and 
efferent  impulses  along  the  fifth  and  facial  nerves  to  and  from 
the  bulb,  and  for  the  latent  period  of  the  contraction  of  the 
orbiscularis  muscle,  there  would  remain  -0555  to  -0471  sec.  for 
the  time  consumed  in  the  central  operations  of  the  reflex  act. 
The  calculations,  however,  necessary  for  this  reduction,  it  need 
not  be  said,  are  open  to  sources  of  error ;  moreover  the  reflex 
act  in  question  is  carried  out  by  the  bulb  and  not  by  the  spinal 
cord  proper.  Blinking  thus  produced  is  a  reflex  act  of  the 
very  simplest  kind ;  but  as  we  have  seen  in  the  preceding 
pages,  reflex  acts  differ  very  widely  in  nature  and  character ; 
and  we  accordingly  find,  as  indeed  we  have  incidentally  men- 
tioned, that  the  time  taken  up  by  a  reflex  movement  varies 
very  largely.      This  indeed  is  seen  in  blinking  itself.      AVhen 


710  mHIBITION   OF   REFLEX   ACTIONS.     [Book  in. 

the  blinking  is  caused  not  by  an  electric  shock  applied  to  the 
eyelid,  but  by  a  flash  of  light  falling  on  the  retina,  in  which 
case  complex  visual  processes,  are  involved,  the  time  is  dis- 
tinctly prolonged  ;  moreover  the  results  in  different  experi- 
ments in  which  light  serves  as  the  stimulus  are  not  nearly  so 
uniform  as  when  the  blinking  is  caused  by  stimulation  of  the 
eyelid.  In  the  "  knee-jerk  "  the  time  is  very  short,  it  may  be 
not  more  than  -03  sec. ;  this  is  one  of  the  reasons  which  have 
led  some  to  regard  the  act  as  not  truly  a  reflex  one. 

In  general  it  may  be  said  that  the  time  required  for  any 
reflex  act  varies  very  considerably  with  the  strength  of  the 
stimulus  employed,  being  less  for  the  stronger  stimuli ;  this  we 
should  expect,  seeing  that  the  efferent  impulses  of  the  reflex  act 
are  not  simply  afferent  impulses  transmitted  through  the  central 
organ,  but  result  from  internal  changes  in  the  central  organ 
started  by  the  afferent  impulse  or  impulses  ;  and  these  internal 
changes  will  naturally  be  more  intense  and  more  rapidly  effected 
when  the  afferent  impulses  are  strong.  It  is  stated  that  when 
the  movement  induced  is  on  the  same  side  of  the  body  as  the 
surface  stimulation  of  which  starts  the  act,  the  time  taken  up 
is  less  than  when  the  movement  is  on  the  other  side  of  the  body, 
allowance  being  made  for  the  length  of  central  nervous  matter 
involved  in  the  two  cases  ;  that  is  to  say  the  central  operations 
of  a  reflex  act  are  propagated  more  rapidly  along  the  cord  than 
across  the  cord.  The  rapidity  of  the  act  varies  of  course  with 
the  condition  of  the  spinal  cord,  the  act  being  greatly  prolonged 
when  the  cord  becomes  exhausted  ;  and  a  similar  delay  has  been 
observed  in  cases  of  disease.  The  time  thus  occupied  by  purely 
reflex  actions  must  not  be  confounded  with  the  interval  required 
when  the  changes  taking  place  in  the  central  nervous  system 
are  of  a  more  complicated  nature,  and  more  or  less  distinctly 
involve  mental  operations  ;  of  the  latter  we  shall  speak  later 
on. 


SEC.   4.     THE   AUTOMATIC   ACTIONS   OF  THE   SPINAL 

CORD. 


§  468.  We  speak  of  an  action  of  an  organ  or  of  a  living  body 
as  being  spontaneous  or  automatic  when  it  apj^ears  to  be  not 
immediately  due  to  any  changes  in  the  circumstances  in  which 
the  organ  or  body  is  placed,  but  to  be  the  result  of  changes 
arising  in  the  organ  or  body  itself  and  determined  by  causes 
other  than  the  influences  of  the  circumstances  of  the  moment. 
Some  automatic  actions  are  of  a  continued  character ;  others, 
like  the  beat  of  the  heart,  are  repeated  in  regular  rhythm  ;  but 
the  most  striking  automatic  actions  of  the  living  body,  those 
which  we  attribute  to  the  working  of  the  will  and  which  we 
call  voluntary  or  A^olitional,  are  characterized  by  their  apparent 
irregularity  and  variableness.  Such  variable  automatic  actions 
form  the  most  striking  features  of  an  intact  nervous  system,  but 
are  conspicuously  absent  from  a  spinal  cord  when  the  brain  has 
been  removed. 

A  brainless  frog  placed  in  a  condition  of  complete  equilib- 
rium in  Avhich  no  stimulus  is  brought  to  bear  on  it,  protected 
for  instance  from  sudden  passing  changes  in  temperature,  from 
a  too  rapid  evaporation  by  the  skin  and  the  like,  remains  per- 
fectly motionless  until  it  dies.  Such  apparently  spontaneous 
movements  as  are  occasionally  witnessed  are  so  few  and  seldom, 
that  we  can  hardly  do  otherwise  than  attribute  them  to  some 
stimulus,  internal  or  external,  which  has  escaped  observation. 
In  the  mammal  (dog)  after  division  of  the  spinal  cord  in  the 
dorsal  region  regular  and  apparently  spontaneous  movements 
may  be  observed  in  the  parts  governed  by  the  lumbar  cord. 
When  the  animal  has  thoroughly  recovered  from  the  operation 
the  hind  limbs  rarely  remain  quiet  for  any  long  period ;  they 
move  restlessly  in  various  ways ;  and  when  the  animal  is  sus- 
pended by  the  upper  part  of  the  body,  the  pendent  hind  limbs 
are  continually  being  drawn  up  and  let  down  again  with  a 
monotonous  rhythmic  regularity,  suggestive  of  automatic  rhyth- 
mic discharges  from  the  central  mechanisms  of  the  cord.  In 
the  newly  born  mammal  too,  after  removal  of  the  brain,  move- 
ments apparently  spontaneous  in  nature  are  frequently  observed. 

711 


712  TONE    OF   SKELETAL   MUSCLES.         [Book  in. 

But  all  these  movements,  even  when  most  highly  developed, 
are  very  diiferent  from  the  movements,  irregular  and  variable 
in  their  occurrence  though  orderly  and  purposeful  in  their  char- 
acter, which  we  recognize  as  distinctly  voluntary.  Even  admit- 
ting that  some  of  the  movements  of  the  brainless  mammal  may 
resemble  voluntary  movements  in  so  far  as  they  are  due  to 
changes  taking  place  in  the  spinal  cord  itself  independent  of 
the  immediate  influence  of  any  stimulus,  we  are  not  thereby 
justified  in  speaking  of  the  spinal  cord  as  developing  a  will  in 
the  sense  that  we  attribute  a  will  to  the  brain. 

§  469.  In  the  case  of  the  beat  of  the  heart,  the  automatic 
rhythmic  discharge  of  energy  appears  to  be  exclusively  the  out- 
come of  the  molecular  nutritive  changes  taking  place  in  the 
cardiac  substance.  The  beat  may  be  modified,  as  we  have  seen, 
by  nervous  impulses  reaching  the  cardiac  substance  along  cer- 
tain nerves  ;  but  the  actual  existence  of  the  beat  is  wholly  inde- 
pendent of  these  extraneous  influences  ;  the  rhythmic  discharge 
continues  when  they  are  entirely  absent.  The  automatic  rhyth- 
mic discharge  of  respiratory  impulses  from  the  respiratory  cen- 
tre is  also  dependent  on  the  intrinsic  molecular  changes  of  the 
centre,  these  being,  as  we  have  seen,  largely  determined  by  the 
character  of  the  blood  streaming  through  it ;  but  in  this  case 
extrinsic  nervous  impulses,  reaching  the  centre  along  the  vagus 
and  other  nerves,  play  a  much  more  important  part  than  do 
similar  impulses  in  the  case  of  the  heart.  They  act  so  continu- 
ally on  the  centre  and  enter  so  largely  into  its  working,  that 
we  are  compelled  to  regard  the  activity  of  the  centre  as  fed,  if 
we  may  use  the  word,  not  only  by  the  intrinsic  molecular  nutri- 
tive processes  of  the  centre  itself,  but  also  by  the  extrinsic  ner- 
vous influences  which  flow  into  the  centre  from  Avithout.  The 
automatism  of  the  spinal  cord  as  a  whole  resembles,  in  this 
aspect,  that  of  the  respiratory  centre  rather  than  that  of  the 
heart.  It  has  for  its  basis  doubtless  the  intrinsic  molecular 
changes  of  the  grey  matter ;  the  metabolic  events  of  this  sub- 
stance are  so  ordered  as  to  give  rise  to  discharges  of  energy ; 
but  the  discharge  appears  to  be  also  intimately  dependent  on  the 
inflow  into  the  grey  matter  of  afferent  impulses  and  influences. 
The  normal  discharge  of  efferent  impulses  from  the  cord  un- 
doubtedly takes  place  under  the  influence  of  these  incoming 
impulses  ;  and  it  may  be  doubted  whether  the  grey  matter  of 
the  cord  Avould  be  able,  in  the  absence  of  all  afferent  impulses, 
to  generate  any  sustained  series  of  discharges  out  of  its  merely 
nutritive  intrinsic  changes.  The  automatic  activity  of  the  cord 
is  fed  not  only  by  intrinsic  nutritive  events,  but  also  by  extrin- 
sic influences. 

In  this  feature  we  may,  moreover,  find  perhaps  the  reason 
why  the  automatic  activity  of  the  spinal  cord  is  so  limited,  as 
compared  with  that  of  the  brain.      In  sjDite  of  certain  striking 


Chap,  i.]  THE   SPINAL   CORD.  713 

but  superficial  characters  of  which  we  shall  speak  later  on,  the 
grey  matter  of  the  brain  presents  no  histological  features  so 
different  from  those  of  the  grey  matter  of  the  cord,  as  to  justify 
us  in  concluding  that  the  one  is  capable  and  the  other  incapable 
of  developing  the  impulses,  Avhicli  we  call  volitional,  out  of  the 
molecular  nutritive  changes  of  its  substance.  We  are,  there- 
fore, led  to  the  conclusion  that  tlie  fuller  automatic  activity  of 
the  brain  is  due  to  the  intrinsic  changes  of  its  substance  being 
so  much  more  largely  assisted  by  the  influx  of  various  afferent 
impulses  and  influences,  notably  those  of  the  special  senses.  To 
tliis  question,  however,  we  shall  have  to  return  later  on. 

§  470.  In  treating  of  the  vascular  system  we  saAv  that  the 
central  nervous  system  exercised  through  the  vaso-motor  nerves 
such  an  influence  on  the  muscular  coats  of  tlie  blood  vessels  as  to 
maintain,  what  we  spoke  of  as  'tone,'  section  of  vaso-constrictor 
fibres  leading  to  "  loss  of  tone."  We  saw  further,  that  arterial 
tone,  though  normally  dependent  on  the  general  vaso-motor 
centre  in  the  bulb,  could  be  kept  up  by  the  cord  itself,  that  for 
instance  a  tone  of  the  blood  vessels  of  the  hind  limbs  could  be 
maintained  by  the  isolated  dorso-lumbar  cord.  This  mainte- 
nance of  arterial  tone  may  be  spoken  of  as  one  of  the  "  auto- 
matic "  functions  of  the  spinal  cord.  We  have  also  seen  that 
plain  muscular  fibres,  other  than  those  of  the  arteries,  notably 
the  fibres  forming  sphincters,  such  as  the  cardiac  and  pyloric 
sphincters  of  the  stomach,  the  sphincter  of  the  bladder,  and 
especially  the  sphincter  of  the  anus,  also  possess  tone,  and  that 
the  tone  of  these  sphincters  is  also  dependent  on  the  spinal  cord, 
or  on  some  part  of  the  central  nervous  system.  AVe  need  not 
repeat  the  discussions  concerning  these  mechanisms  and  other 
instances  of  the  spinal  cord  exercising  an  automatic  influence 
over  various  viscera  ;  we  have  referred  to  them  here,  since  they 
serve  as  an  introduction  to  a  question  wliich  has  been  much  de- 
bated, and  which  has  many  collateral  and  imi)ortant  bearings, 
namely  the  question  whether  the  spinal  cord  exercises  an  auto- 
matic function  in  maintaining  a  tone  of  the  skeletal  muscles. 

The  question  is  not  one  which,  like  tlie  case  of  arterial  tone, 
can  be  settled  off  hand  by  a  simple  experiment.  ]\Iost  observers 
agree  that  the  section  of  a  motor  nerve  does  not  produce  any 
clearly  recognizable  immediate  lengthening  of  a  muscle  supplied 
by  the  nerve,  in  the  same  way  that  section  of  a  vaso-constrictor 
nerve  undoubtedly  gives  rise  to  a  relaxation  of  the  muscular 
fibres  in  the  arteries  governed  by  it ;  and  it  has  been  inferred 
from  this  that  skeletal  tone  does  not  exist.  But  there  are  sev- 
eral facts  to  be  taken  into  consideration  before  we  can  come  to 
a  just  decision. 

The  skeletal  muscles  have  been  described  as  being  placed  "on 
the  stretch"  in  the  living  body.  If  a  muscle  be  cut  away  from 
its  attachments  at  each  end,  it  shortens  ;  if  it  be  cut  across,  it 


714  TONE   OF   SKELETAL  MUSCLES.         [Book  hi. 

gapes.  Ill  other  words,  the  muscle  in  the  living  body  possesses 
a  latent  tendency  to  shorten,  which  is  continually  being  counter- 
acted by  its  disposition  and  attachments.  In  studying  muscular 
contraction  we  saw  (§  81)  that  the  shortening  of  a  contraction 
is  followed  by  a  relaxation  or  return  to  the  former  length,  both 
the  contraction  and  relaxation  being  the  result  of  molecular 
changes  in  the  living  muscular  substance.  We  have  now  to 
extend  our  view  and  to  recognize  that,  apart  from  the  occur- 
rence of  ordinary  contractions,  molecular  changes  are  by 
means  of  nutritive  processes  continually  going  on  in  the  muscle 
in  such  a  way  that  the  muscle,  though  continually  on  the  stretch, 
does  not  permanently  lengthen,  but  retains  the  power  to  shorten 
upon  removal  or  lessening  of  the  stretch,  and  conversely  though 
possessing  this  power  of  shortening  permits  itself  to  lengthen 
when  the  stretch  is  increased.  In  this  way  the  muscle  is  able 
to  accommodate  itself  to  variations  in  the  amount  of  stretch  to 
which  it  is  from  time  to  time  subjected.  When  a  flexor  muscle 
for  instance  contracts,  the  antagonistic  extensor  muscle  is  put  on 
an  increased  stretch  and  is  correspondingly  lengthened ;  when 
the  contraction  of  the  flexor  passes  off  the  extensor  returns  to 
its  previous  length ;  and  so  in  other  instances.  Thus  by  virtue 
of  certain  changes  within  itself  a  muscle  maintains  what  may 
be  called  its  natural  length  in  the  body,  always  returning  to 
that  natural  length  both  after  being  shortened  and  after  being 
stretched.  In  this  the  muscle  does  no  more  than  do  the  other 
tissues  of  the  body  which,  within  limits,  retain  their  natural 
form  under  the  varied  stress  and  strain  of  life  ;  but  the  prop- 
erty is  conspicuous  in  the  muscle ;  and  its  effects  in  skeletal 
muscles  correspond  so  closely  to  those  of  arterial  tone,  that  we 
may  venture  to  speak  of  it  as  skeletal  tone.  Indeed,  the  molec- 
ular changes  at  the  bottom  of  both  are  probably  the  same. 

These  changes  are  an  expression  of  the  life  of  the  muscle; 
they  disappear  when  the  muscle  dies  and  enters  into  rigor  mortis; 
and  moreover,  during  life  they  vary  in  intensity  so  that  the 
'  tone '  varies  in  amount  according  to  the  nutritive  changes 
going  on.  We  have  seen  reason  to  believe  that  the  nutrition  of 
a  muscle  as  of  other  tissues  is  governed  in  some  way  by  the 
central  nervous  system.  We  saw,  in  treating  of  muscle  and 
nerve  (§  78),  that  the  irritability  of  a  muscle  is  markedly 
affected  by  the  section  of  its  nerve,  i.e.  by  severance  from  the 
central  nervous  system;  and  again  (§  439)  in  speaking  of  the 
so-called  trophic  action  of  the  nervous  system,  we  referred  to 
changes  in  the  nutrition  of  muscles  occasioned  by  diseases  of 
the  nervous  system.  And  experience,  especially  clinical  experi- 
ence, shews  that  the  nutritive  changes  which  determine  tone  are 
very  closely  dependent  on  a  due  action  of  the  central  nervous 
system.  When  we  handle  the  limb  of  a  healthy  man,  we  find 
that  it  offers  a  certain  amount  of  resistance  to  passive  move- 


Chap,  i.]  THE   SPINAL  COED.  715 

ments.  This  resistance,  which  is  quite  independent  of,  that  is 
to  say,  which  may  be  clearly  recognized  in  the  absence  of  all 
distinct  mnscular  contractions  of  volitional  or  other  origin,  is 
an  expression  of  muscular  tone,  of  the  effort  of  the  various 
muscles  to  maintain  their  '  natural '  length.  In  many  cases  of 
disease  this  resistance  is  felt  to  be  obviously  less  than  normal; 
the  limb  is  spoken  of  as  "limp"  or  "flabby";  or  as  having  'a 
want  of  tone.'  In  other  cases  of  disease,  on  the  other  hand, 
this  resistance  is  markedly  increased;  the  limb  is  felt  to  be  stiff 
or  rigid;  more  or  less  force  is  needed  to  change  it  from  a  flexed 
to  an  extended,  or  from  an  extended  to  a  flexed  condition;  and, 
in  the  range  of  disease,  we  may  meet  with  very  varying  amounts 
of  increased  resistance,  from  a  condition  which  is  only  slightly 
above  the  normal  to  one  of  extreme  rigidity.  In  some  cases 
the  condition  of  the  muscle  is  such  as  at  first  sight  seems  much 
more  comparable  to  a  permanent  ordinar}-  contraction  than  to  a 
mere  exaggeration  of  normal  tone;  but  all  intermediate  stages 
are  met  with;  and  indeed  these  extreme  cases  may  l)e  taken  as 
indicating  that  the  molecular  processes  which  maintain  what  we 
are  now  calling  tone,  are  at  bottom,  of  the  same  nature  as  those 
which  carry  out  a  contraction;  they  serve  to  shew  the  funda- 
mental identity  of  the  skeletal  tone  with  the  more  obvious 
arterial  tone. 

Clinical  experience  then  shews  that  the  central  nerA'ous 
system  does  exert  on  the  skeletal  muscles  such  an  influence  as 
to  give  rise  to  what  we  may  s^^eak  of  as  skeletal  tone,  changes 
in  the  central  nervous  system,  leading  in  some  cases  to  diminu- 
tion or  loss  of  tone,  in  other  cases  to  exaggeration  of  tone,  mani- 
fested often  as  conspicuous  rigidity.  The  question  why  the 
changes  take  one  direction  in  one  case  and  another  in  another  is 
one  of  great  difficulty  (the  occurrence  of  extreme  rigidity  being 
especially  obscure),  and  camiot  be  discussed  here.  We  have 
called  attention  to  the  facts  simpl}-  because  they  shew  the  exist- 
ence of  skeletal  tone  and  its  dependence  on  the  central  nervous 
system.  This  conclusion  is  confirmed  by  experiments  on  animals, 
and  these  also  afford  proof  that  in  animals  the  spinal  cord  can 
by  itself,  apart  from  the  brain,  maintain  the  existence  of  such  a 
tone.  In  a  frog,  after  division  of  the  cord  below  the  brain,  the 
limbs  during  the  period  of  shock  are  flabb}^  and  toneless;  but 
after  a  while,  as  the  shock  passes  off,  tone  returns  to  the  muscles, 
and  the  limbs  offer  when  handled  a  resistance  like  that  of  the 
limbs  of  an  entire  frog.  AVhen  the  animal  is  suspended  the 
hind  limbs  do  not  hang  perfectly  limp  and  helpless,  but  assume 
a  definite  position  ;  and  that  this  position  is  due  to  some  influ- 
ence proceeding  from  the  s})inal  cord  is  shewn  by  dividing  the 
sciatic  nerve  on  one  side:  the  hind  limb  on  that  side  now  hangs 
quite  helpless.  This  more  pendent  position  shews  that  some  of 
the  flexors  have  lengthened  in  consequence  of  the  section  of  the 


716  imEE-JERK.  [Book  hi. 

nerve,  and  this  result  may  be  taken  as  refuting  the  argument, 
quoted  above,  against  the  existence  of  tone,  which  is  based  on 
the  statement  that  a  muscle  cannot  be  observed  to  lengthen 
after  section  of  its  nerve.  It  may  be  here  remarked  that  if  the 
brainless  frog,  whose  hind  limbs  are  more  or  less  pendent  when 
the  body  is  suspended,  be  placed  on  its  belly  the  hind  limbs  are 
brought  into  a  flexed  position  under  the  body  by  means  of 
obvious  muscular  contraction;  and  from  this  it  might  be  inferred 
that  the  maintenance  of  the  position  of  the  pendent  limb  was 
also  the  result  of  a  feeble  contraction.  But  no  obvious  contrac- 
tions can  be  observed  in  the  latter  case,  as  in  the  former;  and 
when  in  the  former  the  limb  has  once  been  brought  into  the 
flexed  position,  that  position,  like  the  pendent  position,  is  main- 
tained without  obvious  contractions.  As  we  said  above  'tone' 
may  pass  into  something  which  appears  to  be  identical  with  a 
contraction,  but  where  no  obvious  contractions  are  observed  it 
seems  preferable  to  speak  of  the  state  of  the  muscle  as  one 
of  tone. 

In  the  dog,  after  division  of  the  cord  in  the  thoracic  region, 
the  hind  limbs  during  the  period  of  shock  are  limp  and  tone- 
less. In  the  warm-blooded  animal,  as  we  have  said,  the  effects 
of  shock  are  much  more  lasting  than  in  the  cold-blooded  ani- 
mal ;  and  in  the  dog  the  tone  of  the  skeletal  muscle  returns 
much  more  slowly  than  in  the  frog.  Indeed  when  the  division 
of  the  cord  has  taken  place  low  down  the  skeletal  tone  returns 
very  slowly,  and  may  be  manifested  very  feebly,  or  even  be 
absent  altogether.  But  under  favourable  circumstances,  when 
a  sufficient  length  of  cord  has  been  left,  a  fairly  normal  tone  is 
reestablished.  In  man,  in  accordance  Avith  the  facts  previously 
mentioned  (§  464)  skeletal  tone,  which  has  been  lost  through 
the  continuity  of  the  cord  being  broken  by  disease  or  accident, 
appears  rarely  if  ever  to  return  fully  in  the  regions  below  the 
lesion. 

We  may  therefore  on  the  whole  of  the  evidence  conclude 
that  the  maintenance  of  skeletal  tone  is  one  of  the  functions 
of  the  cord  ;  but  we  may  here  repeat  that  the  condition  of  the 
cord,  on  which  depends  the  issue  from  the  cord  along  efferent 
nerves  of  the  influences,  whatever  their  nature,  which  produce 
tone  in  the  muscle,  may  be,  and  indeed  is,  in  its  turn  dependent 
on  afferent  impulses.  In  the  case  of  the  frog  quoted  above 
the  tone  of  the  pendent  limbs  disappears  or  is  greatly  lessened 
when  the  posterior  roots  of  the  sciatic  nerves  are  divided, 
though  the  anterior  roots  be  left  intact.  In  the  absence  of 
the  usual  stream  of  afferent  impulses  passing  into  it,  the  cord 
ceases  to  send  forth  the  influences  which  maintain  the  tone. 
Hence  the  maintenance  of  tone  presents  many  analogies  with  a 
reflex  action  especially  when  we  remember  that,  as  stated  above, 
tone  passes  insensibly  into  contraction  ;  and  it  may  seem  a  mere 


Chap,  i.]  THE   SPIXAL   CORD.  717 

matter  of  words  whether  we  speak  of  the  maintenance  of  tone 
as  an  automatic  or  as  a  reflex  action  of  the  cord.  We  may, 
however,  distinguish  the  part  j)hiyed  by  the  afferent  impulses 
in  assisting  the  cord  to  a  condition  in  which  it  is  capable  of 
maintaining  tone  from  the  part  played  by  an  afferent  impulse 
in  causing  a  reflex  action  ;  in  the  former  the  action  of  the  affer- 
ent impulses  seems  analogous  to  that  of  a  supply  of  arterial 
blood  in  maintaining  an  adequate  irritability  of  the  nervous 
substance,  in  the  latter  the  afferent  impulses  lead  directly  to  a 
discharge  of  energy.  And  it  is  convenient  to  distinguish  the 
two  things  by  different  names. 

§  471.  The  close  connection  between  tone  and  reflex  action 
is  shewn  by  the  fact  that  some  observers  contend  that  the 
'knee-jerk '  and  similar 'tendon-phenomena'  are  not  instances 
of  reflex  action.  They  maintain  that  the  contraction  of  the 
muscle  is  an  example  of  the  direct  stimulation  of  the  muscle 
by  the  vibrations  set  up  in  the  tense  tendon  when  it  is  sharply 
struck  or  suddenly  pulled ;  and  they  explain  the  dependence 
of  the  act  on  the  spinal  cord  by  attributing  variations  in  the 
response  of  the  muscle  to  variations  in  the  tone  of  the  muscle, 
the  tone  being  dejDcndent  on  the  spinal  cord. 

§  472.  Disease  in  man  reveals  other  actions  of  the  spinal 
cord  which  bear  features  different  from  those  of  an  ordinary 
reflex  movement,  and  yet  have  been  described  as  reflex  in 
nature.  For  instance  certain  affections  of  the  cord  are  charac- 
terized by  the  legs  becoming  rigid  in  extreme  extension,  the 
rigidity  of  the  straightened  limbs  being  often  so  great,  that 
when  a  bystander  lifts  up  one  leg  from  the  bed,  the  other  leg 
is  raised  at  the  same  time.  The  rigidity  is  due  to  the  extensor 
muscles  being  thrown  into  a  state  of  contraction,  which  is  so 
uniform  and  long  continued  that  it  may  be  spoken  of  as  a 
"  tonic  "  contraction  ;  such  a  tonic  rigidity  ma};-  however  be 
replaced  by  a  series  of  rhythmic  "  clonic  "  contractions.  It  has 
sometimes  been  observed  that  the  limbs  when  flexed  are  supple 
and  free  from  rigidity,  but  that  rigidity  sets  in  so  soon  as  they 
are  brought  into  the  position  of  extension,  the  leg  becoming 
suddenly  fixed  and  straight  somewhat  in  the  way  that  a  clasp- 
knife  s})vings  back  when  opened.  It  seems  clear  that  the  pecu- 
liar contraction  is  carried  out  by  means  of  the  spinal  cord,  but 
the  whole  action,  though  it  is  often  spoken  of  as  a  '  muscle- 
reflex,'  is  very  unlike  an  ordinary  reflex  movement.  In  an 
ordinary  movement  an  extensor  is  brought  into  action  when  a 
limb  is  flexed,  not  when  it  is  already  extended  ;  and  if  in  a 
reflex  act  the  condition  of  the  muscle  about  to  be  thrown  into 
action  determines  in  any  way  the  discharge  of  impulses  from 
the  reflex  centre,  we  should  expect  that  the  stretcliiug  of  an 
extensor  muscle  by  flexion,  not  its  relaxation  by  extension, 
would  determine   the  discharge  of  extensor  impulses.     In  the 


718  SPASMODIC   EIGIDITY.  [Book  in. 

case  of  tlie  diseases  in  question  just  the  opposite  seems  to  take 
place  ;  the  position  which  appears  to  determine  the  development 
of  the  remarkable  contraction  is  precisely  that  in  which  the 
strain  upon  the  extensors  is  at  its  minimum.  It  may  be  doubted, 
therefore,  whether  the  word  reflex  should  be  used  to  denote 
such  phenomena ;  but  the  phenomena  themselves  deserve  atten- 
tion, especially  perhaps  as  shewing  how  in  the  disorders  of  the 
grey  matter  of  the  cord  due  to  disease  impulses  or  influences 
which  are  latent  only  in  health  become  actual  and  effective. 

It  remains  for  us  to  speak  of  the  part  played  by  the  spinal 
cord,  as  the  instrument  of  the  brain,  in  the  execution  of  volun- 
tary movements  and  in  the  development  of  conscious  sensations  ; 
but  it  will  be  best  to  consider  these  matters  in  connection  with 
the  brain  itself,  to  the  study  of  which  we  must  now  turn. 


CHAPTER  II. 
THE   BRAIN. 


SEC.  1.     ON   THE    PHENOME^TA  EXHIBITED  BY  A:N" 
ANIMAL  DEPEIVED  OF  ITS  CEEEBRAL  HEMISPHERES. 

§  473.  The  cerebral  hemispheres,  as  we  have  more  than 
once  insisted,  seem  to  stand  apart  from  the  rest  of  the  brain.  In 
the  case  of  some  animals  it  is  possible  to  remove  the  cerebral 
hemispheres  and  to  keep  the  animal  not  only  alive,  but  in  good 
health  for  a  long  time,  days,  weeks  or  even  months  after  the 
operation.  In  such  case  we  are  able  to  study  the  behaviour  of 
an  animal  possessing  no  cerebral  hemispheres  and  to  compare  it 
with  that  of  an  intact  animal.  Such  an  experiment  is  best  carried 
out  on  a  frog.  In  this  animal  it  is  comparatively  easy  to  re- 
move the  cerebral  hemispheres,  including  the  parts  correspond- 
ing to  the  corpora  striata,  leaving  behind  intact  and  uninjured 
the  optic  thalami  with  the  optic  nerves,  the  optic  lobes  (or  rep- 
resentatives of  the  corpora  quadrigemina),  the  small  cerebellum 
and  the  bulb.  If  the  animal  be  carefully  fed  and  attended  to, 
it  may  be  kept  alive  for  a  very  long  time,  for  more  than  a  year 
for  instance. 

The  salient  fact  about  a  frog  lacking  the  cerebral  hemis- 
pheres, is  that,  as  in  the  case  of  a  frog  deprived  of  its  whole 
brain,  the  signs  of  the  working  of  an  intelligent  volition  are 
eitlier  wholly  absent  or  extremely  rare.  The  presence  of  the 
bulb  and  the  middle  parts  of  the  brain  (for  so  we  may  con- 
veniently call  the  cerebral  structures  lying  between  the  cerebral 
hemispheres  and  the  bulb)  ensures  the  healthy  action  of  the 
vascular,  respiratory  and  other  nutritive  systems  ;  food  placed 
in  the  mouth  is  readily  and  easily  swallowed ;  the  animal  when 
stimulated  executes  various  movements;  but  if  it  be  left  entirely 
to  itself,  and  care  be  taken  to  sliield  it  from  adventitious  stimuli, 
either  it  remains  perfectl}^  and  permanently  quiescent,  or  the 
apparentlj'^  spontaneous  movements  which  it  carries  out  are  so 

719 


720  WITHOUT    CEREBEAL   HEMISPHERES.     [Book  iii. 

few  and  so  limited  as  to  raise  the  question  whether  they  can 
fairly  be  called  volitional.  Such  a  frog,  for  instance,  after  being 
kept  alive  for  some  time  and  made  to  exhibit  the  phenomena  of 
which  we  are  about  to  speak,  has  been  placed  on  a  table  with  a 
line  drawn  in  chalk  around  the  area  covered  by  its  body,  and 
left  to  itself  has  subsequently  been  found  dead  without  having 
stirred  outside  the  chalked  circle. 

We  must  here  however  repeat  the  caution  laid  down  in 
§  457,  as  to  the  ultimate  effects  of  an  operation  on  the  central 
nervous  system.  The  longer  the  frog  is  kept  alive  and  in  good 
health  after  the  removal  of  the  cerebral  hemispheres,  the  greater 
is  the  tendency  for  apparently  spontaneous  movements  to  shew 
themselves.  For  days  or  even  weeks  after  the  operation  there 
may  be  no  signs  whatever  of  the  working  of  any  volition ;  but 
after  the  lapse  of  months,  movements,  previously  absent,  of  such 
a  character  as  to  suggest  that  they  ought  to  be  called  voluntary, 
may  make  their  appearance.  Still  even  in  their  most  complete 
development  such  movements  do  not  absolutely  negative  the  view 
that  the  frog  in  the  absence  of  the  cerebral  hemispheres  is  want- 
ing in  what  we  ordinarily  call  a  '  will.' 

§  474.  We  have  seen  that  a  frog  from  which  the  whole 
brain  has  been  removed  and  the  spinal  cord  alone  left  appears 
similarly  devoid  of  a  '  will ; '  but  the  phenomena  presented  by 
a  frog  possessing  the  middle  portions  of  the  brain  differ  widely 
from  those  presented  by  a  frog  possessing  a  spinal  cord  only. 
We  may  perhaps  broadly  describe  the  behaviour  of  a  frog  from 
which  the  cerebral  hemispheres  only  have  been  removed,  by 
saying  that  such  an  animal,  though  exhibiting  no  spontaneous 
movements,  can  by  the  application  of  appropriate  stimuli  be 
induced  to  perform  all  or  nearly  all  the  movements  which  an 
entire  frog  is  capable  of  executing.  It  can  be  made  to  swim,  to 
leap,  and  to  crawl.  Left  to  itself  it  assumes  what  may  be  called 
the  natural  posture  of  a  frog,  with  the  fore  limbs  erect,  and  the 
hind  limbs  flexed,  so  that  the  line  of  the  body  makes  an  angle 
with  the  surface  on  which  it  is  resting.  When  placed  on  its 
back,  it  immediately  regains  this  natural  posture.  When  placed 
on  a  board,  it  does  not  fall  from  the  board  when  the  latter  is 
tilted  up  so  as  to  displace  the  animal's  centre  of  gravity :  it 
crawls  up  the  board  until  it  gains  a  new  position  in  which  its 
centre  of  gravity  is  restored  to  its  proper  place.  Its  movements 
are  exactly  those  of  an  entire  frog  except  that  they  need  an 
external  stimulus  to  call  them  forth.  They  differ  moreover  fun- 
damentally from  those  of  an  entire  frog  in  the  following  import- 
ant feature  ;  they  inevitably  follow  when  the  stimulus  is  applied ; 
they  come  to  an  end  when  the  stimulus  ceases  to  act.  By  con- 
tinually varying  the  inclination  of  a  board  on  which  it  is  placed, 
the  frog  may  be  made  to  continue  crawling  almost  indefinitely ; 
but  directly  the  board  is  made  to  assume  such  a  position  that 


Chap,  ii.]  THE   BRAIK  721 

the  body  of  the  frog  is  in  equilibrium,  the  crawling  ceases  ;  and 
if  the  position  be  not  disturbed  the  animal  will  remain  impassive 
and  quiet  for  an  almost  indefinite  time.  When  thrown  into 
water,  the  creature  begins  at  once  to  swim  about  in  the  most 
regular  manner,  and  will  continue  to  swim  until  it  is  exhausted, 
if  there  be  nothing  present  on  which  it  can  come  to  rest.  If  a 
small  piece  of  wood  be  placed  on  the  water  the  frog  will,  when 
it  comes  in  contact  with  the  wood,  crawl  upon  it,  and  so  come  to 
rest.  If  disturbed  from  its  natural  posture,  as  by  being  placed 
on  its  back,  it  immediately  struggles  to  regain  that  posture ; 
only  by  the  application  of  continued  force  can  it  be  kept  lying 
on  its  back.  Such  a  frog,  if  its  flanks  be  gently  stroked,  will 
croak  ;  and  the  croaks  follow  so  regularly  and  surely  upon  the 
strokes  that  the  animal  may  almost  be  played  upon  like  a  musi- 
cal, or  at  least  an  acoustic  instrument.  Moreover,  provided 
that  the  optic  nerves  and  their  arrangements  have  not  been 
injured  by  the  operation,  the  movements  of  the  animal  appear 
to  be  inlluenced  by  light ;  if  it  be  urged  to  move  in  an}^  particu- 
lar direction,  it  seems  in  its  progress  to  avoid  obstacles,  at  least 
such  as  cast  a  strong  shadow ;  it  turns  its  course  to  the  right  or 
left  or  sometimes  leaps  over  the  obstacle.  In  fact,  even  to  a 
careful  observer  the  differences  between  such  a  frog  and  an 
entire  frog  which  was  simply  very  stupid  or  very  inert,  would 
appear  slight  and  unimportant  except  in  this,  that  the  animal 
without  its  cerebral  hemispheres  is  obedient  to  every  stimulus, 
and  that  each  stimulus  evokes  an  appropriate  movement,  whereas 
with  the  entire  animal  it  is  impossible  to  predict  whether  any 
result  at  all,  and  if  so  what  result,  will  follow  the  application  of 
this  or  that  stimulus.  Both  may  be  regarded  as  machines  ;  but 
the  one  is  a  machine  and  nothing  more,  the  other  is  a  machine 
governed  and  checked  by  a  dominant  volition. 

Now  such  movements  as  crawling,  leaping,  swimming,  and 
indeed,  as  we  have  already  urged,  to  a  greater  or  less  extent, 
all  bodily  movements,  are  carried  out  by  means  of  coordinate 
nervous  motor  impulses,  influenced,  arranged,  and  governed  by 
coincident  sensory  or  afferent  impulses.  Muscular  movements 
are  determined  by  afferent  influences  proceeding  from  the  mus- 
cles and  constituting  the  foundation  of  the  muscular  sense  ;  they 
are  also  directed  by  means  of  afferent  impulses  passing  cen- 
tripetally  along  the  sensory  nerves  of  the  skin,  the  eye,  the 
ear,  and  other  organs.  Independently  of  the  particular  afferent 
impulses,  which  acting  as  a  stimulus  call  forth  the  movement, 
very  many  other  afferent  impulses  are  concerned  in  the  genera- 
tion and  coordination  of  tlic  resultant  motor  impulses.  Every 
bodily  movement  such  as  those  of  which  we  are  speaking  is  the 
work  of  a  more  or  less  complicated  nervous  mechanism,  in  which 
there  are  not  only  central  and  efferent,  but  also  afferent  factors. 
And,  putting  aside  the  question  of  consciousness,  with  which 

46 


722  WITHOUT   CEEEBEAL   HEMISPHERES.     [Book  hi. 

we  have  here  no  occasion  to  deal,  it  is  evident  that  in  the  frog 
deprived  of  its  cerebral  hemispheres  all  these  factors  are  pres- 
ent, the  afferent  no  less  than  the  central  and  the  efferent.  The 
machinery  for  all  the  necessary  and  usnal  bodily  movements  is 
present  in  all  its  completeness.  We  may  regard  the  share  there- 
fore which  the  cerebral  hemispheres  take  in  executing  the  move- 
ments of  which  the  entire  animal  is  capable,  as  that  of  putting 
this  machinery  into  action  or  of  limiting  its  previous  activity. 
The  relation  which  the  higher  nervous  changes  concerned  in 
volition  bear  to  this  machinery  may  be  compared  to  that  of  a 
stimulus,  always  bearing  in  mind  that  the  effect  of  a  stimulus 
on  a  nervous  centre  may  be  either  to  start  activity,  or  to  in- 
crease, or  to  curb,  or  to  stop  activity  already  present.  We  might 
almost  speak  of  the  will  as  an  intrinsic  stimulus.  Its  opera- 
tions are  limited  by  the  machinery  at  its  command.  We 
may  infer  that  in  the  frog,  the  action  of  the  cerebral  hemispheres 
in  giving  shape  to  a  bodily  movement  is  that  of  throwing  into 
activity  particular  parts  of  the  nervous  machinery  situated  in 
the  lower  parts  of  the  brain  and  in  the  spinal  cord ;  precisely 
the  same  movement  may  be  initiated  in  the  absence  of  the  cere- 
bral hemispheres  by  applying  such  stimuli  as  shall  throw  pre- 
cisely the  same  parts  of  that  machinery  into  the  same  activity. 
Very  markecl  is  the  contrast  between  the  behaviour  of  such 
a  frog  which,  though  deprived  of  its  cerebral  hemispheres,  still 
retains  the  other  parts  of  the  brain,  and  that  of  a  frog  which 
possesses  a  spinal  cord  only.  The  latter  when  placed  on  its 
back  makes  no  attempt  to  regain  its  normal  posture ;  in  fact, 
it  may  be  said  to  have  completely  lost  its  normal  posture,  for 
even  when  placed  on  its  belly  it  does  not  stand  with  its  fore 
feet  erect,  as  does  the  other  animal,  but  lies  flat  on  the  ground. 
When  thrown  into  water,  instead  of  swimming,  it  sinks  like  a 
lump  of  lead.  When  pinched,  or  otherwise  stimulated,  it  does 
not  crawl  or  leap  forwards ;  it  simply  throws  out  its  limbs  in 
various  ways.  When  its  flanks  are  stroked  it  does  not  croak ; 
and  when  a  board  on  which  it  is  placed  is  inclined  sufficiently 
to  displace  its  centre  of  gravity  it  makes  no  effort  to  regain  its 
balance,  but  falls  off  the  board  like  a  lifeless  mass.  Though, 
as  we  have  seen,  the  various  parts  of  the  spinal  cord  of  the  frog 
contain  a  large  amount  of  coordinating  machinery,  so  that  the 
brainless  frog  may,  by  appropriate  stimuli,  be  made  to  execute 
various  purposeful  coordinate  movements,  yet  these  are  very 
limited  compared  with  those  which  can  be  similarly  carried  out 
by  a  frog  possessing  the  middle  and  lower  parts  of  the  brain  in 
addition  to  the  spinal  cord.  It  is  evident  that  a  great  deal  of 
the  more  complex  machinery  of  this  kind,  especially  all  that 
which  has  to  deal  with  the  body  as  a  whole,  and  all  that  which 
is  concerned  with  equilibrium  and  is  specially  governed  by  the 
higher  senses,  is  seated  not  in  the  spinal  cord  but  in  the  brain. 


Chap,  ii.]  THE  BRAIK  723 

We  do  not  wish  now  to  discuss  the  details  of  this  machinery ; 
all  we  desire  to  insist  upon  at  present  is  that,  in  the  frog  the 
nervous  machinery  required  for  the  execution,  as  distinguished 
from  the  origination,  of  bodily  movements  even  of  the  most 
complicated  kind,  is  present  after  complete  removal  of  the  cere- 
bral hemispheres,  though  these  movements  are  such  as  to  require 
the  cooperation  of  highly  differentiated  afferent  im^iulses. 

§  475.  In  warm-blooded  animals  the  removal  of  the  cere- 
bral hemispheres  is  attended  with  much  greater  difficulties  than 
in  the  case  of  the  frog.  Nevertheless,  in  the  bird  the  operation 
may  be  carried  out  with  approximate  success.  Pigeons  for 
instance  have  been  kept  alive  for  five  or  six  weeks  after  com- 
plete removal  of  the  cerebral  hemisiDheres,  with  the  exception 
of  portions  of  the  crura  and  corpora  striata  immediately  sur- 
rounding the  optic  thalami;  these  parts  were  left  in  order  to 
ensure  the  intact  condition  of  the  latter  bodies. 

When  the  immediate  effects  of  the  operation  have  passed 
off,  and  for  some  time  afterwards,  the  appearance  and  behaviour 
of  the  bird  are  strikingly  similar  to  those  of  a  bird  exceedingly 
sleepy  and  stupid.  It  is  able  to  maintain  what  appears  to  be 
a  completely  normal  posture,  and  can  balance  itself  on  one  leg, 
after  the  fashion  of  a  bird  which  has  in  a  natural  way  gone  to 
sleep.  Left  alone  in  perfect  quiet,  it  will  remain  impassive 
and  motionless  for  a  long  time.  When  stirred  it  moves,  shifts 
its  position;  and  then,  on  being  left  alone,  returns  to  a  natural, 
easy  posture.  Placed  on  its  side  or  its  back  it  will  regain  its 
feet;  thrown  into  the  air,  it  flies  with  considerable  precision 
for  some  distance  before  it  returns  to  rest.  It  frequently  tucks 
its  head  under  its  wings,  and  at  times  may  be  seen  to  clean  its 
feathers;  when  its  beak  is  plunged  into  corn,  it  eats.  It  may 
be  induced  to  move  not  only  by  ordinary  stimuli  applied  to  the 
skin,  but  also  l)y  sudden  loud  sounds,  or  b}'  flashes  of  light;  in 
its  flight  it  will,  though  imperfectly,  avoid  obstacles,  and  its 
various  movements  appear  to  be  to  a  certain  extent  guided  not 
only  by  touch  but  also  by  visual  impressions. 

In  a  certain  number  of  cases  this  sleepy,  drowsy  condition 
passes  off  and  is  succeeded  by  a  phase  in  which  the  bird,  appar- 
ently spontaneously,  without  the  intervention  of  any  obvious 
stimulus,  moves  rapidly  about.  It  does  not  fly,  that  is  to  say, 
it  does  not  raise  itself  from  the  ground  in  flight,  but  walks 
about  incessantly  for  a  long  while  at  a  time,  periods  of  activity 
alternating  with  periods  of  repose.  It  seems,  from  time  to 
time,  to  wake  up  and  move  about,  and  then  to  go  to  sleep 
again;  and  it  has  been  observed  that  during  the  night  it 
appears  to  be  always  asleep.  It  is  obvious,  therefore,  that  tlie 
slee})y,  quiescent  condition  observed  at  flrst  is  not  due  simply 
to  the  absence  of  the  cerebral  hemispheres,  but  is  a  temporary 
effect  of  the  operation,  and  that  spontaneous  movements,  that 


724  WITHOUT   CEEEBRAL   HEMISPHEEES.     [Book  hi. 

is  to  say,  movements  not  started  by  any  obvious  stimulus,  may 
occur  after  removal  of  tlie  cerebral  hemispheres.  But  the 
movements  so  witnessed  differ  from  those  of  an  intact  bird. 
They  are,  it  is  true,  varied;  and  the  variations  are  in  part 
dependent  on  external  circumstances,  the  bird  being  guided  by 
tactile,  and,  as  we  have  said,  visual  sensations,  or,  to  be  more 
exact,  by  impressions  made  upon  the  sensory  nerves  of  the 
skin  and  on  the  retina;  but  they  do  not  shew  the  wide  varia- 
tions of  voluntary  movements.  The  bird  for  instance  never 
flies  up  from  the  ground,  never  spontaneously  picks  up  corn, 
and  its  aimless,  monotonous,  restless  walks,  resembling  the  con- 
tinued swimming  of  the  frog  thrown  into  the  water  after  being 
deprived  of  its  cerebral  hemispheres,  forcibly  suggest  that  the 
activity  is  the  outcome  of  some  intrinsic  impulse  generated  in 
the  nervous  machinery  in  some  way  or  other,  but  not  by  the 
working  of  a  conscious  intelligence  as  in  the  impulse  which  we 
call  the  will. 

Still  we  must  not  shut  our  eyes  to  the  fact  that  spontaneous 
movements,  whatever  their  exact  nature,  are  manifested  by  a 
bird  in  the  absence  of  the  cerebral  hemispheres,  and  become 
the  more  striking  the  more  complete  the  recovery  from  the 
passing  effects  of  the  mere  operation.  Could  such  birds  be 
kept  alive  for  any  considerable  time,  possibly  further  develop- 
ments might  be  witnessed,  and  indeed  cases  are  on  record  where 
birds  have  been  kept  alive  for  months  after  the  operation,  and 
have  shewn  s]3ontaneous  movements  of  a  still  more  varied  char- 
acter than  those  just  described ;  but  in  such  cases  the  removal 
of  the  hemispheres  has  not  been  complete,  portions  of  the  ven- 
tral regions  being  left  behind ;  and,  though  a  mere  remnant 
left  around  the  optic  thalami  can  hardly  be  regarded  as  a  suffi- 
cient cause  for  the  spontaneity  of  which  we  are  speaking,  a 
larger  mass,  still  more  or  less  retaining  its  normal  structure, 
might  have  a  marked  effect.  And  we  may  here  perhaps  remark 
that  all  these  facts  seem  to  point  to  the  conclusion  that  what 
may  be  called  mechanical  spontaneity,  sometimes  spoken  of  as 
'  automatism,'  differs  from  the  spontaneity  of  the  '  will '  in 
degree  rather  than  in  kind.  Looking  at  the  matter  from  a 
purely  physiological  point  of  view  (the  only  one  which  has  a 
right  to  be  employed  in  these  pages),  the  real  difference  between 
an  automatic  act  and  a  voluntary  act  is  that  the  chain  of  phys- 
iological events  between  the  act  and  its  physiological  cause 
is  in  the  one  case  short  and  simple,  in  the  other  long  and  com- 
plex. We  have  seen  that  a  frog  lacking  its  cerebral  hemi- 
spheres, viewed  from  one  standpoint,  appears  in  the  light  of 
a  mechanical  apparatus,  on  which  each  change  of  circumstances 
produces  a  direct,  unvarying,  inevitable  effect.  And  yet  it  is 
on  record  that  such  a  frog,  if  kept  alive  long  enough  for  the 
most  complete  disappearance  of  the  direct  effects  of  the  opera- 


Chap,  ii.]  THE   BEAIK  725 

tion,  will  bury  itself  in  the  eartli  at  the  approach  of  winter, 
and  is  able  to  catch  and  swallow  flies  and  other  food  coming 
in  its  neighbourhood,  although  in  other  respects  it  shews  no 
signs  of  an  intelligent  volition,  and  answers  with  unerring 
mechanical  certainty  to  the  play  of  stimuli.  We  may  add  that 
in  some  fishes  the  removal  of  their  cerebral  hemispheres,  Avhich 
in  these  animals  form  a  relatively  small  part  of  the  wh(jle  brain, 
produces  exceedingly  little  change  in  their  general  behaviour. 

These  however  are  not  the  considerations  on  which  we  wish 
here  to  dwell ;  we  have  quoted  the  behaviour  of  the  bird 
deprived  of  its  cerebral  hemisphere  mainly  to  shew  that  in  this 
warm-blooded  animal,  as  in  the  more  lowly  cold-blooded  frog, 
the  parts  of  the  brain  below  or  behind  the  cerebral  hemispheres 
constitute  a  nervous  machinery  by  which  all  the  ordinary  bodily 
movements  may  be  carried  out.  The  bird,  like  the  frog,  suf- 
fers no  paralysis  when  the  cerebral  hemispheres  are  removed ; 
on  the  contrary,  though  its  movements  have  not  been  studied 
so  closely  as  those  of  the  frog,  the  bird  without  its  cerebral 
hemispheres  seems  capable  of  executing  at  all  events  all  the 
ordinary  bodily  movements  of  a  bird.  And  in  the  bird  as  in 
the  frog,  the  afferent  impulses  passing  into  the  central  nervous 
system,  whether  they  give  rise  to  consciousness  or  no,  play  an 
important  part  not  only  in  originating  but  in  guiding  and 
coordinating  the  efferent  impulses  which  stir  the  muscles  to 
contract,  the  coordination  being  effected  partly  in  the  spinal 
cord,  but  largely  and  indeed  chiefly  in  the  parts  of  the  brain 
lying  behind  the  cerebral  hemispheres.  It  is  further  worthy 
of  notice  that  spontaneity  of  movement  of  the  kind  which  we 
have  described,  is  much  more  prominent  in  the  more  highly 
developed  bird,  than  in  the  more  lowly  frog.  The  cerebral 
hemispheres  are  not  the  only  part  of  the  central  nervous  system 
which  has  undergone  a  greater  development  in  the  bird ;  the 
other  parts  of  the  brain  have  also  acquired  a  far  greater  com- 
plexity than  in  the  frog. 

§  476.  In  the  mammal  the  removal  of  the  cerebral  hemi- 
speres  is  still  more  difficult  than  in  the  bird  ;  the  animal  cannot 
be  kept  alive  for  more  than  a  few  hours;  but  in  some  mammals 
it  is  possible  to  observe  during  those  few  hours  phenomena 
kindred  to  those  witnessed  in  the  bird  and  in  the  frog.  The 
rabbit  or  rat,  from  which  the  whole  of  both  liemis})heres  has 
been  removed  with  the  exception  of  the  parts  immediately  sur- 
rounding the  optic  thalami,  can  stand,  run  and  leap.  Placed 
on  its  side  or  back  it  at  once  regains  its  feet.  Left  alone  it 
generally  remains  as  motionless  and  impassive  as  a  statue,  save 
now  and  then  when  a  passing  impulse  seems  to  stir  it  to  a 
sudden  but  brief  movement;  but  sometimes  it  seems  subject  to 
a  more  continued  impulse  to  move,  in  which  case  death  usually 
follows  very  speedily.     Such  a  rabbit  will  remain  for  minutes 


726  WITHOUT   CEREBRAL   HEMISPHEEES.     [Book  hi. 

together  utterly  heedless  of  a  carrot  or  cahbage-leaf  placed  just 
before  its  nose,  though  if  a  morsel  be  placed  within  its  mouth 
it  at  once  begins  to  eat.  When  stirred  it  will  with  ease  and 
steadiness  run  or  leap  forward;  and  obstacles  in  its  course  are 
very  frequently,  with  more  or  less  success,  avoided.  In  some 
cases  the  animal  (rat)  has  been  described  as  following  by  move- 
ments of  the  head  a  bright  light  held  in  front  of  it  (provided 
that  the  optic  nerves  and  tracts  have  not  been  injured  during 
the  operation),  as  starting  when  a  shrill  and  loud  noise  is  made 
near  it,  and  as  crying  when  pinched,  often  with  a  long  and 
seemingly  plaintive  scream.  So  plaintive  is  the  cry  which  it 
thus  gives  forth  as  to  suggest  to  the  observer  the  existence  of 
passion  ;  this,  however,  is  probably  a  wrong  interpretation  of  a 
vocal  action;  the  cry  appears  plaintive  simply  because,  in  con- 
sequence of  the  completeness  of  the  reflex  nervous  machinery 
and  the  absence  of  the  usual  restraints,  it  is  prolonged. 

Without  insisting  too  much  on  such  results  as  these,  and 
allowing  full  weight  to  the  objection  which  may  be  urged,  that 
in  some  of  these  cases  parts  of  the  cerebral  hemispheres  sur- 
rounding the  optic  thalami  were  left,  there  still  remains  ade- 
quate evidence  to  shew  that  a  mammal  such  as  a  rabbit,  in  the 
same  way  as  a  frog  and  a  bird,  may  in  the  complete  or  all  but 
complete  absence  of  the  cerebral  hemispheres  maintain  a  natural 
posture,  free  from  all  signs  of  disturbance  of  equilibrium,  and 
is  able  to  carry  out  with  success,  at  all  events  all  the  usual  and 
common  bodily  movements.  And  as  in  the  bird  and  frog,  the 
evidence  also  shews  that  these  movements  not  only  may  be 
started  by,  but  in  their  carrying  out  are  guided  by  and  coordi- 
nated by  afferent  impulses  along  afferent  nerves,  including 
those  of  the  special  senses.  But  in  the  case  of  the  rabbit  it  is 
even  still  clearer  than  in  the  case  of  the  bird  that  the  effects  of 
these  afferent  impulses  are  different  from  those  which  result 
when  the  impulses  gain  access  to  an  intact  brain.  The  move- 
ments of  the  animal  seem  guided  by  impressions  made  on  its 
retina,  as  well  as  on  other  sensory  nerves;  we  may  perhaps 
speak  of  the  animal  as  the  subject  of  sensations;  but  there  is 
no  satisfactory  evidence  that  it  possesses  either  visual  or  other 
perceptions,  or  that  the  sensations  which  it  experiences  give 
rise  to  ideas.  Its  avoidance  of  objects  depends  not  so  much  on 
the  form  of  these  as  on  their  interference  with  light.  No  image, 
whether  pleasant  or  terrible,  whether  of  food  or  of  an  enemy, 
produces  an  effect  on  it,  other  than  that  of  an  object  reflecting 
more  or  less  light.  And  we  may  infer  that  it  lacks  the  posses- 
sion of  an  intelligent  will.  But  it  must  always  be  remembered 
that  some  of  the  phenomena  are  due  to  the  operation  producing 
other  results  than  the  mere  absence  of  the  part  removed.  We 
must  bear  in  mind  that  in  all  the  above  experiments  while  the 
positive  phenomena,  the  things  which  the  animal  continues  able 


Chap,  ii.]  THE  BRAIN.  727 

to  do,  are  of  great  value,  the  negative  phenomena,  the  things 
which  the  animal  can  no  longer  do,  are  of  much  less,  indeed  of 
doubtful  value.  The  more  carefully  and  successfully  the  experi- 
ments are  carried  out,  the  narrower  become  what  we  may  call 
the  '  deliciency  phenomena,'  the  phenomena  which  are  alone 
and  directly  due  to  something  having  been  taken  away.  Were 
it  possible  to  keep  the  rabbit  alive  long  enough  for  the  mere 
effects  of  the  operation  to  pass  completely  away,  we  should  not 
only  probably  witness,  as  in  the  case  of  the  bird,  a  greater  scope 
of  movement  and  more  frequent  spontaneity,  but  possibly  find 
a  difficulty  in  describing  the  exact  condition  of  the  animal. 

§  477.  Hitherto  most  attempts  to  witness  similar  phenom- 
ena in  more  highly  organized  mammals  such  as  the  dog  have 
failed;  these  animals  do  not  recover  from  the  operation  of 
removing  the  whole  of  both  their  hemispheres  sufficiently  to 
enable  us  to  judge  whether  they,  like  the  frog,  the  bird  and 
the  rabbit,  can  carry  out  coordinate  bodily  movements  in  the 
absence  of  the  hemispheres,  or  whether  in  them  this  part  of  the 
brain,  so  largely  developed,  has  usurped  functions  which  in 
the  lower  animals  belong  to  other  parts.  When  however  in  a 
dog  the  cerebral  hemispheres  are  removed  not  all  at  once  but 
piecemeal  at  several  operations,  the  animal  may  be  kept  alive 
and  in  good  health  for  a  long  time,  many  months  at  least,  even 
after  the  hemispheres  have  been  reduced  to  a  mere  fragment; 
and  it  is  on  record  that  under  these  circumstances,  the  animal 
is  not  only  able  to  carry  out  with  some  limitations  his  ordinary 
bodily  movements,  but  also  exhibits  a  spontaneit}^  of  movement 
and  a  varied  responsiveness  to  stimuli  suggestive,  at  least,  of 
the  possession  of  a  conscious  volition.  If  we  can  thus  say  little 
about  the  condition  of  a  dog  without  the  cerebral  hemispheres 
we  can  say  still  less  about  the  monkey,  which  in  all  matters 
touching  the  cerebral  nervous  system  serves  as  our  best,  indeed 
our  only  guide  for  drawing  inferences  concerning  man;  but  in 
all  probability  the  monkey  in  this  respect  bears  somewhat  the 
same  relation  to  the  dog  that  the  dog  bears  to  the  bird.  In 
short,  the  more  we  study  the  phenomena  exhibited  by  animals 
possessing  a  part  only  of  their  brain,  the  closer  we  are  pushed 
to  the  conclusion  that  no  sliarp  line  can  be  drawn  between  voli- 
tion and  the  lack  of  volition,  or  between  the  possession  and 
absence  of  intelligence.  Between  the  muscle-nerve  preparation 
at  the  one  limit,  and  our  conscious  willing  selves  at  the  other, 
there  is  a  continuous  gradation  without  a  break;  we  cannot  fix 
on  any  linear  barrier  in  the  brain  or  in  the  general  nervous 
system,  and  say  '  beyond  this  there  is  volition  and  intelligence 
but  up  to  this  there  is  none.' 

This  however  is  not  the  question  with  which  we  are  now 
dealing.  What  we  want  to  point  out  is  that  in  the  higher 
animals,  including  at  least  some  mammals,  as  in  the  frog,  after 


728  WITHOUT   CEREBRAL   HEMISPHERES.     [Book  in. 

the  removal  of  the  cerebral  hemispheres,  even  though  conscious 
volition  and  intelligence  appear  to  be  largely,  if  not  entirely, 
lost,  the  body  is  still  capable  of  executing  all  the  ordinary 
movements  which  the  animal  in  its  natural  life  is  wont  to  per- 
form, in  spite  of  these  movements  necessitating  the  cooperation 
of  various  afferent  impulses;  and  that  therefore  the  nervous 
machinery  for  the  execution  of  these  movements  lies  in  some 
part  of  the  brain  other  than  the  cerebral  hemispheres.  We 
have  reasons  for  thinking  that  it  is  situated  in  the  structures 
forming  the  middle  and  hind  brain;  as  we  shall  see,  interfer- 
ence with  these  parts  produces  at  once  remarkable  disorders  of 
movement. 


SEC.   2.     THE  MACHINERY   OF  COORDINATED 
MOVEMENTS. 

§  478.  We  may  now  direct  our  attention  for  a  while  to  some 
considerations  concerning  the  nature  of  this  complex  nervous 
machinery  for  the  coordination  of  bodily  movements,  and  espe- 
cially concerning  the  part  played  by  afferent  impulses.  Most 
of  our  knowledge  on  this  point  has  been  gained  by  a  study  of 
animals  not  deprived  of,  but  still  possessing  their  cerebral  hemi- 
spheres, or  by  deductions  from  the  data  of  our  own  experience ; 
but  it  is  possible  in  most  cases  to  eliminate  from  the  total  results 
the  phenomena  which  are  due  to  the  working  of  a  conscious 
intelligence. 

Let  us  first  of  all  turn  aside  to  ourselves  and  examine  the 
coordination  of  the  movements  of  our  own  bodies.  When  we 
appeal  to  our  own  consciousness  we  find  that  our  movements 
are  governed  and  guided  by  what  we  may  call  a  sense  of  equi- 
librium, by  an  appreciation  of  the  position  of  our  body  and  its 
relations  to  space.  When  this  sense  of  equilibrium  is  disturbed 
we  say  we  are  dizzy,  and  we  then  stagger  and  reel,  being  no 
longer  able  to  coordinate  the  movements  of  our  bodies  or  to 
adapt  them  to  the  position  of  things  around  us.  What  is  the 
origin  of  this  sense  of  equilibrium?  B}^  what  means  are  we 
able  to  appreciate  the  position  of  our  body?  There  can  be  no 
doubt  that  this  appreciation  is  in  large  measure  the  product  of 
visual  and  tactile  sensations ;  we  recognize  the  relations  of  our 
body  to  the  things  around  us  in  great  measure  by  sight  and 
touch ;  we  also  learn  much  Iw  our  muscular  sense.  But  tlieie 
is  something  besides  these.  Neither  sight  nor  touch  nor  mus- 
cular sense  can  help  us  Avhen,  placed  perfect!}'  flat  and  at  rest 
on  a  liorizontal  rotating  table,  with  the  eyes  sliut  and  not  a 
muscle  stirring,  we  attempt  to  determine  wliether  or  no  the 
table  and  we  with  it  are  being  moved,  or  to  ascertain  how  much 
it  and  we  are  turned  to  the  right  or  to  the  left.  Yet  under 
such  circumstances  we  are  conscious  of  a  change  in  our  posi- 
tion, and  some  observers  have  been  even  able  to  pass  a  tolerably 
successful  judgment  as  to  the  angle  througli  which  they  have 

729 


730  SEMICIECULAR   CANALS.  [Book  hi. 

been  moved.  There  can  be  no  doubt  that  such  a  judgment  is 
based  upon  the  interpretation  by  consciousness  of  afferent  im- 
pulses which  are  dependent  on  the  position  of  the  body,  but 
which  are  not  afferent  impulses  belonging  to  sensations  of  touch 
or  sight,  or  taking  part  in  the  muscular  sense.  It  is  by  help  of 
these  special  afferent  impulses  that  we  are  aware  on  the  one 
hand  of  the  position,  of  the  relation  to  space,  in  which  our  body 
may  at  one  time  happen  to  be,  standing  upright,  lying  down, 
and  the  like,  and  on  the  other  hand,  of  the  nature  and  extent 
of  any  change  of  position  which  our  movements  may  bring 
about.  It  is  by  help  of  these  afferent  impulses  that  we  are  able 
to  coordinate  our  movements  so  as  to  bring  our  body  into  the 
position  we  desire  ;  and  hence  when  these  aft'erent  impulses  are 
disordered  and  abnormal,  the  coordination  of  our  movements, 
the  maintenance  of  equilibrium,  is  imperfect.  Can  we  say  any- 
thing as  to  the  exact  nature  and  origin  of  these  special  afferent 
impulses  ? 

We  learn  much  in  this  respect  by  studying  the  effects  of 
operative  interference  with  certain  parts  of  the  internal  ear. 
When  in  a  pigeon  the  horizontal  membranous  semicircular  canal 
is  cut  through,  the  bird  is  observed  to  be  continually  moving 
its  head  from  side  to  side.  Injury  to  the  bony  canal  alone  is 
insufficient  to  produce  the  symptoms ;  the  membranous  canal 
itself  must  be  divided  or  injured.  If  one  of  the  vertical 
canals  be  cut  through,  the  movements  are  up  and  down. 
The  peculiar  movements  may  not  be  witnessed  when  the 
bird  is  perfectly  quiet,  but  they  make  their  appearance  when- 
ever it  is  disturbed,  or  attempts  in  any  way  to  stir.  When 
the  injury  is  confined  to  one  canal  only  or  even  to  the 
canals  of  one  side  of  the  head  only,  the  condition  after  a 
while  passes  away ;  when  the  canals  of  both  sides  have  been 
divided,  it  becomes  much  exaggerated,  lasts  much  longer,  and  in 
some  cases  is  said  to  remain  permanently.  After  such  injuries 
it  is  found  that  these  peculiar  movements  of  the  head  are  asso- 
ciated with  what  appears  to  be  a  great  want  of  coordination  of 
bodily  movements.  If  the  bird  be  thrown  into  the  air,  it  flutters 
and  falls  down  in  a  helpless  and  confused  manner ;  it  appears 
to  have  lost  the  power  of  orderly  flight.  If  placed  in  a  balanced 
position,  it  may  remain  for  some  time  quiet,  generally  with  its 
head  in  a  peculiar  posture ;  but  directly  it  is  disturbed,  the 
movements  which  it  attempts  to  execute  are  irregular  and  fall 
short  of  their  purpose.  It  has  great  difficulty  in  picking  up 
food  and  in  drinking ;  and  in  general  its  behaviour  very  much 
resembles  that  of  a  person  who  is  exceedingly  dizzy. 

It  can  hear  perfectly  well,  and  therefore  the  symptoms  cannot 
be  regarded  as  the  result  of  any  abnormal  auditory  sensations, 
such  as  '  a  roaring '  in  the  ears.  Besides,  any  such  stimulation 
of  the  auditory  nerve  as  the  result  of  the  section  would  speedily 


Chap,  ii.]  THE   BRAIN.  731 

die  away,  whereas  these  phenomena  may  last  for  at  least  a  very 
considerable  time.  The  movements  are  not  occasioned  by  any 
partial  paralysis,  by  any  want  of  power  in  particular  muscles  or 
group  of  muscles ;  though  removal  of  the  canals  of  one  side  has 
been  described  as  leading  to  diminished  muscular  tone,  especially 
on  the  same  side  of  the  body,  the  mere  diminution  of  force  is 
insufficient  to  explain  the  phenomena.  Nor  on  the  other  hand 
are  the  movements  due  to  any  uncontrollable  impulse ;  a  very 
gentle  pressure  of  the  hand  suffices  to  stop  the  movements  of 
the  head,  and  the  hand  in  doing  so  experiences  no  strain.  The 
assistance  of  a  very  slight  support  enables  movements  otherwise 
impossible  or  most  difficult,  to  be  easily  executed.  Thus,  though 
when  left  alone  the  bird  has  great  difficulty  in  drinking  or  pick- 
ing up  corn,  it  will  continue  to  drink  or  eat  with  ease  if  its  beak 
be  plunged  into  water,  or  into  a  heap  of  baiiey ;  the  slight  sup- 
port of  the  water  or  of  the  grain  seems  sufficient  to  steady  its 
movements.  In  the  same  way  it  can,  even  without  assistance, 
clean  its  feathers  and  scratch  its  head,  its  beak  and  foot  being 
in  these  operations  guided  b}^  contact  with  its  own  body. 

The  amount  of  disorder  thus  induced  differs  in  different 
birds  ;  and  some  movements  are  more  affected  than  others.  As 
a  general  rule  it  may  be  said  that  the  more  complex  and  intri- 
cate a  movement,  the  fuller  and  more  delicate  the  coordination 
needed  to  carry  it  out  successfull}^  the  more  markedly  is  it  dis- 
ordered by  the  operation  ;  thus  after  injury  to  the  canals,  while 
a  pigeon  cannot  fly,  a  goose  is  still  able  to  swim. 

Similar  phenomena  have  been  observed  in  other  classes  of 
animals  than  birds,  in  mammals,  in  frogs  and  in  fishes.  The 
results,  which  are  more  striking  in  some  species  of  animals  than 
in  others,  may  be  described  by  the  following  general  statements. 
When  one  canal  is  severed  the  head  or  the  eyes,  and,  in  certain 
cases,  parts  of  the  body,  in  a  fish  for  instance  the  fins,  make 
certain  definite  movements,  or  take  up  certain  definite  positions, 
or  tend  to  do  so,  the  exact  characters  of  the  movement  or  of 
the  position  taken  up  depending  upon  which  of  the  canals  is 
the  one  operated  on.  After  the  operation,  a  certain  deficiency, 
a  certain  want  of  coordination,  in  the  movements  of  the  animal 
may  be  observed,  the  exact  nature  of  the  loss  differing  with  the 
different  canals ;  but  this  is  generally  slight  and  after  a  while  is 
overcome  by  accommodation.  Section  of  all  three  canals  on 
one  side  is  followed  by  effects  in  the  way  of  movements  and  the 
like  which  may  be  regarded  as  the  sum  of  the  effects  of  the 
severance  of  each  canal ;  the  incoordination  is  made  more  pro- 
nounced, but  may  in  time  apparently  disappear.  When  all 
canals  on  both  sides  are  severed,  the  resultingspecial  movements 
are  not  so  striking  or  may  be  absent,  but  the  loss  of  coordination 
is  still  more  pronounced  and  may  be  permanent.  The  like 
results   have    been   obtained,  at   least   by  some    observers,   by 


732  SEMICIECULAE  CANALS.  [Book  iii. 

section  of  the  eighth  nerve  or  of  its  vestibular  portion  (the 
eighth  nerve  consists  really  of  two  distinct  nerves,  the  vestibu- 
lar nerve  and  the  cochlear  nerve)  the  semicircular  canals  being 
left  intact ;  and  in  cases  where  the  experiment  has  been  possible, 
the  effect  of  dividing  one  semicircular  canal  has  been  reproduced 
by  the  section  of  the  nerve  branch  distributed  to  its  ampulla, 
without  any  injury  to  the  canal  itself. 

Now  in  all  animals  the  three  canals  are  placed  in  the  three 
planes  of  space  in  relation  to  each  other,  though  not  necessarily 
so  that  one  of  them  lies  exactly  in  the  median  plane  of  the  head. 
Hence  whenever  the  head  is  turned  the  cristse  of  the  ampuUse 
are  unequally  affected  by  the  changes  of  pressure  or  of  flow 
which  the  turning  brings  about  in  the  endolymph  of  the  canals, 
and  when  the  head  is  at  rest  the  relations  of  the  endolymph  to 
the  several  canals  are  different  in  the  different  positions  of 
the  head.  And  these  facts  naturally  suggest  the  view  that 
according  to  the  relations  of  the  endolymph  to  the  ampullae, 
impulses  are  generated  in  the  cristas,  which  impulses  pass- 
ing up  to  the  brain  supply  the  data  by  which  the  animal 
becomes  aware  of  the  position  of  his  head  and  so  of  his 
body,  and  enter  into  the  coordination  of  his  movements ;  these 
impulses  in  fact  are  the  special  afferent  impulses  spoken  of 
a  little  while  back.  This  view  is  further  supported  by  the 
following  experimental  results.  Characteristic  movements  of 
the  head  or  eyes  may  be  obtained  by  carefully  laying  bare  a 
canal  and  gently  blowing  over  the  contained  endolymph  with 
a  fine  glass  cannula,  the  movements  differing  according  as  the 
current  of  air  drives  the  endolymph  towards  or  away  from  the 
ampulla.  Similar  characteristic  movements  may  be  brought 
about  by  stimulating  the  cristas  in  various  ways,  as  by  passing 
a  fine  hair  into  the  ampulla,  or  by  suddenly  heating  or  cooling 
the  canal,  or,  in  certain  cases  at  least,  by  passing  an  electric  cur- 
rent through  the  ampulla,  the  last  two  operations  not  necessi- 
tating the  opening  of  the  bony  canal. 

Without  entering  into  any  discussion  as  to  the  exact  way  in 
which  the  impulses  are  generated,  as  to  whether  the  results  for 
instance  of  the  section  of  a  canal  are  due  to  the  lack  of  normal 
impulses  or  to  the  introduction  of  abnormal  ones,  we  may  say 
that  the  evidence  seems  to  shew  beyond  doubt  that  the  cristas 
of  the  ampullae  are  organs  through  which  are  generated,  accord- 
ing to  the  position  of  the  head,  afferent  impulses  which  form 
the  basis  of  the  sense  of  equilibrium  and  enter  into  the  coordi- 
nation of  movements  affecting  that  equilibrium. 

The  three  canals  however  are  only  affected  by  a  turning  of 
the  head ;  when  the  head  otherwise  unmoved  is  carried  directly 
forwards  or  backwards,  or  directly  upwards  or  downwards,  the 
effect  on  all  the  ampullae  is  the  same.  Yet  we  are  as  aware  of 
these  movements  as  of  turning  movements,  and  we  may  con- 


Chap,  ii.]  THE   BRAIK  733 

elude  that  in  them  too  afferent  impulses  supply  the  means  of 
coordination.  Evidence  of  a  nature  similar  to  that  detailed  in 
reference  to  the  cristse  of  the  ampulhe  may  be  brought  forward, 
shewing  that  the  maculae  of  the  utricle  and  saccule  phiy  a  part 
analogous  to  that  of  the  crista).  Indeed  it  is  urged  that  the 
otoliths  are  important  agents  in  this  matter,  a  view  wliich  is 
supported  by  the  result  of  removing  in  various  invertebrata  the 
bodies  called  otoliths  ;  for  this  is  a  want  of  coordination  very 
similar  to  that  which  we  are  discussing.  No  effects  on  the 
coordination  of  bodily  movements  have,  so  far  as  is  at  present 
known,  been  seen  to  follow  removal  of  the  cochlea  alone,  or 
section  of  the  cochlear  nerve  alone.  We  are  thus  led  to  the 
view  that  the  whole  vestibular  nerve  (apart  from  the  sense  of 
hearing  which  we  shall  discuss  later  on)  is  the  agent  of  the 
special  aiferent  impulses  so  essential  to  the  coordination  of  the 
movements  affecting  the  equilibrium  of  the  body,  and  we  may 
speak  of  those  impulses  as  '  vestibular ' ;  the  name  '•  labyrinthine  ' 
(from  the  '  labyrinth '  of  the  ear)  has  also  been  suggested  for 
them. 

§  479.  We  may  here  say  a  few  words  on  the  interpretation 
of  the  subjective  condition  which  we  speak  of  as  giddiness  or 
dizziness  or  vertigo.  We  compared  the  condition  of  the  pigeon 
after  an  injury  to  the  semicircular  canals  to  that  of  a  person 
who  is  giddy  or  dizzy,  and  indeed  vertigo  is  the  subjective  ex- 
pression of  a  disarrangement  of  the  coordination  machinery, 
concerned  in  the  maintenance  of  bodily  equilibrium.  It  may 
be  brought  about  in  many  ways.  When  a  constant  current  of 
adequate  strength  is  sent  through  the  head  from  ear  to  ear,  we 
experience  a  sense  of  vertigo  ;  our  movements  then  ajipear  to  a 
bystander  to  fail  in  coordination,  in  fact  to  resemble  those  of  a 
pigeon  whose  semicircular  canals  have  been  injured ;  and  indeed 
the  effects  are  probably  produced  in  the  same  way  in  tlie  two 
cases.  In  what  is  called  Meniere's  disease  attacks  of  vertigo 
seem  to  be  associated  with  disease  in  the  ear,  being  attributed 
by  many  to  disorder  of  the  semicircular  canals,  and  cases  have 
been  recorded  of  giddiness  as  well  as  deafness  resulting  from 
disease  of  the  eighth  nerve.  Visual  sensations  are  very  potent 
in  producing  vertigo.  Many  persons  feel  giddy  when  they  look 
at  a  waterfall ;  and  this  is  a  case  in  which  both  the  sense  of  sid- 
diness  and  the  disarrangement  of  coordination  is  the  result  of 
the  action  of  a  pure  sensation  and  nothing  else.  In  the  well- 
known  intense  vertigo  whicli  is  caused  by  rapid  rotation  of  the 
body  visual  sensations  play  a  part  when  the  rotation  is  carried 
on  with  the  eyes  open,  but  only  a  part ;  for  vertigo  may  be 
induced,  though  not  so  readily,  by  rotation  with  the  eyes  com- 
pletely shut.  In  the  latter  case  the  vertigo  is  in  part  caused  by 
abnormal  vestibular  impulses,  and  in  the  case  of  some  deaf  per- 
sons, that  is,  persons  whose  eighth  nerve  is  diseased,  is  bi-ought 


734  MACHINEEY  OF   COOEDINATIOX.         [Book  in. 

about  with  difficulty  or  not  at  all,  when  the  eyes  are  shut; 
nevertheless  there  are  reasons  for  thinking  that  it  is  in  part 
caused  by  direct  disturbance  of  the  brain.  When  the  rotation  is 
carried  out  with  the  eyes  open,  the  vertigo  which  is  felt  when  the 
rotation  ceases  is  partly  caused  by  the  visual  sensations,  on  ac- 
count of  the  behaviour  of  the  eyeballs,  ceasing  to  be  in  harmony 
with  the  rest  of  the  sensations  and  afferent  impulses  which  help 
to  make  up  the  coordination.  The  rotation  sets  up  peculiar  oscil- 
lating movements  of  the  eyeballs,  which  continue  for  some  time 
after  the  rotation  has  ceased ;  owing  to  these  movements  of  the 
eyeballs  the  visual  sensations  excited  are  such  as  would  be 
excited  if  external  objects  were  rapidly  moving,  whereas  all  the 
other  sensations  and  impulses  which  are  affecting  the  central 
nervous  system  are  such  as  are  excited  by  objects  at  rest.  In  a 
normal  state  of  things  the  visual  and  the  other  sensations  and 
impulses,  which  go  to  make  up  the  coordinating  machinery,  are 
in  accord  with  each  other  in  reference  to  the  events  in  the 
external  world  which  are  giving  rise  to  them ;  after  rotation 
they  are  for  a  time  in  disaccord,  and  the  coordinating  machinery 
is  in  consequence  disarranged. 

When  we  interrogate  our  own  consciousness,  we  find  that 
we  are  not  distinctly  conscious  of  this  disaccord ;  the  visual 
sensations  are  so  prepotent  in  consciousness,  that  we  really 
think  the  external  world  is  rapidly  whirling  round ;  all  that  we 
are  further  conscious  of  is  the  feeling  of  giddiness  and  our  in- 
ability to  make  our  bodily  movements  harmonize  with  our 
visual  sensations.  So  that  even  in  the  cases  where  the  loss  of 
coordination  is  brought  about  by  distinct  sensations,  what  we 
really  appreciate  by  means  of  our  consciousness  is  the  disar- 
rangement of  the  coordinating  machinery.  It  is  the  appreciation 
of  this  disorder  which  constitutes  the  feeling  of  vertigo ;  both 
the  feeling  of  giddiness  and  the  disordered  movements  are  the 
outcome,  one  subjective  and  the  other  objective,  of  the  same 
thing.  It  is  not  because  we  feel  giddy  that  we  stagger  and 
reel ;  our  movements  are  wrong  because  the  machinery  is  at 
fault,  and  it  is  the  faulty  action  of  the  machinery  which  also 
makes  us  feel  giddy. 

We  may  here  perhaps  remark  that  it  is  an  actually  disordered 
condition  of  the  coordinating  mechanism  which  gives  rise  to  the 
affection  of  consciousness  which  we  call  giddiness,  not  a  mere 
curtailing  of  the  mechanism  or  any  failure  on  its  part  to  make 
itself  effective.  Complete  blindness  limits  the  range  of  activity 
of  the  machinery  but  leaves  the  remainder  intact,  and  no  giddi- 
ness is  felt.  So  again  in  certain  diseases  of  the  nervous  system 
the  muscular  sense  is  interfered  with  over  considerable  regions 
of  the  body,  and  in  these  regions  coordination  fails  or  is  imper- 
fect, but  the  central  machinery  is  not  thereby  affected,  though 
its  area  of  usefulness  is  limited,  and  no  giddiness  is  experienced  ; 
and  so  in  other  instances. 


Chap,  ii.]  THE   BRAIN.  735 

§  480.  Forced  Movements.  So  far  we  have  dwelt  on  disor- 
ders of  the  coordinating  machinery  brought  about  by  the  action 
of  various  afferent  impulses.  We  have  now  to  call  attention 
to  some  peculiar  phenomena  which  result  from  operative  inter- 
ference with  parts  of  the  brain,  and  which  in  some  instances 
at  least  may  be  taken  to  illustrate  how  this  complex  machinery 
works  when  some  of  its  inner  wheels  are  broken. 

All  investigators  who  have  performed  experiments  on  the 
brain  have  observed,  as  the  result  of  injury  to  various  parts  of 
it,  remarkable  movements  which  have  the  appearance  of  being 
irresistible,  compulsory,  forced.  They  vary  much  in  the  extent 
to  which  they  are  developed  ;  some  are  so  slight  as  hardly  to 
deserve  the  name,  while  others  are  strikingly  intense.  One  of 
the  most  common  forms  is  that  in  which  the  animal  rolls  inces- 
santly round  the  longitudinal  axis  of  its  own  body.  This  is 
especially  common  after  section  of  one  of  the  crura  cerebri,  or 
of  the  middle  and  inferior  peduncles  of  the  cerebellum,  or  after 
unilateral  section  of  the  pons,  but  has  also  been  witnessed  after 
injury  to  the  bulb  and  corpora  quadrigemina.  Sometimes  the 
animal  rotates  towards  and  sometimes  away  from  the  side  oper- 
ated on.  Another  form  is  that  in  which  the  animal  executes 
'circus  movements,'  i.e.  continually  moves  round  and  round  in 
a  circle  of  longer  or  shorter  radius,  sometimes  towards  and  some- 
times away  from  the  injured  side.  This  may  be  seen  after  several 
of  the  above-mentioned  operations,  and  in  one  form  or  another 
is  not  uncommon  after  various  unilateral  injuries  to  the  brain. 
There  is  a  variety  of  the  circus  movement,  '  the  clockhand 
movement,'  said  to  occur  frequently  after  lesions  of  the  poste- 
rior corpora  quadrigemina,  in  which  the  animal  moves  in  a 
circle,  with  the  longitudinal  axis  of  its  body  as  a  radius,  and 
the  end  of  its  tail  for  a  centre.  And  this  form  again  may  easily 
pass  into  a  simple  rolling  movement.  In  yet  another  form  the 
animal  rotates  over  the  transverse  axis  of  its  body,  tumbles  head 
over  heels  in  a  series  of  somersaults ;  or  it  may  run  incessantly 
in  a  straight  line  backwards  or  forwards  until  it  is  stopped  by 
some  obstacle.  These  latter  forms  of  forced  movements  are 
sometimes  seen  after  injury  to  the  corpus  striatum,  even  when 
a  very  limited  portion  of  the  grey  matter  is  affected.  And  many 
of  these  forced  movements  may  result  from  injuries  which  appear 
to  be  confined  to  the  cerebral  cortex. 

When  the  phenomena  are  well  developed,  every  effort  of 
the  animal  brings  on  a  movement  of  this  forced  character.  Left 
to  itself  and  at  rest  the  animal  may  present  notliing  abnormal, 
its  posture  and  attitude  may  be  quite  natural ;  but  when  it  is 
excited  to  move  or  when  it  attempts  of  itself  to  move,  it  exe- 
cutes not  a  natural  movement  but  a  forced  one,  turning  round 
or  rolling  over  as  the  case  may  be.  In  severe  cases  the  move- 
ment is  continued  until   the  animal  is  exhausted;    when  the 


736  FOECED   MOVEMENTS.  [Book  in. 

exhaustion  passes  off  the  animal  may  remain  for  some  little  time 
quiet,  but  some  stimulus,  intrinsic  or  extrinsic,  soon  inaugurates 
a  fresh  outbreak,  to  be  again  followed  by  exhaustion. 

In  some  of  the  milder  forms,  that  for  instance  of  the  circus 
movement  with  a  long  radius,  the  curved  character  of  the  pro- 
gression appears  simply  due  to  the  fact  that  in  the  effort  of 
locomotion  volitional  impulses  do  not  gain  such  ready  access  to 
one  side  of  the  body  as  to  the  other,  the  injury  having  caused 
some  obstacle  or  other.  Hence  the  contractions  of  the  muscles 
of  one  side  (the  left  for  instance)  of  the  body  are  more  power- 
ful than  the  other,  and  in  consequence  the  body  is  continually 
thrust  towards  the  other  (the  right)  side.  As  is  well  known, 
we  ourselves,  when  our  walk  is  not  guided  by  visual  sensations, 
tend  to  describe  a  circle  of  somewhat  wide  radius,  the  deviation 
being  due  to  a  want  of  bilateral  symmetry  in  our  limbs ;  and 
the  above  circus  movement  is  only  an  exaggeration  of  this. 

But  the  other  more  intense  forms  of  forced  movements  are 
more  complicated  in  their  nature.  No  mere  blocking  of  voli- 
tional impulses  will  explain  why  an  animal  whenever  it  attempts 
to  move  rolls  rapidly  over,  or  rushes  irresistibly  forwards  or 
backwards.  It  is  not  possible  Avith  our  present  knowledge  to 
explain  how  each  particular  kind  of  movement  is  brought  about; 
and  indeed  the  several  kinds  are  probably  brought  about  in  dif- 
ferent ways,  for  they  differ  so  greatly  from  each  other  that  we 
only  class  them  together  because  it  is  difficult  to  know  where 
to  draw  the  line  between  them.  But  we  may  regard  the  more 
intense  forms  as  illustrating  the  complex  nature  of  what  we 
have  called  the  coordinating  machinery,  the  capabilities  of  which 
are,  so  to  speak,  disclosed  by  its  being  damaged.  Such  gross 
injuries  as  are  involved  in  dividing  cerebral  structures  or  in 
injecting  corrosive  substances  into  this  or  that  part  of  the  brain, 
must,  of  necessity,  partly  by  blocking  the  way  to  the  impulses 
which  in  a  normal  state  of  things  are  continually  passing  from 
one  part  of  the  brain  to  another,  partly  by  generating  new  unu- 
sual impulses,  seriously  affect  the  due  working  of  the  general 
coordinating  machinery.  The  fact  that  an  animal  can,  at  any 
moment,  by  an  effort  of  its  own  will,  rotate  on  its  axis  or  run 
straight  forwards,  shews  that  the  nervous  mechanism  for  the 
execution  of  those  movements  is  ready  at  hand  in  the  brain, 
waiting  only  to  be  discharged ;  and  it  is  easy  to  conceive  how 
such  a  discharge  might  be  affected  either  by  the  substitution  for 
the  will  of  some  potent  intrinsic  afferent  impulse  or  by  some 
misdirection  of  volitional  impulses.  Persons  who  have  experi- 
enced similar  forced  movements  as  the  result  of  disease  report 
that  they  are  frequently  accompanied,  and  seem  to  be  caused, 
by  disturbed  visual  or  other  sensations;  thus  they  attribute  their 
suddenly  falling  forward  to  the  occurrence  of  the  sensation  that 
the  ground  in  front  of  them  is  suddenly  sinking  away  beneath 


Chap,  ii.]  THE  BEAIK.  737 

their  feet.  Without  trusting  too  closely  to  the  interpretations 
the  subjects  of  these  disorders  give  of  their  own  feelings,  and 
remembering  what  was  said  above  concerning  vertigo,  we  may 
at  least  conclude  that  the  unusual  movements  are  in  many  cases 
due  to  a  disorder  of  the  coordinating  mechanism,  brought  about 
by  strange  or  disordered  sensory  impulses.  And  this  view  is 
supported  by  the  fact  that  many  of  these  forced  movements  are 
accompanied  by  a  peculiar  and  wholly  abnormal  position  of  the 
eyes,  which  alone  might  perhaps  explain  many  of  the  phenomena. 

§  481.  The  phenomena  presented  by  animals  deprived  of 
their  cerebral  hemispheres  shew  that  this  machinery  of  coordina- 
tion is  supplied  by  cerebral  structures  lying  between  the  cerebral 
hemisphere  above  and  the  top  of  the  spinal  cord  below.  But 
when  we  ask  the  further  question,  how  is  this  machinery  related 
to  the  various  elements  which  go  to  make  up  this  part  of  the 
brain  ?  the  only  answers  which  we  receive  are  of  the  most 
imperfect  kind. 

In  the  case  of  the  frog  we  can,  after  removal  of  the  cerebral 
hemispheres,  make  an  experimental  distinction  in  the  parts  left 
between  the  optic  thalami  with  the  optic  nerves  and  tracts, 
the  optic  lobes,  and  the  bulb  with  the  rudimentary  cerebellum. 
When  the  optic  thalami  are  removed,  as  might  be  expected,  the 
evidence  of  visual  impressions  modifying  the  movements  of  the 
animal  disappears  ;  and  it  is  stated  that  apparently  spontaneous 
movements  are  much  more  rare  than  when  the  thalami  are 
intact.  When  the  optic  lobes  as  well  as  the  cerebral  hemispheres 
are  removed,  the  power  of  balancing  is  lost ;  when  such  a  frog  is 
thrown  off  its  balance  by  inclining  the  plane  on  which  it  is  placed, 
it  slips  back  or  falls  down  ;  the  special  coordinating  mechanism 
for  balancing  must  therefore  in  this  animal  have  a  special 
connection  with  the  optic  lobes.  But  after  removal  of  these 
organs  the  animal  is  still  capable  of  a  great  variety  of  coordinate 
movements :  unlike  a  frog  retaining  its  spinal  cord  only,  it  can 
swim  and  leap,  it  maintains  a  normal  posture,  and  when  placed  on 
its  back  immediately  regains  the  normal  posture.  The  cerebellum 
of  the  frog  is  so  small,  and  in  removing  it  injury  is  so  likely  to  be 
done  to  the  underlying  parts,  that  it  becomes  difficult  to  say  how 
much  of  the  coordination  apparent  in  a  frog  possessing  cerebellum 
and  bulb  is  to  be  attributed  to  the  former  or  to  the  latter; 
probably,  however,  the  part  played  by  the  former  is  small. 

In  the  case  neither  of  the  bird  nor  of  the  mammal  have  we 
any  exact  information  as  to  the  behaviour  of  the  animal  after 
removal  of  the  parts  behind  the  hemispheres,  in  addition  to  the 
hemispheres  themselves.  Our  knowledge  is  confined  to  the  re- 
sults of  the  ablation,  or  of  the  stimulation  of  parts,  the  cere- 
bellum for  instance,  in  animals  in  which  the  rest  of  the  brain 
has  been  left  intact.  Observations  of  this  kind  have  disclosed 
many  interesting  facts,  besides  the  forced  movements  just  referred 

47 


738  MACHINERY   OF   COORDmATION.         [Book  in. 

to,  but  they  have  not  led  to,  and  indeed  could  hardly  be  expected 
to  lead  to,  any  clear  views  as  to  the  point  which  we  are  now  dis- 
cussing. It  does  not  follow  that  every  part,  injury  or  stimulation 
of  which  interferes  with  coordinated  movements,  or  gives  rise  to 
definite,  forced,  or  other  movements,  is  to  be  considered  as  part 
of  the  machinery  under  consideration.  The  corpora  striata  and 
cerebral  hemispheres  form,  as  we  have  seen,  no  part  of  the 
machinery,  yet  injury  to  them  may  disorder  the  machinery ;  and 
the  fact  that  removal  of,  or  injury  to  the  cerebellum,  disorders 
the  machinery  is  no  proof  by  itself  that  the  cerebellum  is  an 
essential  part  of  the  machinery. 

If  we  may  trust  to  deductions  from  structural  arrangements, 
we  might  be  inclined  to  infer  that  the  anatomical  relations  of 
the  tegmental  region  from  the  bulb  upwards  point  to  its  serving 
as  the  foundation  of  the  machinery  in  question.  Behind,  it  has 
full  connections  with  various  parts  of  the  cord,  while  in  front  by 
means  of  the  optic  thalami  and  anterior  corpora  quadrigemina, 
if  not  by  other  ways  as  well,  it  is  so  far  associated  with  the  optic 
nerves  that  the  path  seems  open  for  visual  impulses  to  gain 
access  to  it.  To  this  foundation,  however,  we  must  add  the 
cerebellum,  on  account  of  its  relations  to  it,  to  the  cord  and  to 
the  bulb  through  the  restiform  bodies,  including  its  ties  with  the 
auditory  nerve.  And  if  we  add  the  cerebellum  we  must  also 
probably  add  the  pons.  We  may  exclude  the  pes  of  the  crus, 
since  this  is  composed  exclusively  of  fibres  bringing  the  cerebral 
hemispheres,  including  the  corpora  striata,  into  connection  with 
the  pons,  bulb  and  cord,  and  so  with  the  coordinating  machinery 
itself,  as  well  as  with  other  parts  of  the  nervous  system.  And 
observation  as  far  as  it  goes  supports  this  deduction  from  ana- 
tomical relationships.  We  will,  however,  defer  what  else  we 
have  to  say  on  this  point  until  after  we  have  discussed  the  car- 
rying out  of  voluntary  movements. 


SEC.  3.     ON"  VOLUNTARY  MOVEMENTS. 

§  482.  When  we  examine  ourselves  we  recognize  certain  of 
our  movements  as  '  voluntary  ' ;  we  say  that  we  carry  them  out 
by  an  effort  of  the  '  will.'  And  when  we  witness  the  movements 
of  other  people  or  of  animals  we  regard  as  also  voluntary  such 
of  those  movements  as  by  their  characters  and  by  the  circum- 
stances of  their  occurrence  seem  to  be  carried  out  in  the  same 
way  as  our  own  voluntary  movements.  Even  in  the  case  of  some 
of  our  own  movements  we  are  not  always  clear  whether  they  are 
really  voluntary  or  no ;  and  in  the  case  of  other  people  and  of 
animals  it  is  still  more  difficult  to  decide  the  question.  It  would 
be  out  of  place  to  attempt  to  discuss  here  how  voluntary  move- 
ments really  differ  from  involuntary  movements,  or  in  other 
words,  what  is  the  nature  of  the  will;  we  must  be  content  to  take 
a  somewhat  rough  use  of  the  words  '  voluntary,'  '  volitional,'  and 
'  will '  as  a  basis  for  physiological  discussion.  We  may  however 
remark  that  so  far  as  the  muscular  side  of  the  act,  if  we  may  use 
such  an  expression,  is  concerned,  a  voluntar}^  movement  does  not 
differ  in  kind  from  an  involuntary  movement.  It  is  perfectly 
true  that  a  skilled  man  may  by  practice  learn  to  execute  mus- 
cular manoeuvres  which  he  would  not  have  learnt  to  execute  had 
not  an  intelligent  volition  been  operative  within  him ;  but  our 
own  experience  teaches  us  that  many  more  or  less  intricate  move- 
ments which  have  undoubtedly  been  learnt  by  help  of  the  will 
may  be  carried  out  under  circumstances  of  such  a  kind  that  we 
feel  compelled  to  regard  them  as,  at  the  time,  involuntar}^;  and 
it  may  at  least  be  debated  whether  every  movement  which  we 
can  carry  out,  by  an  effort  of  the  will,  may  not  appear  under 
appropriate  circumstances  as  part  of  an  involuntary  act.  In  the 
case  of  the  lower  animals,  in  the  frog  deprived  of  its  cerebral 
hemispheres  for  instance,  we  have  seen  that  voluntary  differ  from 
involuntary  movements,  not  by  their  essential  nature  bnt  by  the 
relation  which  their  occurrence  bears  to  circumstances.  We'have 
therefore  to  seek  for  the  distinction  between  voluntary  and  in- 
voluntary, not  in  the  coordination  of  the  muscular  and  nervous 
components  of  a  movement,  but  in  the  nature  of  the  process 
which  starts  the  whole  act. 

739 


740  CORTICAL   MOTOR   REGION.  [Book  iir. 

The  histories,  related  in  a  preceding  section,  of  various  ani- 
mals deprived  of  their  cerebral  hemispheres,  while  they  have 
further  shewn  the  difficulty  of  drawing  a  sharp  line  between  the 
presence  and  absence  of  volition,  such  as  when  we  appeal  to  our 
own  consciousness  we  seem  able  to  draw,  have  taught  us  that 
in  a  broad  sense  the  presence  of  volition  is,  in  the  higher  verte- 
brata,  dependent  on  the  possession  of  the  cerebral  hemispheres ; 
and  we  have  now  to  inquire  what  we  know  concerning  the  way 
in  which  the  cerebral  cortex,  for  this,  as  we  have  seen,  is  the 
important  part  of  the  cerebral  hemisphere,  by  the  help  of  other 
parts  of  the  nervous  system  carries  out  a  voluntary  movement. 

§  483.  With  this  view  we  may  at  once  turn  to  the  results 
of  experimental  interference  with  the  cortex.  When  the  sur- 
face of  the  brain  is  laid  bare  by  removal  of  the  skull  and  dura 
mater,  mechanical  stimulation  of  tlie  cortex  produces  little  or 
no  effect,  thus  affording  a  contrast  with  the  results  of  mechani- 
cally stimulating  other  portions  of  the  brain,  or  other  nervous 
structures.  And  for  a  long  time  the  cortex  was  spoken  of  as 
insensible  to  stimulation.  When,  however,  the  electric  current 
is  employed,  either  the  make  and  break  of  the  constant  current, 
or  the  more  manageable  interrupted  current,  very  marked  results 
follow.  It  is  found  that  certain  movements  follow  upon  electric 
stimulation  of  certain  regions  or  areas.  The  results,  moreover, 
differ  in  different  animals.  It  will  be  convenient  to  begin  with 
the  dog,  on  which  animal  the  observations  of  this  kind  were 
first  conducted. 

When  the  surface  of  the  dog's  brain  is  viewed  from  the  dor- 
sal surface  a  short  but  deep  sulcus  is  seen  towards  the  front, 
running  outwards  almost  at  right  angles  from  the  great  longi- 
tudinal fissure ;  this  is  called  the  crucial  sulcus  (Fig.  121),  the 
gyrus  or  convolution  in  front  and  behind  it,  and  sweeping  round 
its  end  being  called  the  sigmoid  gyrus.  It  will  hardly  be  profit- 
able to  discuss  here  either  the  homology  of  this  sulcus  or  the 
names  of  the  other  sulci  and  convolutions  of  the  dog's  brain. 
We  mention  this  sulcus  because  it  is  found  that  stimulation  of 
the  cortex  in  a  region  which  may  be  broadly  described  as  that 
of  the  neighbourhood  of  this  crucial  sulcus  gives  rise  to  move- 
ments of  various  parts  of  the  body,  whereas  no  such  movements 
result  from  stimulation  of  the  extreme  frontal  region  in  front 
of  the  area  around  the  crucial  sulcus,  or  from  stimulation  of  the 
occipital  region  behind  this  area.  Certain  exceptions  may  be 
made  to  this  broad  statement,  but  these  it  will  be  best  to  discuss 
in  reference  to  the  more  highly  developed  monkey. 

The  region  of  the  cortex  in  the  neighbourhood  of  the  crucial 
sulcus  may  then  be  termed  an  '  excitable '  or  '  motor '  region, 
inasmuch  as  stimulation  of  this  region  leads  to  movements  car- 
ried out  by  skeletal  muscles,  while  stimulation  of  other  regions 
does  not.     Further,  stimulation  of  particular  districts  or  areas 


Chap,  ii.] 


THE   BEArN". 


741 


of  the  region  leads  to  particular  movements  carried  out  by  par- 
ticular muscles.  For  instance,  stimulation  of  the  more  median 
parts  of  the  gyrus  behind  the  crucial  sulcus  (Fig.  121 JJ)  leads 
to  movements  of  the  hind  limb,  whereas  stimulation  of  the  lateral 
part  or  outer  end  of  the  same  gyrus  leads  to  movements  of  the 
fore  limb,  and  we  may  here  distinguish  between  an  area  stimu- 
lation of  which  (Fig.  121  +)  leads  to  flexion  of  the  fore  limb, 
and  an  area  (Fig.  121  +)  stimulation  of  which  leads  to  exten- 
sion of  the  same  limb.     In  a  similar  way  stimulation  of  other 


Fig.  121.   The  Areas  op  the  Cerebral  Convolutions  of  the  Dog,  accord- 
ing   TO    HiTZIG   ANI>    ErITSCH. 

(1)  A  The  area  for  the  muscles  of  the  neck.  (2)  +  The  area  for  the  exten- 
sion and  adduction  of  the  fore  limb.  (3)  +  The  area  for  the  flexion  and  rota- 
tion of  the  fore  limb.  (4)  ft  The  area  for  the  hind  limb.  Running  transversely 
tovpards  and  separating  (1)  and  (2)  from  (3)  and  (4)  is  seen  the  crucial  sulcus. 
(5)  O  The  facial  area. 

areas  within  the  'motor'  region  leads  to  movements  of  this  kind 
or  of  that  kind  of  the  tail,  of  the  eyes,  of  the  mouth,  of  other 
parts  of  the  face,  of  the  tongue,  and  so  on.  Obviously  in  the 
dog  this  region  of  the  cortex  has  connections  with  the  skeletal 
muscles  which  do  not  obtain  between  other  regions  of  the  cor- 
tex and  those  muscles;  and  further,  the  region  in  question  is 
topographically  differentiated,  so  that  certain  areas  or  districts  of 
the  region  are  specially  connected  with  certain  skeletal  muscles 
or  groups  of  muscles.  We  may  speak  of  a  '  localisation  of  func- 
tion '  in  this  region  as  compared  with  other  regions  of  the  cortex, 
and  in  the  several  areas  within  the  region  as  compared  with 
each  other. 

The  muscles  which  are  thus  tlirown  into  contraction  are  the 
muscles  of  the  opposite  side  of  the  body.     When  '  the  fore  limb 


742  CORTICAL  MOTOE  EECI0:N".  [Book  hi. 

area,'  as  we  may  call  it,  of  the  right  hemisphere  is  stimulated,  it 
is  the  left  fore  limb  which  is  moved ;  and  so  with  the  other  areas ; 
it  is  only  in  exceptional  cases,  as  in  certain  movements  of  the 
eyes,  that  the  effect  is  bilateral ;  a  movement  confined  to  the 
same  side  as  that  stimulated  is  never  witnessed. 

The  results  are  most  clear  when  the  current  employed  as  a 
stimulus  is  not  stronger  than  is  just  sufficient  to  produce  the 
appropriate  movement  (roughly  speaking  an  interrupted  current 
just  perceptible  to  the  tongue  of  the  o^Derator  is  in  ordinary 
cases  a  useful  one),  and  when  the  cortex  is  in  good  nutritive 
condition.  In  any  experiment  the  results  obtained  by  the 
earlier  stimulations,  soon  after  the  cortex  has  been  exposed,  are 
the  best ;  after  repeated  stimulations  the  surface  is  apt  to  become 
hypersemic,  and  it  is  then  frequently  observed  that  the  move- 
ments resulting  from  the  stimulation  of  a  particular  area  are  not 
confined  to  the  appropriate  muscles,  but  spread  to  the  corre- 
sponding muscles  of  the  opposite  side,  then  to  muscles  connected 
with  other  cortical  areas,  and  at  last  to  the  muscles  of  the  body 
generally ;  at  the  same  time  the  movements  lose  their  distinctive 
purposeful  character  and  the  animal  is  thrown  into  convulsions 
of  an  epileptiform  kind.  It  not  unfrequentlj'-  happens  that  an 
experiment  has  to  be  stopped  in  consequence  of  the  onset  of 
these  epileptiform  convulsions.  The  response  of  movement  to 
stimulation  may  be  observed  while  the  animal  is  under  the 
moderate  influence  of  an  anaesthetic,  but  a  too  profound  anses- 
thesia  lessens  or  annuls  the  effects. 

In  order  to  carry  out  a  closer  analysis  of  the  phenomena  it  is 
desirable  to  watch  or  record  the  contraction  of  a  particular  group 
of  muscles,  or  perhaps  better  still  a  particular  muscle,  e.  gr.  the 
area  for  extension  of  the  hind  limb  may  be  studied  by  help  of  the 
extensor  digitorum  communis  of  the  limb.  When  this  is  done 
the  following  important  facts  may  be  observed.  The  area  of 
cortex  having  been  found  which  gives  the  best  movements,  and 
the  stimulus  being  no  stronger  than  is  necessary,  isolation  of  the 
area  from  its  lateral  surroundings  by  a  circular  incision  carried  to 
some  little  depth  will  not  prevent  the  development  of  contrac- 
tions in  the  muscle ;  but  these  do  cease,  even  without  the 
circular  incision,  if  by  a  horizontal  section  the  grey  cortex  is 
separated  from  the  subjacent  white  matter.  After  removal  of 
the  cortex,  stimulation  of  the  white  matter  underlying  the  area 
produces  the  appropriate  contraction ;  not  only  however  is  a 
stronger  stimulus  necessary,  but  also  the  latent  period,  that  is 
the  time  intervening  between  the  beginning  of  the  application 
of  the  stimulating  current  and  the  beginning  of  the  muscular 
contraction  is  appreciably  shortened.  The  appropriate  contrac- 
tions not  only  appear  when  the  white  matter  immediately  below 
the  cortex  is  stimulated,  but  by  making  successive  horizontal 
sections  and  stimulating  each  in  turn,  the  effect  may,  so  to 


Chap,  ii.]  THE   BRAIK  743 

speak,  be  traced  through  the  central  white  matter  of  the  hemi- 
sphere down  to  the  internal  capsule.  We  may  conclude  from 
these  results,  that  when  the  current  is  applied  to  the  surface  of 
the  cortex,  certain  parts  of  certain  structures  in  the  grey  matter 
are  stimulated,  the  process  having  a  marked  latent  period,  and 
that  as  the  outcome  of  the  changes  induced  in  the  grey  matter, 
impulses  pass  along  the  fibres  leading  down  from  the  grey 
matter  to  the  internal  capsule,  and  so  by  the  pedal  fibres  of  tlie 
crus  to  the  spinal  cord  and  motor  spinal  roots.  Anatomical 
considerations  lead  us  to  suppose  that  the  fibres  in  question 
belong  to  the  great  pyramidal  tract ;  and  as  we  shall  see,  all  our 
knowledge  confirms  this  view. 

It  must  not,  however,  be  supposed  that  the  several  areas 
stimulation  of  which  produces  each  its  distinctive  movement,  are 
in  the  dog  sharply  defined  from  each  other ;  when  the  term  area 
for  extension  of  the  hind  limb  is  used  it  must  not  be  supposed 
that  the  area  can  be  defined  by  an  outline,  within  which  stimula- 
tion produces  nothing  but  extension  of  the  hind  limb,  and  out- 
side which  stimulation  never  produces  extension  of  the  hind  limb. 
All  that  is  meant  in  that  extension  of  the  hind  limb  is  the  salient 
and  striking  result  of  stimulating  the  area.  When  we  study  the 
various  movements,  and  especially  perhaps  when  we  study,  by 
help  of  a  graphic  record,  the  contractions  of  various  individual 
muscles  resulting  from  the  stimulation  of  various  parts  of  the 
motor  region,  we  find  not  only  that  the  areas  for  particular 
movements  or  particular  muscles  are  very  diffuse,  but  that  the 
several  areas  largely  overlap  each  other.  If  for  instance  we 
were  to  map  out  on  the  same  diagi'am  the  several  areas  belong- 
ing to  four  or  five  muscles  of  different  parts  of  the  body,  such 
as  the  extensors  of  the  digits  of  the  fore  and  of  the  hind  limb, 
the  flexors  of  the  same,  and  the  orbicular  muscle  of  the  eyelid, 
that  is  to  say,  the  several  areas  within  which  in  turn  stimulation 
of  the  cortex  produced  contraction  of  the  particular  muscle,  the 
overlapping  would  be  so  great  that  the  whole  figure  would 
appear  highly  confused.  In  a  simular  way  the  excitable  motor 
region  as  a  whole  would  gi-adually  merge  into,  be  broken  up 
into,  the  unexcitable  frontal,  occipital  and  temporal  regions,  in 
front,  behind  and  below.  In  other  Avords,  the  localization  in  the 
cortex  of  the  dog  is  to  a  marked  degree  imperfect. 

In  this  respect  the  dog,  corresponding  to  its  position  in  the 
animal  hierarchy,  is  intermediate  between  such  animals  as  the 
rabbit,  the  bird,  and  the  frog,  on  the  one  hand,  and  the  more 
highly  developed  monkey  on  the  other ;  and  that  is  one  reason 
why  we  have  taken  the  dog  first  and  dwelt  so  long  upon  it.  In 
the  rabbit,  a  similar  localization  may  be  observed,  but  far  less 
definite,  far  more  diffuse;  it  becomes  still  less  in  the  bird,  and 
is  liardiy  recognizable  in  the  frog.  It  will  not  be  profitable  to 
dwell  on  the  details  of  these  lower  animals;  but  the  phenomena 


744 


COETICAL  MOTOR  EEGION". 


[Book  in. 


of  tlie  monkey,  leading  up  as  they  do  to  those  of  man,  call  for 
special  notice. 

§  484.  When  in  a  monkey,  in  an  individual  for  instance 
belonging  to  the  genus  Macacus,  the  surface  of  the  cerebrum  is 
explored  with  reference  to  the  effects  of  electric  stimulation,  it  is 
found  that  when  the  current  is  applied  to  the  precentral  or 
ascending  frontal  and  the  post-central  or  ascending  parietal 
convolutions  which  lie  respectively  in  front  of  and  behind  the 
important  central  fissure  or  fissure  of  Rolando  (cf.  Fig.  122), 


Fig.  122.  Outline  of  Brain  of  Monkey  (Macacus)  to  shew  principal 
Sulci  (Fissures)  and  Gtri  (Convolutions).  (Natural  size.)  (Sherring- 
ton after  Horsley  and  Schafer.) 

The  brain  figured  is  the  same  as  that  in  Fig.  123,  and  the  two  figures  should 
be  consulted  together.  Over  each  sulcus,  purposely  printed  very  thick,  the  name 
is  written  in  small  ccqntals,  over  each  gyrus  in  italics,  x  indicates  the  small 
depression,  hardly  to  be  called  a  sulcus,  which  is  supposed  to  be  homologous  with 
the  superior  frontal  sulcus  of  man  ;  and  to,  y,  z  similarly  indicate  sulci  whose 
homologies  are  not  certain.     For  some  synonyms  see  Figs.  134,  135. 


movements  of  the  fore  limb  follow.     The  '  motor  area  for  the 
fore  limb '  thus  discovered  is  more  circumscribed  and  definite 


Chap,  ii.] 


THE  BRAIN. 


745 


than  is  the  corresponding  area  in  the  dog.  Its  outline  (Fig. 
123)  is  roughly  that  of  a  truncated  triangle  bisected  by  the 
central  fissure,  with  the  broad  base  at  some  distance  from  the 
mesial  line,  and  the  truncated  apex  reaching  on  the  lateral  sur- 
face of  the  hemisphere  to  a  well-marked  bend  in  the  lower  part 


Fig.  123.  Left  hemisphere  of  the  Cerebrum  of  Macacus  Monkey  viewed 
FROM  ITS  LEFT  SIDE,  AND  FROM  ABOVE.  (Natural  slze.)  (Sherrington 
after  Horsley  and  Beevor.) 

The  fiffure  shews  the  positions  of  the  portions  of  the  cortex  concerned  with 
movement  of  various  parts,  and  with  the  senses  of  sight,  smell,  and  hearing.  The 
cortical  area  connected  with  the  movements  of  the  leg  is  shaded  vertically  across, 
that  with  the  movements  of  the  arm  horizontally,  and  that  with  the  movements 
of  the  trunk  in  a  slanting  direction  ;  the  area  connected  with  movements  of 
the  head  (neck),  face,  and  eyes  is  dotted.  The  course  of  the  chief  fissures  is 
indicated  by  single  lines. 

of  the  central  fissure.  Behind,  it  reaches  as  far  as  the  intra- 
parietal  fissure  which  somewhat  sharply  defines  its  hind  border, 
and  in  front  it  ceases  no  less  definitely  at  some  little  distance 
behind  the  precentral  fissure.  Further  examination  shews  that 
the  whole  area  is  divided  into  areas  corresponding  to  movements 


746  CORTICAL  MOTOR  REGIOK  [Book  iii. 

of  particular  parts  of  the  fore  arm,  and  that  these  are  arranged 
in  a  definite  relation  to  each  other.  In  the  more  dorsal  part  of 
the  area,  at  the  base  of  the  triangle,  stimulation  produces  move- 
ments of  the  shoulder  (Fig.  123);  if  the  electrodes  be  shifted 
ventrally  movements  of  the  elbow  make  their  appearance ;  if 
still  more  ventrally,  movements  of  the  wrist  come  in,  and  these 
are  in  turn  succeeded  ventrally  by  movements  of  the  digits 
generally,  of  the  forefinger,  and  lastly  of  the  thumb.  A  very 
striking  experiment  may  be  made  by  applying  a  current  of  suit- 
able strength,  fii'st  at  the  lower,  ventral  border  of  the  area,  and 
then  gradually  advancing  upwards  towards  the  mesial  line ;  the 
thumb  is  moved  first,  then  the  forefinger,  then  the  rest  of  the 
digits,  then  the  wrist,  next  the  elbow,  and  lastly  the  shoulder. 
Further,  in  certain  parts  of  the  area  the  resulting  movement  is 
flexion  of  the  appropriate  segment  of  the  limb,  in  other  parts 
extension,  in  certain  parts  abduction,  in  other  parts  adduction, 
and  so  on. 

Similar  exploration  shews  that  the  'area  for  the  hind  limb,' 
lies  on  the  median  side  of  the  area  for  the  fore  limb,  stretching 
besides  on  to  the  mesial  surface  along  the  marginal  convolution 
which  forms  the  dorsal  portion  of  the  wall  of  the  great  longi- 
tudinal fissure  ;  it  reaches  as  far  back  as  the  intra-parietal  sulcus, 
and  is  succeeded  in  front  by  the  '  area  for  the  trunk '  (Fig.  124). 
Within  this  general  area  for  the  hind  limb  we  may  similarly  dis- 
tinguish special  areas  for  the  hip  (Figs.  123,  124)  in  the  front 
portion,  for  the  knee  and  ankle  behind  this,  and  for  the  digits 
still  farther  backwards,  the  area  for  the  great  toe  being  however 
in  front  of  the  area  for  the  other  digits. 

In  front  of  the  areas  for  the  limbs  and  trunk,  on  the  median 
dorsal  surface,  dipping  down  into  the  mesial  surface  along  the 
marginal  convolution  (Fig.  124)  and  reaching  laterally  on  the 
dorsal  lateral  surface  to  the  dorsal  extremity  of  the  precentral 
sulcus  (Fig.  123),  is  the  '  area  for  the  head,'  that  is  to  say  for 
movements  of  the  head  brought  about  by  contractions  of  the 
muscles  of  the  neck. 

Ventral  to  this  again,  in  front  of  the  precentral  sulcus  is  the 
'  area  for  the  eyes,'  that  is  to  say,  for  contractions  of  the  ocular 
muscles ;  and  behind  the  precentral  sulcus,  ventral  to  the  arm 
area,  lies  a  small  area  for  movements  of  the  eyelids,  brought 
about  by  contractions  of  the  orbicularis  muscle.  Ventral  to 
this  again  is  the  '  area  for  the  face,'  in  which  we  may  distin- 
guish an  area  for  the  mouth,  that  is  an  area  stimulation  of  which 
produces  changes  in  the  buccal  orifice,  opening,  shutting,  draw- 
ing to  one  side  &c.,  and  an  area  for  movements  of  the  tongue. 
These  two  areas  reach  downwards  to  the  fissure  of  Sylvius,  and 
backwards  to  the  line  of  the  intra-parietal  sulcus.  In  front  of 
them,  occupying  all  the  ventral  part  of  the  precentral  convolu- 
tion and  reaching  forwards  as  far  as  the  precentral  sulcus,  Avhere 


Chap,  ii.] 


THE  BEAIK 


747 


it  meets  the  area  for  the  eyes,  lies  an  area  stimulation  of  which 
produces  movements  of  the  pharynx  or  larynx  as  well  as  the 
mouth  or  face,  and  which  may  be  divided  into  areas  for  mastica- 
tion, for  swallowing,  and  for  the  production  of  the  voice. 

We  might  speak  of  these  several  areas  in  another  way  by 
referring  to  the  nerves  concerned  in  carrying  out  the  several 
movements,  though  in  doing  so  we  must  remember  that  there  is 
not  an  exact  correspondence  between  the  relative  position  of  a 


Fig.  124.  Mesial  aspect  of  the  left  half  of  the  Brain  of  Macacus, 
displayed  by  section  in  the  median  sagittal  plane  and  removal  of 
THE  CEREBELLUM.    Natural  size.     (Sherrington  after  Horsley  and  Beevor.) 

The  hatched  and  stippled  parts  of  the  surface  shew  the  regions  of  the  cortex 
connected  with  movements  of  the  foot^  knee,  hip,  tail,  trunk,  and  7ieck 
respectively.  The  several  positions  of  the  areas  of  cortex  connected  with 
vision  and  smell  and  with  cutaneoiis  sensation  are  indicated  by  the  appro- 
priate words. 

The  plane  of  section  has  passed  through  the  corpus  callosum,  cc,  cc,  cc,  and  through 
the  anterior  commissure,  c,  sparing  the  left  pillar  of  the  fornix,  F ;  behind  it 
has  bisected  the  anterior  part  of  the  pons,  laying  open  the  aqueduct,  Aq. 
(iter  a  tertio  ad  quartum  ventriculum).  Pons,  the  left  half  of  the  pons  in 
frontal  section.     Op.  the  optic  commissure  cut  across. 

III.   the  root  of  the  third  cranial  nerve. 

Fli.  the  frontal  pole,  OC.  the  occipital  pole  ;  Cn.  the  cuneus,  Fen.  the  precu- 
neus ;  G.fn.  G.fn.  G-.  fn.  the  gyrus  fornicatus  ;  the  unlettered  fissure  seen 
to  form  tiie  upper  boundary  of  this  gyrus  in  its  supra-callosal  part  is  the 
callaso-marginal,  Po.  f.  the  parieto-occipital  fissure. 

muscle  along  the  axis  of  the  body  or  along  the  axis  of  a  limb 
and  the  relative  position  along  the  cerebrospinal  axis  of  the  nerve 
or  nerves  governing  the  muscle.  We  may  however,  adopting 
this  method,  note  that  the  sacral  and  lumbar  nerves  are  repre- 
sented by  the  most  mesial  portion  of  tlie  whole  motor  area  and 
by  the  hind  division  of  this  mesial  portion  ;  that  the  lum1)ar  and 
thoracic  nerves  are  represented  by  the  front  division  of  the  same 


748  MOVEMENTS   OE   CORTICAL   ORIGIN.     [Book  in. 

mesial  portion  ;  that  the  upper  thoracic  with  the  lower  cervical 
nerves  belong  to  a  region  lying  lateral  to,  and  the  upper  cervical 
nerves  to  one  lying  in  front  of  the  preceding  area ;  and  lastly 
that  the  remaining  lateral  and  ventral  portions  of  the  whole 
motor  region  appertain  to  the  cranial  nerves.  But  the  topo- 
graphical differentiation  does  not  come  out  so  clearly  by  this 
method,  as  by  that  of  taking  for  our  guide  distinctive  move- 
ments of  the  several  parts  of  the  body. 

It  will  be  observed  that  all  these  areas  taken  together,  repre- 
sented by  the  portion  of  Figs.  123,  124  shaded  in  one  way  or  an- 
other, occupy  chiefly  the  parietal  region  of  the  cerebral  surface 
though  they  also  reach  into  the  frontal  regioii.  Stimulation  of 
the  frontal  region  in  front  of  this  motor  area  or  of  the  occipital 
region  behind,  whether  on  the  lateral  or  on  the  mesial  surface, 
or  of  the  temporal  region,  whether  also  on  the  lateral  or  on  the 
mesial  surface,  or  of  the  gyrus  fornicatus  (Fig.  124)  connecting 
the  frontal  and  occipital  regions  on  the  mesial  surface,  and  run- 
ning ventral  to  the  marginal  gyrus,  does  not  give  rise  to  move- 
ments ;  or  to  be  more  exact,  does  not  give  rise  to  movements 
comparable  to  those  just  described  as  resulting  from  stimulation 
of  various  parts  of  the  motor  region.  Movements  do  take  place 
when  certain  parts  of  the  occipital  or  of  the  temporal  region  are 
stimulated,  but  these  are  not  only  less  striking  and  experiment- 
ally less  certain  than,  but  appear  to  be  of  a  different  nature  from 
those  resulting  from  stimulation  of  the  motor  region ;  it  will  be 
convenient  to  speak  of  the  nature  and  meaning  of  this  kind  of 
movement  when  we  come  to  discuss  the  development  of  sensations. 

§  485.  It  is  obvious  from  the  foregoing  that  the  mechanisms 
for  the  development  of  these  movements  of  cerebral  origin  are 
far  more  highly  differentiated  in  the  monkey  than  in  the  dog. 
But  even  in  the  monkey  (Macacus  and  allied  forms)  the  differ- 
entiation is  still  very  incomplete.  If  we  explore  for  instance  the 
area  for  the  wrist  we  find  that  its  limits  are  ill-defined.  In  some 
parts  of  the  area  we  obtain  movements  of  the  wrist  only,  but  in 
other  parts  of  the  area  stimulation  produces  not  only  movements 
of  the  wrist,  but  also  of  the  shoulder  or  of  the  digits,  or  of  the 
neck  ;  and  so  with  the  other  areas. 

If,  however,  not  a  Macacus  or  other  ordinary  monkey,  but  the 
more  highly  developed  ourang  otang  be  taken  as  the  subject  of 
experiments,  the  differentiation  is  found  to  be  distinctly  ad- 
vanced ;  the  several  areas  are  more  sharply  defined,  and  what  is 
important  to  note,  the  respective  areas  tend  to  be  separated  from 
each  by  portions  of  cortex,  stimulation  of  which  gives  rise  to  no 
movement  at  all. 

The  opportunities  of  stimulating  the  cortex  of  man  himself 
have  been  few  and  far  between,  and  have  for  the  most  part  been 
conducted  under  unfavourable  circumstances ;  but  so  far  as  the 
results  obtained  go,  they  shew  that  the  topographical  distribu- 


Chap,  ii.]  THE  BKAIK  749 

tion  of  areas  for  the  several  movements  is  carried  out  on  the  same 
plan  as  in  the  monkey  (we  are  purposely  confining  ourselves  now 
to  the  results  of  artificial  stimulation)  ;  and  moreover,  justif}"  the 
conclusion,  which  a  priori  reasons  would  lead  us  to  adopt,  that 
in  man  the  differentiation  is  advanced  still  farther  than  in  the 
monkey. 

Thus  when  we  survey  a  series  of  brains  in  succession,  from 
the  more  lowly  frog,  through  the  bird,  the  rabbit,  the  dog,  and 
other  lower  mammals  up  to  the  monkey,  the  anthropoid  ape,  and 
so  to  man  himself,  we  hnd  an  increasing  differentiation  of  the 
cerebral  cortex,  by  which  certain  areas  of  the  cortex  are  brought 
into  special  connection  with  certain  skeletal  or  other  muscles  in 
such  a  way  that  stimulation  of  a  particular  portion  of  the  grey 
matter  gives  rise  to  a  particular  movement  and  to  that  alone. 

§  486.  Before  proceeding  further,  it  will  be  perhaps  advan- 
tageous to  call  to  mind  some  of  the  important  features  of  the  great 
strand  of  fibres  known  as  the  pyramidal  tract.  These  fibres  start 
from  the  cerebral  cortex  of  the  motor  region  which  we  are  study- 
ing ;  they  are  the  axis  cylinder  processes  of  some  of  the  large 
pyramidal  cells  which  are  so  conspicuous  in  this  region  of  the 
cortex.  Passing  downwards  from  the  cortex  through  the  white 
matter  of  the  hemisphere  and  the  corona  radiata  they  are  gath- 
ered up  into  the  internal  capsule  between  the  nucleus  lenticularis 
on  the  one  side  and  the  nucleus  caudatus  with  the  optic  thalamus 
on  the  other  side  (Figs.  125, 126, 127).  The  internal  capsule  has 
the  form  of  a  fan,  the  handle  of  which  passes  into  tlie  cms  cerebri, 
while  the  expansion  stretches  into  the  hemisphere ;  it  is  further 
bent  into  a  rounded  angle  (Fig.  125,  G),  the  'knee,'  which  sepa- 
rates a  front  from  a  hind  limb.  The  fibres  of  the  pyramidal  tract 
occuj^y  the  knee,  a  small  adjoining  portion  of  the  front  limb  and 
a  large  part  of  tlie  hind  limb ;  but  it  must  be  remembered  that 
when  we  examine  the  internal  capsule  by  horizontal  sections 
taken  in  succession  from  the  dorsal  to  the  ventral  regions,  we 
find  that  the  knee  shifts  in  position  and  changes  in  the  width  of 
its  angle,  that  the  two  limbs  vary  in  direction,  in  size  and  in  shape, 
and  that  at  last  the  bent  flattened  capsule  passes  into  the  more 
or  less  rounded  crus  by  the  rapid  disappearance  of  the  fore  limb 
and  the  consequent  extinction  of  the  angle.  When  the  capsule 
has  thus  passed  into  the  crus,  or  rather  into  the  pes  of  the  crus, 
the  pyramidal  tract  is  found  occupying  the  median  region  of 
the  pes  (Fig.  128).  Farther  backward  the  fibres  of  the  tract  are 
found  running  in  the  pons  (Fig.  129,  130,  131),  in  strands  in- 
terwoven with  the  transverse  fibres  of  that  structure,  and  then 
issuing  from  the  pons  are  found  concentrated  again  into  the  an- 
terior pyramids  of  the  bulb  (Fig.  132).  At  the  decussation  of 
the  pyramids,  while  most  of  the  fibres  cross  over  to  form  the 
crossed  pyramidal  tract  of  the  spinal  cord  some  are  continued 
on  as  the  direct  pyramidal  tract,  which  however  speedily  dimin- 


750 


MOVEMENTS   OF   CORTICAL   OEIGIK     [Book  in. 


Fig.  125. 


Outline  of  Horizontal  section  of  Brain,  to  shew  the 
INTERNAL  CAPSULE.     (Natural  size.) 


The  section  is  taken  at  a  level  more  ventral  than  shevrn  in  Fig.  115.  The 
grey  matter  of  the  cortex  and  claustrum  is  left  unshaded,  but  that  of  the  corpus 
striatum  and  optic  thalamus  is  shaded. 

OT.  optic  thalamus,  shev^^ing  the  median,  lateral,  and  anterior  nuclei.  NL. 
nucleus  lenticularis,  shewing  the  putameu  large,  and  the  inner  division  of  the 
globus  pallidus  very  small.  NG.  nucleus  caudatus,  the  large  head  in  front  of, 
and  the  diminishing  tail  behind,  the  thalamus. 

6r.  the  knee  of  the  internal  capsule.  From  '  Eye  '  to  '  Dig,''  marks  the  posi- 
tion of  the  pyramidal  tract  as  a  whole,  and  the  several  letters  indicate  broadly 
the  relative  positions  of  the  several  constituents  of  the  tract,  named  according  to 
the  movements  with  which  they  are  concerned  ;  thus  Eiie  movements  of  the 
eyes  ;  Hd.  of  the  head  ;  Tcj.  of  the  tongue  ;  mtli.  of  tlie  mouth  ;  shl.  of  the  shoul- 
der ;  elh.  of  the  elbow ;  Dig.  of  the  hand ;  Ahd.  of  the  abdomen  ;  Hip,  of  the 
hip  ;  Kn.  of  the  knee  ;  Dig.  of  the  foot. 

8.  the  temporo-occipital  tract,  oc.  fibres  to  the  occipital  lobe.  Op.  optic 
radiation.  At  this  level  the  fibres  of  the  frontal  tract,  in  the  fore  limb  of  the 
capsule  in  front  of  the  pyramidal  tract,  run  almost  horizontally,  parallel  with 
the  plane  of  the  section.     Cf.  Fig.  126,  Fron. 


Chap,  ii.] 


THE  BEAIN. 


751 


cc.  the  rostrum  of  the  corpus  callosum,  Spl.  the  splenium  of  the  same,  both 
cut  across  horizontally.  The  thick  dark  line  indicates  the  boundary  of  the 
cavities  of  the  anterior  and  descending  horns  of  the  lateral  ventricle  and  of  the 
third  ventricle,  the  two  ventricles  being  laid  open  into  one  by  the  removal  of 
the  velum  and  choroid  plexus  &c.  The  oval  outline  in  the  fore  part  of  this  cavity 
indicates  the  fornix. 

Lateral  to  the  nucleus  lenticularis  is  seen  in  outline  the  claustrum,  the  cortex 
of  the  island  of  Reil  and  the  operculum  or  convolution  overlapping  the  island 
of  Reil. 

P  is  inserted  to  shew  which  is  the  hind  part  of  the  section. 

ishes  and  disappears,  through  fibres  leaving  it  to  cross  over  to  the 
other  side  by  the  anterior  commissure.  During  its  progress 
along  the  spinal  cord,  the  fibres  both  of  the  crossed  and  the 
direct  tract  end  by  coming  into  connection  with  the  cells  of  the 
anterior  cornu,  either  by  means  of  their  collaterals  or  by  their  act- 
ual terminations.  In  the  bulb  and  higher  up,  the  tract  gives  off 
fibres  which  crossing  over  to  the  other  side  make  connections  in 
a  similar  way  Avith  nerve  cells  whose  axis  cylinder  processes  are 
motor  fibres  in  the  cranial  nerves.  From  these  relations  of  the 
pyramidal  tract  it  is  obvious  that  the  fibres  of  this  tract  must  be 
concerned  in  the  development  of  the  movements  wliich  we  have 


ti^tl      dorsjum 


Fig.  126.    Outline  of  a  Sagittal  section  through  the  hemisphere.    ^lan, 

(Sherrington.) 

The  section  is  taken  not  far  to  the  right  of  the  median  plane  and  is  one  half 
linear  of  natural  size.  The  grey  matter  of  the  corpus  striatum  and  thalamus  is 
shaded. 

Nc,  iVc,  the  caudate  nucleus  ;  Ft,  the  putamen  and  (T/i,  the  globus  pallidus 
of  the  lenticular  nucleus  ;  OT.  the  optic  thalamus;  C/,  the  internal  capsule  with 
a  streaked  appearance  revealing  appi'oximately  the  direction  taken  by  librc- 
bundles  passing  into  it  from  the  portion  of  corona  radiata  over  it.  In  these  sets 
of  bundles  may  be  broadly  distinguished  a  frontal  system,  fvon.^  a  pyramidal 
system,  PF  (sub-divisible  into  cranial  [r?'nr?i.],  brachial  [fen/c/t.],  dorso-lumbar 
{dors.  Z^nu.],  and  lumbo-sacral  {lum.  sac],  parts)  and  a  temporo-occipital  system. 
sons. ;  the  situation  of  the  genu  of  the  internal  capsule  is  indicated  by  (j.  CB, 
the  eras  cerebri ;  Oc.  the  so-called  optic  radiations  passing  into  the  occipital  lobe  ; 
cc,  the  splcnial  end  of  the  corpus  callosum  ;  v,  r,  v,  the  lateral  ventricle  cut 
across  in  three  different  places  ;  F,  the  fornix  in  cross-section  ;  Op,  the  optic 
tract  in  ci'oss-section.     Tart  of  the  cerebellum  is  seen  in  outline  to  the  right. 


752 


MOVEMENTS   OF  CORTICAL  ORIGIN.     [Book  in. 


Fig.  127.    Outline  of  a  transverse  dorso-venxeal  section  of  the  right 
HALF  OF  THE  BRAIN.     (Natural  size.)     (Sherrington.) 

The  section  which  is  taken  at  the  level  of  the  knee  of  the  capsule  and  is 
therefore  intermediate  between  those  shewn  in  Figs.  116 — 117  is  introduced  to 
illustrate  the  course  of  the  constituents  of  the  pyramidal  tract. 

0.  T.  optic  thalamus  ;  JV.c.  nucleus  caudatus,  the  head  only  appears  in  this 
section.  Ft.  putamen,  Gp",  Gp'  the  two  parts  of  the  globus  pallidus  of  the 
nucleus  lenticularis ;  C.  the  claustrum  ;  C.£^.  the  extemial  capsule ;  /re.  the  island 
of  Reil.  c.a.,  the  anterior  commissure  shaded  to  render  it  distinct  and  the  fibres 
from  the  temporo-sphenoidal  lobe  which  pass  into  it  being  indicated  by  broken 
lines.  Op.  the  optic  tract ;  Ivd.  the  end  of  the  descending  horn  of  the  lateral 
ventricle ;  F.  the  fornix  ;  F'.  the  end  of  the  anterior  pillar  of  the  fornix  in  the 
base  of  the  thalamus  ;  c.c.  corpus  callosum ;  O.P.  anterior  part  of  the  occipital 
lobe. 

f.c.  is  the  central  fissure  or  fissure  of  Rolando.  The  course  of  the  fibres  of 
the  pyramidal  tract  connected  respectively  with  the  trunk,  leg  and  arm,  and 
hence  with  spinal  nerves,  and  of  those  connected  with  the  face  and  hence  with 
cranial  nerves,  is  sliewn  by  broken  lines.  These  are  all  seen  converging  into  the 
internal  capsule  C.I.  This  figure  should  in  respect  to  the  course  of  these  fibres 
be  compared  with  the  horizontal  section  shewn  in  Fig.  125,  and  the  sagittal 
figure  shewn  in  Fig.  120. 


Chap,  ii.] 


THE   BRAIK 


753 


just  described.  When  the  movements  are  brought  about  by 
stimuhxtion  of  the  fibres  in  some  part  of  their  course,  in  the  in- 
ternal cajjsule  for  instance,  there  can  be  no  doubt  that  the  stimu- 
lation starts  impulses  which,  travelling  down  the  tract  to  the 
origins  of  certain  cranial  or  spinal  nerves,  in  some  way  give  rise 
to  coordinate  motor  impulses  along  the  motor  fibres  of  the  nerves; 
and  we  may  with  reason  speak  of  the  impulses  then  passing  along 
the  tract  as  nrotor  or  efferent  in  nature.  When  the  stimulus  is 
applied  direct  to  the  cortex,  we  may  assume  that  processes, 
started  in  the  grey  matter,  eventuate  in  similar  efferent  im- 
pulses along  the  fibres  of  the  tract.  All  the  evidence  leads  us 
to  regard  this  tract  as  an  efferent  tract. 

When  the  spinal  cord  is  divided  in  the  lower  dorsal  region 
and  the  electrodes  of  an  electrometer  are  brought  into  connec- 


FlG.    128.       TlIKOUGH   THE    CrUS   AND    ANTERIOR   CoKPORA    QuADRIGEMIXA. 

(One  half  only  is  shewn.)     (Shemngton.) 
(In  the  line  114,  Fig.  108.) 

Py.  the  pyramidal  portion  of  the  pes.  Fr.  the  region  of  the  pes  occupied  by 
fibres  from  the  frontal  portion  of  the  cortex.  Pr.  0.  the  region  occupied  by 
fibres  coming  from  the  occipital  portion  of  the  cortex,  y.  fibres  coming  from  the 
fillet.  Op.  the  optic  ti-act.  F.  the  fillet,  I.  the  lateral  portion,  m.  the  median 
portion.  I.  the  posterior  longitudinal  bundle.  B.  a.  the  brachium  of  the  anterior 
corpus  quadrigeminum.  x.  fibres  from  the  posterior  commissure  of  the  cerebrum. 
r.  raphe.  S.  n.  substantia  nigra.  P.  n.  red  nucleus.  C.  (j.  I.  lateral,  and  C.  g.  m. 
median  corpus  geniculatum.  Pvr.  pulvinar  of  optic  thalanms.  A.  Q.  7i.  nucleus 
or  grey  matter  of  anterior  corpus  quadrigcminmn.  III.  n.  nucleus  of  III.  third 
nerve.  Ill',  i-ootlets  from  the  dorsal  part  of  III.  7i.  the  nucleus  of  the  third 
nerve  which  cross  the  median  line  to  emerge  with  rootlets  derived  from  the 
nucleus  of  the  opposite  siilc.  s.  m.  superficial  layer  of  fibres  of  the  ant.  corp. 
quad.  d.  m.  deep  layer.  V.  d.  descending  root  of  the  fifth  nerve.  Aq.  aqueduct 
surrounded  by  cerebral  grey  matter. 

tion  with  the  transverse  cut  surface  and  with  some  point  of  the 
longitudinal  surface  above,  the  electrometer  gives  evidence  of 
currents  of  action  (manifested  as  negative  variations  of  a  demar- 

48 


754 


MOVEMENTS    OF   COETICAL   ORIGIN.     [Book  in. 


cation  current  or  current  of  rest,  §  64)  whenever  the  motor  area 
of  the  hind  limb  is  stimulated,  but  not  when  other  parts  of  the 
cortex  are  stimulated.  We  have  already  said  that  stimulation 
of  any  part  of  the  motor  region  may  under  abnormal  conditions 
give  rise  to  general  epileptiform  convulsions ;  when  these  occur 
during  such  an  experiment  as  the  above,  currents  of  action  mani- 
fest themselves  in  the  lower  dorsal  cord,  whether  the  stimulation 
giving  rise  to  the  convulsions  be  applied  to  the  area  for  the  hind 
limb  or  to  any  part  of  the  motor  region.  It  has  been  further 
observed  that  the  currents  of  action  developed  within  the  spinal 
cord  tally  in  a  very  exact  manner  with  the  muscular  movements. 
The  convulsions  begin  with  a  sustained  '  tonic '  contraction  of 
the  muscles,  and  the  electrometer  shews  a  similar  sustained  cur- 
rent of  action ;  this  is  followed  by  rhythmic  movements  of  the 


Fig.   129.     Through  the  Fore  Part  of  the  Pons.     (Sherrington.) 
(In  the  line  113,  Fig.  108.) 

Py.  Pyramidal  fibres.  F.  G.  Fibres  from  the  frontal  cortex.  S.  P.  Superior 
Peduncle  of  the  cerebellum.  F.  m.  median  portion,  F.  I.  lateral  portion  of  the 
Fillet.  I.  posterior  longitudinal  bundles.  P.  G.  Q.  Posterior  corpora  quadri- 
gemina.  y.  Fibres  which  become  detached  from  the  Fillet,  and  further  forward 
from  (the  innermost)  part  of  the  Pes  of  the  Crus.  I.  c.  locus  caeruleus.  n.  P.  Q. 
nucleus  of  the  posterior  corpora  quadrigemina ;  the  outline  is  made  too  sharp. 
IV.  bundles  of  the  fourth  nerve  decussating,  IV.  n.  its  nucleus.  V.  d.  descending 
root  of  the  fifth  nerve.    Aq.  the  aqueduct,     c.  g.  the  region  of  central  grey  matter. 


muscles,  accompanied  by  corresponding  rhythmic  movements  of 
the  mercury  of  the  electrometer.  Without  insisting  too  much 
on  the  exact  interpretation  of  these  results  we  may  take  them  as 
at  least  shewing  that,  when  the  motor  region  of  the  cortex  is 


Chap,  ii.] 


THE  BRAIK 


756 


excited,  nervous  impulses  accompanied  by  '  currents  of  action ' 
pass  downward  along  the  fibres  of  the  p}a-amidal  tract. 

The  results  of  stimulating-  the  fibres  of  the  tract  in  their 
course  through  the  corona  radiata  and  the  internal  capsule  and 


tpp  anp 


Fig.  130.     Through  the  Pons  at  the  exit  of  the  Fifth 

Nerve.     (Sherriugton.) 

(In  the  line  112,  Fig.  108.) 

C.  B.  Remains  of  restiforni  body.  S.  P.  superior  peduncle  of  the  cerebellum, 
F.  in.  median,  F.  I.  lateral  Fillet.  T.  B.  tegmental  reticular  formation,  tr.  P. 
superficial  transverse  fibres  of  the  Pons.  I.  posterior  longitudinal  bundles.  V.  s. 
superior  verniix  ;  sections  of  three  folia  are  shewn,  one  being  detached  ;  between 
them  the  intervening  sulci  laid  open  by  the  section  are  seen.  VL  a.  valve  of 
Vieussens  or  anterior  velum,  r.  raphe.  Py.  Pyramidal  fibres,  r/r.  P.  gi"ey  mat- 
ter of  the  Pons.  s.  o.  superior  olive,  t.  placed  on  the  left  side  indicates  the  posi- 
tion of  a  bundle  of  longitudinal  fibres  which  may  be  traced  forward  into  the 
subthalamic  regions.  V.  m.  motor  nucleus,  V.  s.  sensory  nucleus,  and  V.  roots 
of  the  fifth  nerve. 

4th.  fourth  ventricle  ;  shading  of  central  grey  matter  omitted  as  in  Fig.  131. 

the  results  obtained  by  studying  the  degenerations  following 
upon  injury  to  or  removal  of  the  several  parts  of  the  cortical 
motor  region,  agree  in  marking  out  the  paths  taken  by  the 
several  constituents  of  the  tract  throuerh  the  central  white  mat- 


756 


MOVEMENTS  OF  COETICAL  ORIGIN.     [Book  hi. 


ter  of  the  hemisphere,  the  corona  radiata  and  the  capsule.  Com- 
paring Figs.  123,  124  with  Figs.  125, 126  and  127  it  will  be  seen 
that  the  portion  of  the  tract  destined  for  the  cranial  nerves,  and 
for  the  movements  of  the  eyes,  the   mouth,  face,  tongue, 


so 


pharynx  and  larynx,  starting  from  the  ventral  parts  of  the  more 


tp.P  gp.P 


Fig.   131.     Through  the  widest  part  of  the  Fourth 
Ventricle.     (Sherrington.) 

Taken  in  the  line  111,  Fig.  108. 

Py.  Pyramidal  fibres  cut  transversely,  tr.  P.  the  superficial  (ventral)  trans- 
verse fibres  of  the  pons.  The  shaded  part  of  the  pons  {gr.  P.)  indicates  grey 
matter  mingled  with  the  deeper  transverse  fibres.  F.  the  fillet.  Tp.  the  tra- 
pezium. G.  B.  the  restiform  body  or  inferior  peduncle  of  the  cerebellum,  cut 
across  obliquely.  S.  P.  the  superior  peduncles  of  the  cerebellum,  r.  raphe. 
s.  o.  superior  olive.  C  B.  corpus  dentatum  of  the  cerebellum.  Bf.  n.  the  nucleus 
of  the  roof.  s.  (j.  tubercle  of  Rolando.  V.  S.  section  through  sulcus  in  the  vermis 
superior  of  the  cerebellum,     t.  bundle  from  the  olive  to  the  lenticular  nucleus. 

VIII.  the  eighth  or  auditory  nerve,  its  ventral  or  vestibular  root,  proceeding 
from  VIII.  (i.  the  front  part  of  the  lateral  auditory  nucleus.  VII.  n.  the  nucleus 
of  the  seventh  or  facial  nerve.  VI.  the  nucleus  of  the  sixth  nerve.  VII.  g. 
fibres  of  the  seventh  nerve  cut  across  as  they  sweep  round  the  nucleus  of  the 
sixth  before  issuing  from  the  pons  as  VII. 

4th.  the  fourth  ventricle,  here  roofed  in  by  the  cerebellum  ;  the  shading  of 
the  central  grey  matter  immediately  surrounding  the  ventricle  is,  for  the  sake  of 
eimplicity,  omitted. 


Chap,  ii.] 


THE   BRAIN. 


757 


frontal  district  of  the  motor  region,  take  up  their  position  at  the 
knee  of  the  internal  capsule  ;  and  the  portion  destined  for  those 
upper  cervical  nerves  which  carry  out  movements  of  the  head 
through  the  muscles  of  the  neck,  starting  from  the  extreme 
frontal  and  dorsal  parts  of  the  area,  is  also  apparently  directed 
to  the  knee  of  the  capsule.  The  rest  of  the  tract,  starting  from 
the  part  of  the  area  lying  at  once  behind  and  mesial  to  the  above, 
occupies  in  the  capsule  a  position  posterior  to  them  in  the  hind 
limb  of  the  capsule  ;  and  it  will  be  observed  that  the  tract  for 
the  fore  limb  which  begins  on  the  surface  lateral  of  the  tracts 
for  the  trunk  and  hind  limb,  shifts  its  course  in  i-elation  to  theirs, 
so  that  in  the  capsule  it  is  in  front  of  them,  not  lateral  to  them. 


n.f.t. 


Fig.  132.     Through  the  Bulb  just  behind  the  Pons.     (Sherrington.) 
Taken  in  the  line  110,  Fig.  108. 

Py.  Pyramids.  Ii.  Restiform  Body.  Cbni.  cerebellum.  F.  Fillet.  /.  a.  e. 
external,  /.  a.  i.  internal  arcuate  fibres,  t.  bundle  of  fibres  from  olive  to  the 
lenticular  nucleus.  1.  posterior  longitudinal  bundles,  n.  f.  t.  nucleus  of  fasci- 
culus teres,  s.  o.  superior  olive,  n.  c.  e.  nucleus  centralis  (the  marks  within  it 
are  sections  of  bundles  of  fibres  by  which  it  is  traversed),  s.  g.  substance  of 
Rolando. 

V.  a.  ascending  root  of  fifth  nerve.  VII.  n.  nucleus  of  the  7th  nerve.  YIII. 
auditory  nerve,  chiefly  the  dorsal  or  cochlear  root ;  VIII.  a.  medium  nucleus,  VIII. 
/'^.lateral  nucleus,  VIII.  j.  accessory  nucleus  of  auditory  nerve.  IX.  fibres  of 
root  of  ninth  nerve  passing  through  ascending  root  of  fifth  nerve. 


It  may  further  be  observed  that  while  in  the  tracts  for  the  trunk 
and  hind  limb  the  same  fore  and  aft  order  which  obtains  on 
the  surface  is  reproduced  in  the  capsule,  even  apparently  to  the 
strange  precedence  of  the  ankle  over  the  knee,  the  order  of  the 
several  elements  in  the  fore  limb  tract  which  is  lateral  on 
the  surface  becomes  regularly  fore  and  aft  in  the  ca[)sule.  In 
the  capsule  the  several  elements  are  arranged  in  a  linear  order, 
corresponding  broadly  to  that  of  the  distribution  of  the  muscles 
along  the  longitudinal  axis  of  the  body ;  on  the  cortex  they  are 
disposed  in  an  order  the  cause  of  which  is  at  present  not  very 


758  MOVEMENTS  OF  CORTICAL  OEIGIN.     [Book  hi. 

clear,  but  which  is  probably  determined  by  the  respective  rela- 
tions of  the  several  parts  of  the  motor  region  to  the  functional 
activity  of  the  other  parts  of  the  cortex.  In  the  shifting  from 
the  one  order  to  the  other,  the  several  constituent  fibres,  as  we 
have  said,  describe  a  somewhat  peculiar  course ;  and  when  we 
remember,  that  the  order  shewn  in  Fig.  125  is  only  the  order 
obtaining  at  one  particular  level  of  the  capsule,  and  that  from 
the  dorsal  beginnings  of  the  capsule  in  the  corona  radiata  to  its 
ventral  end  in  the  pes,  the  capsule  is  continually  changing  in 
form,  and  its  fibres  therefore  are  continually  shifting  their  relations 
to  each  other,  the  whole  course  of  the  several  fibres  of  the  tract 
from  their  origin  in  the  cortex  until  they  are  gathered  up  into 
the  central  portion  of  the  pes  (Fig.  128  Py^  must  be  a  very 
complicated  one. 

When  the  area  of  one  hemisphere  is  stimulated,  the  move- 
ment which  results  is  in  most  cases  seen  on  the  other  side  of 
tlie  body,  and  on  that  other  side  alone.  Thus  when  the  area 
for  the  fore  limb  is  stimulated  on  the  left  hemisphere  it  is  the 
right  fore  limb  which  is  moved.  This  is  in  accordance  with 
what  we  have  learnt  of  the  pyramidal  tract  and  its  ulti- 
mate entire  decussation  before  it  reaches  the  motor  nerves, 
the  decussation  either  occurring  massively  as  in  the  case 
of  the  crossed  pyramidal  tract,  or  in  a  more  scattered  man- 
ner along  the  upper  part  of  the  spinal  cord  in  the  case  of 
the  direct  pyramidal  tract ;  and,  as  we  have  seen,  there  is  a 
similar  decussation  for  such  part  of  the  pyramidal  tract  as  is 
connected  with  the  cranial  nerves  above  the  decussation  of  the 
pyramids.  Except  in  the  case  of  certain  areas  for  movements 
naturally  bilateral  of  which  we  shall  speak  presently,  the  move- 
ment is  normally  on  the  crossed  side,  and  on  the  crossed  side 
only.  Under  abnormal  conditions  however  the  limb  of  the 
other  side,  that  is  of  the  same  side  as  the  hemisphere  stimulated, 
may  move  also.  But  such  an  abnormal  movement  of  tlie  same 
side  has  not  the  same  characters  as  the  proper  movement  of  the 
crossed  limb.  Instead  of  being  an  orderl}^  coordinate  movement, 
it  is  a  more  simple,  either  tetanic  or  perhaps  tonic,  or  rhythmic, 
clonic,  contraction  of  the  muscles.  Obviously  its  mechanism  is 
of  a  different  nature  from  that  by  which  the  proper  movement 
of  the  crossed  limb  is  effected ;  but  it  is  important  to  bear  in 
mind  that  a  movement  of  the  uncrossed  limb  may  take  place  ; 
and  further  that,  the  abnormal  conditions  continuing,  similar 
movements  of  an  uncoordinated  character  may  spread  to  the 
hind  limb  and  other  parts  of  the  crossed  side,  though  the  stimu- 
lation be  still  confined  to  the  arm  area,  then  to  other  parts  of 
the  uncrossed  side,  until  as  we  have  said  the  whole  body  is 
thrown  into  epileptiform  convulsions.  This  feature  must  not 
be  forgotten.  In  fact  it  may  be  fairly  insisted  upon  that  while 
we  may  speak  of  a  particular  coordinate  movement  as  being  the 


Chap,  ii.]  THE   BRAIK  759 

normal  outcome  of  an  ordinary  careful  stimulation  of  a  particu- 
lar area  in  a  normal  condition,  it  is  no  less  true  that  diffuse 
uncoordinated  movements,  culminating-  in  general  epileptiform 
convulsions,  are  the  natural  outcome  of  the  stimulation  of  any 
area  in  an  abnormal  condition.  And  in  attempting  to  form  any 
opinion  of  the  nature  of  the  first  act,  we  must  bear  the  second 
in  mind. 

As  we  said  above,  tlie  movements  resulting  from  cortical 
stimulation  are  most  conveniently  described  in  terms  of  parts  of 
the  body,  of  the  arm,  of  the  thumb,  of  the  tongue,  &c.  The 
movements  of  the  same  part  may  be  further  distinguished  by 
means  of  the  nomenclature  usually  adopted  in  speaking  of  mus- 
cular movements,  such  as  flexion,  extension,  abduction,  adduc- 
tion, &c.;  so  that,  within  the  area  bearing  the  name  of  some 
particular  part,  such  as  the  Avrist  for  instance,  we  have  to  distin- 
guish an  area  for  the  flexion,  and  another  for  the  extension  of 
that  joint ;  and  in  like  manner  in  reference  to  other  parts.  But 
it  will  be  readily  understood  that  it  is  easier  to  map  out  the 
area  for  a  particular  part  than  to  distinguish  the  areas  corre- 
sponding to  the  several  movements  of  that  part.  Hence  the 
nomenclature  usually  adopted  in  speaking  of  the  motor  region 
is  one  based  on  the  parts  of  the  body  moved  rather  than  on 
the  character  of  the  movements.  The  more  closely  however 
the  movements  in  question  are  studied,  the  more  probable  it 
appears  that  the  localisation  which  obtains  in  the  coi'tex  is 
essentially  a  localisation  corresponding  not  to  parts  of  the 
body,  or  to  nerves,  or  to  muscles,  but  to  movements.  In  con- 
sidering this  point  it  must  be  remembered  how  rude  and  bar- 
barous a  method  of  stimulation  is  that  of  applying  electrodes 
to  the  surface  of  the  grey  matter  compared  with  the  natural 
stimulation  which  takes  place  during  cerebral  action  ;  the  one 
probably  is  about  as  much  alike  the  other,  as  is  striking  the  kej'S 
of  a  piano  at  a  distance  with  a  broomstick  to  the  execution  of  a 
skilled  musician.  Were  it  in  our  power  to  stimulate  the  cortex 
in  any  way  at  all  approaching  the  natural  method,  we  should  in 
all  probability  arrive  at  two  results  ;  on  the  one  hand  we  should 
be  able  to  produce  at  will  a  variety  of  movements  of  different 
degrees  of  complexity,  some  very  simple,  others  very  complex, 
and  for  these  we  should  have  to  use  names  suggested  by  the  char- 
acters and  purpose  of  each  movement,  and  by  these  alone ;  on 
the  other  hand  we  should  find  very  decided  limits  to  the  num- 
ber and  kind  of  movements  which  we  could  evoke,  limits  fixed 
in  the  case  of  each  subject  partly  by  inherited  organisation, 
partly  by  the  training  of  the  individual. 

Some  such  results  of  refined  experimentation  are  indeed  al- 
ready foreshadowed  by  the  rude  results  of  our  present  rough 
methods.  The  movements  which  usually  follow  stimulation  of 
the  motor  region,  and  which  we  have  described  as  flexion,  &c.. 


760  MOVEMENTS  OE  CORTICAL  ORIGIK     [Book  hi. 

are,  so  to  speak,  the  elementary  factors  of  ordinary  bodily  move- 
ments, the  detaclied  and  imperfect  chords  of  a  musical  piece  ;  and 
in  the  following  facts  relating  to  their  production  we  can  recog- 
nize the  influences  of  organisation  and  habit.  As  we  have  said, 
stimulation  of  the  motor  area  of  one  hemisphere  produces  move- 
ments, as  a  rule,  which  are  limited  to  one  side  of  the  body,  and 
that  the  opposite  side.  Now  both  in  ourselves  and  in  the  higher 
animals  a  large  number  of  bodily  movements,  especially  of  the 
limbs,  are  habitually  unilateral ;  and,  putting  aside  the  question 
why  there  should  be  two  halves  of  the  brain,  and  why  the  one 
half  of  the  brain  should  be  associated  with  the  cross  half  of  the 
body,  we  may  recognize  in  them  unilateral  crossed  movements 
resulting  from  stimulation  of  the  cortex  in  accordance  with  natu- 
ral habits.  But  some  movements  of  the  body  are  ordinarily 
bilateral;  the  two  eyes,  for  instance,  are  ordinarily  moved 
together,  and  the  two  sides  of  the  trunk  move  together  very 
much  more  frequently  than  do  the  two  fore  limbs  or  the  two 
hind  limbs.  And  in  accordance  with  this  we  find  that  stimula- 
tion of  the  motor  area  for  the  eyes  on  either  hemisphere  produces 
movements  of  both  eyes,  and  stimulation  of  the  trunk  area  of 
one  hemisphere  is  also  very  apt  to  produce  bilateral  action  of  the 
trunk  muscles ;  in  such  instances  the  movements  on  both  sides 
are  quite  normal  movements.  We  may  incidentally  remark 
that  removal  of  the  trunk  area  leads  to  a  good  deal  of  bilateral 
degeneration,  that  is,  to  degeneration  of  strands  in  the  pyramidal 
tracts  of  both  sides,  whereas  such  a  bilateral  degeneration  is  com- 
paratively scanty  after  removal  of  the  leg  or  arm  area. 

That  it  is  the  movement  and  not  the  part  moved  which  is,  so 
to  speak,  represented  on  the  cortex  is  further  shewn  by  the  rela- 
tive magnitudes  of  the  several  cortical  areas  when  they  are 
mapped  out  according  to  parts  of  the  body.  The  area  for  the 
arm,  for  instance,  cf.  Figs.  123,  124,  is,  so  to  speak,  enormous 
compared  to  that  of  the  trunk  when  the  relative  bulks  of  these 
two  parts  of  the  body  are  considered ;  and  within  the  arm  area 
itself  the  space  occupied  by  the  thumb  and  forefinger  and  digits 
is,  bulk  for  bulk,  out  of  proportion  to  the  space  allotted  to  the 
shoulder ;  so  also  the  area  for  the  eyes  or  for  the  mouth  is  out 
of  proportion  to  the  size  of  those  organs.  But  these  relative 
sizes  of  the  respective  areas  become  intelligible  when  we  bear  in 
mind  relative  mobility,  nimbleness  and  delicacy  of  execution ; 
in  these  respects  the  shoulder  is  far  behind  the  thumb,  while  the 
eyes  and  mouth  surpass  most  other  parts  of  the  body. 

We  are  brought  yet  a  step  further  when  we  compare,  in 
respect  of  the  cortical  motor  region,  animals  of  different  grades 
of  organisation ;  and  the  results  thus  obtained  lead  us  to  the 
conclusion  that  the  motor  region  is  correlated  not  to  movements 
in  general,  but  to  movements  of  a  particular  kind.  Taking  in 
series  the  rabbit,  the  dog,  the  monkey  and  man,  we  find  in  pass- 


Chap,  ii.] 


THE  BRAIK 


761 


ing  from  one  to  the  other,  an  increase  in  prominence  and  in 
differentiation  of  the  motor  region  accompanied  by  an  increase 
in  the  bulk  of  the  j^yramidal  tract ;  among  the  many  striking 
differences  between  the  brains  of  these  several  animals,  these  two 
features,  the  increasing  complexity  of  the  motor  region,  and  the 
increasing  size  of  the  pyramidal  tract,  are  among  the  most  strik- 
ing. The  size  of  the  pyramidal  tract  is  itself  correlated  to  the 
complexity  of  the  motor  region,  and,  being  the  more  easily 
determined,  may  be  used  as  indicating  both ;  the  difference  in 
the  size  of  the  pyramidal  tract  in  these  animals  is  seen  all  along 
the  whole  length  of  the  cord  (Fig.  133).  Now  as  regards  mere 
quantity  of  movement,  if  we  may  use  such  an  expression,  the 
differences  between  these  animals  are  of  no  great  moment.  If 
we  were  to  take  the  amount  of  energy  expended  as  movement 
in  twenty-four  hours  per  gramme  of  muscle  present  in  the  body 
in  each  of  the  four  cases,  we  should  certain!}^  not  find  any  cor- 


Py.d 


Fig.  133.     Diagram  to  illustrate  the  Relative  Size  of  the  Pyramidal 
Tract  in  the  Uog,  Monkey  and  Man.     (^lierrington.) 

The  figure  shews  in  outline  the  lateral  half  of  the  cord,  at  the  level  of  the 
fifth  thoracic  nerve,  in  A.  Man,  B.  Monkey,  C.  Dog ;  ui  is  a  reproduction  of  Z)^  in 
Fig.  114  ;  B  and  C  are  drawn  of  the  same  size  as  ^4.  Pi/.,  shaded  obliquely,  the 
pyramidal  tract ;  the  depth  of  shading  indicates  that  the  tract  is  more  crowded 
with  true  pyramidal  fibres  as  well  as  larger  in  A  than  in  7>,  and  in  B  than  in  C. 
In  B,  Py'  is  an  outlying  portion  of  the  pyramidal  tract  separated  from  the  rest 
by  the  cerebellar  tract.  Py.d.  the  direct  pyramidal  tract,  present  in  man  only. 
The  grey  matter  seems  relatively  large  in  C  because  the  section  was  taken  from 
a  very  young  puppy. 

respondence  between  that  and  the  size  of  the  pyramidal  tract. 
If  however  we  take  a  particular  kind  of  movement,  what  we  may 
perhaps  call  skilled  movement,  that  is  movement  carried  out  by 
means  of  intricate  changes  in  the  central  nervous  system,  Ave  do 
find  a  remarkable  parallelism  in  the  above  cases  between  the 
amount  of  such  skilled  movement  entering  into  the  daily  life  of 
the  individual  and  the  size  of  the  pyramidal  tract.  In  these  two 
respects  man  is  much  above  the  monkey,  and  the  monkey  far 
above  the  dog.  We  may  conclude  then  that  the  cortical  motor 
region  is  in  some  way  especially  concerned  Avith  the  kind  of 
movement  Avhich  we  haA'e  called  'skilled.' 


762  REMOVAL   OF  CORTICAL   AREAS.       [Book  hi. 

§  487.  These  skilled  movements  are  to  a  large  extent, 
though  not  exclusively,  voluntary  movements.  We  have  in  a 
previous  section  seen  reason  to  believe  that  the  cerebral  cortex 
is  in  some  way  especially  associated  with  the  development  of 
voluntary  movements.  Putting  together  this  conclusion  and 
the  conclusions  just  arrived  at  we  are  naturally  led  to  the 
further  conclusion  that  the  cortical  motor  region,  with  the 
pyramidal  tract  belonging  to  it,  plays  an  important  part  in  carry- 
ing out  voluntary  movements.  Do  other  facts  support  this 
view,  and  if  so,  what  light  do  they  throw  on  the  question  as 
to  what  part  and  what  kind  of  part  the  motor  region  thus 
plays  ? 

In  this  connection  we  naturally  desire  to  know  what  are  the 
results  of  removing  from  an  otherwise  intact  animal  the  whole 
motor  region,  and  more  especially  this  or  that  particular  por- 
tion of  it.  Before  proceeding  further,  however,  we  may  once 
more  call  attention  to  the  caution  given  in  §  457,  and  repeated 
in  §  476 ;  indeed  when  we  consider  the  high  organization  and 
complex  functions  which  obviously  belong  to  the  cortex,  when 
we  bear  in  mind  that  it  appears  to  govern,  and  must  therefore 
be  bound  by  close  ties  to  almost  all  the  rest  of  the  central  ner- 
vous system,  we  must  be  prepared  to  find  after  removing  a  por- 
tion of  cortex  that  the  pure  '  deficiency '  phenomena,  those 
which  result  from  the  mere  absence  of  a  piece  of  the  cortex, 
are  largely  obscured  by  the  other  effects  of  the  operation. 

In  the  rabbit  the  results  have  been  almost  purely  negative. 
When  in  this  animal  the  part  of  the  cortex  which  may  be  con- 
sidered as  the  motor  region  is  removed,  nothing  remarkable  is 
observed  in  the  movements  of  the  animal.  We  can  hardly  sup- 
pose that  the  operations  of  the  central  nervous  system  are  the 
same  in  an  operated  as  in  an  intact  animal,  and  the  differences 
indu(  ed  ought  to  be  betrayed  by  the  movements  of  the  body ; 
but  at  present  they  have  escaped  observation. 

In  the  dog  the  removal  of  an  area  is  followed  by  a  loss  or 
diminution  of  voluntary  movement  in  the  corresponding  part  of 
the  body.  When,  for  instance,  the  area  for  the  fore  limb  is 
removed  from  the  left  hemisphere,  the  right  fore  limb  is  com- 
pletely or  partially  '  paralyzed.'  In  carrying  out  its  ordinary 
movements  the  operated  animal  makes  little  or  no  use  of  its 
right  fore  limb.  Bnt  this  state  of  things  is  temporary  only. 
After  a  while  the  animal  regains  power  over  the  limb,  and  in 
successful  cases  recovery  is  so  complete  that  it  is  impossible  to 
point  out  in  the  limb  any  appreciable  deviation  from  the  nor- 
mal use.  And  careful  examination  after  death  has  shewn  not 
only  that  the  area  had  been  wholly  removed,  but  also  that  there 
was  no  regeneration  of  the  lost  parts ;  the  removal  of  the  cor- 
tex leads  in  such  cases,  as  usual,  to  degeneration  of  the  corre- 
sponding  strand  in  the  pyramidal  tract  right  away  from  the 


Chap,  ii.]  THE   BRAIK  7G3 

cerebral  surface  to  the  endings  of  the  strand  in  the  cervical  and 
dorsal  spinal  cord.  Nor  can  it  be  urged  in  such  cases  tliat 
diffused  remnants  of  the  arm  area  had  been  left  in  the  remain- 
ing parts  of  the  motor  region ;  for  the  whole  motor  region  has 
been  removed,  and  yet  the  animal  has  recovered  to  such  an 
extent  that  a  casual  observer  could  detect  no  differences  between 
the  movements  of  the  two  sides  of  the  body.  Closer  examina- 
tion did  disclose  certain  imperfections  of  movement ;  but  tlie 
operation  had  involved  injury  to  or  produced  changes  in  struc- 
tures other  than  the  motor  region,  and  the  imperfections  might 
have  been  due  to  the  additional  damage.  Nor  can  it  be  urged 
that,  in  such  a  case,  where  one  side  is  removed,  the  remaining 
hemisphere  takes  on  double  functions  ;  for  the  greater  part  of 
the  motor  areas  have  been  removed  on  both  sides,  and  yet  the 
animal's  movements  have  been  so  far  apparently  complete  that 
a  casual  observer  would  see  nothing  strange  in  them.  Again, 
the  Avhole  motor  region  has  been  removed  from  one  hemisphere 
in  a  young  puppy,  and  some  time  later  when  the  movements 
seemed  to  have  recovered  their  normal  condition,  the  removal 
of  the  motor  region  of  the  other  hemisphere  has  produced 
merely  a  paralysis  of  the  crossed  side  of  the  body,  and  that  as 
before  only  of  a  temporary  character. 

Two  things  have  to  be  noted  here.  In  the  first  place  the 
removal  of  an  area  does  affect  the  movements  which  are 
brought  about  by  stimulating  that  area,  it  leads  to  their  dis- 
appearance or  at  least  to  great  diminution  of  them  ;  and  this 
affords  an  additional  argument  that  the  connection  between  the 
area  and  the  movement  is  a  real  and  important  one.  In  the 
second  place,  the  physiological  effect  is  temporary  only,  though 
the  anatomical  results  of  the  operation  are  permanent,  for  the 
cortex  is  never  renewed,  and  the  pyramidal  tract  degenerates 
along  its  whole  length,  never  to  be  restored ;  this  shews  that 
we  have  to  deal  here  with  events  of  a  very  complex  character. 
Wlien  a  particular  movement  results  from  stimulation  of  the 
appropriate  cortical  area,  we  may  be  sure  that  whatever  takes 
place  in  the  cortex  and  along  the  p3a'amidal  tract,  motor  im- 
pulses, duly  coordinated,  pass  along  certain  anterior  roots  to 
certain  muscles;  and  we  know  that  if  we  removed  a  sufficient 
length  of  each  of  those  anterior  roots  that  particular  movement 
would  be  lost  for  tlie  rest  of  tlie  life  of  the  individual.  We  may 
therefore  infer  that  the  events  Avhich,  whatever  be  their  exact 
nature,  talking  place  in  the  cortex  and  along  the  pyramidal 
tract  lead  ultimately  to  the  issue  of  motor  impulses  along  the 
anterior  roots,  differ  essentially  from  the  events  attending  the 
transmission  of  ordinary  motor  impulses. 

In  the  case  of  the  monkey,  the  results  of  removing  parts  of 
the  cortical  inotor  region  have  not  been  so  accordant  as  in  tlie 
case  of  the  dog.     The  two  animals  agree  perfectly  in  so  far  that 


764  COETICAL   MOTOE   EEGIOX   I:^T   MAX.     [Book  in. 

the  removal  of  a  particular  area  leads,  as  an  immediate  result, 
to  the  loss  of  the  corresponding  movement ;  but  while  in  some 
instances  recover}^  of  the  movement  has  in  tlie  monkey  as  in 
the  dog  after  a  while  taken  place,  in  other  instances  the  '  paral- 
ysis '  has  appeared  to  be  permanent.  A'S  a  rule  the  paralysis 
caused  by  a  large  lesion  is  not  only  more  extensive,  but  also 
of  longer  duration  than  that  caused  by  a  small  one ;  and  natural 
bilateral  movements,  as  of  the  eyes,  reappear  earlier  than  uni- 
lateral movements.  The  facts  however  within  our  knowledge 
relating  to  the  permanence  of  the  effect  are  neither  numerous 
nor  exact  enough  to  justify  at  present  a  definite  conclusion. 
On  the  one  hand  the  positive  cases  where  recovery  has  taken 
place  are  of  more  value  than  the  negative  ones,  since  in  the 
latter  the  recovery  may  have  been  hindered  by  concomitant 
events  of  a  nature  which  we  may  call  accidental ;  and  it  is  at 
least  a  j^riori  most  unlikely  that  the  pyramidal  tract  mechanism, 
if  we  may  use  the  expression,  though  it  maj^  differ  in  the  mon- 
key and  the  dog  in  degree  of  development,  differs  so  essentially 
in  kind  that  damage  of  it  leads  in  the  one  case  to  permanent, 
and  in  the  other  to  mere  temporary  loss  of  function.  We  may 
add  that  we  should  further  expect  to  meet  in  the  monkey  with 
more  prominent  and  more  lasting  complications  due  to  the  sub- 
sidiary effects  of  the  operation,  and  it  may  be  doubted  whether 
in  any  of  the  recorded  experiments  the  animal  has  been  allowed 
to  live  a  sufficient  time  for  these  subsidiar}^  events  to  have 
wholly  cleared  away,  leaving  only  what  we  have  called  the 
'  deficiency '  phenomena,  due  to  the  loss  of  the  cortical  area 
alone.  On  the  other  hand  it  must  be  remembered  that  the 
movements  of  the  monkey  are  more  intricate  in  origin,  more 
'  skilled '  than  those  of  the  dog ;  it  may  be  that  differences  in 
the  characters  of  movements  determine  the  possibility  of  their 
recovery ;  and  undoubtedly  the  coarser  movements  return  first, 
the  finer,  more  skilled  movements  reappear  later  or  not  at  all. 
Thus,  after  the  removal  of  an  arm  area  in  the  monkey,  a  cer- 
tain awkwardness  in  the  movements  of  the  thumb  is  one  of  the 
lasting  effects  of  the  operation. 

§  488.  So  far  we  have  spoken  of  changes  in  movements  as 
if  these  were  the  only  effects  produced  by  removal  of  the  motor 
area  or  of  parts  of  it.  But  as  a  matter  of  fact  changes  in  sen- 
sations are  as  prominent  results  of  such  operations  as  changes 
in  movements,  and  this  fact  opens  up  a  different  view  of  the 
matter.  Before  however  we  proceed  any  further  in  the  discus- 
sion, it  will  be  of  advantage  to  turn  aside  to  what  is  known 
concerning  the  cortical  motor  region  in  man.  As  we  have 
already  said,  theoretical  considerations  lead  us  to  believe  that 
the  cortical  motor  region  in  man  is  disposed  in  accordance  with 
the  plan  of  the  anthropoid  ape  as  ascertained  experimentally, 
but  with  the  differentiation  carried  still  further;  and  observa- 


Chap,  ii.]  THE  BRAIN.  765 

tion  supports  this  view.  On  the  one  hand  in  certain  cases  (and 
the  number  of  these  is  increasing)  it  has  been  possible  to  ap- 
ply an  electric  current  to  the  human  brain  laid  bare  or  covered 
only  with  the  membranes ;  the  results  obtained  distinctly  cor- 
roborate the  above  view.  We  may  note  in  passing  that  in  such 
cases  it  has  been  found  necessary  to  apply  a  relatively  strong 
current.  On  the  other  hand  corroboration  is  also  afforded  by 
cases  of  disease,  by  the  phenomena  attending  circumscribed  affec- 
tions of  the  cortex,  such  as  tumours  and  the  like,  and  that  in 
spite  of  the  advantages  of  dealing  with  one  of  ourselves  being 
counterbalanced  by  the  disadvantages  due  to  disease  being  so 
often  anatomically  diffuse  and  physiologicall}^  changeful  and 
progressive. 

We  said  above  that  during  experiments  on  animals  stimu- 
lation of  any  part  of  the  motor  region  may  under  abnormal 
conditions  lead  to  general  epileptiform  convulsions.  Now  clin- 
ical study  has  shewn  that  in  man  certain  kinds  of  epileptic 
attacks  are  of  similar  cortical  origin.  In  these  cases  it  has  been 
observed  that  the  attack  begins  in  a  particular  movement,  by 
contractions  of  particular  muscles,  or  of  the  muscles  of  a  par- 
ticular region  of  the  body,  of  the  hand,  foot,  toe,  thumb,  &c., 
and  then  spreads  in  a  definite  order  or  '  march '  over  the  mus- 
cles of  other  regions  until  the  whole  body  is  involved.  When 
in  an  experiment  on  an  animal  epileptiform  convulsions  super- 
vene, they  similarl}'  start  from  the  region  of  the  body,  the 
motor  area  of  which  is  beneath  the  electrodes  at  the  time,  and 
similarly  spread  by  a  definite  '  march '  over  the  whole  bod3\ 
Hence  in  the  human  epileptiform  attacks  of  which  Ave  are 
speaking,  it  has  been  inferred  that  the  immediate  exciting  cause 
of  tlie  attack  is  to  be  sought  in  events  taking  place  in  that  part 
of  the  cortex  which  serves  as  the  area  for  the  movement  which 
ushers  in  the  attack.  Further  inquiry  has  not  only  confirmed 
this  view,  but  has  also  shewn  that  the  topograph}'  of  the  corti- 
cal areas  in  man,  as  thus  determined,  very  closely  follows  that 
of  the  monkey. 

Other  diseases  of  the  cortex  have  been  marked,  among  other 
symptoms,  by  loss  or  impairment  of  particular  movements.  In 
most  of  such  cases,  the  cortical  lesion  has  been  of  such  an 
extent  as  to  involve  a  number  of  special  areas  at  the  same  time, 
and  so  to  lead  to  loss  or  impairment  of  movement  over  relatively 
considerable  regions  of  the  body,  such  as  the  whole  of  one  arm ; 
and  in  general  the  teaching  of  these  cases  of  disease,  while  con- 
firming the  deductions  from  the  monkey,  and  giving  us  some 
general  idea  of  the  topogra[)hy  of  tlie  human  motor  cortical 
region,  lias  at  present  given  us  ai)proximate  results  onl}-.  Figs. 
136  and  137  shew  in  broad  diagrammatic  manner  the  position 
and  relative  extent  of  the  motor  areas  for  the  leg,  arm  and  face 
in  man,  so  far  as  has  yet  been  ascertained.    To  assist  the  reader 


766  CORTICAL   MOTOE   REGION  IN   MAK      [Book  in. 

we  give  at  the  same  time  diagrams  Figs.  134,  135  illustrating 
the  nomenclature  of  the  surface  of  the  human  brain. 

One  area  is  of  special  and  instructive  interest.  Speech  is 
an  eminentl}^  '  skilled '  movement.  We  have  seen  that  in  the 
monkey  the  area  for  the  mouth  and  tongue  lies  at  the  ventral 
end  of  the  central  fissure  or  fissure  of  Rolando,  ventral  to  the 
arm  area,  and  that  the  extreme  ventral  and  front  part  of  the 
motor  region  just  above  the  fissure  of  Sylvius  supplies  an  area 
which  we  marked  as  that  of  phonation  (Fig.  123).  In  the  monkey 
the  area  of  phonation  is  determined  by  experimental  stimulation ; 
in  man,  in  a  similar  position,  on  the  third  or  lowest  frontal  con- 
volution, sometimes  called  Broca's  convolution,  ventral  to  and  in 
front  of,  and  probably  overlapping  backwards  the  area  which  in 
Fig.  136  is  marked  '  face '  and  which  includes  the  mouth  and 
tongue,  clinical  study  has  disclosed  the  existence  of  an  area  which 
may  be  spoken  of  as  the  area  of  '  speech.'  Lesions  of  the  cortex 
in  this  area  cause  a  loss  of  or  interference  with  speech,  the  con- 
dition being  known  as  aphasia  ;  to  this  we  shall  presently  return. 
In  Fig.  136  this  area  is  shewn  in  an  approximate  manner. 

The  movements  of  speech  are  essentially  bilateral  movements. 
In  the  dog  and  monkey  various  bilateral  movements  may  be 
excited  by  stimulation  of  the  appropriate  area  in  either  hemi- 
sphere ;  and  analogy  would  lead  us  to  suppose  that  in  man,  the 
movements  of  speech  would  be  connected  with  the  speech  area 
in  both  one  and  the  other  hemisphere.  The  results  of  lesions 
however  shew  that  it  is  in  most  cases  especially  the  left  hemi- 
sphere which  is  connected  with  speech :  it  is  a  lesion  in  the  third 
frontal  convolution  of  the  left  hemisphere,  often  associated  with 
other  lesions  of  the  same  hemisphere  leading  to  paralysis  of  the 
right  side  of  the  body  and  face,  which  causes  aphasia,  it  being 
only  in  exceptional  cases  that  the  condition  results  from  a  lesion 
of  the  corresponding  area  of  cortex  on  the  right  hemisphere. 

In  man,  then,  clinical  study  corroborates  the  conclusions 
deduced  from  the  experimental  investigation  of  the  dog  and  of 
the  monkey,  but  still  leaves  us  in  uncertainty  as  to  the  question 
what,  and  what  alone  are  the  absolutely  permanent  effects  of 
the  loss  of  a  cortical  area  and  of  nothing  else.  On  the  one  hand, 
in  the  cases  in  which  recovery  of  a  movement  follows  upon  its 
loss  or  impairment,  it  is  open  for  us  to  suppose  that  the  lesion 
itself  was  temporary,  and  that  with  the  cure  of  the  malady  the 
cortical  area  regained  its  normal  condition.  On  the  other  hand, 
where  the  disease  continues,  the  permanency  of  the  loss  of  any 
movement  may  be  attributed  to  the  disease  doing  more  than 
merely  suspend  the  function  of  the  cortical  area.  Aphasia, 
especially  in  young  persons,  has  been  followed  by  recovery,  but 
in  such  cases  it  has  been  supposed  that  the  dormant  area  on  the 
right  side  has  been  awakened  to  activity  by  the  loss  of  the  left 
area;  and  in  support  of  this  view  cases  have  been  recorded  in 


Chap,  ii.] 


THE   BRAIN. 

FRO    N_T    A     ^ 


767 


Fig.  134.     Diagram  of  the  Gyri  (coxvolutions)  sulci,  (Fissures  on  the 

LATERAL    SURFACE    OF    THE    lllGHT    HEMISPHERE    OF    MaN.       (GoWeiS.) 


O    B^ 


/:  Rolando 


■TTKrro?>^ 

Fig.  135.     The  same  ox  the  Mesial  Surface.     (Gowers.) 

In  both  figures  the  sulci  are  indicated  by  italic  and  the  convolutions  by 
roman  type. 

The  following  list  of  some  synonyms  may  perhaps  be  of  use  in  connection 
with  these  figures  and  those  of  the  brain  of  the  monkey,  Figs.  123,  124. 

Gyri,  or  Convolutions.  Precentral  or  anterior  central  =  ascending  frontal. 
Postcentral  or  posterior  central  =  ascending  parietal.  Superior  temporal  =  infra- 
marginal  =  first  temporal.  Triangular  lobule  =  cuneus.  Central  lobe  =  Island 
of  lieil.  Taracentral  lobule  =  the  mesial  face  of  the  ascending  frontal,  within 
the  marginal  gyrus.  Cinguluni  =  the  part  of  the  gyrus  fornicatus  which  adjoins 
the  Corpus  callosum.  Gyrus  Hippocampi  =  uncinate  gyrus,  though  the  latter 
name  is  sometimes  restricted  to  tlie  front  part  of  the  hippocampal  gyrus  ;  the  two 
may  be  considered  as  a  continuation  of  the  gyrus  fornicatus,  and  the  three 
together,  forming  a  series,  have  been  called  '  the  great  limbic  lobe.' 

Sulci  or  Fissures.  Central  =  Rolandic,  or  of  Kolaudo.  Perpendicular  = 
parieto-occipital.     Parietal  =  intraparietal  or  sometimes  interparietal. 

Temporo-sphenoidal  lobe  =  temporal  lobe. 


768 


CORTICAL   MOTOR   REGIOi^   IN   MAN.     [Book  m. 


Fr.L 


Te.LT 


Fig.  136.  The  Lateral  Surface  of  the  Right  Cerebral  Hemisphere  of  Man 
IN  OUTLINE,  TO  ILLUSTRATE  THE  CoRTicAL  Areas.     Reduced  from  nature. 

The  position  of  the  areas  of  the  cortex  concerned  with  movements  of  the 
face,  arm,  and  leg,  and  witli  tlae  senses  of  sight  and  hearing  are  approximately 
shewn.  The  position  of  the  area  connected  with  speech  (Broca's  centre)  is  also 
shewn  for  the  sake  of  comparison  of  it  with  tiie  position  of  the  other  areas ; 
the  representation  of  speech  in  the  cortex  cerebri  lies  however  in  the  left  liemis- 
phere  chiefly. 

Oc.  L.  Occipital  lobe  ;  Fr.  L.  Frontal  lobe  ;  Te.  L.  Temporal  lobe  ;  Sy.  f. 
the  fissure  of  Sylvius  ;  G.f.  the  central  fissure  (Rolandic)  ;  Cm.  f.  indicates  the 
position  of  the  posterior  end  of  the  calloso-marginal  fissure. 


Fr.L 


Oc.L 


Te.L 

Fig.  137.    The  Mesial  Surface  of  the  Right  Cerebral  Hemisphere  of  Man 
IN  outline,  to  illustrate  the  Cortical  Areas. 

The  areas  shewn  are  those  connected  with  the  movements  of  the  leg,  and 
with  the  senses  of  sight  and  smell. 

Fr.  L.  the  frontal  pole  of  the  hemisphere;  Oc.  L.  the  occipital  pole,  Te.  L. 
the  temporal  pole.  Gm.  f.  the  calloso-marginal  fissure  separating  the  marginal 
gyrus  above  from  the  gyrus  fornicatus  below.  Cf.  marks  the  situation  of  the 
central  fissure,  the  fissure  itself  not  being  apparent  on  the  mesial  aspect  of  the 
hemisphere.  The  corpus  callosum  and  the  anterior  commissure  are  seen  in 
cross  section. 


Chap,  ii.]  THE   BKAIK  769 

whicli  a  lirst  aphasia,  due  to  a  lesion  on  the  left  side,  has  been 
followed  by  a  second  aphasia  due  to  a  sequent  lesion  occurring 
on  the  right  side.  On  the  whole  perhaps  the  evidence  of  clini- 
cal study  tends  to  shew  that  in  man  the  loss  of  movement  due 
to  the  destruction  by  disease  of  an  area  is  a  permanent  one, 
though  actual  demonstration  of  this  is  wanting. 

§  489.  We  may  now  return  to  the  discussion  of  the  ques- 
tion, what  is  the  part  played  by  a  motor  area,  and  by  the  con- 
tribution from  that  area  to  the  pyramidal  tract  in  carrying  out 
the  movements  with  which  the  area  is  associated? 

We  may  premise  that  the  evidence  points  very  distinctly  to 
the  conclusion  that  whatever  be  the  nature  of  the  whole  chain  of 
events  of  which  the  cortical  area  seems  to  be  a  sort  of  centre, 
the  fibres  of  the  pyramidal  tract  serve  as  the  channel  of  processes 
which  we  must  regard  as  efferent  in  nature.  The  characters  of 
the  fibres,  axis  cylinder  processes  terminating,  so  far  as  we  can 
ascertain,  in  connection  with  motor  cells,  the  fact  that  the  degen- 
eration of  the  fibres  is  a  descending  one,  though  this  cannot  be 
trusted  by  itself  to  prove  that  the  direction  in  which  the  fibres 
carry  impulses  is  only  that  from  the  cortex  downwards,  and  above 
all  the  fact  that  when  the  fibres  of  the  tract  are  stimulated  at 
any  part  of  their  course,  movements,  the  signs  of  the  occurrence 
of  efferent  centrifugal  impulses,  are  produced,  these  things 
together,  leave  no  doubt  as  to  the  tract  being  one  of  efferent 
fibres.  Hence  we  may  infer  that  whatever  be  the  nature  of  the 
events  taking  place  in  a  motor  area  during  the  carrying  out  of 
a  movement,  the  part  played  by  the  fibres  of  the  pyi'amidal  tract 
is  that  of  carrying  efferent  impulses  from  the  area  to  the  muscles 
concerned. 

Let  us  consider  first  the  movements  of  speech  in  man,  the 
evidence  touching  the  connection  of  which  with  an  area  on  the 
third  frontal  convolution  appears  so  very  clear.  Speech  is 
eminently  a  'skilled'  movement;  it  involves  the  most  delicate 
coordination  of  several  muscular  contractions,  and  we  may  cer- 
tainly say  of  it  that  it  has  to  be  '  learnt.'  The  whole  chain  of 
coordinated  events  by  which  the  utterance  of  a  sentence,  a  word, 
or  any  vocal  sign  is  accomplished  consists  of  many  links,  the 
breaking  of  any  of  which  will  lead  to  failure  of  one  kind  or 
another  in  the  act.  Something  may  go  wrong  in  the  glossal  or 
other  muscles,  in  the  nerve  endings  in  those  muscles,  or  in  the 
fibres  of  the  nerves,  hypoglossal  and  others,  between  the  central 
nervous  system  and  the  muscles,  or  something  may  go  wrong  in 
that  })ai-t  of  tlie  central  nervous  system,  the  bulb  to  wit,  in  which 
a  certain  amount  of  cooi'dination  is  carried  out  just  previous  to 
the  issue  of  the  motor  impulses.  Damage  done  to  any  of  tliese 
parts  of  the  mechanism  may  lead  to  dumbness  or  to  imperfect 
speech.  In  the  latter  case  the  imperfections  have  a  certain  char- 
acter ;  if  we  are  at  all  able  to  gather  the  wish  of  the  speaker, 

49 


770  VOLUNTAEY   MOVEMENTS.  [Book  in. 

we  recognize  that  lie  is  attempting  to  utter  the  right  words  in 
the  right  sequence,  but  that  liis  efforts  are  frustrated  by  imper- 
fect coordination  or  imperfect  muscuhar  action ;  his  speech  is 
'  tliick,'  the  syllables  are  blurred  and  the  like.  Disease  of  the 
bulb  at  times  leads  to  imperfect  speech  of  this  kind  in  which  the 
imperfection  may  be  recognized  as  due  to  the  lack  of  proper 
coordination  of  motor  impulses.  The  affection  of  speech,  known 
as  '  aphasia,'  which  is  caused  by  lesions  of  the  cortex  is  of  a  dif- 
ferent character,  and  the  forms  of  imperfect  speech  caused  by 
bulbar  disease  have  justly  been  distinguished  from  true  aphasia 
by  the  use  of  other  terms.  Cases  of  complete  aphasia  in  which 
all  power  of  speech  is  lost,  do  little  more  than  help  us  to  ascer- 
tain the  topographical  position  in  the  cortex  of  the  '  speech  '  area, 
but  cases  of  partial  aphasia  are  especially  instructive.  Without 
attempting  to  go  into  the  details  of  the  subject  and  into  the 
many  considerations  which  have  to  be  had  in  mind  in  dealing 
with  it,  for  there  are  different  kinds  of  aphasia,  we  may  venture 
to  say  that  the  striking  feature  of  partial  aphasia  is  the  failure 
to  say  certain  words  or  syllables,  and  the  tendency  to  substitute 
some  wrong  word  or  syllable  for  the  right  one.  The  words  or 
syllables  which  are  uttered  are  rightly  pronounced  without  defect 
of  articulation  ;  and  in  many  cases,  though  the  right  word  can- 
not be  produced  as  a  direct  effort  of  the  will,  it  may  be  uttered 
under  the  influence  of  an  emotion,  or  indeed  sometimes  as  the 
result  of  some  psychical  processes  more  complex  than  those  in- 
volved in  the  mere  volitional  effort  to  say  the  word.  An  instruc- 
tive case  is  recorded  of  a  man  suffering  from  slight  aphasia,  who 
after  several  failures  to  say  the  word  '  no '  by  itself,  at  last  said, 
'  I  can't  say  no,  sir.' 

From  the  phenomena  of  partial  aphasia  we  may  on  the  one 
hand  draw  the  deduction  that  the  cortical  speech  area  does  not 
carry  out  the  whole  of  the  coordination  of  the  impulses  involved 
in  articulation.  That  coordination  is  exceedingly  complex,  and 
we  ought  perhaps  to  recognize  in  it  more  than  one  degree  or 
kind  of  coordination.  We  must  of  course  admit  that  a  great 
deal  of  coordination  of  a  certain  kind  takes  place  in  the  cortex, 
for  the  bulb  cannot  by  itself  be  made  to  speak.  But  the  failure 
of  articulation  in  disease  of  the  bulb  shews  that  a  certain  amount 
of  coordination  takes  place  there  also;  for  the  affections  of  speech 
due  to  bulbar  disease  are  not  the  same  as  those  resulting  fi'om 
the  mere  loss  of  this  or  that  muscle  or  nerve.  The  word  spoken 
does  not  start,  so  to  speak,  ready  made  in  the  cortex  ;  it  is  not  that 
a  group  of  impulses  start  from  the  cortex  with  their  coordination 
fully  achieved,  and  pass  along  certain  nerve  fibres  to  certain 
muscles  making  their  way  without  change  through  the  tangle  of 
the  bulb,  as  if  this  were  merely  a  bundle  of  lines  offering  paths 
for,  but  exercising  no  influence  over  the  impulses.  We  must 
rather  suppose  that  something  takes  place  in  the  cortex  of  the 


CiiAP.  II.]  THE   BRAIK  771 

third  frontal  convolution,  as  the  result  of  which  efferent  impulses 
pass  along-  the  appropriate  fibres  of  the  pyramidal  tract  to  the 
bulb,  and  there  start  a  series  of  events  leading  to  the  issue  of  the 
coordinated  impulses  by  which  the  word  is  spoken.  And,  since 
we  have  no  reason  to  think  that  the  cortical  area  for  speech  differs 
in  its  fundamental  characters  from  other  divisions  of  the  motor 
region,  we  may  appl}^  the  same  reasoning  to  other  motor  areas. 

On  the  other  hand,  the  phenomena  of  aphasia  illustrate 
another  view  of  the  nature  of  the  motor  areas  to  which  we  must 
now  turn.  We  said  that  there  are  different  kinds  of  aphasia. 
We  may  in  a  broad  way  distinguish  two  classes.  In  considering 
speech  we  have  to  deal  on  the  one  hand,  with  the  efferent  motor 
factors,  the  framing  and  utterance  of  the  word,  and  on  the  other 
hand  with  the  afferent,  in  a  broad  sense,  sensory  factors  which 
lead  to  and  guide  the  framing  of  the  word.  And  in  aphasia  we 
may  recognize  a  class  in  which  the  failure  is  on  the  motor  side 
of  the  business  so  to  speak  and  a  class  in  which  the  failure  is  on 
the  afferent  side.  The  dumbness  of  those  who  are  born  deaf  is 
an  extreme  illustration  of  the  latter  class.  Now  as  we  said  above, 
when  a  so-called  motor  area  is  removed  from  the  cortex,  the 
results  are  not  purely  motor,  that  is  to  say  the  loss  or  impair- 
ment of  movement  is  not  the  only  effect;  a  loss  or  impairment 
of  sensation  is  also  produced  in  the  part  of  the  body,  the  move- 
ment of  which  is  affected.  When,  for  instance,  in  the  monkey 
the  area  for  the  arm  is  removed,  not  only  is  the  arm,  for  the  time 
being,  paralyzed,  but  also  the  animal  does  not  shew  signs  of  sen- 
sation, or  shews  only  feeble  signs  of  sensation  when  the  skin  of 
the  arm  is  pricked  or  otherwise  stimulated.  And  so  with  other 
areas.  In  all  cases  the  loss  of  movement  is  accompanied  by  a 
corresponding  loss  of  sensations,  of  tactile  sensations,  of  sensa- 
tions of  heat  and  cold,  and  even  of  pain.  And  the  loss  or  impair- 
ment of  sensation  runs  more  or  less  parallel  in  point  of  time  to 
the  loss  or  impairment  of  movement,  and  shews  the  same  ten- 
dency to  be  in  part  lasting.  Moreover  in  the  cases  mentioned 
above  where  it  has  been  possible  to  stimulate  the  cortex  of  the 
human  brain,  and  where  this  has  been  done  while  the  sul)ject 
was  conscious,  the  production  of  sensations,  often  described  as 
tingling,  in  the  part  of  the  body  corresponding  to  the  particular 
area,  has  been  at  least  as  striking  a  result  as  the  production  of 
movement  in  that  part.  And  this  is  in  harmony  with  the  fact 
that  in  e})ileptic  attacks  which,  as  we  said,  illustrate  the  action 
of  cortical  areas,  the  movements  which  are  the  objective  factors 
of  an  attack  are  preceded  by  peculiar  sensations,  the  so-called 
'aura,'  and  these  equally  with  the  movement  have  definite 
relations  to  the  area  of  the  cortex,  disease  of  which  causes  the 
attack.  In  fact  there  is  increasing  evidence  that  the  region  of 
the  cortex  which  we  have  called  the  motor  area,  is  connected 
not  only  with  the  movements  carried  out  by,  but  also  with  the 


772  VOLUNTARY   MOVEMENTS.  [Book  in. 

sensations  derived  from  the  several  parts  of  the  body ;  we  seem 
justified  in  speaking  of  a  topographical  distribution  of  cortical 
"  sensory  "  areas,  if  we  may  so  call  them,  following  very  closely 
that  of  the  motor  areas ;  and  we  know  that  by  the  numerous 
fibres  passing  from  the  cortex  to  the  optic  thalamus,  if  not  in 
other  ways,  an  anatomical  path  appears  to  be  afforded  for  sen- 
sory impulses.     But  to  this  point  we  shall  return  later  on. 

Meanwhile  we  may  conclude  that  the  loss  of  movement  which 
follows  the  removal  of  a  motor  area  is  not  due  merely  to  the 
loss  of  motor  elements,  it  may  be  as  much  due  to  the  loss  of 
sensory  elements.  Indeed  it  has  been  maintained  by  some  that 
the  loss  or  impairment  of  movement  in  question  is  not  a  motor 
business  at  all,  but  is  simply  due  to  a  loss  of  the  muscular  sense. 
We  have  seen,  however,  reasons  for  thinking  that  the  pyramidal 
tract  is  certainly  an  efferent  tract,  and  injury  to  it  in  its  begin- 
ning in  the  cortex  must  lead  to  failure  of  efferent  impulses. 
Moreover,  though  removal  of  the  cortex  does  appear  to. interfere 
with  muscular  sense,  it  also,  and  even  more  clearly,  interferes 
with  cutaneous  and  other  sensations.  The  conclusion  which 
we  ought  to  draw  from  the  above  facts  is  perhaps  rather  this, 
that  the  relations  of  the  cerebral  cortex  are  manifold,  and  that 
the  carrying  out  even  of  a  simple  voluntary  movement  is  a  very 
complicated  matter ;  even  if  we  assume  that  the  cell  in  the  cor- 
tex giving  origin  to  a  fibre  of  the  pyramidal  tract  is  in  nature  a 
motor  cell,  we  must  also  recognize  that  its  work  is  determined 
by  ties  which  bind  it  to  other  elements  of  the  cortex  and  through 
them  to  other  parts  of  the  nervous  system  and  indeed  of  the 
body.  The  connections  of  a  sensory  nature  between  a  motor 
area  and  the  part  to  whose  movements  it  is  related  is  strikingly 
shewn  by  results  which  may  make  their  appearance  when  stimu- 
lation of  the  cortex  is  carried  on  while  the  animal  (dog)  is  in  a 
particular  stage  of  the  influence  of  moriDhia.  If  a  subminimal 
stimulus  be  found,  that  is  a  current  of  such  intensity  that  applied 
to  a  motor  area  it  will  produce  no  movement,  but  if  increased 
ever  so  slightly  will  give  a  feeble  contraction  of  the  appropriate 
muscles,  it  may  be  observed  that  a  slight  stimulus,  such  as  gently 
stroking  the  skin  over  the  muscles  in  question,  will  render  the 
previous  subminimal  stimulus  effective  and  so  call  forth  a  move- 
ment. Thus  if  the  area  experimented  on  be  that  connected  wi,th 
the  lifting  of  the  forepaw,  and  the  subminimal  stimulus  be  applied 
to  the  area  at  intervals,  after  several  applications  followed  by  no 
movements,  a  gentle  stroke  or  two  over  the  skin  of  the  paw  will 
lead  to  the  paw  being  lifted  the  next  time  the  stimulus  is  applied 
to  the  area.  A  similar  result,  but  less  sure  and  striking,  may  fol- 
low upon  the  stimulation  of  parts  of  the  body  other  than  the  part 
corresponding  to  the  area  stimulated.  Then  again  it  has  been 
observed  that  in  certain  other  stages  of  the  influence  of  morphia, 
the  cortex  and  the  rest  of  the  nervous  system  are  in  such  a  con- 


Chap,  ii.]  THE   BRAIN.  773 

ditioii  that  the  application  of  even  a  momentary  stimulus  to  an 
area  leads  not  to  a  simple  movement  but  to  a  long-continued 
tonic  contraction  of  the  appi'opriate  muscles.  Under  these  cir- 
cumstances, a  gentle  stimulus,  such  as  stroking  the  skin,  or  blow- 
ing on  the  face,  applied  immediately  after  the  application  of  the 
electric  stimulus  to  the  area,  suddenly  cuts  short  the  contraction, 
and  brings  the  muscles  at  once  to  rest  and  normal  flaccidity. 

§  490.  The  carrying  out  of  a  voluntary  movement  is  in  fact 
a  very  complex  proceeding,  and  the  motor  cortex  with  the  pyra- 
midal tract  is  only  one  part  of  the  whole  mechanism.  This 
complexity  is  illustrated  by  the  fact  that  after  removing  of  a 
motor  area  not  only  purely  voluntary  but  also  reflex  and  other 
movements  are  for  a  while  abolished  or  impaired ;  and  even 
making  every  allowance  for  the  effects  of  '  shock '  (§  457)  we 
cannot  account  for  the  latter  to  the  exclusion  of  the  former,  by 
api^ealing  to  such  effects.  It  is  further  shewn  by  the  fact  that 
in  the  case  of  most  voluntary  movements  at  least,  after  removal 
of  an  area  recovery  is  after  a  while  complete,  though  there  is  no 
regeneration  either  of  the  area  or  the  strand  of  the  pyramidal 
tract  belonging  to  it ;  the  will  finds  some  other  way  to  the 
muscles  and  to  mechanism  coordinating  the  movements  of  those 
muscles.  By  the  following  reflection  the  complexity  of  the 
matter  is  also  shewn  in  a  different  direction.  When  a  gymnast 
executes  a  skilled  voluntary  movement  in  which  all  his  four 
limbs  and  other  parts  as  well  perhaps  of  his  body  are  involved, 
it  is  probably  the  case  that  changes  of  the  nature  of  efferent 
impulses  sweep  down  his  pyramidal  tract,  and  that  these  im- 
pulses, starting  in  a  definite  order  from  his  cortex,  that  is  to  say 
having  undergone  a  certain  amount  of  initial  coordination  at 
their  very  origin,  meet  with  further  coordination  in  the  spinal 
grey  matter,  which  serves  as  a  set  of  nuclei  of  origin  for  the 
motor  nerves  concerned  in  the  movement,  before  they  issue  as 
ordinary  motor  impulses  along  the  anterior  roots.  But  this  is 
not  all.  Should  the  gymnast's  semicircular  canals  happen  to  be 
injured  and  his  cerebellum  thereby  be  troubled,  or  mischief  fall 
on  some  other  part  of  the  brain  which  like  this  has  no  direct 
connection  with  either  the  p^numidal  tract  or  the  motor  cortex, 
the  movement  fails  through  lack  of  coordination,  thougli  both 
the  cortex,  the  pyramidal  tract,  and  the  spinal  motor  mechanisms 
remain  as  they  were  before. 

Lastly  we  may  note  that  in  the  above  discussion  we  have 
used  the  word  '  will '  in  a  general  sense  only.  A  man  may  be 
brought  into  a  condition,  for  instance  in  certain  hypnotic  phases, 
in  winch  he  can  carry  out  all  the  various  skilled  movements  which 
he  has  inherited  or  which  he  has  learnt ;  and  yet,  according  to 
some  definitions  of  the  word  '  will,'  those  movements  could  not 
be  said  to  be  initiated  by  his  will.  It  can  hardly  be  doubted 
that  in  such  cases  the  motor  cortex  and  pyramidal  tract  play 


774        VOLITIONAL  IMPULSES  IN"  THE  COED.     [Book  hi. 

their  usual  part.  But  we  may  pass  from  such  cases  as  these 
through  others,  until  we  come  to  cases  where  a  skilled  move- 
ment which  has  been  learnt  and  practised  by  the  working  of  an 
intelligent  will,  may  continue  to  be  carried  out  under  circum- 
stances which  seem  to  preclude  the  intervention  of  any  conscious 
will  at  all ;  and  the  transition  from  one  case  to  another  is  so 
gradual,  that  it  is  impossible  to  suppose  that  there  has  been  any 
shifting  of  the  machinery  employed  for  carrying  out  the  move- 
ment. So  that  a  volitional  origin  is  not  an  essential  feature  of 
these  so-called  voluntary  movements,  and  the  machinery  of  the 
motor  cortex  and  pyramidal  tract  is  available  for  otlier  things 
than  pure  volitional  impulses. 

§  491.  The  preceding  discussion  v/ill  enable  us  to  be  very 
brief  concerning  a  question  which  has  from  time  to  time  been 
much  discussed,  and  which  has  acquired  perhaps  factitious  im- 
portance, viz.  the  question  as  to  how  volitional  impulses  leading 
to  voluntary  movements  travel  along  the  spinal  cord.  The  con- 
clusion at  which  we  have  arrived,  namely,  that  in  the  normal 
carrying  out  of  voluntary  movements  the  chief  part  is  played  by 
efferent  impulses  passing  along  the  pyramidal  tract,  carries  with 
it  the  answer  that  volitional  impulses  travel  in  the  spinal  cord 
along  the  pyramidal  tract. 

In  the  dog,  in  which  the  whole  pyramidal  tract  crosses  at  the 
decussation  of  the  pyramids,  we  should  expect  to  find  that  a 
break  in  the  pyramidal  tract  of  one  side  of  the  cord  at  any  point 
along  its  length  caused  loss  of  voluntar}'"  movement  on  tlie  same 
side  below  the  level  of  the  break.  And  experiments  as  far  as 
they  go  support  this  view.  No  one  it  is  true  has  so  far  succeeded 
in  dividing  or  otherwise  causing  to  break  in  the  pyramidal  tract 
alone,  leaving  the  rest  of  the  cord  intact ;  and  indeed,  even  if 
an  injury  were  limited  to  the  area  marked  out  as  the  pyramidal 
tract,  fibres  other  than  pyramidal  fibres  would  be  injured  at  the 
same  time,  since  the  tract  is  never  a  'pure'  one.  But  it  has 
been  found  that  a  section  of  a  lateral  half  of  the  cord,  a  lateral 
hemisection,  or  a  section  limited  to  the  lateral  column  of  one 
side  has  for  one  of  its  principal  effects  loss  of  voluntary  move- 
ment on  the  same  side  in  the  parts  supplied  by  motor  nerves 
leaving  the  cord  below  the  level  of  the  section.  We  say  '  one 
of  its  principal  effects '  because,  besides  the  concomitant  inter- 
ference with  sensations  concerning  which  we  shall  speak  pres- 
ently, the  loss  of  voluntary  movement  is  not  absolutely  confined 
to  the  same  side  ;  there  is  some  loss  of  power  on  the  crossed 
side,  at  least  in  a  large  number  of  cases.  We  must  not  lay  stress 
on  this  crossed  paralysis  because  it  is  possibly  one  of  the  effects 
of  the  mere  operation,  not  a  pure  'deficiency'  phenomenon,  and 
indeed  appears  soon  to  pass  away.  But  talcing  into  considera- 
tion what  was  said  above  concerning  the  effects  of  removing 
cortical  areas,  it  is  important  to  note  that  in  the  experience  of 


Chap,  ii.]  THE   BRAIX.  775 

many  experimenters  the  loss  of  voluntaiy  power  on  the  operated 
side  diminishes  after  a  while,  and  that  the  animal  if  kept  alive 
and  in  good  health  long  enough  appears  to  regain  almost  full 
voluntary  power  over  the  affected  parts.  In  such  cases,  as  in 
other  operations  on  the  central  nervous  system,  there  is  no  re- 
generation of  nervous  tissue  ;  the  two  surfaces  of  the  section 
unite  by  connective  not  nervous  tissue,  and  the  tracts  which  as 
the  result  of  the  section  degenerate  downwards  or  upwards  ai-e 
permanently  lost.  Hence  even  if  we  admit  that  in  the  intact 
animal  a  voluntary  movement  is  chiefl}''  carried  out  by  means  of 
efferent  impulses  passing  along  the  pj-ramidal  tract  right  down 
to  the  motor  mechanisms  of  the  cord  immediately  connected 
with  the  motor  nerves,  we  must  also  admit  that  the  'will '  under 
changed  circumstances  can  hnd  other  channels  for  gaining  access 
to  the  same  mechanisms. 

It  has  been  further  observed  that  if  in  the  dog  a  hemisection 
be  made  at  one  level,  for  instance  in  the  lower  thoracic  region  of 
the  cord,  and  then,  after  waiting  until  the  voluntary  powei'  over 
the  hind  limb  of  that  side  has  returned,  a  second  hemisection, 
this  time  on  the  other  side,  be  made  at  a  higher  level,  this  second 
operation  is  followed  by  results  similar  to  those  of  the  first;  there 
is  loss  of  voluntary  power  on  the  side  operated  on,  with  some 
loss  of  power  on  the  crossed  side,  and  as  in  the  first  case  this 
loss  of  power  not  only  on  the  crossed  but  also  on  the  same  side 
may  eventually  disappear.  This  shews  among  other  things  that 
the  recovery  after  the  first  operation  was  not  due  to  the  remain- 
ing pyramidal  tract  doing  the  work  of  both.  Further,  the  hemi- 
section may  be  repeated  a  third  time,  the  third  hemisection  being 
on  the  same  side  as  the  first,  and  in  this  case  also  there  may  be 
at  least  very  considerable  return  of  power  over  both  limbs.  That 
is  to  say,  under  such  abnormal  circumstances  voluntary  impulses 
may,  so  to  speak,  thread  their  way  in  a  zigzag  manner  from  side 
to  side  along  the  mutilated  cord  until  they  reach  tlie  appropri- 
ate spinal  motor  mechanisms.  Such  an  abnormal  state  of  things 
does  not  however  really  militate  against  the  view  that  under 
normal  circumstances  volitional  impulses  normally  travel  along 
the  pyramidal  tract ;  but  it  does  shew,  what  indeed  has  already 
been  shewn  by  the  phenomena  of  strychnia  poisoning,  §  461, 
that  in  the  central  nervous  system  the  passage  of  nervous  im- 
[)ulses  (using  those  words  in  the  general  sense  of  changes  prop- 
agated along  nervous  material)  is  not  rigidly  and  unalterably 
fixed  by  the  anatomical  distribution  of  tracts  of  fibres  ;  in  all 
such  discussions  as  those  in  which  Ave  are  engaged  we  must  bear 
in  mind  that  physiological  conditions  as  well  as  anatomical  con- 
nections are  potent  in  determining  the  passage  of  these  impulses. 

§  492.  When  we  refiect  on  the  great  prominence  of  the 
pyramidal  tract  in  the  spinal  cord  of  man  as  comjjared  with  that 
of  the  dog,  we  may  justly  infer  not  only  that  the  pyramidal  tract 


776  VOLUNTAEY  MOVEMENTS.  [Book  in. 

is  under  normal  circumstances  more  exclusively  the  channel  of 
volitional  impulses  in  man  than  in  such  lower  animals,  but  also, 
bearing  in  mind  the  discussion  in  a  previous  chapter,  §  464, 
concerning  the  activities  of  the  spinal  cord  of  man,  that  the 
potential  alternatives  presented  by  the  spinal  cord  of  the  dog 
are  greatly  reduced  in  that  of  man.  And  such  clinical  histories 
of  disease  or  accidental  injury  in  man  as  we  possess  support  this 
conclusion.  Lesions  confined  to  one  half  of  the  cord,  or  even 
lesions  confined  to  the  lateral  column  of  one  half,  appear  to  lead 
to  loss  of  voluntary  power  on  the  same  side,  and  the  same  side 
only,  in  the  parts  below  the  level  of  the  lesion ;  and  the  same 
symptoms  have  been  observed  to  accompany  disease  limited 
apparently  to  the  pyramidal  tract  of  one  side.  Moreover, 
though  eases  of  recovery  of  power  have  been  recorded,  we  have 
not  such  satisfactory  evidence  as  in  animals  of  the  volitional 
impulses  ultimately  making  their  way  along  an  alternative 
route ;  but  here  the  same  doubts  may  be  entertained  as  were 
expressed  in  discussing  the  reflex  acts  of  the  cord  in  man. 

When  we  say  that  the  loss  of  voluntary  power  is  seen  on 
the  side  of  the  lesion  only,  we  should  add  that  this  statement 
appears  to  apply  chiefly  to  the  thoracic  and  lower  parts  of  the 
cord.  We  have  seen  that  in  man,  in  the  upper  regions  of  the 
cord,  the  pyramidal  tract  is  only  partly  crossed;  a  variable  but 
not  inconsiderable  number  of  the  pyramidal  fibres  do  not  cross 
at  the  decussation  of  pyramids,  but  running  straight  down  as 
the  direct  pyramidal  tract  effect  their  crossing  lower  down  in 
the  cervical  and  upper  thoracic  regions.  Hence  we  should  in- 
fer that  a  hemisection  of,  or  a  lesion  confined  to  one  side  of  the 
cervical  cord,  would  affect  the  voluntary  movements  of  the 
crossed  side  as  well  as  of  the  same  side,  though  not  to  the  same 
extent.  But  we  have  no  exact  information  as  to  this  point. 
And  indeed  the  purpose  of  the  direct  tract  is  not  clear ;  there 
is  no  adequate  evidence  for  the  view  which  has  been  held  that 
these  direct  fibres  are  destined  for  the  upper  limbs  and  upper 
part  of  the  body ;  since  they  are  the  last  to  cross  we  should 
a  priori  be  inclined  to  suppose  that  they  were  distributed  to 
lower  rather  than  higher  parts. 

§  493.  We  may  now  briefly  summarize  what  we  know  con- 
cerning voluntary  movements.  And  it  will  be  convenient  to 
trace  the  events  in  order  backwards. 

Certain  muscles  are  thrown  into  a  contraction  which  even  in 
the  briefest  movements  is  probably  of  the  nature  of  a  tetanus. 
In  almost  every  movement  more  than  one  muscle  as  defined  by 
the  anatomists  is  engaged,  and  in  many  movements  a  part  of 
several  muscles  is  employed,  and  not  the  whole  of  each.  It  is 
perhaps  partly  owing  to  the  latter  fact  that  a  muscle  which  has 
become  tired  in  one  kind  of  movement,  may  shew  little  or  no 
fatigue  when  employed  for  another  movement,  though  we  must 


Chap,  ii.]  THE  BRAIN.  777 

bear  in  mind  that  in  a  voluntary  movement  fatigue  is  much 
more  of  nervous  than  of  muscuhir  origin. 

Besides  the  active  muscles,  if  we  may  so  call  them,  which 
directly  carry  out  the  movement,  the  metabolism  of  which  sup- 
plies the  energy  given  out  as  work  done,  other  muscles,  some  of 
which  are  antagfonistic  to  the  active  muscles  and  some  of  which 
may  be  spoken  of  as  adjuvant,  enter  into  the  whole  act.  In 
flexion  for  instance  of  the  forearm  on  the  arm  it  is  not  the 
flexor  muscles  onl}^  but  the  extensors  also  which  are  engaged. 
According  to  the  immediatel}''  preceding  position  and  use  of  the 
arm,  and  according  to  the  kind  and  amount  of  flexion  which  is 
to  be  carried  out,  the  extensors  will  be  either  relaxed,  that  is  to 
say  inhibited,  or  thrown  into  a  certain  amount  of  contraction. 
And  in  some  of  the  more  complicated  voluntary  movements  the 
part  played  by  adjuvant  muscles  is  considerable.  Hence  in  a 
voluntary  movement  the  will  has  to  gain  access  not  only  to  the 
active  muscles,  but  also  to  the  antagonistic  and  adjuvant  mus- 
cles ;  and  every  voluntary  movement,  even  one  of  the  simjjlest 
kind,  is  a  more  or  less  complex  act. 

The  impulses  which  lead  to  the  contraction  of  the  active 
muscles  reach  the  muscles  along  the  fibres  of  the  anterior  roots, 
(we  may  for  the  sake  of  simplicity  take  spinal  nerves  alone, 
neglecting  the  peculiar  cranial  nerves,)  and  such  evidence  as 
we  possess  goes  to  shew  that  the  impulses  governing  the  antag- 
onistic and  adjuvant  muscles  travel  by  the  anterior  roots  also; 
the  question  whether  the  inhibition  of  the  antagonistic  muscles 
when  it  takes  place,  is  carried  out  by  inhibitory  impulses  passing 
as  such  along  the  fibres,  or  simply  by  /central  inhibition  of  pre- 
viously existing  motor  impulses  need  not  be  considered  now. 
These  anterior  roots  are  connected  as  we  have  seen  with  the 
grey  matter  of  the  cord,  and  in  each  hypothetical  segment  of 
the  cord  we  may  recognize  the  existence  of  an  area  of  grey 
matter  which,  though  we  cannot  define  its  limits,  we  may,  led 
by  the  analogy  of  the  cranial  nerves,  call  the  nucleus  of  the 
nerve  belonging  to  the  segment;  and  we  may  further  recognize 
in  such  a  nucleus  what  we  may  call  its  efferent  and  its  afferent 
side. 

Every  voluntary  movement,  even  the  simplest,  is  as  we  have 
repeatedly  insisted  a  coordinated  movement,  and  in  its  coordi- 
nation afferent  impulses  play  an  important  part.  The  study  of 
reflex  actions,  §  462,  has  led  us  to  suppose  that  each  spinal  seg- 
ment presents  a  nervous  mechanism  in  Avhich  a  certain  amount 
of  coordination  is  already  present,  in  which  efferent  impulses 
are  adjusted  to  afferent  im[)ulses.  But  the  results  obtained  by 
stimulating  separate  anterior  nerve  roots  shew  that,  in  the  case 
of  most  muscles  at  all  events,  the  especially  active  muscles  of 
the  limbs  for  instance,  each  muscle  is  supplied  by  fibres  coming 
from  more  than  one  nerve  root,  that  is  to  say  the  spinal  nucleus, 


778  VOLUNTARY   MOVEMENTS.  [Book  hi. 

or  at  least  the  spinal  motor  meclianism  for  any  one  muscle, 
extends  over  two  or  three  segments.  Hence  d  fortiori  in  a  vol- 
untary movement,  involving  as  this  does  in  most  cases  more 
than  one  muscle,  the  si^inal  mechanism  engaged  in  the  act 
spreads  over  at  least  two  or  three  segments,  thus  allowing 
of  increased  coordination.  In  that  coordination  the  impulses 
servino-  as  the  foundation  of  muscular  sense  play  an  important 
part,  but  other  afferent  impulses,  such  as  those  from  the  adjoin- 
ing skin,  also  have  their  share  in  the  matter;  and  it  is  worthy 
of  notice  that  not  only  is  the  skin  overlying  a  muscle  served, 
broadly  speaking,  by  nerve  roots  of  the  same  segment  as  the 
muscle  itself,  afferent  in  one  case,  efferent  in  the  other,  but 
in  the  parts  of  the  body  where  coordination  is  especially  com- 
plex, in  the  fingers  for  instance,  not  only  is  each  muscle  sup- 
plied from  more  than  one  segment,  but  also  each  piece  of  skin 
is  supplied  in  the  sanae  way  by  the  posterior  roots  of  more  than 
one  nerve. 

In  the  case  of  the  frog  it  is  clear  that  in  reflex  movements 
a  large  amount  of  coordination  is  carried  out  by  these  various 
spinal  mechanisms  ;  and  as  we  have  urged,  we  may  safely  infer 
that  in  the  voluntary  movements  of  the  frog,  the  will  makes 
use  of  this  already  existing  coordination,  whatever  be  the  exact 
path  by  which  in  this  animal  the  will  gains  access  to  the  spinal 
mechanisms.  In  the  dog  we  may  conclude  that  in  voluntary 
movements  the  spinal  mechanisms,  with  coordinating  functions, 
are  also  set  in  action,  in  this  case  by  impulses  passing  straight 
from  the  cortex  to  the  mechanisms  by  the  pyramidal  tract, 
though,  apparently  in  the  absence  of  the  pyramidal  tract, 
the  will  can  work  upon  the  mechanisms  by  changes  travelling 
through  other  parts  of  the  cerebrospinal  axis.  And  in  the 
monkey  and  man,  subject  to  the  doubts  already  expressed  as 
to  the  potentialities  of  the  human  spinal  cord,  we  may  prob- 
ably also  infer  that  in  each  voluntary  movement  some,  perhaps 
we  may  say  much,  of  the  coordination  is  carried  out  by  the 
spinal  meclianism  set  into  action  through  impulses  along  the 
pyramidal  tract.  We  may  probably  further  infer  that  a  care- 
ful adjustment  obtains  between  the  beginnings  of  the  pyra- 
midal tract  in  the  cortex  and  its  endings  in  the  cord,  so  that 
the  topography  of  '  areas '  or  '  foci '  in  the  cortex  above  is  an 
image  or  projection  of  the  spinal  mechanisms  below. 

The  complex  character,  on  which  we  insisted  just  now,  of 
almost  every  voluntary  movement  necessitates  that  in  every 
such  movement  a  large  area  of  spinal  mechanism  is  involved. 
But  this  is  not  all.  The  movements  of  any  part,  of  the  legs 
for  instance,  are  not  determined,  nor  is  the  coordination  of  the 
movements  effected,  simply  by  what  is  going  on  in  the  legs  and 
the  part  of  the  spinal  cord  belonging  to  them.  The  discussion 
in  a  previous  section  has  shewn  that  much  of  the  coordination 


Chap,  ii.]  THE   BRAIK  779 

of  the  body  is  carried  out  by  the  middle  portions  of  the  brain, 
and  on  these  the  motor  area  must  have  its  hold  as  well  as  on 
the  spinal  mechanisms. 

The  details  of  the  nature  of  that  hold  are  at  present  un- 
known to  us ;  but  it  must  be  remembered  that  not  all  the 
fibres  passing  down  from  the  motor  region,  not  all  those  even 
proceeding  from  the  densest  and  most  clearly  defined  motor 
areas,  are  pyramidal  fibres.  With  the  pyramidal  fibres  are 
mingled  fibres  having  other  destinations,  and  some  of  these 
probably  pass  to  the  thalamus  and  so  join  the  great  tegmental 
region.  ^loreover  th^  motor  region  must  have  close  ties  with 
other  regions  of  the  cortex  whence  fibres  pass  to  the  pons  to 
make  connections  with  the  cerebellum.  On  the  other  hand, 
the  cerebellum  is  especially  connected  with  what  we  may  fairly 
consider  the  afferent  side  of  the  spinal  cord  and  bulb.  These 
facts  must  merely  be  taken  as  indicating  the  possibilities  by 
which  the  motor  region  is  kept  in  touch  Avith  the  great  coordi- 
nating mechanism ;  it  would  be  venturesome  at  present  to  say 
much  more. 

In  an  ordinary  voluntary  movement  an  intelligent  conscious- 
ness is  an  essential  element.  But  many  skilled  movements 
initiated  and  repeated  by  help  of  an  intelligent  conscious  Aboli- 
tion may,  when  the  nervous  machinery  for  carrying  them  out 
has  acquired  a  certain  facility,  (and  in  all  the  higher  j^rocesses 
of  the  brain  we  must  recognize  that,  in  nervous  mateiial  at 
all  events,  action  determines  structure,  meaning  by  structure 
molecular  arrangement  and  disposition)  be  carried  out  under 
appropriate  circumstances  with  so  little  intervention  of  distinct 
consciousness  that  the  movements  are  then  often  spoken  of  as 
involuntary.  All  the  arguments  which  go  to  shew  that  the 
distinctl}'  conscious  voluntary  skilled  movement  is  carried  out 
by  help  of  the  appropriate  motor  area,  go  to  shew  that  the 
motor  area  must  play  its  part  in  the  involuntary  skilled  move- 
ments also.  So  that  distinct  consciousness  is  not  a  necessary 
adjunct  to  the  activity  of  a  motor  area.  And  it  is  worthy  of 
notice  that  some  of  these,  in  their  origin,  purely  voluntary 
skilled  movements,  which  by  long-continued  training  have  be- 
come almost  as  purely  involuntary,  are  hampered  rather  than 
assisted  by  being  '  thought  about.' 

Lastl}^  without  attempting  to  enter  into  psychological  ques- 
tions we  may  at  least  say  that  the  birth-place  of  wliat  we  call 
the  '  will,'  is  not  conterminous  with  the  motor  area ;  the  will 
arises  from  a  complex  series  of  events,  some  of  which  take 
place  in  other  regions  of  the  cortex,  and  probably  in  other  parts 
of  the  brain  as  well.  With  these  parts  the  motor  area  has  ties 
concerned  not  in  the  carrying  out  of  volition,  but  in  the  genera- 
tion of  the  will.  So  that,  looking  round  on  all  sides,  it  is 
obvious,  as  we  have  said,  that  the  motor  area  is  a  mere  link  in 


780  VOLUNTAEY   MOVEMENTS.  [Book  hi. 

a  complex  chain.  It  is  moreover  a  link  of  such  a  kind,  that 
while  the  changes  which  the  breaking  of  it  makes  in  the  daily 
life  of  a  lowly  animal,  such  as  the  dog,  in  whom  the  experience 
of  the  individual  adds  relatively  little  to  the  nervous  and  psy- 
chical storehouse  transmitted  from  his  ancestors,  can  hardly 
be  appreciated  by  a  bystander,  those  which  the  breaking  of  it 
makes  in  the  daily  life  of  a  man,  whose  brain  at  any  moment  is 
not  only  a  machine  fitted  for  present  and  future  work  but  a 
closely  packed  record  of  his  insist  life,  are  obvious  not  only  to 
the  individual  himself,  but  to  his  fellows. 


SEC.  4.  ON  THE  DEVELOPMENT  WITHIN  THE  CEN- 
TRAL NERVOUS  SYSTEM  OF  VISUAL  AND  OF 
SOME   OTHER   SENSATIONS. 

Visual  Sensations. 

§  494.  In  the  chain  of  events  through  which  some  influence 
brought  to  bear  on  the  periphery  of  a  sensory  nerve  gives  rise  to 
a  sensation,  we  are  able,  with  more  or  less  success,  to  distinguish 
between  those  events  which  are  determined  by  the  changes  at 
the  periphery  and  those  which  are  the  expression  of  changes 
induced  in  the  central  nervous  system.  Thus  when  certain 
rays  of  light  proceeding  from  an  object  and  falling  upon  the  eye 
give  rise  to  visual  perception  of  the  object,  two  sets  of  events 
happen  ;  the  rays  of  light,  by  help  of  the  mechanisms  of  the  eye, 
partly  dioptric,  partly  nervous,  give  rise  to  certain  changes  in  the 
fibres  of  the  optic  nerve,  which  we  may  call  visual  impulses  ;  and 
these  visual  impulses  reaching  the  brain  along  the  optic  nerve 
give  rise  to  visual  sensations  and  so  to  visual  perception  of  the 
object.  We  shall  later  on,  under  the  heading  of  "  the  senses," 
deal  chiefly  with  the  peripheral  events,  and  have  now  to  consider 
some  points  connected  with  the  central  events,  to  learn  what  we 
know  concerning  how  the  various  sensor}^  impulses  travelling 
along  the  several  kinds  of  sensory  nerves  behave  witliin  the 
central  nervous  system.  In  doing  so  we  shall  have  from  time 
to  time  to  refer  to  peripheral  events,  but  only  occasionally,  and 
never  in  any  great  detail.  It  will  be  convenient  to  begin  with 
the  special  sense  of  sight,  and  we  must  first  briefly  call  atten- 
tion to  a  few  points  which  we  shall  liave  to  study  in  fuller 
detail  hereafter. 

The  eye  is  so  constructed  that  images  of  external  objects  are 
brought  to  a  focus  on  the  retina,  the  stimulation  of  which  by 
light  starts  the  visual  impulses  along  the  fibres  of  the  optic 
nerve ;  and  the  distinctness  with  which,  by  means  of  the  visual 
sensations  arising  out  of  these  visual  impulses,  we  perceive 
external  objects  is  dependent  on  the  sharpness  of  the  retinal 
images.     The  eye  is  farther  so  constructed  that,  in  an}'-  posi- 

781 


782  VISUAL   SENSATIONS.  [Book  hi. 

tion  of  the  eye,  the  rays  of  light  proceeding  from  a  portion 
only  of  the  external  world  fall  upon  the  retina ;  or  in  other 
words  in  any  one  position  of  the  eye  only  a  portion  of  the 
external  world  is  visible  at  the  same  time.  The  portion  so 
seen  is  spoken  of  as  the  visual  field  for  that  position. 

The  image  thrown  on  the  retina  is  an  inverted  one,  so  that  the 
top  of  an  actual  object  is  represented  by  the  lower,  and  the  bot- 
tom by  the  upper  part  of  the  retinal  image  ;  similarly  the  actual 
left-hand  side  of  the  retinal  image  corresponds  to  the  right- 
hand  side  of  the  actual  object,  and  the  right-hand  side  to  the 
left-hand  side.  Hence  the  right-hand  half  of  the  visual  field 
corresponds  to  the  left-hand  side  of  the  retina,  and  the  left-hand 
half  to  the  right-hand  side. 

The  eye  can  be  moved  in  various  directions,  and  since  in  the 
visual  field  the  portion  of  external  nature  which  can  be  seen  at 
the  same  time  differs  with  each  different  position,  a  large  range 
of  vision  is  thus  secured;  and  this  can  be  further  increased  by 
movements  of  the  head.  Moreover  we  normally  make  use  of 
two  eyes,  our  normal  vision  is  binocular ;  and  the  visual  field 
of  the  right  eye  differs  from  that  of  the  left  eye.  There  is  one 
striking  difference  which  must  always  be  borne  in  mind.  A 
section  carried  through  the  eye  in  a  vertical  and  front-to-back 
plane,  through  what  we  shall  learn  to  call  the  optic  axis  (Fig. 
138,  ox)  (the  exact  details  of  the  plane  may  be  left  for  the 
present),  will  divide  the  retina  into  two  lateral  halves,  and  in 
each  retina  one  half  will  be  on  the  nasal  side  next  to  the  nose, 
and  the  other  half  will  be  on  the  malar  or  temporal  side, 
next  to  the  cheek  or  temple.  It  must  be  remembered  that  the 
nasal  halves  and  temporal  halves  of  the  two  retinas  do  not 
occupy  corresponding  positions  in  space.  The  temporal  half  of 
the  left  retina  is  on  the  left  side  of  its  own  eye,  whereas  the  tem- 
poral half  on  the  right  retina  is  not  on  the  left  but  on  the  right 
side  of  its  eye  ;  and  so  with  the  nasal  halves.  Now  in  the  right 
eye,  the  right-hand  side  of  the  visual  field  corresponds  to  the 
nasal  half  of  the  retina,  and  the  left-hand  side  of  the  visual 
field  to  the  temporal  half  of  the  retina,  whereas  in  the  left  eye 
the  right-hand  side  of  the  visual  field  corresponds  to  the  tem- 
poral half  of  the  retina,  and  the  left-hand  side  to  the  nasal  half. 
This  is  shewn  in  Fig.  138,  where  the  left-hand  visual  field  and 
the  retinal  area  concerned  are  shewn  shaded  in  each  eye. 

When  we  look  at  an  object  with  the  two  eyes,  though  two 
retinal  images  are  produced,  one  in  one  eye  and  one  in  the  other, 
we  perceive  one  object  only,  not  two.  This  is  the  essential  fact 
of  binocular  vision  ;  when  certain  parts  of  each  retina  are  stimu- 
lated at  the  same  time  we  are  conscious  of  one  sensation  only, 
not  two;  and  the  parts  of  the  two  retinas  which,  stimulated  at 
the  same  time,  give  rise  to  one  sensation  are  spoken  of  as  "  cor- 
responding  parts."     From  the  structure  and   relations  of  the 


Chap,  ti.] 


THE  BflAIN. 


783 


Fig.  138 


784  VISUAL   SENSATIONS.  [Book  in. 

Fig.   138.     Diagram   to   illustrate  the  Nervous  Apparatus  of  Vision  in 
Man.     (Sherrington.) 

L.  the  left  eye,  B.  the  right  eye,  o.x.  the  optic  axis.  F.  the  outline  of  the 
face  between  the  eyes,  0/5.7".  the  right  optic  tract  (shaded)  supplying,  through 
Op.  Be.  the  optic  decussation,  the  temporal  side  of  the  retina  of  the  right  eye 
and  the  nasal  side  of  the  retina  of  the  left  eye.  L.  F.  L.  and  L.  F.  B.  the  left 
visual  fields  of  the  left  and  right  eye  respectively  ;  the  two  fields  and  the  parts 
of  the  two  retinas  whose  excitation  produces  vision  over  the  fields  are  shaded, 
the  object  a  in  the  field  of  the  right  side  giving  rise  to  an  image  at  o',  and  a  on 
the  left  side  an  image  at  a' . 

The  right  optic  tract  is  represented  as  ending  in  GL.  the  lateral  corpus  geni- 
culatum,  in  Pn.  the  pulvinar,  and  in  AQ.  the  anterior  corpus  quadrigeminum,  all 
three  stippled  ;  op.  racl.  the  optic  radiation  from  these  bodies  to  B.  Oc.  the  right 
occipital  lobe,  whose  stippled  cortex  indicates  the  'visual  area.'  d.  the  'direct' 
tract  to  the  cortex,  c.  c.  corpus  callosum,  cut  across  at  the  splenium,  I.  v.  d. 
descending  horn  of  the  lateral  ventricle. 

The  left  side  has  been  utilized  to  indicate  at  F.  shaded  with  lines,  the  corti- 
cal motor  area  for  the  eyes;  fm.  c.  indicates  the  path  from  it  to  III.  IV.  VI.  the 
nuclei  of  the  third,  fourth  and  sixth  nerves,  p.  b.  the  posterior  longitudinal 
bundle,  shewn  as  a  broken  line.  NG.  the  nucleus  caudatus,  LN.  the  nucleus 
lenticularis  and  Til.  optic  thalamus  shewn  in  outline,  Cia.  the  front  limb,  Cig. 
the  knee,  and  Cip.  the  hind  limb  of  the  internal  capsule.  The  outlines  of  the 
fourth  ventricle  4th.  Vn.  and  of  the  posterior  corpora  quadrigemina  are  shewn 
by  dotted  lines,  that  of  the  bulb  is  shewn  by  a  fine  line.    p.  the  pineal  gland. 

two  eyes  it  follows  that  the  temporal  side  of  the  right  and  the 
nasal  side  of  the  left  eye  are  such  corresponding  parts,  while 
the  nasal  side  of  the  right  eye  corresponds  to  the  temporal  side 
of  the  left  eye.  But  the  whole  of  each  retina  is  not  employed  in 
binocular  vision.  Owing  to  the  position  of  the  two  eyes  in  rela- 
tion to  the  nose,  it  comes  about  that  an  object  held  very  much 
on  one  side,  to  the  left-hand  side  for  instance,  while  it  is  capable 
of  producing  an  image  on  the  extreme  nasal  side  of  the  left  eye, 
and  can  be  seen  therefore  by  that  eye,  cannot  produce  an  image 
on  the  temporal  side  of  the  right  eye ;  the  nose  blocks  the  way. 
It  is  therefore  not  seen  by  the  right  eye,  and  the  vision  of  it  is 
monocular,  by  the  left  eye  only.  In  Fig.  138  it  may  be  seen 
that  the  left  visual  field  of  the  left  eye  (^L.F.L.^  extends  more 
to  the  left,  and  is  larger  than  the  left  visual  field  of  the  right 
eye  (^L.F.R.^  and  that  the  right  retinal  area,  corresponding  to 
the  left  visual  field,  extends  farther  along  the  nasal  side  of  the 
left  side  («')'  than  it  does  along  the  temporal  side  of  the  right 
eye  (a')i  the  difference  being  due  to  the  presence  of  the 
nose  (i^).  And  similar  conditions  obtain  with  regard  to  the 
extreme  right-hand  side  of  the  visual  field. 

§  495,  After  these  preliminary  statements,  we  may  now  turn 
to  consider  some  anatomical  facts  concerning  the  ending  of  the 
optic  nerve  in  the  brain. 

The  optic  nerve  of  each  eye  consists  of  nerve  fibres  coming 
from  all  parts  of  the  retina  of  that  eye  ;  but  the  two  optic  nerves 
meet,  ventral  to  the  floor  of  the  third  ventricle,  cross  each  other 
at  the  optic  chiasma  (Fig.  138,  op.  -De),  and  are  thence  continued 
on  under  the  name  not  of  optic  nerves  but  of  optic  tracts  (Op.T.^. 


Chap,  ii.]  THE   BRAIX.  785 

The  decussation  of  fibres  which  takes  phice  in  the  chiasma  has 
peculiar  characters.  At  their  decussation  (we  are  speaking  now 
of  man)  the  fibres  in  the  optic  nerve  belonging  to  the  temporal 
half  of  tlie  eye  in  which  the  nerve  ends  pass  into  one  optic  tract, 
namely,  the  optic  tract  of  the  same  side,  while  the  fibres  belong- 
ing to  the  nasal  half  pass  into  another  optic  tract,  namely,  the 
optic  tract  of  the  opposite  side.  Thus  the  fibres  of  the  temporal 
half  of  the  right  eye  and  of  the  nasal  half  of  the  left  eye  pass 
into  the  riglit  optic  tract,  and  the  fibres  of  the  nasal  half  of  the 
right  eye  and  of  the  temporal  half  of  the  left  eye  pass  into  the 
left  optic  tract.  Compare  Fig.  138,  in  which  the  fibres  forming 
the  right  optic  tract  are  shaded  while  those  forming  the  left 
oj^tic  tract  are  left  unshaded.  Now,  the  nasal  half  of  one  retina 
and  the  temporal  half  of  the  other  retina  are  'corresponding' 
parts.  Hence,  while  each  optic  tract  contains  fibres  belonging 
to  half  of  each  eye,  the  two  halves  thus  represented  in  each  tract 
are  corresponding  halves. 

The  amount  and  character  of  the  decussation  taking  place 
in  the  optic  chiasma  differs  in  different  animal  types,  the  differ- 
ences having  relation  to  the  amount  of  binocular  vision,  which 
in  turn  depends  on  the  position  of  the  eyes  in  the  head,  that  is, 
on  the  prominence  of  the  face  between  the  eyes.  In  the  fish  for 
instance,  Avith  laterally  placed  eyes,  no  binocular  vision  at  all  is 
possible,  and  the  decussation  is  complete ;  the  whole  optic  nerve 
of  each  eye  crosses  over  to  the  other  optic  tract.  Between  this 
and  the  arrangement  in  man  just  described,  various  stages  obtain 
in  various  animals. 

The  chiasma  also  contains  at  its  liinder  part  fibres  which 
have  no  connection  with  the  optic  nerves  or  the  eyes,  but  are 
simply  commissural  tracts  passing  from  one  side  of  the  brain, 
namely,  from  the  median  corpus  geniculatum  (this  has  no  con- 
nection with  the  optic  nerves  and  is  not  concerned  in  vision) 
along  one  optic  tract,  through  the  chiasma  to  the  other  optic 
tract,  and  so  to  the  median  corpus  geniculatum  of  the  other  side 
of  the  brain.  These  fibres  are  spoken  of  as  the  inferior  or  pos- 
terior (optic)  commissure  or  arcuate  commissure,  or  Gudden's 
commissure.  It  was  once  thought  that  in  a  similar  Avay  fibres 
passed  from  one  retina  along  one  optic  nerve,  through  the  front 
part  of  the  chiasma  to  the  other  optic  nerve,  and  so  to  the  other 
retina  forming  an  anterior  (optic)  commissure  ;  but  this  seems 
to  be  an  error. 

The  optic  tract  also  contains  fibres  taking  origin  from  the 
grey  matter  in  the  floor  of  the  third  ventricle  and  forming  what 
is  sometimes  spoken  of  as  Meynert's  commissure  ;  tliese  fibres 
which  belong  neither  to  tlie  optic  nerves  nor  to  the  inferior  com- 
missure, join  the  optic  tracts  for  a  while,  but  eventually  leave 
them  to  pass  to  the  pes. 

§  496.     That  part  of  the  optic  tract  which  consists  exclu- 

50 


786  VISUAL   SENSATIONS.  [Book  iii. 

sively  of  fibres  coming  from  the  retinas  of  the  two  eyes,  and  it 
is  this  part,  and  this  part  only,  wliich  is  concerned  in  vision, 
ends  in  three  main  Avays,  as  shewn  diagrammatically  in  Fig.  138. 
In  the  first  place  part  of  the  tract  ends  in  the  lateral  corpus 
geniculatum  (^GrL).  In  the  second  place,  a  very  large  number 
of  fibres  passing  the  corpus  geniculatum  on  its  ventral  and  lat- 
eral surfaces  spread  out  into  the  pulvinar  (P  V) .  In  the  third 
place  others,  in  considerable  number,  taking  a  more  median  direc- 
tion, reach  the  anterior  corpus  quadrigeminum  (^AQ^.  These 
three  sets  end  apparently  in  connection  with  the  nerve  cells  of 
the  respective  bodies.  Thus  the  really  optic  fibres  of  the  optic 
tract  end  in  one  of  three  collections  of  grey  matter,  the  lateral 
corpus  geniculatum,  the  pulvinar,  and  the  anterior  corpus  quad- 
rigeminum. Further,  we  have  reasons  for  thinking  that  a  con- 
siderable part  at  all  events  of  the  grey  matter  of  these  three 
bodies,  especially  of  the  first  two,  is  associated  with  and,  in  a 
certain  sense,  dependent  on  the  fibres  of  the  optic  nerves ;  the 
reasons  are  as  follows.  We  know  that  when  a  nerve  fibre  is  cut 
away  from  its  trophic  centre  it  degenerates  ;  but  the  division,  and 
the  loss  of  the  peripheral  degenerating  portion,  has  no  obvious 
effect  on  the  trophic  centre  ;  when  a  spinal  nerve,  for  instance, 
is  divided  below  the  spinal  ganglion,  though  the  nerve  below 
the  section  degenerates,  the  ganglion  and  the  piece  of  nerve  in 
connection  with  it  remain  very  much  as  before,  We  have  it, 
however,  in  our  power  to  bring  about  changes  of  a  deeper  and 
wider  character,  a  cessation  of  growth  amounting  to  atrophy,  by 
operative  interference  with  nervous  structures  before  they  are 
fully  developed.  Thus  in  an  adult  animal,  a  section  of  an  optic 
nerve  or  removal  of  the  eye  leads  to  degeneration  in  the  optic 
nerve  and  optic  tract ;  the  optic  fibres  have  their  trophic  centre 
in  certain  cells  of  the  retina,  of  which  we  shall  speak  in  treating 
of  vision,  and  cut  away  from  that  centre  they  degenerate ;  by 
this  means  the  nature  of  the  optic  decussation  in  animals,  and 
indeed  in  man,  has  been  ascertained.  But  if  the  ej^es  be  removed 
(removal  of  both  eyes  being  desirable  on  account  of  the  charac- 
ters of  the  optic  decussation),  in  a  new-born  animal,  not  only 
do  both  the  optic  nerves  and  the  greater  part  of  both  optic 
tracts  cease  to  be  further  developed  and  degenerate,  but  the 
bodies  mentioned  above,  the  two  lateral  corpora  geniculata,  the 
pulvinar  on  each  side,  and  the  two  anterior  corpora  quadrigemina 
do  not  fully  develope  ;  certain  parts  of  them  undergo  atrophy. 
The  development  of  these  nervous  structures  seems  therefore 
to  be  largely  dependent  on  their  functional  connection  with  the 
eyes  by  means  of  the  optic  tracts  and  nerves. 

The  same  method  confirms  the  view  expressed  above  that 
the  median  corpus  geniculatum  has  no  connection  with  vision^ 
When  the  eyes  of  new-born  animals  are  extirpated  neither  the 
median  corpora  geniculata  nor  the  ]30sterior  corpora  quadrigemina 


Chap,  ii.]  THE   BRAIN.  .  787 

shew  any  sign  of  atrophy,  and  the  part  of  the  optic  tract  which 
does  not  degenerate  is  the  inferior  commissure  connecting  the 
two  median  corpora  geniculata.  Obviously  these  parts  are  asso- 
ciated with  functions  of  the  brain  other  than  those  of  sight. 
The  lateral  corpora  geniculata,  the  pulvinar  and  the  anterior 
corpora  quadrigemina,  are,  we  may  repeat,  alone  to  be  I'egarded 
as  the  chief  central  parts  in  which  the  optic  nerves  end.  We 
may  also  repeat  that  owing  to  the  peculiarity  of  the  optic  decus- 
sation each  optic  nerve  thus  finds  its  endings  in  both  sides  of 
the  brain. 

§  497.  Though  the  above  three  bodies  are  undoubtedly  the 
chief  endings  of  the  optic  nerve,  three  primary  visual  centres, 
if  we  may  so  call  them,  it  is  also  believed  that  some  fibres  of 
the  optic  tract,  making  connections  with  neither  of  these  three 
bodies,  pass  by  the  cms  cerebri  straight  to  certain  parts  of  the 
cerebral  hemisphere  (Fig.  138,  (/)  ;  but  this  fourth  ending  is  by 
no  means  so  clearly  established  as  are  the  other  three.  And 
undoubtedly  the  main  connection  of  the  cerebral  hemisphere 
with  the  optic  tract  is  not  a  direct  one,  but  an  indirect  one, 
through  the  three  bodies  in  question.  A  number  of  fibres  pro- 
ceeding from  the  occipital  cortex  and  reaching  the  thalamus 
through  the  hind  limb  of  the  internal  capsule  form  \\hat  is 
called  the  'optic  radiation,'  These  fibres  beginning  (or  ending) 
in  the  cortex  of  the  occipital  region,  end  (or  begin),  (Fig.  138, 
op.  racT)  to  a  large  extent,  in  the  pulvinar  and  in  the  lateral 
corpus  geniculatum,  but  also  in  the  anterior  corpus  quadrigem- 
inum.  reaching  it  by  the  anterior  brachium.  When  in  a  new- 
born animal  the  occipital  cortex,  or  even  a  certain  part  of  the 
occipital  cortex  is  removed,  these  three  bodies  atroph}-,  and 
the  atrophy  extends  to  the  optic  tract  and  nerve ;  conversely 
removal  of  both  eyes  in  a  new-born  animal  leads  not  only  to 
atrophy  of  the  three  bodies  in  question,  but  also  to  imperfect 
growth  of  the  occipital  lobes.  Not  only  so,  but  even  in  the  full- 
grown  animal  removal  of  the  occipital  cortex  entails  a  degenera- 
tion and  atrophy  of  these  three  bodies,  the  details  differing  in 
different  animals  according  to  their  kind,  the  degeneration  in 
some  animals  reacliing  into  the  optic  tract  and  nerve.  ^Vnd 
even  in  man  degeneration  of  the  external  corpus  geniculatum,  of 
the  pulvinar,  and  to  some  extent  of  the  anterior  corpus  quadri- 
geminum  and  of  the  optic  tract  has  been  observed  to  result  from 
disease  of  the  occipital  lobe.  This  is  a  remarkable  circum- 
stance ;  for  we  may  assume  that  the  fibres  carrying  impulses 
from  say  the  corpus  geniculatum  to  the  occij^ital  lobe,  are  axis 
cylinder  processes  starting  from  cells  in  the  former  structure 
to  end  in  the  latter,  and  therefore  having  their  trophic  centres 
in  the  former.  The  degeneration  occurring  in  the  corpus  gen- 
iculatum as  a  result  of  the  removal  of  or  disease  in  the  occipital 
cortex  is  a  different  thintj  from  the  deereneration  Avhich  takes 


788  .  VISUAL   SENSATIONS.  [Book  in. 

place  in  the  optic  tract,  after  removal  of  tlie  retina;  it  seems 
rather  a  degeneration  from  ]3ure  want  of  functional  activity,  as 
if  the  cells  in  the  corpus  geniculatum  are  unable  to  act  as  the}^ 
ought  to  do  on  the  arrival  of  visual  impulses  along  the  optic 
fibres,  if  their  ties  with  the  occipital  cortex  are  broken.  We  may 
therefore  conclude  that  in  the  complex  act  of  vision  two  orders 
of  central  apparatus  are  involved ;  we  may  speak  of  two  kinds 
of  centres  for  vision,  the  primary  or  lower  visual  centres  sup- 
plied by  the  three  bodies  of  which  we  are  speaking,  and  a  sec- 
ondary or  higher  visual  centre  supplied  by  the  cortex  in  the 
occipital  region  of  the  cerebrum.  And  experimental  results 
accord  with  this  view. 

Before  we  proceed  to  discuss  those  results,  one  or  two  pre- 
liminary observations  may  prove  of  use. 

In  the  first  place,  as  we  have  previously  urged,  the  interpre- 
tation of  the  results  of  an  experiment  in  which  we  have  to  judge 
of  sensory  effects,  are  far  more  uncertain  than  when  we  have 
to  judge  of  motor  effects,  that  is  of  course  when  the  experi- 
ment is  conducted  on  an  animal.  We  can  estimate  the  motor 
effect  quantitatively,  we  can  measure  and  record  the  contrac- 
tion of  the  muscles  ;  but  in  estimating  a  sensory  effect  we  have 
to  depend  on  signs,  our  interpretation  of  which  is  based  on 
analogies  which  may  or  may  not  be  misleading.  We  are  on 
safer  ground  when  we  can  appeal  to  man  himself  in  the  experi- 
ments instituted  by  disease ;  but  the  many  advantages  thus 
secured  are  often  more  than  counterbalanced  by  the  diffuse 
characters,  or  the  complex  concomitants  of  the  lesion.  In 
dealing  with  sensory  effects  we  must  expect  and  be  content 
for  the  present  with  conclusions  less  definite  and  more  uncer- 
tain even  than  those  gained  by  the  study  of  motor  effects. 

In  the  second  place,  in  dealing  with  vision,  it  will  be  desir- 
able to  know  the  meaning  which  we  are  attaching  to  the  words 
which  we  employ.  By  blindness,  that  is  '  complete '  or  '  total ' 
blindness,  we  mean  that  the  movements  and  other  actions  of 
the  body  are  in  no  way  at  all  influenced  by  the  amount  of  light 
falling  on  the  retina.  Of  partial  or  incomplete  or  imperfect 
vision,  using  the  word  vision  in  its  widest  sense,  there  are  many 
varieties ;  and  we  may  illustrate  some  of  the  defects  of  the 
visual  machinery,  regarded  as  a  whole,  with  its  central  as  well 
as  its  peripheral  parts,  by  referring  to  certain  defects  of  vision 
due  to  changes  in  the  eye  itself.  The  eye  may  fall  into  such 
a  condition,  that  the  mind  can  only  appreciate,  and  that  to  a 
varying  degree,  the  difference  between  light  and  darkness ;  the 
mind  is  aware  that  the  retina  (or  it  may  be  part  of  the  retina) 
is  being  stimulated  to  a  less  or  greater  degree,  but  cannot  per- 
ceive that  one  part  of  the  retina  is  being  stimulated  in  a  differ- 
ent way  from  another  part ;  a  sensation  of  light  is  excited,  but 
not  a  set  of  visual  sensations  corresponding  to  the  sets  of  pen- 


CHAr.  II.]  THE  BRAIK  789 

oils  of  luminous  rays,  which,  reflected,  or  emanating  from 
external  objects  in  a  definite  order,  are  falling-  upon  the  eye. 
The  eye  again  may  fall  into  another  condition,  in  wliich 
such  sets  of  visual  sensations  are  excited,  but  on  account  of 
dioptric  imperfections  or  for  other  reasons,  the  several  sensa- 
tions are  not  adequately  distinct;  the  mind  is  aware  through 
the  eye  of  the  existence  of  'things,'  but  cannot  adequately 
recognize  the  characters  of  those  things ;  the  visual  images  are 
blurred  and  indistinct.  And  a  large  number  of  gradations  are 
possible  between  the  extreme  condition  in  which  only  those 
objects  which  present  tlie  strongest  contrast  with  their  sur- 
roundings are  visible,  to  a  condition  which  only  just  falls  short 
of  normal  vision.  Imperfections  of  this  kind,  of  varying  de- 
gree, may  result  from  failure  not  in  the  peripheral  apparatus, 
not  in  the  retina,  or  optic  nerve  or  other  parts  of  the  eye,  but 
in  the  central  apparatus;  the  retinal  image  may  be  sharp,  the 
retina  and  the  optic  fibres  may  be  duly  responsive,  but  from 
something  wrong  in  some  part  or  other  of  the  brain,  the  visual 
sensations  excited  by  the  visual  impulses  may  fail  in  distinct- 
ness, and  that  in  varying  degree :  imperfections  of  vision 
whether  of  central  or  peripheral  origin,  in  which  visual  sensa- 
tions fail  in  distinctness  are  generally  spoken  of  under  the  not 
wholly  unexceptionable  name  of  amhh/opia. 

If  one  optic  nerve  be  divided,  total  blindness  of  one  eye  will 
result ;  but  if  one  optic  tract  be  divided,  it  follows  from  what 
has  been  said  above,  that  half-blindness  in  the  corresponding 
halves  of  both  ej^es  will  result.  If,  for  instance,  the  right  optic 
tract  (Fig.  138,  Op.  T.)  be  divided,  the  left  visual  fields  of 
both  eyes  will  be  blotted  out.  The  same  condition  will  be 
brought  about  by  failure  in  the  optic  tract  at  its  central  ending, 
provided  of  course  the  mischief  be  confined  to  the  ending  of  the 
one  tract.  Such  a  half-blindness  or  half-vision  is  spoken  of  as 
hemianopsia,  or  hemianopia  or  hemiopia ;  the  words  left  and 
right  are  generally  used  in  reference  to  the  visual  field ;  thus 
left  hemianopsia  is  the  blotting  out  of  both  left  visual  fields, 
througli  failure  of  the  right  optic  tract.  If  instead  of  the  whole 
optic  nei"ve  being  divided,  certain  bundles  onl}'"  were  cut  across, 
partial  blindness  in  that  eye  would  be  the  result,  a  portion  of 
tlie  visual  field  of  that  eye  would  be  blotted  out.  If  not  the 
optic  nerve  but  the  optic  tract  were  so  treated,  mischief 
limited  to  a  few  bundles  of  one  tract  would  lead  to  correspond- 
ing blots  in  the  corresponding  halves  of  the  visual  fields  of  both 
eyes.  Further,  an  affection  of  lialf  the  retina  or  of  a  limited  area 
in  the  retina  might  occur  of  sucli  a  character  as  to  lead  not  to 
complete,  but  to  partial  blindness,  to  a  hemi-amblyopia  or  to  a 
partial  amblyopia.  The  i)art  of  the  retina  so  affected  might  be 
central,  or  peripheral,  and  might  be  a  quadrant,  or  au}^  patch  of 
any  size,  form  and  relative  position.     And  we  may  further  im- 


790  VISUAL   SEE:SATI0NS.  [Book  III. 

agine  it  at  least  possible  that  mischief  in  the  brain  might  be  so 
limited  as  to  produce  any  of  the  above  partial  effects,  though  the 
retina,  optic  nerve,  and  optic  tracts  all  remained  intact. 

The  above  visual  imperfections  we  have  illustrated  by  changes 
in  the  peripheral  apparatus,  but  there  is  a  kind  of  imperfection 
which  we  may  still  call  a  visual  imperfection,  though-  it  is  of 
purely  central  origin.  In  a  normal  state  of  things  a  visual  sen- 
sation, excited  in  the  brain,  is  or  may  be  linked  on  to  a  chain  of 
psychical  events ;  we  often  then  speak  of  it  as  a  visual  idea. 
When  we  see  a  dog,  the  visual  sensation,  or  rather  the  group  of 
sensations  making  up  the  visual  perception  of  the  dog,  does  not 
exist  by  itself,  apart  from  all  the  other  events  of  the  brain ;  it 
joins  and  affects  them,  and  among  the  events  which  it  so  affects 
may  be  and  often  are  psychical  events  ;  the  visual  perception 
'  enters  into  our  thoughts '  and  modifies  them.  Between  the 
visual  impulse  as  it  travels  along  the  optic  nerve  or  tract  and  its 
ultimate  psychical  effect  a  whole  series  of  events  intervene  ;  and 
we  may  take  it  for  granted  that  the  chain  may  be  broken  or  spoilt 
at  any  of  its  links,  at  the  later  as  well  as  at  the  earlier  ones. 
We  may  therefore  consider  it  possible  that  the  break  or  damage 
may  occur  at  the  links  by  which  the  fully  developed  visual  sensa- 
tion joins  on  to  psychical  operations.  We  may  suppose  that  an 
object  is  seen  and  yet  does  not  affect  the  mind  at  all  or  affects  it 
in  an  abnormal  way. 

These  foregoing  considerations  emphasize  the  difficulty  and 
uncertainty  of  interpreting  the  visual  condition  of  an  animal 
which  has  been  experimented  upon.  When  for  instance,  after 
an  operation,  an  animal  ceases  to  be  influenced  in  its  previous 
normal  manner  by  the  visual  effects  of  external  objects,  a  most 
careful  psychical  analysis  is  often  necessary  to  enable  us  to  judge 
whether  the  newly  introduced  disregard  of  this  or  that  object  is 
due  to  the  mere  visual  sensations  being  blurred  or  blunted,  or  to 
some  failure  in  the  psychical  appreciation  of  the  sensations ;  and 
in  most  cases  such  an  analysis  is  beyond  our  reach.  The  greatest 
caution  is  needful  in  drawing  conclusions  from  experiments  of 
this  kind,  especially  from  such  as  appear  to  have  been  hastily 
carried  out  or  hastily  observed  ;  and  we  must  be  content  here  to 
dwell  on  some  of  the  broader  features  only  of  the  subject. 

§  498.  Since  we  have  in  this  matter  to  trust  so  much  to  an- 
alogies with  our  own  experience,  we  may  turn  at  once  to  the 
monkey,  as  being  more  instructive  than  any  of  the  lower  animals. 
We  have  already  said  that  electrical  excitation  of  the  occipital 
cortex  behind  the  motor  region  may  produce  movements,  but  that 
these  movements  are  in  character  different  from  those  caused  by 
stimulation  of  the  motor  region  itself.  In  the  monkey  stimulation 
of  parts  of  the  occipital  region,  the  occipital  lobe  and  the  angu- 
lar gyrus  for  instance,  may  give  rise  to  movements  of  the  eyes, 
of  the   eyelids,  and  of  the  head,  that  is  of  the  neck,  all  the 


Chap,  ii.]  THE   BRAIK  791 

movements  so  produced  being  such  as  are  ordinarily  connected 
with  vision.  It  will  not  be  prolitable  to  enter  here  into  the  de- 
tails concerning  the  exact  topography  of  the  excitable  parts  or 
of  the  special  characters  of  the  movements  so  called  forth.  But 
it  is  important  to  note  that  these  movements  are  unlike  the  move- 
ments excited  by  stimulation  of  the  appropriate  motor  area  in  as 
much  as  their  occurrence  is  far  less  certain,  they  need  a  stronger 
stimulus  to  bring  them  out,  when  evoked  they  are  feeble,  being 
easily  antagonized  by  ap})ropriate  stimulation  of  the  motor  area, 
and  they  have  a  much  longer  latent  period.  They  are  not  due 
to  any  indirect  stimulation  of  the  motor  area,  through  '  associa- 
tion '  fibres  connecting  the  spot  stimulated  with  the  motor  area 
or  otherwise,  since  they  persist  after  removal  of  the  motor  area. 
Movements  of  this  kind  may  also  be  witnessed  in  the  dog.  They 
are  obviously  the  result  of  impulses  transmitted  in  some  direct 
manner  from  the  cortex  to  some  parts  below,  and  may  be  taken 
as  an  indication  that  the  parts  of  the  cortex  in  question  are  in 
some  way  connected  with  vision.  The  exact  manner  however  in 
which  they  are  brought  about  is  at  present  not  clear. 

§  499.  The  results  of  removal  of  the  cortex  in  the  monkey 
also  shew  clearly  that  the  hind  region  of  the  cortex  is  specially 
connected  wdtli  vision,  though  there  has  been  and  indeed  still  is 
much  discordance  among  the  various  observers  both  as  to  the 
particular  results  and  especially  as  to  their  interpretation. 

Many  observers  have  found  that  removal  of  the  occipital  lobe 
on  one  side,  the  region  marked  '  vision '  in  Figs.  123, 124,  caused 
hemiopia,  the  effect  on  the  visual  fields  being  a  crossed  one ; 
w^hen  the  right  lobe  for  instance  was  removed  there  was  blind- 
ness in  the  left  visual  fields,  that  is  in  the  right  halves  of  the 
retinas  of  both  eyes  i  in  other  words  the  visual  impulses  passing 
along  the  right  optic  tract  failed  to  produce  their  usual  effect,  so 
that  the  animal  disregarded  objects  on  its  left-hand  side.  We 
may  remark  that  the  decussation  of  the  optic  nerves  in  the  monkey 
is  very  similar  to  that  in  man.  When  both  occipital  lobes  were 
removed,  total  blindness  resulted.  Such  a  result  seems  perhaps 
at  first  sight,  not  only  in  accordance  with  the  anatomical  connec- 
tions spoken  of  above,  but  also  simple  and  satisfactory.  Visual 
impulses  originating  in  the  corresponding  halves  of  the  two  re- 
tinas travel  along  the  appropriate  optic  tract  to  the  primary  vis- 
ual centres  of  one  side,  and  thence  pass  by  the  optic  radiation  to 
the  occipital  lobe  of  the  same  side  (for  there  is  no  decussation 
along  the  whole  line  save  at  the  optic  chiasma),  where  they  give 
rise  to  visual  perceptions.  But  even  a  little  consideration  will 
raise  difficulties.  The  eyes  are  preeminently  organs  of  bilateral 
use ;  and  if,  when  w^e  are  looking  with  the  two  eyes  at  an  object 
whose  image  occupies  parts  of  the  two  halves  of  each  retina, 
the  impulses  generated  in  the  corresjionding  halves  of  the  two 
retinas,  affect  exclusively  the  one  side  of  the  brain  only,  there 


792  VISUAL   SENSATIONS.  [Book  iii. 

must  exist  remarkably  delicate  and  exact  arrangements  between 
the  two  halves  of  the  brain  by  which  in  each  case  as  it  occurs,  the 
two  sets  of  visual  sensations  should  be  accurately  pieced  into  a 
whole  perception  in  which  we  can  subjectively  recognize  no 
halves.  It  is  at  least  strange  that  while  the  motor  nervous  mech- 
anism for  the  two  eyes  is  finely  bilateral,  the  sensory  mechanism 
should  be  baldly  unilateral.  We  need  not  then  be  surprised  to 
meet  with  other  experimental  results  discordant  with  the  above 
mentioned  apparently  simple  result.  In  the  first  place  not  only 
is  the  hemiopia,  caused  by  the  removal  of  one  occipital  lobe,  often 
transient,  but  also,  according  to  some  observers,  the  lost  vision 
may  return  after  the  total  removal  of  both  lobes,  though  some 
impairment  may  be  noticed  long  afterwards,  so  long  in  fact  as 
the  animal  is  kept  alive.  In  the  second  place  in  the  hands 
of  other  observers  destruction  not  of  the  occipital  lobe  but 
of  the  angular  gyrus  of  one  side  (Fig.  122)  has  led  to  hemi- 
opia, failure  in  the  left  (or  right)  visual  fields,  indicating 
failure  in  the  central  endings  of  the  right  (or  left)  optic 
tract,  being  caused  by  removal  of  the  right  (or  left)  gyrus, 
and  destruction  of  both  angular  gyri  has  led  to  total  blindness, 
not  only  the  hemiopia  but  the  total  blindness  being,  however, 
apparently  transitory.  And  cases  have  been  observed  in  which 
the  blindness  due  to  removal  of  the  occipital  lobe  which  would  be 
by  itself  transient  has  been  made  permanent  by  the  subsequent 
removal  of  the  angular  gyrus.  In  other  cases  again  destruction 
of  one  angular  gyrus  has  produced,  not  hemiopia,  but  crossed 
blindness  or  crossed  amblyopia,  that  is  to  say  has  affected  the 
whole  of  the  retina  of  one  eye,  and  that  the  crossed  eye,  the  eye 
of  the  same  side  not  being,  or  being  supposed  not  to  be,  at  all 
affected ;  and  indeed  similar  results  have  also  been  stated  to 
follow  upon  removal  of  one  occipital  lobe. 

In  man  clinical  histories  so  far  conform  to  the  results  of  ex- 
periments on  the  monkey  as  to  associate  the  occipital  cortex,  and 
more  particularly  the  cuneus  (see  Figs.  134,  135)  with  vision. 
A  very  large  and  increasing  number  of  cases  have  been  recorded 
in  which  hemiopia,  a  blotting  out  of  the  corresponding  halves  of 
the  visual  fields  of  the  two  eyes  (homonymous  hemiopia  as  it  is 
called)  has  been  associated  with  disease  of  the  occipital  lobe 
namely  of  the  cuneus,  or  of  adjoining  parts  of  the  lingual  lobe, 
in  the  neighbourhood  of  the  calcarine  fissure,  or  of  adjoining  por- 
tions of  the  occipital  convolutions;  and  there  have  been  similar 
cases  where  not  the  half,  but  a  part  only,  a  quadrant  it  may  be 
or  less,  of  each  visual  field  has  been  blotted  out.  The  teaching 
of  such  cases  is  in  full  accord  with  the  anatomical  leading,  but 
leaves  untouched  the  difficulty  mentioned  above  as  attaching  to 
such  unilateral  cerebral  action.  It  is  worthy  of  note  that  in  many 
such  cases  of  hemiopia,  the  macula  lutea,  the  region  of  distinct 
vision  is  left  intact  in  both  eyes ;  and  this  has  led  some  to  the 


Chap,  ii.]  THE   BRAIX.  793 

view,  that  the  macula  of  each  eye  unlike  the  rest  of  the  retina  is 
represented  in  both  cerebral  hemispheres.  But  this  is  not  very 
satisfactory.  Certain  cases  on  the  other  hand  have  been  met 
with,  which  like  some  of  the  experiments  on  monkeys  point  to 
at  least  some  share  being  taken  by  the  angular  gyrus. 

Many  experiments  have  been  made  on  the  dog,  and  the  results 
obtained  have  been  interpreted  by  some  observers  as  shewing  that 
in  this  animal  not  only  is  an  area  in  the  occipital  region  of  the 
cortex  specially  connected  with  vision,  but  even  that  particular 
parts  of  the  area  correspond  to  particular  parts  of  the  field  of 
vision,  the  several  parts  of  the  retina  being  as  it  were  projected 
on  to  the  cortex.  We  need  not  enter  here  upon  the  details  of 
this  view  which  needs  some  special  exposition  since  in  the  dog 
vision  is  much  less  binocular  than  in  man ;  it  will  become  desir- 
able to  do  so  should  evidence  be  forthcoming  that  a  similar  pro- 
jection obtains  in  man :  but,  though  some  clinical  histories  have 
been  held  to  indicate  this,  others  are  opposed  to  it.  In  short 
though  it  is  clear  that  the  occipital  cortex  is  concerned  in  vision, 
our  present  knowledge  does  not  afford  a  consistent  view  of  the 
exact  way  in  which  it  is  concerned. 

v^  500.  We  may  perhaps  say  a  few  words  on  the  question 
what  is  it  which  actually  takes  place  in  the  cortex  during 
vision?  Are  we  to  conceive  of  it  as  if  a  visual  impulse  set 
going  along  the  fibres  of  the  optic  tract  underwent  no  essen- 
tial change  until  it  reached  the  cortex,  as  if  it  there  suddenly 
developed  into  a  '  visual  sensation '  ?  We  can  hardly  suppose 
this.  Between  the  cortex  and  the  optic  tract,  the  lower  visual 
centres,  the  tegmental  masses,  intervene ;  and  we  can  hardly 
suppose  that  interference  with  these  bodies  produces  the  same 
effect  on  vision  as  simple  section  of  the  optic  tract.  We  have 
seen  in  a  previous  section  that  the  frog  and  the  bird  certainly, 
and  according  to  some  observers  also  the  rabbit,  are  in  the 
absence  of  the  cerebral  hemispheres  not  totally  blind,  since 
their  movements  appear  to  be  guided  by  retinal  impressions ; 
and  cases  are  recorded  of  the  dog  being  obviously  still  guided 
in  some  measure  by  retinal  impressions  after  the  occipital  lobes 
and  indeed  the  greater  part  of  the  brain  had  been  removed-.  This 
is  a  matter  of  no  little  difficulty ;  it  is  perhaps  possible  for  sim- 
ple afferent  impulses  to  determine  even  complex  movements 
without  the  intervention  of  '  consciousness,'  and  we  may  be  jus- 
tified in  speaking  of  the  effects  of  light  on  a  brainless  animal  as 
being  mei-e  instances  of  '  mechanical '  reflex  action  ;  still  we  are 
probably  justified  in  assuming  that  the  simple  visual  impulses, 
travelling  along  the  fibres  of  the  optic  tract,  undergo  important 
transformations  in  the  tegmental  masses,  and  that  the  changes 
which  are  propagated  along  the  fibres  of  the  optic  radiation, 
constitute  something  quite  different  from  the  impulses  along 
the   optic  tract   or   nerve.     We  may  perhaps  assume  that   in 


794  OLFACTOEY   SENSATIONS.  [Book  iii. 

vision  the  cortical  events  are  psychical  in  nature,  and  that  the 
function  of  the  optic  radiation  is  to  furnish  what  we  may  call 
crude  visual  sensations  for  further  psychical  elaboration.  Or 
perhaps  we  do  wrong  in  attempting  to  dissociate  in  any  such 
distinct  way  the  cortical  and  the  tegmental  events.  The  re- 
markable dependence  as  regards  nutrition  between  the  occipital 
cortex  and  the  tegmental  masses,  of  which  we  spoke  a  little 
while  back,  shews  that  the  two  work  together  in  the  closest 
way.  Possibly  we  are  wrong  in  thinking  that  in  the  generation 
of  a  visual  sensation,  a  something  passing  upwards  from  the  one 
is  transformed  into  something  else  in  the  other,  and  ought 
rather  to  suppose  that  the  sensation  is  a  product  of  the  two 
working  together  in  a  manner  to  the  understanding  of  which 
our  ordinary  conceptions  of  nervous  impulses  gathered  from 
the  study  of  ordinary  efferent  and  afferent  nerves  afford  us 
little  help.  Possibly  also  the  ties  between  the  cortical  and 
tegmental  processes  are  drawn  tighter  in  the  higher  animals 
as  vision  becomes  more  complex,  so  that  after  total  removal  of 
the  cerebral  cortex  while  the  bird  and  possibly  even  the  dog 
may  be  said  still  to  'see,'  man  and  probably  the  higher  monkeys 
appear,  as  experiments  and  clinical  histories  shew,  to  be  not 
only  psychically  but  in  every  way  blind,  and  that  long  before 
degenerative  changes  in  the  tegmental  masses  can  have  become 
marked. 

As  to  the  several  parts  played  by  the  individual  tegmental 
masses  we  at  present  know  little  or  nothing.  It  is  worthy  of 
note  that  when  we  compare  various  animals,  we  find  that  the 
connections  of  the  cortex  with  the  corpus  quadrigeminum  are 
the  more  prominent  in  the  lower  animals,  and  in  the  higher 
animals  more  and  more  give  place  to  those  with  the  pulvinar 
and  especially  with  the  corpus  geniculatum.  Some  facts  per- 
haps may  be  taken  as  indicating  that  the  corpus  quadrigeminum 
is  especially  concerned,  when  visual  sensations  influence  the 
coordination  of  movements ;  but  this  is  not  certain. 

Sensations  of  Smell. 

§  501.  In  many  animals  in  whom  the  sense  of  smell  is  acute, 
a  portion  of  the  cortex,  known  as  the  '  pyriforra  lobe  '  or  '  hippo- 
campal  lobule,'  and  which  is  anatomicall}^  continuous  with  the 
front  end  of  the  hippocampal  gyrus  (the  part  to  which  the  name 
uncinate  gyrus  is  often  restricted),  acquires  relatively  large 
dimensions.  This  and  the  anatomical  relations  just  mentioned 
would  lead  us  to  suppose  that  a  part  of  the  cortex  which  is 
continuous  with  the  front  end  of  the  hippocampal  gyrus  is  in 
some  way  connected  with  smell.  The  argument  from  compara- 
tive anatomy,  however,  is  one  which  must  be  used  with  caution ; 
since,  besides  the  great  difficulty  of  determining  the  homologies 


Chap,  ii.]  THE  BRAIK  795 

of  parts  of  the  brain  in  different  animals,  relative  increase  in  the 
part  in  question  might  be  correhited  to  other  things  than  the 
power  of  smell,  and  might  be  determined  by  circumstances  hav- 
ing no  relation  to  smell. 

The  experimental  evidence,  though  on  the  whole  it  gives 
support  to  the  view,  is  conflicting ;  and  when  the  difficulty  of 
determining  whether  a  '  dumb  animal '  can  or  cannot  smell  is 
borne  in  mind,  this  will  not  be  wondered  at.  The  observation 
that  electrical  stimulation  of  the  region  in  question  gives  rise  to 
movements  of  the  nostrils,  which  have  been  interpreted  as  sniffing 
in  response  to  subjective  olfactory  sensations,  cannot  have  much 
weight;  and  while  some  observers  have  found  that  the  removal 
of  this  part  of  the  brain  destroys  the  sense  of  smell,  others  have 
obtained  negative  results. 

Tlie  few  clinical  histories  which  bear  upon  the  matter  are 
perhaps  more  trustworth3^  These  seem  to  shew  that  a  lesion 
involving  the  cortex  of  this  region,  but  leaving  the  olfactory  bulb 
and  tract,  as  well  as  other  parts  of  the  brain,  intact,  may  destroy 
or  greatly  impair  smell.  And  we  may  perhaps  give  particular 
weight  to  the  cases  in  which  epileptiform  attacks,  preceded  by 
an  '  aura '  in  the  form  of  a  peculiar  smell,  have  been  associated 
with  disease  limited  to  this  region  ;  for  the  phenomena  of  '  aura  ' 
seem  to  be  connected  with  cortical  processes. 

Though  the  evidence  on  the  whole  goes  to  shew  that  the 
cortex  at  the  front  end  of  the  hippocampal  gyrus  is  especially 
connected  with  smell,  and  we  have  so  marked  it  in  Fig.  137,  yet 
the  whole  matter  stands  on  a  somewhat  different  footing  from 
the  sense  of  sight.  In  man  the  relations  of  smell  to  the  other 
operations  of  the  brain  (though,  as  we  shall  see  in  dealing  witli 
the  senses,  somewhat  peculiar)  are  far  more  limited  than  are 
those  of  vision,  and  the  psj^chical  development  of  simple  olfactory 
sensations  is  extremely  scanty. 

Sensations  of  Taste. 

§  502.  This  special  sense  though  so  closely  associated  with 
smell  stands,  together  with  the  special  sense  of  hearing,  on  a 
different  tooting  from  the  two  preceding  special  senses,  since  the 
nerves  concerned  belong  to  the  category  of  ordinary  cranial 
nerves,  and  we  lack,  in  reference  to  them,  the  anatomical  lead- 
ing which  is  offered  to  us  in  the  case  of  the  optic  and  olfactory 
nerves. 

We  shall  see  in  dealing  with  the  senses  that  the  fifth  nerve 
and  the  glosso})haryngeal  nerve  have  been  considered  as  nerves 
of  taste,  but  that  the  matter  is  one  subject  to  controversy ;  tlie 
gustatory  function  of  the  fifth  is  attributed  to  the  peculiar 
chorda  tympani  nerve,  and  other  questions  have  been  raised. 
Whatever  view  we  take,  however,  the  nerves  of  taste  are  ordi- 


796  SENSATIOXS  OF  HEAEIXG.  [Book  hi. 

naiy  cranial  nerves,  and  we  have  no  anatomical  guidance  as  to 
the  fibres  of  either  of  the  above  two  nerves  making  special  con- 
nections with  an}^  part  of  the  cortex.  Though  sensations  of 
taste  enter  largely  into  the  life  of  animals,  and  indeed  of  man 
himself,  we  have  no  satisfactory  indications  which  will  enable 
us  to  connect  this  special  sense  with  any  part  of  the  cortex ;  the 
view  indeed  has  been  put  forward  that  some  part  of  the  cortex 
in  the  lower  portion  of  the  temporal  lobe,  not  far  from  the  centre 
for  smell,  serves  as  a  centre  for  taste ;  but  the  arguments  in 
favour  of  this  view  are  not,  as  yet  at  least,  convincing. 


Sensatio7is  of  Hearing. 

§  503.  The  cochlear  division  of  the  eighth  or  auditory 
nerve  may  be  assumed  to  be  a  nerve  of  the  special  sense  of  hear- 
ing, and  of  that  alone  ;  the  vestibular  division  serves,  as  we  have 
seen,  for  other  functions  than  those  of  hearing,  §  478,  but  as  we 
shall  urge  in  dealing  with  the  senses  is  not  to  be  regarded  as 
wholly  useless  for  the  purposes  of  that  sense.  The  cochlear 
division  may  be  traced  into  the  bulb,  and  the  vestibular  division 
into  the  lateral  auditory  nucleus  (which  perhaps  may  be  regarded 
as  a  continuation  or  segmental  repetition  forwards  of  the  cuneate 
nucleus  or  of  part  of  that  nucleus),  and  into  the  cerebellum, 
the  cerebellar  continuation  being  probably  the  part  of  the  nerve 
which  serves  for  coordinating"  functions.  The  connections  of 
the  auditory  nerve  with  the  cerebral  hemisphere  belong  to  the 
same  category  as  those  of  other  afferent  cranial,  and  we  may  add 
spinal  nerves ;  we  have  no  very  clear  anatomical  guide  towards 
any  particular  part  of  the  cortex. 

When  we  turn  to  the  empirical  results  furnished  by  experi- 
ment and  clinical  observations,  we  find  that  these,  though  even 
less  definite  and  less  accordant  than  in  the  case  of  the  senses  of 
sight  and  smell,  point  to  part  of  the  first  or  superior  temporal 
(temporo-sphenoidal)  convolation  (Figs.  123,  134,  136)  lying  in 
the  temporal  lobe  just  ventral  to  the  Sylvian  fissure,  as  being 
specially  concerned  in  hearing  in  some  such  way  as  the  occipital 
lobe  is  concerned  in  vision. 

Electrical  stimulation  of  this  region  of  the  cortex  gives  rise 
to  'pricking  of  the  ears,'  and  other  movements  such  as  are 
frequently  connected  with  auditory  sensations;  but  such  phe- 
nomena are  in  this  instance  perhaps  to  be  depended  upon  even 
less  than  in  other  similar  instances.  While  some  observers 
maintain  that  this  convolution,  the  operation  including  other 
portions  of  the  temporal  lobe  as  well,  may  be  removed  from  a 
monkey  without  producing  any  certain  signs  of  deafness,  other 
observers  have  found  that  removal  of  it  on  one  side  affected  the 
hearing  of  the  ear  on  the  opposite  side,  and  removal  on  both 


Chap,  ii.]  THE   BKAIN".  797 

sides  bronglit  the  animal  into  a  condition  in  which,  without 
being  perhaps  absolutely  deaf,  it  reacted  towards  sound  in  a 
very  imperfect  manner  indeed,  very  different  from  its  normal 
behaviour.  The  scanty  clinical  histories  bearing  on  this 
matter  are  not  very  decisive ;  for  though  deafness  has  been 
observed  in  connection  with  disease  affecting  the  suj)erior  tem- 
poral convolution,  the  lesion  has  usually  invaded  other  parts 
as  well,  and  the  deafness  has  been  associated  with  other  S3niip- 
toms,  notably  aphasia.  An  auditory  'aura'  has  however  at 
times  been  observed  in  connection  with  disease  of  this  region, 
as  also  a  peculiar  psychical  failure,  known  as  '  word  deafness,' 
in  which,  though  sounds  are  heard,  that  is  to  sa}^  auditory  sen- 
sations are  felt,  it  may  be  even  as  usual,  the  perception  or  psy- 
chical appreciation  of  the  sounds  is  lacking,  and  a  spoken  word 
is  not  recognized. 

Lastly,  we  may  add  that,  though  as  we  said  the  anatomical 
leading  is  not  definite,  observers  have  found  that,  in  new-born  ani- 
mals, on  the  one  hand  destruction  of  the  part  of  the  cortex  prob- 
ably corresponding  to  the  region  mentioned  above,  leads  to 
atrophy  of  the  median  corpus  geniculatum,  and,  to  some  extent, 
of  the  posterior  corpus  quadrigeminum  ;  and  on  the  other  hand 
destruction  of  the  internal  ear  leads  to  an  atrophy  of  part  of  the 
lateral  fillet  of  the  opposite  crossed  side  which  may  be  traced 
to  the  posterior  corpus  quadrigeminum,  and  thence  to  the 
median  corpus  geniculatum ;  and  section  of  the  lateral  fillet 
on  one  side  leads,  among  other  results,  to  atrophy  of  the 
striae  acusticae  and  tuberculum  acusticum  of  the  crossed 
side.  This  suggests  that  the  path  of  auditory  impulses  is 
along  the  cochlear  nerve  to  the  lateral  fillet  of  the  crossed  side, 
and  so  b}'  the  posterior  corpus  quadrigeminum  and  median 
corpus  geniculatum  to  the  cortex  of  the  temporal  lobe  of  that 
crossed  side,  the  two  latter  bodies  bearing  towards  hearing  a 
relation  somewhat  like  that  borne  towards  sight  by  the  ante- 
rior corpus  quadrigeminum  and  lateral  corpus  geniculatum. 
But  the  matter  needs  farther  investio-ation. 

Thei'e  remains  the  special  sense  of  touch,  but  this  we  had 
better  consider  in  connection  with  sensations  in  general. 


SEC.  5.  ON  THE  DEVELOPMENT  OF  CUTANEOUS  AND 
SOME  OTHEE  SENSATIONS. 

§  504.  The  sensations  with  which  we  have  just  dealt  arise 
througli  impulses  passing  along  special  nerves  or  parts  of  spe- 
cial nerves,  the  optic  nerve,  the  olfactory  nerve,  &c. ;  we  have 
now  to  deal  with  sensations  arising  through  impulses  along  the 
nerves  of  the  body  generally.  These  are  of  several  kinds.  In 
the  first  place  there  are  sensations  which  we  may  speak  of  as. 
'  cutaneous  sensations,'  the  impulses  giving  rise  to  which  are 
started  in  the  skin  covering  the  body,  or  in  the  so-called  mucous 
membrane  lining  certain  passages.  These  sensations,  which  as 
we  shall  see  in  dealing  with  the  senses  are  dependent  on  the 
existence  of  special  terminal  organs  in  or  near  the  skin,  are 
sensations  of  '  touch,'  in  the  narrower  meaning  of  that  word, 
by  which  we  appreciate  contact  with  and  pressure  on  the  skin, 
and  the  sensations  of  '  temperature '  which  again  we  may,  as 
we  shall  see,  divide  into  sensations  of  'heat'  and  sensations  of 
'  cold.'  These  sensations  may  be  excited  in  varying  degree  by 
impulses  passing  along  any  nerve  branches  of  which  are  sup- 
plied to  the  skin.  Then  there  are  the  sensations  constituting 
the  '  muscular  sense,'  to  which  we  have  already  referred,  and 
these  again  may  be  excited  in  any  nerve  having  connections 
with  the  skeletal  muscles. 

As  we  shall  see  in  dealing  with  the  senses,  when  a  nerve  is 
laid  bare  and  its  fibres  are  stimulated  directly  either  by  pres- 
sure, such  as  pinching,  or  by  heat,  or  by  cold,  or  in  other  ways, 
the  sensations  which  are  caused  do  not  enable  us  to  appreciate 
whether  the  stimulation  is  one  of  contact  or  pressure,  or  of  tem- 
perature, or  of  some  other  kind ;  we  only  experience  a  '  feeling,' 
which  at  all  events  when  it  reaches  a  certain  intensity  we  speak 
of  as  'pain.'  And  we  have  reason  to  think  that  at  least  from 
time  to  time  impulses  along  various  nerves  give  rise  to  sensations 
which  have  been  spoken  of  as  those  of  'general  sensibility,' 
by  which  in  addition  to  other  sensations,  such  as  those  of  touch 
and  of  the  muscular  sense,  we  become  aware  of  changes  in  the 
condition  and  circumstances  of  our  body.     When  the  stimula- 

798 


Chap,  ii.]  THE   BRAIN.  .  799 

tion  of  the  skin  exceeds  a  certain  limit  of  intensity,  the  sense 
of  touch  or  temperature  is  lost  in,  that  is  to  sa}^,  is  not  appreci- 
ated as  separate  from  the  sense  of  pain ;  and  under  abnormal 
circumstances  acute  sensations  of  pain  are  started  b}^  changes 
in  parts,  for  example  tendons,  the  condition  of  which  under 
normal  circumstances  we  are  not  conscious  of  appreciating 
through  any  distinct  sensations,  though  it  maj^  be  that  these 
parts  do  normally  give  rise  to  feeble  impulses  contributing  to 
'general  sensibility.'  It  may  be  debated  whether  'pain'  is  a 
phase  of  all  sensations,  or  of  general  sensibility  alone,  or  a  sen- 
sation sui  generis.  We  shall  have  something  further  to  say  on 
this  matter  when  we  treat  of  the  senses;  meanwliile  it  Avill  be 
convenient  for  present  purposes  if  we  consider  that  the  sensa- 
tions we  have  to  deal  with  just  now  are  the  sensations  of  touch 
and  of  temperature,  those  of  the  muscular  sense,  and  those  of 
general  sensibility  including  those  of  pain. 

§  505.  The  fairly  convincing  evidence  that  the  occipital 
cortex  has  special  relations  with  vision,  and  the  less  clear  evi- 
dence that  other  regions  have  special  relations  with  smell  and 
hearing,  suggest  that  special  parts  of  the  cortex  have  special 
relations  with  the  sensations  now  under  consideration.  But  in 
the  cases  of  the  senses  of  sight  and  smell  we  had  a  distinct  ana- 
tomical leading ;  and  we  have  seen  how  uncertain  is  the  evidence 
where  such  an  anatomical  leading  fails  or  is  deficient,  as  in 
hearing  and  taste.  In  the  case  of  sensations  of  the  body  at 
large,  the  anatomical  leading  similarly  fails  us.  Moreover,  if 
our  judgment  concerning  the  visual  sensations  of  animals  oper- 
ated on  is  difficult,  how  much  more  difficult  must  be  our  judg- 
ment concerning  their  sensations  of  touch  and  temperature,  and 
even  of  pain? 

As  we  have  already  urged  (§§  488,  489)  observations  made 
on  man  himself  whether  in  the  cases  where  it  has  been  possible 
to  stimulate  the  cortex  by  an  electric  current,  or  in  respect  to 
the  phenomena  of  disease,  such  as  the  aura  of  epileptic  attacks 
and  the  like,  shew  most  distinctly  that  the  so-called  motor  region 
of  the  cortexis  closely  associated  with  sensatiojis,  and  that  par- 
ticular areasof  this  region  are"especially  associated  with  sensa- 
tions originating  in  particular  parts  of  the  body.  AVe  have 
also  seen  that  experiments  on  monkeys  confirm  this  view;  the 
removal  of  a  motor  area,  that  for  instance  of  the  hand,  entails 
not  only  loss  of  movement  in  the  hand,  but  also  loss  or  impair- 
ment of  sensation  in  the  hand,  lasting  as  long  at  least  as  the 
loss  or  impairment  of  the  movements ;  moreover,  so  far  as  can 
be  learnt,  ;ill_  semaations  are  affected  by  the  removaLof .  the  cortex, 
those  of  pressure  and  temperature  as  well  as  those  of  the  muscu- 
lar sense  and  of  pain.  Similar  results  have  also  been  obtained  in 
the  dog.  So  that  the  evidence  seems  convincing  that  the  parietal 
region  of  the  cortex,  while  it  has  special  connections  with  volun- 


800  CUTANEOUS   SENSATIONS.  [Book  hi. 

taiy  movements,  has  at  the  same  time  special  connections  with 
cutaneous  and  other  sensations,  and  in  both  relations  may  be 
mapped  out  into  areas  corresponding  to  parts  of  the  body. 

But  this  matter  of  the  sensations  is  one  of  even  greater  com- 
plexity than  that  of  voluntary  movements.  This  is  shewn  among 
other  things  by  the  fact  that  the  sensations  with  which  we  are 
now  dealing  may  be  profoundly  affected  by  operations  on  parts 
of  the  cortex  other  than  the  above  region.  In  the  dog,  for  in- 
stance, according  to  many  observers,  removal  of  almost  any  part 
of  the  cortex  impairs  cutaneous  sensations,  the  amount  and 
duration  of  the  effect  being  broadly  proportionate  to  the  extent 
of  cortex  removed,  though  operations  on  the  frontal  region  have 
the  least  effect.  Other  observers  again  have  found  that  in  the 
monkey  removal  or  destruction  of  the  gyrusfornicatus  (Figs.  122, 
124)  on  the  mesiaT^surface  of  the"  braih^  ventfaFto^  the  calloso- 
marginal  sulcus  which  forms  on  the  mesial  surface  the  ventral 
limit  of  the  motor  region  (an  operation  of  very  great  difficulty), 
has  brought  the  whole  of  the  opposite  side  of  the  body  to  a  con- 
dition which  has  been  described  as  an  anesthesia,  that  is  a  loss 
of  all  cutaneous  tactile  sensations,  and  an  analgesia,  that  is  a 
loss  of  sensations  of  pain,  the  condition  being  accompanied  by 
little  or  no  impairment  of  voluntary  movements  and,  though 
apparently  diminishing  as  time  went  on,  lasting  until  the  death 
of  the  animal  some  weeks  afterwards.  Again,  removal  of  the 
continuation  of  the  gyrus  fornicatus  into  the  gyrus  hippocampi 
has  in  other  instances  led  to  a  more  transient  anesthesia  also  of 
the  whole  or  greater  part  of  one  side  of  the  body.  And  it  is 
asserted  that  removal  of  no  other  region  of  the  cortex  interferes 
with  cutaneous  and  painful  sensations  in  so  striking  and  lasting 
a  manner  as  does  the  removal  of  parts,  or  of  the  whole  of  this 
mesial  region.  These  contradictory  results  shew  how  complex 
and  difficult  the  subject  is. 

§  506.  We  may  now  attack  the  problem  in  a  different  way, 
and  instead  of  beginning  with  the  cortex  begin  with  afferent 
impulses  started  along  afferent  nerves  from  their  peripheral 
endings,  and  attempt  to  trace  them  central AV&^rds.  And  first 
we  may  call  to  mind  what  anatomical  guidance  we  possess. 

We  have  seen  (§  452)  that  the  fibres  of  posterior  roots,  the 
channels  of  afferent  impulses,  end  in  the  spinal  cord  in  at  least 
two  main  ways.  One  set  are  continued  on,  not  broken  by  any 
relays,  in  the  median  posterior  tract,  and  by  this  tract  represen- 
tatives of  all  the  spinal  nerves  are  connected  with  the  gracile 
nucleus  in  which  the  median  posterior  column  ends.  The  other 
fibres  of  a  posterior  root  end  in  the  grey  matter  of  the  cord  not 
far  from  their  entrance  ;  we  have  reason  to  think  that  they  are 
brought  into  contact  by  arborescent  endings  collateral  or  ter- 
minal, with  the  bodies  or  processes  of  certain  cells  in  the  grey 
matter.     Putting  aside  as  foreign  to  our  present  subject  the 


Chap,  ii.]  THE   BRAIN.  801 

probable  endings  in  contact  with  motor  cells  which  seem  to 
afford  the  mechanism  for  many  reflex  actions,  we  may  distin- 
guish at  least  two  other  kinds  of  ending.  We  have  reason  to 
think  that  some  of  the  fibres  make  connections  by  contact  with 
the  cells  of  the  vesicular  cylinder,  Clarke's  column.  We  have 
further  reason  to  think  that  axis-cylinder  processes  of  these 
cells  of  the  vesicular  cylinder,  go  to  form  the  cerebellar  tract, 
which  we  may  assume  to  be  an  afferent  tract,  and  which  may 
be  traced  through  the  restiform  body  to  the  cerebellum  in 
connection  with  certain  cells  of  which,  especially  those  of  the 
nucleus  dentatus,  the  fibres  of  the  tract  end.  Now  thougli  the 
cerebellum  is  connected,  in  an  indirect  way  it  is  true,  with 
the  cerebral  cortex  we  have  no  grounds  for  thinking  that  the 
cerebellum  is  concerned  with  the  development  of  the  sensations 
with  which  we  are  now  concerned :  when  the  whole  cerebellum 
is  removed  there  is  no  apparent  affection  of  cutaneous  sensations. 
We  may  therefore  dismiss  the  cerebellar  tract;  the  afferent 
impulses  passing  along  it  have  probably  to  do  with  the  coordi- 
nation of  movements  affecting  equilibrium,  but  they  are  not 
the  impulses  which  become  developed  into  conscious  sensations 
of  touch  or  of  pain. 

The  other  fibres  of  the  posterior  root  of  which  we  are 
speaking  end  in  connection  with  other  cells  of  the  grey  matter. 
From  these  cells  we  have  reason  to  think  fibres  originate  and 
pass  upward  in  the  white  matter.  Some  of  these  we  may 
distinguish  as  the  '  ascending  antero-lateral '  tract ;  but  our 
knowledge  here  becomes  less  definite.  Some  of  the  processes 
of  the  cells,  with  which  the  posterior  root  fibres  make  connec- 
tion, end  as  they  begin  within  the  grey  matter  of  the  cord  itself, 
in  connection  with  other  cells  of  the  grey  matter;  and  even 
those  which  travel  beyond  the  cord  appear  to  go  no  farther  than 
the  bulb,  ending  in  connection  with  cells  in  that  structure,  more 
particularly  with  the  cells  of  the  gracile  and  cuneate  nuclei. 
So  far  as  our  present  knowledge  goes  there  is  no  definite  tract 
from  various  parts  of  the  cord  to  the  brain  comparable  to  the 
pyramidal  tract  from  the  brain  to  various  parts  of  the  cord. 
The  passage  of  afferent  impulses  on  their  way  to  become  con- 
scious sensations  is  from  the  first  a  system  of  relays.  There  is 
a  first  relay  where  the  ending  of  the  root  fibre  makes  connec- 
tions with  a  nerve  cell  in  the  adjoining  grey  matter,  or  in  the 
gracile  nucleus.  There  is  in  the  former  case  a  further  rehi}-  in 
the  bulb  at  least,  and  possibly  in  some  instances  other  relays 
in  the  grey  matter  of  the  cord  on  the  way.  The  path  to  the 
bulb  thus  supplied  by  the  fibres  and  their  relays,  keeps  to  a 
large  extent  on  the  same  side  of  the  cord,  though  some  cross- 
ing is  observed. 

From  the  bulb  onwards  one  definite  path  only,  and  that  a 
narrow  one,  is  marked  out  anatomically  by  the  median  fillet 

51 


802  CUTANEOUS   SENSATIONS.  [Book  hi. 

(we  may  as  urged  above  neglect  the  ties  between  tbe  bulb  and 
the  cerebellum).  This,  starting  from  the  gracile  and  cuneate 
nuclei  and  crossing  at  the  interolivary  layer,  passes  on  through 
the  mid-brain  to  the  optic  thalamus  with  the  cells  of  which 
its  fibres  seem  to  make  connections,  though  according  to  some 
observers  part  of  the  fillet  is  continued,  without  relays,  right  to 
the  cerebral  cortex.  But  if,  as  is  stated,  the  destruction  of  the 
two  nuclei  followed  by  complete  degeneration  of  the  fillet  en- 
tails no  obvious  loss  of  sensation,  the  afferent  impulses  passing 
along  it  must  be  of  a  peculiar  kind.  Other  fibres  of  the  anterior 
lateral  columns,  some  at  least  of  which,  probably  most,  if  not  all, 
we  may  regard  as  afferent  in  nature,  pass  into  the  reticular  for- 
mation of  the  bulb,  and  thence  onward,  probably  more  or  less 
by  relays  into  the  tegmentum. 

§  507.  How  do  experimental  results  and  clinical  histories 
accord  with  such  an  anatomical  programme? 

We  may  first  call  attention  to  a  somewhat  old  experiment. 
We  have  seen,  §  153,  that  afferent  impulses  started  in  afferent 
fibres,  in  those  for  instance  of  the  sciatic  nerve,  so  affect  the 
vaso-motor  centre  in  the  bulb  as  to  cause  a  rise  of  blood-pressure, 
at  least  in  an  animal  under  urari.  Those  afferent  impulses 
must  pass  by  some  path  or  other  from  the  roots  which  sujDply 
the  sciatic  nerves  with  afferent  fibres  along  the  thoracic  and 
cervical  cord  to  the  bulb.  If  the  path  be  blocked,  the  stimula- 
tion of  the  sciatic  nerve  will  fail  to  produce  the  usual  rise  of 
blood-pressure.  Now  in  a  rabbit,  the  amount  of  rise  of  blood- 
pressure  following  upon  the  stimulation  of  one  sciatic  nerve  with 
a  certain  strength  of  current  having  been  ascertained,  it  is  found 
that  a  much  less  rise  of  blood-pressure  or  none  at  all  follows  the 
same  stimulation  after  division  of  certain  parts  of  the  cord  in 
the  mid  or  upper  thoracic  region ;  that  is  to  say,  the  section  of 
the  cord  has  partially  or  completely  blocked  the  path  of  the 
afferent  impulses.  Further,  the  block  is  conspicuous  when  the 
lateral  column  is  divided,  and  is  not  increased  by  other  parts  of 
the  cord  being  divided  at  the  same  time ;  when  both  lateral 
columns  are  divided  the  block  is  almost  complete.  And  further, 
supposing  one  sciatic,  say  the  right,  is  the  one  which  is  stimu- 
lated, a  block  occurs  both  when  the  lateral  column  of  the  same, 
right,  side  and  when  that  of  the  crossed,  left,  side  is  divided, 
but  is  greater  when  the  division  is  on  the  crossed  than  when  it 
is  on  the  same  side.  And  similar  results  were  obtained  when, 
the  experiment  being  conducted  in  a  similar  way,  signs  of  pain 
instead  of  variations  in  blood-pressure  were  taken  as  the  tokens 
of  the  blocking  of  impulses.  We  may  infer  that  the  impulses, 
which  reach  the  lumbar  cord  by  the  roots  of  the  sciatic  nerve, 
travel  up  the  cord,  or  rather  give  rise  within  the  lumbar  cord 
to  nervous  impulses,  which  travel  up  the  cord  in  such  a  manner 
that  in  the  lower  thoracic  region  they  pass  almost  exclusively 


Chap,  ii.]  THE   BRAIK  803 

along  the  fibres  of  the  lateral  column,  some  having  kept  to  the 
same  side  of  the  cord,  but  more  having  crossed  over  to  the  oppo- 
site side,  before  reaching  the  thoracic  region.  Though  these 
vaso-motor  experiments  have  a  certain  value,  inasmuch  as  the 
i-esults  gained  by  them  are  more  or  less  distinctly  quantitative 
and  measurable,  many  objections  may  be  urged  against  their 
validity  as  affording  a  general  proof  of  the  course  taken  in  the 
cord  by  impulses  giving  rise  to  sensations.  They  were  conducted 
on  rabbits,  animals  low  in  scale  and  especially  so  perhaps  in  re- 
spect to  the  spinal  cord,  they  were  limited  to  one  region  of  the 
cord,  the  observations  were  made  immediately  after  the  division 
of  the  cord,  before  the  immediate  effects  of  the  operation  had 
passed  off ;  and  further,  it  may  be  urged  that  impulses  affecting 
the  vaso-motor  centre  may  not  be  identical  with  those  giving  rise 
to  sensations. 

Many  experiments  have  been  made  on  dogs ;  and  if  we  con- 
tent ourselves  with  making  no  distinction  between  the  different 
kinds  of  afferent  impulses,  and  in  the  case  of  these  animals  it 
would  hardly  be  profitable  to  attempt  to  make  a  distinction,  we 
may  say  that  the  several  experiments  so  far  agree  that  they  point 
to  the  lateral  columns  as  being  the  chief  paths  of  afferent,  sensory, 
impulses,  or  to  speak  more  exactly,  to  the  passage  of  these  im- 
pulses being  especially  blocked  by  section  of  the  lateral  columns. 
Some  observers  find  that  in  the  dog  a  section  of  the  lateral  column 
on  one  side,  or  at  least  a  hemisection  of  the  cord,  produces  'loss 
of  sensation '  on  the  opposite  side  greater  than  on  the  same  side, 
or  confined  to  the  opposite  side,  and  even  accompanied  by  an  ex- 
altation of  sensation,  a  hyperesthesia,  on  the  same  side.  Other 
observers  again,  and  these  certainly  competent  observers,  find  that, 
in  the  dog,  section  of  one  side  affects  sensation  on  both  sides,  and 
indeed  chiefly  on  the  same  side.  We  may  perhaps  once  more  re- 
peat the  warning  how  difficult  is  the  quantitative  and  qualitative 
determination  of  sensations  in  such  an  animal  as  the  dog;  and  may 
remark  that  in  all  these  cases  of  unilateral  section  the  increased 
blood  supply  due  to  failure  of  the  normal  vaso-constrictor  tone 
must  influence  the  peripheral  development  of  sensory  impulses. 

In  these  experiments,  as  in  those  on  voluntary  movements, 
it  is  most  important  to  distinguish  between  immediate  or  tem- 
porary and  more  lasting  effects  ;  and  observers  have  found  that 
the  loss  of  sensation  following  a  hemisection  of  the  cord,  like 
the  loss  of  voluntary  movement,  is  temporary  only,  and  event- 
ually disappears,  though  the  recovery  is  slower  and  less  complete 
than  is  the  case  with  movements.  As  with  voluntary  movement 
(§  491)  so  with  sensation,  recovery,  though  less  complete  than 
that  of  movement,  is  possible  when  a  hemisection  on  one  side 
has  been  at  a  later  date  followed  by  a  hemisection  on  the 
other  side. 

The  experiments  on  monkeys  are  in  like  manner   neither 


804  CUTANEOUS   SEIs^SATlONS.  [Book  hi. 

accordant  nor  decisive;  and  even  in  these  animals  with  their 
more  varied  signs  of  sensations,  the  interpretation  of  these  signs 
is  beset  with  fallacies.  Some  observers  have  found  that  ahemi- 
section  (in  the  thoracic  region)  produced  loss  of  sensation  on 
the  crossed  side,  accompanied  by  little  or  no  loss  on  the  same 
side ;  other  observers  again  have  failed  to  obtain  after  a  hemisec- 
tion  satisfactory  proof  of  any  such  marked  loss  on  the  crossed 
side ;  they  find  on  the  contrary  that  impulses  giving  rise  to  sen- 
sations of  touch  and  of  temperature,  as  well  as  those  concerned 
in  the  muscular  sense  pass  up  the  same  side,  while  those  giving 
rise  to  pain  seem  to  pass  up  both  sides,  that  is  to  say  probably 
travel  along  the  grey  matter.  Further,  large  portions  of  the 
lateral  column,  the  more  internal  parts  adjacent  to  the  grey 
matter  being  left,  have  been  removed  without  any  very  obvious 
and  certainly  without  any  lasting  defects  of  sensation  on  the 
one  side  or  on  the  other. 

§  508.  Turning  now  to  man  we  find  that  clinical  experience 
shews  that  in  him  the  integrity  of  the  cerebral  hemispheres, 
and  of  the  connection  of  the  hemispheres  with  the  rest  of  the 
central  nervous  system,  is  essential  to  the  full  development  of 
sensations ;  and  that  in  this  respect  each  hemisphere  is  related 
to  the  crossed  side  of  the  body.  A  very  common  form  of  paraly- 
sis or  'stroke '  is  that  due  to  a  lesion  of  some  part  of  one  hemi- 
sphere (the  exact  position  of  the  lesion  need  not  concern  us 
now),  frequently  caused  by  rupture  of  a  blood  vessel,  in  which 
the  patient  loses  all  power  of  voluntary  movement  and  all  sensa- 
tions on  the  crossed  side  of  his  body  (including  the  face);  he 
is  said  to  be  suffering  from  hemiplegia,  'one  sided  stroke.' 
Not  only  do  voluntary  impulses  fail  to  reach  the  muscles  of  the 
affected  side,  but  sensory  impulses,  such  as  those  which,  started 
for  instance  in  the  skin,  would  under  normal  conditions  lead  to 
sensations  of  touch,  of  heat  or  cold,  or  of  pain,  fail  to  effect  con- 
sciousness, when  they  originate  on  the  affected  side ;  the  patient 
cannot  on  that  side  feel  a  rough  surface,  or  a  hot  body,  or  the 
prick  of  a  pin.  The  same  is  true  when  the  loss  of  sensation  is 
not  complete,  but  partial. 

Farther,  though  perhaps  anatomical  considerations  would 
lead  us  to  expect  that  a  great  deal  of  the  crossing  took  place  in 
the  spinal  bulb,  clinical  histories  moreover  agree,  at  least  to 
large  extent,  in  shewing  that  much  takes  place  in  the  cord,  so 
that  when  the  lesion  is  confined  to  one  half  of  the  cord,  the  sensa- 
tions affected  in  the  parts  below  the  level  of  the  lesion  are  chiefly 
or  even  exclusively  those  of  the  crossed  side.  But  there  is  not 
entire  accordance  among  observers,  especially  as  to  the  crossing 
being  complete.  And  with  regard  to  the  muscular  sense  there 
is  a  distinct  conflict  of  opinion ;  the  majority  of  cases  seem  to 
shew  that  in  unilateral  disease  or  injury  to  the  cord,  the  muscu- 
lar sense  in  company  with  the  voluntary  movements,  fails  on  the 


Chap,  ii.]  THE    BRAIX.  805 

same  side ;  but  cases  have  been  recorded  in  which  the  muscular 
sense  in  company  with  other  sensations,  seemed  to  be  affected 
on  the  crossed  side ;  it  must  be  remembered  however  that  it 
is  very  difficult  to  appreciate  a  deficiency  of  muscular  sense 
mingled  with  deficiencies  in  other  sensations,  and  we  should 
d  priori  expect  the  muscular  sense  to  run  parallel  with  motor 
impulses. 

The  clinical  histories  of  diseases  of  the  spinal  cord  in  man 
bring  to  light  in  a  fairly  clear  manner  a  fact  of  some  importance, 
namel}^  that  the  several  impulses  which  form  the  bases  of  the 
several  kinds  of  sensations,  of  touch,  heat,  cold,  and  pain,  and 
of  the  muscular  sense,  are  transmitted  along  the  cord  in  different 
ways  and  presumably  by  different  structures.  For  disease  may 
impair  one  of  these  sensations  and  leave  the  others  intact.  Thus 
cases  of  spinal  disease  are  recorded,  in  which  on  one  side  of  the 
body  or  in  one  limb  ordinary  tactile  sensations  seemed  to  be  little 
impaired,  and  yet  sensations  of  pain  were  absent ;  when  a  needle 
was  thrust  into  the  skin  no  pain  was  felt,  though  the  patient  was 
aware  that  the  needle  had  been  pressed  upon  the  skin  at  a  par- 
ticular spot ;  and  conversely  in  other  cases  pain  has  been  felt 
upon  the  insertion  of  a  needle,  though  mere  contact  with  or 
pressure  on  the  skin  could  not  be  appreciated.  Again,  cases  are 
recorded  in  which  the  skin  was  sensitive  to  touch  or  pain,  but 
not  to  variations  of  temperature  ;  it  is  further  stated  that  cases 
have  been  met  with  in  which  cold  could  be  appreciated  but  not 
heat,  and  vice  versa;  and  there  are  some  facts  which  point  to 
sensations  of  pain  being  more  closely  associated  with  those  of 
heat,  and  tactile  sensations  with  those  of  cold,  than  those  of  pain 
with  those  of  touch  or  those  of  heat  with  those  of  cold.  Cases 
of  spinal  disease  are  also  recorded  in  which  the  muscular  sense 
appeared  to  be  affected  apart  from  other  sensations.  We  shall 
return  to  these  matters  later  on  in  dealing  with  the  senses ;  we 
refer  to  them  now  simply  as  shewing  that  disease,  limited  as  far 
as  can  be  ascertained  to  the  spinal  cord,  may  affect  the  several 
sensations  separately,  and  therefore  as  suggesting  that  the  sev- 
eral kinds  of  impulses,  forming  the  bases  of  the  several  kinds  of 
sensation,  are  transmitted  in  different  ways  and  follow  different 
"  paths  '  along  the  spinal  cord. 

When  however  we  appeal  to  clinical  histories  or  indications 
as  to  the  several  paths  within  the  spinal  cord  taken  by  these 
several  impulses,  the  answer  is  a  most  uncertain  one,  as  indeed 
might  be  expected  from  the  too  often  diffuse  character  of  the 
lesions  of  disease ;  and  it  is  perhaps  not  too  much  to  say  that 
no  satisfactory  deductions  at  all  can  be  made. 

§  509.  Whether  then  we  turn  to  experiments  on  animals  or 
to  the  study  of  disease,  the  teachings  with  regard  to  sensation, 
in  contrast  to  those  with  regard  to  voluntary  movement,  are  in 
the  highest  degfiee  uncertain  and  obscure.     A  few  general  reflec- 


806  CUTANEOUS   SENSATIONS.  [Book  iii. 

tions  will  perhaps  help  us  to  appreciate  the  value  of  such  facts 
as  we  possess. 

We  have  seen  reason  to  think  that  in  every  movement 
whether  voluntary  and  of  cortical  origin,  or  involuntary  and 
started  either  as  a  simple  spinal  reflex  or  through  the  working 
of  some  part  or  other  of  the  brain,  the  motor  impulses,  which 
sweep  down  the  motor  fibres  to  the  muscles,  issue  marshalled 
and  coordinated  from  the  grey  matter  of  the  cord  (for  the  sake 
of  clearness  we  may  omit  the  cranial  nerves),  from  what  we  have 
called  the  motor  mechanisms  of  the  cord.  Analogy  would  lead 
us  to  suppose  that  the  afferent  impulses,  forming  the  bases  of 
the  several  kinds  of  sensations,  similarly  left  the  afferent  fibres 
to  join  the  grey  matter  of  the  cord  in  what  we  may  call  the  sen- 
sory mechanism.  And  such  anatomical  leading  as  we  possess 
seems  to  support  this  view ;  with  the  exception  of  the  median 
posterior  tract,  to  which  we  will  return  immediately,  all  the  fibres 
of  a  posterior  root  seem  to  end  in  the  grey  matter  not  very  far 
from  the  entrance  of  the  root.  We  have  seen  that  a  coordinate 
reflex  movement  may  be  carried  out  by  at  least  a  few  segments 
of  the  cord ;  that  a  reflex  movement  may  be  started  by  stimuli 
of  various  kinds  and  therefore  presumably  by  afferent  impulses  of 
various  kinds ;  and  that  impulses  forming  the  basis  of  the  mus- 
cular sense  are  essential  to  the  coordination  of  the  movement. 
All  our  knowledge  goes  to  shew  that  in  a  reflex  movement  car- 
ried out  by  a  few  segments  of  the  cord,  the  whole  chain  of  events 
between  the  arrival  of  the  afferent  impulses  along  the  posterior 
root  and  the  issue  of  efferent  impulses  along  the  anterior  root 
may  be  carried  out  by  grey  matter,  and  grey  matter  alone.  We 
may  further  infer  that,  while  on  the  one  hand  the  same  proce- 
dure might  obtain  not  through  a  few  segments  only  but  along 
the  whole  length  of  the  cord,  there  would  be  an  advantage,  espe- 
cially in  respect  to  the  rapidity  of  transmission,  in  employing 
internuncial  tracts  of  fibres  between  the  several  segments,  the 
advantage  being  greater  the  more  distant  the  segments  which 
have  to  work  together. 

We  might  further  suppose  that  it  would  be  of  advantage  to 
possess  some  direct  path  between  the  cerebral  cortex  and  the 
spinal  sensory  mechanism  immediately  connected  with  the  pos- 
terior root,  such  as  is  afforded  by  the  pyramidal  tract  between 
the  cortex  and  the  spinal  motor  mechanism  immediately  con- 
nected with  the  anterior  root.  But  no  anatomical  evidence  of 
such  a  tract  is  forthcoming ;  and,  as  we  have  before  remarked, 
along  all  the  tracts  which  seem  to  be  sensory  in  nature,  in  con- 
trast to  what  takes  place  in  the  motor  tracts,  relays  of  grey 
matter  are  continually  being  interpolated. 

The  median  posterior  tract,  since  it  gathers  up  representa- 
tives of  successive  nerves,  presents  itself  as  the  nearest  approach 
to  such  a  sensory  homologue  of  the  pyramidal  tract,  though  it 


Chap,  ii.]  THE   BKAIX.  807 

ends  in  the  bulb,  and  is  not  continned  on  directly  to  the  cortex. 
And  possibly  it  does  play  a  somewhat  analogous  part,  in  so  far 
as  it  serves  as  a  special  connection  between  the  brain  and  the 
whole  series  of  spinal  nerves.  But  we  are  wholly  ignorant  as 
to  what  it  really  does ;  and  whatever  be  the  exact  nature  of  the 
part  which  it  plays,  there  is  no  adequate  evidence  either  from 
clinical  histories  or  from  experiment  that  it  has  relations  to  one 
kind  of  sensation  only.  It  has  indeed  been  supposed  by  some 
to  be  especially  a  tract  for  the  impulses  of  the  muscular  sense ; 
but  neither  experiment  nor  clinical  study  affords  adequate 
proof  of  this  view.  The  condition  known  as  locomotor  ataxy, 
the  salient  feature  of  which  is  loss  or  impairment  of  muscu- 
lar sense,  is  associated  Avith  disease  of  the  posterior  root  and 
of  its  entrance  into  the  cord,  not  with  disease  confined  ex- 
clusively to  the  median  posterior  column.  Moreover  the  tract 
cannot  carry  all  the  impulses  of  muscular  sense,  since  some 
of  them  must  pass  at  once  into  the  grey  matter,  to  take  part 
in  the  coordination  of  reflex  movements,  and  must  therefore 
travel  by  fibres  Avhich  do  not  form  this  tract.  Similarly  is  there 
no  adequate  proof  of  the  tract  being  an  exclusive  channel  for 
tactile  or  for  painful  sensations.  Possibly  it  has  some  special 
relations  to  all  the  different  kinds  of  sensation. 

We  may  also  perhaps  urge  similar  considerations  with  regard 
to  the  cerebellar  tract,  which  though  starting  from  a  relay  of 
grey  matter  is  thence  onward  to  the  cerebellum  a  continuous 
tract.  This  tract  also  has  been  supposed  to  carry  impulses  of 
a  special  kind,  and  more  particularly  those  of  muscular  sense. 
But  even  admitting  that  the  tract  does  convey  impulses  derived 
from  the  muscles  and  their  appendages,  which  impulses  the 
cerebellum  makes  use  of  in  its  work  of  coordinating  movements, 
especiall}^  those  concerned  in  equilibrium,  it  cannot  be  the  chan- 
nel for  the  ordinary  impulses  of  muscular  sense,  since  these 
remain  after  total  removal  of  the  cerebellum.  Nor  does  either 
experiment  or  clinical  study  afford  in  other  ways  any  clear 
proof  that  this  tract  is  solely  or  even  especially  concerned  with 
the  muscular  sense. 

With  regard  to  the  antero-lateral  or  other  ascending  tracts 
our  knowledge  is  too  imperfect  to  justify  us  in  supposing  that 
any  one  is  the  special  or  exclusive  channel  for  aiiy  one  kind 
of  sensation,  or  indeed  in  drawing  any  conclusions  at  all  con- 
cerning it. 

But  when  we  subtract  from  the  white  matter  of  the  cord 
these  continuous  tracts  of  ascending  degeneration  of  presumably 
sensory  or  afferent  function,  and  the  continuous  tracts  of  descend- 
ing degeneration,  which  we  may  confidently  speak  of  as  motor 
or  at  least  efferent,  there  are  left  only  tlie  libres  which  we  may 
suppose  to  be  longitudinal  commissural  or  internuncial  fibres 
between  successive  segments.     We  are  thus  driven  to  the  pro- 


808  CUTANEOUS   SENSATIONS.  [Book  in. 

yisional  conclusion,  that  sensory  impulses  pass  either  by  the 
grey  matter  alone,  or  by  a  series  of  steps  as  it  were,  by  relays 
of  grey  matter  connected  by  internuncial  tracts  of  fibres,  whose 
length  we  do  not  know,  but  which  may  be  short.  That  such 
internuncial  tracts  intervene  is  rendered  probable,  on  the  one 
hand  by  the  fact  that  section  of  the  white  matter,  leaving  the 
grey  untouched,  does  affect  sensations,  and  on  the  other  hand 
by  the  fact  that  the  several  kinds  of  sensation  appear  to  travel 
along  the  cord  by  separate  paths,  or  at  least  may  be  separately 
blocked.  But,  if  we  accept  this  view,  we  must  at  the  same  time 
admit  that,  in  animals  at  least,  the  lines  provided  by  the  inter- 
nuncial tracts  and  their  relays  are  not  rigid,  that  within  limits 
and  under  circumstances  alternative  routes  are  possible. 

§  510.  We  may  here  perhaps  raise  once  more,  and  this  time 
more  pointedly  than  before,  the  doubt  whether  we  are  justified 
in  assuming,  as  we  generally  do  assume,  that  the  events  which 
take  place  in  the  fibres  connecting  relays  of  grey  matter  within 
the  central  nervous  system,  are  exactly  the  same  as  those  which 
take  place  in  the  fibres  of  nerves  outside  the  central  system, 
during  the  passage  of  what  we  call  a  nervous  impulse.  Most  of 
our  knowledge  of  a  nervous  impulse  has  been  gained  by  the  study 
of  the  motor  nerve  of  a  muscle-nerve  preparation.  Our  know- 
ledge of  the  processes  in  afferent  nerves  is  much  more  imperfect. 
And,  with  regard  to  the  processes  taking  place  in  fibres  within 
the  central  nervous  system  we  have  hardly  any  exact  experimental 
knowledge  at  all.  It  has  been  maintained  by  many  observers  that 
not  only  the  grey  matter  but  also  the  tracts  of  white  matter  in 
the  spinal  cord,  while  they  are  capable  of  conveying  impulses  in 
one  direction  or  the  other,  are  incapable  of  being  so  excited  by 
artificial  stimuli  as  to  generate  new  impulses.  These  observers 
maintain  that,  when  movements  or  signs  of  sensation  follow  the 
direct  stimulation  of  various  parts  of  the  cord,  the  effects  are  due 
to  issuing  motor  fibres  or  entering  sensory  fibres  having  been 
stimulated,  and  not  to  a  stimulation  of  the  intrinsic  substance  of 
the  parts  themselves  ;  they  propose  accordingly  to  call  these  parts 
'  kinesodic '  and  '  sesthesodic '  respectively,  that  is  to  say,  serv- 
ing as  paths  for  motor  or  sensory  impulses  without  being  them- 
selves either  motor  or  sensory.  The  evidence  on  the  whole  goes 
to  shew  that  this  view  is  a  mistaken  one,  that  the  various  tracts  of 
the  spinal  cord,  like  the  pyramidal  tract  and  indeed  other  parts  of 
the  brain,  are  excitable  towards  artificial  stimuli.  The  question 
cannot,  however,  be  considered  as  definitely  closed ;  and  the  very 
fact  that  it  has  been  raised  illustrates  the  point  on  which  we  are 
now  dwelling.  We  may  further  quote,  in  similar  illustration  of 
the  same  point,  the  following  remarkable  fact  which  was  observed 
in  the  series  of  experiments  referred  to  in  §  491  on  the  effects  of 
repeated  hemisection  of  the  spinal  cord  in  dogs.  The  animal  had 
partially  recovered  voluntary  movements  in  his  hind  limbs  after 


Chap,  ii.]  THE   BRAIN.  809 

a  third  heniisection  of  the  thoracic  cord,  and  yet  when,  at  his 
death,  a  strong  tetanizing  current  was  directed  through  the  bulb 
and  cervical  cord,  no  movements  of  the  hind  limbs  followed: 
the  impulses  started  by  artificial  stimulation  could  not  pass  the 
bridge  which  sufficed  for  volitional  impulses  of  natural  origin. 
It  is  not  too  much  to  say  that  our  experimental  knowledge  as  to 
the  events  which  accompany  the  activity  of  the  structures  within 
the  central  nervous  system  is  almost  entirely  limited  to  the 
recognition  of  the  '  currents  of  action  '  referred  to  in  §  486.  We 
are  already  going  beyond  our  tether  when  we  assume  on  the 
strength  of  this  that  the  processes  started  in  the  fibres  of  the 
pyramidal  tract  by  artificial  stimulation  are  in  all  respects  identi- 
cal with  those  started  in  the  fibres  of  a  motor  nerve.  We  are 
going  still  more  beyond  our  tether  when  we  assume  that  the 
processes  started  in  the  same  pyramidal  fibres  as  the  outcome  of 
natural  events  in  the  motor  cortex  are  of  the  same  kind.  But 
these  assumptions  are  trifles  compared  with  the  assumption  that 
the  events  taking  place  in  the  fibres  of  the  optic  radiation,  pass- 
ing from  the  pulvinar  to  the  occipital  cortex  are  identical  with 
the  events  taking  place  in  the  fibres  of  the  optic  tract  on  the  way 
to  the  pulvinar,  or  that  the  events  travelling  along  the  spinal 
cord  to  the  brain  as  the  result  of  a  prick  of  the  little  finger  are 
identical  with  those  which  the  prick  has  started  in  the  fibres 
of  the  ulnar  nerve.  Of  the  latter  events  we  know  a  little ;  of 
the  former  events  we  know  next  to  nothing.  And  we  may  here 
ask  the  question  what  is  the  meaning  of  these  continual  relays 
of  grey  matter  along  the  sensory  tract  unless  it  be  that  at  each 
relay,  some  transformation,  some  further  elaboration  of  the 
impulses  takes  place,  until  what  were  the  relativel}^  but  only 
relatively,  simple  impulses  along  the  fibres  of  the  peripheral 
nerve  are  by  successive  steps  changed  in  the  complex  events 
which  we  call  a  conscious  sensation?  We  have  no  reason  to 
think  that  the  afferent  impulses  started  at  the  periphery  of  a 
cutaneous  nerve-fibre  change  essentially  in  character  as  they 
travel  over  the  fibre  along-  the  stretch  of  nerve  of  which  it  is  a 
])art.  Nor  have  we  any  satisfactory  evidence  that  any  change  in 
tlie  character  of  the  impulses  is  effected  by  the  nerve  cell  in  the 
root-ganglion  with  which  the  fibre  is  connected  by  means  of  a 
T  piece,  though  this  is  possible.  Within  the  cord  things  are 
different.  The  arborescent  ending  of  the  fibre  of  the  posterior 
root  Avithin  the  spinal  cord  (or  bulb)  is  in  contact  with,  not  in 
continuity  with,  the  cell-substance  of  the  cell,  or  the  processes  of 
the  cell  on  which  it  impinges.  Of  course  it  is  possible  that  the 
ending  should  set  up  in  that  cell-substance  molecular  changes 
identical  with  those  which  constitute  the  impulse  passing  along 
the  fibre ;  but  it  is  much  more  probable  that  the  changes  which 
it  sets  up  are  of  a  different  order,  the  transition  being  in  a  rough 
way  comparable  to  the  transition  from  a  nervous  impulse  reach- 


810  CUTANEOUS   SENSATIONS.  [Book  in. 

ing  a  motor  end  plate  to  the  contraction  in  the  muscular  fibre 
which  that  impulse  brings  about.  The  new  changes  started  in 
the  first  relay  cell  may  be  very  different  from  those  coursing 
along  the  posterior  root-fibre ;  and  these  again  in  a  similar  way 
may  start  still  other  changes  in  the  next  relay  cell ;  and  so  on. 
We  may  therefore  well  hesitate  to  speak  of  or  consider  all  the 
events  in  the  central  nervous  system  as  either  motor  or  sensory 
in  nature.  It  is  perhaps  not  an  exaggeration  to  represent  the 
views  of  some  observers  as  if  they  supposed  that  afferent  impulses, 
say  tactile  impulses,  that  is  impulses  eventually  giving  rise  to 
tactile  sensations,  travelled  unchanged  from  the  skin  to  the 
cortex  and  there  suddenly  blossomed  into  sensations.  If  such  a 
view  were  true,  undoubtedly  the  chief  task  of  physiology,  almost 
the  only  one,  would  be  to  ascertain  the  tract  along  which  these 
impulses  passed.  But  if  on  the  other  hand  the  views  just  now 
urged  have  any  real  foundation,  the  question  of  tracts  or  paths 
sinks  into  insignificance  compared  with  the  almost  untouched 
problems  as  to  what  are  the  several  successive  changes  by  which 
simple  impulses  are  developed  into  full  sensations,  and  when  and 
how  the  changes  are  effected. 

§  511.  Seeing  how  unsatisfactory  is  our  present  knowledge 
with  regard  to  the  tracts  or  paths  of  sensations  in  the  relatively 
simple  spinal  cord,  it  would  be  useless  to  attempt  any  discussion 
as  to  their  paths  in  the  much  more  complex  brain.  If  it  be 
probable  that  the  passage  is  effected  by  relays  of  grey  matter  in 
the  former,  the  same  method  is  much  more  probable  in  the  latter ; 
and  if  neither  experiment  nor  clinical  study  throws  much  light 
on  the  path  up  to  the  bulb,  these  cannot  be  expected  to  give 
much  help  in  the  maze  of  grey  matter  and  fibres  by  which  the 
bulb  is  joined  to  the  cortex.  The  several  defined  areas  or  col- 
lections of  grey  matter,  and  the  several  strands  and  tracts  of 
fibres  must  have  of  course  a  meaning ;  but  it  may  be  doubted 
whether  we  have  even  so  much  as  a  correct  glimpse  of  that 
meaning  in  any  case,  if  we  except  those  which  are  in  immediate 
connection  with  the  cranial  nerves  and  their  nuclei.  Seeing  that 
the  thalamus  appears  on  the  one  hand  to  be  connected  with  all 
or  nearly  all  parts  of  the  cortex,  and  on  the  other  hand  to  serve 
as  the  front  of  the  tegmental  system,  it  is  tempting  to  suppose 
that  it  plays  an  important  part  in  sensations  pertaining  to  the 
body  generally,  as  part  of  it,  the  pulvinar,  certainly  does  with 
reference  to  the  special  sense  of  sight ;  but  we  have  no  decisive 
indications  as  to  what  part  it  plays.  And  the  part  which  it  plays, 
whatever  that  may  be,  is  not  an  exclusively  sensory  one,  since 
both  experimental  and  morbid  lesions  of  the  thalamus  are  apt  to 
produce  disorders  of  movement  as  well  as  other  efferent  effects. 
We  ought  perhaps  to  say  the  parts  which  it  plays ;  for  it  is  a 
complex  body,  having  many  ties  and  probably  performing  many 
duties. 


Chap,  it.]  THE   BRAIK  811 

We  have  already  spoken  of  the  fillet,  as  seeming  to  be  a 
special  internuncial  tract  connecting  what  appear  to  be  more 
particularly  afferent  or  sensory  parts  of  the  bulb,  such  as  the 
gracile  and  cuneate  nuclei,  with  such  parts  of  the  middle  brain 
as  the  optic  thalamus  and  thus  indirectly  with  the  cortex,  and 
of  its  value  as  a  probable  path  of  sensations  of  one  kind  or 
another  from  the  body  at  large. 

A  conspicuous  part  of  the  brain,  namely  the  cerebellum, 
naturally  arrests  our  attention  on  account  of  its  large  connec- 
tions with  what  appear  to  be  afferent  structures ;  what  may  be 
said  concerning  this  will  be  said  in  the  next  section. 


SEC.   6.     SOME    OTHEE   ASPECTS    OE   THE   EUNC- 
TIONS   OE   THE  BEAIN. 

§  512.  It  is  difificult  to  say  anything  definite  concerning  the 
transmission  of  sensory  impulses  and  the  development  of  sensa- 
tions ;  it  is  still  more  difiicult  to  say  anything  definite,  beyond 
what  has  been  already  incidentally  said,  concerning  the  parts 
played  in  the  work  of  the  brain  by  the  various  aggregations  of 
grey  matter  and  tracts  of  fibres  forming  the  middle  part  of  the 
brain.  Neither  experiment  nor  clinical  study  has  as  yet  afforded 
any  clear  or  sure  leading. 

Let  us  first  speak  of  the  cerebellum. 

The  connections  of  this  body  with  the  rest  of  the  nervous 
system  are  strikingly  manifold.  By  the  inferior  peduncle,  the 
fibres  of  which  end  largely  in  the  nucleus  dentatus,  it  has  an 
uncrossed  grip  on  afferent  structures  of  the  spinal  cord  and 
bulb ;  by  the  cerebellar  tract  it  is  connected  through  the  vesicu- 
lar cylinder  with  the  posterior  roots  of  spinal  nerves  of  the  same 
side ;  it  has  a  like  connection  with  the  cuneate  and  gracile 
nuclei  of  the  same  side,  and  fibres  from  the  eighth  (vestibular) 
nerve,  as  well  probably  as  from  other  afferent  cranial  nerves 
of  the  same  side,  pass  into  the  peduncle.  The  same  inferior 
peduncle  has  a  crossed  connection  with  the  lower  olive  of  the 
other  side,  but  this  is  probably  efferent  in  nature,  and  there  are 
probably  other  efferent  connections.  By  its  middle  peduncle, 
the  fibres  of  which  are  especially  connected  with  the  superficial 
grey  matter,  the  cerebellum  has  large  connections  with  the 
opposite  side  of  the  pons  and,  through  the  relay  of  the  cells 
forming  the  grey  matter  of  the  pons,  with  fibres  passing  from 
the  frontal  and  temporo-occipital  regions  (possibly  from  scattered 
elements  of  the  parietal  region)  of  the  cerebral  cortex  to  the 
pons.  Thus  each  lateral  half  of  the  cerebellum  has  wide  crossed 
connections  with  the  opposite  half  of  the  cerebrum.  Whether 
these  connections  are  afferent,  that  is  leading  from  the  cere- 
bellum to  the  cerebral  cortex,  or  vice  versa,  or  have  both  char- 
acters, it  is  difficult  to  say.     Besides  this  the  superior  peduncle, 

812 


Chap,  ii.]  THE   BRAIK  813 

starting  largely  from  the  nucleus  dentatus,  affords  another 
connection  with  the  cerebrum,  largely  crossed,  though  possibly 
partly  uncrossed;  but  this  is  an  indirect  connection  so  far  as  the 
cerebral  cortex  is  concerned,  since  the  peduncle  ends  in  the  red 
nucleus  and  other  tegmental  structures.  This  connection  is 
presumably  the  channel  of  impulses  passing  from  the  cerebellum 
to  the  cerebrum ;  but  we  can  only  say  '  presumably.' 

The  above  connections  are  too  complex  to  enable  one  to 
draw  from  them  any  precise  physiological  deductions ;  and  when 
we  turn  to  the  results  of  experiment  and  clinical  observation, 
we  find  even  these  by  no  means  clear. 

Electrical  stimulation  of  the  surface  of  the  cerebellum,  in  the 
monkey  and  in  other  animals,  has  led  to  movements  of  the  eyes, 
and  of  other  parts  of  the  head ;  but  we  cannot  from  such  results 
draw  any  satisfactory  inferences. 

The  results  of  removing  either  portion  of  or  the  whole  of 
the  cerebellum  have  been  partly  accordant,  partly  discordant. 
They  agree  in  shewing  that  the  organ  has  no  special  connections 
with  the  sexual  functions,  a  view  once  largely  held.  They  also 
appear  to  agree  in  so  far  that  the  effects  produced  by  removing 
a  lateral  half  are  largely  if  not  whoUj^  confined  to  the  same  side 
of  the  body ;  the  influence  exerted  by  one  half  of  the  cerebellum  / 
tells _on_one  half  of  the  body,  and  that  on  the  same  side.  They ' 
further  agree  in  that  removal  of  even  the  whole  of  the  cerebellum 
has  no  obYioiAS4}iSXchical  effects,  and  does  not  appear  to  interfere 
with  ^he  full  development  of  cutaneous  and  other  sensation^. 
Lastly,  while  they  agree  in  indicating  that  the  cerebellum  plaj'-s 
a  special  part  in  the  coordination  of  movements  affecting  equil- 
ibrium, and  this  is  also  shewn  by  clinical  histories,  they  are  not 
agreed  as  to  how  that  part  is  exactly  played. 

The  effects  produced  by  removal  of  the  cerebellum  are  in 
part  immediate  and  temporary,  in  part  of  a  more  lasting  char- 
acter. Characteristic  among  the  former  are  some  of  the  *  forced 
movejnents '  treated  of  in  §  480,  such,  for  instance,  as  rotation 
round  the  longitudinal  axis  of  the  body ;  this  seems  especially 
to  occur  after  division  of  the  middle  peduncle.  And  the  move- 
ments of  the  body  in  general  are  for  a  while  in  a  remarkable 
degree  incoordinate- and  irregular,  so  that  locomotion  and  even 
the  maintenance  of  a  natural  attitude  are  for  the  time  impossible. 
Touic  spasms  of  various  muscles  are  also  observed,  those  of  the 
trunk  leading  to  curvature  of  the  body.  Later  on  this  condition 
subsides,  but  there  remains  as  a  lasting  effect  an  imperfection 
of  movement,  an  unsteadiness  of  gait,  an  irregularity  and  short- 
coming in  muscular  actions,  accompanied  frequently  by  mus- 
cular tremors.  This  unsteadiness  in  movement  is  also  seen  in 
many  cases  of  cerebellar  disease,  the  gait  of  the  patient  in  many 
respects  resembling  that  of  a  person  who  is  drunk.  Obviously 
the  cerebellum  has  some  important  influence  over  the  coutrac- 


814  THE    CORPOEA   QUADRIGEMIi^A.       [Book  hi. 

tions  of  the  skeletal  muscles  (the  visceral  muscles  do  not  seem 
to  be  affected) ;  but  observers  are  not  agreed  as  to  what  that 
influence  exactly  is  or  exactly  how  it  is  brought  about.  The 
view  has  been  put  forward  that  influences  from  the  cerebellum 
reinforce  so  to  speak  the  voluntary  impulses  proceeding  to  the 
muscles,  regulate  the  fusion  of  the  several  constituent  simple 
contractions  into  the  sustained  muscular  effort,  which  as  we 
have  seen  is  of  the  nature  of  a  tetanus,  and  further,  so  affect  the 
nutrition  of  the  muscle,  as  to  keep  up  that  proper  tone  of  the 
muscle  (§  470)  which  is  a  necessary  condition  of  successful 
muscular  action.  But  according  to  some  observers  an  animal 
which  from  removal  of  the  cerebellum  has  become  highly  irreg- 
ular in  its  general  movements,  can  carry  out  a  particular  muscu- 
lar act  with  complete  success,  can  for  instance  grasp  powerfully 
and  firmly  with  the  hand ;  in  such  an  act  the  voluntary  impulses 
must  reach  the  muscle  in  full  force,  the  muscular  sense  belong- 
ing to  the  muscles  used  must  be  in  full  play,  and  it  may  even 
be  argued  that  no  marked  loss  of  tone  in  the  particular  muscles 
can  be  present.  If  this  be  so,  obviously  the  unsteadiness  of  the 
movements  in  general  must  have  some  other  explanation  than 
the  one  given  above. 

Though  some  have  maintained  that  injury  to  the  cerebellum 
must  be  unsymmetrical,  must  be  on  one  side  only  or  more  on 
one  side  than  the  other,  in  order  to  produce  its  effects,  this  does 
not  seem  to  be  the  case.  Again  while  some  have  maintained 
that  injury  to  the  median  region  is  especially  effective,  others 
deny  this.  Indeed  so  far  as  we  can  judge  at  present,  there  is 
no  localization  of  function  in  the  various  regions  of  the  cere- 
bellar cortex ;  all  the  cortex  may  be  said,  as  used  to  be  said  of 
the  cerebral  cortex,  to  'act  as  a  whole.'  Lastly,  in  respect  to 
the  relations  of  the  cerebellum  to  the  cerebrum,  it  should  be 
noted  that  removal  of  one  half  of  the  cerebellum  has  appeared 
to  produce  an  effect  on  the  crossed  cerebral  hemisphere,  of  such 
a  kind  that  that  hemisphere  is  more  readily  excitable  towards 
electric  and  other  stimuli,  the  effect  lasting  long  after  the  opera- 
tion of  removal.  In  this  connection  it  is  worthy  of  notice,  that 
congenital  deficiency,  or  atrophy  of  the  cerebral  hemisphere  of 
one  side,  is  frequently  accompanied  by  a  corresponding  defi- 
ciency of  the  crossed  cerebellar  hemisphere. 

§  513.  Both  the  anterior  and  posterior  corpora  quadrigemina 
are  complex  in  structure ;  not  only  do  they  differ  from  each 
other,  but  also  in  each  the  grey  matter  differs  in  different  parts, 
both  as  to  its  nature  and  appearance  and  as  to  its  connections 
with  tracts  of  fibres.  If  we  have  little  right  to  speak  of  the 
'  functions  of  the  cerebellum,'  we  have  even  less  right  to  speak 
of  the  '  functions  of  the  corpora  quadrigemina '  or  of  either  pair 
of  them.  The  anterior  pair,  as  we  have  seen,  has  to  do  in  some 
way  with  vision ;  but  we  have  reason  to  think  that  a  part  only  of 


CiiAP.  II.]  THE  BRAIK  815 

the  whole  body  is  thus  concerned ;  and  there  is  some  foundation 
for  the  view  that  of  this  part,  one  portion  belongs,  so  to  speak, 
to  the  optic  tract  and  another  portion  to  the  cortical  fibres  of  the 
optic  radiation.  Possibly  still  another  part  is  concerned  in 
bringing,  as  we  have  (§  500)  suggested,  visual  impulses  to  bear 
on  the  coordination  of  movements. 

Stimulation  of  the  surface  of  the  posterior  pair,  besides  giv- 
ing rise  to  movements  of  various  parts  of  the  body,  has  in  monkeys 
and  some  other  animals,  the  singular  effect  of  producing  a  vocal 
utterance  in  the  form  of  a  cry  or  bark.  But  we  cannot  make 
much  use  of  these  results  for  the  purpose  of  drawing  conclusions 
as  to  what  share  these  bodies  take  in  the  whole  work  of  the 
brain.  In  the  frog,  the  optic  lobes  correspond  to  the  two  pair 
of  corpora  quadrigemina  together ;  and  the  cry  just  mentioned 
may  perhaps  be  put  side  by  side  with  the  fact  that  in  the  frog 
the  optic  lobes  seem  to  furnish  a  mechanism  for  croaking ;  when 
the  optic  lobes  are  destroyed,  the  characteristic  reflex  croaking 
is  done  away  with.  The  probable  connection  of  the  posterior 
corpora  quadrigemina  with  hearing  is  also  interesting  in  this 
connection  ;  but  we  have  no  satisfactory  evidence  of  an}^  special 
ties  between  the  bodies  in  question  and  either  the  cortical  area 
for  phonation  or  the  vocal  mechanism  in  general ;  the  occurrence 
of  the  cry  remains  so  far  an  isolated  fact. 

In  frogs,  in  which  the  cerebellum  is  very  small,  the  optic 
lobes  seem  to  be  particularly  concerned  in  the  coordination  of 
movements.  When  the  brain  is  removed  by  means  of  a  section 
behind  the  optic  lobes  the  animal  loses  the  power  of  balancing 
itself  (§  474),  which  it  possesses  when  the  section  passes  in 
front  of  the  optic  lobes ;  and  injury  to  the  optic  lobes  produces 
incoordination  of  movement  and  often  'forced  movements.'  It 
has  been  maintained  that  the  loss  of  coordination  is  in  these 
cases  due  to  removal  of  or  injury  to  the  central  grey  matter  in 
the  walls  of  the  third  ventricle,  and  not  to  mere  removal  of  or 
injury  to  the  optic  lobes  ;  but  the  whole  evidence  goes  to  shew 
that  in  the  frog  and  in  the  bird  the  optic  lobes  do  play  a  part 
in  the  coordination  of  movement,  though  lesions  of  the  central 
grey  matter  around  the  third  ventricle,  or  indeed  of  the  thala- 
mus or  other  parts  of  the  tegmentum,  may  give  rise  to  loss  of 
coordination  or  to  '  forced  movements. ' 

In  the  mammal  removal  of  or  injury  to  the  posterior  cor- 
pora quadrigemina  does  not  cause  blindness,  but  may,  like  a 
lesion  of  the  anterior  pair,  give  rise  to  loss  of  coordination  or 
to  forced  movements ;  the  effect,  however,  is  in  most  instances 
very  temporary.  The  connection  of  the  anterior  pair  with 
vision  suggests  a  clue  as  to  how  this  pair  takes  part  in  coordi- 
nation ;  but  as  to  how  the  posterior  pair  could  intervene  in  the 
matter  we  have  hardly  so  much  as  a  hint ;  for,  even  if  we  admit 
a  connection    between   them   and   the  sense    of   hearing,   and, 


816     SPLANCH:>^IC  FUNCTIUI^S   OF  THE  BilAlK    [Book  in. 

remembering  that  a  loud  sound  will  often  cause  a  person  to 
reel,  further  admit  that  purely  auditory  impulses,  as  distinct 
from  what  we  have  called  ampullar  impulses,  may  take  part  in 
the  general  coordination  of  bodily  movements  and  in  the  main- 
tenance of  equilibrium,  as  they  certainly  do  in  the  special  coor- 
dination of  laryngeal  movements,  still  we  are  not  much  nearer 
an  understanding  of  the  matter.  We  may  add  that  section  of 
the  lateral  fillet,  which  appears  as  a  conspicuous  tie  between 
the  posterior  corpora  quadrigemina  and  the  parts  of  the  nervous 
system  behind  them,  does  not  appear  to  have  any  marked  effect 
in  producing  incoordination. 

In  fine,  beyond  the  broad  facts  on  which  we  dwelt  in  a  pre- 
vious section,  namely,  that  we  maintain  our  equilibrium  and 
carry  out  complex  movements  involving  often  several  parts  of 
our  body,  through  what  we  call  coordination,  that  afferent 
impulses  supply  important  factors  of  this  coordination,  and  that 
the  cerebellum,  through  the  vestibular  nerves  in  part  at  all 
events,  together  with  other  portions  of  the  middle  brain,  are  in 
some  way  its  chief  instruments,  we  as  yet  know  very  little. 
We  have  certainly  no  adequate  knowledge  as  to  how  either 
pair  of  corpora  quadrigemina  exactly  intervene  in  the  matter,  or, 
indeed,  as  to  what  other  parts  they  play  in  the  general  work  of 
the  brain. 

With  regard  to  other  tracts  of  fibres  or  areas  of  grey  matter 
we  have  nothing  to  say,  except  as  regards  those  which  are  more 
or  less  immediately  connected  with  certain  of  the  cranial  nerves, 
such  for  instance  as  the  nerves  for  movements  of  the  eyes,  and 
these  it  will  be  best  to  consider  when  we  have  to  deal  with  the 
nerves  themselves. 

§  514.  Besides  the  somatic  functions  which  in  previous 
discussions  we  have  chiefly  had  in  view,  the  brain  as  a  whole 
undoubtedly  carries  out  splanchnic  functions ;  concerning  these, 
however,  we  must  be  very  brief. 

Of  the  respiratory  and  vaso-motor  functions  of  the  bulb  we 
have  already  treated  in  their  appropriate  places,  and  we  have 
referred  (§  429)  to  the  experimental  evidence  that  a  lesion  of 
the  corpus  striatum,  or  of  the  front  part  of  the  optic  thalamus 
has  a  remarkable  influence  on  the  development  of  heat  in  the 
body.  We  have  further  seen  that  the  higher  parts  of  the 
brain,  acting  through  the  bulb,  exercise  powerful  influences  on 
respiration,  on  the  vaso-motor  system,  and  on  the  beat  of  the 
heart.  Daily  experience  affords  abundant  instances  of  actions 
such  as  these,  as  well  as  of  the  influence  of  the  brain  on 
other  organic  functions.  We  can  bring  our  will  to  bear  on 
the  mechanism  of  micturition  (§  348)  which  is  almost  wholly, 
and  on  the  mechanism  of  deftecation  (§  225)  which  is  largely, 
splanchnic  in  nature.  These  movements,  however,  are  not 
skilled  movements ;  and  as  we  explained  in  dealing  with  them, 


Chap,  ii.]  THE   BKAIN".  817 

the  action  of  the  brain  as  regards  them  seems  limited  to  aug- 
menting or  inhibiting  the  activity  of  spinal  centres.  We  should 
therefore  hardly  expect  them  to  be  specially  represented  in  the 
cortical  motor  region.  But  emotions  have  a  much  wider  and 
more  powerful  influence  over  the  splanchnic  functions  than  has 
the  will,  and  have  the  power  of  affecting  the  work  of  certain 
organs,  for  instance  the  heart  and  secreting  glands,  which  the 
will  is  unable  to  touch.  And  since  we  have  every  reason  to 
believe  that  the  cortex  is  closely  associated  with  the  emotions, 
we  may  naturally  infer  that  the  elements  of  the  cortex  supply 
a  link  in  the  chain  through  which  an  emotion  influences  this  or 
that  splanchnic  activity;  we  may,  accordingly,  expect  to  find 
that  stimulation  of  some  part  or  other  of  the  cortex  produces 
splanchnic  effects.  The  results  of  experimental  investigation, 
however,  are  both  scanty  and  discordant;  but  the  greater 
weight  should  perhaps  be  attached  to  the  positive  results. 
Thus,  some  observers  find  that  stimulation  of  the  cortex,  the 
locality  being  in  the  dog  some  part  of  the  sigmoid  gyrus,  pro- 
duces movements  of  the  bladder;  and  they  trace  the  path  of 
this  influence  through  the  front  part  of  the  thalamus  and  the 
tegmentum  to  the  bulb  and  so  to  the  cord,  excluding  the  cere- 
bellum, which  other  observers  believed  to  be  concerned  in  the 
matter.  Some  observers  a^ain  find  that  stimulation  of  the  cor- 
tex  produces  a  flow  of  '  chorda  saliva,'  while  others  maintain 
that  the  secretion,  when  it  does  occur,  is  an  indirect  and  not  a 
direct  effect  of  the  cortical  stimulation  ;  and  it  may  be  remarked 
that  the  cortical  area,  which  is  claimed  to  be  a  'salivation 
area,'  lying  in  the  dog  on  the  convolutions  dorsal  to  and  in 
front  of  the  Sylvian  fissure,  is  not  either  the  area  connected 
with  the  facial  nerve,  or  that  allotted  to  taste  or  smell. 

Similarl}^  stimulation  of  parts  of  the  cortex  has  in  the  hands 
of  various  observers  led  to  movements  or  to  arrest  of  movements 
of  the  intestines,  to  changes  in  the  beat  of  the  heart,  and  to 
various  vaso-motor  and  other  effects ;  but  it  will  not  be  profit- 
able to  enter  into  any  further  details.  We  may,  however,  add 
the  remark  that  when  the  cortical  motor  area  for  a  limb  is 
removed,  or  suffers  a  lesion,  the  temporary  paralysis  which  is 
thereby  caused  is  accompanied  by  a  rise  of  temperature  in  the 
limb ;  this  may  be  at  times  very  great  indeed ;  in  the  monkey 
for  instance,  the  hand  or  foot  on  the  paralysed  side  may  be  as 
much  as  5°  C.  higher  than  that  of  the  other  side.  The  effect  is 
partly  due  to  vaso-motor  paralysis,  but,  especially  considering 
that  the  muscles  of  the  limb  are  relativelj^  quiescent  and  so  pro- 
ducing less  heat  than  usual,  cannot  be  due  to  that  alone.  The 
remarkable  result  may  be  taken  as  still  further  illustrating  the 
complexity  of  the  processes  connected  with  the  cortical  motor 
area ;  the  area  is  in  some  way  associated  with  the  vascular 
arrangements  and  nutrition  of  the  muscles  with  whose  move- 

52 


818    SPLANCHNIC  FUNCTIONS  OF  THE  BEAIN.  [Book  hi, 

ments  it  is  concerned.  We  may  add  that  in  the  cases  of  the 
dogs  kept  alive  for  some  time  after  nearly  complete  removal 
of  both  cerebral  hemispheres,  a  vastly  increased  bodily  metabo- 
lism was  a  marked  symptom ;  the  animals  had  to  be  fed  with 
enormous  quantities  of  food. 


SEC.    7.     ON  THE   TIME  TAKEN  UP  BY   CEREBRAL 
OPERATIONS. 


§  515.  We  have  already  seen  (§  467)  that  a  considerable 
time  is  taken  up  in  a  purely  reflex  act,  such  as  that  of  winking, 
though  this  is  perhaps  the  most  rapid  form  of  reflex  movement. 
When  the  movement  which  is  executed  in  response  to  a  stimulus 
involves  cerebral  operations  a  still  longer  time  is  needed :  and 
the  interval  between  the  application  of  the  stimulus  and  the 
commencement  of  the  muscular  contraction  varies  according  to 
the  nature  of  the  mental  labour  involved. 

The  simplest  case  is  that  in  which  a  person  makes  a  signal 
immediately  that  he  perceives  a  stimulus,  ex.  gr.  closes  or  opens 
a  galvanic  circuit  the  moment  that  he  feels  an  induction  shock 
applied  to  the  skin,  or  sees  a  flash  of  light,  or  hears  a  sound. 
By  arrangements  similar  to  those  employed  in  measuring  the 
velocity  of  nervous  impulses,  the  moment  of  the  application  of 
the  stimulus  and  the  moment  of  the  making  of  the  signal  are 
both  recorded  on  the  same  travelling  surface,  and  the  interval 
between  them  is  carefully  measured.  This  interval,  which  has 
been  called  '  the  reaction  period '  or  '  reaction  time,'  may  be 
divided  into  three  stages :  (1)  The  time  during  which  afferent 
impulses  are  generated  in  the  peripheral  sense  organs  and  trans- 
mitted along  the  afferent  nerves  to  the  central  nervous  system ; 
this  may  be  called  the  "  afferent  stage."  (2)  The  time  during 
which,  through  the  operations  of  the  central  nervous  system,  the 
afferent  impulses  are  transformed  into  efferent  impulses ;  this 
may  be  called  the  "  central  stage."  (3)  The  time  taken  up  by 
the  passage  of  the  efferent  impulses  along  the  efferent  nerves 
and  the  transformation  of  the  nervous  impulses  into  muscular 
contractions;  this  may  be  called  the  "efferent  stage."  In  the 
efferent  stage  the  events  are  comparatively  simple,  and  though 
not  absolutely  constant,  do  not  vary  largely ;  we  are  able  to  form 
a  fairly  satisfactory  estimate  of  its  duration  and  so  of  the  share 
in  the  whole  reaction  period  which  may  be  allotted  to  it.  The 
events  of  the  afferent  stage  are  much  more  complex  and  the 
estimates  of  its  duration,  being  arrived  at  in  an  indirect  manner, 

819 


820      DUEATIOK  OF  PSYCHICAL  PEOCESSES.     [Book  in. 

and  chiefly  based  upon  calculations  of  the  whole  reaction  time, 
are  very  uncertain.  Hence  all  attempts  to  estimate  the  length 
of  the  "  central "  stage,  the  "  reduced  reaction  period  "  as  it  is 
sometimes  called,  by  subtracting  the  efferent  and  afferent  stages, 
must  be  subject  to  much  error.  But  a  good  deal  may  be  learnt 
by  studying  the  variations  under  different  circumstances  of  the 
reaction  period  as  a  whole. 

Taking  first  of  all  the  cases  in  which  the  events  of  the  central 
stage  are  simple,  such  as  those  where  the  subject  has  merelj'-  to 
make  a  signal  upon  feeling  a  sensation,  we  find  that  the  length 
of  the  reaction  period  is  dependent  on  the  intensity  of  the  stim- 
ulus, being  shorter  with  the  stronger  stimulus.  But  variations 
in  the  strength  of  the  stimulus,  especially  in  the  case  of  minimal 
stimuli,  have  a  much  more  striking  effect  in  determining  the 
certainty  of  the  reaction  than  in  affecting  the  length  of  the 
period.  Thus  when  the  signal  is  made  in  response  to  some 
visual  sensation,  upon  seeing  an  electric  spark  for  instance,  if 
the  spark  be  a  very  weak  one  the  subject  of  the  experiment  often 
fails  to  make  the  signal  at  all,  though  he  may  rarely  fail  if  the 
spark  be  a  strong  one. 

Some  of  the  most  marked  variations  in  the  length  of  the 
reaction  period  are  determined  by  the  individuality  of  the  sub- 
ject. Thus  with  the  same  stimulus  applied  under  the  same  cir- 
cumstances the  reaction  period  of  one  person  will  be  found  very 
different  from  that  of  another. 

The  length  of  the  reaction  period  varies  also  according  to  the 
nature  and  disposition  of  the  peripheral  organs  stimulated.  In 
general  it  may  be  said  that  cutaneous  sensations  produced  by 
the  stimulus  of  an  electric  shock  applied  to  the  skin  (the  signal 
for  instance  being  made  by  the  right  hand  when  the  shock  is  felt 
by  the  left  hand)  are  followed  by  a  shorter  reaction  period  than 
are  auditory  sensations,  while  the  period  of  these  is  in  turn 
shorter  than  that  of  visual  sensations  produced  by  luminous 
objects ;  on  the  other  hand,  the  shortest  period  of  all  is  said  to 
be  that  of  visual  sensations  produced  by  direct  electrical  stimu- 
lation of  the  retina.  Roughly  speaking  we  may  say  that  the 
reaction  period  is  for  cutaneous  sensations  ^th,  for  hearing  ith, 
and  for  sight  -|^th  of  a  second. 

Practice  materially  shortens  the  reaction  period ;  indeed,  after 
long  practice,  making  the  signal,  at  first  a  distinct  effort  of  the 
will,  takes  on  the  characters  of  a  reflex  act,  with  a  correspond- 
ingly shortened  interval.  Lastly,  we  may  add  that  in  the  same 
individual  and  with  the  same  stimulus,  the  length  of  the  period 
will  vary  according  to  circumstances,  such  as  the  time  of  year, 
the  weather,  and  the  like,  as  well  as  according  to  the  condition 
of  the  individual,  whether  fresh  or  fatigued,  fasting  or  replete, 
having  taken  more  or  less  alcohol,  and  the  like. 

The  reaction  period  of  vision  has  long  been  known  to  astrono- 


Chap,  ii.]  THE   BRAI]!^.  821 

mers.  It  was  early  found  that  when  two  observers  were  watch- 
ing the  appearance  of  the  same  star,  a  considerable  discrepancy 
existed  between  their  respective  reaction  periods,  and  that  the 
difference,  forming  the  basis  of  the  so-called  '  personal  equation,' 
varied  from  time  to  time  according  to  the  personal  conditions  of 
the  observers. 

§  516.  The  events  taking  place  in  the  central  stage  are  of 
course  complex,  and  this  stage  may  be  subdivided  into  several 
stages.  Without  attempting  to  enter  into  psychological  ques- 
tions, we  may  at  least  recognize  certain  elementary  distinctions. 
The  afferent  impulses  started  by  the  stimulus,  whatever  be  their 
nature,  when  they  reach  the  central  nervous  system  undergo 
changes,  and  as  we  have  seen,  probably  complex  changes  before 
they  become  sensations ;  and  further  changes,  now  of  a  more 
distinctly  psychical  character,  are  necessary  before  the  mind  can 
duly  appreciate  the  characters  of  these  sensations  and  act  accord- 
ingly. Then  come  the  psychical  processes  through  which  these 
appreciated  sensations,  or  perceptions,  or  apperceptions  as  they 
are  sometimes  called,  determine  an  act  of  volition.  Lastly,  there 
are  the  executive  processes  of  volition,  the  processes  which, 
psychical  to  begin  with,  end  in  the  issue  of  coordinate  motor 
impulses,  or,  in  other  words,  start  the  distinctly  physiological 
processes  of  the  efferent  stage.  We  may  thus  speak  of  the  time 
required  for  the  perception  of  the  stimulation,  of  the  time  re- 
quired for  the  action  of  the  will,  and  of  the  time  required  for 
the  complex  psychical  processes  which  link  these  two  together. 
Accepting  this  elementary  analysis,  it  is  obvious  that  the  total 
length  of  the  central  stage  may  be  varied  by  differences  in  the 
length  of  each  of  these  parts ;  and  a  more  complete  analysis 
would  of  course  open  the  way  for  further  distinctions.  Hence, 
by  studying  the  variations  of  the  whole  reaction  time  under 
varying  forms  of  psychical  activity,  we  may  form  an  estimate  of 
time  taken  up  by  various  psychical  processes. 

We  may  take  as  an  instance  the  case  in  which  the  subject 
of  the  experiment  has  to  exercise  discrimination.  The  mode 
of  making  the  signal  being  the  same,  and  the  stimulus  being  of 
the  same  order  in  each  trial,  that  is  to  say,  visual,  or  cutaneous, 
or  auditory,  &c.,  and  general  circumstances  remaining  the  same, 
two  different  stimuli  are  employed,  and  the  subject  is  required 
to  make  a  signal  in  response  to  the  one  stimulus,  but  not  to 
the  other ;  the  subject  has  to  discriminate  between  the  psychi- 
cal effects  of  the  two  stimuli.  Suppose,  for  example,  the 
stimulus  is  the  sound  of  a  spoken  or  sung  vowel,  and  the  sub- 
ject is  required  to  make  a  signal  when  a  is  spoken  or  sung,  but 
not  when  o  is  spoken  or  sung.  If  the  subject's  whole  reaction 
period  be  determined  (i)  in  the  usual  wa3%  with  either  a  or  o 
spoken  (and  the  result  will  be  found  not  to  differ  materially 
whether  a  or  o  be  used),  the  subject  knowing  that  only  a  or 


822      DURATION   OF   PSYCHICAL   PROCESSES.     [Book  in. 

only  0  will  be  spoken,  and  then  be  determined  again  (ii)  when 
he  has  to  discriminate  in  order  that  he  may  make  the  signal  when 
a  is  spoken  but  not  when  o  is  spoken,  he  not  knowing  which 
is  about  to  be  spoken,  the  whole  reaction  period  will  be  found  to 
be  distinctly  longer  in  the  second  case.  The  experiment  may 
be  varied  by  making  use  of  all  the  vowel  sounds  taken  irregu- 
larly as  the  stimulus,  the  subject  responding  by  a  signal  to  one 
only,  as  arranged  beforehand.  And  of  course  other  orders  of 
stimulus  may  be  used,  either  visual,  the  signal  being  made 
when  a  red  light  is  shewn  but  not  when  other  colours  are 
shewn,  or  tactile,  the  signal  being  made  when  one  part  of  the 
body  is  touched  but  not  when  other  parts  are  touched,  and 
the  like. 

In  such  experiments  where  the  subject  has  to  distinguish, 
to  discriminate  between  two  or  more  events,  the  prolongation 
of  the  reaction  period  is  obviously  due  to  the  longer  time  re- 
quired for  the  psychical  processes  taking  place  during  what  we 
have  called  the  central  stage.  In  the  two  cases,  one  without  and 
the  other  with  discrimination,  not  only  are  the  afferent  and 
efferent  stages  the  same  in  both,  but  we  have  no  reason  to  sup- 
pose that  in  the  central  stage  is  there  any  difference  between 
the  two  cases  as  to  the  time  taken  up  by  the  transformation  of 
simple  sensory  impulses  into  perceptions,  or  as  to  that  taken  up 
by  the  will  in  gaining  access  to  the  motor  apparatus  and  so 
starting  the  processes  of  the  efferent  stage ;  the  delay  takes 
place  in  the  psychical  processes  intervening  between  these  two 
parts,  and  the  amount  of  delay  is  the  measure  of  the  time 
needed  for  the  processes  involved  in  the  discrimination.  This 
"discrimination  period"  has  been  found  to  differ  in  the  same 
individual  according  to  the  sensation  employed,  visual,  audi- 
tory, &c.,  and  according  to  the  kind  of  difference  in  the  sensa- 
tion which  has  to  be  discriminated,  for  instance  in  visual 
sensations  between  colours  or  between  objects  in  different  parts 
of  the  field  of  vision.  In  a  series  of  observations  made  in  this 
way,  the  discrimination  period,  i.e.  the  prolongation  of  the 
simple  reaction  period  due  to  having  to  discriminate,  was  found 
to  range  from  0-011  sec.  to  0-062  sec. 

Another  series  of  observations  may  be  made  in  the  follow- 
ing way.  The  signal  being  one  made  with  the  hand,  the 
simple  reaction  period  for  a  stimulus  is  determined  with  the 
signal  given  by  the  right  hand.  Two  kinds  of  stimuli  are  then 
employed,  both  of  the  same  order,  two  vowel  sounds  for  in- 
stance, and  the  subject  is  directed  to  respond  to  one  vowel 
with  the  right  hand  and  to  the  other  with  the  left  hand.  It 
is  found,  the  subject  being  right-handed,  that  the  reaction 
period  is  greater  when  the  signal  is  made  with  the  left  hand. 
In  this  case  the  delay  takes  place  not  in  the  recognition  of  the 
effects  of  the  stimulus,  nor  in  the  processes  through  which  the 


Chap,  ii.]  THE   BRAIN.  823 

will  is  formed  upon  that  lecoguition ;  these  are  the  same  in 
the  two  cases ;  it  takes  place  in  the  processes  by  which  the  will 
is  brought  to  bear  on  the  nervous  motor  apparatus  for  making 
the  signal,  on  the  cortical  origin,  for  example  of  the  pyi'amidal 
tract ;  these  processes  take  a  longer  time  in  the  case  of  the 
unaccustomed  left  hand  than  in  the  case  of  the  usual  right 
hand.  In  this  way  we  obtain  a  measure  of  so  to  speak  the 
volitional  side  of  psychical  processes. 

In  a  somewhat  similar  way  we  may  obtain  a  measure  of  the 
time  required  for  perception.  A  strong  sensation  following 
too  closely  upon  a  weak  one  will  prevent  the  psychical  recog- 
nition of  the  weaker  one.  If,  for  instance,  two  or  three  letters 
in  white  on  a  black  background  be  presented  to  the  eye,  and 
a  large  white  surface  be  presented  afterwards  at  an  interval 
which  is  made  successively  shorter  and  shorter,  it  is  found  that 
when  the  interval  is  made  very  brief  indeed  the  letters  cannot 
be  perceived  at  all.  In  proportion  as  the  interval  is  prolonged, 
the  recognition  of  the  letters  increases,  until  at  an  interval  of 
about  -05  sec.  they  are  fully  and  clearly  recognized.  That  is 
to  say,  the  time  required  for  perception  is  in  such  a  case 
of  about  that  length. 

The  duration  of  all  these  psychical  processes,  as  of  the 
simple  reaction  period  itself,  varies  of  course  under  different 
circumstances,  and  the  discrimination  period  may  be  conve- 
niently used  for  measurements  of  the  varying  effects  of  circum- 
stances. Practice  shortens  the  discrimination  period  as  it  does 
the  simple  reaction  period.  One  of  the  most  powerful  influ- 
ences is  that  of  attention.  And  it  is  stated  that  the  shortening 
of  the  period  is  greater  when  the  attention  is  concentrated  on 
the  making  of  the  signal  than  when  it  is  more  especially 
directed  to  recognition  of  the  stimulus ;  in  other  words,  the 
volitional  processes  are  more  amenable  than  are  the  perceptive 
processes  to  the  psychical  action  which  we  call  attention.  On  the 
other  hand,  the  period  is  distinctly  prolonged  if  the  observer  be 
distracted  by  concomitant  sensations.  For  example,  the  period 
for  discriminating  between  two  visual  sensations  is  prolonged 
if  powerful  auditor}'-  sensations  be  excited  at  the  same  time. 

The  same  method  of  measurement  may  be  used  in  other 
ways  and  under  other  circumstances  with  reference  to  psychi- 
cal processes.  It  must  be  remembered,  however,  that  all  such 
observations  are  open  to  many  fallacies  and  need  particular 
caution.  It  not  unfrequently  happens  that  false  results  are 
obtained ;  for  instance,  the  subject,  expecting  the  stimulus  to 
be  brought  to  bear  upon  him  and  straining  his  attention,  makes 
the  signal  before  the  stimulus  actually  comes  off.  And  the 
interpretation  of  the  results  obtained  are  in  many  cases  very 
difficult ;  but  it  would  be  out  of  place  to  dwell  upon  these 
matters  any  further  here. 


SEC.   8.     THE  LYMPHATIC   AEEANGEMENT   OF   THE 
BEAIN   AND   SPINAL   COED. 


§  517.  The  Cerebrospinal  Fluid.  The  specimens  of  cerebro- 
spinal fluid  which  have  been  examined  as  to  their  composition 
are  not  quite  comparable  with  each  other,  since  while  some 
(such  as  those  obtained  from  cases  where  a  fracture  of  the  base 
of  the  skull  has  placed  the  subarachnoid  space  at  the  base  of 
the  brain,  where  it  is  largely  developed,  in  communication  with 
the  external  meatus,  and  the  fluid  escapes  by  the  ear)  may  be 
regarded  as  normal,  others  (such  as  those  obtained  from  cases 
of  hydrocephalus  where  the  ventricles  contain  an  unusual  quan- 
tity of  fluid,  or  from  cases  of  spinal  malformations)  must  be 
considered  as  abnormal.  In  most  of  the  more  complete  analy- 
ses, the  fluid  examined  has  belonged  to  the  latter  class ;  and 
the  following  statements  apply,  strictly  speaking,  to  them  alone. 

With  this  caution  we  may  say  that  the  cerebro-spinal  fluid  is 
a  transparent,  colourless  or  very  slightly  yellowish  fluid,  of  faint 
alkaline  reaction,  free  from  histological  elements.  The  specific 
gravity  is  about  1010  or  less,  the  amount  of  solids  being  on  an 
average  1  p.c.  Of  these  by  far  the  greater  part,  -8  or  -9  p.c,  is 
supplied  by  salts,  the  total  quantity  of  which  as  well  as  the 
relative  amount  of  the  several  constituents  being  about  the  same 
as  obtain  in  blood  and  lymph.  The  comparative  deficiency  of 
solids  is  due  to  the  scantiness  of  the  proteids,  which  rarely 
exceed  -1  p.c.  These  are  chiefly  globulin  and  a  form  of  albu- 
mose,  or  even  peptone ;  albumin  is  said  to  be  generally  absent. 
The  fluid,  save  apparently  in  exceptional  cases,  does  not  clot, 
and  contains  neither  fibrogenous  factors,  nor  fibrin  ferment.  It 
very  frequently  contains  a  substance  which  like  dextrose  reduces 
Fehling's  solution  but  which  is  not  a  sugar ;  it  appears  to  be  pyro- 
catechin  or  a  closely  allied  body. 

Seeing  that  a  fluid  of  such  a  composition  is  of  a  different 
nature  from  ordinary  lymph,  furnished  entirely  in  the  ordinary 
way,  we  might  be  inclined  to  infer  that  probably  a  very  large 
part  of  the  whole  mass  of  the  fluid  is  furnished  by  the  secreting 
epithelium  of  the  choroid  plexus.     But  it  must  be  borne  in 

824 


Chap,  ii.]  THE   BRAIX.  825 

mind,  that  the  foregoing  analyses  refer  chiefly  to  fluid  appear- 
ing under  abnormal  circumstances,  and  it  would  be  hazardous 
to  draw  any  wide  inference  from  them.  We  have  little  or  no 
exact  exj^erimental  evidence  as  to  how  much  fluid  is  actually 
secreted  by  the  choroid  j)lexuses ;  and  if  the  fluids  which  have 
been  analyzed  do  represent  a  mixture  of  ordinary  lymph  sup- 
plied through  the  pia  mater  with  the  peculiar  secretion  of  the 
choroid  plexus  and  cerebro-spinal  canal,  some  further  change 
beyond  the  mere  mingling  of  the  two  fluids  is  needed  to  explain 
the  remarkable  absence  of  albumin  which  has  been  so  strongly 
insisted  upon  by  various  authors. 

§  518.  We  may  fairly  suppose  that  during  life  the  fluid  is 
continually  being  supplied,  from  the  one  source  or  the  other; 
but  we  have  no  very  exact  knowledge  as  to  the  rate  at  which  it 
is  furnished.  In  che  dog,  the  fluid  has  been  observed  to  escape 
at  a  rate  varying  very  largely  under  different  circumstances, 
and  ranging  from  1  c.c.  in  40  minutes  to  as  much  as  1  c.c,  in 
6  minutes,  the  total  quantity  discharged  in  24  hours  varying 
from  36  c.c.  to  240  c.c.  In  the  cases  of  fracture  of  the  base 
of  the  skull  mentioned  above,  a  very  considerable  flow  has  been 
frequently  observed ;  but  it  may  be  doubted  whether  the  abnor- 
mal circumstances  of  such. cases  have  not  raised  the  secretion 
above  the  normal.  The  rate  of  flow  was  found  in  the  dog  to 
be  much  increased  by  the  injection  of  substances  (normal  saline 
solution)  into  the  blood,  but  to  be  relatively  little  influenced  by 
artificial  heightening  of  arterial  pressure.  This  has  been  put 
forward  as  indicating  that  the  fluid  is  chiefly  furnished  as  a 
secretion  and  not  as  an  ordinary  transudation  of  lymph  ;  but  it 
cannot  be  regarded  as  affording  a  valid  argument.  The  pressure 
under  which  the  fluid  exists  is  also  very  variable ;  it  is  closely 
dependent  on  the  vascular  arrangements  of  which  we  shall  have 
to  speak  presently.  In  the  dog  the  average  pressure  has  been 
estimated  at  about  10  mm.  of  mercury. 

If  the  fluid  is  thus  continually  formed  it  must  always  find  a 
means  of  escape.  This  is  probably  supplied  by  the  tubular  pro- 
longations of  the  subarachnoid  space  along  the  nerve  roots; 
these  are  continuous  with  the  lymphatic  vessels  of  the  nerves, 
and  so  with  the  lymphatics  of  the  body  generally ;  and  in  the 
skull,  the  passages  of  this  kind  along  the  cranial  nerves,  especially 
along  the  two  optic  nerves  into  the  orbits,  afford  a  ready  means 
of  escape.  It  is  also  urged  that  some  of  the  fluid  escapes  through 
the  Pacchionian  glands  directly  into  the  blood  of  the  venous 
sinuses.  In  a  dead  body  fluid  introduced  into  the  subarachnoid 
space  through  an  opening  over  the  bulb,  disappears  at  even  a 
very  low  pressure  with  great  rapidity.  The  circumstances  then 
are,  however,  not  the  same  as  in  life ;  and  the  few  experiments 
which  have  been  made  seem  to  shew  that,  during  life,  a  some- 
what high  pressure  is  required  to  secure  the  escape   of  fluid 


826  THE   CEREBRO-SPINAL  FLUID.  [Book  hi. 

introduced  in  addition  to  that  naturally  secreted.  Thus  it  is 
stated  that  when  in  a  dog  normal  saline  solution  is  introduced 
into  the  subarachnoid  cavity  at  the  lower  end  of  the  spinal  cord 
very  little  resorption  takes  place  so  long  as  the  pressure  remains 
as  low  as  about  10  c.c.  of  mercury;  as  the  pressure  is  increased 
beyond  this  resorption  quickly  increases.  But  it  may  be  doubted 
whether  the  resorption  of  added  fluid  is  a  fair  test  of  the  escape 
of  fluid  naturally  present ;  and  the  experiment  is  of  value  rather 
as  shewing  simply  that  there  are  means  of  escape  than  as  afford- 
ing a  measure  of  the  rate  of  escape.  Besides,  the  immediate 
effects  of  applying  pressure  at  the  caudal  end  of  the  spinal  cord 
are  not  the  same  as  those  of  applying  pressure  within  the  skull. 
The  rate  of  possible  escape  is  not  without  importance  as 
regards  the  mechanical  importance  of  the  cerebro-spinal  fluid. 
Thus  it  has  been  urged  that  when  an  extra  quantity  of  blood  is 
driven  into  the  skull,  any  injurious  intercranial  compression  is 
prevented,  not  only  by  the  transference  of  a  corresponding  quan- 
tity of  cerebro-spinal  fluid  through  the  foramen  of  Majendie 
from  the  cranium  into  the  spinal  canal,  the  walls  of  which  are 
less  rigidly  complete,  but  also  by  the  direct  escape  of  the  fluid 
from  the  cavity  of  the  skull  along  the  cranial  nerves  in  the 
manner  described.  It  has  also  been  urged  that  the  fluid  at  the 
base  of  the  skull,  in  the  large  subarachnoid  spaces  of  which  it 
gathers  in  larger  quantity  than  elsewhere,  acts  as  a  sort  of  pro- 
tective water  cushion  to  the  delicate  cerebral  substance,  and  that, 
in  general,  the  presence  of  the  fluid  is  mechanically  useful  to 
the  welfare  of  the  brain,  removal  of  the  fluid  by  aspiration  being 
said  to  lead  to  haemorrhage  from  the  pia  mater  and  to  various 
nervous  disorders.  But  our  knowledge  as  to  the  part  which 
the  fluid  plays  is  at  present  very  imperfect ;  and  its  very  peculiar 
chemical  characters  suggest  that  it  has  some  chemical  as  well 
at  least  as  mechanical  functions. 


SEC.    9.       THE    VASCULAR    ARRANGEMENTS     OF    THE 
BRAIN  AND   SPINAL   CORD. 


§  519.  In  the  brain  two  important  features  of  the  distribu- 
tion of  the  arteries  deserve  special  attention.  In  the  first  place, 
the  quadruple  supply  by  the  right  and  left  vertebral  and  internal 
carotid  arteries  is  made  one  by  remarkable  anastomoses  forming 
the  circle  of  Willis.  Blood  can  pass  along  this  circle  in  various 
ways ;  from  the  basilar  artery  along  the  right  posterior  commu- 
nicating artery  to  the  right  internal  carotid,  and  so  by  the  right 
anterior  cerebral  arter}-  and  anterior  communicating  artery  to  the 
left  side  of  the  circle,  and  similarly  from  the  basilar  artery  along 
the  left  side  to  the  right,  or  from  the  right  or  from  the  left  carotid 
through  the  circle,  to  the  right  hand  or  to  the  left  liand  in  each 
case.  Since  the  channel  of  the  circle  is  a  fairly  wide  one,  the 
passage  in  various  directions  is  an  eas}'  one  ;  all  the  vessels  radi- 
ating from  the  circle,  including  the  basilar  arterj-  and  its  branches, 
can  be  supplied  b}-  the  carotids  alone,  or  b}'  the  vertebrals  alone, 
or  even  by  one  carotid  or  one  vertebral  alone.  In  this  way  an 
ample  supply  of  blood  to  the  brain  is  secured  in  the  face  of  any 
hindrance  to  the  flow  of  blood  along  any  one  of  the  four-  channels. 
In  what  may  perhaps  be  considered  the  usual  arrangement,  the 
calibre  of  the  posterior  communicating  arteries  is  rather  smaller 
than  the  other  parts  of  the  circle,  so  that,  other  things  being  equal, 
most  of  the  vertebral  blood  will  pass  by  the  posterior  cerebral 
arteries,  while  the  carotid  blood  passes  to  the  middle  and  ante- 
rior cerebral  arteries ;  but  many  variations  are  met  with.  We 
may  also  here  perhaps  call  to  mind  the  fact  that  the  left  carotid 
coming  off  from  the  top  of  the  aorta,  offers  a  more  straight  path 
for  the  blood  than  does  the  right  carotid  which  comes  off  from 
the  innominate  artery. 

Another  special  feature  of  the  arterial  supply  to  the  brain  is 
that  the  three  large  cerebral  arteries,  posterior,  middle  and  ante- 
rior, are  distributed  almost  exclusively  to  the  cortex  and  to  the 
subjacent  white  matter,  while  the  deeper  parts  of  the  hemisphere, 
the  nucleus  caudatus,  thalamus  and  the  like,  with  the  capsule  and 
other  adjoining  white  matter  are  supplied  by  smaller  arteries 


828  THE  VENOUS    SINUSES.  [Book  in. 

coming  direct  from  the  circle  of  Willis,  or  from  the  very  begin- 
nings of  the  three  cerebral  arteries.  It  is  stated  that  these  two 
systems  make  no  anastomoses  with  each  other ;  but  different  in- 
dividuals in  respect  to  this  appear  to  vary  much.  We  may  add 
that  the  anterior  cerebral  artery  supplies  the  cortex  of  the  dorsal 
aspect  of  the  frontal  lobe  as  well  as  the  front  and  middle  portions 
of  the  whole  mesial  surface  of  the  hemisphere ;  while  the  middle 
cerebral,  always  large,  is  distributed  to  the  side  of  the  brain,  that 
is,  the  parietal  lobe,  with  the  ventral  part  of  the  frontal  lobe  and 
the  dorsal  part  of  the  temporal  lobe  ;  the  posterior  cerebral  sup- 
plying the  rest  of  the  cortex,  that  is  to  say,  the  occipital  lobe 
including  the  hind  part  of  the  mesial  surface  of  hemisphere, 
together  with  the  ventral  part  of  the  temporal  lobe.  The  dis- 
tribution of  these  arteries  therefore  does  not  correspond  to 
functional  divisions,  for  while  the  middle  cerebral  supplies  a 
large  part  of  the  motor  region,  it  does  not  supply  the  whole 
of  it,  and  does  supply  parts  outside  it.  Though  the  small 
arteries  as  they  run  in  the  pia  mater  on  the  surface  of  the  cortex 
anastomose  freely,  there  is  very  little  anastomosis  between  the 
small  arteries  which  leaving  the  pia  mater  dip  down  into  the 
substance  of  the  brain  ;  hence  when  these  latter  arteries  are 
blocked,  the  nutrition  of  the  part  of  the  cortex  supplied  by 
them  is  apt  to  be  impaired. 

§  520.  The  venous  arrangements  of  the  brain  have  very 
special  characters. 

The  channels  for  the  venous  blood  of  the  brain  are  not 
veins  but  sinuses,  not  so  much  tubes  for  maintaining  a  uniform 
current  as  longitudinal  reservoirs,  which  while  affording  an 
easy  onward  path  can  also  be  easily  filled  and  easily  emptied, 
and  in  which  the  blood  can  move  to  and  fro  without  the 
restrictions  of  valves.  This  arrangement  is  correlated  to  the 
peculiar  surroundings  of  the  brain,  which  is  not  like  other 
organs  protected  merely  by  skin  or  other  extensible  and  elastic 
tissue,  but  is  encased  by  a  fairly  complete  inextensible  envelope, 
the  skull.  As  a  consequence  of  this,  when  at  any  time  an 
extra  quantity  of  blood  is  sent  from  the  heart  to  the  brain, 
room  must  be  made  for  it  by  the  increased  exit  of  the  fluids 
already  present;  for  any  pressure  on  the  brain-substance  beyond 
a  certain  limit  is  injurious  to  its  welfare  and  activity.  Some 
room  may,  as  we  have  seen  (§  518),  be  provided  by  the  escape 
of  cerebro-spinal  fluid  from  the  skull.  But,  within  the  limits 
of  the  normal  cerebral  circulation,  the  characteristic  venous 
sinuses  especially  serve  to  regulate  the  internal  pressure ;  they 
form  temporary  reservoirs  from  which  a  comparatively  large 
quantity  of  blood  can  be  rapidly  discharged  from  the  cranium, 
the  flow  from  the  sinuses  being  greatly  assisted  by  the  low  or 
negative  pressure  obtaining  in  the  veins  of  the  neck  at  each 
inspiratory  movement  of  the  chest.     The  injurious  effects  of  too 


Chap,  ii.]  THE  BEAIK  829 

great  a  pressure  on  the  brain-substance  are  seen  in  certain  mal- 
adies, where  blood  passing  by  rupture  of  the  blood  vessels  out 
of  its  normal  channels  remains  effused  on  the  surface  of  the 
brain  or  elsewhere,  and  thus  taking  up  the  room  of  the  proper 
brain-substance  leads,  by  '  compression '  as  it  is  called,  to  paral- 
ysis, loss  of  consciousness,  or  death.  They  are  also  shewn  by 
experiments  on  animals.  When  by  driving  an  excess  of  fluid 
into  the  subdural  cavity  through  a  hole  in  the  cranium  or  by 
other  means  a  certain  amount  of  pressure  is  established  in  the 
cranial  cavity  both  the  respiration  and  the  circulation  are 
affected.  The  breathing  is  slowed  and  eventually  arrested,  but 
may  in  certain  cases  be  quickened.  The  heart  is  slowed  by 
vagus  inhibition,  and  a  rise  of  blood  pressure  due  to  vaso-con- 
striction  is  observed  unless  the  slowing  of  the  heart  be  sufiScient 
to  neutralize  this.  These  phenomena  point  of  course  especially 
to  an  influence  exerted  on  the  spinal  bulb;  but  besides  these 
changes  in  the  pupil  and  other  effects  are  met  with. 

§  521.  The  supply  of  blood  to  the  brain  seems  at  first  sight 
not  to  correspond  to  the  importance  of  this  the  chief  organ  of  the 
body.  In  the  rabbit  it  would  appear  that  hardly  more  than  one 
per  cent,  of  the  total  quantity  of  the  blood  of  the  body  is  pres- 
ent at  any  one  time  in  the  brain,  a  quantity  but  little  more  than 
half  that  which  is  found  in  the  kidneys ;  and  while  the  weight 
of  blood  in  the  brain  at  any  one  time  amounts  to  about  five  per 
cent,  of  the  total  weight  of  the  organ,  being  about  the  same  as 
in  the  muscles,  in  the  kidney  it  amounts  to  nearly  twelve  per 
cent.,  and  in  the  liver  to  as  much  as  nearly  thirty  per  cent. 
Making  every  allowance  for  the  relative  small  size  and  func- 
tional importance  of  the  rabbit's  brain,  the  blood-supj)ly  of  even 
the  human  brain  must  still  be  small ;  and  making  every  allow- 
ance for  rapidity  of  current,  the  interchange  between  the  blood 
and  the  nervous  elements  must  also  be  small.  In  other  words, 
the  metabolism  of  the  brain-substance  is  of  importance  not  so 
much  on  account  of  its  quantity  as  of  its  special  qualities. 

The  circulation  in  the  brain  may  be  studied  by  help  of  various 
methods.  A  manometer  may  be  connected  with  the  periplieral 
end  of  the  divided  internal  carotid  artery,  a  second  manom- 
eter being  attached  in  the  usual  way  to  the  central  portion. 
Since  the  peripheral  manometer  records  the  blood-pressure  in 
the  circle  of  Willis  transmitted  along  the  peripheral  portion  of 
the  carotid  artery,  variations  of  pressure  in  the  circle  of  Willis 
may  thus  be  studied;  and  a  comparison  of  the  peripheral  Avith 
the  central  manometer  will  indicate  what  oeneral  changes  are 
taking  place  in  the  circulation  through  the  brain.  Thus  a  fall 
of  pressure  in  the  peripheral  manometer  unaccompanied  by  any 
corresponding  fall  in  the  central  manometer  would  shew  that 
the  "peripheral  resistance"  in  the  brain  was  being  lowered,  in 
other  words,  that  the  vessels  were  being  dilated. 


830  THE  VENOUS   SINUSES.  [Book  hi. 

In  another  method,  in  the  dog,  the  outflow  of  venous 
blood  from  the  lateral  sinus  through  the  posterior  facial  vein  has 
been  measured.  The  freedom  with  which  blood  passes  along 
the  sinuses  justifies  the  assumption  that  the  outflow  through 
the  open  vein  gives  an  approximate  measure  of  the  rate  of  flow 
under  natural  conditions ;  still  the  results  are  only  approximate, 
and  besides,  the  continued  loss  of  blood  introduces  error. 

A  third  method  is  a  plethysmographic  one.  The  skull  is 
made  to  serve  as  the  box  of  the  plethysmograph  or  oncometer 
(§  330)  ;  a  small  piece  of  the  roof  having  been  removed  by  the 
trephine,  a  membrane  is  fitted  to  the  hole,  and  the  movements 
of  the  membrane  are  recorded  by  help  of  a  piston  and  lever  or 
directly  by  a  lever.  In  young  subjects,  the  fontanelle,  or  por- 
tion of  the  cranium  not  yet  ossified,  may  be  utilized  as  a 
natural  membrane,  and  its  movements  recorded  in  a  similar 
manner.  When  the  instrument  is  fitted  to  the  hole  in  a  water- 
tight manner,  this  method  records  variations  in  internal  pres- 
sure ;  and  we  may  take  it  for  granted,  unless  otherwise  indicated, 
that  greater  or  less  pressure  is  due  to  more  or  less  blood  pass- 
ing to  the  brain.  But  the  amount  of  pressure  brought  to  bear 
on  the  recording  instrument  will  also  depend  on  the  readiness 
with  which  the  cerebro-spinal  fluid  escapes  from  the  cavity  of 
the  skull ;  if  there  be  a  hindrance  to  the  escape,  or  on  the  other 
hand  an  increased  facility  of  escape,  the  same  increase  of  sup- 
ply of  blood  will  produce  in  one  ease  a  less,  in  the  other  a 
greater  movement  of  the  lever.  If  the  membrane  be  attached 
loosely  to  the  hole  so  as  to  allow  free  escape  of  the  cerebro- 
spinal fluid,  the  lever  practically  resting  on  the  surface  of  the 
cerebral  hemisphere,  the  method  records  variations  in  the  dorso- 
ventral  diameter  of  the  hemisphere,  and  these  may  be  taken  as 
measuring  variations  in  the  volume  of  the  brain  and  so  in  the 
blood  supply.  In  neither  form,  however,  does  the  method  by 
itself  give  us  all  the  information  which  we  want.  An  increase 
of  blood  in  the  brain,  and  therefore  an  expansion  of  the  brain, 
and  so  a  movement  of  the  recording  instrument,  may  result 
either  from  a  fuller  arterial  supply  or  from  hindrance  to  the 
venous  outflow;  the  former  condition  is,  at  least  in  most 
cases  favourable  to,  the  latter  always  and  distinctly  injurious 
to,  the  activity  of  the  nervous  structures.  Hence  the  teach- 
ings of  the  lever  must  be  interpreted  by  help  of  a  simultaneous 
observation  of  the  general  arterial  pressure  and  of  the  blood- 
pressure  in  the  veins  of  the  neck ;  or  the  pressure  in  the  sinuses 
themselves  may  be  measured  by  introducing  a  cannula  directly 
into  the  torcular  Herophili.  Moreover,  the  argument  which 
we  used  (§  337)  in  reference  to  the  kidney  may  be  applied 
here  and  probably  with  equal  force,  namely,  that  the  value  of 
the  blood  stream  for  the  nutrition  of  the  tissue  is  dependent 
not  alone  on  the  amount  of  blood-pressure,  but  also  and  espe- 


Chap,  ii.]  THE   BRAIK  831 

cially  on  the  rapidity  of  the  flow ;  indeed  this  second  factor  is 
of  particular  importance  in  view  of  the  need  of  supplying  the 
nervous  elements  with  an  adequate  interchange  of  gases.  Now 
of  the  lapidit}^  of  flow  the  plethysmographic  method  can  give 
us  indirect  information  onl3^ 

§  522.  By  one  or  other  or  all  of  these  methods,  certain 
important  facts  have  been  made  out.  The  volume  of  the  brain, 
as  determined  by  the  amount  of  blood  present  in  it,  is  contin- 
ually undergoing  changes  brought  about  by  various  causes. 
Each  heart-beat  makes  itself  visible  on  the  cerebral  as  on  the 
renal  plethysmographic  tracing,  and  as  we  have  seen  in  speak- 
ing of  respiration,  the  diminution  of  pressure  in  the  great  veins 
of  the  neck  during  inspiration  leads  to  a  shrinking,  and  the. 
reverse  change  during  expiration  to  a  swelling  of  the  brain. 
The  plethysmograph  also  shews  variations,  larger  and  slower 
than  the  respiratory  undulations,  and  brought  about  by  various 
causes,  such  as  the  position  of  the  head  in  relation  to  the 
trunk,  movements  of  the  limbs,  modifications  of  the  respira- 
tor}^ movements,  and  apparently  phases  of  activity  of  the  brain 
itself,  as  in  waking  and  sleeping ;  undulations  corresponding  to 
the  Traube-Hering  variations  (§  315)  of  blood-pressure  may 
not  unfrequently  be  observed. 

All  the  various  methods  show  that  the  flow  tlii-ough  the 
brain  is  largely  determined  by  a  vaso-motor  action  of  some 
kind  or  another.  And  this  we  might  indeed  infer  from  ordi- 
nary experience.  When  the  head  is  suddenly  shifted  from  the 
erect  to  a  hanging  position,  there  must  be  a  tendency  for  the 
blood  to  accumulate  in  the  cranial  cavity,  and  conversely  when 
the  head  is  suddenly  shifted  from  a  hanging  to  an  erect  posi- 
tion, there  must  be  a  tendency  for  the  supply  of  blood  within 
the  cranium  to  be  for  a  while  less  than  normal.     Either  chancre 

«...  ^ 

of  position,  and  especially  perhaps  the  latter,  would  lead  to 
cerebral  disturbances,  which  in  turn  would  in  ourselves  be  re- 
vealed by  affections  of  our  consciousness.  That  a  perfectly 
healthy,  and  especially  young  organism  whose  vaso-motor 
mechanisms  are  at  once  effective  and  delicately  responsive,  can 
pass  swiftly  from  one  position  of  the  head  to  the  other  without 
inconvenience,  whereas  those  in  whom  the  vaso-motor  mechan- 
isms have  by  age  or  otherwise  become  imperfect  are  giddy  when 
they  attempt  such  rapid  changes,  is  in  itself  adequate  evidence 
of  the  importance  of  the  vaso-motor  arrangements  affecting  the 
circulation  through  the  brain.  The  several  methods  asfree  in 
shewing  that  increased  general  arterial  pressure,  such  as  that 
for  instance  induced  by  stimulation  of  a  sensory  nerve,  leads  to 
a  greater  flow  of  blood  to  the  brain ;  the  volume  of  the  brain 
is  increased  and  the  venous  outflow  by  the  lateral  sinus  is 
quickened.  Conversely,  a  lowering  of  arterial  pressure  leads 
to  a  lessened  flow  of  blood  to  the  brain. 


832  THE   CIECULATION   IK   THE   BRAIN.     [Book  in. 

Seeing  that  the  cerebral  arteries  have  well-developed  muscu- 
lar coats,  the  basilar  artery  in  fact  being  conspicuous  in  this 
respect,  one  would  be  led  to  suppose  that  the  brain  possessed 
special  vaso-motor  nerves  of  its  own;  and  recognising  the 
importance  of  blood  supply  to  rapid  functional  activity  one 
would  perhaps  anticipate  that  by  special  vaso-motor  action,  the 
supply  of  blood  to  this  or  that  particular  part  of  the  brain 
might  be  regulated  apart  from  changes  in  the  general  supply. 
The  various  observations,  however,  which  have  hitherto  been 
made  have  failed  to  demonstrate  Avith  certainty  any  such  special 
vaso-motor  nerves  or  fibres  directly  governing  cerebral  vessels. 
It  would  be  hazardous  to  insist  too  much  on  this  negative  result, 
especially  since  the  observations  have  been  chiefly  directed  to 
the  nerves  of  the  neck,  the  experimental  difficulties  of  investi- 
gating the  presence  of  vaso-motor  fibres  in  the  cranial  nerves 
being  very  great.  Still  it  may  be  urged  and  indeed  has  been 
urged  that  the  flow  of  blood  through  the  brain  is  so  delicately 
responsive  to  the  working  of  the  general  vaso-motor  mechanism 
just  because  it  has  no  vaso-motor  nerves  of  its  own.  In  such 
an  organ  as  the  kidney,  an  increase  of  general  blood-pressure, 
as  we  have  more  than  once  insisted,  may  or  may  not  lead  to  a 
greater  flow  through  the  kidney  according  as  the  vessels  of  the 
kidney  itself,  through  the  action  of  the  renal  vaso-motor  nerves, 
are  dilated  or  constricted;  and,  as  we  have  seen,  a  constriction 
of  the  renal  vessels  may  be  one  of  the  contributors  to  the  in- 
creased general  pressure.  In  the  brain,  on  the  other  hand,  an 
increase  of  general  arterial  pressure  seems  always  to  lead  to 
increase  of  flow.  Thus  in  the  Traube-Hering  undulations  just 
mentioned,  the  expansions  of  the  brain  are  coincident  with  the 
rises  of  the  general  pressure,  whereas  in  the  normal  kidney  and 
in  other  organs  the  local  Traube-Hering  undulation  reverses  the 
general  one,  the  shrinkings  are  synchronous  with  the  rises  of 
pressure,  the  local  constriction  being  one  of  the  factors  of  the 
general  rise.  It  is  argued,  that  in  the  absence  of  vaso-motor 
nerves  of  their  own,  the  cerebral  vessels  are  wholly,  so  to 
speak,  in  the  hands  of  the  general  vaso-motor  system,  so  that 
when  the  blood-pressure  is  high  owing  to  a  large  vaso- constric- 
tion in  the  abdominal  viscera,  more  blood  must  necessarily  pass 
to  the  brain,  and  when  again  the  blood-pressure  falls  through 
the  opening  of  the  splanchnic  flood-gates  (§  151)  less  blood 
necessarily  flows  along  the  cerebral  vessels.  And  indeed  one 
may  recognize  here  a  sort  of  self-regulating  action ;  for  dimin- 
ishing the  supply  of  blood  to  the  vaso-motor  centre  in  the  bulb 
acts,  as  we  know,  as  a  powerful  stimulus  in  producing  vaso- 
constriction, and  so  leads  to  a  rise  of  blood-pressure ;  but  this 
very  rise  of  blood-pressure  drives  more  blood  to  the  brain,  in- 
cluding the  bulb,  and  thus  the  injurious  effects  to  the  brain 
threatened  by  an  anaemic  condition  are  warded  off  by  the  very 


Chap,  ii.]  THE   BRAIN.  833 

beginning  of  the  ansemia  itself.  All  these  advantages  are, 
however,  quite  compatible  with  the  coexistence  of  special  vaso- 
motor mechanisms. 

§  523.  Moreover  the  flow  of  blood  to,  and  consequent  change 
in  the  bulk  of,  the  brain,  and  indeed  the  flow  of  blood  through 
the  brain,  as  measured  by  the  venous  outflow,  may  be  modified 
independently  of  changes  in  the  general  blood-pressure.  For 
instance,  stimulation  of  the  motor  region  of  the  cortex  quickens 
the  venous  outflow,  without  producing  any  marked  change  in 
the  general  blood-pressure ;  this  feature  becomes  very  striking 
at  the  onset  of  epileptiform  convulsions  when  these  make  their 
appearance.  It  is  difficult  not  to  connect  such  a  result  of  func- 
tional activity  with  some  special  vaso-motor  nervous  arrange- 
ment comparable  to  that  so  obvious  in  the  case  of  a  secreting 
gland.  Again,  it  has  been  observed  that  certain  drugs  have  an 
effect  on  the  volume  of  the  brain,  quite  incommensurate  with 
their  effect  on  the  vaso-motor  system ;  thus  in  particular  the 
injection  into  the  general  blood  stream  of  a  weak  acid  produces 
a  large  and  immediate  expansion  of  the  brain,  while  the  intro- 
duction of  a  weak  alkali  similarly  gives  rise  to  similar  consider- 
able shrinking.  It  is  suggested  that  these  effects  are  produced 
by  the  acid  or  alkali  acting  directly  on  the  muscular  coats  of 
the  minute  arteries  and  so  leading  to  relaxation  or  contraction 
respectively.  Observations  go  to  show  that  the  grey  matter 
of  the  cortex  is  faintl}^  alkaline  during  life  and  under  normal 
conditions,  but  becomes  acid  after  death  or  when  its  blood- 
supply  is  interfered  with ;  and  it  has  been  urged  that  nervous 
grey  matter  like  muscular  substance  developes  acidity  during 
activity,  as  well  as  upon  death,  the  acidity  being  probably  due 
in  each  case  to  some  form  of  lactic  acid.  And  just  as  it  has  been 
suggested  that  the  dilation  of  the  minute  arteries  of  a  skeletal 
muscle,  accompanying  or  following  the  contraction  of  the  mus- 
cle, is  brought  about  by  the  acid  generated  during  the  contrac- 
tion causing  a  relaxation  of  the  muscular  coats  of  the  minute 
arteries,  so  it  has  been  suggested  that  a  similar  acidity,  the 
product  of  nervous  activity,  similarly  leads  in  nervous  tissue  to 
a  dilation  of  the  vessels  of  the  part.  The  existence  of  special 
vaso-motor  mechanisms  would,  however,  afford  a  more  satisfac- 
tory explanation  of  these  and  other  phenomena ;  in  spite  of  the 
negative  results  so  far  obtained,  the  matter  is  obviously  one 
needing  further  investigation.  Meanwhile  we  have  abundant 
evidence  that,  however  brought  about,  the  flow  of  blood  through 
the  brain,  and  probabl}^  through  particular  parts  of  the  brain,  is 
varied  in  accordance  with  the  needs  of  the  brain  itself  and  the 
events  taking  place  elsewhere  in  the  body. 


63 


CHAPTER  III. 


SIGHT. 


SEC.  1.     ON  THE   GENEEAL   STEUCTUKE   OF  THE  EYE, 
AND  ON  THE  FOEMATION  OF  THE  EETINAL  IMAGE. 

§  524.  In  dealing  with  the  brain  we  have  been  incidentally- 
obliged  to  deal  with  some  of  the  facts  connected  with  the  senses  ; 
but  we  must  now  study  the  details  of  the  subject.  And,  for  the 
very  reason  that  it  is  the  most  highly  developed  and  differenti- 
ated sense,  it  will  be  convenient  to  begin  with  the  sense  of 
sight ;  we  shall  find  that  the  study  of  it  throws  more  light  on 
the  simpler  and  more  obscure  senses  than  the  study  of  them 
throws  on  it. 

A  ray  of  light  entering  the  eye  and  falling  on  the  retina 
gives  rise  to  what  we  call  a  sensation  of  light ;  but  in  order  that 
distinct  vision  of  any  object  emitting  or  reflecting  rays  of  light 
may  be  gained,  an  image  of  the  object  must  be  formed  on  the 
retina,  and  the  better  defined  the  image  the  more  distinct  will 
be  the  vision.  Hence  in  studying  the  physiology  of  vision,  our 
first  duty  is  to  examine  into  the  arrangements  by  which  the  for- 
mation of  a  satisfactory  image  on  the  retina  is  effected ;  these 
we  may  call  briefly  the  dioptric  mechanisms.  We  shall  then 
have  to  inquire  into  the  laws  according  to  which  rays  of  light 
impinging  on  the  retina  give  rise  to  nervous  impulses,  and  into 
the  laws  according  to  which  the  sensory  impulses  thus  gene- 
rated, which  we  will  call  visual  impulses,  give  rise  in  turn  to 
visual  sensations.  Here  we  shall  come  upon  the  difficulty  of 
distinguishing  between  the  events  which  are  of  physical  origin, 
due  to  changes  in  the  retina  and  optic  fibres,  and  those  which 
are  of  psychical  origin,  due  to  features  of  our  own  consciousness ; 
for  many  of  our  conclusions  are  based  on  an  appeal  to  conscious- 
ness. We  shall  find  our  difficulties  further  increased  by  the  fact, 
that  in  appealing  to  our  own  consciousness  we  are  apt  to  fall 
into  error  by  failing  to  distinguish  between  those  affections  of 

834 


Chap,  hi.]  SIGHT.  835 

consciousness  Avhich  are  the  primary  and  direct  results  of  the 
stimulation  of  the  retina  and  those  secondary,  more  recondite, 
affections  of  consciousness  to  which  the  former,  through  the 
intricate  working  of  the  central  nervous  system,  give  rise,  or,  in 
familiar  language,  by  confounding  what  we  see  with  \vhat  we 
think  we  see.  These  two  things  we  will  briefly  distinguish  as 
visual  sensations  and  visual  judgments ;  and  we  shall  And  that 
even  in  vision  with  one  eye,  though  more  especially  in  binocular 
vision,  visual  judgments  form  a  very  large  part  of  what  we  fre- 
quently speak  of  as  our  '  sight.' 

§  525.  In  the  structure  of  the  eye  we  may  distinguish  two 
parts :  the  one  is  the  retina,  in  which  visual  impulses  are  gene- 
rated ;  the  other  comprises  all  the  rest  of  the  eyeball,  for  all  the 
other  structures  serve  either  as  a  dioptric  mechanism  or  as  a 
means  of  nourishing  the  retina.  This  distinction  is  readily  seen 
when  we  trace  out  the  early  history  of  the  eye. 

The  first  of  the  three  primary  cerebral  vesicles,  that  which 
is  the  forerunner  of  the  third  ventricle,  buds  out  on  each  side 
the  stalked  and  hollow  optic  vesicle.  The  wall  of  this  optic 
vesicle,  like  that  of  the  rest  of  the  medullary  tube,  consists  of 
epithelium,  and  the  cavity  of  the  vesicle  is  at  first  continuous, 
through  the  canal  of  the  hollow  stalk,  with  that  of  the  medul- 
lary tube.  The  whole  is  covered  over  by  the  layer  of  epiblast 
which,  with  scanty  underlying  mesoblast,  is  the  rudiment  of  the 
future  skin. 

Very  soon  the  vesicle  is  doubled  back  upon  or  folded  in 
upon  itself  so  that  the  originally  more  or  less  spherical  hollow 
single-walled  vesicle  is  converted  into  a  more  or  less  hemispher- 
ical cup  with  a  double  wall,  one  the  hind  or  outer  wall  corre- 
sponding to  the  hind  half,  and  the  other  the  front  or  inner  ^A'all 
to  the  front  half  of  the  vesicle,  the  two  walls  of  the  cup  coming 
eventually  into  contact  so  that  the  cavity  of  the  vesicle  is  oblit- 
erated. The  folding  is  somewhat  peculiar,  inasmuch  as  it  takes 
place  not  only  at  the  front  but  also  and  indeed  chiefly  at  the 
side,  forming  at  the  side  a  cleft,  the  choroidal  fissure,  the  edges 
of  which  ultimately  unite.  We  cannot  enter  into  the  details 
of  the  matter  here,  and  indeed  only  refer  to  the  character  of  the 
folding  in  order  to  point  out  tliat  it  involves  the  stalk  as  well 
as  the  cup.  The  stalk  is  first  flattened  and  then  doubled  up 
lengthwise,  a  quantity  of  mesoblastic  tissue  being  thrust  into 
the  hollow  of  the  fold ;  and  eventually  the  originally  hollow 
stalk  becomes  a  solid  stalk  having  within  it  a  core  of  mesoblas- 
tic tissue,  carrying  blood  vessels.  This  core  of  vascular  meso- 
blast, the  origin  of  the  future  centi'al  arter}'  of  the  retina,  is 
continuous  with  a  quantity  of  mesoblast  which  enters  into  the 
hollow  of  the  cup  at  the  time  of  folding,  and,  as  we  shall  see, 
the  central  artery  of  the  stalk  is  up  to  a  certain  stage  of  devel- 
opment carried  forward  through  the  centre  of  the  cup.     The 


836 


STEUCTUEE   OF   THE   EYE. 


[Book  hi. 


cup  becomes  what  we  may  speak  of  broadly  as  the  retina,  and 
we  may  call  it  the  optic  or  retinal  cup ;  the  solid  stalk  becomes 
the  optic  nerve  ;  and  the  other  parts  of  the  eyeball  are  formed 
round  this  retinal  cup,  which  remains  as  the  essential  part  of 
the  eye. 


Fig. 


139.     Diagrammatic  outline  of  a  horizontal  section  of  the  eye, 
TO  illustrate  the  relations  of  the  various  parts. 


The  figure  is  to  be  regarded  as  very  diagrammatic,  more  or  less  distortion  of 
the  relative  sizes  of  the  various  parts  and  of  the  relative  thickness  of  the  coats 
being  unavoidable  in  the  effort  to  secure  simplicity. 

Sr2.  the  sclerotic  coat,  shaded  longitudinally,  continuous  with  the  (unshaded) 
body  of  the  cornea,  ex.  the  epithelium  of  the  cornea  continuous  with  e.cj.  the 
epithelium  of  the  conjunctiva. 

Ch.  the  choroid  coat  with  G.  P.  the  ciliary  process  and  I.  the  body  of  the  iris, 
all  stippled  to  indicate  that  they  are  all  parts  of  the  same  vascular  investment. 

B.  the  retina  or  inner  wall,  and  P.  E.  the  pigment  epithelium  or  outer  wall  of 
the  retinal  cup.  In  front  of  the  wavy  line  OS.,  marking  the  position  of  the  ora 
serrata,  the  retina  proper  changes  into  the  pars  ciliaris  retinee,  p.  c.  B.  Both  the 
pigment  epithelium  and  the  pars  ciliaris  retinse  are  represented  as  continued  over 
the  back  of  the  iris  as  well  as  over  the  ciliary  process. 

L.  the  lens.  sp.  I.  the  suspensory  ligament.  The  broken  line  round  the  lens, 
shewn  on  one  side  only,  represents  the  membrana  capsulo-pupillaris  ;  and  the 
straight  continuation  of  it  through  V.  H.  the  vitreous  humour  to  0.  N.  the  optic 
nerve  indicates  the  embryonic  continuation  of  the  central  artery  of  the  retina. 

o.  X.  the  optic  axis,  in  this  case  made  to  pass  through  the  fovea  centralis  /c. 


The  front  or  inner  wall  of  the  retinal  cup  is  from  the  first 
distinctly  thicker  than  the  hind  or  outer  wall  (Fig.  139)  ;  it  soon 
consists  of  more  than  one  layer  of  epithelium,  and  it  alone,  or, 
more  strictly  speaking,  part  of  it  alone,  becomes  the  retina  proper. 
The  hind  or  outer  wall  remains  thin,  and  continues  to  consist  of  a 
single  layer  of  epithelium,  the  cells  of  which  are  never  developed 


Chap,  hi.]  SIGHT.  837 

into  nervous  elements  but  soon  become  loaded  with  pigment,  and 
the  greater  part  of  it  is  known  in  the  adult  eye  as  the  pigment 
epithelium  of  the  retina,  which,  as  we  shall  see,  is  in  close  func- 
tional connection  with  the  nervous  elements  of  the  retina  proper. 
The  fibres  of  the  optic  nerve,  as  they  are  developed  in  the  stalk 
of  the  retinal  cup,  become  connected  with  the  elements  of  the 
inner  or  retinal  wall  only  of  the  cup ;  they  pierce  the  outer  wall 
of  pigment  epithelium,  making  no  connections  with  the  cells  of 
that  outer  wall. 

The  retina  then,  in  which  by  the  action  of  light  visual  impulses 
are  generated,  is  in  reality  a  part  of  the  brain,  removed  to  some 
distance  from  the  rest  of  the  brain  but  remaining  connected  with 
it  by  means  of  the  tract  of  white  matter  which  we  call  the  optic 
nerve ;  and,  as  we  shall  see,  the  retina  is  in  structure  similar  to 
parts  of  the  grey  matter  of  the  brain.  The  optic  nerve  is  not  like 
other  nerves  an  outgrowth  from  the  central  nervous  system,  but 
like  the  olfactory  tract  a  commissure  of  white  matter  between 
two  parts  of  the  brain,  namely,  between  the  outlying  retina  and 
the  internally  placed  corpus  geniculatum,  pulvinar,  and  corpus 
quadrigeminum.  We  shall  find  accordingly  that  in  structure  it 
differs  from  ordinary  cranial  or  spinal  nerves. 

Into  the  mouth  of  the  retinal  cup  there  is  thrust  a  rounded 
mass  of  epithelium,  an  involution  from  the  superficial  epiblast ; 
this  becomes  the  lens.  The  hollow  of  the  retinal  cup  is  occupied, 
as  we  have  said,  by  mesoblast ;  this  ultimately  becomes  modified 
into  the  vitreous  humour.  The  mesoblastic  tissue  surrounding 
the  cup  is  developed  into  an  investment  of  two  coats  ;  an  inner, 
somewhat  loose  and  tender,  vascular  and  in  part  muscular  coat, 
which  on  the  one  hand  serves  to  nourish  the  retina,  and  on  tlie 
other  hand  carries  out  certain  movements  of  the  dioptric  appa- 
ratus, and  an  outer,  firmer  and  denser  coat,  which  affords  protec- 
tion to  the  whole  of  the  structures  within.  The  inner  vascular 
coat,  which  may  be  compared  to  the  pia  mater,  is  called  the  cho- 
roid (Fig.  139  (7/i.),  and  in  the  front  part  of  the  eye,  at  about 
the  level  of  the  lens,  is  thrown  into  a  number  of  radiating  folds 
or  plaits,  the  ciliary  processes  Q.P.  The  outer  coat,  which  may 
be  compared  to  the  dura  mater,  is  called  the  sclerotic  (Fig.  139 
Scl.).  Over  the  greater  part  of  the  eyeball  the  two  coats  are  in 
apposition,  or  separated  only  by  narrow  lymphatic  spaces,  which 
may  be  compared  with  the  subarachnoid  spaces,  but  towards  the 
front  they  diverge  ;  the  choroid  is  bent  inwards  towards  the  cen- 
tral axis  of  the  eye  to  form  the  diaphragm  called  the  iris  (Fig. 
139  /.),  while  the  sclerotic  is  continued  forwards  to  form,  beneath 
the  epidermis  into  which  the  superficial  epiblast  is  developed,  the 
basis  of  the  cornea  (Fig.  139  C).  At  the  angle  of  divergence 
of  the  two  coats  is  developed  a  small  mass  of  muscular  fibres,  the 
ciliary  muscle  of  Avhich  we  shall  speak  in  detail  presently. 

The  inner  or  front  wall  of  the  retinal  cup  becomes  as  we  have 


838  STRUCTURE   OF  THE  EYE.  [Book  hi. 

said  thick,  and  is  developed  into  the  retina ;  but  this  takes  place 
only  over  about  the  hind  three-fourths  of  the  cup.  Along  a 
meridian  round  the  eye,  at  a  wavy  boundary  line  called  the  ora 
serrata  (Fig.  139  O.S.')^  the  retina  proper  ceases  and  the  inner  wall 
of  the  retinal  cup  in  front  of  the  ora  serrata  is  continued  on  as 
a  much  thinner  structure  (Fig.  139  p.c.E.')  consisting  of  a  single 
layer  only  of  cells ;  this  is  spoken  of  as  the  pars  ciliaris  retince. 
The  outer  or  hind  wall  of  the  retinal  cup  consists  throughout 
of  a  single  layer  of  epithelium  cells  loaded  with  pigment.  Behind 
the  ora  serrata,  that  is,  in  the  region  of  the  retina  proper,  these 
cells  have,  as  we  shall  see,  peculiar  features,  but  in  front  of  the 
ora  serrata  they  lose  these  features  and  become  ordinary  cubical 
cells,  though  still  loaded  with  pigment. 

Hence  the  choroid  may  be  described  as  having  a  double  lining. 
Over  the  hind  part  of  the  eye,  behind  the  ora  serrata,  it  is  lined 
by  the  single  layer  of  pigment  epithelium  and  the  retina.  In  front 
of  the  ora  serrata  it,  including  the  ciliary  processes,  is  lined  by 
the  same  layer  of  pigment  epithelium  representing  the  outer 
wall,  and  by  the  single  layer  of  cells,  free  from  pigment,  repre- 
senting the  inner  wall  of  the  retinal  cup,  the  latter  being  called, 
as  we  have  said,  the  pars  ciliaris  retinse.  And  as  the  ciliary  part 
of  the  choroid  passes  on  to  form  the  iris,  these  two  layers  are 
also  continued  on  to  line  the  back  of  the  iris,  coming  to  an  end 
at  the  margin  of  the  pupil  or  central  opening  of  the  iris,  which 
may  accordingly  be  taken  as  marking  the  extreme  lip  of  the 
retinal  cup.  Fig.  139.  Here  however,  as  we  shall  see,  the  two 
layers  are  not  so  easily  and  distinctly  recognized  as  in  the  ciliary 
region  ;  and  the  nature  of  the  structures  forming  the  back  of  the 
iris  has  been  a  matter  of  much  controversy. 

At  an  early  stage  the  mesoblastic  tissue,  which  fills  up  the 
hollow  of  the  retinal  cup  and  surrounds  the  lens,  is  continuous 
at  the  mouth  of  the  retinal  cup  with  the  outer  investment  of  the 
cup  ;  it  here  forms  around  the  lens  the  membrana  capsulo-pupil- 
laris,  and  at  the  margin  of  the  iris  the  membrana  pupillaris  block- 
ing up  the  future  opening  of  the  pupil.  The  arteria  centralis 
retinse,  which  during  the  folding  of  the  cup  and  stalk  is  carried 
into  the  core  of  the  optic  nerve,  does  not  at  this  early  stage  stop 
at  the  retina,  but  is  continued  forward  through  the  middle  of  the 
vitreous  humour  to  the  membrana  capsulo-pupillaris,  and  furnishes 
the  developing  lens  with  an  abundant  supply  of  blood.  But 
neither  layer  of  the  retinal  cup  stretches  over  the  pupillary 
membrane  ;  they  both  stop,  as  we  have  said,  at  the  margin  of  the 
iris.  Before  birth  takes  place,  the  membrana  pupillaris  is,  in  man, 
absorbed  and  the  pupil  is  thus  established ;  at  the  same  time  the 
central  artery  in  the  vitreous  humour  is  obliterated  beyond  the 
retina,  and  the  vascular  membrana  capsulo-pupillaris  gives  place 
to  the  non-vascular  capsule  of  the  lens  and  the  suspensory  liga- 
ment of  which  we  shall  speak  hereafter. 


Chap,  hi.]  SIGHT.  839 

Between  the  iris,  which  is  the  extreme  front  of  the  choroid 
investment,  and  the  cornea,  which  is  the  extreme  front  of  the 
sclerotic  investment,  the  lym^jhatic  spaces  which  over  the  rest  of 
the  eye  are  narrow  and  linear  are  developed  into  a  large  con- 
spicuous chamber,  the  anterior  chamber  of  the  eye,  which  upon  the 
establishment  of  the  pupil  by  the  absorj)tion  of  the  pupillary 
membrane  becomes  continuous  with  the  smaller  "posterior  cham- 
ber "  of  the  eye  or  space  between  the  back  surface  of  the  iris  and 
ciliary  processes  on  the  outside  and  the  suspensory  ligament  with 
the  lens  on  the  inside.  The  cavity  of  the  conjoined  anterior  and 
posterior  chambers,  being  a  continuation  and  enlargement  of  the 
flatter  spaces  between  the  choroid  or  pia  mater  of  the  eye,  and 
sclerotic  or  dura  mater  of  the  eye,  may  be  likened  to  the  sub- 
arachnoid space,  and  like  that  space  contains  a  peculiar  fluid ;  this, 
which  is  called  the  aqueous  humour,  like  the  cerebro-spinal  fluid, 
differs  fiom  ordinary  lymph,  and  is  probably,  to  a  large  extent, 
furnished  by  the  ciliary  processes  in  some  such  way  as  the  cerebro- 
spinal fluid  is  furnished  by  the  choroid  plexuses  (§  517). 


The  Formation  of  the  Retinal  Image. 

§  526.  The  iris  and  choroid  coat  contain,  as  we  have  said, 
muscular  elements,  and  by  means  of  these  muscular  elements 
changes  in  the  form  and  relations  of  some  of  the  parts  of  the  eye 
are  brought  about ;  hence  we  have  to  distinguish  between  the  eye 
at  rest,  and  the  eye  which  is  undergoing  one  or  other  of  these 
changes. 

The  eye  is  a  camera,  consisting  of  a  series  of  surfaces  and 
media  arranged  in  a  dark  chamber,  the  iris  serving  as  a  diaphragm  ; 
and  the  object  of  the  apparatus  is  to  form  on  the  retina  a  distinct 
image  of  external  objects.  That  a  distinct  image  is  formed  on  the 
retina,  may  be  ascertained  by  removing  the  sclerotic  from  the 
back  of  an  eye,  and  looking  at  the  hinder  surface  of  the  transparent 
retina  while  rays  of  light  proceeding  from  an  external  object  are 
allowed  to  fall  on  the  cornea.  To  understand  how  such  an  image 
is  formed,  we  must  call  to  mind  a  few  optical  principles. 

A  dioptric  apparatus  in  its  simplest  form  consists  of  two  media 
of  different  refractive  power  separated  by  a  (spherical)  surface ; 
and  the  optical  properties  of  such  an  apparatus  depend  upon  (1) 
the  degree  of  curvature  of  the  surface,  (2)  the  relative  refractive 
powers  of  the  media. 

Such  a  simple  optical  system  is  represented  in  Fig.  140,  where 
aph  represents,  in  section,  a  curved  (spherical)  surface  separating 
a  less  refractive  medium,  on  the  left  hand  towards  0,  from  a 
more  refractive  medium  on  the  right  hand  towards  A.  The 
surface  in  question  is  symmetrically  placed  as  regards  the  line 
Ovl,  which  falling  normal  (perpendicular)  to  tlie  surface  at  p 


840  FOEMATION    OF   RETINAL   IMAGES.       [Book  in. 

passes  through  the  centre  n  of  the  sphere  with  whose  surface  we 
are  dealing.     This  line  is  called  the  optic  axis. 

All  rays  of  light  which,  in  passing  from  the  first  less  refrac- 
tive to  the  second  more  refractive  medium,  cut  the  surface  nor- 
mally, such  as  the  one,  Op,  in  the  line  of  the  optic  axis,  and 
others,  such  as  md,  m'e,  undergo  no  refraction  ;  all  such  rays  are 
continued  on  as  straight  lines,  and  all  pass  through  n  the  centre 
of  the  sphere  or  nodal  point.  All  other  rays  passing  from  the 
jfirst  to  the  second  medium  are  refracted.  Of  these  all  those 
which  lie  in  the  first  medium  parallel  to  the  optic  axis,  such  as 
cd,  are  so  refracted  as  to  meet  in  the  second  medium  at  a  point, 
-Fg,  on  the  optic  axis ;  this  is  called  the  principal  posterior  (or 
second)  focus.  On  the  optic  axis  in  the  first  medium  there  is 
another  important  point,  F^,  the  rays  of  light  passing  from  which, 


Fig.   140.   Diagram  of  Simple  Optical  System. 

such  as  F-^e,  are  so  refracted  in  passing  into  the  second  medium 
as  to  become  parallel,  e/,  to  the  optic  axis  ;  this  point  is  called 
the  principal  anterior  (or  first)  focus.  The  point  at  which  the 
optic  axis  cuts  the  surface  is,  for  reasons  which  we  shall  see 
presently,  called  the  principal  point.  The  above  points,  viz.  the 
posterior  and  the  anterior  principal  foci,  the  nodal  point,  and 
the  principal  point  are  the  cardinal  points  of  such  an  optical 
system. 

Such  a  simple  system,  however,  does  not  represent  the  optical 
conditions  of  the  eye,  for  this  consists  of  several  media  bounded 
by  several  surfaces,  the  latter  differing  from  each  other  in  curva- 
ture, though  being  approximately  spherical.  Rays  of  light  in 
passing  from  an  external  object  to  the  retina  traverse  in  succes- 
sion the  following  surfaces  and  media  :  —  the  anterior  surface  of 
the  cornea,  the  substance  of  the  cornea,  the  posterior  surface 
of  the  cornea,  the  aqueous  humour,  the  anterior  surface  of  the 
lens,  the  substance  of  the  lens,  the  posterior  surface  of  the  lens, 


Chap,  hi.]  SIGHT.  841 

and  the  vitreous  humour  ;  so  that  we  have  to  deal  with  four  sur- 
faces, and,  including  the  external  air,  four  media.  Indeed  the 
matter  is  in  reality  still  more  complicated,  for  the  structure  of 
the  lens,  as  we  shall  see,  is  such  that  the  substance  of  the  lens 
differs  somewhat  in  refractive  power  in  different  parts,  the 
central  parts  being  more  refractive  than  the  peripheral  parts ; 
moreover  the  lens  is  covered  in  front  by  a  capsule  different  in 
structure  from  the  lens  itself.  We  may,  however,  neglect,  with- 
out fear  of  serious  error,  these  smaller  differences,  and  consider 
the  lens  as  one  medium  of  uniform  refractive  power  bounded  by 
an  anterior  and  a  posterior  surface.  The  cornea  again,  as  we 
shall  see,  is  not  absolutely  uniform  in  structure,  but  this  we  may 
also  neglect  and  consider  the  cornea  as  a  medium,  also  of  uni- 
form refractive  power,  bounded  by  an  anterior  and  a  posterior 
surface.  Moreover,  the  posterior  surface  of  the  cornea  is  parallel 
to  (concentric  with)  the  anterior  surface  or  nearly  so.  Now 
when  the  two  surfaces  which  bound  a  medium  are  parallel  to 
each  other  we  may,  in  dealing  with  refraction,  neglect  the  thick- 
ness of  the  medium  entirely,  we  may  suppose  it  to  be  absent  and 
treat  the  two  surfaces  as  if  they  were  one.  We  may  therefore, 
without  serious  error,  neglect  the  substance  of  the  cornea,  and 
consider  the  cornea  as  affording  one  surface,  its  anterior  surface, 
bounding  the  air  in  front  from  the  aqueous  humour  behind. 
Lastly,  the  aqueous  humour  differs  in  refractive  power  so  little 
from  the  vitreous  humour  that  we  may  consider  the  two  as  form- 
ing one  medium. 

We  have  therefore  to  deal  with  three  surfaces  separating 
three  media,  viz.:  —  first,  the  anterior  surface  of  the  cornea,  at 
which  considerable  refraction  takes  place  as  the  rays  of  light 
pass  from  the  less  refractive  air  into  the  more  refractive  aqueous 
humour;  secondly,  the  anterior  surface  of  the  lens,  at  Avhich 
again  considerable  refraction  takes  place  as  the  rays  pass  from 
the  less  refractive  aqueous  humour  into  the  more  refractive  sub- 
stance of  the  lens ;  and  lastly,  the  posterior  surface  of  the  lens, 
at  which  refraction  takes  place  as  the  rays  pass  from  the  more 
refractive  substance  of  the  lens  into  the  less  refractive  vitreous 
humour.  The  three  surfaces,  differing  in  curvature,  are  all 
approximately  centred,  symmetrically  disposed  around,  the  optic 
axis  of  the  system.  This  optic  axis  meets  the  retina,  according 
to  some  authorities,  not  quite  at  the  part  of  the  retina  which, 
under  the  name  oi  fovea  cetitralis,  we  shall  hereafter  speak  of  as 
the  centre  of  the  retina,  but  a  little  above  and  to  the  nasal  side 
of  that  part ;  other  autliorities,  however,  maintain  that  it  does 
cut  the  retina  at  the  fovea  centralis. 

§  527.  The  eye,  therefore,  even  with  the  simplifications 
which  we  have  introduced,  presents  a  much  more  complex  op- 
tical system  than  the  one  described  above.  It  has,  however,  been 
shewn  mathematically  that  a  complex  optical  system  consisting 


842  FOEMATION    OF   RETINAL   IMAGES.       [Book  hi, 

of  several  surfaces  and  media  centred  on  one  optical  axis  may  be 
treated  as  if  it  were  a  more  simple  system  consisting  of  two  sur- 
faces only.  In  such  a  simplified  system  each  of  the  two  (ideal) 
surfaces  has  its  own  nodal  point  and  its  own  principal  foci,  an- 
terior and  posterior ;  moreover,  the  two  points  where  the  two 
surfaces  cut  the  optic  axis  are  called  principal  points  (and  ver- 
tical planes  drawn  through  those  ^oiniB  principal  planes),  first,  or 
anterior,  and  second  or  posterior.  Hence  the  cardinal  points  of 
such  a  simplified  complex  system  are  six  in  number,  namely,  the 
anterior  and  posterior  principal  foci,  the  anterior  and  posterior 
principal  points,  and  the  anterior  and  posterior  nodal  points. 
(When  such  a  system  is,  by  removal  of  surfaces  and  media,  con- 
verted into  the  still  more  simple  system  of  one  surface  separating 
two  media,  the  two  nodal  points  become  coincident  in  one  point, 
namely,  the  centre  of  the  sphere,  and  the  two  principal  points 
become  coincident  in  one  point,  namely,  the  point  at  which  the 
optic  axis  cuts  the  surface.) 

In  order  to  effect  such  a  simplification  of  a  complex  optical 
system,  it  is  requisite  to  know :  —  (1)  The  refractive  index  of 
each  medium.  (2)  The  radius  of  curvature  of  each  surface. 
(3)  The  distance  along  the  optic  axis  between  the  first  surface 
on  which  the  rays  fall  and  the  succeeding  surfaces.  These  can 
be  and  have  been  determined  for  the  human  eye,  and  the  follow- 
ing table  gives  the  several  values  usually  adopted  with  some  re- 
cent corrections,  the  latter  being  placed  in  brackets. 

Refractive  index  of  aqueous  or  vitreous  humour  1-3376  (1-3365) 

Mean  refractive  index  of  lens 1-4545  (1-4371) 

Radius  of  curvature  of  cornea  8  (7-829)  mm. 

"  "  of  anterior  surface  of  lens  10  " 

"  "  of  posterior    "         "         ...  6  " 

Distance  from  anterior  surface  of  cornea  to  ante-  " 

rior  surface  of  lens 4  (3-6)  " 

Thickness  of  lens 4  (3-6) 

By  means  of  these  measurements  the  optical  system  of  the 
eye  may  be  simplified  into  an  optical  system  of  two  surfaces.  In 
this  '  schematic,  or  diagrammatic,  eye  of  Listing,'  as  it  is  gener- 
ally called,  the  two  (ideal)  surfaces,  and  the  principal  points 
where  these  cut  the  optic  axis  (Fig.  14:l,p\p'\  the  two  surfaces 
being  indicated  by  dotted  lines),  lie  close  together  in  the  front 
part  of  the  aqueous  humour,  and  the  nodal  points,  n\  w^,  lie,  also 
close  together,  in  the  back  part  of  the  lens. 

Further,  the  two  principal  surfaces  lie  so  close  together  that, 
for  practical  purposes,  no  serious  error  is  introduced,  if  instead 
of  two  such  surfaces  we  assume  the  existence  of  one  surface  lying 
midway  between  the  two.  In  this  way  we  arrive  at  the  '  reduced 
diagrammatic  eye,'  or  'the  reduced  eye '  as  it  is  called,  in  which 


Chap,  hi.] 


SIGHT. 


843 


the  several  surfaces  and  media  of  the  actual  eye  are  replaced  by 
one  (ideal)  spherical  surface  (Fig.  141,  P),  having  one  nodal 
point,  iV ;  the  two  media  which  the  surface  separates  are  sup- 
posed to  be  air  on  the  one  side  and  water  on  the  other. 

The  several  positions  of  the  cardinal  points  of  this  'reduced 
eye  '  are  as  follows  : 

The  principal  j)oint,  where  the  one  surface  of  the  system  cuts 
the  optic  axis,  lies  in  the  aqueous  humour,  2-3448  mm.  behind  the 
anterior  surface  of  the  cornea. 

The  nodal  point  lies  in  the  back  part  of  the  lens,  -4764  mm. 
in  front  of  the  posterior  surface  of  the  lens. 


Fig.  141.     Diagram  of  the  Schematic  or  Diagrammatic  Eye. 


The  poster  io7'  principal  focus  lies  22-647  (22-819)  mm.  behind 
the  anterior  surface  of  the  cornea,  that  is  to  say,  practically  lies 
on  the  retina. 

The  anterior  principal  focus  lies  12'8326  mm.  in  front  of  the 
anterior  surface  of  the  cornea. 

The  radius  of  curvature  of  the  (ideal)  surface  is  5-1248  mm. ; 
(that  of  the  cornea  is  8  mm.  and  of  the  anterior  surface  of  the 
lens  10  mm.). 

§  528.  By  help  of  this  '  reduced  eye '  we  are  enabled  to  trace 
out  the  paths  of  raj^s  of  light  through  the  actual  eye,  and  to  study 
the  formation  of  imagoes  on  the  retina.  When  an  iraao-e  of  an  ex- 
ternal  object,  such  as  an  arrow  (Fig.  142),  is  formed  in  such  an 
eye,  each  point  of  the  object  is  considered  as  sending  out  a  pencil 
of  diverging  rays,  which  by  the  system  are  made  to  converge 
again  into  the  point  in  the  image  which  corresponds  to  the  point 
in  the  object.  One  such  pencil  of  rays  proceeds  from  the  point  at 
the  extreme  tip  of  the  arrow,  another  from  the  extreme  point  at 
the  other  end,  and  other  pencils  from  all  the  points  between  these 


844  FOEMATION   OF   RETINAL   IMAGES.      [Book  in. 

two.  Each  such  pencil  has  for  its  core  a  ray  called  the  principal 
ray,  a,  a',  around  which  are  arranged,  with  increasing  divergency, 
the  other  rays  of  the  pencil,  such  as  h,  b',  c,  c' .  When  such  a 
pencil  of  rays  falls  on  the  refracting  surface,  such  as  the  '  prin- 
cipal surface  '  of  the  reduced  eye,  the  principal  ray  of  the  pencil, 
a,  being  normal  to  that  surface,  is  not  refracted  at  all,  but  passes 
straight  on  through  the  nodal  point  n,  while  the  other  rays  of  the 
pencil,  6,  c,  undergoing  refraction  according  to  their  respective 


Fig.  142.     Diagram  of  the  Formation  of  a  Retinal  Image. 

divergencies,  are  made  to  converge  together  at  some  point  on  the 
path  of  the  principal  ray,  and  thus  form  at  that  spot  the  image 
of  the  point  from  which  the  pencil  proceeded.  The  exact  posi- 
tion on  the  line  of  the  principal  ray,  at  which  convergence  takes 
place  and  at  which  the  image  is  formed,  will  depend  on  the  re- 
fractive power  of  the  optical  system  in  relation  to  the  amount  of 
divergence  of  the  pencil ;  the  refractive  power  of  the  system  re- 
maining the  same,  it  will  be  nearer  to,  or  farther  from,  the  nodal 
point  according  as  the  rays  are  less  or  more  divergent ;  and  the 
divergence  of  the  rays  remaining  the  same,  it  will  be  nearer  to, 
or  farther  from,  the  nodal  point  according  as  the  refractive  power 
of  the  system  is  greater  or  less. 

Hence  supposing  the  eye  to  be  in  that  condition  in  which  a 
distinct  image  of  the  arrow  is  formed  on  the  retina,  we  can  find 
the  position  on  the  retina  of  the  image  of  the  extreme  point 
of  the  tip  of  the  arrow,  by  simply  drawing  a  straight  line  from 
■that  extreme  point  of  the  arrow  X  through  the  nodal  point  n 
of  the  'reduced'  eye.  Such  a  straight  line  represents  the  path 
of  the  '  principal  ray '  of  the  pencil  proceeding  from  the  extreme 
tip  of  the  arrow,  and  when  an  image  is  formed  on  the  retina  the 
other  diverging  rays  of  that  pencil  will  be  so  refracted  as  to 
converge  at  the  point  x,  where  that  line  meets  the  retina ;  all 
the  rays  will  form  together  there  the  image  of  the  extreme  point 
of  the  arrow.  In  a  similar  way  a  straight  line  drawn  through 
the  nodal  point  from  the  extreme  point  of  the  other  end  of  the 
arrow,  and  continued  until  it  meets  the  retina  at  y,  will  give  us 
the  position  of  the  image  of  tlie  other  end  of  the  arrow ;  and  in 
like  manner  lines  drawn  from  other  points  of  the  arrow  through 


Chap,  hi.]  SIGHT.  845 

the  same  nodal  point  will  give  us  the  position  on  the  retina  of 
the  images  of  those  other  points.  In  this  way  the  construction 
of  the  reduced  eye  enables  us  to  ascertain  the  position,  magnitude 
and  features  of  the  retinal  image  of  an  object. 

§  529.  A  ray  of  light,  tliat  is  to  say  a  series  of  waves  of 
ether,  falling  upon  a  point  of  the  retina  stimulates  certain 
structures  in  the  retina  and  gives  rise,  as  we  have  said,  to  visual 
impulses  and  so  to  a  sensation  of  light ;  this  we  may  consider 
as  a  visual  sensation  in  its  simplest  form.  When  a  number  of 
different  points  of  the  retina  are  thus  stimulated  at  the  same 
time,  as  Avhen  an  image  of  an  external  object  falls  in  proper 
focus  on  the  retina,  the  total  result  is  a  complex  group  of  visual 
impulses  and  thus  a  complex  sensation,  by  which  we  perceive, 
as  we  say,  the  object ;  and  we  frequently  speak  of  this  complex 
sensation  as  a  visual  image  corresponding  to  the  retinal  image. 
The  term  is  perhaps  not  a  very  desirable  one,  since  it  seems  to 
imply  an  identity  between  the  former  which  is  a  psychical 
matter,  and  the  latter  wliich  is  a  physical  matter ;  whereas,  the 
one  thing  we  may  be  sure  about  is  that  the  psychical  thing, 
though  it  is  a  sign  and  token  of,  is  wholly  different  from  the 
physical  thing. 

It  will  be  as  well  perhaps  thus  early  to  call  attention  to  the 
fact  that,  as  indeed  is  shewn  in  Fig.  142,  the  image  on  the  retina 
is  an  inverted  one.  What  is  the  upper  part  of  the  object  in  the 
external  world  is  represented  in  the  lower  part  of  the  retinal 
image,  what  is  on  the  right-hand  side  of  the  object  is  represented 
on  the  left-hand  side  of  the  image.  In  the  visual  judgment 
which  is  based  upon  the  visual  sensation,  the  retinal  image  is,  as 
it  were,  reinverted ;  we  take  the  left-hand  side,  or  the  bottom  of 
the  retinal  image,  as  a  token  or  sign  of  the  right-hand  side 
or  the  top  of  the  object  seen.  We  shall  return  to  this  matter 
latei-  on ;  but  in  studying  the  dioptrics  of  the  eye  this  inversion 
of  the  retinal  image  must  always  be  borne  in  mind. 


SEC.  2.     THE  FACTS   OF   ACCOMMODATION. 

§  530.  When  an  object  emitting  or  reflecting  light,  a  lens, 
and  a  screen  to  receive  the  image  of  the  object,  are  so  arranged 
in  reference  to  each  other,  that  the  image  upon  the  screen  is 
sharp  and  distinct,  the  rays  of  light  proceeding  from  each  lumi- 
nous point  of  the  object  are  brought  into  focus  on  the  screen  in 
a  point  of  the  image  corresponding  to  the  point  of  the  object. 
If  the  object  be  then  removed  farther  away  from  the  lens,  the 
rays  proceeding  in  a  pencil  from  each  luminous  point  will  be 
brought  to  a  focus  at  a  point  in  front  of  the  screen,  and,  subse- 
quently diverging,  will  fall  upon  the  screen  as  a  circular  patch 
composed  of  a  series  of  circles,  the  so-called  diffusion  circles^ 
arranged  concentrically  round  the  principal  ray  of  the  pencil. 
If  the  object  be  removed,  not  farther  from,  but  nearer  to  the  lens, 
the  pencil  of  rays  will  meet  the  screen  before  they  have  been 
brought  to  focus  in  a  point,  and  consequently  will  in  this  case 
also  give  rise  to  diffusion  circles.  When  an  object  is  placed 
before  the  eye,  so  that  the  image  falls  into  exact  focus  on  the 
retina,  and  the  pencils  of  rays  proceeding  from  each  luminous 
point  of  the  object  are  brought  into  focus  in  points  on  the  retina, 
the  sensation  called  forth  is  that  of  a  distinct  image.  When  on 
the  contrary  the  object  is  too  far  away,  so  that  the  focus  lies  in 
front  of  the  retina,  or  too  near,  so  that  the  focus  lies  behind  the 
retina,  and  the  pencils  fall  on  the  retina  not  as  points,  but  as 
systems  of  diffusion  circles,  the  sensation  produced  is  that  of  an 
indistinct  and  blurred  image.  In  order  that  objects  both  near 
and  distant  may  be  seen  with  equal  distinctness  by  the  same 
dioptric  apparatus,  the  focal  arrangements  of  the  apparatus  must 
be  accommodated  or  adjusted  to  the  distance  of  the  object,  either 
by  changing  the  refractive  power  of  the  lens,  or  by  altering  the 
distance  between  the  lens  and  the  screen. 

That  the  eye  does  possess  such  a  power  of  accommodation  or 
adjustment  is  shewn  by  every-day  experience.  If  two  needles  be 
fixed  upright,  some  two  feet  or  so  apart,  into  a  long  piece  of 
wood,  and  the  wood  be  held  before  the  eye,  so  that  the  needles 
are  nearly  in  a  line,  it  will  be  found  that  if  attention  be  directed 

846 


Chap,  hi.]  SIGHT.  847 

to  the  far  needle,  the  near  one  appears  blurred  and  indistinct^ 
and  that,  conversely,  when  the  near  one  is  distinct,  the  far  one 
appears  blurred.  By  an  effort  of  the  will  we  can  at  pleasure 
make  either  the  far  one  or  the  near  one  distinct ;  but  not  both 
at  the  same  time.  When  the  eye  is  arranged  so  that  the  far 
needle  appears  distinct,  the  image  of  that  needle  falls  exactly  on 
the  retina,  and  each  pencil  of  rays  reflected  from  each  point  of 
the  needle  unites  in  a  point  upon  the  retina ;  but  when  the  far 
needle  is  seen  distinctly,  the  focus  of  the  near  needle  lies  behind 
the  retina,  and  each  pencil  from  each  point  of  this  needle  falls 
upon  the  retina  in  a  series  of  diffusion  circles ;  hence  the  image 
of  the  near  needle  is  blurred.  Similarly,  when  the  eye  is 
arranged  so  that  the  near  needle  is  distinct,  the  image  of  that 
needle  falls  upon  the  retina  in  such  a  way,  that  each  pencil  of 
rays  from  each  point  of  the  needle  unites  in  a  point  on  the  retina, 
while  each  pencil  from  each  point  of  the  far  needle  unites  at  a 
point  in  front  of  the  retina,  and  then  diverging  again  falls  on  the 
retina  in  a  series  of  diffusion  circles,  and  the  far  needle  is  noAv 
seen  indistinctly.  If  the  near  needle  be  gradually  brought 
nearer  and  nearer  to  the  eye,  it  will  be  found  that  greater  and 
greater  effort  is  required  to  see  it  distinctly,  and  at  last  a  point 
is  reached  at  which  no  effort  can  make  the  image  of  the  needle 
appear  anything  but  blurred.  The  distance  of  this  point  from 
the  eye  marks  the  near  limit  of  accommodation  for  near  objects. 
Similarlj^  if  the  person  be  short-sighted,  the  far  needle  may  be 
moved  away  from  the  eye,  until  a  point  is  reached  at  which  it 
ceases  to  be  seen  distinctly,  and  appears  blurred ;  the/ar  limit  of 
accommodation  is  reached.  In  the  one  case,  the  eye,  with  all  its 
power,  is  unable  to  bring  the  image  of  the  needle  sufficiently  for- 
ward to  fall  on  the  retina:  the  focus  lies  permanently  behind 
the  retina.  In  the  other,  the  eye  cannot  bring  the  image  suffi- 
ciently backward  to  fall  on  the  retina :  the  focus  lies  perma- 
nently in  front  of  the  retina.  In  both  cases  the  pencils  of  raj's 
from  the  needle  strike  the  retina  in  diffusion  circles. 

§  531.  The  same  phenomena  may  be  shewn  with  greater 
nicety  by  what  is  called  Scheiner's  Experiment.  If  two  smooth 
holes  be  pricked  in  a  card,  at  a  distance  from  each  other  less 
than  the  diameter  of  the  pupil,  and  the  card  be  held  up,  with  the 
holes  horizontal  before  one  eye,  the  other  being  closed,  and  a 
needle  placed  vertically  be  looked  at  through  the  holes,  the  fol- 
lowing facts  may  be  observed.  When  attention  is  directed  to 
the  needle  itself,  the  image  of  the  needle  appears  single.  When- 
ever the  gaze  is  directed  to  a  more  distant  object,  so  that  the  eyQ 
is  no  longer  accommodated  for  the  needle,  the  image  of  the 
needle  appears  double  and  at  the  same  time  blurred.  It  also 
appears  double  and  blurred  when  the  eye  is  accommodated  for  a 
distance  nearer  than  that  of  the  needle.  When  only  one  needle 
is  seen,  and  the  eye  therefore  is  properly  accommodated  for  the 


848 


ACCOMMODATIOK 


[Book  hi. 


distance  of  the  needle,  the  only  effect  produced  by  blocking  up 
one  hole  of  the  card  is  that  the  needle  and  indeed  the  whole 
field  of  vision  seems  dimmer.  When,  however,  the  image  is 
double  on  account  of  the  eye  being  accommodated  for  a  distance 
greater  than  that  of  the  needle,  blocking  the  left-hand  hole 
causes  a  disappearance  of  the  right-hand  or  opposite  image,  and 
blocking  the  right-hand  hole  causes  the  left-hand  image  to  dis- 
appear. When  the  eye  is  accommodated  for  a  distance  nearer 
than  that  of  the  needle,  blocking  either  hole  causes  the  image 
on  the  same  side  to  vanish.  The  following  diagram  will  explain 
how  these  results  are  brought  about. 


v2 

~ii 

\ 

II 

,  n 

f\ 

Z  <rL 

\v, 

11 

w 

Fig.  143.     Diagram  of  Scheiner's  Experiment. 


Let  a  (Fig.  143)  be  a  point  in  the  needle,  and  ae,  af  the 
extreme  right-hand  and  left-hand  rays  of  the  pencil  of  rays 
proceeding  from  that  point,  and  passing  respectively  through 
the  right-hand  e,  and  left-hand/,  holes  in  the  card.  (The  figure 
is  supposed  to  be  a  horizontal  section  of  the  eye,  and  a  forms 
part  of  a  transverse  section  of  the  vertically  placed  needle.) 
When  the  eye  is  accommodated  for  a,  the  rays  e  and  /  meet 


Chap,  hi.]  SIGHT.  849 

together  in  the  point  c,  the  retina  occupying  the  position  of  the 
plane  nn  ;  the  point  in  the  needle  appears  as  one  point,  and  the 
needle  will  appear  as  one  needle.  When  the  eye  is  accommo- 
dated for  a  distance  bej'ond  a,  the  retina  may  be  considered  to 
lie^  no  longer  at  im,  but  nearer  the  lens,  at  mm  for  example; 
the  rays  ae  will  cut  this  plane  at  jt?,  and  the  rays  q/  at  ^ ;  hence 
the  point  in  the  needle  will  no  longer  appear  single,  but  will  be 
seen  as  two  points,  or  rather  as  two  systems  of  diffusion  circles, 
and  the  single  needle  will  appear  as  two  blurred  needles.  The 
rays  passing  through  the  right-hand  hole  e,  will  cut  the  retina  at 
p,  i.e.  on  the  right-hand  side  of  the  optic  axis ;  but,  as  we  have 
already  (§  529)  said,  the  image  on  the  right-hand  side  of  the 
retina  is  referred  by  the  mind  to  an  object  on  the  left-hand  side 
of  the  person ;  hence  the  affection  of  the  retina  at  jo,  produced 
by  the  rays  ae  falling  on  it  there,  gives  rise  to  an  image  of  the 
spot  a  at  P,  and  similarly  the  left-hand  spot  q  corresponds  to  the 
right-hand  Q.  Blocking  the  left-hand  hole,  therefore,  causes  a 
disappearance  of  the  right-hand  image,  and  vice  versa.  Simi- 
larly when  the  eye  is  accommodated  for  a  distance  nearer  than 
the  needle,  the  retina  may  be  supposed  to  be  removed  to  II,  and 
the  right-hand  ae  and  left-hand  af  rays,  after  uniting  at  c,  will 
diverge  again  and  strike  the  retina  in  diffusion  circles  at  p'  and 
q' .  The  blocking  of  the  hole  e  will  now  cause  the  disappearance 
of  the  image  q'  on  the  left-hand  side  of  the  retina,  and  this  will 
be  referred  by  the  mind  to  the  right-hand  side,  so  that  Q  will 
seem  to  vanish. 

If  the  needle  be  brought  gradually  nearer  and  nearer  to  the 
eye,  a  point  will  be  reached  within  which  the  image  is  always 
double.  This  point  marks  with  considerable  exactitude  the 
near  limit  of  accommodation.  With  short-sighted  persons,  if 
the  needle  be  removed  farther  and  farther  away,  a  point  is 
reached  beyond  which  the  image  is  always  double ;  this  marks 
the  far  limit  of  accommodation. 

The  experiment  may  also  be  performed  with  the  needle 
placed  horizontally,  in  which  case  the  holes  in  the  card  should 
be  vertical. 

The  determination  of  the  accommodation  of  the  eye  for  near 
or  far  distances  may  be  assisted  by  using  two  needles,  one  near 
and  one  far.  In  this  case  one  needle  should  be  vertical,  and  the 
other  horizontal,  and  the  card  turned  round  so  that  the  holes  lie 
horizontally  or  vertically  according  to  whether  the  vertical  or 
horizontal  needle  is  being  made  to  appear  double. 

§  532.  In  what  may  be  regarded  as  the  normal  eye,  the  so- 
called  emmetropic  eye,  the  near  limit  of  accommodation  is  about 
10  or  12  cm.,  and  the  far  limit  may  be  put  for  practical  purposes 

1  Of  course,  in  the  actual  eye,  as  we  shall  see,  accommodation  is  effected  by  a 
change  in  the  lens,  and  not  by  an  alteration  in  the  position  of  the  retina  j  but  for 
convenience  sake,  we  may  here  suppose  the  retina  to  be  moved. 

54 


850  ACCOMMODATIOK  [Book  in. 

at  an  infinite  distance.  The  '  range  of  distinct  vision'  therefore 
for  the  emmetropic  eye  is  very  great.  In  the  myopic,  or  short- 
sighted eye,  the  near  limit  is  brought  much  closer  (5  or  6  cm.) 
to  the  cornea ;  and  the  far  limit  is  at  a  variable  but  not  very 
great  distance,  so  that  the  rays  of  light  proceeding  from  an 
object  not  many  feet  away  are  brought  to  a  focus  in  the  vitreous 
humour  instead  of  on  the  retina.  The  range  of  distinct  vision  is 
therefore  in  the  myopic  eye  very  limited.  In  the  hypermetropic, 
or  long-sighted  eye,  the  rays  of  light  coming  from  even  an  infi- 
nite distance  are,  in  the  passive  state  of  the  eye,  brought  to  a 
focus  beyond  the  retina.  The  near  limit  of  accommodation  is 
at  some  distance  off,  and  a  far  limit  of  accommodation  does  not 
exist.  The  presbyopic  eye,  or  eye  of  advanced  years,  resembles 
the  hypermetropic  eye  in  the  near  point  of  accommodation  being 
at  some  distance,  but  differs  from  it  inasmuch  as  the  former  is 
an  essentially  defective  power  of  accommodation,  whereas  in  the 
latter  the  power  of  accommodation  may  be  good  and  yet,  from 
the  internal  arrangements  of  the  eye,  be  unable  to  bring  the 
image  of  a  near  object  on  to  the  retina.  When  an  eye  becomes 
presbyopic,  the  far  limit  may  remain  the  same,  but  since  the 
power  of  accommodating  for  near  objects  is  weakened  or  lost, 
the  change  is  distinctly  a  reduction  of  the  range  of  distinct 
vision.  When  no  effort  of  accommodation  is  made,  the  princi- 
pal posterior  focus  of  the  eye  lies  in  the  normal,  emmetropic  eye 
on  the  retina,  in  the  myopic  eye  in  front  of  it,  and  in  the  hyper- 
metropic eye  behind  it. 

§  533.  By  what  changes  in  the  eye  are  we  thus  able,  within 
the  above  mentioned  limitations,  to  see  distinctly  objects  at  differ- 
ent distances  ?  In  directing  our  attention  from  a  far  to  a  very 
near  object  we  are  conscious  of  a  distinct  effort,  and  feel  that 
some  change  has  taken  place  in  the  eye ;  when  we  turn  from  a 
very  near  to  a  far  object,  if  we  are  conscious  of  any  change  in 
the  eye,  it  is  one  of.  a  different  kind.  The  former  is  the  sense 
of  an  active  exertion ;  the  latter,  when  it  is  felt,  is  the  sense  of 
relaxation  after  exertion. 

Since  the  far  limit  of  an  emmetropic  eye  is  at  an  infinite 
distance,  no  such  thing  as  active  accommodation  for  far  distances 
need  exist.  The  only  change  which  need  take  place  in  the  eye 
in  turning  from  near  to  far  objects  will  be  a  mere  passive  undoing 
of  the  accommodation  previously  made  for  the  near  object.  And 
that  no  such  active  accommodation  for  far  distances  takes  place 
is  shewn  by  the  following  facts ;  the  eye,  when  opened  after 
being  closed  for  some  time,  is  found  adjusted  not  for  moderately 
distant  but  for  far  distant  objects  ;  we  are  conscious  of  no  effort 
in  turning  from  moderately  distant  to  far  distant  objects ;  and 
when  the  power  of  the  eye  to  accommodate  is  impaired  or 
abolished,  as  we  shall  see  it  may  be,  by  atropin  or  nervous  dis- 
ease, the  vision  of  distant  objects  may  be  unaffected.    The  sense 


Chap,  hi.]  SIGHT.  851 

of  effort  often  spoken  of  by  myopic  persons  as  being  felt  when 
they  attempt  to  see  things  at  or  beyond  the  far  limit  of  their 
range  seems  to  arise  from  a  movement  of  the  eyelids,  and  not 
from  any  internal  changes  taking  place  in  the  eye. 

What  then  are  the  changes  which  take  place  in  the  eye, 
when  we  accommodate  for  near  objects  ?  It  might  be  thought, 
and  indeed  once  was  thought,  that  the  curvature  of  the  cornea 
was  changed,  becoming  more  convex,  with  a  shorter  radius  of 
curvature,  for  near  objects.  This  is  disproved  by  the  fact  that 
accommodation  takes  place  as  usual  when  the  eye  (and  head) 
is  immersed  in  water.  Since  the  refractive  powers  of  aqueous 
humour  and  water  are  very  nearly  alike,  the  cornea,  with  its 
parallel  surfaces,  placed  between  these  two  fluids,  can  have 
little  or  no  effect  on  the  direction  of  the  rays  passing  through 
it  when  the  eye  is  immersed  in  water.  Moreover  we  have  it  in 
our  power  to  detect  any  change  in  the  curvature  of  the  cornea 
which  may  take  place.  If  a  luminous  body  such  as  a  candle  be 
held  in  front  of  a  convex  surface  like  the  cornea  an  image  of 
the  body  is  seen  reflected  from  the  surface ;  and,  with  the  body 
at  a  certain  distance,  the  image  will*  be  of  a  certain  size.  If 
now  the  curvature  of  the  surface  be  increased,  if  the  surface  be 
made  more  convex,  the  image  will  diminish  in  size ;  if  the 
curvature  of  the  surface  be  diminished  the  image  will  increase 
in  size.  Indeed  by  measuring  carefully  the  changes  in  the  size 
of  the  image  we  may  determine  the  amount  of  change  in  the 
curvature  of  the  surface.  And  accurate  measurements  of  the 
dimensions  of  an  image  on  the  cornea  have  shewn  that  these 
undergo  no  change  during  accommodation,  and  that  therefore 
the  curvature  of  the  cornea  is  not  altered.  Nor  is  there  any 
change  in  the  form  of  the  bulb ;  for  any  variation  in  this  would 
necessarily  produce  an  alteration  in  the  curvature  of  the  cornea, 
and  pressure  on  the  bulb  would  act  injuriously  by  rendering  the 
retina  ansemic  and  so  less  sensitive.  In  fact,  there  are  only  two 
changes  of  importance  which  can  be  ascertained  to  take  place  in 
the  eye  during  accommodation  for  near  objects. 

One  is  that  the  pupil  contracts.  When  we  look  at  near 
objects,  the  pupil  becomes  small;  when  we  turn  to  distant 
objects,  it  dilates.  This  however  cannot  have  more  than  an 
indirect  influence  on  the  formation  of  the  image ;  the  chief  use 
of  the  contraction  of  the  pupil  in  accommodation  for  near 
objects  is  to  cut  off  the  more  divergent  circumferential  rays 
of  light. 

The  other  and  really  efficient  change  is  that  the  anterior 
surface  of  the  lens  becomes  more  convex.  If  a  light  be  held 
before  the  eye,  three  reflected  images  may,  with  care  and  under 
proper  precautions,  be  seen  by  a  bystander :  one  (Fig.  144  A,  a) 
a  very  bright  one  caused  by  the  anterior  surface  of  the  cornea, 
a  second  less  bright,  b,  by  the  anterior  surface  of  the  lens,  and  a 


852  ACCOMMODATIOK  [Book  in. 

third  very  dim,  <?,  by  the  posterior  surface  of  the  lens ;  when  the 
images  are  those  of  an  object,  such  as  the  flame  of  a  candle,  in 
which  a  top  and  bottom  can  be  recognized,  the  two  former  images 
are  seen  to  be  erect,  but  the  third  inverted.  When  the  eye  is 
accommodated  for  near  objects,  no  change  is  observed  in  the 
first,  and  none,  or  a  very  insignificant  one,  in  the  third  of  these 
images ;  but  the  second,  that  from  the  anterior  surface  of  the 
lens,  is  seen  to  become  distinctly  smaller,  shewing  that  the  sur- 
face has  become  more  convex.  When,  on  the  contrary,  vision 
is  directed  from  near  to  far  objects,  the  image  from  the  anterior 

A  B 

a     b    c  .  a       b      c 

Fig.  144.     Diagram  of  Images  eeflected  from  the  Eye. 

In  A  are  seen  the  three  images  of  a  candle  reflected  from  a,  the  anterior 
surface  of  the  cornea,  6,  the  anterior  surface  of  the  lens,  and  c  the  posterior 
surface  of  the  lens,  a  is  bright  and  erect,  h  also  erect,  is  larger  but  less  bright, 
c  inverted  is  small  and  dim. 

B  shews  the  images,  two  squares,  as  seen  in  the  phakoscope  when  the  eye 
is  directed  to  a  far  object.  C  the  same  when  the  eye  is  accommodated  for  a 
near  object.  The  pair  h  are  in  C,  smaller  and  closer  together  than  in  B,  shewing 
an  increase  of  curvature. 

surface  of  the  lens  grows  larger,  indicating  that  the  convexity 
of  the  surface  has  diminished,  while  no  change  takes  place  in 
the  image  from  the  cornea,  and  none,  or  hardly  any,  in  that 
from  the  posterior  surface  of  the  lens.  And  accurate  measure- 
ments of  the  size  of  the  image  from  the  anterior  surface  of  the 
lens  have  shewn  that  the  changes  in  curvature  which  do  take 
place  are  considerable ;  the  radius  of  curvature  of  the  lens 
accommodated  for  near  objects  is  6  mm.,  for  far  objects  10  mm. ; 
and  this  difference  is  sufficient  to  account  for  the  power  of 
accommodation  which  the  eye  possesses. 

The  observation  of  these  reflected  images  is  facilitated  by  the 
simple  instrument  introduced  by  Helmholtz  and  called  a  Phakoscope. 
It  consists  of  a  small  dark  chamber,  with  apertures  for  the  observed 
and  observing  eyes ;  a  needle  is  fixed  at  a  short  distance  in  front  of 
the  former,  to  serve  as  a  near  object,  for  v^hich  accommodation  has  to 
be  made ;  and  a  lamp  or  candle  is  so  disposed  as  to  throw  an  image 
on  each  of  the  three  surfaces  of  the  observed  eye.  Since  a  change 
in  the  distance  between  two  images  is  more  readily  appreciated  than 
is  a  simple  change  of  size  of  a  single  image,  two  prisms  are  employed 
so  as  to  throw  a  double  image  in  the  form  of  bright  squares  on  each 


Chap,  in.]  SIGHT.  853 

of  the  three  surfaces,  Fig.  144  B,  C.  When  the  anterior  surface  of 
the  lens  becomes  more  convex  the  two  images  reflected  from  that 
surface  approach  each  other,  C,  when  it  becomes  less  convex  they 
retire  from  each  other,  B. 

These  observations  leave  no  doubt  that  the  essential  change 
by  which  accommodation  is  effected,  is  an  alteration  of  the  con- 
vexity of  the  anterior  surface  of  the  lens.  And  that  the  lens  is 
the  agent  of  accommodation,  is  further  shewn  by  the  fact  that 
after  removal  of  the  lens,  as  in  the  operation  for  cataract,  the 
power  of  accommodation  is  lost.  In  the  cases  which  have  been 
recorded,  where  eyes  from  which  the  lens  had  been  removed 
seemed  still  to  possess  some  accommodation,  we  must  suppose 
that  no  real  accommodation  took  place,  but  that  the  pupil  con- 
tracted when  a  near  object  was  looked  at,  and  so  assisted  in 
making  vision  more  distinct. 


SEC.   3.     THE   MECHANISM    OF    ACCOMMODATION    AND 
THE  MOVEMENTS   OF  THE  PUPIL. 


§  534.  How  is  this  increase  in  the  curvature  of  the  ante- 
rior surface  of  the  lens  during  accommodation  for  near  objects 
brought  about  ? 

It  has  been  supposed  to  be  due  to  a  compression  of  the  cir- 
cumference of  the  lens  by  a  contraction  of  the  sphincter  muscle 
of  the  iris,  which,  as  we  shall  see,  is  the  cause  of  the  narrowing 
of  the  pupil  attendant  upon  accommodation  for  near  objects ; 
but  this  is  disproved  by  the  fact  that  normal  accommodation 
may  take  place  in  eyes  from  which  the  iris  is  congenitally  absent 
or  has  been  wholly  removed  by  operation.  It  has  also  been 
attributed  to  vaso-motor  changes,  to  increased  fulness  of  the 
vessels  of  the  iris  or  ciliary  processes,  surrounding  and  pressing 
upon  the  lens ;  but  this  also  is  disproved,  not  only  by  the  fact 
just  mentioned  but  as  well  by  the  fact  that  accommodation  may 
be  effected,  after  death,  in  an  eye  which  is  practically  bloodless, 
by  stimulating  the  ciliary  ganglion  or  short  ciliary  nerves  with 
an  interrupted  current  or  by  other  means  ;  as  we  shall  see,  these 
nerves  govern  the  accommodation  mechanism.  The  real  nature 
of  the  mechanism  seems  to  be  as  follows : 

The  lens  is  a  body  of  considerable  elasticity.  When  the 
curvature  of  the  anterior  surface  of  the  lens  is  determined,  as 
may  be  done  by  appropriate  means  (by  measurements  of  images 
seen  by  reflection  from  it),  in  its  natural  position  in  the  eye  at 
rest,  and  then  again  determined,  after  the  lens  has  been  removed 
from  the  eye,  the  anterior  surface  is  found  to  be  more  convex  in 
the  latter  than  in  the  former  case.  There  seems  to  be  in  the 
eye  in  its  natural  condition  at  rest  some  agency  at  work,  keep- 
ing the  anterior  surface  of  the  lens  somewhat  flattened.  All 
that  is  needed  is  some  means  of  counteracting  this  agency,  and 
thereby  allowing  the  lens  through  its  elasticity  to  assume  its 
natural  form.  And  the  arrangements  of  the  suspensory  liga- 
ment described  in  a  previous  section  afford  an  explanation  of 
what  is  the  agency  in  question,  and  how  it  is  counteracted. 

The  cavity  of  the  eyeball  behind  the  suspensory  ligament  is 
filled  with  the  vitreous  humour.     If  this  is  sufficiently  abundant 

854 


Chap,  hi.] 


SIGHT. 


855 


it  will  distend  the  cavity  and  render  the  suspensory  ligament 
tense.  But  since  the  suspensory  ligament  passes  obliquely  for- 
wards, all  round,  from  the  ciliary  processes  to  the  front  of  the 
lens,  tension  of  the  ligament  will  tend  to  flatten  the  lens,  altering 
its  shape  but  not  its  bulk. 

The  choroid,  of  which  the  ciliarj^  processes  form  the  forward 
continuation,  is  loosely  attached  to  the  sclerotic  along  the  line 
of  the  lamina  fusca  and  suprachoroideal  membrane ;  the  one  can 
to  a  certain  extent  be  slipped  backwards  and  forwards  beneath 
the  other. 


sp.ch. 


Fig.  145.   Diagram  of  the  ciliary  muscle   as  seen  in  a  vertical  radial 
section  of  the  ciliary  region. 

E.cj.  epithelium  of  the  conjunctiva,  d.cj.  dermis  of  the  conjunctiva.  Scl.  Scle- 
rotic, sp.ch.  suprachoroidal  layer.  Ch.  Choroid,  p.e.  pars  ciliaris  retinae  and 
pigment  epithelium  represented  as  one  layer.  C.P.  Ciliary  processes.  /.  Iris. 
<ig.h.  anterior  chamber.  E.p.  ligamentum  pectinatum.  c.S.  canal  of  Schlemm, 
and  X  tissue  to  inside  of  it. 

I. cm.  longitudinal,  and  c.c.m.  circular  ciliary  muscle.  ;/  bundles  of  the  longi- 
tudinal muscle  cut  across  as  they  are  taking  a  circular  direction. 


The  (longitudinal)  ciliary  muscle  (Fig.  145)  is  attached  on 
the  one  hand  to  the  junction  of  the  sclerotic  and  cornea,  and  on  the 
other  hand  to  the  front  part  of  the  choroid.  If  we  suppose  the 
former  to  be  a  fixed  point,  the  contraction  of  the  muscle  would 
pull  the  moveable  choroid  and  ciliary  processes  somewhat  for- 
ward. But  the  pulling  forward  of  these  structures  would  slacken 
the  suspensory  ligament  by  bringing  its  ciliary  attachment  more 
forward.  And  a  slackening  of  the  suspensory  ligament  b}'  reliev- 
ing the  pressure  on  the  elastic  lens  would  allow  the  front  surface 
to  become  more  convex.  This  is  shewn  diagrammatically  in 
Fig.  146,  one-half  of  which,  the  left  half,  is  intended  to  repre- 
sent the  eye  directed  towards  distant  objects,  while  the  other 


856 


ACCOMMODATION. 


[Book  hi. 


half  represents  the  change  taking  place  during  accommodation 
for  a  nearer  object. 

§  535.  It  seems  possible  then  that  accommodation  for  near 
objects  may  be  brought  about  by  a  contraction  of  the  (longi- 
tudinal) ciliary  muscle  dragging  forwards  the  choroid  and  ciliary 
processes,  thus  slackening  the  suspensory  ligament,  and  so  per- 
mitting the  compressed  elastic  lens  to  bulge  forward.  And 
experimental  evidence  shews  that  this  is  what  does  take  place. 
The  ciliary  muscle  is  governed,  as  we  shall  see  presently,  by  the 
ciliary  nerves.  If  in  a  living  animal  (dog)  or  in  an  eye  imme- 
diately after  removal  from  the  body,  an  opening  be  made  in  the 
sclerotic  in  order  to  watch  the  choroid,  it  may  be  seen  that  when 
the  ciliary  nerves  are  stimulated  the  choroid  does  move  forward 
at  the  same  time  that  the  front  surface  of  the  lens  becomes  more 
convex ;  a  needle,  the  point  of  which  is  carefully  lodged  in  the 


FAR 


NEAR 


Fig.  146.    Diagram  to  illustrate  Accommodation.     (After  Helmholtz.) 

C.P.  Ciliary  process.     I.  Iris.     Sp.l.  suspensory  ligament,    l.c.m.  longitudi- 
nal ciliary  muscle,    c.c.m.  circular  ciliary  muscle.     G.S.  canal  of  Schlemm. 

The  left  half  represents  the  arrangement  for  viewing  far  objects  and  the 
right  half  that  for  viewing  near  objects. 


choroid,  moves  in  such  a  way  as  to  shew  that  the  choroid  moves 
forward,  though  no  appreciable  movement  can  be  seen  in  a  needle 
thrust  into  the  front  part  of  the  ciliary  muscle  itself.  If  the  cor- 
nea be  cut  away  so  as  to  leave  only  at  one  place  a  small  fragment 
still  connected  to  the  junction  of  the  sclerotic  and  cornea,  this 
piece  moves  backward  when  the  ciliary  nerves  are  stimulated, 
shewing  that  the  ciliary  muscle  does  pull  on  the  point  of  junc- 
tion of  the  sclerotic  with  the  cornea.  When,  however,  the  cor- 
nea is  intact,  or  even  when  a  sufficiently  large  part  of  it  is  left, 
the  junction  becomes  a  fixed  point,  at  least  relatively  to  the 
moveable  choroid.  Moreover  not  only  the  contraction  of  the 
ciliary  muscle  and  movement  of  the  choroid,  but  the  actual 
slackening  of  the  suspensory  ligament  and  change  in  the  curva- 


Chap,  hi.]  SIGHT.  857 

ture  of  the  lens  may  be  observed  to  follow  upon  stimulation  of 
the  ciliary  nerves.  We  may  conclude,  therefore,  that  the  pos- 
sible explanation  given  above  is  the  actual  one. 

One  or  two  additional  points  are  worth  mentioning.  During 
accommodation  for  near  objects  the  pupil  is  narrowed;  we  shall 
speak  of  this  presently.  A  narrowing  of  the  pupil  means  that 
the  edge  and  inner  part  of  the  iris  moves  over  the  front  surface 
of  the  lens  toward  the  centre  of  the  pupil.  In  becoming  more 
convex,  the  front  surface  of  the  lens,  especially  the  central  por- 
tion, projects  further  forward  into  the  anterior  chamber,  and  in 
so  doing  carries  with  it  the  pupillary  edge  and  inner  part  of  the 
iris ;  for  the  iris  lies  close  upon  and  indeed  in  contact  with  the 
anterior  surface  of  the  lens.  And  when  the  eye  is  carefully 
watched  sideways  this  projection  forwards  of  the  pupillary 
margin  of  the  iris  may  be  observed.  While  the  edge  of  the 
pupil  thus  moves  forward,  and  the  body  of  the  iris  increases  in 
a  radial  direction,  becoming  correspondingly  thinner  (cf.  Fig. 
146),  the  circumferential  edge  of  the  iris  is  carried  slightly 
backwards,  owing  to  the  giving  way  to  a  certain  extent  of  the 
elastic  ligamentum  pectinatum  on  which  the  ciliary  muscle  pulls ; 
and  thus  additional  space  is  afforded  in  the  anterior  chamber 
for  the  aqueous  humour  driven  aside  by  the  projection  of  the 
anterior  surface  of  the  lens. 

The  action  of  the  circular,  equatorial  fibres  of  the  ciliary 
muscle  (Fig.  145)  and  of  the  fibres  intermediate  between  these 
and  the  longitudinal  meridional  fibres,  is  not  quite  so  clear. 
We  may,  however,  suppose  that  the  circular  fibres  acting  in  con- 
cert with  the  longitudinal  fibres  would  bring  the  ciliary  processes 
nearer  to  the  lens,  and  so  assist  in  slackening  the  suspensory 
ligament.  But  no  very  decisive  explanation  has  been  given 
why  the  circular  fibres  are  often  largely  developed  in  some  eyes, 
it  is  said  hypermetropic  or  long-sighted  eyes,  and  scantily 
present  in  others,  myopic  or  short-sighted  eyes.  And  indeed 
there  are  several  points  in  the  whole  action  of  accommodation 
which  still  require  to  be  cleared  up. 

Accommodation  is  in  a  certain  sense  a  voluntary  act ;  we  can 
by  looking  at  near  or  far  objects  bring  about  the  change  when- 
ever we  please.  Since,  however,  the  change  in  the  lens  is  always 
accompanied  by  movements  in  the  iris,  it  will  be  convenient  to 
consider  the  latter  before  we  speak  of  the  nervous  mechanism  of 
the  former. 

Tlie   Changes  in  the  Pupil. 

§  536.  Although  by  looking  at  near  or  far  objects,  and  so 
voluntarily  bringing  about  changes  in  the  accommodation 
mechanism,  we  can  call  forth  the  accompanying  changes  in  the 
iris,  and  can  thus  at  pleasure  produce  a  constriction,  narrowing, 
or  a  dilation,  widening,  of  the  pupil ;  it  is  not  in  our  power  to 


858  CHANGES    IN    THE   PUPIL.  [Book  in. 

bring  the  will  to  act  directly  on  the  iris  by  itself.  This  fact 
alone  indicates  that  the  nervous  mechanism  of  the  pupil  is  of  a 
special  character,  and  such  indeed  we  find  it  to  be. 

The  pupil  is  constricted,  contracted,  narrowed,  (1)  when  the 
retina  (or  optic  nerve)  is  stimulated,  as  when  light  falls  on  the 
retina,  the  brighter  the  light  the  greater  being  the  contraction ; 
(2)  when  we  accommodate  for  near  objects.  The  pupil  is  also 
constricted  when  the  eyeball  is  turned  inwards,  when  the  aqueous 
humour  is  deficient,  in  the  early  stages  of  poisoning  by  chloro- 
form, alcohol,  and  similar  substances,  in  nearly  all  stages  of 
poisoning  by  morphia,  physostigmin,  and  some  other  drugs,  in 
the  early  part  of  the  day,  in  deep  slumber,  in  the  epileptic 
seizure,  and  in  certain  nervous  diseases.  The  pupil  is  dilated, 
widened,  (1)  when  stimulation  of  the  retina  (or  optic  nerve)  is 
diminished  or  arrested,  as  in  passing  from  a  bright  into  a  dim 
light  or  into  darkness,  (2)  when  the  eye  is  adjusted  for  far 
objects.  Dilation  also  occurs  when  there  is  an  excess  of  aqueous 
humour  distending  the  anterior  chamber,  during  dyspnoea,  dur- 
ing violent  muscular  efforts,  as  the  result  of  stimulation  of  sen- 
sory nerves,  as  an  effect  of  emotions,  as  an  effect  of  fatigue,  in 
the  later  stages  of  poisoning  by  chloroform,  alcohol  and  similar 
substances,  in  all  stages  of  poisoning  by  atropin  and  some  other 
drugs,  and  in  certain  nervous  diseases. 

§  537.  We  may  say  at  once  that  we  are  able  to  recognize 
two  separate  nervous  mechanisms  regulating  these  changes  in  the 
pupil.  One  of  these  regulates  the  size  of  the  pupil  according 
to  the  amount  of  light  falling  upon  the  retina,  and  is  by  far 
the  more  important  of  the  two ;  through  the  other,  the  size  of 
the  pupil  is  modified  by  other  influences.  "We  will  consider  the 
former  mechanism  first.  During  the  action  of  this,  constriction 
of  the  pupil  is  undoubtedly  caused  by  contraction  of  the  circu- 
larly disposed  muscular  fibres  which  form  within  the  iris  the 
sphincter  muscle.  The  more  or  less  spongy  body  of  the  iris 
being  extensible,  the  shortening  of  the  fibres  and  bundles  of 
fibres  of  the  sphincter  must  necessarily  narrow  the  ring  of  the 
pupil  of  which  the  sphincter  forms  the  almost  immediate  margin. 
Conversely,  the  body  of  the  iris  being  elastic  as  well  as  exten- 
sible, a  relaxation  of  the  muscular  fibres  of  the  sphincter, 
assisted  by  the  return  to  their  natural  position  of  structures 
displaced  by  the  contraction,  will  lead  to  a  widening  of  the  pupil. 
We  may  in  respect  to  this  mechanism  at  all  events  consider  the 
constricted  pupil  as  the  result  of  a  contraction  of  the  sphincter 
muscle,  and  the  dilated  pupil  as  the  result  of  a  diminution  of 
that  contraction.  Whether  a  dilated  pupil  is  always  a  mere 
negative  result,  due  to  a  lessening  of  the  activity  of  the  sphinc- 
ter, or  whether  in  certain  cases  an  active  dilator  muscle  is 
brought  into  play  we  will  discuss  later  on  in  connection  with 
the  other  mechanism. 


Chap,  hi.] 


SIGHT. 


859 


§  538.  Before  proceeding  it  will  be  desirable  to  recall  to 
mind  the  nervous  supply  of  the  eyeball,  omitting  for  the  pres- 
ent the  nerves  governing  the  six  ocular  muscles  which  move 
the  eyeball  as  a  whole. 

The  eyeball  is  supplied,  in  the  first  place,  by  the  short  ciliary 
nerves  (Fig.  147  s.c.)  coming  from  the  ophthalmic  or  lenticular, 
or  ciliary  ganglion  (l.cJ)  which  is  connected  by  means  of  its 
three  roots,  (1)  through  the  so-called  '  short  root '  with  the 
third  nerve  (r.b.),  (2)  with  the  cavernous  sympathetic  plexus 


Fig.  147.  Diagrammatic  representation  of  the  nerves  governing  the  PtrpiL. 

II.  Optic  nerve,  l.g.  ciliary  ganglion,  r.b.  its  short  root  from  ///.  oc.m., 
third  or  oculo-motor  nerve,  sym.  its  sympathetic  root.  r.l.  its  long  root  from  V. 
ophthm.  the  nasal  branch  of  the  ophthalmic  division  of  the  fifth  nerve,  s.c.  the 
short  ciliarj'  nerves  from  the  lenticular  ganglion.  I.e.  the  long  ciliary  nerves 
from  the  nasal  branch  of  the  ophthalmic  division  of  the  fifth  nerve. 

and  so,  along  the  internal  carotid  artery,  with  the  cervical  sym- 
pathetic nerve  (sijm.'),  and  (3)  through  the  so-called  'long  root' 
with  the  nasal  branch  of  the  oplithalmic  division  of  the  lifth 
nerve  (?•.?.).  Besides  the  short  ciliary  nerves,  the  eyeball  is 
supplied  by  the  long  ciliary  nerves  (Z.t\)  coming  direct  from 


860  MOVEMENTS    OF   THE   PUPIL.  [Book  hi. 

the  nasal  branch  of  the  ophthalmic  division  of  the  fifth  nerve. 
The  short  ciliary  nerves,  which  are  the  most  numerous,  pierce 
the  sclerotic  at  the  hind  part  of  the  eyeball  and  are  distributed 
on  the  one  hand  to  the  blood  vessels  of  the  choroid,  ciliary  pro- 
cesses and  iris,  and  on  the  other  hand  to  the  ciliary  muscle  and 
to  the  sphincter  of  the  pupil.  The  less  numerous  long  ciliary 
nerves,  piercing  the  sclerotic  somewhat  nearer  the  front  of  the 
eye,  are  distributed  to  the  muscles  of  the  iris,  and  probably  to 
the  ciliary  muscle. 

The  third  or  oculo-motor  nerve  we  may  trace  back  to  its 
nucleus  in  the  floor  of  the  aqueduct ;  the  sympathetic  root  we 
may  trace  back  along  the  cervical  sympathetic  to  the  spinal 
connections  of  that  nerve,  on  which  we  have  so  often  dwelt ; 
the  remarkable  ophthalmic  division  of  the  fifth  nerve  we  may 
trace  back  to  the  nucleus  of  the  fifth  nerve ;  this  nerve  is  exceed- 
ingly complex,  and  indeed  we  have  reason  to  consider  its 
ophthalmic  division  as  an  independent  nerve,  which  in  the 
course  of  evolution  has  become  annexed  to  other  nerves  to 
form  what  we  call  '  the  fifth '  nerve. 

§  539.  We  may  now  make  the  broad  statement,  qualifica- 
tions of  which  we  will  consider  later  on,  that  constriction  of  the 
pupil,  brought  about  by  light  falling  on  the  retina,  is  a  reflex 
act,  of  which  the  optic  is  the  afferent  nerve,  the  third  or  oculo- 
motor the  efferent  nerve,  and  the  centre  some  portion  of  the 
brain  lying  in  the  front  part  of  the  floor  of  the  aqueduct  at  the 
level  of  the  anterior  corpora  quadrigemina.  This  is  shewn  by 
the  following  facts.  When  the  optic  nerve  is  divided,  light 
falling  on  the  retina  of  that  eye  no  longer  causes  a  constriction 
of  the  pupil :  we  are  supposing  that  the  observations  are  con- 
fined to  one  eye.  When  the  third  nerve  is  divided,  stimulation 
of  the  retina  or  of  the  optic  nerve  no  longer  causes  constriction ; 
but  direct  stimulation  of  the  peripheral  portion  of  the  divided 
third  nerve  causes  constriction  of  the  pupil  which  may  be  ex- 
treme. If  the  region  of  the  brain  spoken  of  above  as  the  centre 
be  carefully  stimulated,  constriction  of  the  pupil  will  take  place 
even  when  no  light  falls  on  the  retina  or  after  the  optic  nerve 
has  been  divided.  After  destruction  of  the  same  region  stimu- 
lation of  the  retina  is  ineffectual  in  narrowing  the  pupil.  But 
if  the  centre  and  its  connections  with  the  optic  nerve  and  third 
nerve  be  left  intact  and  in  thoroughly  sound  condition,  con- 
striction of  the  pupil  will  occur  as  a  result  of  light  falling  on 
the  retina,  though  all  other  parts  of  the  brain  be  removed. 

It  might  be  imagined  that  this  cerebral  centre  acted  as  a 
tonic  centre,  whose  action  was  simply  increased,  not  originated, 
by  the  stimulation  of  the  retina ;  but  this  is  disproved  by  the 
fact  that  if  (still  dealing  with  one  eye)  the  optic  nerve  be 
divided  subsequent  section  of  the  third  nerve  produces  no  fur- 
ther dilation. 


Chap,  hi.]  SIGHT.  861 

When  the  rootlets  of  the  third  nerve  are  separately  divided 
as  they  leave  the  brain,  it  is  found  that  section  of  those  placed 
more  anteriorly  interferes  with  accommodation  and  constriction 
of  the  pupil,  while  section  of  the  hinder  ones  affects  the  ocular 
muscles.  Moreover  if  the  hind  part  of  the  floor  of  the  third 
ventricle  and  front  part  of  the  floor  of  the  aqueduct  be  carefully 
explored  (in  the  dog)  by  means  of  the  interrupted  current,  the 
following  movements  may  be  observed  in  succession  as  the  elec- 
trodes are  shifted  from  the  front  backwards;  first  movements 
of  accommodation,  next  constriction  of  the  pupil,  and  then  con- 
tractions of  the  ocular  muscles.  Now  in  this  region  lies  the 
elongated  nucleus  of  the  third  nerve ;  and  it  would  appear  that 
while  the  fibres  of  the  third  nerve  concerned  in  accommodation 
arise  from  the  extreme  front  of  the  nucleus,  those  which  act 
upon  the  pupil  start  from  a  succeeding  part,  the  remaining 
hinder  part  giving  rise  to  the  fibres  which  govern  the  ocular 
muscles.  It  seems  therefore  natural  to  regard  the  part  of  the 
nucleus  from  which  the  pupil-constricting  fibres  spring,  as  the 
centre  of  the  reflex  pupil-constricting  mechanism,  as  the  pupil- 
constrictor  centre. 

There  is  no  difliculty  as  to  the  connection  of  the  centre  with 
the  efferent  limb  of  the  reflex  chain.  The  pupil-constrictor 
fibres  pass  from  the  nucleus  to  the  trunk  of  the  third  nerve  of 
the  same  side,  and  so  by  the  short  root  to  the  ciliary  ganglion 
(Fig.  147  r.5.),  whence  they  reach  the  pupil  by  the  short  ciliary 
nerves  ;  section  of  the  short  ciliary  nerves  breaks  the  reflex  chain 
of  which  we  are  speaking,  and  stimulation  of  them  or  of  their 
peripheral  ends  causes  narrowing  of  the  pupil. 

But  considerable  difficulties  are  met  with  in  determining  the 
connection  of  the  optic  fibres,  the  afferent  limb  of  the  chain, 
with  the  centre.  We  should  perhaps  naturally  suppose  that  the 
afferent  nervous  impulses  which  affected  the  pupil  were  the 
same  as,  or  at  least  took  the  same  course  as  those  which  gave 
rise  to  visual  sensations ;  but  visual  sensations  may  be  inter- 
fered with  or  even  abolished,  leaving  the  pupil-constrictor 
mechanism  still  active,  and  on  the  other  hand  the  afferent  limb 
of  the  latter  may  fail,  without  any  impairment  of  visual  sensa- 
tions. The  afferent  impulses  by  means  of  wliicli  light  constricts 
the  pupil,  seem  therefore  to  take  a  path  of  their  own ;  but  the 
matter  is  not  as  3''et  fully  worked  out. 

It  is  desirable  to  remember  one  important  difference  as  to  the 
behaviour  of  the  pupil  which  obtains  between  man  and  some  of 
the  higher  mammals  on  the  one  hand,  and  the  lower  mammals 
as  well  as  other  vertebrates  on  the  other.  In  the  former,  the 
pupil-constricting  nervous  mechanisms  of  the  two  eyes  are  not 
completely  independent;  there  is  a  functional  communion  be- 
tween tlie  two  sides,  so  that  when  one  retina  is  stimulated  both 
pupils  contract,  and  indeed,  in  man,  as  a  rule,  contract  equally. 


862  MOVEMENTS   OE  THE  PUPIL.  [Book  hi. 

Hence,  when  a  change  in  the  pupil  of  one  eye  is  brought  about 
by  some  means  other  than  the  one  we  are  now  considering,  the 
pupil  of  the  other  eye  is  affected ;  when  for  instance  one  pupil 
is  dilated  with  atropin,  the  larger  amount  of  light  thus  admitted 
into  that  eye  causes  a  narrowing  of  the  pupil  of  the  other  eye, 
and  thus  increases  the  difference  between  the  pupils  of  the  two 
eyes.  In  the  lower  mammals  and  other  vertebrates,  the  mech- 
anisms in  question  are  independent,  stimulation  of  one  retina 
produces  no  effect  on  the  pupil  of  the  other  eye. 

It  is  by  means  of  this  reflex  mechanism  of  which  we  have 
just  given  a  sketch  that  the  changes  of  the  pupil  which  take 
place  in  actual  life  are  to  a  large  extent  carried  out ;  a  con- 
stricted pupil  indicates  in  the  majority  of  instances  an  activity 
of  the  reflex  mechanism,  and  a  dilated  pupil  the  absence  of  or 
diminution  of  that  activity.  In  the  normal,  healthy  organism 
the  activity  of  the  mechanism  is  in  the  first  instance  dependent 
on  the  amount  of  light  falling  on  the  retina ;  but  even  in  the 
normal  condition,  and  still  more  in  an  abnormal  condition  of  the 
organism,  other  influences  may  become  dominant.  The  activity 
of  the  centre  may  be  exalted  or  depressed  by  nervous  or  other 
actions ;  the  retina  or  optic  nerves  may  be  affected  by  the  same 
amount  of  light  to  a  degree  less  than  or  greater  than  the  normal, 
and  the  efferent  limb  of  the  chain  may  be  less  or  more  effective. 

§  540.  Besides,  however,  all  the  various  changes  which  may 
thus  be  induced  by  playing  upon  the  optic-oculo-motor  reflex 
mechanism,  other  agencies  are  able  to  act  on  the  pupil  quite  apart 
from  this  reflex  mechanism ;  some  of  these  act  through  the  second 
mechanism  of  which  we  spoke,  and  to  which  we  can  now  turn. 

If  the  cervical  sympathetic  in  the  neck  be  divided,  all  other 
influences  which  could  possibly  affect  the  pupil  being  avoided,  a. 
constriction  of  the  pupil  will  be  seen  to  take  place  ;  this  however 
is  at  times  (in  animals)  not  very  well  marked ;  but,  whether  it 
be  so  or  no,  if  the  peripheral  portion  of  the  nerve  (i.e.  the  upper 
portion  still  connected  with  the  head)  be  stimulated,  a  well-de- 
veloped dilation  is  the  result.  The  cervical  sympathetic  has,  it 
will  be  observed,  an  effect  on  the  pupil,  the  opposite  of  that  which 
it  exercises  on  the  blood  vessels  of  the  head  and  neck ;  when  it 
is  divided,  the  pupil  becomes  constricted  but  the  blood  vessels 
dilate,  and  when  it  is  stimulated  the  pupil  is  dilated  while  the 
blood  vessels  are  constricted.  This  pupil-dilating  influence  of 
the  cervical  sympathetic  may,  as  in  the  case  of  the  vaso-con- 
strictor  action  of  the  same  nerve,  be  traced  backwards  down  the 
neck  to  the  upper  thoracic  ganglion,  and  thence  to  the  spinal 
cord  alonor  the  rami  communicantes  and  anterior  roots  of  certain 
thoracic  nerves.  In  all  the  higher  animals  the  chief  channel  is 
the  second  thoracic  nerve  ;  in  the  cat,  dog,  monkey,  and  probably 
in  man  some  of  the  impulses  pass  by  the  first  thoracic  nerve  and 
a  few  by  the  third ;  in  the  rabbit  a  very  few  pass  by  the  first 


Chap,  hi.]  SIGHT.  863 

but  a  good  many  by  the  third  nerve.  In  the  frog  the  channel 
is  the  fourth  spinal  nerve.  Along  the  spinal  cord  the  dilating 
influence  may  be  further  traced  up  through  the  bulb  to  a  centre, 
which  appears  to  be  placed  in  the  floor  of  the  front  part  of  the 
aqueduct  not  far  from  and  apparently  lateral  to  the  centre  for 
constriction  of  the  pujjil.  Some  authors  have  supposed  that  a 
part  of  the  spinal  cord  in  the  lower  cervical  or  upper  thoracic 
region  above  the  origin  of  the  second  thoracic  nerve  has  a  special 
share  in  carrying  out  the  dilating  action  and  hence  have  called 
this  region  the  centrum  ciliospinale  inferius  ;  but  this  seems  very 
doubtful.  Since,  as  a  rule,  a  very  decided  amount  of  narrowing 
of  the  pupils  follows  upon  mere  section  of  the  cervical  sympa- 
thetic, we  may  infer  that,  unlike  the  case  of  the  puj^il-constrictor 
mechanism,  tonic  impulses  habitually  proceed  from  the  pupil- 
dilator  centre. 

We  may  trace  the  path  of  dilating  impulses  in  the  other 
direction  upwards  along  the  cervical  sympathetic,  not  to  the 
sympathetic  root  of  the  ciliary  ganglion  and  so  to  the  short 
ciliary  nerves,  but  to  fibres  which,  passing  over  the  Gasserian 
ganglion  apply  themselves  to  the  ophthalmic  division  of  the  fifth 
nerve,  and  from  thence  along  the  nasal  branch  to  the  long  ciliary 
nerves,  and  so  to  the  iris ;  while  the  short  ciliary  nerves  are  the 
channels  for  pupil-constrictor  impulses,  the  long  ciliary  nerves 
are  the  channels  of  pupil-dilator  impulses. 

§  541.  Buit  while  the  mode  of  action  of  the  pupil-constrictor 
impulses  seems  clear,  since  these  have  simply  to  throw  into  con- 
traction, or  increase  the  contraction  of,  the  fibres  of  the  sphincter, 
the  mode  of  action  of  the  pupil-dilator  impulses  is  a  matter  which 
has  been  and  still  is  disputed.  In  the  first  place,  considering 
how  vascular  the  iris  is,  it  does  not  seem  unreasonable  to  inter- 
pret some  of  the  variations  in  the  condition  of  the  pupil  as  the 
results  of  simple  vascular  turgescence  or  depletion  brought 
about  by  vaso-raotor  action  or  otherwise.  When  the  blood  vessels 
are  dilated  and  filled  they  will  cause  the  iris  to  encroach  on  the 
pupil,  making  the  latter  small  and  narrow,  and  conversely  a 
constricted  and  emptied  condition  of  the  blood  vessels  would 
lead  to  the  pupil  being  large  and  wide.  And  indeed  slight  oscil- 
lations of  the  pupil,  due  to  greater  or  less  fulness  of  the  blood 
vessels,  may  be  observed  synchronous  with  the  heart-beat,  and 
others  synchronous  with  the  respiratoiy  movements.  Hence, 
remembering  how  conspicuous  a  channel  for  vaso-constrictor 
impulses  is  the  cervical  sympathetic,  it  seems  very  natural  to 
suppose  that  the  widening  of  the  pupil  Avhich  follows  upon 
stimulation  of  the  cervical  sympathetic  is  sim^)!}'  the  result  of 
the  constriction  of  the  blood  vessels  of  the  iris,  and  conversely 
that  the  narrowing  of  the  pupil  observed  after  section  of  the 
cervical  sympathetic  is  simply  the  effect  of  a  greater  fulness  of 
the  iridic  blood  vessels  resulting  from  the  falling  away  of  the 


864  MOVEMENTS    OF   THE   PUPIL.  [Book  iir. 

usual  vaso-constrictor  impulses.  A  further  support  to  this  view 
is  afforded  by  the  observations  that  the  cervical  sympathetic 
does  contain  vaso-constrictor  fibres  for  the  blood  vessels  of  the 
iris,  and  that  these  leave  the  spinal  cord  by  the  same  paths  as 
the  pupil-dilator  impulses,  that  is  to  say  somewhat  higher  up 
than  the  vaso-constrictor  fibres  for  the  ear.  Nevertheless  it 
seems  clear  that  the  pupil-dilating  influence  exerted  by  the  sym- 
pathetic is  something  quite  different  from  its  vaso-constrictor 
influence  ;  for  the  dilating  effects  of  stimulating  the  sympathetic 
may  be  witnessed  in  a  bloodless  eye,  in  which  vaso-motor  changes 
could  not  produce  their  effect.  Further,  the  changes  in  the 
pupil  and  in  the  calibre  of  the  iridic  blood  vessels  are  not  coin- 
cident; Avhen  the  sympathetic  is  stimulated  the  widening  of 
the  pupil  begins  some  time  before  the  constriction  of  the  blood 
vessels  and  indeed  may,  with  a  brief  stimulation,  be  over  and 
past,  before  the  latter  has  reached  its  maximum.  Again,  in 
the  extreme  widening  of  the  pupil  which  as  we  shall  see  is 
brought  about  by  atropin,  and  which  seems  to  be  of  the  same 
nature  as  the  widening  caused  by  stimulation  of  the  sympathetic, 
the  blood  vessels  of  the  iris  need  not  be  in  the  least  constricted. 
Moreover,  it  is  stated  that  the  long  ciliary  nerves  whicli  act  as 
pupil-dilators  carry  no  vaso-constrictor  impulses ;  it  is  said  that 
stimulation  of  the  long  ciliary  nerves  while  it  widens  the  pupil 
produces  no  vaso-motor  effects,  and  after  the  division  of  the 
long  ciliary  nerves  stimulation  of  the  cervical  sympathetic, 
while  it  produces  vaso-constriction  in  the  eye  as  in  other  parts 
of  the  head  and  face,  gives  rise  to  no  widening  of  the  pupil. 
The  impulses  along  the  fibres  of  the  cervical  sympathetic,  which 
cause  widening  of  the  pupil,  must  act  in  some  manner  other 
than  by  giving  rise  to  vascular  changes. 

Did  there  exist  in  the  eyes  of  animals  an  arrangement  of 
muscular  fibres  disposed  radially  from  the  circumference  of  the 
iris  to  the  pupil  as  conspicuous  as  the  circular  muscle,  since 
such  a  muscular  arrangement  would  act  as  a  dilator,  there  would 
probably  be  a  general  agreement  that  the  widening  which  results 
from  stimulation  of  the  sympathetic  is  brought  about  by  con- 
traction of  these  dilator  muscular  fibres.  But  it  is  only  in  the 
case  of  one  or  two  kinds  of  animals  that  any  such  distinct  radial 
muscles  are  present  in  the  iris,  and  even  in  these  cases  the 
muscles  are  not  conspicuous.  In  all  other  animals  including 
man,  the  only  structure  in  the  iris  which  can  be  appealed  to  as 
a  dilator  muscle,  is  a  peculiar  nucleated  layer  at  the  hinder  sur- 
face, just  in  front  of  the  pigment  epithelium.  The  absence  of 
any  clear  indubitable  dilator  muscle  has  led  some  to  explain  the 
pupil-dilating  influence  of  the  sympathetic  as  due  to  the  impulses 
along  that  nerve  inhibiting  the  previously  existing  contraction 
of  the  sphincter.  These  argue  that  the  sphincter  ma}^  be  com- 
pared to  the  heart,  inasmuch  as  it  possesses  an  automatic  power 


Chap,  in.]         *  SIGHT.  865 

of  contraction,  manifested  however  not  in  a  rhythmic  but  in  a 
tonic  manner,  and  that  like  the  heart  its  action  may  be  either 
augmented  or  inhibited  by  nervous  impulses  ;  and  we  have  seen 
(§  347)  that  a  similar  view  may  be  taken  of  the  actions  of  the 
plain  muscular  fibres  of  the  alimentary  canal  and  of  the  bladder. 
According  to  this  view  the  sphincter  of  the  iris,  when  removed 
from  all  influences,  is  in  a  state  of  tonic  contraction,  pulling 
against  the  radial  strain  of  the  elastic  tissue  of  the  iris  and  so 
maintaining  a  pupil  of  a  certain  size.  Under  the  influence  of 
light  falling  on  the  retina,  impulses  reaching  the  sphincter  by 
the  short  ciliary  nerves  augment  its  contraction,  and  narrow  the 
pupil  in  proportion  to  their  intensity.  On  the  other  hand,  im- 
pulses reaching  the  sphincter  from  the  sympathetic  by  the  long 
ciliary  nerves  inhibit  the  activity  of  the  sphincter,  diminish  the 
force  with  which  it  is  pulling  against  the  elastic  tissue  of  the 
iris,  and  so  lead  to  a  widening  of  the  pupil,  thus  either  diminish- 
ing the  constriction  which  is  being  caused  by  the  action  of  light 
on  the  retina  or  otherwise,  or,  in  the  absence  of  all  external 
constricting  influences,  causing  the  pupil  to  become  wider  than 
it  naturally  would  when  removed  from  all  extrinsic  influences 
whatever.  In  support  of  such  a  view  it  is  pointed  out  that  the 
muscular  tissue  forming  the  sphincter  is  peculiar,  since  a  slip 
of  it  when  directly  stimulated  by  a  weak  interrupted  current 
elongates ;  in  this  respect  it  shews  a  further  analogy  ^ith  the 
heart  whose  activity  may  similarly  be  inhibited  by  the  inter- 
rupted current.  Again,  in  the  extirpated  eye,  or  even  in  the 
isolated  iris,  cold  dilates  and  warmth  constricts  the  pupil,  the  one 
relaxing,  and  the  other  increasing  the  contraction  of  the  sphincter. 
On  the  other  hand  the  following  facts  seem  to  shew  that  the 
dilation  which  we  are  discussing  cannot  be  simply  a  matter  of 
inhibition  but  must  be  due  to  the  radial  pull  of  some  or  other 
contractile  elements.  By  stimulating  one  ciliary  nerve  at  a 
time,  or  by  partial  direct  stimulation  of  the  iris  with  an  electric 
current,  the  two  electrodes  being  placed  close  together  on  the 
sclerotic  near  the  outer  margin  of  the  iris,  an  unequal  change, 
in  the  iris,  an  uneven  pupil  may  be  produced;  and  this,  since  it 
may  be  brought  about  even  while  the  sphincter  is  contracting, 
must  be  due  to  a  radial  pull.  Further,  a  mere  radial  slip  of 
the  iris,  a  slip  cut  out  by  two  radial  incisions  carried  from  the 
margin  of  the  pupil  towards  the  sclerotic,  the  sli[)  remaining 
connected  with  the  sclerotic,  may  be  made  by  stimulation  to 
shorten.  Lastly,  since  this  shortening  does  not  appear,  Avhen 
the  hind  surface  of  the  iris  has  been  previously  shai-ply  brushed, 
so  as  to  injure  it,  we  may  conclude  that  the  la3'er  spoken  of 
above,  peculiar  though  its  characters  be,  is  really  a  radially  dis- 
posed contractile  tissue,  and  b}'  its  contraction  dilates  the  pupil. 
Whatever  be  the  view  adopted  as  to  the  exact  mode  of  action 
of  the  sympathetic  there  remains  the  broad  fact  that  the  pupil  is 

65 


866  MOVEMENTS   OE   THE  PUPIL.-         [Book  m. 

luicler  tlie  dominion  of  two  antagonistic  mechanisms  :  one  a  con- 
stricting mechanism,  reflex  in  nature,  the  third  nerve  serving 
as  the  efferent,  and  the  optic  as  the  afferent  tract ;  the  otlier  a 
dilating  mechanism,  apparently  tonic  in  nature,  but  subject  to 
augmentation  from  various  causes,  and  of  this  the  cervical  sym- 
pathetic is  the  efferent  channel.  Hence,  when  the  third  or  optic 
nerve  is  divided,  not  only  do  constricting  impulses  cease  to  be 
manifest,  but  the  effect  of  their  absence  is  increased,  on  account 
of  the  tonic  dilating  influence  of  the  sympathetic  being  left  free 
to  work.  When,  on  the  other  hand,  the  sympathetic  is  divided, 
this  tonic  dilating  influence  falls  away,  and  constriction  results. 
When  the  optic  or  third  nerve  is  stimulated,  the  dilating  effect 
of  the  sympathetic  is  overcome,  and  constriction  results  ;  and 
when  the  sympathetic  is  stimulated,  any  constricting  influence 
of  the  third  nerve  which  may  be  present  is  overcome,  and  dila- 
tion ensues. 

The  former,  optic  oculo-motor  mechanism  is  the  instrument 
by  means  of  which  the  pupil  is  adapted  to  the  amount  of  light, 
the  latter,  sympathetic  mechanism  appears  to  be  employed  when 
other  influences  are  brought  to  bear  on  the  pupil.  Thus  the 
characteristic  pupil-dilating  effects  of  emotions  such  as  fear,  of 
the  painful  stimulation  of  sensory  nerves,  of  dyspnoea,  and  in 
part  of  some  drugs,  appear  to  be  carried  out  through  the  sym- 
pathetic mechanism. 

We  may  add  that  both  these  mechanisms  may  be  thro'wn 
into  action  by  stimulation  of  certain  parts  of  the  '  ocular '  area 
in  the  cerebral  cortex  ;  constriction  or  dilation  ixiiij  be  obtained 
by  stimulation  of  the  appropriate  spot.  That  the  dilation  which 
is  observed  is  brought  about  by  means  of  the  sympathetic  mech- 
anism, is  shewn  by  the  fact  that  it  fails  if  the  cervical  sympa- 
thetic be  previously  divided. 

§  542.  In  the  case  of  many  drugs,  however,  the  effect  pro- 
duced is  either  in  part  or  wholly  independent  of  both  these  ner- 
vous mechanisms.  A  small  quantity  of  atropin  introduced  into 
the  system,  or  even  directly  into  the  eje,  causes  a  dilation  of  the 
pupil  which  may  be  so  great  that  the  iris  is  reduced  to  a  mere 
rim,  while  physostigmin  (eserin)  similarly  introduced  into  the 
system  or  eye  produces  a  constriction  of  the  pupil  which  may  be 
so  great  that  the  pupil  is  narrowed  to  a  mere  pin's  point.  Since 
both  these  drugs  may  produce  their  full  effects  after  division  of 
the  optic  oculo-motor  and  the  sympathetic  nerves,  and  indeed 
may  produce  their  effects  in  an  extirpated  eyeball,  it  is  obvious 
that  those  effects  are  not  due  to  the  drugs  acting  on  the  central 
parts  of  the  above  mechanisms.  Their  action  is  a  local  one. 
They  do  not  act  by  means  of  the  ciliary  ganglion,  for  both  drugs 
continue  to  produce  their  effects  to  a  most  marked  degree  after 
the  ganglion  has  been  excised.  Nor  have  we  any  evidence 
that  their  action  is  dependent  on  any  other  local  nervous  mech- 


Chap,  hi.]  SIGHT.  8G7 

anism,  such  as  might  be  afforded  by  the  nerve  cells  lying  in  the 
choroid  or  even  in  the  iris.  They  appear  to  act  directly  on  the 
sphincter,  atropin  paralyzing  it  or  producing  relaxation,  and 
physostigmin  increasing  or  producing  contraction,  both  often 
of  an  extreme  character.  Whether  the  drugs  act  on  the  actual 
muscular  tissue  itself  or  on  the  endings  of  the  nerve  fibres  in  the 
muscular  tissue,  or  on  both  together,  and  how  far  their  effect 
is  due  to  changes  in  the  special  dilator  muscle,  are  questions 
which  we  need  not  discuss  here.  The  im^jortant  point  is  that 
the  action  of  both  these  drugs  is  a  local  one  ;  hence,  when  they 
have  produced  their  full  effects,  the  normal  nervous  mechanisms 
on  which  we  have  been  dwelling  are  of  little  or  no  use  ;  even  an 
abundance  of  light  leads  to  no  constriction  in  the  full  atropinized 
eye,  and  removal  of  light  produces  little  or  no  dilation  in  an  eye 
fully  under  the  influence  of  physostigmin. 

We  may  here  mention  the  fact  that  in  certain  animals  at  all 
events,  for  instance  the  eel,  light  falling  into  the  eye,  even  into 
an  extirpated  eye,  will  cause  constriction  of  the  pupil  ;  and  this 
seems  to  be  brought  about  by  means  of  some  nervous  connection 
between  the  retina  and  the  iris,  for  the  effect  ceases  when  the 
retina  is  destroyed.  But  this  peculiar  action  has  not  yet  been 
satisfactorily  explained. 

The  share  of  the  fifth  nerve  in  the  work  of  the  iris  seems  to 
be  chiefly  at  least  a  sensory  one  ;  the  iris  is  sensitive,  and  the 
sensory  impulses  which  are  generated  in  it  pass  from  it  along 
the  fibres  of  the  fifth  nerve. 

^^"e  may  sum  up  the  nervous  mechanism  of  the  pupil  then 
somewhat  as  follows.  The  salient  and  most  frequently  repeated 
event,  the  constriction  of  the  pupil  upon  exposure  to  light,  is  a 
reflex  act,  the  centre  of  which  is  placed  in  the  brain  ;  and  the 
correlative  widening  of  the  pupil  upon  diminution  of  light  is  due 
to  the  tonic  action  of  the  sympathetic  making  itself  felt  upon 
the  waning  of  its  antagonist.  The  dilating  effects  of  emotions, 
of  sensory  impressions,  especially  painful  ones,  and  of  dyspnoea 
appear  to  be  brought  about  by  an  increased  activity  of  the  dila- 
ting centre,  assisted  possibly  in  the  latter  instance  by  a  depres- 
sion of  the  constricting  centre.  The  constriction  of  tlie  pupil 
in  the  earlier  stages  of  the  action  of  alcohol  and  chloroform  and 
in  slumber  is  probably  due  to  an  increased  action  of  the  con- 
stricting centre,  but  the  narrow  pupil  caused  by  such  a  drug  as 
physostigmin  is  due,  chiefly  if  not  exclusively,  to  a  local  action. 
The  constricted  pupil  of  morphia  a})pears  to  be  due  partly  to 
central  and  partly  to  local  action.  The  dilating  effects  of  such 
a  drug  as  atropin  are  chiefly  if  not  exclusively  due  to  a  local 
action,  but  in  the  widened  pupil  of  the  later  stages  of  alcohol 
poisoning  and  of  some  other  drugs  we  can  probably  trace  the 
effects  of  an  exhaustion  of  the  constricting  centre,  assisted  pos- 
sibly by  an  increased  activity  of  the  dilating  centre. 


868  MOVEMENTS   OF   THE   PUPIL.  [Book  in. 

§543.  Tlie  nervous  mechanism  of  accommodation.  The  ciliary 
muscle  which  brings  about  accommodation  is  governed  in  this 
action  by  fibres  which  may  be  traced,  through  the  short  ciliary 
nerves  and  ciliary  ganglion,  along  the  third  nerve,  to  a  centre 
which  lies  (in  dogs)  in  the  extreme  front  of  the  floor  of  the 
aqueduct,  or  rather  perhaps  in  the  extreme  hind  part  of  the  floor 
of  the  third  ventricle,  and  which  is  especially  connected  with  the 
extreme  front  of  the  nucleus  of,  and  so  with  the  most  anterior 
bundles  of  the  root  of,  the  third  nerve.  As  we  have  already 
said  stimulation  of  this  centre,  or  of  the  third  nerve,  or  of  the 
short  ciliary  nerves,  leads  to  a  contraction  of  the  ciliary  muscle 
and  to  accommodation  for  near  objects. 

This  nervous  mechanism,  unlike  that  for  the  pupil,  is  under 
the  command  of  the  will,  though  the  will  needs  to  be  assisted 
by  visual  sensations  ;  it  is  moreover  only  brought  into  play  by 
the  direct  action  of  the  will ;  we  are  not  led  to  accommodate  by 
any  other  influence  than  the  desire  to  see  distinctly  near  or  far 
objects.  The  mechanism  may,  however,  be  affected  by  the  local 
action  of  drugs.  Such  drugs  as  atropin  and  physostigmin  which 
have  a  special  action  on  the  pupil,  also  affect  the  mechanism  of 
accommodation.  Atropin  paralyzes  it,  so  that  the  eye  remains 
adjusted  for  far  objects  ;  and  physostigmin  throws  the  eye  into  a 
condition  of  forced  accommodation  for  near  objects.  This  double 
action  has  been  explained  by  the  supposition  that,  by  acting  on 
the  muscular  fibres,  or  on  the  nerve  endings,  or  on  both,  atropin 
inhibits  the  contraction  of  or  paralyzes,  while  physostigmin 
throws  into  contraction  or  augments  the  contraction  of  the 
ciliary  muscle.  But  the  phenomena,  on  further  inquiry,  are 
found  to  be  more  complicated  than  they  appear  to  be  at  first 
sight.  There  are  also  other  facts  which  indicate  that  our  knowl- 
edge of  the  mechanism  of  accommodation  is  far  from  being  com- 
plete. For  instance,  so  far  as  we  know  at  present,  when  we  pass 
from  accommodation  for  a  near  object  to  that  for  a  far  object,  we 
simply  '  let  go '  the  previous  effort ;  we  cease  to  contract  the 
ciliary  muscle,  and  the  return  of  the  suspensory  ligament  and 
other  parts  is  simply  the  passive  result  of  the  cessation  of  the 
contraction  of  the  ciliary  muscle.  If,  now  the  change  from  near 
to  far  be  such  a  mere  passive  relaxation  of  a  previous  contrac- 
tion we  should,  judging  from  our  experience  of  ordinary  mus- 
cular contractions,  expect  the  time  taken  up  by  it  to  be  greater, 
or  at  least  not  less  than  the  time  taken  up  by  the  change  from 
far  to  near  ;  but  as  a  matter  of  fact  it  is  very  much  shorter, 
indeed  the  act  is  an  exceedingly  rapid  one. 

§  544.  There  remains  a  word  to  be  said  concerning  the  con- 
striction of  the  pupil  which  takes  place  when  the  eye  is  accom- 
modated for  near  objects,  and  when  the  pupil  is  turned  inwards 
(the  two  being  closely  allied,  since  the  two  eyes  converge  to  see 
near  objects),  and  the  return  to  the  more  dilated  condition  when 


Chap,  hi.]  SIGHT.  8G9 

the  eye  returns  to  rest  and  regains  the  condition  adaj^ted  for 
viewing  far  objects.  These  are  instances  of  what  are  called 
"associated  movements."  A  similar  instance  is  afforded  by  cer- 
tain cases  of  blindness  of  one  eye  due  to  atrophy  of  the  optic 
nerve ;  in  such  cases  the  pupil  of  the  blind  eye  may  be  wholly 
insensible  to  light,  and  yet  becomes  narrowed  when  the  subject 
looks  at  a  near  object  with  the  sound  eye.  In  so  doing  he  throws 
into  action  the  accommodation  mechanism,  and  with  that  the 
pupil-constricting  mechanism  6f  both  eyes.  Two  movements  are 
thus  spoken  of  as  "associated"  when  the  special  central  nervous 
mechanism  employed  in  carrying  out  the  one  act  is  so  connected 
by  nervous  ties  of  some  kind  or  other  with  that  employed  in  carry- 
ing out  the  other,  that  when  we  set  the  one  mechanism  in  action 
we  unintentionally  set  the  other  in  action  also.  In  this  constric- 
tion of  the  pupil  associated  with  accommodation  the  nervous  ties 
between  the  parts  of  the  central  nervous  system  concerned  in 
the  generation  of  the  will,  the  centre  for  accommodation,  and  the 
centre  for  the  constriction  of  the  pupil,  are  such  that  wdienever 
the  will  stirs  up  the  impulses  necessary  to  carry  out  accommoda- 
tion, it  at  the  same  time  stirs  up  corresponding  impulses  in 
the  pupil-constrictor  mechanism.  More  than  this  we  cannot  at 
present  say. 

We  can,  as  we  have  said,  accommodate  at  will ;  few  persons 
only  can  effect  the  necessary  change  in  the  eye  unless  they  direct 
their  attention  to  some  near  or  far  object,  as  the  case  may  be,  and 
thus  assist  their  will  by  visual  sensations.  By  practice,  however, 
the  aid  of  external  objects  may  be  dispensed  with  ;  and  it  is 
when  this  is  achieved  that  the  pupil  may  seem  to  be  made  to 
dilate  or  contract  at  pleasure,  accommodation  being  effected 
without  the  eye  being  directed  to  any  particular  object. 


SEC.  4.   IMPERFECTIOISrS   IK   THE   DIOPTRIC 
APPARATUS. 


§  545.  Imperfections  of  accommodation.  The  emmetropic  eye, 
in  which  the  principal  posterior  focus  lies  on  the  retina,  may,  as 
we  have  said,  be  taken  as  the  normal  eye.  The  myopic,  in  which 
the  principal  posterior  focus  lies  in  front,  and  the  hypermetropic 
eye,  in  which  it  lies  beyond  the  retina,  may  be  considered  as  im- 
perfect eyes,  though  the  former  possesses  an  advantage  over  the 
normal  eye  in  so  far  that  it  can  see  minute  objects  more  distinctly 
than  can  the  normal  eye,  since  these  can  be  brought  so  near  the 
eye  as  to  give  a  relatively  large  retinal  image  and  yet  remain 
within  the  limits  of  accommodation.  An  eye  may  be  myopic 
from  too  great  a  convexity  of  the  cornea,  or  of  the  anterior  surface 
of  the  lens,  or  from  permanent  spasm  of  the  accommodation- 
mechanism,  or  from  too  great  a  length  of  the  long  axis  of  the  eye- 
ball. The  last  appears  to  be  the  usual  cause.  Similarly,  the  cause 
of  hypermetropism  is  in  most  cases  the  possession  of  too  short  a 
bulb.  In  presbyopia  the  failure  or  loss  of  accommodation  may 
be  due  either  to  a  loss  of  elasticity  of  the  lens,  or  to  increasing 
weakness  of  the  ciliary  muscle,  or  to  the  parts  becoming  rigid ; 
the  first  appears  to  be  the  more  common  cause  ;  the  change,  which 
may  affect  not  only  normal  but  also  other  eyes,  generally  begins 
in  the  fifth  decade  of  life. 

These  several  defects  may  be  remedied  by  the  use  of  appro- 
priate lenses,  by  wearing  proper  spectacles.  The  myopic  eye 
needs  for  distant  objects  the  rays  of  which  fall  parallel  on  the 
cornea  (or  at  least  so  little  divergent  that  they  still  are  brought 
to  a  focus  in  front  of  the  retina)  a  concave  glass,  of  such  a  refrac- 
tive power,  of  such  a  focal  length,  as  to  give  to  parallel  rays, 
before  they  fall  on  the  cornea,  sufficient  divergence  to  enable  the 
dioptric  mechanism  of  the  eye  to  bring  them  to  a  focus  on,  and 
no  longer  in  front  of,  the  retina. 

The  hypermetropic  eye  needs  a  convex  glass  of  such  a  focal 
length  as  will  give  to  parallel  rays,  before  they  fall  on  the  cornea, 
sufficient  convergence  to  enable  the  eye  to  bring  them  to  a  focus 
on  the  retina. 

870 


Chap,  hi.]  SIGHT.  871 

The  presbyopic  eye  similarly  needs  a  convex  glass  the  focal 
length  of  which  must  de^^end  on  the  amount  of  accommodation 
still  possessed  by  the  eye ;  it  must  give  the  rays  just  so  much 
convergence  that  the  weakened  mechanism  is  able  to  bring  them 
to  a  focus  on  tlie  retina,  the  convexity  or  refractive  power  of  the 
glass  being  increased,  that  is  to  say  its  focal  length  diminished, 
as  the  loss  of  accommodation  increases. 

§  546.  Spherical  aberration.  In  a  spherical  lens  the  rays 
wliich  are  refracted  by  the  circumferential  parts  are  brought  to 
a  focus  sooner  than  those  which  pass  through  the  more  central 
parts  ;  in  consequence  the  rays  proceeding  from  a  luminous  point 
are  no  longer  brought  to  a  single  focus  at  one  point,  but  form  a 
number  of  foci  at  different  distances.  Hence,  when  rays  are 
allowed  to  fall  on  the  whole  of  the  lens,  the  image  formed  on  a 
screen  placed  in  the  focus  of  the  more  central  rays  is  blurred  by 
the  diffusion-circles  caused  by  the  circumferential  rays  Avhich  have 
been  brought  to  a  premature  focus.  In  an  ordinary  optical  instru- 
ment spherical  aberration  is  obviated  by  a  diaphragm  which  shuts 
off  the  more  circumferential  rays.  In  the  eye  the  iris  is  an 
adjustable  diaphragm ;  and  when  the  pupil  contracts  in  near 
vision  the  more  divergent  rays  proceeding  from  a  near  object, 
which  tend  to  fall  on  the  circumferential  parts  of  the  lens,  are  cut 
off.  The  lens  however,  as  we  have  seen,  is  not  uniform  in  struc- 
ture, and  the  refraction  which  it  exercises  does  not,  as  in  the  case 
of  the  ordinary  lens,  increase  regularly  and  progressively  from 
the  circumference  to  the  centre,  but  varies  most  irregularly ; 
hence  the  purpose  of  the  narrowing  of  the  pupil  cannot  be  simply 
to  obviate  spherical  aberration ;  and  indeed  the  other  optical 
imperfections  of  the  eye  are  so  great,  that  such  spherical  aber- 
rations as  are  actually  caused  by  the  lens  produce  no  obvious 
effect  on  vision. 

§  547.  Astigmatism.  We  have  hitherto  treated  the  eye  as  if 
its  dioptric  surfaces  were  all  jjarts  of  perfect  spherical  surfaces. 
In  reality  this  is  rarely  the  case,  either  with  the  lens  or  w.ith  the 
cornea.  Slight  deviations  from  the  spherical  shape  do  not  produce 
any  marked  effect,  but  there  is  one  deviation,  known  as  regular 
astigmatism,  which,  present  to  a  certain  extent  in  most  e3^es  and 
very  lai-gely  developed  in  some,  frequentl}-  leads  to  very  imper- 
fect vision.  This  defect  is  due  to  one  or  other  of  the  dioptric 
surfaces  being  not  spherical  but  more  convex  along  one  meridian 
than  another,  more  convex,  for  instance,  along  the  vertical  than 
along  the  horizontal  meridian.  When  this  is  the  case  with  the 
dioptric  surface  of  an  optical  system  the  rays  proceeding  from  a 
luminous  point  are  not  brought  to  a  single  focus  at  a  point,  but 
possess  two  linear  foci,  one  nearer  than  the  normal  focus  and 
corresponding  to  the  more  convex  surface,  the  other  farther  than 
the  normal  focus  and  corresponding  to  the  less  convex  surface. 
If  the  vertical  meridians  of  the  surface  be  more  convex  than  the 


872  ASTIGMATISM.  [Book  hi. 

horizontal,  then  the  nearer  linear  focus  will  he  horizontal  and  the 
farther  linear  focus  will  be  vertical  and  vice  versa.  (This  can  be 
shewn  much  more  effectually  on  a  model  than  in  a  diagram  in 
which  we  are  limited  to  two  dimensions.)  Now,  in  order  to  see 
distinctly  a  vertical  line,  it  is  much  more  important  that  the  rays 
which  diverge  from  the  line  in  a  series  of  horizontal  planes  should 
be  brought  to  a  focus  properly  than  those  which  diverge  in  the 
vertical  plane  of  the  line  itself ;  for  the  former  contribute  to  a  far 
greater  extent  than  do  the  latter  to  the  sum  of  rays  which  go  to 
form  the  retinal  image  of  and  so  to  excite  the  sensation  of  the 
line.  Similarly,  in  order  to  see  a  horizontal  line  distinctly  it  is 
much  more  important  that  the  rays  which  diverge  from  the  line 
in  a  series  of  vertical  planes  should  be  brought  to  a  focus  properly 
than  those  which  diverge  in  the  horizontal  plane  of  the  line  itself. 
When  a  horizontal  line  is  held  before  an  astigmatic  dioptric  sur- 
face, more  convex  in  the  vertical  meridian,  it  will  give  rise  to  a 
strong  image  of  a  horizontal  line  at  the  nearer  focus  where  the 
many  vertical  rays  diverging  from  the  line  are  brought  to  a  linear 
horizontal  focus,  and  to  a  weak  image  of  a  vertical  line  at  the 
farther  focus  where  the  fewer  horizontal  rays  are  brought  to  a 
linear  vertical  focus.  Similarly,  a  vertical  line  held  before  the 
same  surface  will  give  rise  to  a  strong  image  of  a  vertical  line 
at  the  farther  focus  where  the  horizontal  rays  diverging  from  the 
vertical  line  are  brought  to  a  linear  vertical  focus,  and  to  a  weak 
image  of  a  horizontal  line  at  the  nearer  focus.  But  in  the  case 
of  an  astigmatic  eye  trying  to  see  a  horizontal  or  vertical  line  or 
rod  such  as  a  horizontal  or  vertical  needle,  the  mind  will  neglect 
the  weaker  image,  and  take  the  stronger  image  as  the  only  image 
of  the  object.  Hence  if  a  horizontal  and  a  vertical  needle  be 
placed  at  the  same  distance  before  an  astigmatic  eye,  which  is 
more  convex  in  the  vertical  meridian,  that  eye  will  see  a  hor- 
izontal needle  distinctly  when  the  nearer,  and  a  vertical  needle 
distinctly  when  the  farther  of  the  two  foci  falls  on  the  retina ; 
it  will  require  a  different  accommodation  to  see  the  one  and  the 
other  distinctly.  If  the  astigmatism  is  such  that  the  horizontal 
meridian  be  the  more  convex,  the  vertical  needle  will  be  seen 
most  distinctly  at  the  nearer,  and  the  horizontal  at  the  farther 
focus.  In  both  forms  of  astigmatism  the  horizontal  and  the 
vertical  lines  which  go  to  make  up  the  features  of  the  surface 
of  an  object  will  fail  of  being  seen  distinctly  at  the  same  time ; 
and  the  vision  of  the  object  will  be  imperfect. 

Rays  of  light  proceeding  from  a  line,  which  is  neither  ver- 
tical nor  horizontal  but  oblique,  give  rise  in  an  astigmatic 
system  to  a  number  of  foci  arranged  in  so  complex  a  manner 
that  no  distinct  image  can  be  formed  on  the  retina ;  the  pres- 
ence of  these  lines  accordingly  adds  to  the  imperfection  of  the 
vision  of  any  object. 

Most  eyes  are  thus  more  or  less  '  regularly '  astigmatic,  and 


Chap,  hi.] 


SIGHT. 


873 


generally  with  a  greater  convexity  along  the  vertical  meridian. 
If  a  set  of  horizontal  or  vertical  lines  be  looked  at,  or  if  the 
near  point  of  accommodation  be  determined  by  Scheiner's  exper- 
iment, for  the  needle  placed  first  horizontally  and  then  verti- 
cally, the  distance  from  the  eye  at  which  tlie  horizontal  lines 
or  needle  are  seen  distinctly  Avill  be  found,  in  most  cases,  to 
be  appreciably  and  in  many  cases  considerably  shorter  than 
that  at  which  the  vertical  lines  or  needle  are  seen  with  equal 
distinctness.  In  other  words,  in  the  case  of  most  eyes,  a  ver- 
tical line  must  be  farther  from  the  eye  than  a  horizontal  one, 
if  both  are  to  be  seen  distinctly  at  the  same  time.  The  cause 
of  astigmatism  is,  in  the  great  majorit}^  of  cases,  the  unequal 
curvature  of  the  cornea  ;  but  sometimes  the  fault  lies  in  the 
lens,  as  was  the  case  with  the  philosopher  Young. 

Regular  astigmatism  may  be  remedied  by  the  use  of  cylin- 
drical glasses,  that  is  to  say,  glasses  which  are  convex  along 
one  meridian  but  plane  along  the  other.  Thus  the  ordinary 
astigmatic  eye  with  the  greater  curvature  along  the  vertical 
meridian  will  be  benefited  by  a  cylindrical  glass,  plane  in  the 
vertical  plane  but  possessing  such  convexity  in  the  horizontal 
plane  as  will  make  up  for  the  relatively  deficient  horizontal 
curvature  of  the  cornea. 

When  the  curvature  of  the  cornea  or  of  the  lens  differs  not 
in  two  meridians  only  but  in  several,  irregular  astigmatism  is 
the  result.  A  certain  amount  of  irregular  astigmatism,  due  to 
the  cornea  or  lens,  exists  in  most  eyes,  thus  causing  the  image 
of  a  bright  point,  such  as  a  star,  to  be  not  a  round  dot  but  a 
radiate  figure ;  in  some  cases  the  irregularity  is  so  great  that 
several  imperfect  images  are  formed  of  every  object. 

§  548.  Chromatie  aberration.  The  dilferent  ra3^s  of  the 
spectrum  are  of  different  refrangibility,  those  towards  the 
violet  end  of  the  spectrum  being  brought  to  a  focus  sooner 
than  those  near  the  red  end.     This  in  optical  instruments  is 


Y    f 


Fig.  148.     Diagram  illustrating  Chromatic  Aberration. 

hh  is  the  dioptric  surface,  hv  represents  the  bhie,  aiul  hr  the  red  raj's  ;  Fis  the  focal 
plane  of  the  blue,  li  of  the  red  ray. 


obviated  by  using  compound  lenses  made  up  of  various  kinds 
of  glass.  In  the  eye  we  have  no  evidence  that  the  lens  is  so 
constituted  as  to  correct  this  fault ;   still  the  total  dispersive 


874  CHEOMATIC   ABEEKA.TIOX.  [Book  in. 

power  of  the  instrument  is  so  small,  that  snch  amount  of  chro- 
matic aberration  as  does  exist  attracts  little  notice.  Never- 
theless some  slight  aberration  may  be  detected  by  careful 
observation.  When  the  spectrum  is  observed  at  some  dis- 
tance off  the  violet  end  will  not  be  seen  in  focus  at  the  same 
time  as  the  red  end.  Again,  if  a  luminous  point  be  looked 
at  through  a  narrow  orifice  covered  by  a  piece  of  violet  glass, 
which  while  shutting  out  the  yellow  and  green  allows  the  red 
and  blue  rays  to  pass  through,  there  will  be  seen  alternately 
an  image  having  a  blue  centre  with  a  red  fringe,  or  a  red  centre 
with  a  blue  fringe,  according  as  the  image  of  the  point  looked 
at  is  thrown  on  one  side  or  other  of  the  true  focus.  Thus 
supposing  /  (Fig.  148)  to  be  the  plane  of  the  mean  focus  of 
A,  the  violet  rays  will  be  brought  to  a  focus  in  the  plane  V, 
and  the  red  rays  in  the  plane  M.  If  the  rays  be  supposed  to 
fall  on  the  retina  between  F'and  /,  the  diverging  or  blue  rays 
will  form  a  centre  surrounded  by  the  still  converging  red  rays  ; 
whereas  if  the  rays  fall  on  the  retina  between  /  and  R,  the 
converging  red  rays  will  form  a  centre  with  the  still  diverging 
blue  rays  forming  a  fringe  round  them.  If  the  rays  fall  on 
the  retina  at  /,  the  two  kinds  of  rays  will  be  mixed  together ; 
as  will  be  seen  from  the  figure,  the  circumferential  still  con- 
verging red  ray  hr  as  it  cuts  the  plane  of  the  retina  is,  in 
ordinary  vision,  accompanied  by  the  diverging  violet  ray  hv, 
and  thus  by  a  sort  of  compensation,  we  see  together,  though 
not  in  absolutely  proper  focus,  even  the  raj's  which  differ  most 
in  refraction.  The  experiment  may  be  varied  by  blocking  up 
one  half  of  the  pupil  with  a  piece  of  card  and  using  the  uncov- 
ered half  of  the  pupil  to  look  through  a  piece  of  violet  glass  at 
a  wliite  surface  or  a  candle  flame.  The  red  strip  will  be  seen 
to  have  a  blue  edge. 

§  549.  Entoptic  phenomena.  The  various  media  of  the  eye 
are  not  uniformly  transparent ;  the  raj^s  of  light  in  passing 
through  them  undergo  local  absorption  and  refraction,  and 
thus  various  shadows  are  throAvn  on  the  retina,  of  which  we 
become  conscious  as  imperfections  in  the  field  of  vision,  espe- 
cially when  the  eye  is  directed  to  a  uniformly  illuminated  sur- 
face. These  are  spoken  of  as  entoptic  phenomena,  and  are 
very  varied,   many  forms  having  been  described. 

Tears  on  the  cornea,  or  temporary  unevenness  of  the  ante- 
rior surface  of  the  cornea  after  the  eyelid  has  been  pressed  on 
it,  may  give  rise  to  retinal  images  and  so  to  visual  sensations ; 
but  in  these  cases  the  cause  lies  outside  the  eye  and  the  result 
can  hardly  be  spoken  of  as  entoptic. 

Changes  in  the  margin  of  the  pupil  appear  in  the  shadow 
of  the  iris  which  bounds  the  field  of  vision.  If  we  look  at  a 
bright  object  or  luminous  surface  through  a  pin-hole  in  a  card 
placed  close  in  front  of  the  eye  (in  order  to  get  the  best  image 


Chap,  hi.]  SIGHT.  875 

on  the  retina,  the  pin-hole  should  occupy  the  position  of  the 
principal  anterior  focus),  the  dark  circle  which  bounds  the  field 
of  vision  is  the  image  caused  by  the  shadow  of  the  margin  not 
as  might  at  first  be  supposed  of  the  j^in-hole  but  of  the  iris. 
This  is  at  once  shewn  by  the  changes  which  it  can  be  made  to 
undergo,  while  the  pin-hole  remains  motionless,  by  alternately 
closing  and  opening  the  other  e3'e ;  the  field  of  vision  of  the 
eye  which  is  looking  through  the  pin-hole  may  be  observed  to 
contract  when  light  enters,  and  to  expand  when  the  light  is 
shut  off  from  the  other  eye ;  for  as  we  have  seen  (§  539)  light 
falling  on  one  retina  leads  to  consensual  narrowing  of  the  pupil 
of  the  other  eye.  Other  changes  or  irregularities  in  the  iris 
may  be  observed  by  this  method. 

Imperfections  in  the  lens  or  in  its  capsule  may  also  give  rise 
to  entoptic  images.  Not  unfrequently  a  radiate  figure  corre- 
sponding to  the  arrangement  of  the  fibres  of  the  lens  makes 
its  appearance. 

The  most  common  entoptic  phenomena  are  those  caused  by 
the  presence  of  floating  bodies  in  the  vitreous  humour,  the  so- 
called  muscce  voUtantes.  These  are  readily  seen  when  the  eye 
is  turned  towards  a  uniform  surface,  and  are  frequently  very 
troublesome  in  looking  through  a  microscope.  They  assume 
the  form  of  rows  and  groups  of  beads,  of  single  beads,  of 
streaks,  patches  and  granules,  and  may  be  recognized  by 
their  almost  continual  movement,  especially  when  the  head  or 
eye  is  moved  up  and  down.  When  an  attempt  is  made  to 
fix   the   vision   upon   them   they   immediately   float   away. 

Since  the  images  on  the  retina  are  in  these  cases  shadows 
and  since  the  strongest  shadows  are  cast  by  parallel  rays,  the 
images  are  best  seen  when  the  rays  of  light  giving  rise  to  the 
shadows  on  the  retina  traverse  the  vitreous  humour  in  parallel 
lines ;  hence  the  best  illumination  for  examining  the  phenom- 
ena is  one  placed  in  the  principal  anterior  focus,  the  rays 
diverging  from  which  fall  parallel  on  the  retina  (§  526,  Fig. 
140).  The  sharpness  of  the  images  is  also  increased  by  using 
a  small  but  bright  source  of  light,  as  by  looking  at  a  bright 
light  through  a  small  hole  in  a  screen. 

The  sensations  which  these  objects  in  the  vitreous  humour 
excite  by  means  of  the  retinal  images  to  which  they  give  rise 
do  not  tell  us  that  the  objects  are  in  the  vitreous  humour.  As 
we  shall  see  we  refer  all  affections  of  the  retina,  all  visual  sen- 
sations to  some  changes  in  the  external  world  ;  and  if  we  trusted 
to  our  sensations  alone  in  the  cases  of  these  entoptic  phenom- 
ena we  should  suppose  that  the  causes  existed  outside  ourselves. 
It  is  only  by  means  of  inferences  drawn  from  the  features  and 
behaviour  of  tlie  sensations  that  we  arrive  at  the  conclusion 
that  the  causes  lie  in  the  vitreous  humour. 

The  accompanying  diagram  (Fig.  149)  illustrates  how  the 


876  ENTOPTIC  PHENOMENA.  [Book  in. 

position  of  objects  in  the  eye  may  be  determined  by  the  move- 
ments of  their  shadows  on  the  retina.  It  represents  the  reduced 
diagrammatic  eye  seen  in  vertical  section,  with  n  the  nodal  point, 
p  the  principal  plane,  and  F  the  plane  of  the  principal  anterior 
focus.  1  represents  an  object  in  the  anterior  chamber,  2  another 
in  the  substance  of  the  lens,  and  3  a  third  in  the  vitreous  humcmr. 
If  a  bright  light  be  looked  at  through  a  pin-hole  in  a  card  placed 


Fig.  149.    Diagram  to  illustrate  Entoptical  Images. 

in  the  plane  of  the  principal  anterior  focus  F  so  that  the  hole 
is  at  the  principal  anterior  focus  a,  the  rays  of  light  may  be 
considered  as  diverging  from  a,  and  we  may  draw  them  as 
refracted  at  the  principal  plane  p,  and  then  passing  parallel 
through  the  vitreous  humour.  The  image  on  the  retina  in  this 
case  may  be  represented  by  a'.  The  field  of  vision,  limited  by 
the  shadow  of  the  iris,  will  be  circular  ;  the  shadow  of  2  will  lie 
close  to  the  optic  axis,  that  of  1  a  little  above  it,  and  that  of  3 
some  little  way  below  it.  It  will  of  course  be  remembered  that 
in  the  apparent  image  all  the  features  will  be  inverted  (§  707). 
If  now  the  card  be  moved  upward  so  that  the  light  emanates 
from  the  pin-hole  at  6,  and  the  paths  of  the  rays  of  light  be  drawn 
as  before,  the  image  resulting  will  be  that  shewn  at  h' .  The 
shadow  of  2  has  changed  very  little  in  position  ;  but  that  of  1 
has  moved  downwards,  while  that  of  3  has  moved  upwards  so 
that  all  three  lie  closer  together.  If,  on  the  contrary,  the  card 
be  moved  downward  to  c  the  result  will  be  that  shewn  in  c'  ;  the 
shadow  of  2,  as  before,  has  moved  but  little,  while  that  of  1  has 
moved  upward,  and  that  of  3  downwards,  so  that  the  three 
shadows  are  farther  apart. 

Thus  while  the  shadows  of  objects  in  the  anterior  chamber 
move  in  a  direction  the  opposite  to  that  of  the  movement  of  the 


Chap,  hi.]  SIGHT.  877 

source  of  an  illiimination  placed  in  the  plane  of  the  principal 
anterior  focus,  the  shadows  of  objects  in  the  vitreous  humour 
move  in  the  same  direction  as  the  source  of  illumination.  Hence, 
by  observing  the  direction  of  the  movement  of  an  entoptic  image 
resulting  from  the  movement  of  the  illumination,  the  position  in 
the  eye  of  the  object  giving  rise  to  the  image  may  be  determined, 
regard  of  course  always  being  had  to  the  so-called  mental  inver- 
sion of  the  retinal  image.  Stated  more  strictly  the  rule  would 
run  thus.  The  shadows  of  objects  in  front  of  the  nodal  jDoint 
(§  527)  in  the  lens  move  in  a  direction  contrary  to  and  those  of 
objects  behind  the  nodal  point  in  the  same  direction  as  the  move- 
ment of  the  illumination  ;  moreover  the  more  distant  the  object 
from  tlie  nodal  point  the  greater  the  movement  of  the  shadow 
caused  by  the  same  movement  of  the  illumination. 

In  this  connection  we  may  refer  to  one  or  two  matters  which 
however  cannot  be  called  dioptric  imperfections. 

If  a  white  sheet  or  Avliite  cloud  be  looked  at  in  daylight 
through  a  Nicol's  prism,  ,a  somewhat  bright  double  cone  or  double 
tuft,  with  the  apices  touching,  of  a  faint  blue  colour,  is  seen  in 
the  centre  of  the  held  of  vision,  crossed  by  a  similar  double  cone 
of  a  somewhat  yellow  darker  colour.  These  are  spoken  of  as 
Haidinger's  brushes  ;  they  rotate  as  the  prism  is  rotated,  and  are 
supposed  to  be  due  to  the  unequal  absorption  of  the  polarized 
light  in  that  part  of  the  retina  which  we  shall  study  presently 
as  "the  yellow  spot."  The  prism  must  be  frequently  rotated, 
since  when  the  prism  remains  at  rest  the  phenomena  fade.  We 
may  here  remark  that  the  media  of  the  eye  are  fluorescent  :  a 
condition  which  favours  the  perception  of  the  ultraviolet  rays. 
There  are  other  entoptic  phenomena  due  to  features  of  the  retina, 
of  which  we  shall  speak  in  treating  of  the  development  of  visual 
impulses. 

Lastly,  returning  to  dioptric  imperfections,  we  may  add  that 
the  optical  arrangements  are  also  to  a  certain  extent  imperfect  in- 
asmuch as  the  dioptric  surfaces  are,  according  to  most  observers, 
not  truly  centred  on  the  o^jtic  axis. 


SEC.    5.     OX    SOME    GENERAL    EEATUEES    OF    VISUAL 

SENSATIONS. 

§  550.  When  light  falls  upon  the  retina  it  produces,  under 
favourable  circumstances,  a  change  in  our  consciousness  which 
we  call  a  sensation  of  light,  a  visual  sensation.  The  immediate 
effect  of  the  light  is  to  stir  up  certain  changes  in  the  retina ; 
these  retinal  changes  give  rise  in  turn  to  nervous  changes  in  the 
optic  fibres ;  these  latter,  which  we  have  called  'visual  impulses,' 
start  in  the  brain  a  further  series  of  events,  one  effect  of  which 
is  a  change  in  our  consciousness ;  and  it  is  this  change  in  our 
consciousness  which  Ave  call  a  sensation.  We  may,  and  often 
do,  speak  of  light  as  a  'stimulus'  to  the  retina,  the  result  of  the 
stimulation  being  visual  impulses ;  but  we  may  also  speak  of 
light  as  a  stimulus  to  the  whole  visual  apparatus,  central  as 
well  as  retinal,  regarding  the  sensation  as  if  it  were  the  direct 
and  immediate,  instead  of  being  the  indirect  and  ultimate  effect 
of  the  stimulus.  We  may,  by  observing  certain  general  features 
of  visual  sensations,  such  as  can  be  ascertained  by  means  of  a 
direct  and  simple  appeal  to  our  own  consciousness,  study  the 
relations  which  obtain  between  the  characters  of  the  stimulus 
on  the  one  hand  and  those  of  the  sensation  on  the  other.  There 
are  certain  advantages  indeed  in  doing  this  before  we  proceed 
to  discuss  the  nature  of  the  changes  in  the  retina  through  which 
rays  of  light  give  rise  to  visual  impulses  in  the  optic  fibres.  But 
in  taking  this  course  we  must  bear  in  mind  how  complex  is  the 
whole  process  through  which  the  stimulus  gives  rise  to  the 
sensation.  We  must  remember  that,  as  we  have  already  said, 
though  some  of  the  characters  of  a  visual  sensation  are  impressed 
upon  it  while  it  is  as  yet  immature,  as  yet  in  the  stage  of  visual 
impulses,  others  are  introduced  later  on  in  the  course  of  the 
cerebral  changes.  Since  we  are  now  dealing  for  the  first  time 
with  sensory  impulses  studied  in  this  Avay,  we  may  venture  to 
enter  into  some  details,  for  the  deductions  which  may  be  drawn 
concerning  visual  sensations  will  apply  to  a  large  extent  to  other 
sensations. 

To  simplify  matters  we  will  in  the  first  instance  suppose  that 
the  luminous  object,  the  object  emitting  or  reflecting  light,  is  so 

878 


Chap,  hi.]  SIGHT.  879 

small  that  the  image  of  it  on  the  retina  may  be  considered  as  a 
mere  point ;  we  may  speak  of  it  as  a  luminous  point.  If  for  the 
sake  of  illustration  or  otherwise  we  have  to  consider  a  larger 
luminous  object,  we  shall  do  so  without  regard  to  the  size  of  tlie 
image  on  the  retina  unless  this  is  specially  mentioned. 

We  may  begin  with  the  preliminary  remark  that  in  dealing 
with  light  as  a  stimulus  of  visual  sensations,  we  have  to  consider 
not  only  the  intensity  of  the  stimulus  but  also  its  duration.  A 
luminous  point  may  appear  dim  and  feeble,  that  is  to  say,  the 
waves  of  light  from  it  have  a  small  amplitude  and  so  bring  little 
energy  to  bear  on  the  retina,  or  it  may  appear  bright  and  strong, 
that  is  to  say,  the  waves  of  light  have  a  large  amplitude  and  so 
bring  much  energy  to  bear  on  the  retina.  Whether  dim  or 
bright,  the  luminous  point  may  act  on  the  retina  for  a  longer 
or  a  shorter  time  ;  and,  moreover,  during  its  action  may  remain 
steady,  not  varying  in  intensity,  or  may  A'ar}'^  in  intensity  and 
become  unsteady  or  flickering.  In  estimating  the  total  visual 
effect  of  a  luminous  point,  we  have  to  consider  both  these  feat- 
ures, its  intensity  or  brightness  and  its  duration. 

Neglecting  for  the  present  the  feature  of  duration,  we  find 
that  a  luminous  point  must  possess  a  certain  amount  of  bright- 
ness in  order  to  produce  any  conscious  sensation  at  all,  in  order 
to  be  visible.  If  the  waves  of  light  fall  on  the  retina  with  less 
than  a  certain  amplitude,  if  their  energy  sinks  below  a  certain 
minimum,  they  fail  to  give  rise  to  visual  impulses,  or  at  least  to 
such  as  can  affect  consciousness ;  for  we  may  suppose  that  visual 
impulses  might  be  generated  and  yet  be  so  feeble  as  not  to  pro- 
duce in  the  cerebral  centre  changes  sufficiently  great  to  affect 
consciousness.  It  will  be  understood,  of  course,  that  the  exact 
degree  of  brightness  at  which  the  luminous  point  becomes  visible 
depends  on  the  greater  or  less  irritability,  on  the  sensitiveness, 
of  the  retina.  The  same  amount  of  luminous  energy  which,  fall- 
ing on  one  retina  or  on  one  part  of  a  retina,  produces  a  distinct 
sensation,  may,  falling  on  a  less  sensitive  retina  or  on  a  less  sen- 
sitiA'e  part  of  the  same  retina,  produce  no  sensation  whatever. 

From  the  minimum  onwards  the  intensity  of  the  sensation 
increases  with  the  luminous  intensity  of  the  object ;  a  Avax 
candle  appears  brighter  than  a  rushlight,  and  the  sun  brighter 
than  any  candle  ;  we  are  dealing  now  with  the  intensity  of  the 
light  quite  apart  from  the  size  of  the  luminous  object.  The 
ratio,  however,  of  the  sensation  to  the  stimulus  is  not  a  simple 
one.  If  the  luminosity  of  an  object  be  gradually  increased  from 
a  very  feeble  stage  to  a  very  bright  one,  it  will  be  found  that, 
though  the  corresponding  sensations  likewise  gradually  increase, 
the  increments  of  the  sensations  due  to  the  increments  of  the 
luminosity  gradually  diminish,  and  at  last  an  increase  of  the  lu- 
minosity produces  no  appreciable  increase  of  sensation  ;  a  light, 
when  it  reaches  a  certain  brightness,  appears  so  bright  that  if  it 


880  VISUAL   SENSATIONS.  [Book  iii. 

becomes  brighter  we  do  not  recognize  that  it  is  brighter.  Hence 
it  is  much  easier  to  distinguish  a  slight  difference  of  brightness 
between  two  feeble  lights  than  the  same  difference  between  two 
bright  lights ;  we  can  easily  tell  the  difference  between  a  rush- 
light and  a  wax  candle  ;  but  two  suns,  or  even  two  bright  lamps, 
one  of  which  compared  with  the  other  gave  out  just  that  addi- 
tional amount  of  light,  just  that  additional  quantity  of  luminous 
energy,  which  a  wax  candle  gives  out  in  addition  to  that  given 
out  by  a  rushlight,  would  appear  to  us  to  have  exactly  the  same 
brightness.  In  a  darkened  room  an  object  placed  before  a  candle 
will  throw  what  we  consider  a  deep  shadow  on  a  sheet  of  paper 
or  any  white  surface.  If,  however,  sunlight  be  allowed  to  fall 
on  the  paper  at  the  same  time  from  the  opposite  side,  the  shadow 
is  no  longer  visible.  The  difference  between  the  total  light 
reflected  from  that  part  of  the  paper  where  the  shadow  was, 
and  which  is  illuminated  by  the  sun  alone,  and  that  reflected 
from  the  rest  of  the  paper  which  is  illuminated  by  the  candle 
as  well  as  by  the  sun,  remains  the  same  ;  yet  we  can  no  longer 
appreciate  that  difference  because  the  whole  surface  has  become 
so  bright. 

On  the  other  hand,  when  we  carefully  compare  the  visual 
sensations  excited  by  measurable  differences  of  luminosity,  Ave 
come  upon  the  following  remarkable  result.  If  we  place  two 
candles  so  as  to  throw  two  shadows  of  some  object  on  a  white 
surface,  the  shadow  caused  by  each  light  will  be  illuminated 
by  the  other  light,  and  the  rest  of  the  surface  will  be  illuminated 
by  both  lights.  If  now  we  move  one  candle  away  we  shall  reach 
a  point  at  which  the  shadow  caused  by  it  ceases  to  be  visible, 
that  is  to  say,  we  fail  at  this  point  to  appreciate  the  difference 
between  the  surface  illuminated  by  the  near  light  alone  and  that 
illuminated  by  the  near  light  and  the  far  light  together.  If 
now,  having  noted  the  distance  to  which  the  candle  had  to  be 
moved,  we  repeat  the  same  experiment  with  two  bright  lamps, 
moving  one  lamp  away  until  the  shadow  it  casts  ceases  to  be 
visible,  we  shall  find  that  the  lamp  has  to  be  moved  just  as  far 
as  the  candle  ;  that  is  to  say,  the  least  difference  between  the 
illumination  of  the  bright  lamps  which  we  can  appreciate  is  the 
same  as  in  the  case  of  the  dimmer  candles.  Many  similar 
examples  might  be  given  shewing  a  similar  result  ;  in  fact,  it  is 
found  by  careful  observation  that,  within  tolerably  wide  limits, 
the  smallest  difference  of  light  which  we  can  appreciate  by  visual 
sensations  is  a  constant  fraction  (about  ^^th)  of  the  total  lumi- 
nosity employed.  As  we  shall  see,  the  same  relation  holds  good 
with  regard  to  the  other  senses  as  well.  It  may  be  put  in  a 
general  form,  as  a  law  of  sensation,  often  called  Weber's  law, 
somewhat  as  follows  :  The  smallest  change  in  the  magnitude  of 
a  stimulus  which  we  can  appreciate  through  a  change  in  our 
sensation  always  bears  the  same  proportion  to  the  whole  magni- 


Chap,  hi.]  SIGHT.  881 

tude  of  the  stimulus.  It  should  however  be  stated  that  this  law 
holds  good  within  certain  limits  oiily  ;  it  fails  when  the  stimulus 
is  either  above  or  below  a  certain  range  of  intensity. 

Hence,  if  we  take  the  smallest  difference  which  we  can 
appreciate  in  a  stimulus  as  a  measure  of  our  sensibility  to 
differences  in  the  stimulus,  we  may  say  that  on  the  one  hand  in 
respect  to  absolute  differences,  such  as  that  between  two  lamps 
and  that  between  two  rushlights,  our  sensibility  varies  inversely 
as  the  magnitude  of  the  stimulus  ;  Ave  are  more  sensible  of  the 
same  absolute  difference  when  that  is  a  difference  between  two 
rushlights  than  when  it  is  a  difference  between  two  lamps.  On 
the  other  hand,  in  regard  to  relative  differences,  our  sensibility 
is  independent  of  the  magnitude  of  the  stimulus  ;  the  difference 
of  Avhich  we  are  sensible  in  the  case  of  the  lamp  bears  the  same 
proportion  to  the  whole  luminosity  of  the  lamp  as  the  difference 
of  which  we  are  sensible  in  the  case  of  the  rushlight  bears  to  the 
whole  luminosity  of  the  rushlight. 

§  551.  Returning  now  to  consider  the  duration  of  the  sti- 
mulus, as  distinguished  from  its  intensity,  we  find  that  a  stimulus 
of  extremely  brief  duration  may  give  rise  to  a  distinct  sensation  ; 
the  flash  of  an  electric  spark,  for  instance,  is  readily  visible. 
There  is  probably  a  limit  in  respect  to  duration  within  which 
the  stimulus  fails  to  produce  a  sensation  ;  it  is  probable,  for 
instance,  that  a  certain  number  of  undulations  in  succession  must 
fall  on  the  retina  in  order  to  give  rise  to  a  visual  sensation,  and 
that  a  single  undulation  of  the  ether  falling  on  the  retina,  if  such 
a  thing  were  possible,  would  produce  no  visual  effect  ;  but  the 
exact  limit  will  depend  on  the  intensity  and  nature  of  the  light, 
and  we  need  not  enter  upon  these  details  here. 

It  is  of  more  importance  to  note  that  the  visual  sensation 
caused  by  a  very  brief  stimulus  lasts  a  considerable  time  ;  the 
sensation  has  a  duration  much  greater  than  that  of  the  stimulus. 
The  sensation  of  a  flash  of  light,  for  instance,  lasts  for  a  much 
longer  time  than  that  during  which  luminous  vibrations  are 
falling  on  the  retina.  In  this  respect,  we  may  roughly  compare 
a  visual  sensation  to  a  simple  muscular  contraction  caused  by 
such  a  stimulus  as  a  single  induction  shock.  We  might  indeed 
construct  a  "  visual  sensation  curve  "  very  much  after  tlie  fashion 
of  a  "muscle  curve."  We  should  find  that  after  a  very  obvious 
latent  i)eriod  the  sensation  began,  rose  to  a  maximum  and  then 
declined.  This  latent  period  forms  an  important  part  of  the 
"reaction  period,"  on  which  we  dwelt  in  a  former  part  of  this 
work  (§  515).  As  we  have  said,  in  all  the  sensations  with  which 
we  are  now  dealing,  we  have  to  distinguish  at  least  two  parts, 
the  peripheral  part,  the  events  taking  place  in  the  retina,  and 
the  central  part,  the  events  taking  place  in  the  brain,  the  two 
being  united  by  means  of  the  visual  impulses  passing  along  the 
optic  nerve.     And  within  the  latent  period  are  comprised  the 

56 


882  VISUAL   SENSATIONS.  [Book  in. 

changes  in  the  retina  which  start  the  visual  impulses,  the  passage 
of  these  impulses  along  the  optic  fibres,  and  the  changes  in  the 
brain  antecedent  to  consciousness  beginning  to  be  affected  ;  of 
these  the  retinal  changes  probably  take  up  the  most  time,  but 
into  this  point  we  cannot  enter  now. 

The  length  of  the  sensation,  as  compared  with  that  of  the 
stimulus,  is  illustrated  by  viewing  objects  in  motion  under  a 
very  brief  illumination,  such  as  that  of  a  single  electric  spark. 
In  such  a  case  the  light  reflected  from  the  object  is  sufficient  to 
generate  a  distinct  sensation,  to  give  rise  to  a  distinct  image  of 
the  object,  but  it  ceases  before  the  object  can  make  any  appreci- 
able change  in  its  position,  and  the  image  accordingly  is  that 
of  a  motionless  object.  When  a  moving  body  is  illuminated  by 
several  rapid  flashes  in  succession,  several  distinct  images  cor- 
responding to  the  positions  of  the  body  during  the  several  flashes 
are  generated ;  this,  as  we  shall  see  presently,  is  because  the 
images  of  the  body  corresponding  to  the  several  flashes  fall  on 
different  parts  of  the  retina. 

The  duration  of  the  stimulus  remaining  the  same,  the  char- 
acters of  the  sensation  and  the  form  of  the  sensation  curve  will, 
in  accordance  with  what  was  stated  above,  vary  with  the  inten- 
sity of  the  stimulus ;  a  bright  flash  will  produce  a  sensation 
greater  and  of  longer  duration  than  that  produced  by  a  feeble 
flash,  the  curve  will  be  higher  and  more  extended.  We  have 
reason  to  think,  too,  that  the  form  of  the  curve  is  dependent  on 
the  intensity  of  the  stimulus  in  such  a  way  that  the  decline  from 
the  maximum  begins  earlier  and  at  all  events  in  the  first  part  of 
its  course,  is  more  rapid  with  the  stronger  than  with  the  feebler 
stimulus. 

When  the  stimulus  is  not  a  mere  flash,  but  is  of  some  dura- 
tion leading  to  a  prolonged  sensation,  we  can  readily  distin- 
guish between  that  part  of  the  sensation  which  is  going  on 
while  the  light  is  still  falling  into  the  eye,  and  that  part  which 
goes  on  after  the  light  has  ceased  to  fall  on  the  retina ;  this 
latter  part  is  often  spoken  of  as  the  after-image.  When  the 
light  is  very  bright  this  "after-image"  frequently  becomes  very 
prominent  even  after  a  very  brief  exposure.  Thus,  if  we  look, 
even  for  a  moment  only,  at  the  sun,  and  then  immediately  shut 
the  eye,  an  intense  visual  sensation,  a  bright  visual  image  of  the 
sun,  remains  for  some  considerable  time.  After-images,  espe- 
cially as  they  are  vanishing,  are  marked  by  certain  features, 
which  we  shall  study  later  on,  and  which,  as  we  shall  see,  are 
related  to  the  fatigue  or  exhaustion  of  the  retina ;  for  the  retina, 
or  rather  the  whole  visual  apparatus,  is,  we  need  hardly  say, 
subject  to  fatigue. 

Careful  observation  moreover  has  shewn  that  the  visual  sen- 
sation curve  is  not  a  smooth  one  but  broken  in  a  remarkable 
manner.     When  the  retina  is  momentarily  stimulated  with  a 


Chap,  hi.]  SIGHT.  883 

bright  light,  the  sensation  almost  immediately  that  it  has  begun 
suddenly  diminishes  or  even  disappears  and  then  is  immediately 
again  re-established. 

§  552.  From  the  prolonged  duration  of  visual  sensations  it 
results  that  when  two  or  more  stimuli,  such  as  two  or  more 
flashes  of  light,  follow  each  other  at  a  sufficiently  short  interval, 
the  two  sensations  or  the  several  successive  sensations  are  fused 
into  one  more  or  less  uniform  sensation.  Thus  a  luminous  point 
moving  rapidly  round  in  a  circle  gives  rise  to  the  sensation  of  a 
continuous  circle  of  light.  We  might,  in  a  very  general  manner, 
compare  this  with  the  way  in  which  a  series  of  sim23le  muscular 
contractions  resulting  from  rapidly  repeated  induction  shocks 
are  fused  into  a  fairly  uniform  tetanus.  When  the  stimuli 
succeed  each  other  so  rapidly  that  each  sensation  begins  before 
its  predecessor  has  had  time  to  appreciably  decline,  the  total 
sensation  is  as  completely  uniform  as  if  the  stimulus  were  con- 
stant. If  the  interval  between  each  two  stimuli  be  just  so  long 
that  each  sensation  in  turn  has  had  time  to  distinctly  diminish 
before  the  next  sensation  begins,  the  result  is  a  "  flickering  "  ; 
and  there  are  of  course  many  degrees  of  flickering  between  a 
perfectly  steady  and  an  obviously  intermittent  light.  The  inter- 
val at  which  fusion  takes  place,  that  is,  the  interval  between 
successive  stimuli  which  must  be  exceeded  in  order  that  suc- 
cessive distinct  sensations  may  be  produced,  varies  according  to 
the  intensity  of  the  light,  being  shorter  with  the  stronger  light; 
with  a  faint  light  it  is  about  ^j  sec,  with  a  strong  light  -^  or 
-^-Q  sec.  This  may  be  shewn  by  rotating  rapidl}-  before  the  eye 
a  disc  arranged  Avith  alternate  black  and  Avhite  sectors  of  equal 
width.  With  a  faint  illumination,  the  flickering  indicative  of  the 
successive  sensations  from  the  white  sectors  not  being  com- 
pletely fused,  ceases  when  the  rotation  becomes  so  rapid  that 
each  pair  of  black  and  white  sectors  takes  only  -^^  sec.  in  pass- 
ing before  the  eye.  When  a  brighter  illumination  is  used  the 
rapidity  must  be  increased  before  the  flickering  disappears  ;  this 
is  owing  to  the  decline  of  the  stronger  sensation,  as  stated  above, 
beginning  earlier  and  being  more  rapid  than  that  of  the  weaker 
sensation. 

§  553.  When  a  luminous  point  excites  the  retina,  we  recog- 
nize in  the  sensation  not  only  the  features  of  intensity,  duration 
and  constancy  or  steadiness,  but  also  a  character  which  is  de- 
pendent on  th  position  in  the  retina  of  the  image  of  the  lumi- 
nous point.  We  recognize  the  sensation  caused  by  a  luminous 
point  whose  image  falls  on  the  temporal  side  of  the  retina,  as 
different  and  distinct  from  the  sensation  caused  by  a  luminous 
point  whose  image  falls  on  the  nasal  side  of  the  retina,  and  so 
with  other  positions  of  the  images  ;  indeed,  as  we  shall  see  pres- 
ently, we  are  able  to  distinguish,  to  recognize  as  dift'erent  and 
distinct,  two  sensations  excited  by  two   luminous  points,  the 


884  FUSION   OF  VISUAL   SENSATIONS.     [Book  hi. 

images  of  wliicli  lie  very  close  indeed  to  each  other  on  the 
retina.  We  distinguish  the  sensations,  however,  not  by  refer- 
ence to  the  parts  of  the  retina  affected,  but  by  reference  to  the 
relations  in  space  of  the  luminous  points  giving  rise  to  the  sen- 
sations. Since  this  is  a  matter  of  some  importance  we  may  treat 
of  it  in  some  detail. 

In  the  vast  majority  of  cases  the  changes  in  the  retina  which 
give  rise  to  visual  impulses,  and  so  to  visual  sensations,  are 
brought  about  by  light  falling  on  the  retina.  But  the  retina 
may  be  stimulated  by  other  agencies  than  that  of  light.  When 
this  is  the  case  the  changes  in  the  retina,  however  produced,  if 
they  are  able  to  affect  consciousness  at  all,  give  rise  to  visual 
sensations,  and  to  visual  sensations  only.  A  mechanical  stimu- 
lation of  the  retina,  as  when  a  blow  is  struck  on  the  eye,  pro- 
duces a  visual  sensation,  a  sensation  of  light  ;  pressure  exerted 
on  the  eyeball  so  as  to  produce  pressure  on  the  retina  gives  rise 
to  visual  sensations  in  the  form  of  rings  of  light,  of  coloured 
light,  known  as  '  phosphenes  ' ;  and  when  the  retina  is  subjected 
in  various  ways  to  stress  or  strain,  as  by  rapid  accommodation,  or 
by  rapid  movement  of  the  eyeball  from  side  to  side,  there  often 
result  visual  sensations  in  the  form  of  light  of  some  kind  or 
other,  best  appreciated  when  objective  light  is  cut  off  from  the 
retina  and  when  the  retina  has  by  long  repose  been  rendered 
unusually  sensitive.  Electrical  stimulation  also  gives  rise  to 
visual  sensations  ;  not  only  is  the  induced  current,  or  the  break 
and  make  of  a  constant  current,  thus  able  to  excite  the  retina, 
but  during  the  whole  time  of  the  passage  of  a  constant  current 
of  adequate  strength,  even  though  it  remain  of  uniform  inten- 
sity, visual  impulses,  and  thus  visual  sensations,  are  being  gener- 
ated ;  in  this  respect  the  retina  resembles  sensory  and  differs 
from  motor  nerves.  It  is  stated  that  when  the  current  is 
directed  from  the  layer  of  optic  fibres  to  the  layer  of  rods  and 
cones,  the  sensation  is  a  positive  one,  a  sensation  of  light  or  of 
increased  light,  but  that  a  current  in  the  reverse  direction  gives 
rise  to  a  negative  sensation,  a  sensation  of  diminished  light,  a 
sensation  of  blackness. 

That  the  stimulation  of  retinal  structures  by  other  agents 
than  light  may  thus  give  rise  to  visual  sensations,  and  appar- 
ently to  visual  sensations  alone,  may  be  verified  by  experiment 
at  any  time.  The  occasions  on  the  other  hand  are  rare  in  which 
evidence  can  be  gained  as  to  whether  stimulation  of  the  optic 
nerve  apart  from  the  retina,  whether  stimulation  of  the  optic 
fibres  themselves,  and  not  of  their  special  endings  in  the  retina, 
also  gives  rise  to  visual  sensations  and  to  visual  sensations  alone. 
In  certain  cases  of  removal  of  the  eye  it  has  been  stated  that 
when  the  optic  nerve  was  divided  in  the  absence  of  anesthetics, 
the  patient  "  saw  a  great  light "  accompanied  by  no  more  pain 
than  could  be  accounted  for  by  the  filaments  of  the  fifth  nerve 


Chap,  hi.]  SIGHT.  885 

which  are  distributed  to  the  optic  nerve  as  nervi  nervorum.  Such 
experiences  are  urged  in  support  of  the  view  that  all  impulses 
passing  along  the  optic  nerve  however  generated,  whether  by 
retinal  changes  or  by  other  means,  are  visual  impulses  and  visual 
impulses  only  ;  they  give  rise  to  visual  sensations  and  to  visual 
sensations  alone.  On  the  other  hand,  in  other  cases  of  removal 
of  the  eye  in  the  absence  of  anesthetics,  neither  section  of  the 
optic  nerve  nor  subsequent  stimulation  of  the  stump  has  given 
rise  to  visual  sensations.  We  shall  return  to  this  question  later 
on  when  we  have  to  speak  of  what  is  known  as  the  "  specific 
energy  of  nerves,"  and  have  only  referred  to  it  incidentally  now. 
§  554.  Visual  sensations  then  may  be  produced  in  many  other 
ways  than  by  the  falling  of  light  on  the  retina  ;  and  the  point 
to  which  we  wish  to  call  attention  now  is  that  we  are  unable  to 
distinguish  a  sensation  thus  produced  from  the  visual  sensation 
produced  by  light  itself.  We  cannot  by  the  help  of  the  mere 
sensation  alone  recognize  the  nature  of  the  agency  which  has  pro- 
duced the  changes  in  the  retina  giving  rise  to  the  sensation.  The 
identity  of  sensations  due  to  mechanical  stimulation  with  those 
due  to  luminous  stimulation  may  be  illustrated  by  the  story  of 
the  witness  in  a  case  of  assault,  who  swore  that  he,  in  the  dark, 
recognized  his  assailant  by  help  of  the  flash  of  light  produced 
by  a  blow  on  his  eye.  Since  light  emitted  or  reflected  from 
external  objects  is  the  normal  stimulus  for  visual  sensations,  all 
our  visual  sensations  seem  to  us  to  be  produced  by  rays  of  light 
proceeding  from  external  objects  ;  we  look  for  their  cause  not 
in  the  retina  itself,  but  in  the  external  world  ;  and  when  we  wish 
to  know  why  we  have  felt  the  sensation  of  a  flash  of  light,  we 
ignore  the  retina  and  seek  at  once  in  the  external  world  for  some 
source  of  the  rays  of  light  corresponding  to  the  sensation. 
Hence,  also,  when  in  a  particular  part  of  the  retina,  in  a  spot 
for  instance  on  the  nasal  side  of  the  right  eye,  changes  take  place 
such  as  would  be  produced  by  the  image  of  a  luminous  point  fall- 
ing on  that  spot,  though  we  recognize  the  sensation  which  results 
as  having  a  certain  feature,  owing  to  its  being  started  in  that 
particular  spot,  we  do  not  through  the  sensation  learn  anything 
about  the  retina  itself,  we  do  not  through  it  recognize  that  the 
nasal  side  of  the  retina  or  any  particular  spot  in  the  nasal  side 
has  been  affected  ;  what  we  do  recognize,  or  infer,  is  the  exist- 
ence in  the  external  world  of  such  a  luminous  point  as  would 
give  rise  to  the  sensation  in  question.  The  dioptric  arrange- 
ments of  the  eye  are,  as  we  have  seen  (§  529),  such  that  a  lumi- 
nous point  in  order  to  give  rise  to  an  image  in  the  spot  in 
question,  and  so  to  the  sensation  in  question,  must  occupy  a 
particular  position  on  what  we  call  the  right-hand  side  of  the 
external  Avorld.  We  accordingly  recognize  the  sensation  as  hav- 
ing been  caused  by,  or  refer  the  sensation  to,  a  luminous  point 
having  that  position  on  our  right  hand.     And  so  with  the  sensa- 


886     LOCALIZATION  OF  VISUAL  SENSATIONS.     [Book  hi. 

tions  similarly  generated  in  all  other  spots  in  the  retina  ;  we 
recognize  them  as  caused  by  luminous  points  occupying  such 
positions  in  the  external  world  that  their  images  fall  on  those 
spots.  In  each  case  we  ignore  the  retina  itself,  and  the  changes 
taking  place  in  it  are  to  us  simple  tokens  of  luminous  events  in 
the  external  world.  When  with  the  right  eye  we  see  a  luminous 
point  on  our  right-hand  side,  if  we  know  that  changes  are  taking 
place  on  the  nasal  side  of  the  retina  of  that  eye,  it  is  not  because 
we  are  directly  aware  that  the  nasal  part  of  the  retina  is  being 
affected,  but  because  our  knowledge  of  the  dioptrics  of  the  eye 
teaches  us  that  the  image  of  the  luminous  point  is  falling  on  the 
nasal  side  of  the  retina.  If  we  are  suffering  from  right-sided 
hemiopia  (§  497)  all  that  our  sensations  can  of  themselves  tell  us 
is  that  we  cannot  see  things  on  the  right-hand  side  ;  they  do  not 
tell  us  anything  about  the  retina  itself  ;  they  cannot  even  tell  us 
whether  the  deficiency  of  vision  is  due  to  changes  failing  to  be 
set  up  in  the  retina  or  to  the  cerebral  centres  failing  to  be  affected 
by  the  retinal  changes  ;  such  questions  we  have  to  decide  by 
some  other  means  than  a  simple  examination  of  our  sensations, 
and  by  a  similar  roundabout  way  only  are  we  able  to  conclude 
that  in  such  a  hemiopia  it  is  the  nasal  side  of  the  right  retina, 
and  the  temporal  side  of  the  left  retina,  which  fail  to  give  rise  to 
visual  sensations.  Our  sensations,  in  fact,  tell  us  of  themselves 
nothing  about  the  optical  image  on  the  retina  ;  they  do  not  tell 
us  whether  the  retinal  image  is  inverted  or  no  ;  the  fact  that  the 
retinal  image  is  an  inverted  one  does  not  in  itself  influence  our 
visual  sensations,  and  hence  the  inversion  needs  no  correction  on 
our  part. 

§  555.  As  we  have  just  said,  if  the  images  of  two  luminous 
objects,  two  luminous  points,  fall  on  the  retina  at  a  certain  dis- 
tance apart,  the  consequent  sensations  are  distinct.  If,  however, 
the  two  objects  are  made  to  approach  each  other,  a  point  will  be 
reached  at  which  the  two  sensations  are  fused  into  one.  Two 
stars  at  a  certain  distance  apart  may  be  seen  distinctly  as  two 
stars,  while  two  stars  nearer  each  other  appear  to  be  one  star ; 
we  cannot  analyze  the  latter  sensation  into  its  constituent 
parts. 

Similarly,  when  the  images  of  a  number  of  luminous  points, 
of  equal  luminosity,  fall  on  the  retina  sufficiently  near  each 
other,  the  effect  is  not  a  number  of  sensations  of  luminous 
points,  but  one  sensation,  that  of  a  luminous  surface.  This 
introduces  a  new  feature  of  visual  sensations,  namely,  that  of 
size.  If  the  luminous  points  be  few,  so  as  to  involve  a  small 
area  of  the  retina,  the  sensation  is  that  of  a  small  surface ;  if 
the  luminous  points,  equally  near  to  each  other  as  before,  be 
numerous,  so  as  to  involve  a  large  area  of  the  retina,  the  sensa- 
tion is  that  of  a  large  surface.  Moreover,  such  a  sensation  of  a 
surface  will  be  referred  to  some  position  in  space  corresponding. 


Chap,  hi.]  SIGHT.  887 

as  we  have  just  seen,  to  the  region  of  the  retina  affected,  and 
will  possess  features  determined  by  the  relative  positions  of,  that 
is,  by  the  figure  formed  by  the  luminous  points  ;  it  will  be  the 
sensation  of  a  surface  of  a  certain  form,  round,  square  or  the  like  ; 
thus  the  retinal  area  stimulated  supplies  data,  which  are  used, 
in  a  manner  which  we  shall  study  later  on,  for  judging  the  size 
and  form  as  well  as  the  position  of  visible  objects. 

When  the  images  qf  two  luminous  points  are  at  a  certain 
distance  apart  on  the  retina,  the  two  sensations  may  have  no  • 
appreciable  effect  whatever  on  each  other  ;  but  when  they  are 
within  a  certain  distance  from  each  other,  the  sensations  do 
affect  each  other,  in  a  manner  which  we  shall  study  later  on. 
Meanwhile  we  will  merely  say  that  when  two  images  approach 
so  closely  that  the  two  sensations  become  fused  into  one,  such 
a  mutual  influence  is  exerted  that  the  intensity  of  the  total  sen- 
sation produced  is  greater  than  that  of  either  of  the  sensations 
caused  by  a  single  image,  though  less  than  the  sum  of  the  two. 
A  number  of  luminous  points  scattered  over  a  wide  surface 
would  ajDpear  each  to  have  a  certain  brightness ;  each  would 
give  rise  to  a  sensation  of  a  certain  intensity.  If  thej^  were  all 
gathered  into  one  spot,  that  spot  Avould  appear  brighter  than 
any  of  the  previous  points  ;  the  intensity  of  the  sensation  would 
be  greater. 

§  556.  The  region  qf  distinct  vision.  The  distance  at  which 
two  images  must  be  apart  from  each  other  in  order  that  the  two 
sensations  may  be  separate  is  not  the  same  for  the  whole  area 
of  the  retina.  If  two  luminous  points  lie  near  the  optic  axis,  so 
that  their  images  fall  on  the  fovea  centralis  or  on  the  j'ellow 
spot,  they  will  be  seen  as  two  distinct  points,  even  when  their 
images  lie  very  close  indeed  to  each  other.  If  the  luminous 
points  be  moved  aside,  so  that  the  images  fall  on  the  retina  out- 
side the  yellow  spot,  the  two  luminous  points,  though  at  the 
same  distance  apart  from  each  other,  will  give  rise  to  one  sen- 
sation only,  and  be  seen  as  one  point ;  they  may  be  moved  even 
farther  apart  from  each  other  and  still  give  rise  to  one  sensa- 
tion ;  and  if  the  two  points  be  placed  so  much  on  one  side  that 
their  respective  images  fall  on  the  extreme  peripheral  jjarts  of 
the  retina  near  the  ora  serrata,  the  two  images  may  be  separated 
from  each  other  a  very  considerable  distance  and  yet  give  rise 
to  one  sensation  only.  We  may  vary  the  experiment  by  making 
use  of  a  negative  sensation,  and  take  two  black  dots  on  a  white 
surface  only  just  so  far  apart  that  they  can  be  seen  distinctly  as 
two  when  placed  near  the  axis  of  vision  so  that  their  images  fall 
on  or  near  tlie  fovea,  and  then,  keeping  the  axis  lixed,  move  the 
two  points  outwards,  so  that  their  images  travel  outwards  from 
the  fovea  ;  it  will  be  found  that  the  two  soon  ajjpear  as  one. 
The  two  sensations  become  fused,  as  they  would  do  if  brought 
nearer  to  each  other  in  the  centre  of  the  field.     The  farther 


883  EEGIOI^   OF  DISTINCT  VISIOK  [Book  in. 

away  from  the  centre  of  tlie  field,  the  farther  apart  must  two 
points  be  in  order  they  may  be  seen  as  two. 

It  is  obvious  that  the  more  sharply  we  can  distinguish  the 
several  sensations  produced  by  the  images  of  the  several  pomts 
of  which  any  external  object  may  be  supposed  to  be  made  up, 
the  more  distinct  will  be  our  vision  of  the  object.  In  the  fovea 
centralis  our  power  of  thus  distinguishing  sensations  is  at  its 
maximum ;  in  the  outer  parts  of  the  yellow  spot  around  the 
fovea  it  is  less ;  just  outside  the  yellow  spot  it  is  much  less ; 
and  thence  diminishes  more  gradually  towards  the  periphery  of 
the  retina.  Hence  we  speak  of  the  fovea  centrahs,  including 
more  or  less  of  the  whole  yellow  spot,  as  the  "region  of  distinct 
vision ;  "  and  when  we  wish  to  examine  closely  the  features  of 
an  external  object,  we  so  direct  the  eye,  we  so  'look'  at  the 
object,  that  its  image  falls  as  far  as  possible  on  the  fovea 
centralis.  The  diminution  of  distinctness  does  not  take  place 
equally  from  the  centre  to  the  circumference  along  all  meri- 
dians. The  outline  described  by  a  line  uniting  the  points 
where  two  spots  at  a  certain  distance  apart  cease  to  be  seen  as 
two  when  moved  along  different  radii  from  the  centre,  is  a  very 
irregular  figure ;  it  differs  very  much  in  different  individuals  ; 
is  often  not  the  same  in  the  two  eyes  of  the  same  person,  and 
does  not  necessarily  correspond  to  the  figure  of  "•  the  field  of 
vision  "  to  which  we  shall  later  on  refer.  We  may  add  that  the 
power  of  distinguishmg  two  points  in  the  peripheral  parts  of  the 
retina  is  much  increased  by  practice. 

As  we  have  just  said,  when  we  look  intently  at  an  object 
such  as  a  star  in  the  heavens  we  so  direct  the  eye  that  the  image 
of  the  object  falls  on  the  fovea  centralis.  In  the  case  of  most 
people,  two  stars  so  looked  at  appear  to  become  one  when  the 
angle  subtended  by  the  distance  between  them  becomes  less  than 
60  seconds  or  one  minute ;  when  they  are  nearer  than  this  the 
two  sensations  become  one.  And  similar  measurements  are 
obtained  when  other  images  are  made  to  fall  on  the  fovea,  such 
as  those  of  parallel  white  streaks  on  a  black  ground  or  black 
streaks  on  a  white  ground.  In  the  case  of  an  acute  and  trained 
observer  this  minimum  distance  maybe  diminished  to  50  seconds; 
in  many  cases,  on  the  other  hand,  it  is  not  less  than  73  seconds 
and  may  be  more.  Now  the  distance  between  two  points  sub- 
tended by  an  angle  of  50  seconds,  corresponds  in  the  diagram- 
matic eye  (§  527)  to  a  distance  of  3-65  //,  in  the  retinal  image, 
and  of  73  seconds  to  5-36  fi.  Hence  in  the  fovea  centralis  the 
elements  of  the  retina  excited  by  light,  must  lie  3*65  /*  or  5-36  /* 
apart,  or  in  round  numbers  about  4  yu,  apart,  in  order  that  the 
two  sensations,  excited  at  the  same  time,  may  remain  distinct. 

In  the  periphery  of  the  retina  the  distance  must  be  much 
greater ;  thus  at  the  extreme  periphery,  two  black  dots  distant 
apart  about  15  mm.  viewed  at  a  distance  of  20  cm.  and  there- 


Chap,  hi.]  SIGHT.  889 

fore  giving  a  distance  of  more  than  a  millimeter  in  the  retinal 
image,  are  still  seen  as  one  point. 

§  557.  In  accordance  with  the  above,  we  may  suppose  the 
retina  to  be  divided  into  areas,  stimulation  of  the  retina  within 
which  gives  rise  to  a  single  sensation  ;  we  might  speak  of  these 
as  visual  areas,  and  of  the  stimulation  of  a  visual  area  as  a  sensa- 
tional unit.  The  areas  are  very  small,  and  the  sensational  units 
very  numerous  in  the  fovea  centralis  and  yellow  spot ;  the  areas 
are  larger,  and  the  sensational  units  fewer,  over  the  rest  of  the 
retina,  increasingly  so  towards  the  periphery.  The  smaller  or 
larger  the  areas,  the  more  numerous  or  fewer  the  sensational 
units  in  any  retina  or  in  any  part  of  a  retina,  the  more  or  less 
distinct  will  be  the  vision. 

Now  in  the  human  eye  50  cones  may  be  counted  along  a  line 
of  200  yu,  in  length  drawn  through  the  centre  of  the  yellow  spot ; 
this  would  give  4  fu,  for  the  distance  between  the  centres  of  two 
adjoining  cones  in  the  yellow  spot,  the  average  diameter  of  a 
cone  at  its  widest  part  being  here  about  3  /i  and  there  being 
slight  intervals  between  neighbouring  cones.  Hence  if  we  take 
the  centre  of  a  cone  as  the  centre  of  an  anatomical  retinal  area, 
these  anatomical  areas  correspond  very  fairly  in  the  region  of 
distinct  vision  to  the  physiological  visual  areas  just  spoken  of. 
If  two  points  of  the  retinal  image  are  less  than  4  ix  apart,  they 
may  both  lie  within  the  area  of  a  single  cone ;  and  it  is  just 
when  they  are  less  than  about  4  /x  apart  that  they  cease  to  give 
rise  to  two  distinct  sensations.  It  must  be  remembered,  how- 
ever, that  the  fusion  or  distinction  of  the  sensations  is  ultimately 
determined  by  the  brain.  The  retinal  area  must  be  carefully 
distinguished  from  the  sensational  unit,  for  the  sensation  is  a 
process  whose  arena  stretches  from  the  retina  to  certain  parts 
of  the  brain,  and  the  circumscription  of  the  sensational  unit, 
though  it  must  beefin  as  a  retinal  area,  must  also  be  continued 
as  a  cerebral  area,  the  latter  corresponding  to,  and  being  as  it 
were  the  projection  of,  the  former.  Two  points  of  the  retinal 
image  less  than  4  jx  apart  might  lie  both  within  the  area  of  a 
single  cone  ;  but  the  reason  why,  under  such  circumstances,  they 
give  rise  to  one  sensation  only  is  not  because  one  cone-hbre  only 
is  stimulated.  For,  two  points  of  a  retinal  image  might  lie,  one 
on  the  area  of  one  cone  and  another  on  the  area  of  an  adjoining 
cone,  and  still  be  less  than  4  fju  apart ;  in  such  a  case  two  cone- 
fibres  would  be  stimulated ;  and  yet  only  one  sensation  would 
be  produced. 

In  the  case  where  the  two  points  lie  entirely  within  the  area 
of  a  single  cone,  it  is  exceedingly  probable  that,  even  if  the 
adjacent  cones  or  cone-fibres  in  the  retina  are  not  at  the  same 
time  stimulated,  impulses  radiate  from  the  cerebral  ending  of 
the  excited  cone  into  the  neighbouring  cerebral  endings  of  the 
neighbouring  cones ;   in  other  words,  the  sensation-area  in  the 


890  DISTINCT  VISIOX.  [Book  hi. 

brain  does  not  exactly  correspond  to  and  is  not  sharply  defined 
like  the  retinal  area,  but  gradually  fades  away  into  neighbouring 
sensation-areas.  We  may  imagine  two  points  of  the  retinal 
image  so  far  apart  that  even  the  extreme  margins  of  their  re- 
spective cerebral  sensation-areas  do  not  touch  each  other  in  the 
least ;  in  such  a  case  there  can  be  no  doubt  about  the  two  points 
giving  rise  to  two  sensations.  We  might,  however,  imagine 
a  second  case  where  two  points  were  just  so  far  apart  that  their 
respective  sensation-areas  should  coalesce  at  their  margins,  and 
5^et  that,  in  passing  from  the  centre  of  one  sensation-area  to  the 
centre  of  the  other,  we  should  find  on  examination  a  consider- 
able fall  of  sensation  at  the  junction  of  the  two  areas  ;  and  in 
a  third  case  we  might  imagine  the  two  centres  to  be  so  close  to 
each  other  that  in  passing  from  one  to  the  other  no  appreciable 
diminution  of  sensation  could  be  discovered.  In  the  last  case 
there  would  be  but  one  sensation,  in  the  second  there  might 
still  be  two  sensations  if  the  marginal  fall  were  great  enough, 
even  though  the  areas  partially  coalesced. 

That  the  ultimate  differentiation  of  the  sensations  rests  with 
the  brain  is  still  more  clear  in  the  case  of  sensations  started  in 
the  periphery  of  the  retina ;  two  points  of  a  retinal  image  might 
stimulate  two  cones  a  considerable  distance  apart,  or  several 
cones,  to  say  nothing  of  the  intervening  rods,  might  be  stimu- 
lated, and  yet  one  sensation  only  result. 

Thus,  the  distinction  or  fusion  of  visual  sensations  is  ulti- 
mately determined  by  the  disposition  and  condition  of  the  cere- 
bral centres.  Hence  the  possibility  of  increasing  by  exercise 
the  faculty  of  distinguishing  two  sensations,  since  by  use  the 
cerebral  sensation-areas  become  more  and  more  differentiated, 
though  the  mosaic  of  rods  and  cones  fixes  for  the  power  of  dis- 
crimination of  each  individual  a  limit  beyond  which  exercise 
cannot  carry  improvement.  This  effect  of  exercise  is  however 
shewn  in  touch  even  more  strikingly  than  in  sight. 


SEC.   6.     OX   COLOUR   SEXSATIOXS. 

§  558.  The  sensation  excited  by  a  luminous  point  possesses 
still  another  character  besides  those  of  intensity,  duration,  con- 
stancy, and  localization,  namely  the  one  which  we  speak  of  as 
colour. 

When  we  allow  sunlight  reflected  from  a  white  cloud  or  from 
a  sheet  of  white  paper  to  fall  into  the  eye,  we  have  a  sensation 
which  we  call  that  of  white  light.  When  we  look  at  the  same 
light  through  a  prism  and  allow  different  parts  of  the  spectrum 
to  fall  in  succession  into  the  eye,  we  have  a  series  of  sensations, 
differing  in  character  from  the  sensation  of  white  light  and  from 
each  other ;  these  we  call  '  colour  sensations,'  sensations  of  red, 
yellow,  and  the  like.  In  the  latter  case  the  luminous  undula- 
tions are  dispersed  in  a  linear  series  according  to  their  wave- 
lengths, from  the  short  waves  of  the  extreme  violet  to  the  long 
waves  of  the  extreme  red ;  and  we  learn  from  the  spectrum,  on 
the  one  hand,  that  undulations  having  different  Avave-lengths 
produce  different  sensations,  and  on  the  other  hand  that  undula- 
tions having  wave-lengtlis  longer  than  that  of  the  extreme  red, 
about  A-  T(JU,i  or  shorter  than  that  of  the  extreme  violet,  about 
A,  390,  are  unable  to  excite  the  retina  and  are  therefore  invisible. 
When  we  look  directly  at  a  white  object  all  this  dispersion  is 
absent,  and  the  retina  is  excited  at  the  same  time  by  undula- 
tions of  all  the  above  wave-lengths.  A  sensation  of  '  colour ' 
then  is  a  sensation  evoked  by  undulations  of  particular  wave- 
lengths, a  sensation  of  '  while '  is  the  sensation  which  results 
when  the  retina,  or  a  j)art  of  it,  is  simultaneously  excited  by 
undulations  of  all  wave-lengths  which  are  able  to  affect  it,  that 
is  by  the  whole  visible  spectrum.  When  we  direct  our  e3'es  to 
an  object  in  such  a  way  that  the  rays  of  light  proceeding  from 
it  might  fall  on  the  retina  when  we  bring  the  object  within  our 
field  of  vision,  and  yet  experience  from  it  neither  any  sensation 
of  white  nor  any  of  the  various  colour  sensations,  we  call  the 
resulting  affection  of  consciousness  a  sensation  of  '  black,'  we 
say  that  Ave  see  'black.'  Sometimes  the  word  'colour'  is  eon- 
fined  to  the  sensations  other  than  those  of  white  and  black,  some- 
times it  is  used  to  comprise  tliese  two  sensations  as  well. 

1  \  signifies  a  millionth  of  a  millimeter  or  -001  fx. 
891 


892  MIXING  OF   COLOURS.  [Book  hi. 

When  we  examine  the  spectrum  we  are  able  to  perceive  a 
very  large  number  of  different  colours,  we  experience  a  multi- 
tude of  sensations,  no  two  of  which  are  exactly  alike.  There 
are  certain  broad  differences  which  we  express  by  common 
names,  such  as  red,  orange,  yellow  and  the  like.  But  we  can 
go  much  further  than  this.  If  we  take  any  part  of  the  spec- 
trum, the  green  for  instance,  we  find  that  a  very  slight  change 
in  the  wave-length  produces  a  change  in  the  character  of  the 
sensation.  For  convenience'  sake  we  call  a  whole  group  of  sen- 
sations green  ;  but  we  are  obliged  to  admit  that  there  are  several 
kinds  of  green,  several  distinct  kinds  of  sensations,  though  we  do 
not  possess  names  for  all  of  them  ;  a  trained  eye  will  recognize 
that  within  the  green  of  the  spectrum,  the  sensation  produced 
by  one  part  is  a  different  sensation  from  that  produced  by  an 
adjoining  part  differing  in  wave-length  from  the  former  by  an 
exceedingly  small  amount.  The  same  is  the  case  with  other 
parts  of  the  spectrum.  And  in  general  we  may  say  that  any 
change  in  the  wave-length  will  produce  a  change  in  the  sensa- 
tion, so  that  we  might  speak  of  almost  each  wave-length  as  pro- 
ducing a  separate  sensation. 

On  the  other  hand  we  also  easily  recognize  that  the  sensa- 
tions produced  by  the  spectrum  are  not  all  wholly  unlike,  that 
some  are  allied  to  others,  and  that  in  some  cases  one  sensation 
is  intermediate  between  two  other  sensations  and  partakes  of 
the  nature  of  both.  We  recognize  the  sensation  produced  by 
the  part  of  the  spectrum  lying  between  the  green  and  the 
yellow  as  partaking  on  the  one  hand  of  the  nature  of  the 
sensation  of  green  and  on  the  other  hand  of  yellow,  and  call 
it  yellowish  green  or  greenish  yellow  ;  we  similarly  recognize 
a  greenish  blue  or  a  bluish  green,  and  so  on.  This  suggests 
that  our  colour  sensations  are  in  reality  mixed  sensations,  that 
the  multitude  of  different  sensations  to  which  the  spectrum 
gives  rise  are  brought  about  not  by  each  wave-length  giving 
rise  to  a  separate  and  independent  sensation,  but  by  means  of 
a  certain  smaller  number  of  primary  sensations  excited  in  dif- 
ferent degrees  by  different  wave-lengths  and  mixed  in  various 
proportions. 

§  559.  This  view  is  confirmed  when  Ave  study  in  a  syste- 
matic manner  the  results  of  mixing  or  fusing  together  colour 
sensations. 

The  best  method  of  fusing  colour  sensations  is  that  of 
allowing  two  different  parts  of  the  spectrum  to  fall  on  the 
same  part  of  the  retina  at  the  same  time.  We  may  also  make 
use  of  surfaces  coloured  with  pigments,  but  in  doing  so  we 
must  bear  in  mind  the  nature  of  the  colour  of  pigments.  A 
pigment  possesses  colour  because  when  white  light  falls  upon 
it  some  of  the  rays  are  absorbed  while  others  are  reflected. 
Thus  gamboge  absorbs  the  blue  rays  very  largely  as  well  as  to 


Chap,  hi.]  SIGHT.  893 

a  slight  extent  the  red  rays,  but  reflects  the  yellow  rays  and 
with  these  many  of  the  green  rays  ;  indigo  on  the  other  hand 
absorbs  the  red  and  yellow  but  reflects  the  blue  and  a  good 
deal  of  the  green.  Hence  when  we  look  at  a  yellow  gamboge 
patch  our  retina  is  excited  not  by  those  rays  alone  which  form 
the  yellow  of  the  spectrum,  but  by  many  other  rays  as  Avell  ; 
the  colour  is  not  a  ^  pure '  colour,  does  not  correspond  to  one 
of  the  colours  of  the  spectrum,  but  is  a  mixture  of  more  than 
one.  And  this  is  the  case  with  most  pigments  ;  hence  when 
they  are  employed  in  experiments  on  the  mixture  of  sensa- 
tions, difficulties  and  even  errors  arise  which  are  avoided  by 
the  use  of  the  colours  of  the  prism.  We  may  here  incidentally 
remark  that  mixing  the  sensations  excited  by  looking  at  pig- 
ments  gives  very  different  results  from  mixing  the  pigments 
themselves.  Thus  when  gamboge  and  indigo  are  mixed  the 
mixture  is  green  because  the  gamboge  absorbs  the  blue  and 
the  indigo  absorbs  the  red  and  yellow,  while  both  reflect  the 
green.  We  shall  see  presently  that  when  the  sensation  excited 
by  gamboge  is  mixed  with  the  sensation  excited  by  indigo  the 
result  is  a  sensation  not  of  green  but  of  white  ;  and  we  shall 
see  why  this  is.  What  we  have  just  said  with  regard  to  sur- 
faces coloured  with  pigments  applies  also  to  glasses  stained 
with  pigment,  it  being  understood  that  the  colour  of  stained 
glass,  seen  as  a  transparent  object,  corresponds  to  the  rays 
which  it  does  not  absorb.  When  pure  pigments,  i.e.  pigments 
corresponding  as  closely  as  possible  to  the  prismatic  colours, 
are  used,  satisfactory  results  may  be  gained,  either  by  using 
the  reflected  image  of  one  pigment,  and  arranging  so  that  it 
falls  on  the  retina  at  the  same  spot  as  the  direct  image  of  the 
other  pigment,  or  by  allowing  the  image  of  one  pigment  to  fall 
on  the  retina  before  the  sensation  produced  by  the  other  has 
passed  away.  The  first  result  is  easily  reached  by  the  simple 
method  of  placing  two  pieces  of  coloured  paper  a  little  dis- 
tance apart  on  a  table,  one  on  each  side  of  a  glass  plate  in- 
clined at  an  angle.  By  looking  with  one  eye  down  on  the 
glass  plate  the  reflected  image  of  the  one  paper  may  be  made 
to  coincide  with  the  direct  image  of  the  other,  the  angle  which 
the  glass  plate  makes  with  the  table  being  adjusted  to  the  dis- 
tance between  the  pieces  of  paper.  In  the  second  method,  the 
'  colour  top '  is  used  ;  sectors  of  the  colours  to  be  investigated 
are  placed  on  a  disc  made  to  rotate  very  rapidly,  and  the  image 
of  one  colour  is  thus  brought  to  bear  on  the  retina  so  soon 
after  the  image  of  another  that  the  two  sensations  are  fused 
into  one. 

i5  560.  When  b}"  any  of  the  above  methods  sensations  corre- 
sponding to  the  red  and  yellow  of  the  spectrum  are  mixed 
together  in  certain  proportions  the  result  is  a  sensation  of 
orange,  quite  indistinguishable  from  the  orange  of  the  spec- 


894  CHAEACTERS   OE   COLOUES.  [Book  hi. 

trum  itself.  Now  the  latter  is  produced  by  rays  of  certain 
wave-lengths,  whereas  the  rays  of  red  and  of  yellow  are  respec- 
tively of  quite  different  wave-lengths.  The  orange  of  the  spec- 
trum cannot  be  made  up  by  any  mixture  of  the  red  and  the 
yellow  of  the  spectrum  in  the  sense  that  the  red  and  yellow  rays 
can  unite  together  to  form  rays  of  the  same  wave-lengths  as  the 
orange  rays ;  the  three  things  are  absolutely  different.  It 
is  simply  the  mixed  sensation  of  the  red  and  yellow  which  is 
indistinguishable  from  the  sensation  of  orange  ;  the  mixture  is 
entirely  and  absolutely  a  subjective  one.  In  the  same  way  we 
may  by  appropriate  mixtures  produce  the  sensations  correspond- 
ing to  other  parts  of  the  spectrum.  Now  we  must  suppose  that 
rays  of  different  wave-lengths  affect  the  retina  in  different  ways 
and  so  give  rise  to  different  visual  impulses,  that,  for  instance, 
the  visual  impulses  generated  by  orange  rays  are  different  from 
those  generated  by  red  rays  or  by  yellow  rays.  Hence  we  are 
led  by  the  fact  of  mixed  sensations  being  identical  with  other 
apparently  simple  sensations  to  infer  that  the  visual  impulses 
and  hence  the  visual  sensations  which  any  ray  originates  are 
of  a  complex  character.  We  conclude,  for  instance,  that  the 
impulses  which  a  ray  in  the  middle  of  the  orange  gives  rise  to 
are  not  simple  impulses  answering  exclusively  to  the  colour  of 
that  ray,  but  complex  impulses,  parts  of  which  may  be  excited 
by  rays  other  than  the  particular  orange  ray  in  question.  In 
saying  this  we  must  bear  in  mind  that  we  possess  no  direct 
information  of  the  nature  of  visual  impulses,  our  knowledge  of 
these  being  limited  to  what  we  learn  through  the  sensations  to 
which  they  give  rise  ;  the  complexity  of  the  sensation  may  be, 
and  indeed  probably  is,  of  a  different  order  from  that  of  the 
visual  impulse  ;  to  this  point  we  shall  return. 

The  view  that  our  ordinary  colour  sensations  are  mixtures 
of  simpler  sensations  is  further  confirmed  by  an  examination  of 
the  colours  of  external  nature.  For,  though  we  see  around  us 
very  many  colours  besides  those  present  in  the  spectrum,  yet 
we  find  that  the  sensations  of  all  these  colours  may  be  repro- 
duced by  mixtures  of  sensations  excited  by  various  parts  of  the 
spectrum.  Thus  the  colour  purple,  which  is  so  abundant  in 
the  external  world  and  yet  so  conspicuous  by  its  absence  from 
the  spectrum,  may  be  at  once  reproduced  by  fusing  in  proper 
proportions  the  sensations  of  red  and  of  blue.  And  very  many 
other  colours  present  in  the  external  world  but  not  seen  in  the 
spectrum  itself  may  be  produced  by  mixing  various  spectral 
colours  in  various  proportions. 

Other  colours  in  nature  may  be  reproduced  by  mixing  spec- 
tral colours  with  white  or  with  black.  When  by  means  of  a 
slit  we  allow  a  certain  limited  part  of  the  spectrum,  say  in  the 
green,  to  fall  on  a  certain  area  of  the  retina,  the  rays  exciting 
that  area  have  certain  wave-lengths,  lying  within  certain  limits, 


Chap,  m.]  SIGHT.  895 

say  from  A,  525  to  \  535 ;  no  rays  but  these  are  affecting  the 
retina  at  the  time,  and  the  result  is  the  sensation  which  we 
call  spectral  green.  But  we  might  easily  so  arrange  matters 
that  a  certain  amount  of  white  light,  that  is  of  light  of  all 
wave-lengths  of  the  visible  spectrum,  should  fall  on  the  area  in 
question  at  tlie  same  time  that  the  green  is  falling  upon  it  ;  the 
result  would  be  a  mixed  sensation,  a  sensation  of  spectral  green 
mixed  with  the  sensation  of  white,  and  we  should  recognize 
this  sensation  as  different  from  the  sensation  of  spectral  green. 
Further  by  varying  the  proportion  of  white  to  green  falling  on 
the  area  in  question  at  the  same  time  we  should  have  a  whole 
series  of  different  sensations  from  a  green  in  which  there  was 
hardly  any  white  to  a  white  in  which  there  was  hardly  any 
green.  In  such  a  series  of  colour  sensations  we  recognize  a  hue 
supplied  by  the  spectral  colour,  and  we  use  the  phrase  more  or 
less  "  saturated  "  to  express  the  proportion  of  wliite  light ;  when 
very  little  white  is  present,  we  speak  of  the  colour  as  being 
highly  saturated.  It  need  hardly  be  said  that  not  only  indi- 
vidual spectral  colours,  but  all  mixtures  of  these  also,  may  be 
thus  "  mixed  Avitli  white." 

Again,  taking  a  given  area  of  the  retina  we  may,  on  the  one 
hand,  throw  on  to  the  area  a  small  amount  of  a  spectral  colour 
in  such  a  way  that  all  the  elements  of  the  retina  in  the  area  are 
excited,  to  a  slight  degree,  giving  rise  to  a  feeble  sensation  of 
that  colour ;  but  we  may,  on  the  other  hand,  so  scatter  a  few 
rays  OA^er  the  area  that  while  some  elements  are  excited  others 
remain  at  rest  and  yet  in  such  way  that  the  excitation  of  the 
whole  area  still  gives  rise  to  one  sensation  only.  We  may  speak 
of  each  of  these  sensations  as  one  in  which  the  sensation  of  the 
spectral  colour  is  mixed  or  fused  with  the  sensation  which  we 
call  black  ;  or  we  may  distinguish  the  former  as  merely  a  feeble 
sensation  and  the  latter  as  more  strictly  mixed  with  black. 
Many  of  the  colours  of  the  external  world  are  of  this  nature ; 
thus  the  colours  which  we  call  "browns  "  are  mixtures  of  yel- 
low or  of  red  or  of  both  (and  possibly  of  other  spectral  colours 
also)  with  more  or  less  black.  In  a  similar  way  we  may  mix, 
not  a  spectral  colour,  but  white  with  black,  various  mixtures 
forming  A^arious  "greys." 

§  561.  Putting  aside  these  more  or  less  peculiar  cases  of 
mixture  with  black,  Ave  may  say  that  the  character  of  a  colour 
depends  (1)  on  the  wave-lengths  of  the  particular  rays  which, 
either  alone  or  in  excess  of  other  rays,  are  falling  on  a  given 
area  of  the  retina ;  (2)  on  the  amount  of  this  coloured  light 
falling  on  that  area  in  a  giA'en  time ;  and  (3)  on  the  amount  of 
Avhite  light  falling  on  that  area  at  the  same  time.  The  lirst 
determines  AAdiat  Ave  call  the  hue,  the  second  the  intensity,  and 
the  third  the  amount  of  saturation.  Our  common  phrases  do 
not  distinguish  Avith  sufficient  accuracy  these  three  conditions, 


896  COMPLEMEKTAEY  COLOUES.  [Book  hi. 

which  obviously  may  exist  under  various  combinations.  On 
the  one  ha.nd  we  frequently  use  wholly  unlike  names  for  colours 
which  differ  only  in  degree  of  saturation,  such  as  carmine  and 
pink ;  on  the  other  hand  we  often  use  the  same  adjectives  for 
quite  different  conditions.  It  is  desirable  to  employ  the  word 
'  pale,'  to  mean  little  saturated,  largely  mixed  with  white,  and 
the  word  '  deep '  or  '  rich  '  to  mean  highly  saturated,  slightly 
mixed  with  white.  The  word  '  tint '  might  be  used  to  express 
various  degrees  of  saturation,  the  word  '  hue '  being  reserved 
to  denote  the  dominant  wave-length.  '  Tone '  is  frequently 
employed  to  express  variations  of  wave-length  within  a  named 
colour,  as  for  instance  different  tones  of  red.  The  word  '  bright ' 
is  often  used  somewhat  loosely,  but  it  is  desirable  to  employ  it 
exclusively  as  identical  with  '  luminous,'  that  is  to  say,  as  indi- 
cating the  intensity  of  the  sensation  ;  a  colour  is  more  or  less 
bright  according  to  the  amount  of  luminous  energy  which  is 
being  expended  on  the  retina.  We  may  remark,  in  passing, 
that  while  we  can  easily  compare  the  brightness  or  luminosity 
of  two  white  lights  or  of  the  same  part  of  the  spectrum  under  a 
feeble  and  under  a  strong  illumination,  we  may  feel  some  diffi- 
culty in  comparing  the  amount  of  brightness  of  one  colour  with 
that  of  another,  the  brightness  for  instance  of  a  given  yellow 
with  that  of  a  given  red.  Conversely  the  word  '  dark  '  is  used 
to  denote  feeble  intensity,  or  admixture  with  black.  Lastly, 
our  appreciation  of  the  colours  of  external  objects  is  modified 
by  the  nature  of  the  surface  which  is  coloured,  and  features  so 
arising  receive  various  names ;  but  these  are  in  reality  outside 
actual  colour  sensations. 

§  562.  Admitting  that  our  colour  sensations  may  be  consid- 
ered to  be  much  fewer  in  number  than  those  which  we  appear 
to  have  when  we  look  on  the  colours  of  the  spectrum  or  of 
nature,  admitting  that  rays  of  light  awake  in  us  certain  "  pri- 
mary" colour  sensations,  which  mixed  in  various  proportions 
reproduce  all  our  colour  sensations,  we  have  now  to  ask  the 
question.  What  is  the  nature  or  what  are  the  characters  of 
these  primary  colour  sensations  ? 

In  view  of  the  answer  to  this  question  we  must  call  attention 
to  certain  results  which  may  be  obtained  by  a  further  study  of 
the  mixing  of  colours,  meaning  by  that  the  mixing  of  colour 
sensations. 

We  have  seen  that  all  the  colours  of  the  spectrum  mixed 
together  make  white.  We  have  now  to  add  that  white  may 
also  be  produced  by  mixing  two  colours  only,  provided  that 
these  are  properly  chosen.  If  we  take  a  part  of  the  red  of  the 
spectrum,  and  by  any  of  the  methods  given  in  §  559,  mix  it 
with  successive  parts  of  the  spectrum,  we  shall  find  that  the 
mixture  with  a  particular  part  of  the  green  or  blue  green  gives 
white.     These  two  colours  are  said  to  be  complementary  to  each 


Chap,  hi.]  SIGHT.  897 

other.  In  order  to  get  a  complete  white,  that  is  a  white  free 
from  all  colour,  a  certain  proportion  between  the  relative 
amounts  of  red  and  green  light,  that  is  to  say  between  the 
intensities  of  the  two  sensations,  must  be  observed.  And  it 
will  Ije  understood  that  the  white  thus  produced  by  two  small 
parts  of  the  spectrum  is  not  equal  in  intensity  to  the  white 
which  would  be  produced  by  the  combined  effect  of  the  whole 
of  the  same  spectrum.  The  following  may  be  taken  as  char- 
acteristic complementary  colours,  the  respective  wave-lengths 
being  given : 

Red,  A,  656,  Blue  Green,  \  492, 

Orange,  X  608,  Blue,  X  490, 

Gold  Yellow,  X  574,  Blue,  \  482, 

Yellow,  X  564,  Indigo-blue,  X  462, 

Greenish  Yellow,  X  564,  Violet,  X  433. 

It  will  be  understood  that  the  above  are  not  the  only  comple- 
mentary colours  ;  as  we  pass  from  the  red  end  of  the  spectrum 
towards  the  green,  each  successive  part  of  the  spectrum  has  its 
complementary  part  on  the  other,  blue  side  of  the  spectrum, 
each  wave-length  on  the  red  side  has  its  complementary  wave- 
length on  the  blue  side.  When  we  reach  the  greenish  yellow 
at  X  564,  the  complementary  colour  is  on  the  very  margin  of 
the  violet  end  of  the  visible  spectrum.  But  Ave  may  go,  so  to 
speak,  outside  the  spectrum,  for  the  green  of  the  spectrum  has 
for  its  complementary  colour,  purj)le.  Or,  to  put  it  in  another 
way,  while  each  end  of  the  spectrum  has  its  complementary 
colour  at  the  other  end,  the  complementary  colour  of  the  mid- 
dle of  the  spectrum  is  a  combination  of  the  two  ends. 

The  bearing  of  these  facts  on  the  theory  of  primary  colour 
sensations  is  obvious.  Two  complementary  colours  excite 
between  them  all  the  primar}-  sensations  which  are  excited  by 
white  light,  though  not  to  the  same  intensity.  Rays  of  the 
wave-length  X  656  falling  on  the  retina  give  rise  to  the  sensa- 
tion which  we  denote  as  a  particular  kind  of  red  ;  they  do  this 
however,  not  by  the  simple  and  exclusive  stimulation  of  a  par- 
ticular red  sensation,  but  by  exciting  all  the  primary  sensations 
which  are  not  excited  by  the  wave-length  X  492.  Conversely 
rays  of  the  wave-length  X  492,  produce  the  sensation  of  blue 
green  b}-  exciting  all  the  primary  sensations  which  are  not  ex- 
cited by  X  656.  Similarly  com})lex  is  the  effect  of  other  wave- 
lengths. We  may  roughly  describe  each  of  two  com])lementary 
wave-lengths  as  stirring  up  about  half  the  whole  of  the  primary 
sensations  which  can  be  excited  by  rays  of  all  wave-lengths. 

§  563.  To  produce  white  out  of  two  colours,  out  of  two  parts 
of  the  spectrum,  we  are  limited  to  certain  pairs  ;  if  we  take 
one  colour,  we  are  limited  to  one  other  colour,  to  its  pair  ;  we 

57 


898  THEORIES   OE   COLOUR  VISION.        [Book  in. 

have  no  choice.  If  however  we  are  allowed  three  colours 
instead  of  two,  we  have  a  much  greater  range.  If  we  take  any 
three  colours,  provided  only  that  they  lie  a  certain  distance 
apart  along  the  spectrum,  we  can  produce  white  by  mixing 
them  in  certain  proportions.  If  we  take  any  red,  green  and 
blue,  we  can  by  adjusting  the  amount  of  each,  that  is  the  in- 
tensity of  each,  produce  white. 

We  may  go  further  than  this.  By  adjusting  the  amounts  of 
each  of  the  three  colours  we  can  reproduce  all  the  colours  of 
the  spectrum.  If  we  take,  for  instance,  a  red  of  a  certain 
wave-length,  a  green  of  a  certain  wave-length,  and  a  blue  of  a 
certain  wave-length,  we  can,  without  calling  to  our  aid  any 
other  wave-lengths,  by  varying  the  relative  intensities  of  the 
three,  produce  not  only  white  light,  but  also  orange,  yellow, 
and  violet,  with  all  the  intermediate  tints,  that  is  to  say,  pro- 
duce all  the  colours  of  the  spectrum  ;  and  we  may  in  the  same 
way  produce  the  non-spectral  purple.  Our  choice  however  is 
to  a  certEdn  extent  limited  ;  the  three  colours  which  we  choose 
must  be  spread  over  the  spectrum,  for  we  cannot  obtain  these 
results  with  three  colours  taken  from  the  red  and  yellow  alone, 
or  from  the  green  and  blue  alone.  Moreover,  the  result  is  not 
a  complete  one  ;  the  colour  which  we  thus  produce  by  combin- 
ing three  spectral  colours  differs  from  a  true  spectral  colour  in 
not  being  saturated  ;  it  is  "  mixed  with  white,"  more  so  in  some 
cases  than  in  others  ;  in  relation  to  this  deficiency  of  satura- 
tion, the  green  region  of  the  spectrum  behaves  differently  from 
the  red  end  and  the  blue  end. 

§  564.  These  results  shew  that  the  primary  colour  sensa- 
tions out  of  which  our  recognized  colour  sensations  originate, 
may  be  reduced  to  three  in  number.  If  we  suppose  that  we 
possess  three  primary  sensations  so  disposed  in  reference  to  the 
spectrum,  so  arranged  so  to  speak  along  the  spectrum,  that  a 
ray  of  light  affects  each  of  the  three  differently  according  to 
its  wave-length,  we  can  understand  how  all  our  multitudinous 
colour  sensations  may  arise  from  the  varied  excitation  of  these 
primary  sensations.  There  may  be  more  than  three  of  these 
primary  sensations,  but  if  so  they  must  behave  as  if  they  were 
three  ;  they  cannot  be  less,  since  as  we  have  seen  the  results 
of  mixing  two  sensations  only  are  extremely  limited.  We 
may  therefore  speak  of  our  vision  as  trichromic,  as  based  on 
three,  or  the  equivalent  of  three,  primary  sensations. 

When  we  attempt  to  inquire  further  into  the  nature  of  these 
primary  sensations,  we  find  ourselves  in  the  face  of  two  rival 
theories. 

The  one,  propounded  by  Young  but  more  fully  elaborated 
by  Helmholtz  and  Maxwell,  and  known  as  the  Young-Helmholtz 
theory,  teaches  that  there  are  three  and  only  three  such  primary 
sensations.     As  we  have  just  seen,  any  three  parts  of  the  spec- 


Chap,  hi.] 


SIGHT. 


899 


triim,  with  certain  restrictions,  might  be  chosen  as  correspond- 
ing to  these  three  primary  sensations  so  far  as  concerns  the 
reproduction,  by  means  of  them,  of  all  other  colour  sensations  ; 
hence  in  determining  the  nature  of  the  primary  sensations 
we  must  have  recourse  to  other  considerations.  We  may  for 
instance  very  naturally  suppose  that  two  of  the  three  correspond 
to  the  two  ends  of  the  spectrum,  and  may  therefore  be  spoken 
of  as  more  or  less  closely  corresponding  to  our  recognized  sensa- 
tions of  red,  and  of  violet.  If  red  and  violet  be  tlius  two  of 
the  sensations  the  third  one  must  correspond  to  green,  for  only 
a  sensation  corresponding  to  green  would  give  white  when 
mixed  with  the  other  two  sensations.  Or  again,  choosing  green 
in  the  first  instance  as  one  of  the  primary  sensations  foV  the 
reason  that  it  stands  apart  from  the  others  in  its  complement, 
purple,  not  being  a  spectral  colour,  we  may  decide  that  the  two 
other  primary  sensations  ought  to  differ  as  much  as  possible 
from  each  other,  and  therefore  choose  red  and  blue  rather  than 
red  and  violet  since  violet  is  obviously  more  allied  to  red  than 
is  blue ;  indeed  Ave  may  perhaps  regard  violet,  on  account  of 
its  relations  to  red,  as  the  beginning  of  a  second  spectrum  the 
greater  part  of  which  is  invisible.  The  decision  between  these 
two  forms  of  the  same  theory  rests  on  a  number  of  considera- 
tions, into  the  discussion  of  Avhicli  we  cannot  enter  here. 
Unless  we  sjDecially  call  attention  to  the  difference  between 
them,  which  acquires  importance  on  certain  occasions  only,  we 
shall  treat  them  as  identical,  and  use  the  words  blue  and  violet 
in  this  connection  indifferently. 

Such  a  view  of  three  primary  colour  sensations  is  represented 
in  the  diagram  (Fig.  150).     Thus  the  red  x^rimary  sensation, 


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Fig.  150.     Diagram  of  Three  Primary  Colour  Sensations. 

1  is  the  so-called  'red,'  2  'green,'  and  3  'violet'  primary  colour  sensation, 
i?,  0,  Y,  &c.  represent  the  red,  orange,  yellow,  &c.,  colour  of  the  spectrum.  The 
diagram  illustrates,  by  the  height  of  the  curve  in  each  case,  how  the  several 
primary  colour  sensations  are  respectively  excited  to  dift'ei'ent  extents  bj'  vibra- 
tions of  different  wave-lengths.  Rut,  in  this,  and  also  in  Fig.  l;jl,  the  curves 
are  to  be  understood  nnt  as  careful  curves  of  actual  variations  in  the  intensity 
of  the  several  changes,  but  as  simply  serving  to  illustrate  roughly  the  nature  of 
the  theory. 


900  THEORIES   OF  COLOUE  VISION.       [Book   hi. 

excited  to  a  certain  extent  by  the  rays  at  the  extreme  red  end, 
is  most  powerfully  affected  by  the  rays  at  a  little  distance  from 
that  end,  the  rays  from  this  point  onwards  towards  the  blue  end 
producing  less  and  less  effect.  The  curve  of  the  green  primary 
sensation  begins  later  and  reaches  its  maximum  in  the  green  of 
the  spectrum,  while  the  violet  or  blue  primary  sensation  is  still 
later  and  only  reaches  its  maximum  towards  the  blue  end  of  the 
spectrum.  Each  ray  calls  forth  each  primary  sensation  though 
to  a  different  degree,  and  the  total  result  of  each  ray,  or  of  each 
group  of  rays,  is  determined  by  the  proportionate  amount  of 
the  three  sensations.  Thus  the  sensation  of  orange  {0  in  the 
figure)  is  brought  about  by  a  mixture  of  a  great  deal  of  the 
primary  red  with  much  less  of  the  primary  green,  and  hardly 
any  of  the  primary  violet ;  the  orange  sensation  is  converted 
into  a  yellow  sensation  by  diminishing  the  primary  red  and 
largely  increasing  the  primary  green,  the  primary  violet  under- 
going also  some  slight  increase.  And  similarly  with  all  the 
other  sensations.  When  all  the  three  primary  sensations  are 
together  excited,  each  to  its  whole  extent,  as  when  ordinary 
light  falls  on  the  retina,  the  result  is  a  sensation  of  white. 
According  to  this  theory,  black  is  simply  the  absence  of  sensa- 
tion from  the  visual  apparatus. 

In  the  view,  as  originally  put  forward  by  Young,  the  three 
primary  sensations  were  supposed  to  be  represented  ])y  three  sets 
of  fibres,  each  set  of  fibres  being  differently  affected  by  different 
rays  of  light,  and  the  impulses  passing  to  the  brain  along  each 
set  awakening  a  distinct  sensation.  No  such  distinction  of  fibres 
can  be  found  in  the  retina;  but  an  anatomical  basis  of  this  kind 
is  not  necessary  for  the  theory;  we  can  easily  conceive  of  the 
same  fibre  transmitting  three  distinct  kinds  of  impulses;  and 
indeed,  as  we  shall  see  later  on,  there  are  more  ways  than  one 
by  which  we  can  imagine  the  sensations  to  be  differentiated. 

§  565.  Another  theory,  that  of  Hering,  starts  from  the 
observation  that  when  we  examine  our  own  sensations  of  light 
we  find  that  certain  of  these  seem  to  be  quite  distinct  in  nature 
from  each  other,  so  that  each  is  something  sui  generis^  whereas 
we  easily  recognize  all  other  colour  sensations  as  various  mix- 
tures of  these.  Thus  red  and  yellow  are  to  us  quite  distinct: 
we  do  not  recognize  any  thing  common  to  the  two;  but  orange 
is  obviously  a  mixture  of  red  and  yellow.  Green  and  blue  are 
equally  distinct  from  each  other  and  from  red  and  yellow,  but 
in  violet  and  purple  we  recognize  a  mixture  of  reel  and  blue. 
White  again  is  quite  distinct  from  all  the  colours  in  the  nar- 
rower sense  of  that  word,  and  black,  which  we  must  accept  as 
a  sensation,  as  an  affection  of  consciousness,  even  if  we  regard 
it  as  the  absence  of  sensation  from  the  field  of  vision,  is  again 
distinct  from  everything  else.  Hence  the  sensations,  caused 
by  different  kinds  of  light  or  by  the  absence  of  light,  which 


Chap,  hi.]  SIGHT.  901 

thus  appear  to  us  distinct,  and  wliicli  we  may  speak  of  as 
'  native '  or  '  fundamental '  sensations,  are  white,  bhick,  red, 
yellow,  green,  blue.  Each  of  these  seems  to  us  to  have  noth- 
ing in  common  with  any  of  the  others,  whereas  in  all  other 
colours  we  can  recognize  a  mixture  of  two  or  more  of  these. 

This  result  of  common  experience  suggests  tlie  idea  that 
these  fundamental  sensations  are  the  primary  sensations,  con- 
cerning which  we  are  inquiring.  And  Hering's  theory  at- 
tempts to  reconcile,  in  some  such  way  as  follows,  the  various 
facts  of  colour  vision  with  the  sujiposition  that  we  possess 
these  six  fundamental  sensations.  The  six  sensations  readily 
fall  into  three  pairs,  the  members  of  each  pair  having  analogous 
relations  to  each  other.  In  each  pair  the  one  colour  is  com- 
plementary to  the  other;  white  to  black,  red  to  green,  and 
yellow  to  blue. 

The  little  we  know  about  the  actual  nature  of  sensations 
leads  us  to  believe  that  the  nervous  processes  which  are  at  the 
bottom  of  sensations  are,  like  other  nervous  processes,  the  out- 
come of  metabolic  changes  in  nervous  substance.  We  shall 
presently  call  attention  to  the  view  that  vision  originates  in 
the  metabolic  changes  of  a  certain  substance  (or  substances) 
in  the  retina,  that  the  metabolism  of  this  substance,  "wdiicli  has 
been  called  visual  substance,  is  especially  affected  by  the  inci- 
dence of  light,  and  that  the  metabolic  changes  so  induced  deter- 
mine the  beginnings  of  visual  impulses  and  thus  of  visual 
sensations.  In  the  metabolism  of  living  substance,  we  recog- 
nize (§  30)  two  phases,  the  upward  constructive  anabolic  phase, 
and  the  downward  destructive  katabolic  phase;  we  may  accord- 
ingly, in  the  absence  of  any  distinct  leading  to  the  contrary,  on 
the  one  liand  suppose  that  different,  rays  of  light,  rays  differ- 
ing in  their  wave-length,  may  affect  the  metabolism  of  the 
visual  substance  in  different  ways,  some  promoting  anabolic, 
others  promoting  katabolic  changes,  and  on  the  other  hand 
that  different  changes  in  the  metabolism  of  the  visual  sub- 
stance may  give  rise  to  different  sensations. 

We  may  therefore  regard  ourselves  as  at  liberty  to  suppose 
that  there  may  exist  in  the  retina  a  visual  substance  of  such  a 
kind  that  when  rays  of  light  of  certain  wave-lengths,  the  longer 
ones  for  instance  of  the  red  side  of  the  spectrum,  fall  u})on  it, 
katabolic  changes  are  induced  or  encouraged,  while  anabolic 
changes  are  similarly  promoted  by  the  incidence  of  rays  of  other 
wave-lengths,  the  shorter  ones  of  the  blue  side.  But,  as  we  have 
already  said,  it  is  dillicult  in  these  matters  of  sensation,  to 
distinguish  between  periplieral,  retinal,  and  central,  cerebral 
events;  we  may  accordingly  extend  the  above  view  to  the 
whole  visual  apparatus,  central  as  well  as  peripheral,  and  sup- 
pose that  when  rays  of  a  certain  wave-length  fall  upon  the 
retina,  they  in  some  way  or  other,  in  some  part  or  other  of  the 


902 


THEORIES   OF  COLOUR  VISION        [Book  in. 


visual  apparatus,  induce  or  promote  katabolic  changes  and  so 
give  rise  to  a  sensation  of  a  certain  kind,  while  rays  of  another 
wave-length  similarly  induce  or  promote  anabolic  changes  and 
so  give  rise  to  a  sensation  of  a  different  kind. 

The  theory  of  Hering,  of  which  we  are  now  speaking,  applies 
this  view  to  the  six  fundamental  sensations,  and  supposes  that 


Fig.  151.     Diagram  to  illustrate  Hering's  Theory  of  Colour  Vision. 

The  lines  B.O.Y.G.B.  V.  indicate,  as  in  Fig.  150,  the  position  on  the  spectrum 
of  red,  orange,  yellow,  green,  blue  and  violet. 

The  line  r.g.,  which  indicates  a  space,  shaded  vertically,  is  intended  to  rep- 
resent the  effect  of  rays  of  different  wave-lengths  on  the  red-green  visual  sub- 
stance. In  the  red,  orange  and  yellow  up  to  the  line  Y.,  the  effect  is  katabolic, 
one  of  dissimilation  (red  sensation) .  Y.  marks  the  position  of  equilibrium ; 
beyond  this  the  effect  is  anabolic,  one  of  assimilation  (green  sensation).  Beyond 
the  blue,  B.  the  effect  (indicated  by  a  broken  line)  is  represented  as  once  more 
katabolic. 

The  line  y.h.  similarly  represents  the  behaviour  of  the  yellow-blue  substance, 
shaded  horizontally,  katabolic  (yellow)  up  to  G.,  anabolic  (blue)  beyond. 

The  line  iv.  similarly  indicates  the  white-black  substance,  unshaded,  kata- 
bolic (sensation  of  white)  along  the  whole  length  of  the  spectrum. 

each  of  the  three  pairs  is  the  outcome  of  a  particular  set  of 
katabolic  and  anabolic  changes;  these  we  may  provisionally  speak 
of  as  changes  in  a  distinct  visual  substance,  without  attemj)ting 
to  decide  whether  the  changes  are  retinal  or  cerebral  or  both. 
The  theory  supposes  the  existence  of  what  we  may  call  a  red- 
green  visual  substance,  of  such  a  nature  that  so  long  as  its  me- 
tabolism is  normal,  katabolic  and  anabolic  changes  being  in 
equilibrium,  we  experience  no  sensation,  but  that  when  katabo- 
lic changes  (changes  of  dissimilation  is  Hering's  own  term)  are 
increased,  we  experience  a  sensation  of  (fundamental)  red,  and 
when  anabolic  changes  (changes  of  assimilation)  are  increased 


Chap,  hi.]  SIGHT.  903 

we  experience  a  sensation  of  (fundamental)  green.  A  similar 
yellow-blue  visual  substance  is  supposed  to  furnisli  through 
katabolic  changes,  a  3-ellow,  through  anabolic  changes  a  blue 
sensation  ;  and  a  white-black  visual  suljstance  similarl}^  provides 
for  a  katabolic  sensation  of  white  and  an  anabolic  sensation  of 
black.  The  two  members  of  each  pair  are  therefore  not  only 
complementary  but  also  antagonistic.  Further  these  substances 
are  of  such  a  kind  that  while  the  white-black  substance  is 
influenced  in  the  same  way  though  to  different  degrees  by  rays 
along  the  wliole  range  of  the  spectrum,  the  two  other  substances 
are  differently  influenced  by  rays  of  different  wave-length  (see 
Fig.  151^.  Thus  in  the  part  of  the  spectrum  which  we  call 
red,  the  ra3-s  promote  a  large  katabolism  of  the  red-green  sub- 
stance with  comparatively  slight  effect  on  the  3'ellow-blue  sub- 
stance ;  hence  our  sensation  of  red.  In  that  part  of  tlie  spectrum 
which  we  call  yellow  the  rays  effect  a  large  katabolism  of  the 
yellow-blue  substance  but  their  action  on  the  red-green  sub- 
stance does  not  lead  to  an  excess  of  either  katabolism  or  anabo- 
lism,  this  substance  being  neutral  to  them ;  hence  our  sensation 
of  yellow.  The  green  rays,  again,  promote  anabolism  of  the 
red-green  substance,  leaving  the  anabolism  of  the  yellow-blue 
substance  equal  to  its  katabolism  ;  and  similarly  blue  rays  cause 
anabolism  of  the  yellow-blue  substance,  and  leave  the  red-green 
substance  neutral.  Finally  at  the  extreme  blue  end  of  the  spec- 
trum, the  ra^'s  once  more  p'rovoke  katabolism  of  the  red-green 
substance,  and  by  adding  red  to  blue  give  violet.  When  orange 
rays  fall  on  the  retina,  there  is  an  excess  of  katabolism  of  both 
the  red-green  and  the  yellow-blue  substance  ;  when  greenish- 
blue  ra3^s  are  perceived  there  is  an  excess  of  anabolism  of  both 
these  substances ;  and  other  intermediate  hues  correspond  to 
var3dng  degrees  of  katabolism  or  anabolism  of  the  several  visual 
substances. 

When  all  the  ra3's  together  fall  on  the  retina,  the  red -green 
and  3"ellow-blue  substance  remain  in  equilibrium,  but  the  white- 
black  substance  undergoes  great  katabolic  changes  ;  and  we  say 
the  light  is  white. 

Such  are  the  two  main  tlieories  of  colour  vision ;  and  much 
ma3'  be  said  in  favour  of  both  of  them  ;  at  the  same  time  both 
of  them  i^resent  difticiilties.  We  ma)'  perhaps  regard  as  the 
distinctive  feature  of  Hering's  theory  the  view  that  white  is  an 
independent  sensation,  and  not,  as  according  to  the  Young- 
Helmholtz  theor)^,  the  secondar^^  result  of  the  mixture  of  pri- 
mary sensations.  In  Ilering's  theor3'  rays  of  all  wave-lengths 
(within  the  range  of  tlie  visible  spectrum)  give  rise  to  the  sen- 
sation of  white,  whatever  ma3^  be  the  colour  sensation  produced 
at  the  same  time  ;  a  fully  saturated  colour,  one  wholly  unmixed 
with  white,  according  to  this  view  does  not  exist.  This  assump- 
tion enables  us  to  explain  much  more  readil3'  than  does  the 


904  THEORIES   OE   COLOUR  VISION.       [Book  hi. 

Young-Helmlioltz  theory  the  occurrence  under  certain  circum- 
stances of  white  sensations  replacing  or  accompanying,  that  is 
to  say  diminishing  the  saturation  of,  colour  sensations.  On  the 
other  hand  it  introduces  what  appears  to  many  minds  a  grave 
difficulty  in  reference  to  black.  The  theory  supposes  that  the 
sensation  of  black  is  the  result  of  the  predominance  of  anabolic 
changes  in  the  white-black  substance.  But  what  name  are  we 
to  give  to  the  sensation  when  the  white-black  substance  is  in  a 
condition  of  equilibrium?  We  cannot  investigate  the  corre- 
sponding conditions  of  equilibrium  in  the  red-green,  or  in  the 
yellow-blue  substance,  because  we  can  never  study  these  by 
themselves.  When  either  of  them  occurs,  as  when  rays  limited 
to  certain  wave-lengths  are  falling  on  the  retina,  we  are  by 
hypothesis  at  the  same  time  subject  to  changes  in  the  white- 
black  substance ;  we  may  therefore  leave  these  two  conditions 
of  equilibrium  on  one  side.  But  we  are  constantly  experienc- 
ing the  condition  of  equilibrium  of  the  white-black  substance, 
unaccompanied  by  any  stimulation  of  either  the  red-green  or 
yellow-blue  substance ;  we  do  so  when  the  influence  of  light 
has  for  some  time  been  wholly  removed  from  the  eye,  or  again 
taking  the  view,  which  is  the  more  probable  one,  that  the 
changes  of  which  we  are  speaking  are  cerebral  changes,  when 
the  retina  by  disease  or  injury  has  become  insensible  to  light. 
Under  such  circumstances  we  must  suppose  that  the  previous 
katabolic  excitement  of  the  white-black  substance  has  died 
away,  and  that  the  substance  is  in  equilibrium.  Now  when  we 
examine  our  sensation  under  these  circumstances,  we  find  that 
though  it  is  one  of  darkness  it  is  one  which  differs  from  a  sen- 
sation of  intense  blackness.  So  distinct  is  the  difference  that 
the  sensation  in  question  has  been  spoken  of  under  the  phrase 
"the  intrinsic  light  of  the  retina."  And  that  we  may  experi- 
ence sensations  of  black  different  from  this  sensation  due  to  the 
retina  being  at  rest  may  be  shewn  in  several  ways.  When  we 
close  and  shade  the  eyes  after  they  have  been  exposed  to  a  very 
bright  sunlight,  we  first  experience  a  sensation  of  blackness,  but 
this  soon  gives  way  to  the  sensation  of  mere  darkness  corre- 
sponding to  the  "intrinsic  light  of  the  retina."  Again  if  we 
stare  for  some  time  at  a  white  disc  on  a  black  field  and  then 
close  the  eyes,  what  we  shall  speak  of  presently  as  a  negative 
after  image  is  developed  ;  the  part  of  the  field  of  vision  corre- 
sponding to  the  white  disc  appears  as  a  black  disc,  which  by  its 
blackness  stands  out  in  fairly  strong  contrast  to  the  rest  of  the 
field  of  vision,  which  corresponding  to  the  area  of  the  retina 
previously  free  from  the  stimulus  of  light,  now  yields  the  sen- 
sation of  the  "intrinsic  light  of  the  retina."  And  other 
examples  of  a  similar  kind  might  be  given.  Admitting  then 
that  the  "  intrinsic  light  of  the  retina  "  corresponds  to  a  condi- 
tion of  equilibrium  of  the  white-black  substance,  we  may  speak 


Chap,  hi.]  SIGHT.  905 

of  this  as  the  neutral  condition  on  one  side  of  which  we  have 
sensations  of  white  and  on  the  other  side  sensations  of  black. 
Such  a  neutral  condition  has  been  spoken  of  as  a  "  neutral  grey," 
but  the  word  grey  is  so  often  associated  with  a  mixture  of  white 
and  black  sensations  coexisting  at  the  same  time  rather  than 
with  a  neutral  condition,  that  the  term  seems  unsuitable.  i\Iany 
minds  find  it  difficult  to  realize  that  the  condition  of  Avhich  we 
are  speaking  is  a  true  neutral  condition,  the  various  degrees  of 
blackness  being  insignificant  compared  with  the  various  degrees 
of  intensity  of  Avhite,  and  accordingly  find  it  difficult  to  accept 
Hering's  theory. 

Both  theories  conform  to  the  conclusion  (§  504)  that  nor- 
mal vision  is  trichromic  in  the  sense  of  being  made  up  of  three 
factors;  for  the  three  pairs  of  fundamental  sensations  of  the 
one  theory  (the  two  members  of  each  pair  being  reciprocally 
antagonistic,  the  positive  and  negative  phase  of  the  same 
thing),  play  the  same  part  in  the  equations  of  mixtures  as  the 
three  primary  sensations  of  the  other  theory.  Indeed  it  will 
be  found  on  examination  that  all  the  results  of  the  mixtures  of 
colours  are  equally  explicable  on  both  theories.  In  comparing 
the  two  theories,  iioAvever,  especially  in  reference  to  the  results 
of  mixtures,  we  must  bear  in  mind  that  "  brightness  "  or  "  lumi- 
nosity "  does  not  possess  the  same  meaning  in  the  two  theories. 
In  the  Young-Helmholtz  theory  brightness  is  dependent  on  the 
extent  to  which  the  primary  sensation  is  excited,  on  the  amount 
of  energy  expended  in  the  physical  substratum,  whatever  that 
may  be,  of  the  primary  sensation.  The  red  of  the  extreme  red 
end  of  the  spectrum  has  a  minimum  of  brightness  since  the 
extreme  red  rays  excite  the  red  sensation  to  a  minimum  and 
the  other  two  sensations  hardly  or  not  at  all.  As  we  pass 
bluewards  the  brightness  increases,  partly  because  tlie  red  sen- 
sation is  more  powerfully  excited,  but  also  because  to  the  bright- 
ness of  the  red  sensation  there  is  now  added  the  brightness  of 
the  green  sensation.  And  the  brightness  of  a  saturated  yellow, 
such  as  that  of  the  spectrum,  is  the  sum  of  the  brightnesses  of 
the  red  and  green  sensations  and  nothing  else;  we  neglect  for 
the  sake  of  simplicity  the  minute  adjunct  of  the  blue  sensation. 
In  Hering's  theory  the  _case  is  different.  The  lack  of  bright- 
ness at  the  red  end  of  the  spectrum  is  due  not  merely  to  the 
feeble  development  of  the  red  sensation,  to  tlie  feeble  (katabolic) 
excitation  of  the  red-green  substance,  but  also  to  the  feeble 
develoi)ment  of  the  white  sensation,  to  the  feeble  (katabolic) 
excitation  of  the  Avhite-bhick  substance;  and  the  brightness  of 
the  yellow  of  the  spectrum  is  due  not  merely  to  the  large  devel- 
opment of  the  yellow  sensation  but  also  to  the  large  increase  of 
the  white  sensation.  When,  moreover,  we  come  to  examine 
this  feature  of  '  brightness '  or  ''  luminosit}^ "  more  closely,  we 
find  that  many  questions  of  great  complexity  are  raised;  and 


906  C0L0UE-BLINDNE3S.  [Book  hi. 

many  statements  regarding  the  results  of  mixing  sensations, 
sucli  as  those  respecting  complementary  colours  (§  562)  have  to 
be  qualified  by  considerations  touching  the  luminosity  of  the 
constituents;  but  into  these  questions  we  cannot  enter  here. 

We  may  here  remark  when  the  extreme  red  end  of  the 
spectrum  is  examined  it  is  found  that  along  a  certain  length, 
between  A,  760  and  X  655,  there  is  no  change  in  the  sensation 
as  regards  hue  but  only  as  regards  luminosity  ;  the  red  remains 
exactly  the  same  kind  of  red,  it  only  becomes  brighter  and 
more  readily  seen.  Similarly  at  the  other  end  from  A,  430  to 
A-  390  the  sensation  of  violet  remains  of  the  same  hue  though 
differing  in  luminosity.  And  these  facts  have  been  brought 
forward  on  the  Young-Helmholtz  theory  in  support  of  violet 
being  a  primary  sensation  ;  it  is  urged  that  the  red  and  violet 
which  thus  do  not  change  in  hue  but  only  in  luminosity  cor- 
respond to  the  actual  primary  sensations.  The  behaviour  at 
the  red  end  is  quite  intelligible  on  Hering's  theory,  since,  as 
the  waves  shorten  in  length  both  the  red  and  the  white  sensa- 
tions are  supposed  to  increase,  though  probably  in  this  part  of 
the  spectrum  the  white  sensation  is  very  feeble,  rapidly  increas- 
ing a  little  farther  on.  The  behaviour  at  the  violet  end  presents 
difficulties,  since  if  the  violet  be  due  to  admixture  with  a  second 
octave  so  to  speak  of  red,  the  violet  should  change  in  hue, 
become  more  red,  as  the  rays  shorten.  But  the  same  difficulty 
presents  itself  to  the  Young-Helmholtz  theory  if  blue  be 
accepted  as  a  primary  sensation.  Moreover  observations  on 
this  part  of  the  spectrum  are  exceedingly  difficult.  We  cannot, 
however,  attempt  to  discuss  the  contending  theories  properly ; 
this  would  carry  us  beyond  the  limits  of  this  book.  We 
must  content  ourselves  with  incidental  reference  to  some  con- 
clusions, which  are  suggested  by  the  study  of  some  other 
features  of  colour  sensations  as  well  as  of  abnormal  colour 
vision,  and  to  these  we  may  now  turn. 

§  566.  Variations  in  Colour  Vision.  Qolour-Blindness.  Per- 
sons differ  very  much  in  their  power  of  appreciating  and  dis- 
criminating colours,  and  that  quite  independently  of  their  ability 
to  give  expression  to  their  colour  sensations,  that  is  to  say,  of 
their  skill  in  naming  colours.  One  person  will  regard  as  iden- 
tical two  colours  Avhich  another  person  recognizes  as  different. 
In  many  cases  such  differences  in  the  power  of  discriminating 
colours  are  slight,  but  in  some  cases  they  are  great.  Certain 
persons  are  met  with  who  regard  as  quite  alike,  or  nearly  alike, 
colours  which  to  most  people  are  glaringly  distinct ;  such 
persons  are  said  to  be  "colour-blind." 

The  most  common  token  of  "  colour-blindness "  is  the 
inability  to  distinguish,  or  the  difficulty  in  distinguishing,  red 
and  green.  The  great  chemist  Dalton,  who  was  colour-blind, 
found  great   difficulty  in  recognizing   at   a   distance   his   red 


Chap,  hi.]  SIGHT.  907 

(GlasgoAv)  college  gown  when  it  was  lying  on  the  college  grass- 
plot  ;  the  colour-blind  can  tell  a  cherry  among  the  leaves  on  a 
tree  much  more  by  its  form  than  by  its  colour ;  and  when  such 
persons  are  asked  to  '  make  matches '  bet^\'eeu  coloured  objects, 
such  as  skeins  of  coloured  wools,  they  will  put  together  a  red 
skein  and  a  green  skein  as  being  of  the  same  colour.  ]\Iost 
colour-blind  people  more  or  less  confound  red  and  green ;  but 
when  a  number  of  such  colour-blind  persons  are  tested  in 
making  matches  either  between  skeins  of  wool  or  otherwise,  it  is 
found  that  they  do  not  all  make  the  same  matches ;  they  do  not 
agree  as  to  the  particular  red  and  green  Avhich  they  regard  as 
identical,  and  they  disagree  in  various  other  matches.  But 
they  all  agree  in  this  that  when  they  are  tested  by  the  method 
of  mixing  colours  it  is  found  that  all  the  colour  sensations 
which  they  experience,  including  white,  may  be  reproduced  by 
mixtures  of  two  colours  only,  whereas  as  Ave  have  seen  (§  564) 
normal  vision  requires  three.  For  instance  all  the  colours 
which  they  see  may  be  reproduced  by  var3-ing  mixtures  of 
yellow  and  blue.  The  vision  of  these  colour-blind  people  is 
therefore  dichromic  not  trichromic.  All  their  colour  sensa- 
tions are  compounded  of  two  not  three  (or  two,  not  three  pairs 
of)  primary  sensations. 

On  further  examination  it  is  found  that  these  ordinary 
colour-blind  persons  ma}^  be  more  or  less  successfully  divided 
into  two  classes.  The  members  of  one  class  ha^-e  the  folloAving 
characters.  The  spectrum  seems  to  them  shortened  at  the  red 
end ;  that  is  to  say  they  fail  to  receive  any  visual  sensation 
from  the  rays  of  extremely  long  wave-length  which  still  give 
to  the  normal  eye  a  distinct  sensation  of  red.  The  blue-green 
of  the  spectrum  seems  to  them  less  deeply  coloured  than  the 
rest  of  the  spectrum  either  on  the  red  or  on  the  blue  side ; 
this  part  gives  rise  in  them  to  a  sensation  like  that  caused  by 
feeble  white  light ;  they  have  a  difhculty  in  recognizing  any 
hue  in  it  and  they  often  speak  of  it  as  grey,  Avhile  in  the 
remainder  of  the  spectrum  both  to  the  blue  and  to  the  red 
side  they  have  distinct  sensations  of  colour.  AVe  may  call  this 
region  of  the  spectrum  the  'neutral  band';  and  it  is  one  of  the 
characters  of  this  class  that  they  see  such  a  neutral  band  in 
the  blue-green.  They  confound,  as  we  have  said,  reds  and 
greens,  but  when  asked  to  make  an  exact  match  between  a  red 
and  a  green  they  choose  a  bright  red  and  a  dark  green ;  they 
are  more  or  less  uncertain  about  all  colours  containing  red  or 
green,  and  when  asked  to  match  a  purple  they  generally  select 
a  blue  or  a  violet. 

To  the  members  of  the  other  class,  the  spectrum  is  not 
shortened  ;  they  receive  sensations  as  far  to  the  red  side  as  does 
the  normal  eye.  They  also  see  a  '  neutral '  band,  but  tins  is 
placed  in  the  green,  that  is  to  say,  nearer  the  red  end  than  is 


908  COLOUR-BLINDNESS.  [Book  hi. 

the  case  witli  the  first  class.  When  asked  to  make  an  exact 
match  between  red  and  green  they  choose  a  dark  red  and  a 
bright  green ;  when  asked  to  match  a  purple  they  generally 
select  a  green  or  a  grey. 

Persons  whose  vision  belongs  to  one  or  other  of  these 
classes  are  sometimes  spoken  of  as  '  totally '  colour-blind ;  for 
there  are  grades  of  difference  between  such  a  kind  of  vision 
and  normal  vision,  and  some  eyes  may  be  called  '  partially ' 
colour-blind.  Moreover,  even  among  these  '  totally  '  colour- 
blind persons  individual  differences  occur  in  each  class ;  indeed 
not  a  few  cases  are  met  with  which  do  not  seem  to  fit  into  either 
class,  since  they  unite  in  themselves  some  of  the  characters  of 
each  class.  But  even  if  we  make  allowance  for  these  excep- 
tions, the  existence  of  the  two  classes  with  their  respective 
features  seems  to  offer  a  strong  support  to  the  Young-Helmholtz 
theory.  In  both  classes  vision  is  dichromic  not  trichromic, 
that  is  to  say  according  to  that  theory  in  both  classes  one  of 
the  three  primary  sensations  is  missing.  Since  the  character- 
istic mistake  which  they  both  make  is  to  confound  red  and 
green,  we  may  infer  that  the  missing  primary  sensation  is  not 
blue  but  either  red  or  green.  If  we  further  suppose  that  in 
the  first  class  red  is  missing,  in  the  second  green,  all  the 
features  of  the  two  classes  seem  intelligible. 

On  this  view  all  the  visual  sensations  Avhich  the  first  class 
experience  are  made  up  of  green  and  blue  ;  and  their  vision 
might  be  represented  by  Fig.  160  with  the  upper  curve  (1) 
omitted.  Owing  to  the  absence  of  the  red  sensation,  the  ex- 
treme red  rays  hardly  affect  them  at  all.  Since  all  their  visual 
sensations  are  made  up  of  various  mixtures  of  the  primary  green 
and  primary  blue  sensations,  and  since  the  sensation  which  they 
call  white  light  (whatever  it  may  be  when  compared  subjec- 
tively with  that  of  the  normal  eye)  is  the  sensation  produced 
when  rays  of  all  the  wave-lengths  of  the  visible  spectrum  are 
falling  on  the  retina  at  the  same  time,  that  is  to  say  when  both 
of  the  two  primary  sensations  are  being  equally  excited  at  the 
same  time,  it  follows  that  any  particular  wave-lengths  which 
equally  excite  both  the  two  sensations  should  also  produce  a 
sensation  which  to  them  is.  identical  with  that  of  white  light. 
Now  the  blue-green  rays  do  excite  equally  both  the  green  and 
the  blue  sensation  (cf.  Fig.  150)  ,  and  it  is  just  at  this  part  of 
the  spectrum  that  these  persons  see  the  '  neutral  band  '  spoken 
of  above.  Further,  the  matches  which  eyes  of  this  class  make 
are  such  as  we  might  imagine  would  be  made  if  the  sensation 
of  red  were  absent,  and  the  two  remaining  sensations  when 
mixed  together  made  white.  Hence  members  of  this  class  are 
spoken  of  as  being  "red-blind." 

In  eyes  of  the  second  class,  since  red  is  present  though  green 
is  wanting,  the  spectrum  extends  redwards  as  far  as  in  the 


Chap,  hi.]  SIGHT.  909 

normal  eye  ;  the  least  coloured  part  of  the  spectrum,  the  'neu- 
tral band,'  occupies  about  the  same  position  as  the  green  seen 
by  the  normal  eye,  for  here  the  red  sensation  and  blue  sensa- 
tion are  excited  to  about  the  same  extent ;  and  the  matches 
made  by  eyes  of  this  class  are  such  as  might  be  expected  in  tlie 
absence  of  the  green  sensation.  ]Members  of  this  class  are 
accordingly  spoken  of  as  "green-blind." 

It  miglit  appear  at  first  sight  that  the  lack  of  a  primary 
sensation,  that  is  to  say,  the  want  of  a  third  of  all  visual  sensa- 
tions, would  lead  to  a  general  deficiency  of  vision  ;  for  the  lack 
of  one-third  of  visual  sensations  would  be  equivalent  to  a  dim- 
inution of  the  total  illumination  of  external  objects  to  the  extent 
of  one-third,  and  this,  unless  we  suppose  that  the  normal  eye 
lives  in  a  superfluity  of  light  must,  especially  in  feeble  light, 
lead  to  dim  vision ;  moreover  a  vision  which  has  to  trust  to 
two-fold  differences  must  be  less  sure  than  one  based  on  three- 
fold dii^erences.  But  this  does  not  necessarily  follow  ;  the  two 
remaining  sensations  might  become  more  highly  developed, 
might  so  to  speak  expand  in  the  absence  of  the  thii'd.  Or  Ave 
may  suppose,  as  indeed  has  been  supposed,  that  in  these  cases 
neither  the  red  sensation  nor  the  green  sensation  is  absent,  but 
that  the  two  sensations  coincide.  We  may  imagine  that  the 
visual  substance,  or  whatever  it  be.  changes  in  which  give  rise  to 
the  red  sensation,  is  affected  by  light  of  different  wave-lengths 
in  the  same  way  as  is  the  visual  substance  changes  in  which 
give  rise  to  the  green  sensation.  This  might  be  illustrated  by 
making  the  two  curves  for  red  and  green  in  Fig.  150  coincide. 
If  the  red  curve  were  moved  bluewards  so  as  to  coincide  with 
the  green  curve,  the  figure  would  illustrate  a  red-blind  case  ; 
if  the  green  curve  were  moved  redwards  to  coincide  with  the 
red  curve,  the  figure  would  illustrate  a  green-blind  case.  And 
as  a  matter  of  fact  the  general  vision  of  colour-blind  people 
seems  to  be  as  good  as  that  of  normal  eyes ;  moreover,  within 
the  range  of  the  colours  which  they  can  see,  colour-blind  people 
are  if  anything  more  acute  than  most  people ;  though  they 
regard  as  more  or  less  alike  two  colours  which  seem  to  the 
normal  eye  wholly  unlike,  they  can  moi'e  easily  detect  minute 
differences  such  as  those  of  shade  or  tone,  within  each  of  the 
two  colours. 

§  567.  The  phenomena,  however,  of  these  two  classes  of 
colour-blind  eyes  can  also  be  interpreted  on  Hering's  theory. 
In  both  of  them  the  red-green  substance  may  be  supposed  to  be 
missing,  and  their  dichromic  vision  to  be  made  up  exclusively 
*of  changes  in  the  yellow-blue  and  white-black  substances.  Since 
they  are  thus  supposed  to  have  neither  red  nor  green  sensations, 
they  must  necessarily  confound  red  and  green  ;  and  the  smaller 
differences,  which,  as  we  have  seen,  divide  into  two  classes  all 
those  which  confound  red  with  green  may  be  explained  as  follows. 


910  COLOUE-BLINDNESS.  [Book  hi. 

Even  in  eyes  which  may  be  considered  normal  as  regards 
colonr  vision,  eyes  which  certainly  cannot  be  called  colour- 
blind, considerable  differences  will,  on  closer  examination,  be 
found  in  regard  to  sensations  of  yellow.  If  by  means  of  a 
special  arrangement  we  bring  a  certain  amount  of  the  red  part 
of  the  spectrum  and  a  certain  amount  of  the  green  part  of  the 
spectrum  on  to  the  eye  at  the  same  time,  the  result  is  a  sensa- 
tion of  yellow ;  according  to  the  Young-Helmholtz  theory  yel- 
low is  a  mixture  of  red  and  green.  By  the  same  arrangement 
we  can  bring  on  to  the  eye  at  the  same  time  a  certain  amount 
of  the  actual  yellow  of  the  spectrum.  In  this  way  we  can 
make  a  match  l3etween  a  mixture  of  spectral  red  and  green  on 
the  one  hand,  and  spectral  yellow  on  the  other,  comparing  the 
mixed  sensation  derived  from  two  parts  of  the  spectrum  with 
the  sensation  derived  from  a  single  (yellow)  part.  We  have  to 
adjust  the  quantities  of  red  light  and  green  light  until  the  mix- 
ture seems  of  the  same  hue  and  the  same  brightness  as  the  yel- 
low, not  shewing  either  a  reddish  or  a  greenish  tone.  When 
this  is  done  it  is  found  that  different  people  differ  very  materi- 
ally as  to  the  proportion  of  red  and  green,  the  proportion  of  the 
intensities  of  the  two  sensations,  necessary  to  make  the  match 
with  yellow ;  with  the  same  quantity  of  red  some  need  more 
green,  others  less  green,  to  make  the  match.  This,  on  the 
Young-Helmholtz  theory,  is  interpreted  as  meaning  that  the 
development  of  the  reel  and  green  primary  sensations  differs 
even  in  people  whose  colour  vision  is  considered  normal.  But 
on  Hering's  theory,  in  which  yellow  is  a  fundamental  sensation, 
it  may  be  interpreted  as  meaning  that  in  passing  along  the  spec- 
trum towards  the  red  end,  the  point  at  which  the  yellow-blue 
substance  ceases  to  be  affected  by  rays  of  light,  is  placed  much 
nearer  the  red  end  in  some  people  than  in  others.  By  Hering's 
hypothesis  the  green  of  the  spectrum  affects  not  only  the  red- 
green  substance,  cf.  Fig.  151  (in  way  of  anabolism),  but  also  to 
some  extent  the  yellow-blue  substance  (in  way  of  katabolisra)  ; 
the  red  rays  on  the  other  hand  affect  the  yellow-blue  substance 
very  slightly,  while  the  (pure)  j^ellow  rays  are  neutral  to  the 
red-green  substance  producing  neither  katabolic  red  nor  ana- 
bolic green,  but  simply  yellow  by  katabolic  action  on  tlie  yellow- 
blue  substance.  This  at  least  represents  the  condition  of  the , 
majority  of  eyes.  If,  howe-ver,  we  suppose  that  in  other  eyes 
the  yellow-blue  substance  is  considerably  affected  by  red  rays, 
if  in  Fig.  151,  we  suppose  the  curve  representing  the  yellow 
sensation  to  be  considerably  extended  towards  the  red  end,  in 
these  ejea  the  red  rays  would  give  rise  to  a  sensation  of  yellow 
at  the  same  time  that  they  excited  a  sensation  of  red,  the  red 
would  be  mixed  with  yellow ;  hence  in  such  ej^es  a  certain 
amount  of  red  being  already  mixed  with  yellow  would  need 
less  green  (with  its  necessarily  accompanying  yellow)  to  pro- 


CHAr.  III.]  SIGHT.  911 

duce  a  certain  amount  of  yellow  as  the  result  of  the  mutual 
neutralization  of  the  I'ecl  and  green.  In  such  cases  we  may 
suppose  not  that  the  whole  relation  of  the  yellow-blue  substance 
to  wave-lengths  is  altered,  but  merely  tliat  the  sensitiveness  to 
long  wave-length  is  increased  ;  the  curve  of  the  yellow-blue  is 
not  shifted  bodily  along  the  spectrum,  but  the  form  of  the  curve 
is  altered  so  that,  the  maximum  of  yellow  remaining  the  same, 
the  3^ellow  end  of  the  curve  extends  further  into  the  red.  Not 
only  this  match  of  red  and  green  with  yellow,  but  other  matches 
of  a  similar  nature,  shew  that  in  different  eyes  the  yellow  sen- 
sation (in  Heriiig's  sense)  is  more  prominent  in  some  people 
than  in  others,  that  some  people  so  to  speak  are  more  yellow 
sighted  than  others. 

The  application  of  this  fact  to  the  colour-blind  cases  is  obvious. 
In  the  one  class,  the  red-blind  of  the  Young-Helmholtz  theory, 
the  relations  of  the  primary  sensations,  the  distribution  along  the 
spectrum  of  the  visual  substances  are  the  same  as  in  the  normal 
eye  save  that  the  red-green  substance  and  the  corresponding 
sensations  are  missing;  and  since  the  visibility  of  the  red  end  of 
the  spectrum  is  chiefly  affected  by  the  red  sensation,  the  white- 
black  substance  being  as  compared  with  the  red-green  substance 
but  slightly  sensitive  to  the  extreme  rays,  the  spectrum  is  short- 
ened. The  feeble  white  visual  impulses  excited  are  insufficient 
to  affect  consciousness  unless  supported  by  red  Adsual  impulses. 
In  the  second  class,  the  yellow-blue  substance  has  undergone  an 
expansion  similar  to  but  probably  greater  than  that  which  obtains 
in  the  yellow-sighted  but  otherwise  normal  eyes  mentioned  above, 
it  is  sensitive  to  even  the  rays  at  the  red  end  of  the  spectrum ; 
hence  the  spectrum  to  eyes  of  this  class  seems  of  the  ordinary 
visible  length. 

§  568.  So  far  then  both  theories  may  be  made  to  explain 
the  ordinary  phenomena  of  colour-blindness ;  but  it  is  obvious 
that  the  subjective  condition  of  the  colour-blind  must  be  different 
according  to  one  theory  from  what  it  is  according  to  the  other. 
According  to  the  Young-Helmholtz  theory  the  red-blind  person 
does  not  experience  in  any  degree  the  sensation  of  either  red  or 
yellow;  from  the  green  of  the  spectrum  to  the  red  end  lie  only 
sees  some  sort  of  green.  Indeed  along  the  whole  spectrum,  the 
sensations  which  he  experiences  are  only  various  kinds  of  green 
and  blue,  with  various  amounts  of  the  sensation  whatever  it  be, 
whether  white  or  simply  green-blue,  or  some  other  sensation 
unknown  to  the  normal  eye,  which  results  from  the  mixture  of 
the  green  and  blue  sensations.  The  green-blind  person,  accord- 
ing to  the  same  theory,  has  only  the  sensations  of  red  and  blue, 
with  the  sensation  whatever  it  may  be  derived  from  the  mixture 
of  these  two,  he  never  has  the  sensation  of  either  green  or  yellow. 
Obviously  the  sensations  of  the  two  classes  ought  to  differ  very 
■widely.     According  to  Hering's  theory,  both  classes  agree  in 


912  COLOUR-BLINDNESS.  [Book  hi. 

seeing  neither  red  nor  green,  all  their  sensations  are  made  up  of 
3'ello\v  and  blue,  with  white  and  black,  and  the  only  difference 
between  the  two  classes  is  that  the  one,  the  green-blind  of  the 
other  theory,  see  more  yellow  than  do  the  other. 

§  569.  We  have  treated  of  the  colour-blind  as  if  they  were 
confined  to  those  who  confounded  red  with  green.  According 
to  the  Young-Helmholtz  theory,  another  class  of  colour-blind  is 
possible,  the  violet-  or  blue-blind,  those  who  while  possessing  red 
and  green  sensations  lack  the  third,  violet  or  blue  sensation. 
Such  a  kind  of  vision  is  impossible  on  Hering's  theory ;  accord- 
ing to  that  theory,  if  a  person  fails  to  see  blue,  he  must  also  fail 
to  see  yellow. 

Lastly  we  may  remark  that  absolute  colour-blindness,  a  con- 
dition in  which  shades  of  black  and  white  alone  indicate  the 
features  of  external  objects,  while  possible  on  Hering's  theory, 
is  impossible  on  the  Young-Helmholtz  theory.  According  to 
the  latter  a  person  reduced  to  one  primary  sensation  must  see 
either  red,  green,  or  blue ;  this  one  sensation  is  excited  in  him 
both  by  objects  which  we  called  coloured  and  by  objects  which 
we  call  white.  He  would  probably  call  it  white  :  but  it  would 
be  either  red,  green,  or  blue.  According  to  Hering's  theory 
he  might  still  see  white  and  black  in  the  total  absence  of  both 
the  red-green  and  the  yellow-blue  substance.  Now,  cases  do 
undoubtedly  occur,  though  they  are  relatively  rare,  of  marked 
colour-blindness,  which  are  neither  red-blind  nor  green-blind. 
Cases  further  occur  which  may  be  described  as  cases  of  total 
colourless  blindness ;  the  subject  recognizes  only  differences  of 
luminosity.  Cases,  moreover,  occur  in  which  one  eye  only  is 
affected,  the  other  being  normal,  so  that  the  subject  can  describe 
the  sensations  of  the  affected  eye  by  help  of  those  of  the  normal 
eye ;  and  a  not  small  number  of  cases  occur  in  which  the  defi- 
ciency of  colour  sensation  is  not  congenital,  affecting  the  whole 
of  the  eye,  but  the  result  of  disease,  and  limited  to  a  part  of 
the  retina,  the  rest  of  which  may  be  normal.  And  it  might  be 
thought  that  by  an  examination  of  these  several  cases  it  would 
be  easy  to  decide  definitely  between  the  two  theories.  But 
when  this  examination  is  carried  out,  many  difficulties  arise  in 
the  way  of  reaching  such  a  decision ;  and  indeed  some  of  the 
facts  observed  seem  compatible  with  neither  theory.  A  full 
discussion  of  these  difficulties  —  and  only  a  full  discussion 
would  be  satisfactory  —  is  impossible  here.  We  may  simply 
remark  that  when  we  said  that  absolute  colour-blindness,  the 
limitation  of  visual  sensations  to  those  of  black  and  white,  was 
impossible  on  the  Young-Helmholtz  theory,  we  sliould  have 
added,  unless  it  be  supposed  that  in  such  cases  all  the  tlu^ee 
primary  sensations  coincide,  none  of  them  being  absent,  much 
in  the  same  way  that  two  may  be  supposed  to  coincide  in  red  or 
green  blindness. 


Chap,  hi.]  SIGHT.  913 

§  570.  What  we  have  said  concerning  colour  vision  refers  to 
the  central  parts  of  the  retina  only.  If  a  coloured  object  be 
moved  so  that  its  image  travels  from  the  central  to  the  periph- 
eral parts  of  the  retina,  the  colour  sensations  change  and  the 
peripheral  parts  may  be  spoken  of  as  colour-blind.  Thus  the 
sensation  of  red  is  lost  towards  the  periphery,  which  may  be 
sjDoken  of  as  red-blind,  while  in  the  same  region  other  sensa- 
tions, at  all  events  that  of  blue,  are  still  felt.  At  the  extreme 
periphery  even  blue  is  wanting,  that  is,  all  the  primary  sensa- 
tions are  wanting,  and  yet  we  receive  by  it  uncoloured  sensa- 
tions, sensations  of  black  and  white.  But  these  phenomena  of 
peripheral  colour  vision  also  need  a  fuller  discussion  than  we 
can  afford  to  give  them  here. 

§  571.  Influence  of  the  ptipnent  of  the  yelloiv  spot.  In  the 
macida  lutea,  or  yellow  spot,  the  yellow  pigment  which  is  dif- 
fused through  the  retinal  structures  in  this  region  absorbs  some 
of  the  greenish-blue  rays  of  the  light  which  falls  upon  it.  We 
may  use  this  feature  of  the  yellow  spot  for  the  purpose  of 
making  the  spot,  so  to  speak,  visible  to  ourselves,  by  the  follow- 
ing experiment.  A  solution  of  chrome  alum,  which  only  trans- 
mits red  and  greenish-blue  rays,  is  held  up  between  the  eye  and 
a  white  cloud.  The  greenish-blue  rays  are  absorbed  by  the 
yellow  spot,  and  here  the  light  gives  rise  to  a  sensation  of  red ; 
whereas  in  the  rest  of  the  held  of  vision,  the  sensation  is  that 
ordinarily  produced  by  the  purplish  solution.  The  yellow  spot 
is  consequently  marked  out  as  a  rosy  patch.  This  very  soon 
however  dies  away. 

Though,  when  we  wish  our  vision  to  be  most  acute,  we  use 
the  fovea  centralis  in  which  the  pigment  is  extremely  scanty  or 
absent  owing  to  the  thinness  or  absence  of  all  retinal  layers  ex- 
cept the  cones  and  cone  fibres,  still  in  ordinary  vision  we  make 
large  use  of  the  whole  yellow  spot,  and  our  sensations  of  the 
colour  of  external  objects  must  be  to  a  certain  extent  influenced 
by  the  pigment  of  the  spot.  The  light  which  reaches  the  rods 
and  cones  of  this  region  from  objects  which  we  call  white,  is  in 
reality  more  or  less  tinged  with  yellow ;  in  other  words  what  we 
call  white  is  more  or  less  yellow.  Indeed  variations  in  the 
amount  of  pigment  present  in  the  yellow  spot  have  been  offered 
in  explanation  of  some  of  the  differences  in  colour  vision  dis- 
cussed above. 

§  572.  In  speaking  of  the  relation  between  a  visual  sensa- 
tion and  the  intensity  of  the  stimulus  (§  550)  we  were  confining 
our  remarks  to  white  light ;  when  we  inquire  into  the  behaviour 
of  our  colour  sensations  under  variations  in  the  intensity  of  the 
stimulus,  we  come  upon  results  which  are  in  many  ways  com- 
plicated. We  must  be  content  with  pointing  out  one  or  two 
only  of  these. 

Each  of  our  colour  sensations,  when  the  light  giving  rise  to 

58 


914  COLOUR  VISION.  [Book  in. 

it  reaches  a  certain  intensity,  ceases  to  be  a  colour  sensation 
and  becomes  a  sensation  of  white.  The  theory  of  three  primary 
colour  sensations  may  be  used  to  explain  this.  Thus,  taking 
violet  as  a  primary  sensation,  a  violet  light  of  moderate  intensity 
appears  violet  because  it  excites  the  primary  sensation  of  violet 
much  more  than  those  of  green  and  red.  If  the  stimulus  be 
increased  the  maximum  of  violet  stimulation  will  be  reached, 
while  the  stimulation  of  green  will  continue  to  be  increased  and 
even  that  of  red  to  a  slight  degree.  The  result  will  be  that  the 
light  appears  violet  mixed  with  green,  that  is  to  say,  appears 
blue.  If  the  stimulus  be  still  further  increased  while  the  green 
and  violet  are  both  still  largely  excited  the  red  stimulation  may 
be  increased  until  the  result  is  violet,  green,  and  red  in  the  pro- 
portions which  make  white  light.  And  so  with  light  of  other 
colours.  But  the  same  facts  may  also  be  explained  on  Hering's 
theory,  for  this  supposes  that  the  stock,  so  to  speak,  of  white- 
black  substance  is  far  greater  than  that  of  either  of  the  other 
two  visual  substances ;  hence  under  violent  stimulation  the 
white  sensation  wholly  overpowers  any  accompanying  colour 
sensation. 

Conversely  when  the  intensity  of  the  stimulus  is  diminished, 
colour  sensations  may  disappear  before  all  sensation  of  light  is 
lost.  When  the  light  is  very  dim  we  cease  to  recognize  the 
colour  of  coloured  objects  though  we  continue  to  see  the  objects. 
And  this  is  not  merely  because  the  white  light  reflected  from 
the  object  (and  it  is  through  this  that  we  chiefly  become  aware 
of  the  form  of  an  object)  is  more  powerful  than  the  particular 
rays  which  give  the  object  colour;  since  even  a  saturated  colour 
behaves  in  the  same  way.  If  with  a  feeble  illumination  we  allow 
a  very  small  part  of  the  spectrum  to  fall  on  the  retina,  we  are 
much  more  distinctly  conscious  of  a  sensation  of  light  than  of 
any  particular  colour  sensation ;  indeed  the  minimum  sensation 
thus  felt  has  been  called  a  'grey'  for  all  jiarts  of  the  spectrum. 
Moreover  the  colour  which  is  first  recognized  upon  gradually 
increasing  the  illumination,  appears  less  saturated,  that  is  to  say 
apparently  more  mixed  with  white  than  when  a  large  amount  of 
light  of  the  same  refrangibility  falls  on  the  retina ;  and  such 
distinct  colour  sensation  as  may  be  felt  at  the  first  moment  of 
looking  at  such  a  light  soon  diminishes,  giving  way  to  a  mere 
sensation  of  light. 

When  we  attempt  to  compare  one  colour  sensation  with 
another  in  reference  to  their  behaviour  towards  variations  in  the 
intensity  of  the  stimulus  we  find  the  results  to  a  certain  extent 
conflicting.  When  we  diminish  the  intensity  of  the  stimulus  by 
diminishing  general  illumination,  when  we  look  for  instance  at 
objects  in  nature  under  light  of  varing  intensity,  we  find  that 
the  colours  change  unequally  as  the  light  diminishes ;  as  is  well 
known  the  colours  of  flowers  look  very  different  when  night  is 


Chap,  hi.]  SIGHT.  915 

falling  from  what  they  do  under  bright  daylight.  In  particular 
we  find  that  as  the  light  diminishes  red  sensations  and  also  yel- 
low sensations  disappear  earlier  than  blue  sensations.  Hence  in 
dim  lights,  as  those  of  evening  and  moonlight,  blues  preponder- 
ate, reds  and  yellows  being  less  obvious,  whereas  in  bright  lights 
yellows  and  reds  become  prominent. 

On  the  other  hand,  if  we  test  our  sensitiveness  to  different 
colours  in  a  different  way  we  get  results  which  are  opposed  to 
the  above.  If  for  instance  we  determine  the  distance  at  which 
we  cease  to  recognize  the  colour  of  a  piece  of  coloured  paper,  say 
1  cm.  square,  we  find  that  the  blue  goes  first,  then  green  and 
next  yellow,  red  being  recognizable  at  the  longest  distance, 
though  the  difference  between  red  and  yellow  is  not  very  great. 
It  will  be  understood  of  course  that  in  this  experiment  we  are 
dealing  not  only  with  diminished  energy,  with  diminished  ampli- 
tude of  the  luminous  waves,  but  also  with  a  diminished  area  of 
retinal  stimulation. 

Or  again,  if  we  take  the  heating  effects  of  rays  of  different 
wave-lengths  as  a  measure  of  their  energy,  we  may  determine 
the  amount  of  energy  needed,  in  the  case  of  the  several  colours, 
to  produce  a  given  visual  effect.  When  this  is  done  it  is  found 
that  the  rays  in  the  green,  about  wave-length  A,  530,  are  the  most 
effective ;  from  this  part  of  the  spectrum  the  efficiency  declines 
both  towards  the  violet  and  the  red. 

The  three  several  methods  lead  to  three  different  results, 
the  one  teaches  that  blue,  the  other  that  red  or  j^ellow,  and  the 
third  that  green  is  the  colour  to  which  the  eye  is  most  sensitive. 
It  would  be  hazardous  to  found  important  conclusions  on  any 
of  them. 

There  are  several  other  facts  of  considerable  importance 
bearing  on  the  theory  of  colour  vision,  but  it  will  be  best  to 
consider  these  in  connection  with  certain  modifications  of  visual 
sensations  with  which  we  shall  have  presently  to  deal.  Mean- 
while having  acquired  some  general  notions  of  visual  sensa- 
tions, we  may  turn  from  the  study  of  the  little  we  know 
concerning  the  way  in  which  these  sensations  originate  through 
retinal  changes,  to  the  study  of  the  way  in  which  light  falling 
on  the  retina  gives  rise  to  visual  impulses. 


SEC.  7.    ON  THE  DEVELOPMENT  OF  VISUAL  IMPULSES. 

§  573.  We  have  already  called  attention  to  the  important 
fact  that  the  changes  which  give  rise  to  visual  impulses  begin 
on  the  outer  side  of  the  retina,  that  the  rays  of  light  pass 
through  the  inner  layers  of  the  retina  without,  as  far  as  we 
know,  producing  any  effect,  and  do  not  begin  their  work  until 
they  reach  the  region  of  the  rods  and  cones.  It  is  in  this 
region  that  the  energy  of  light  is  transformed  into  energy  of 
another  kind ;  and  the  processes  here  started  travel  back  to  the 
layer  of  fibres  in  the  inner  surface  of  the  retina  and  thence  pass 
as  visual  impulses  along  the  optic  nerve.  That  on  the  one 
hand  the  optic  fibres  themselves  are  insensible  to  light  and  that 
on  the  other  hand  visual  impulses  do  begin  in  the  region  of  the 
rods  and  cones  is  shewn  by  the  phenomena  of  the  blind  spot 
and  of  Purkinje's  figures  respectively. 

The  Blind  Spot.  There  is  one  part  of  the  retina  on  which 
rays  of  light  falling  give  rise  to  no  sensations ;  this  is  the  en- 
trance of  the  optic  nerve,  and  the  corresponding  area  in  the 
field  of  vision  is  called  the  blind  spot.  If  the  visual  axis  of  one 
eye,  the  right  for  instance,  the  other  being  closed,  be  fixed  on 
a  black  spot  in  a  white  sheet  of  paper,  and  a  small  black  object, 
such  as  the  point  of  a  quill  pen  dipped  in  ink,  be  moved  gradu- 
ally from  the  black  spot  sideways  over  the  paper  away  towards 
the  outside  of  the  field  of  vision,  at  a  certain  distance  the  black 
point  of  the  quill  will  disappear  from  view.  On  continuing 
the  movement  still  farther  outward  the  point  will  again  come 
into  view  and  continue  in  sight  until  it  is  lost  in  the  periphery 
of  the  field  of  vision.  If  the  pen  be  used  to  make  a  mark  on 
the  paper  at  the  moment  when  it  is  lost  to  view  and  at  the 
moment  when  it  comes  into  sight  again,  and  if  similar  marks  be 
made  along  the  other  meridians  as  well  as  the  horizontal,  an 
irregular  outline  will  be  drawn  circumscribing  an  area  of  the 
field  of  vision  within  which  rays  of  light  produce  no  visual 
sensations.  This  is  the  blind  spot.  The  dimensions  of  the  fig- 
ure drawn  vary  of  course  with  the  distance  of  the  paper  from 
the  eye.  If  this  distance  be  known,  the  size  as  well  as  the 
position  of  the  area  of  the  retina  corresponding  to  the  blind  spot 

916 


Chap,  hi.]  SIGHT.  917 

may  be  calculated  from  the  cliagramraatic  eye  (§  527).  The 
position  thus  determined  coincides  exactly  with  the  entrance  of 
the  optic  nerve,  and  the  dimensions  (about  1'5  mm.  diameter)  also 
correspond;  the  exact  size  and  shape  of  the  blind  spot  differs 
however  in  different  individuals.  While  drawing  the  outline 
as  above  directed  the  indications  of  the  large  brandies  of  the 
retinal  vessels  as  they  diverge  from  the  entrance  of  the  nerve 
can  frequently  be  recognized.  The  existence  of  the  blind  spot 
is  also  shewn  by  the  fact  that  an  image  of  light,  sufficiently 
small,  thrown  upon  the  optic  nerve  by  means  of  the  ophthal- 
moscope, gives  rise  to  no  sensations. 

The  existence  of  the  blind  spot  proves  that  the  optic  fibres 
themselves  are  insensible  to  light,  that  light  can  stimulate  them 
only  through  the  agency  of  the  retinal  structures  in  which  tliey 
end. 

§  574.  PurJci7ijS\s  Figures.  If  one  enters  a  dark  room  with 
a  candle  and  while  looking  at  a  plain  (not  parti-coloured)  wall, 
moves  the  candle  up  and  down,  holding  it  on  a  level  with  the 
eyes  by  the  side  of  the  head,  there  will  appear  in  the  field  of 
vision  of  the  eye  of  the  same  side,  projected  on  the  wall,  an 
image  of  the  retinal  vessels,  similar  to  that  seen  on  looking  into 
an  eye  with  the  ophthalmoscope.  The  field  of  vision  is  illumi- 
nated with  a  glare,  and  on  this  the  branched  retinal  vessels 
appear  as  shadows.  In  this  mode  of  experimenting  the  light 
enters  the  eye  through  the  cornea,  and  an  image  of  the  candle 
is  formed  on  the  nasal  side  of  the  retina;  it  is  the  light  emanat- 
ing from  this  image  which  throws  shadows  of  the  retinal  vessels 
on  to  the  rest  of  the  retina.  In  Fig.  153  the  light  a  forms  an 
image  on  the  retina  at  b;  the  light  reflected  from  this  spot  casts 
a  shadow  of  the  retinal  vessel  v  on  to  another  part  of  the  retina 
at  c,  and  the  image  of  this  shadow  appears  in  the  field  of  vision 
at  d.  A  far  better  method  is  for  a  second  person  to  concentrate 
the  rays  of  light,  Avith  a  lens  of  low  power,  on  to  the  outside 
of  the  sclerotic  Avhere  this  is  thin  just  behind  the  cornea;  the 
light  in  this  case  emanates  from  the  illuminated  spot  on  the 
sclerotic  and  passing  straight  through  the  vitreous  humour  throws 
a  direct  shadow  of  the  vessels  on  to  the  retina.  Thus  the  rays 
passing  througli  the  sclerotic  at  l\  Fig.  152,  in  the  direction  bu, 
will  throw  a  shadow  of  the  vessel  v  on  to  the  retina  at  /3 ;  this 
will  appear  as  a  dark  line  at  B  in  the  glare  of  the  field  of  vision. 
This  proves  that  the  structures  in  which  visual  impulses  origi- 
nate must  lie  behind  the  retinal  vessels,  otherwise  the  shadows 
of  these  could  not  be  perceived. 

If  the  light  be  moved  from  b  to  a,  the  shadow  on  the  retina 
will  move  from  /3  to  a,  and  the  dark  line  in  the  field  of  vision 
will  move  from  B  to  A.  If  the  distance  BA  be  measured  when 
the  whole  image  is  projected  at  a  known  distance,  A-B  from  the 
e5^e,  k  being  the  nodal  point  (§  527)  of  the  reduced  diagram- 


918 


PURKINJ]fi'S  FIGURES. 


[Book  hi. 


matic  eye,  then,  knowing  the  distance  k^  in  the  diagrammatic 
eye,  the  distance  ^a  can  be  calculated.  But  if  the  distance  /3a 
be  thus  estimated,  and  the  distance  ha  be  directly  measured,  the 
distances  ^v,  av,  hv,  av  can  be  calculated ;  and  if  the  appearance 
in  the  field  of  vision  is  really  caused  by  the  shadow  of  v  falling 
on  /S,  these  distances  ought  to  correspond  to  the  distances  of  the 
retinal  vessels  v  from  the  sclerotic  h  on  the  one  hand  and  from 
that  part  of  the  retina  /3  where  visual  impressions  begin,  on  the 


B    A 


X  /J 


Fig.  152.    Diagram   illustrating   the  Formation  op  Purkinje's  Figures 
WHEN  THE  Illumination  is  directed  through  the  Sclerotic. 


other.  When  this  is  done  it  is  found  that  the  distance  /3y  thus 
calculated  corresponds  fairly  well  to  the  distance  of  the  retinal 
vessels  from  the  layer  of  rods  and  cones.  Thus  Purkinje's  fig- 
ures prove  in  the  first  place  that  the  sensory  impulses  which 
form  the  commencement  of  visual  sensations  originate  in  some 
part  of  the  retina  behind  the  retinal  vessels,  i.e.  somewhere 
between  them  and  the  choroid  coat;  and  calculations  based  on 
the  movements  of  the  shadows  following  movements  of  the  illu- 
mination, even  if  they  do  not  give  absolutely  exact  results,  at 
least  go  far  to  shew  that  these  impulses  originate  at  the  outer- 
most part  of  the  retina,  viz.  the  layer  of  rods  and  cones. 

In  the  second  method  of  experimenting,  where  the  light  passes 
through  the  sclerotic,  the  image  always  moves  in  the  same  direc- 
tion as  the  light,  as  it  obviously  must  do,  when  the  spot  of  light 
on  the  sclerotic  is  moved  from  a  to  h  (Fig.  152)  the  shadow  on 
the  retina  moves  from  a  to  /3,  and  the  (inverted)  image  moves 
from  A  to  B.  In  the  first  method,  where  the  light  enters  through 
the  cornea,  the  image  moves  in  the  same  direction  as  the  light 
when  the  light  is  moved  from  side  to  side,  provided  the  move- 
ment does  not  extend  beyond  the  middle  of  the  cornea,  but  in 


Chap,  hi.] 


SIGHT. 


919 


the  opposite  direction  to  the  light  when  the  latter  is  moved  up 
and  down.  In  Fig.  153,  which  represents  a  horizontal  section  of 
an  eye,  if  a  be  moved  to  a,  h  (the  illuminated  spot  on  the  retina, 
the  light  reflected  from  which  casts  a  shadow  of  v  on  to  c)  will 
move  to  /3,  the  shadow  on  the  retina  c  to  7,  and  the  image  d  to  8. 
If  on  the  other  hand  a  be  supposed  to  move  above  the  plane  of 
the  paper,  h  will  move  below,  in  consequence  c  will  move  above, 
and  d  will  appear  to  move  below,  i.e.  d  will  sink  as  a  rises. 


Fig.   153.     Diagram  illustrating   the   Formation  of  Purkinjk's  Figures 
WHEN  THE  Illumination  is  directed  through  the  Cornea. 

It  is  desirable  in  these  cases  to  keep  moving  the  light  to  and 
fro,  especially  in  the  first  method,  since  the  retina  soon  becomes 
tired,  and  the  image  fades  away.  To  give  rise  to  a  conscious 
sensation  of  the  slight  difference  between  shadow  and  absence 
of  shadow  the  retina  must  be  extremely  sensitive  ;  if  the  shadow 
remains  motionless,  the  sensitiveness  rapidly  decreases  in  the 
parts  which  are  not  in  shadow,  until  the  visual  sensations  from 
these  parts  are  no  stronger  than  those  from  the  parts  in  shadow  ; 
when  the  light  is  moved  the  parts  which  were  in  shadow,  not 
having  been  so  much  stimulated,  are  sufficiently  sensitive  to 
the  light  which  now  falls  on  them,  while  those  parts  which  had 
been  previously  fatigued  recover  their  sensitiveness  by  resting 
in  the  shadow.  The  experiment,  like  the  experiment  by  which 
the  yellow  spot  (§  571)  is  made  visible,  is  incidentally  useful 
as  shewing  how  extremely  sensitive  and  how  soon  fatigued  are 
the  retinal  structures. 

Some  observers  can  recognize  in  the  axis  of  vision  a  faint 
shadow  corresponding  to  the  edge  of  the  depression  of  the  fovea 
centralis. 

The  retinal  vessels  may  also  be  rendered  visible  by  looking 
through  a  small  orifice  such  as  a  pin-hole  in  a  card  placed  close 
to  the  eye,  in  the  position  of  the  principal  anterior  focus,  at  a 
bright  surface  such  as  a  white  cloud,  and  moving  the  orifice 
very  rapidly  from  side  to  side  or  up  and  down.  If  the  move- 
ment be  from  side  to  side,  the  vessels  which  run  vertical  will  be 
seen ;  if  up  and  down,  the  horizontal  vessels.     In  this  case,  as  in 


920  OEIGIN  OF  VISUAL  IMPULSES.         [Book  hi. 

the  similar  instance  of  shadows  cast  by  objects  in  the  vitreous 
humour  (§  549),  the  shadow  is  cast  by  the  rays  passing  parallel 
through  the  vitreous  humour;  hence  the  change  from  shadow  to 
absence  of  shadow  is  more  marked  with  the  vertical  vessels 
when  the  movement  is  sideways  and  with  the  horizontal  vessels 
when  it  is  up  and  down.  The  fine  capillary  vessels  are  seen 
more  easily  in  this  way  than  by  Purkinje's  method.  The  same 
appearances  may  also  be  produced  by  looking  through  a  micro- 
scope from  which  the  objective  has  been  removed  and  the 
eye-piece  only  left  (or  in  which  at  least  there  is  no  object 
distinctly  in  focus  in  the  field),  and  moving  the  head  rapidly 
from  side  to  side  or  backwards  and  forwards.  Or  the  micro- 
scope itself  may  be  moved ;  a  circular  movement  of  the  field 
will  then  bring  both  the  vertically  and  horizontally  directed 
vessels  into  view  at  the  same  time. 

§  575.  It  being  admitted  that  the  processes  which  give  rise 
to  visual  impulses  begin  somewhere  in  the  region  of  the  rods 
and  cones,  we  have  to  ask  the  question.  How  do  they  begin  and 
what  is  their  nature  ?  We  are  accustomed  to  consider  light 
as  the  undulations  of  an  ether ;  a  nervous  impulse  is,  so  far  as 
we  can  understand,  a  molecular  change  propagated  along  the 
substance  of  the  axis  cylinder  of  a  nerve  fibre ;  and,  though  as 
we  have  seen  our  knowledge  of  the  subject  is  very  limited,  still 
the  analogy  of  a  muscular  contraction,  and  of  other  responses  of 
living  substance  to  a  stimulus,  lead  us  to  conclude  that  chemical 
changes  play  a  part  in  this  molecular  change.  By  what  steps 
does  the  undulation  of  the  ether  give  rise  to  the  material 
molecular  change?  In  attempting  to  answer  this  question  we 
may  adopt  one  or  other  of  two  views. 

On  the  one  hand  we  may  suppose  that  the  vibrations  of  the 
ether  are  able,  through  the  means  of  the  retinal  apparatus  of  the 
rods  and  cones  for  example,  to  give  rise  in  some  more  or  less 
direct  manner  to  the  molecular  vibrations  which  are  the  begin- 
nings of  the  nervous  impulses  in  the  optic  nerve.  And  the  rapid- 
ity with  which  events  must  come  and  go  in  the  retina  in  order 
that  the  eye  may  be,  what  it  is,  an  instrument  for  appreciating 
rapidly  repeated  minute  changes,  lends  support  to  this  view. 
But  the  present  state  of  our  knowledge  of  physical  phenomena 
does  not  afford  us  an  adequate  explanation  of  how  such  a  direct 
transformation  can  be  effected.  The  recent  progress  of  science 
tends,  it  is  true,  more  and  more  to  lay  bare  the  close  relations 
which  obtain  between  optical  and  electric  phenomena,  and  the 
latter,  as  we  have  so  often  seen,  play  an  important  part  in  the 
generation  of  nervous  impulses.  Then  again  many  of  the  phe- 
nomena of  fluorescence  seem  to  supply  a  bridge  between  the 
vibrations  of  ether,  and  the  vibrations  of  molecules.  But  in 
neither  of  these  directions  is  it  possible,  at  present  at  all  events, 
to  frame  a  hypothesis  which  can  be  satisfactorily  applied  to 
retinal  processes. 


Chap,  hi.]  SIGHT.  921 

On  tlie  other  hand  we  may  perhaps  more  naturally  turn  to  a 
chemical  explanation.  We  are  familiar  with  the  fact  that  rays  of 
light  are  able  to  bring  about  the  decomposition  of  very  many 
chemical  substances ;  and  we  accordingly  speak  of  these  sul> 
stances  as  being  sensitive  to  light.  All  the  facts  dwelt  on  in 
this  book  illustrate  the  great  complexity  and  corresponding 
instability  of  the  composition  of  living  matter.  And  we  might 
reasonably  suppose  that  living  matter  itself  would  be  sensitive 
to  light;  that  is  to  say  that  rays  of  light  falling  on  even  undif- 
ferentiated protoplasmic  substance  might  set  up  a  decomposition 
of  that  substance  and  so  bring  about  a  molecular  disturbance; 
in  other  words,  that  light  might  act  as  a  direct  stimulus  to  living 
matter.  As  a  matter  of  fact,  however,  we  meet  with  very  little 
evidence  of  this,  especially  when  we  make  a  distinction  between 
thermic  rays,  rays  which  though  they  produce  ph^'sical  results 
are  to  us  invisible,  and  luminous  ra3's  which  alone  when  they 
fall  on  our  retina  give  rise  in  us  to  the  sensation  of  light.  Nor 
can  we  be  surprised  at  this  apparent  indifference  of  living  matter 
towards  light  when  we  reflect  that  living  matter  in  what  we  may 
call  its  purest  form  is  remarkable  for  its  transparency,  that  is  to 
say  the  rays  of  light  pass  through  it  with  exceedingly  little  ab- 
sorption. But  in  order  that  light  may  produce  chemical  effects, 
it  must  be  absorbed;  its  energy  must  be  spent  in  doing  tlie  chemi- 
cal Avork.  Accordingly  the  first  step  towards  the  formation  of 
an  organ  of  vision,  that  is  to  say  an  organ  through  which  the 
body  of  a  living  being  reacts  towards  light,  is  the  differentiation 
of  a  portion  of  the  substance  of  the  body  into  a  pigment  at  once 
capable  of  absorbing  light,  and  sensitive  to  light,  i.e.  undergoing 
decomposition  upon  exposure  to  light.  An  organism,  a  portion 
of  whose  body  had  thus  become  differentiated  into  sucli  a  pig- 
ment, would  be  able  to  react  towards  light.  The  light  falling 
on  the  organism  would  be  in  part  absorbed  by  the  pigment,  and 
the  rays  thus  absorbed  would  produce  a  chemical  action  and  set 
free  chemical  substances  which  before  Avere  not  present.  We 
liave  only  to  suppose  that  the  chemical  substances  thus  produced 
are  of  such  a  nature  as  to  induce  other  chemical  changes,  or  in 
some  way  or  other  to  act  as  a  stimulus  to  other  parts  of  the  or- 
ganism, (and  Ave  have  manifold  evidence  of  the  exquisite  sensi- 
tiveness of  living  matter  in  general  to  chemical  stimuli,)  in  order 
to  see  hoAV  rays  of  light  falling  on  the  organism  might  excite 
movements  in  it,  or  modify  movements  A\'hich  were  being  carried 
on,  or  might  otherwise  affect  tlie  organism  in  Avhole  or  in  part. 
A  comparatively  simple  illustration  of  this  is  afforded  by  some 
of  the  loAvly  organisms  called  bacteria,  especially  by  the  one 
which  has  been  called  bacterium  photometricum.  This  organism 
is  remarkably  sensitive  to  light,  and  especially  reacts  towards 
certain  rays  of  light.  It  is  coloured  Avith  a  purple  pigment, 
apparently  allied  to  chlorophyll;  and  the  rays  of  light,  to  Avhich 


922  PHOTOCHEMISTRY   OF  THE   EETINA.     [Book  in. 

it  is  especially  sensitive,  are  just  those  which  are  absorbed  by 
the  pigment. 

§  576.  Photochemistry  of  the  Retina.  Such  considerations 
as  the  foregoing  may  be  ap]3lied  to  even  the  complex  organ  of 
vision  of  the  higher  animals.  If  we  suppose  that  the  actual 
terminations  of  the  optic  nerve  are  surrounded  by  substances 
sensitive  to  light,  then  it  becomes  easy  to  imagine  how  light 
falling  on  these  sensitive  substances  should  set  free  chemical 
bodies  possessed  of  the  property  of  acting  as  stimuli  to  the  actual 
nerve-endings  and  thus  give  rise  to  visual  impulses  in  the  optic 
fibres.  We  say  "  easy  to  imagine,"  but  we  are,  at  present,  far 
from  being  able  to  give  definite  proofs  that  such  an  explanation 
of  the  origin  of  visual  impulses  is  the  true  one,  probable  and 
enticing  as  it  may  appear.  And  it  must  be  remembered  that  in 
such  chemical  changes  electrical  events  may  intervene  and  that 
in  a  special  way. 

One  of  the  most  striking  features  in  the  structure  of  the 
retina  is  the  abundance  of  black  pigment,  fuscin,  in  the  retinal 
epithelium.  It  is  difficult  to  suppose  that  the  sole  function  of 
this  pigment  is  to  absorb  the  superfluous  rays  of  light,  and  that 
the  rays  thus  absorbed  are  put  to  no  use  and  simply  wasted. 
And  indeed  it  has  been  shewn  that  the  pigment  is  sensitive  to 
light ;  but  the  changes  in  it  induced  by  light  are  excessively 
slow.  Moreover  its  presence  cannot  be  of  fundamental  impor- 
tance, since  vision  is  not  only  possible  but  fairly  distinct  with 
albinos  in  which  this  pigment  is  absent. 

Then  again,  in  the  vast  majority  of  vertebrate  animals,  the 
outer  limbs  of  the  rods  are  suffused  with  a  purplish-red  pigment, 
the  so-called  visual  purple,  which  is  so  eminently  sensitive  to 
light  that  images  of  external  objects  may  by  appropriate  means 
be  photographed  in  it  on  the  retina.  When  the  eye  of  a  frog  or 
of  a  rabbit  is  examined  in  an  ordinary  way,  with  full  exposure 
to  light,  the  retina  appears  colourless.  But  if  the  eye  be  kept 
in  the  dark  for  some  time  before  it  is  examined,  the  retina,  if 
removed  rapidly,  will  be  found  to  be  of  a  beautiful  purplish-red 
or  pink  colour.  Upon  exposure  to  light  the  colour  changes  to 
yellow  and  then  fades  away,  leaving  however  the  retina,  not 
only  white  but  more  opaque  than  it  was  before.  Upon  exami- 
nation with  the  microscope  it  is  found  that  the  purple  colour  is 
confined  exclusively  to  the  rods  and  to  the  outer  limbs  of  the 
rods,  the  inner  limbs  being  wholly  devoid  of  it. 

The  colour  of  the  rods  is  due  to  the  presence  of  a  distinct 
pigment,  the  "  visual  purple,"  diffused  through  the  substance  of 
the  outer  limbs  ;  and  this  may  be  extracted  from  the  rods  by  dis- 
solving these  in  an  aqueous  solution  of  bile  salts.  A  clear  purple 
solution  is  thus  obtained,  which  is  capable  of  being  bleached  by 
the  action  of  light,  and  in  its  general  features  and  behaviour  is 
similar  to  the  pigment  as  it  naturally  exists  in  the  retina. 


Chap,  hi.]  SIGHT.  923 

Visual  purple  is  found  as  we  have  said  exclusively  in  the 
outer  limbs  of  the  rods ;  it  has  never  yet  been  found  in  the 
cones,  and  it  is  accordingly  absent  from  (or  exceedingly  scanty 
in)  the  retinas  (such  as  those  of  snakes)  which  are  composed  of 
cones  only  (or  contain  very  few  rods),  and  from  the  greater 
part  of  the  macula  lutea  and  the  whole  of  the  fovea  centralis  of 
the  retinas  of  man  and  the  ape.  The  intensity  of  the  coloration 
varies  in  different  animals,  and  the  retinas  even  of  some  animals 
possessing  rods  (bat,  dove,  hen)  seem  to  be  wholly  devoid  of  the 
visual  purple ;  it  is  generally  well  marked  in  retinas  in  which 
the  outer  limbs  of  the  rods  are  well  developed.  Its  absence  or 
presence  is  not  dependent  on  nocturnal  habits,  since  the  intense 
colour  of  the  retina  of  the  owl  is  in  strong  contrast  to  the 
absence  of  colour  in  the  bat.  It  has  been  found  in  the  retina 
of  the  embryo. 

The  visual  purple  is  bleached  not  onl}^  by  white  but  also  by 
monochromatic  light.  Of  the  various  prismatic  rays  the  most 
active  are  the  greenish-yellow  rays,  those  to  the  blue  side  of 
these  coming  next,  the  least  active  being  the  red.  Now  it  is 
precisely  the  greenish-yellow  rays  which  are  most  readily  ab- 
sorbed by  the  colour  itself.  A  natural  colour  retina  or  a  solu- 
tion of  visual  purple  gives  a  diffuse  spectrum  without  any 
defined  absorption  bands,  and  according  to  the  amount  of  colour- 
ing material  through  which  the  light  passes,  absorption  is  seen 
either  to  be  limited  to  the  greenish-yellow  part  of  the  spectrum 
or  to  spread  thence  towards  the  blue  and,  to  a  much  less  extent, 
towards  the  red.  Thus  the  various  prismatic  rdjs  produce  a 
photochemical  effect  on  the  visual  purple  in  proportion  as  they 
are  absorbed  by  it.  Under  the  action  of  light  the  visual  purple, 
whether  in  solution,  or  in  its  natural  condition  in  the  rods,  passes 
through  a  purplish  orange  to  a  yellow,  and  finally  becomes  colour- 
less ;  and  we  appear  to  be  justified  in  speaking  of  a  "visual  yel- 
low "  and  "  visual  white "  as  products  of  the  photochemical 
changes  undergone  by  the  visual  purple. 

For  the  restoration  of  the  visual  purple,  after  it  has  been 
destroyed  by  light,  the  maintenance  of  the  circulation  of  the 
blood  through  the  tissues  of  the  eye  is  not  essential.  The  reti- 
nal epithelium  has  by  itself,  provided  that  it  still  retains  its  tis- 
sue life,  the  power  of  regenerating  the  purple.  If  a  portion  of 
the  retina  of  an  excised  eye  be  raised  from  its  epithelial  bed, 
bleached,  and  then  carefully  restored  to  its  natural  position,  the 
purple  will  I'eturn  if  the  eye  be  kept  in  the  dark. 

If  the  image  of  some  bright  object  such  as  a  lamp  or  a  win- 
dow be  thrown  on  to  the  retina,  either  of  an  eye  in  its  natural 
position  or  of  one  recently  excised,  care  having  been  taken  to 
keep  tlie  retina  for  some  time  previous  away  from  all  rays  of 
light,  the  portion  of  the  retina  on  which  the  rays  have  fallen 
will  be  found  to  be  bleached,  the  rest  of  the  retina  remaining 


924  PHOTOCHEMISTRY   OF   THE   RETINA.     [Book  hi. 

purple.  In  fact  an  "  optogram  "  of  external  objects  may  thus 
be  obtained;  and  if  the  retina  be  removed  and  treated  with  a 
4  p.c.  solution  of  potash  alum  before  the  retinal  epithelium  has 
had  time  to  obliterate  the  bleaching  effects,  the  retina  may- 
remain  permanently  in  that  condition :  the  photochemical  effect 
may,  as  the  photographers  say,  be  "fixed." 

It  seemed  very  tempting,  especially  upon  the  first  discovery 
of  it,  to  suppose  that  this  visual  purple  is  directly  concerned  in 
vision.  If  we  suppose  that  visual  purple  itself  is  inert  towards, 
produces  no  effect  on,  the  endings  of  the  optic  nerve,  but  that 
either  visual  yellow  or  visual  white,  i.e.  some  product  of  the 
action  of  light  on  visual  purple,  may  act  as  a  stimulus  to  those 
endings,  the  way  seems  opened  to  understanding  how  rays  of 
light  can  give  rise  to  sensory  impulses  in  the  optic  nerve.  And 
such  a  view  receives  incidental  support  from  the  fact  that  the 
visual  efficiency  of  rays  of  different  wave-lengths  corresponds 
very  closely  to  their  photochemical  efficiency  towards  visual 
purple ;  the  greenish-yellow  rays  which  are  most  active  towards 
visual  purple  are  precisely  those  which  seem  to  us  the  brightest, 
most  luminous,  which  produce  the  greatest  effect  on  our  con- 
sciousness. But  visual  purple  is  absent  from  the  cones,  it  is  in 
ourselves  absent  from  the  fovea  centralis,  the  region  of  most 
distinct  vision ;  it  is  further  entirely  wanting  in  some  animals 
which  undoubtedly  see  very  well ;  and  lastly  animals  such  as 
frogs,  naturally  possessing  the  pigment,  continue  to  see  very 
well  and  even  apparently  to  see  colours  when  their  visual  pur- 
ple has  been  absolutely  bleached,  as  it  may  be  by  prolonged 
exposure  of  the  eyes  to  strong  light.  We  cannot  therefore,  at 
present  at  least,  explain  the  origin  of  visual  impulses  by  the  help 
of  visual  purple.  It  is  difficult  to  suppose  that  it  plays  no  part 
in  the  origination  of  visual  impulses ;  but  even  in  a  photochemi- 
cal theory  of  vision  we  cannot  allot  to  it  more  than  a  subsidiary 
function,  possibly  something  analogous  to  the  "sensitizer"  of 
the  photographer.  At  the  same  time  its  history  suggests  that 
some  substances,  sensitive  like  it  to  light,  but  unlike  it,  colour- 
less and  therefore  escaping  observation,  may  exist,  and  by  photo- 
chemical changes  be  the  means  of  exciting  the  optic  nerves ; 
but  if  so  we  must  suppose  that  these  substances,  though  colour- 
less, are  capable  of  absorbing  light,  since  otherwise  they  would 
not  be  acted  upon  by  it.  Apart  from  their  providing  visual 
purple  the  cells  of  the  retinal  epithelium,  with  their  remarkable 
amoeboid  pigment-carrying  filamentous  processes,  have  probably 
in  other  wa,js  to  do  with  vision,  though  we  cannot  at  present 
state  what  their  exact  function  in  this  respect  is.  Their  impor- 
tance in  vision  is  indicated  by  their  behaviour  towards  light. 

If  an  eye  be  fully  exposed  to  light  before  removal  and  ex- 
amination, the  processes  carrying  pigment  are  found  to  stretch 
a  long  way  inwards  between  the  outer  limbs  of  the  rods  and 


Chap,  hi.]  SIGHT.  925 

cones,  investing  these  outer  limbs  with  a  sheath  of  pigment,  and 
even  reaching  between  the  inner  limbs.  If  on  the  contrary  the 
eye  be  kept  in  the  dark  before  removal  and  examination,  the 
processes  are  found  to  be  short  and  to  stretch  a  little  way  only 
inwards,  not  reaching  much  farther  than  the  tops  of  tlie  outer 
limbs  of  the  rods  and  cones.  The  substance  of  the  cell  has  in 
fact  the  power  of  amceboid  movement,  at  one  time  throwing  out 
long  filamentous  processes  inwards  between  the  rods  and  cones, 
and  at  another  time  retracting  the  processes  into  the  body  of  the 
cell.  As  they  move  to  and  fro  these  processes  carry  with  them 
the  crystals  of  pigment  with  which  the}^  are  studded;  hence  in 
the  extended  condition  much  of  the  pigment  is  carried  away 
from  the  body  of  the  cell  inwards  between  the  rods  and  cones, 
leaving  the  nucleus  less  covered  with  pigment,  while  in  the 
retracted  condition  the  pigment  is  carried  back  to  the  body  of 
the  cell  and  the  nucleus  becomes  obscured.  Further,  while  va- 
rious circumstances  may  determine  wdiether  the  processes  are 
extended  or  retracted,  the  falling  of  light  on  the  retina  has  the 
most  marked  and  potent  effect.  When  light  falls  on  the  retina 
the  processes  hurry  inwards  and  envelope  the  outer  limbs  of  the 
rods  and  cones  with  pigment ;  when  the  light  is  shut  off  from 
the  retina  the  processes  carry  back  the  pigment  to  the  body  of 
the  cell. 

Hence  in  an  eye  exposed  to  light  the  processes  and  pigment 
being  largely  jammed  in  between  the  outer  limbs  of  the  rods,  and 
these  outer  limbs  at  the  same  time  swelling,  the  pigment  epithe- 
lium adheres  closely  to  the  retina,  and  wdien  the  retina  is 
removed  is  carried  away  with  it.  In  an  eye  kept  in  the  dark, 
the  processes  being  withdrawn,  and  the  outer  limbs  of  the  rods 
shrinking  again,  the  attachment  of  the  retina  to  the  epithelium 
is  much  less,  and  the  retina  can  be  more  readily  removed  so  as 
to  leave  the  pigment  epithelium  adherent  to  the  choroid. 

Urari  has  an  effect  on  these  cells  of  the  pigment  epithelium 
of  such  a  kind  that  they  cease  to  throw  out  their  processes ; 
they  seem  to  be  paralyzed.  Hence  in  the  eye  of  a  urarized 
animal  the  pigment  epithelium  readily  separates  from  the 
retina. 

We  may  add  that  in  frogs  at  least,  this  shifting  of  the  pig- 
ment may  be  seen  to  be  accompanied  by  a  change  of  form  in  the 
inner  limbs  of  the  cones.  Under  the  influence  of  light  the  inner 
limb  becomes  shorter  and  broader,  in  fact  contracts,  and  when 
the  influence  of  the  light  is  removed  elongates  to  its  original 
length.  Moreover  these  changes  in  the  cones  may  be  induced, 
not  only  by  light  falling  on  the  retina  but  also,  through  a 
mechanism  not  at  present  fully  understood,  as  the  result  of 
stimulation  of  the  skin,  by  light  or  otherwise ;  in  these  latter 
cases  the  change  of  form  of  the  cone  is  not  necessarily  accom- 
panied by  migration  of  the  pigment. 


926  FUNCTIONS   OF   EODS   AND   CONES.     [Book  hi. 

§  577.  Whatever  view  we  adopt,  whether  photochemical 
or  otlier,  as  to  the  changes  which  lead  to  stimulation  of  the  real 
endings  of  the  retinal  nervous  mechanism,  we  cannot  at  present 
state  anything  definite  concerning  those  nerve-endings  or  the 
manner  of  their  stimulation. 

Each  outer  limb  of  a  rod  is  a  cylinder  of  highly  refractive 
material,  closely  packed  round  with  the  black  pigment  of  the 
retinal  epithelium.  When  an  image  of  an  external  object,  such 
as  a  candle-flame,  is  formed  on  the  retina,  at  or  near  the  layer 
of  rods  and  cones,  the  rays  of  light  diverge  again  beyond  the 
focal  plane  in  tlie  form  of  pencils  of  rays  from  each  point  of  the 
image.  Of  these  some  passing  between  the  rods  are  absorbed 
by  the  pigment,  while  others  pass  into  the  outer  limbs  of  the 
rods ;  of  these  latter  some  traversing  the  whole  length  of  tlie 
limb,  are  absorbed  by  the  pigment  beyond,  while  others  undergo 
"  total  reflection  "  at  the  sides,  or  are  absorbed  by  the  pigment 
after  reflection.  Hence  of  all  the  rays  which  fall  on  the  layer 
of  rods  and  cones,  a  small  number  only  are  reflected  back  into 
the  vitreous  humour  and  so  through  the  pupil ;  hence  the  eye 
when  looked  into  usually  looks  black.  In  the  case  of  the  coni- 
cal outer  limbs  of  the  cones  the  amount  of  light  thus  thrown 
back  into  the  vitreous  humour  must  be  still  less.  We  may 
fairly  assume  that  the  light  which  thus  disappears,  partly  in  the 
actual  outer  limbs  of  the  rods  and  cones,  partly  in  their  immedi- 
ate surrounding,  sets  up  changes  which,  whatever  be  tlieir  exact 
nature,  either  are  or  in  some  way  assist  the  very  beginnings  of 
visual  impulses.  It  also  seems  probable  that  these  clianges, 
so  long  as  they  are  confined  to  the  region  of  the  outer  limbs, 
ought  not  to  be  considered  as  nervous  in  nature,  it  seems  prob- 
able that  they  do  not  take  on  a  nature  analogous  to  that  of  a 
nervous  impulse,  until  they  have  passed  the  conspicuous  break 
which  divides  tlie  outer  from  the  inner  limbs.  But  on  these 
matters  we  liave  no  certain  knowledge. 

We  may  here  turn  aside  for  a  moment  to  remark  that  when 
an  image  of  a  candle-flame  is  formed  on  the  retina  the  rays 
reflected  back,  as  stated  above,  from  the  retina  through  the 
pupil  form  a  second  image  in  the  position  of  the  candle-flame  ; 
hence  to  see  an  image  of  an  illuminated  retina  the  observing 
eye  must  be  placed  in  the  position  of  the  source  of  illumination. 
This  is  the  principle  of  the  ophthalmoscope. 

Tliere  are  many  forms  of  this  instrument,  but  the  accom- 
panying diagram  (Fig.  154)  will  illustrate  its  essential  feat- 
ures. The  rays  from  the  lamp  L  (or  other  source  of  illumina- 
tion) are  reflected  by  tlie  concave  mirror  ili,  iff,  and  brought  to 
a  focus  at  a.  The  rays  diverging  from  a  are,  by  means  of  the 
lens  Z,  rendered  parallel,  and  thus,  through  natural  dioptric 
arrangements  of  the  observed  eye  B^  are  brought  to  a  focus  on 
the  retina  at  a'.     The  rays  reflected  back  from  the  part  a'  of 


Chap,  hi  J 


SIGHT. 


927 


the  retina  thus  illuminated,  will,  as  stated  above,  follow  the 
same  path  as  on  entering,  and  so  return  to  the  focus  a.  Hence 
the  rays  reflected  from  a  number  of  i)oints  on  the  retina,  such 
as  those  forming  the  arrow  at  a',  will  be  brought  to  a  focus  in  a 


Fig.  154.     Diagram   to   illustrate   the   Principles   of  a  simple   Form  of 

Ophthalmoscope. 


corresponding  number  of  points  at  «,  i.e.  will  form  an  (inverted) 
image  of  the  arrow  at  a.  And  the  observing  e3'e  placed  at  A 
behind  the  hole  in  the  mirror  will  see  at  a  an  inverted  image 
of  the  illuminated  retina. 

§  578.  As  to  the  meaning  of  the  difference  between  rods 
and  cones  no  satisfactory  statement  can  be  made.  It  has,  it 
is  true,  been  suggested  that  the  cones  subserve  the  vision  of 
colour  and  the  rods  that  of  form  only.  This,  however,  is  in 
flagrant  contradiction  to  both  the  theories  of  colour  vision  dis- 
cussed above.  For  colourless  vision  of  form  is  the  appreciation 
of  differences  in  black  and  white  ;  and  according  to  the  Young- 
Helmholtz  theory,  white  is  simply  a  combination  of  colour  sen- 
sations. Sensations  of  white,  apart  from  colours  ordinarily  so 
called,  are  only  admitted  by  liering's  theor}-,  and  an  extension 
of  this  theory  in  the  direction  that  the  rods  are  connected 
exclusively  with  the  white  and  black  substance,  and  the  cones 
exclusively  with  the  red-green  and  yellow-blue  substances,  lands 
us  at  once  in  absurdity.  Moreover  since  it  is  in  the  fovea 
centralis  that  we  have  the  most  acute  vision  of  both  form  and 
colour,  the  cones  alone  must  be  able  to  serve  as  the  instruments 
of  all  visual  sensations.  The  argument  that  in  nocturnal  ani- 
mals the  rods  are  developed  almost  to  the  exclusion  of  cones, 
because  such  animals  do  not  need  colour  sensations,  is  one 
which  can  be  turned  against  itself,  since  it  may  be  urged  that 


928  FUNCTIONS   OF  EODS   AND   CONES.     [Book  iii. 

the  dim  light  in  which  these  creatures  move  calls  for  increased 
and  not  diminished  appreciation  of  small  differences  of  colour. 
The  coloured  globules  intercalated  between  the  outer  and  inner 
limbs  of  cones  in  some  of  the  lower  animals,  such  as  birds  and 
reptiles,  have  probably  no  closer  relation  to  colour  vision  than 
has  the  yellow  pigment  of  our  own  macula  lutea. 

The  close  resemblance  in  their  general  features,  apart  from 
form,  between  the  rods  and  cones,  suggests  that  their  functions 
differ  in  degree  rather  than  in  kind,  and  this  view  is  supported 
by  the  rod-like  character  assumed  by  the  cones  in  the  macula 
lutea  and  especially  in  the  fovea  centralis.  But  we  can  hardly 
expect  to  be  able  to  differentiate  the  functions  of  the  two,  so 
long  as  we  know  so  little  about  either. 

With  regard  to  what  goes  on  in  the  other  layers  of  the  retina 
our  ignorance  is  complete.  We  may  fairly  suppose  that  the 
events  which  take  place  in  the  inner  limbs  of  the  rods  and 
cones  are  different  from  those  which  take  place  in  the  optic 
fibres.  We  may  conclude  that  the  latter  are  of  the  nature  of 
nervous  impulses,  though  we  ma}^  here  repeat  what  we  have 
already  urged,  namely,  that  it  is  hazardous  to  infer  that  the 
little  we  know  of  motor  nervous  impulses  may  be  applied  with 
little  or  no  modification  to  sensory  nervous  impulses ;  but  as  to 
the  nature  of  the  events  in  the  inner  limbs  of  rod  and  cones,  or 
as  to  what  happens  in  the  intervening  layers  of  the  retina,  we 
know  nothing. 

§  579.  The  little  objective  knowledge  which  we  possess 
concerning  retinal  processes  is  almost  limited  to  the  detection 
of  electric  currents.  The  retina  and  optic  nerve  like  other 
nervous  structures  develope  electric  currents  which  may  be 
spoken  of  as  currents  of  rest  and  currents  of  action.  They 
may  be  shewn  by  placing  one  electrode  on  the  retina  of  a 
bisected  eye,  or  on  the  cornea  of  a  whole  one,  and  the  other  on 
the  optic  nerve,  or  hind  part  of  the  eye-ball  or  on  the  cortical 
visual  centre  or  even  on  some  distant  part  of  the  body.  They 
are  also  manifested  by  the  isolated  retina  itself.  The  phenom- 
ena appear  somewhat  complicated  by  the  appearance  now  of 
positive,  now  of  negative  variations  ;  but  this  fact  comes  out 
clearly  that  the  incidence  of  light  on  the  irritable  retina  devel- 
opes  an  electric  change,  the  magnitude  of  which  is  to  a  certain 
extent  proportionate  to  the  intensity  of  the  light  acting  as  a 
stimulus.  The  changes  gradually  diminish  and  disappear  as 
the  retina  gradually  loses  its  irritability.  We  may  add  that 
these  electric  phenomena  appear  to  be  quite  independent  of  the 
condition  of  the  visual  purple. 


SEC.   8.     ON   SOME  FEATUEES   OF   VISUAL   SENSATIONS 
ESPECIALLY  IN  RELATION  TO  VISUAL  PERCEPTIONS. 

§  580.  In  our  previous  study  of  visual  sensations  we  dealt 
chiefly  with  the  more  simple  and  fundamental  characters  of 
sensations ;  we  considered  each  sensation  by  itself  and  discussed 
its  features  irrespective  of  the  influence  of  other  sensations 
excited  at  the  same  time,  except  so  far  as  it  became  necessarj'", 
in  treating  of  the  localization  of  sensations,  to  speak  of  the  cir- 
cumstances which  determined  the  fusing  of  two  neighbouring 
sensations  into  one.  It  very  rarely  occurs  however  that  any 
object  or  event  in  the  external  world  gives  rise  to  a  simple 
sensation  such  as  those  on  which  we  have  dwelt;  each  part 
of  the  external  world,  each  external  object  such  as  a  tree,  is 
the  source  of  many  distinct  sensations  differing  from  each  other 
in  intensity  and  other  characters.  In  looking  at  a  tree  we  are 
conscious  of  many  sensations  of  different  colours  and  intensities, 
each  having  a  definite  localization ;  but  these  are  coordinated 
in  our  consciousness  into  a  whole  and  we  say  we  "  see  a  tree." 
The  effect  which  the  whole  visible  world  has  upon  us  is  not 
that  of  a  multitude  of  single  sensations  each  separate  from  and 
independent  of  the  other,  but  of  a  smaller  though  still  large 
number  of  groups  of  sensations  corresponding  to  what  we  call 
the  objects  of  nature.  And  we  have  now  to  turn  our  attention 
to  certain  faces  concerning  vision  which  become  especiall}^  prom- 
inent when  we  are  the  subject  not  of  an  isolated  single  visual 
sensation  but  of  complex  groups  of  simultaneous  visual  sensa- 
tions. The  sum  of  visual  sensations  and  groups  of  sensations 
which  are  excited  by  images  falling  on  the  retina  at  any  one 
time,  we  call,  as  we  have  already  said  (§  494),  the  '  field  of 
vision,'  or  '  visual  field.' 

§  581.  Before  we  proceed  any  further  however  it  will  be 
well  to  call  to  mind  that  in  studying  vision  as  we  are  now 
doing  by  means  of  an  appeal  to  our  own  consciousness,  we  are 
deserting  the  ordinary  methods  of  physiology  for  the  methods 
which  are  more  strictly  speaking  those  of  psychology.  Or 
rather  in  our  study  of  vision  we  are  using  both  methods,  sud- 
59  929 


930    PSYCHICAL   FEATUEES   OF   SENSATIONS.    [Book  in. 

denly  turning  from  one  to  the  other.  We  are  using  ordinary 
physiological  methods  when  we  are  studying  how  the  various 
rays  of  light  proceeding  from  a  tree  form  an  image  of  the  tree 
on  the  retina,  and  how  these  rays  thus  falling  on  the  retina  give 
rise  to  visual  impulses.  But  when  we  study  the  change  in  our 
consciousness  which  is  brought  about  by  the  visual  impulses 
thus  excited  through  the  image  of  the  tree  falling  on  the  retina, 
we  are  dealing  with  psychological  problems.  The  object,  the 
tree  itself,  and  our  vision  of  it,  the  one  being  commonly  spoken 
of  as  the  cause  of  the  other,  are  connected  by  a  chain  of  events ; 
one  end  of  the  chain  we  study  by  physiological,  the  other  end 
by  psychological  methods ;  and  the  difficulty  of  our  task  arises 
from  the  fact  that  we  have  to  use  these  two  different  methods 
for  a  common  purpose,  namely  that  of  explaining  how  the  tree 
gives  rise  to  the  vision  of  it. 

When  we  turn  to  the  physiological  side  of  the  problem  we 
cannot  at  present  say  much  more  than  that  the  rays  of  light  pro- 
ceeding from  the  tree  give  rise  to  the  changes  in  the  optic  fibres 
which  we  have  called  visual  impulses.  We  have  seen  in  deal- 
ing with  the  brain  reason  to  think  (§  478)  that  visual  impulses, 
like  other  sensory  impulses,  may  influence  the  working  of  the 
central  nervous  system  without  producing  any  such  change  of 
consciousness  as  can  be  studied  by  psychological  methods  ;  and 
we  further  suggested  (§  500)  that  in  the  structures  of  the  mid- 
brain which  we  called  the  primary  visual  centres  a  visual  impulse 
underwent  a  development  by  which  it  became  no  longer  a  mere 
impulse  but  something  more,  and  that  the  changes  in  these 
primary  visual  centres  transmitted  to  the  occipital  cortex  gave 
rise  there  to  the  changes  with  which  the  distinct  affection  of 
consciousness  is  associated.  It  is  undesirable  to  speak  of  the 
events  in  the  primary  visual  centres  as  "sensations,"  since  it 
is  convenient  to  reserve  this  term  for  the  psychical  events,  the 
changes  of  consciousness  of  which  we  can  become  aware  by 
examining  our  own  minds ;  nor  is  there  at  present  any  need  to 
give  them  any  name  at  all ;  but  it  is  important  when  we  are 
using  the  psychological  method  to  remember  that  between  the 
physiological  visual  impulses  and  the  psychological  sensation 
there  are  events  which  must  not  be  ignored. 

Turning  now  to  the  psychological  side  of  the  problem  we 
find  that  the  psychical  events  are  also  complex,  and  that  the 
psychical  effects  due  to  the  same  visual  impulses  are  not  all  of 
the  same  kind.  This  is  seen  even  in  the  case  of  simple  and 
isolated  visual  sensations.  Taking  the  effect  of  a  luminous 
point,  shining  for  a  moment  only,  as  a  simple  form  of  visual 
sensation,  we  must  distinguish  what  we  may  call  the  mere 
change  of  consciousness,  the  mere  sensation  of  light,  from  the 
further  psychical  effect  of  which  we  have  already  spoken  and 
through  which  we  associate  the  sensation  with  a  luminous  point 


Chap,  hi.]  SIGHT.  931 

occupying  a  particular  position  in  external  nature.  Though 
the  latter  always  accompanies  the  former,  though  whenever  we 
experience  a  visual  sensation  we  refer  it  to  its  cause  in  the 
external  world,  we  can  dissociate  the  two  in  our  minds,  and 
can  sjDeak  of  the  mere  sensation  independently  of  the  further 
psychical  action.  When  we  have  vision  not  of  such  a  simple 
object  as  a  luminous  point,  which  we  may  consider  as  giving 
rise  to  a  single  sensation,  but  of  a  tree  which  gives  rise  to  a 
complex  group  of  sensations,  the  psychical  actions  which  accom- 
pany the  mere  sensations  are  manifold  and  become  prominent 
in  the  total  visual  effect  produced  by  the  tree.  That  total 
visual  effect  is  determined  not  only  by  the  sensations  to  which 
the  retinal  image  of  the  tree  is  at  the  time  giving  rise,  but  also 
by  various  psychical  events  dependent  on  the  previous  knowl- 
edge of  the  nature  of  trees  which  we  have  gained  by  touch  as 
well  as  by  sight,  and  on  other  circumstances.  In  common 
language  we  distinguish  between  the  mere  sensation  and  the 
further  psj'chical  visual  effect  by  saying  that  we  '  feel '  a  sensa- 
tion and  'perceive'  an  object;  and,  though  the  term  'percep- 
tion '  has  been  employed  in  different  meanings  by  different 
writers,  we  may  here  make  use  of  it,  in  what  is  perhaps  its  most 
usually  accepted  meaning,  to  denote  the  further  visual  effect  to 
which  we  have  just  called  attention  as  distinguished  from  the 
immediate  sensation.  We  feel  a  sensation  of  light,  and  we  may 
feel  at  one  and  the  same  time  a  number  of  such  sensations  of 
different  intensity  and  quality ;  we  perceive  an  object,  it  may 
be  a  simple  object  such  as  a  mere  transient  flash  of  light  or  a 
complex  object  such  as  a  tree  or  a  scene. 

From  what  we  have  said  above  it  follows  that,  although  it  is 
perfectly  true  as  we  have  insisted  (§  524  ),  that  our  perception 
of  external  objects  is  based  on  the  optical  sharpness  of  the 
retinal  image,  and  on  the  distinctness  of  the  several  sensations 
which  the  retinal  image  excites,  we  should  be  wrong  in  sup- 
posing that  when  an  image  of  an  object  is  formed  on  the  retina 
the  visual  impulses  correspond  exactly  to  the  retinal  image,  the 
sensations  correspond  exactly  to  the  impulses,  and  the  perception 
corresponds  exactly  to  the  sensations,  so  that  the  perception  is 
as  it  were  a  "  mental  image  "  corresponding  exactly  to  the  reti- 
nal image  and  hence  to  the  object  itself.  The  truth  lies  in 
the  contrary  direction  ;  things  are  not  what  they  look,  or,  since 
the  same  applies  to  other  senses  besides  vision,  what  they  seem  ; 
and  one  object  of  philosoph}'  is  to  ascertain  the  exact  relations 
between  things  as  they  are  and  things  as  we  think  them  to  be. 
We  must  of  course  confine  ourselves  here  to  pointing  out,  in 
regard  to  vision,  some  of  the  more  salient  differences  which 
obtain  between  the  actual  features  of  an  object  and  our  percep- 
tion of  the  object. 

Of  these  differences  some  are   clearly  of  psychical  origin. 


932 


IREADIATIOK 


[Book  hi. 


Our  perception  of  a  tree  is  in  part  determined  by  events  other 
than  the  actual  sensations,  by  psychical  processes  arising  out  of 
our  previous  experiences  of  trees,  and  in  other  ways.  Some  of 
these  psychical  processes  we  shall  consider  a  little  later  on. 

Other  differences  are  either  clearly  or  possibly  of  physiologi- 
cal origin ;  the  view  may  at  least  be  argued  that  they  arise 
either  during  the  retinal  changes  through  which  visual  impulses 
are  developed  or  during  the  subsequent  cerebral  changes,  spoken 
of  above,  through  which  the  visual  impulses  give  rise  to  visual 
sensations  ;  and  it  is  to  some  of  these  that  we  wish  first  to  call 
attention. 

§  582.  Irradiation.  A  white  patch  on  a  dark  ground  ap- 
pears larger,  and  a  dark  patch  on  a  white  ground  smaller,  than 
it  really  is.     In  Fig.  155,  the  white  square  on  the  right  hand 


EiG.  155. 

side  looks  larger  than  the  black  square  on  the  left  hand  side 
though  both  are  exactly  of  the  same  size.  So  also  neighbouring 
white  surfaces  tend  to  melt  together.  The  effect  is  increased 
when  the  object  is  somewhat  out  of  focus,  and  may  be  then 
partly  explained  by  the  diffusion  circles  which,  in  each  case, 
encroach  from  the  white  upon  the  dark.  But  over  and  beyond 
this,  any  sensation  coming  from  a  given  retinal  area  occupies 
a  larger  share  of  the  field  of  vision,  when  the  rest  of  the  retina 
and  central  visual  apparatus  are  at  rest,  than  when  they  are 
simultaneously  excited.  It  is  as  if  the  neighbouring,  either 
retinal  or  cerebral,  structures  were  sympathetically  thrown  into 
action  at  the  same  time.  In  this  way  a  certain  difference  is 
established  between  the  retinal  image  and  the  perception. 

§  583.  Simultaneous  contrast.  If  a  white  strip  be  placed 
between  two  black  strips,  the  edges  of  the  white  strip,  near  to 
the  black,  will  appear  whiter  than  its  medium  portion ;  and  if  a 
white  cross  be  placed  on  a  black  background,  the  parts  close  to 
the  black  will  appear  sometimes  so  white,  compared  with  the 
centre  of  the  cross,  that  the  latter  will  seem  dim  or  even  shaded. 
This  effect  which  occurs  even  when  the  object  is  well  in  focus, 
is  spoken  of  as  one  of  '  simultaneous  contrast ' ;  the  increased 


Chap,  hi.]  SIGHT.  933 

sensation  of  light  which  causes  the  apparent  greater  whiteness 
of  the  borders  of  the  cross  is  regarded  as  the  result  of  the  '  con- 
trast '  with  the  black  placed  immediately  close  to  it.  Still  more 
striking  results  are  seen  with  coloured  objects.  If  a  book,  or 
pencil,  be  placed  vertically  on  a  sheet  of  white  paper,  and  illumi- 
nated on  one  side  by  the  sun,  and  on  the  other  by  a  candle,  two 
shadows  will  be  produced,  one  from  the  sun  which  will  be  illumi- 
nated by  the  yellowish  light  of  the  candle,  and  the  other  from 
the  candle  which  will  in  turn  be  illuminated  by  the  white  light 
of  the  sun.  The  former  naturally  appears  yellow ;  the  latter, 
however,  appears  not  white  but  blue ;  it  assumes,  by  contrast,  a 
colour  complementary  to  that  of  the  candle-light  which  sur- 
rounds it.  If  the  candle  be  removed,  or  its  light  shut  off  by  a 
screen,  the  blue  tint  disappears,  but  returns  when  the  candle  is 
again  allowed  to  produce  its  shadow.  If,  before  the  candle  is 
brought  back,  vision  be  directed  through  a  narrow  blackened  tube 
at  some  part  falling  entirely  within  the  area  of  what  will  be  the 
candle's  shadow,  the  area,  which  in  the  absence  of  the  candle 
appears  white,  will  continue  to  appear  white  when  the  candle  is 
made  to  cast  its  shadow,  and  it  is  not  until  the  direction  of  the 
tube  is  changed  so  as  to  cover  part  of  the  ground  outside  the 
shadow,  as  well  as  part  of  the  shadow,  that  the  latter  assumes  its 
blue  tint.  If  a  small  piece  of  grey  paper  be  placed  on  a  sheet 
of  pale  green  paper,  and  both  covered  with  a  sheet  of  thin  tissue 
paper,  the  grey  paper  will  appear  of  a  pink  colour,  the  comple- 
mentary of  the  green.  This  effect  of  contrast  is  far  less  striking, 
or  even  wholly  absent,  when  the  small  piece  of  paper  is  white 
instead  of  gre}^  and  generally  disappears  when  the  thin  cover- 
ing of  tissue  paper  is  removed.  It  also  vanishes  if  a  bold  broad 
black  line  be  drawn  round  the  small  piece  of  paper,  so  as  to 
isolate  it  from  the  ground  colour.  And  many  other  instances 
of  this  kind  of  contrast  might  be  given.  It  is  obvious  that 
whenever  in  vision  this  effect  intervenes,  a  discrepancy  is  intro- 
duced between  the  features  of  an  object  and  our  perception  of 
them. 

§  584.  After-images.  Successive  Contrast.  As  we  have  al- 
ready (§  551)  seen  the  visual  sensation  lasts  much  longer  than 
the  stimulus,  and  under  certain  circumstances  the  sensation 
is  so  prolonged  that  it  is  spoken  of  as  an  after-image.  Such 
after-images  are  best  developed  when  an  eye,  which  has  for  some 
time  been  removed  from  the  influence  of  light,  is  momentarily 
exposed  to  a  somewhat  strong  stimulus.  Thus  if  immediately 
on  waking  from  sleep  in  the  morning  the  eye  be  directed  to  a 
window  for  an  instant  and  then  closed,  an  image  of  the  window 
with  its  bright  panes  and  darker  sashes,  the  various  parts  being 
of  the  same  colour  as  the  object,  will  remain  for  an  appreciable 
time. 

When,  however,  the  eye  has  been  for  some  time  subjected 


934  AFTEE-IMAGES.  [Book  in. 

to  a  stimulus,  the  sensation  which  follows  the  withdrawal  of  the 
stimulus  is  of  a  different  kind ;  the  result  is  what  is  called  a 
negative  after-image^  or  negative  image,  to  distinguish  it  from 
a  positive  after-image,  like  the  one  mentioned  above,  which  is 
simply  a  continuation  of  the  sensation  primarily  excited  with  all 
its  characters  unchanged  except  that  of  intensity.  If,  after  look- 
ing steadfastly  at  a  white  patch  on  a  black  ground,  the  eye  be 
turned  to  a  white  ground,  a  grey  patch  is  seen  for  some  little 
time.  A  black  patch  on  a  white  ground  similarly  gives  rise 
when  the  eye  is  subsequently  turned  towards  a  grey  ground  to 
a  negative  image  in  the  form  of  a  white  patch.  This  may  be 
explained  as  the  result  of  exhaustion.  When  the  white  patch 
has  been  looked  at  steadily  for  some  time,  that  part  of  the  retina 
on  which  the  image  of  the  patch  fell  has  become  tired ;  hence 
the  white  light,  coming  from  the  white  ground  subsequently 
looked  at,  which  falls  on  this  part  of  the  retina,  does  not  produce 
so  much  sensation  as  in  other  parts  of  the  retina ;  and  the  image, 
consequently,  appears  grey.  And  so  in  the  other  instance ;  in 
this  case,  the  whole  of  the  retina  is  tired,  except  at  the  patch ; 
here  the  retina  is  for  a  while  most  sensitive,  and  hence  the  white 
negative  image.  In  speaking  of  the  retina  being  tired  we  are 
using  these  words  for  simplicity's  sake.  We  have  no  right  to 
suppose  that  the  exhaustion  takes  place  in  the  retinal  structures 
only ;  it  may  occur  in  the  central  cerebral  structures  during  the 
development  of  visual  impulses  into  sensations  ;  indeed  the  chief 
part  of  it  is  probably  of  such  a  cerebral  origin. 

When  a  red  patch  is  looked  at,  and  the  eye  subsequently 
turned  to  a  white  or  to  a  grey  ground,  the  negative  image  is  a 
greenish  blue ;  that  is  to  say,  the  colour  of  the  negative  image 
is  complementary  to  that  of  the  object.  Thus  also  orange  pro- 
duces a  blue,  green  a  pink,  yellow  an  indigo-blue,  negative  image, 
and  so  on ;  the  negative  image  is  in  each  case  complementary  to 
the  primary  one. 

Similarly,  when  the  eye,  after  looking  at  a  coloured  patch, 
is  turned  not  to  a  white  or  grey  but  to  a  coloured  ground,  the 
colour  of  the  negative  image  is  a  mixture  of  the  colour  comple- 
mentary to  the  primary  image  with  the  colour  of  the  ground ;  if 
a  yellow  ground  be  chosen  after  looking  at  a  green  object,  the 
negative  image  will  appear  as  a  mixture  of  red  and  yellow,  a 
reddish  yellow ;  and  so  on. 

Again,  when  a  patch  of  coloured  light  is  made  to  travel 
through  the  visual  field  with  sufficient  rapidity,  as  when  a  patch 
of  light  or  of  colour  placed  near  the  margin  of  a  rotating  disc  is 
looked  at,  the  image  of  the  patch  as  the  disc  revolves  is  followed 
by  a  negative  image  in  the  shape  of  a  sort  of  ghost  having  a 
colour  more  or  less  but  not  exactly  complementary  to  that  of 
the  patch. 

Though  these  negative  images  only  become  striking  after  a 


Chap,  hi.]  SIGHT.  935 

prolonged  or  intense  excitation  of  the  retina,  such  as  rarely 
occurs  in  ordinary  vision,  still  the  effect  must  intervene,  even  if 
to  a  slight  extent  only,  in  our  daily  sight,  and  proportionately 
contribute  to  the  discrepancy  between  the  perception  and  the 
object. 

§  585.  The  phenomena  of  '  simultaneous  '  and  '  successive 
contrast'  are  further  of  interest  in  relation  to  the  theory  of 
colour  vision.  The  mere  occurrence  of  the  negative  images  can 
be  explained  as  a  result  of  exhaustion  on  either  hypothesis  of 
colour  vision.  According  to  the  Young-Helmholtz  theory  when 
the  coloured  patch  is  looked  at,  one  of  the  three  primary  colour 
sensations  is  much  exhausted,  and  the  other  two  less  so  in  vary- 
ing proportions,  according  to  the  exact  nature  of  the  colour  of 
the  patch ;  and  the  less  exhausted  sensations  become  prominent 
in  the  after-image.  Thus,  the  red  patch  exhausts  the  red  pri- 
mary sensation,  and  the  negative  image  is  made  up  chiefly  of 
green  and  blue  sensations,  that  is,  appears  to  be  greenish-blue, 
or  bluish-green,  according  to  the  particular  hue  or  tone  of  the 
red.  So  also  the  yellow  patch  exhausts  both  the  red  and  green 
sensations  leaving  the  blue  only  to  make  itself  felt.  On  Hering's 
hypothesis,  we  may  suppose  that,  owing  to  the  continued  effect 
of  looking  at  the  red  patch,  the  katabolic  changes  of  the  red- 
green  substance  become  less  and  less,  leading  to  a  prominence 
and  indeed  to  an  actual  increase  of  anabolic  changes  in  the 
same  substance;  hence,  the  sensation  of  green  dominating  in 
the  negative  image ;  and  we  may  suppose  that  like  events  occur 
in  the  yellow-blue  substance. 

The-  Young-Helmholtz  theorj'-  does  not  explain  so  readily  as 
does  Hering's  theory  why  negative  images  often  follow  upon 
positive  images  without  any  stimulation  of  the  retina  subsequent 
to  the  primary  one.  As  we  have  already  said,  if  a  Avhite  patch 
on  a  black  ground  be  looked  at  for  some  time,  and  the  eyes  be 
then  shut,  a  negative  image  of  the  spot  will  be  seen  on  the 
ground  of  the  'intrinsic  light'  of  the  retina  much  blacker  than 
the  ground,  and  having  in  its  immediate  neighbourhood  a  sort  of 
bright  corona.  Conversely  a  black  patch  on  a  white  ground  will 
give  rise  to  a  patch  of  exaggerated  'intrinsic  light'  in  contrast 
to  the  blackness  of  the  rest  of  the  field.  So  also,  if  a  window  be 
looked  at  and  the  eyes  then  closed,  the  positive  after-image  with 
bright  panes  and  dark  sashes  gives  way  to  a  negative  after-image 
with  bright  sashes  and  dark  panes.  Looking  at  a  bright  red 
spot  gives  rise  to  a  green  after-image,  and  so  with  other  colours. 
Moreover,  the  eyes  being  still  shut,  there  may  be  a  series  of 
after-images  ;  the  negative  after-image  with  its  black,  green, 
&c.,  corresponding  to  the  white,  red,  &c.,  of  the  positive  image, 
may  give  way  to  a  return  of  the  positive  image  with  all  its 
original  features,  to  be  succeeded  by  a  second  negative  image 
like  the  first,  and  thus  often  by  a  whole  series  of  alternate  pos- 


936  AFTEE-IMAGES   AND  CONTKAST.      [Book  in. 

itive  and  negative  images,  each  gradually  becoming  fainter  and 
more  obscure.  These  and  similar  phenomena  are  more  or  less 
satisfactorily  explained  on  Hering's  theory  as  the  results  of 
rhythmic  oscillations  between  katabolic  and  anabolic  changes ; 
on  the  other  theory  we  have  to  have  recourse  to  psycholog- 
ical explanations.  This  is  especially  the  case  with  the  phe- 
nomena of  simultaneous  contrast.  In  the  case  for  instance  of 
the  grey  patch  seen  as  pink  in  the  midst  of  a  green  field,  it  is 
argued  that  the  patch  does  not  actually  excite  a  sensation  of 
pink  but  that  we  think  it  is  pink  because  we  attribute  the  green- 
ness of  the  whole  field  to  the  covering  tissue  paper,  and  seeing 
the  patch  shine  through  this  judge  the  patch  to  be  reflecting 
just  those  rays,  namely  pink,  which  mixing  with  the  green 
would  give  rise  to  white,  that  is  to  a  colourless  grey.  Hering's 
theory  on  the  other  hand  offers  a  direct  physiological  explana- 
tion of  the  effect;  it  supposes  that  when  one  part  of  the  retina 
is  stimulated,  the  neighbouring  portions  of  the  field  of  vision 
are  affected  at  the  same  time  in  a  manner  which  may  be  roughly 
but  only  roughly  compared  to  electric  induction,  so  that  they 
undergo  changes  antagonistic  or  complementary  to  those  going 
on  in  the  part  of  the  field  of  vision  corresponding  to  the  portion 
of  the  retina  actually  stimulated.  Thus  in  the  case  of  the  grey 
patch  on  the  green  field,  the  anabolism  of  the  red-green  sub- 
stance in  the  green  field  surrounding  the  grey  patch  leads  to  a 
certain  amount  of  katabolic  action  of  the  red-green  substance 
within  the  grey  patch,  and  so  gives  rise  to  a  red  sensation. 

§  586.  We  have  seen  (§  553)  that  visual  sensations  may  be 
produced  in  other  ways  than  by  light  falling  on  the  retina.  In 
such  cases  the  effect  which  is  produced  upon  our  consciousness 
is  wholly  misleading.  A  mechanical  or  electrical  stimulation 
of  the  retina  may  give  rise  to  a  visual  sensation  identical  with 
that  which  would  be  produced  by  the  rays  from  a  flash  of  light 
falling  upon  a  part  of  the  retina.  In  both  cases  we  should  have 
a  perception  of  a  flash  of  light  occurring  in  a  certain  part  of  the 
field  of  vision ;  and  so  far  as  the  perception  itself  is  concerned 
we  could  not  distinguish  between  the  latter  which  is  a  real  and 
the  former  which  is  a  false  perception. 

Not  only  single  and  simple  sensations,  but  also  complex 
groups  of  sensations  may  be  excited  by  other  means  than  that  of 
light  falling  on  the  retina,  and  we  may  thus  experience  varied 
and  intricate  perceptions  which  have  no  objective  reality  at  all. 
Many  people  when  they  close  their  eyes  at  night,  or  indeed  at 
other  times,  see  images  of  faces  or  other  objects:  and  though 
under  such  circumstances  it  is  easy  to  recognize  the  subjective 
origin  of  the  perception,  that  conclusion  is  reached  by  reasoning 
upon  the  circumstances,  and  not  because  the  perception  itself 
differs  in  character  from  a  like  perception  caused  by  looking  at 
an    external    object.     In  such  cases  it  is  probable  that  some 


Chap,  hi.]  SIGHT.  937 

causes  or  other  of  a  physiological  nature  give  rise  either  in  the 
lower  visual  centres  or  in  the  cerebral  cortex  to  just  such 
changes  as  would  be  induced  by  corresponding  visual  impulses, 
though  those  impulses  are  wholly  wanting;  in  other  words  the 
causes  in  question  give  rise  to  visual  sensations,  in  the  physio- 
logical meaning  of  that  word,  which  produce  a  psychological 
effect  identical  with  that  of  visual  sensations  produced  in  the 
ordinary  way  through  the  action  of  light  on  the  retina.  In 
some  cases  perhaps  the  process  may  begin  even  in  the  retina 
itself;  abnormal  changes  in  one  or  other  of  the  retinal  structures 
may  lead  to  the  development  of  complex  coordinate  visual 
impulses. 

Sometimes  the  sensations  and  perceptions  thus  occurring, 
especially  those  which  are  met  with  on  closing  the  eyes  at  night, 
may  be  recognized  as  revivals,  more  or  less  altered,  of  sensations 
experienced  during  the  day ;  something  sets  going  again  the 
series  of  cerebral  events  which  were  set  going  by  actual  rays 
of  light.  These  are  generally  spoken  of  as  "recurrent  sensa- 
tions." 

At  other  times,  there  is  no  history  of  any  like  sensation 
having  been  felt  in  the  immediate  past;  the  psychical  effect 
appears  to  have  no  objective  cause  at  all.  Moreover  such  false 
sensations  and  perceptions  having  a  distinctness  which  gives 
them  an  apparent  objective  reality  quite  as  striking  as  that  of 
ordinary  visual  perceptions,  may  occasionally  be  experienced 
not  only  when  the  eyes  are  closed,  but  even  when  the  eyes  are 
open,  and  when  therefore  ordinary  visual  perceptions  are  being 
generated,  with  which  they  mingle  and  with  which  they  are 
often  confused.  They  are  then  spoken  of  as  ocular  phantoms  or 
hallucinations.  They  sometimes  become  so  frequent  and  obtru- 
sive as  to  be  distressing,  and  form  an  important  element  in  some 
kinds  of  delirium,  such  as  delirium  tremens. 

It  is  probable,  as  we  have  just  suggested,  that  these  false 
perceptions  may  be  started  by  events,  which  in  ordinary  lan- 
guage may  be  called  physiological ;  but  the  whole  chain  of 
events  between  the  visual  impulse  or  even  the  immediate  effect 
of  the  impulse  which  we  may  consider  as  the  physiological  sen- 
sation, and  the  terminal  psychological  perception  is  long  and 
complex ;  the  discordance  between  the  perception  and  its  ap- 
parent cause,  in  other  words,  the  falsity  of  the  perception,  may 
be  introduced  in  the  later,  psychological,  links  of  the  chain. 
And  an  hallucination  may  have  such  an  origin  that  it  may  fitly 
be  spoken  of  as  purely  psychological. 

This  naturally  leads  to  the  remark  that  a  perception  may  be 
revived  in  the  mind,  without  the  usual  physiological  antece- 
dents, as  the  result  of  purely  psychological  processes  ;  it  is  then 
generally  spoken  of  as  an  '  idea.'  And  we  find,  upon  exam- 
ination, that  each  new  perception  which  we  experience  is  more 


938  OCULAR  PHANTOMS.  [Book  ni. 

or  less  modified  by  memories  and  ideas  resulting  from  bygone 
perceptions  of  a  like  kind.  But  we  have  already  determined 
to  defer  the  consideration  of  these  and  other  more  or  less  dis- 
tinctly psychical  modifications  of  perceptions  until  we  have 
studied  certain  results  arising  from  the  use  of  two  eyes. 


SEC.  9.     BINOCULAR  VISION. 

§  587.  So  far  we  have  treated  of  vision  as  if  it  were  carried 
out  by  means  of  one  eye  and  have  only  incidentally  referred  to 
our  possessing  two  eyes.  Our  ordinary  vision  is,  however,  carried 
out  by  means  of  two  eyes,  our  vision  is  binocular  not  monocular ; 
and  to  the  characters  of  this  binocular  vision  we  must  now  turn. 
In  dealing  with  monocular  vision  we  rarely  have  occasion  to 
refer  specially  to  the  movements  of  the  eyeball ;  but  in  binocular 
vision  these  play  an  important  part ;  and  even  before  we  go  into 
details,  it  will  be  desirable  to  point  out  not  only  certain  general 
facts,  but  also  the  meaning  of  certain  terms  which  we  shall  have 
to  use. 

The  eye  is  virtually  a  ball  placed  in  a  socket,  the  bulb  or 
eyeball  and  the  orbit  forming  a  ball-and-socket  joint.  In  its 
socket  joint  the  eyeball  is  capable  of  various  movements,  but 
these  are  limited  to  those  of  rotation  within  the  socket;  the 
eyeball  cannot  by  any  voluntary  effort  be  moved  out  of  its 
socket.  It  is  stated  that  by  a  very  forcible  opening  of  the  eye- 
lids the  eyeball  may  be  slightly  protruded;  but  this  trifling 
locomotion  may  be  neglected.  By  disease,  however,  the  position 
of  the  eyeball  in  the  socket  may  be  materially  changed. 

The  movements  of  rotation  to  which  the  eyeball  is  thus 
limited  are  carried  out  round  a  centre  in  the  eye  which  is  termed 
the  cent7'e  of  rotation^  and  which  has  been  determined  to  lie  in 
the  vitreous  humour  about  13.5  mm.  behind  the  anterior  surface 
of  the  cornea,  not  quite  2  mm.  behind  what,  though  the  eyeball 
is  not  a  sphere,  may  be  considered  as  the  geometric  centre  of  the 
eyeball ;  it  is  of  course  quite  different  from  the  optical  centre 
or  nodal  point  of  the  diagrammatic  eye  (§  527). 

When  we,  in  looking,  direct  our  vision  to  a  point,  a  line 
drawn  from  such  a  point,  which  we  may  call  the  fixed  point  of 
vision,  to  the  centre  of  rotation,  is  called  the  visual  axis ;  pro- 
longed past  the  centre  of  rotation  it  meets  the  retina  in  the 
centre  of  the  fovea  centralis ;  hence  in  the  view  of  those  who 
hold  that  the  optic  axis,  the  line  on  which  the  dioptric  surfaces 
of  the  eye  are  centred,  meets  the  retina  on  one  side  of  the 
fovea,  the  visual  axis  does  not  coincide  with,  and  is  different 

939 


940  FIELD   OF   SIGHT.  [Book  iii. 

from  the  optic  axis.  When  with  both  eyes  we  look  straight- 
forwards  to  the  far  distance,  the  visual  axes  of  the  two  eyes  are 
parallel ;  when  we  direct  the  two  eyes  to  the  same  fixed  point, 
the  two  visual  axes  converge  to  the  fixed  point,  the  amount  of 
convergence  being  the  greater  the  nearer  the  fixed  point  to  the 
observer. 

The  horizontal  plane  in  which  the  two  visual  axes  lie  is 
called  the  visual  plane  ;  and  a  vertical  plane  at  right  angles  to 
this,  midway  between  the  two  eyes,  or  more  exactly  bisecting  a 
line,  sometimes  called  the  "  base  line  "  or  "  fundamental  line  " 
joining  the  nodal  points  of  the  two  eyes,  is  called  the  median 
plane. 

§  588.  As  we  have  seen,  the  sum  of  the  sensations  which 
we  can  receive  from  the  retina  at  the  same  time  is  spoken  of  as 
the  "  visual  field "  or  "  field  of  vision."  The  term  therefore 
has  properly  a  subjective  meaning,  but  it  is  sometimes  used  in 
an  objective  sense  to  denote  the  space  or  area  of  the  external 
world,  rays  of  light  from  which  are  capable  of  exciting  the 
retina  at  any  one  time ;  where  we  wish  to  distinguish  between 
the  two,  we  may  call  the  latter  the  "field  of  sight."  The 
dimensions  of  the  field  of  sight  for  one  eye  will  even  in  the 
same  individual  vary  with  the  width  of  the  pupil  and  other 
dioptric  arrangements  of  the  eye ;  individual  variations  are  also 
considerable ;  but  the  ordinary  dimensions  may  be  stated  as 
subtending  an  angle  of  about  145°  in  the  horizontal  and  about 
100°  in  the  vertical  meridian,  the  former  being  distinctly  greater 
than  the  latter.  When  an  external  object  lies  outside  the  area 
subtending  these  angles  we  say  that  it  is  outside  the  field  of 
sight  for  that  position  of  the  eye  ;  it  may  of  course  be  brought 
into  the  field  of  sight  by  moving  it  or  by  moving  the  eye.  The 
outline  of  the  field  is  an  irregular  one,  and  stretches  farther 
towards  the  temporal  side  of  the  fixed  point,  that  is,  towards 
the  nasal  side  of  the  retina,  than  on  the  other  side ;  it  is  some- 
what larger  and  of  a  different  form  when  the  eye  is  turned 
towards  the  temporal  side  than  when  the  eye  is  directed  straight 
forwards,  cf.  Fig,  156.  It  will  be  understood  that  the  two 
visual  fields  of  the  two  eyes  are  unlike,  cf.  Fig.  157. 

When  we  use  both  eyes  a  large  part  of  the  visual  field  -of 
each  eye  overlaps  that  of  the  other ;  that  is  to  say,  the  rays  of 
light  proceeding  from  a  large  part  of  the  field  of  sight  of  each 
eye  fall  upon  and  affect  both  retinas.  But  at  the  same  time  a 
certain  part  of  each  visual  field  does  not  so  overlap  any  part 
of  the  other.  If  the  right  hand  be  held  up  above  the  right 
shoulder  and  brought  a  little  forward  it  soon  becomes  distinctly 
visible  to  the  right  eye,  it  enters  into  the  field  of  sight  of  the 
right  eye.  But  if  the  right  eye  be  closed,  we  find  that  the  right 
hand  kept  in  the  former  position  is  not  visible  to  the  left  eye ; 
it  is  outside  the  field  of  sight  of  that  eye  ;  it  has  to  be  brought 


Chap,  hi.]  SIGHT.  941 

much  further  forward  until  it  comes  into  the  field  of  sight  of 
the  left  eye  ;  the  profile  of  the  face  and  especially  of  the  nose 
prevent  the  rays  reflected  from  the  hand  gaining  access  to  the 
left  retina  until  the  hand  is  brought  a  certain  distance  forward. 
The  right-hand  side  of  the  objective  field  of  sight  of  the  right 

Upper 


Nasal     -V^iiil^^--^ 


Lower 

FiGo  156.     The  visual  field  of  the  right  ete.     (Aubert.) 

The  figure  represents  the  visual  field  projected  into  space  and  therefore  cor- 
responds to  the  objective  field  of  sight ;  the  temporal  side  of  the  figure  corre- 
sponds to  the  nasal  side  of  the  retina.  The  shaded  part  indicates  tlie  increase 
gained  by  looking  outwards  towards  the  temporal  side.    /,  fovea ;  a;,  blind  spot, 

eye,  corresponding  to  the  nasal  side  of  the  retina  of  that  eye, 
extends  much  farther  to  the  right  than  does  the  right-hand  side 
of  the  field  of  sight  of  the  left  eye,  which  corresponds  to  the 
temporal  side  of  the  retina  of  that  eye.  Cf.  Fig.  157.  Simi- 
larly, the  left-hand  side  of  the  field  of  sight  of  the  left  eye 
extends  farther  to  the  left  than  does  that  of  the  right  eye. 
Hence  on  the  one  hand  the  total  field  of  sight  of  the  two  eyes 
together  is  increased  in  the  horizontal  diameter,  subtending  on 
an  average  an  angle  of  180°  instead  of  145° ;  and  on  the  other 
hand  while  a  certain  right-hand  and  left-hand  part  of  the  united 
fields  of  sight  belong  respectively  to  the  right  and  left  eye  only, 
the  remainder  of  the  field  is  common  to  the  two  eyes.  The 
area  common  to  the  two  eyes  when  the  visual  axes  converge  to 
the  same  fixed  point,  is  shewn  as  the  shaded  part  in  Fig.  157. 
Rays  of  light  from  objects  in  the  common  part  affect  the  retinas 
of  both  eyes  at  the  same  time,  vision  is  here  binocular ;  rays  of 
light  from  objects  at  the  extreme  right  and  left  aflFect  only  the 
right  and  left  retina  respectiveljs  vision  in  these  parts  of  each 
eye  is  never  binocular,  always  monocular.  The  amount  of  each 
retina  which  is  thus  cut  oif  from  binocular  vision  is  determined 
by  the  jMominence  of  the  nose  and  profile  between  the  eyes;  in 
some  of  the  lower  animals  the  position  of  the  eyes  is  so  com- 
pletely lateral  that  no  rays  of  light  proceeding  from  the  same 


942 


CORRESPONDING  POINTS. 


[Book  hi. 


object  can  fall  on  any  part  of  the  two  retinas  at  the  same  time, 
and  in  these  creatures  vision  is  wholly  monocular. 


Fig. 


157.     The   visual   fields  (fields  of   sight)  of   the  two   eyes  when 

THE  eyes  converge  TO  THE  SAME  FIXED  POINT.   (Aubert.) 


The  shaded  part  is  that  common  to  the  two  eyes.  /,  the  fixed  point,  corre- 
sponding to  the  fovea  of  each  eye  ;  x,  the  blind  spots  of  the  two  eyes. 

§  589.  Corresponding  or  Identical  Points.  Though  when 
we  use  two  eyes,  we  must  receive  from  every  object  in  the  field 
of  sight  common  to  the  two  eyes  two  sets  of  visual  impulses, 
indeed  we  may  say  two  sets  of  sensations,  our  perception  of  the 
object  is  under  ordinary  circumstances  a  single  one ;  we  see 
one  object,  not  two.  By  putting  either  eye  into  an  unusual 
position,  as  by  squinting,  we  can  render  the  perception  double ; 
we  see  two  objects  where  one  only  exists.  This  singleness  of  the 
sensation  under  ordinary  circumstances  shews  that  certain  parts 
of  each  retina  are  so  related  to  each  other  that  when  an  image  of 
an  object  falls  on  these  parts  at  the  same  time,  the  two  sets  of  sen- 
sations excited  in  the  two  parts  are  blended  into  one  ;  such  parts 
are  spoken  of  as  corresponding  parts ;  they  have  also  been  called 
identical  parts.  Since  in  the  ordinary  movements  of  the  eyes 
we  see  objects  single,  and  do  not  receive  double  impressions 
unless  we  move  the  eyes  in  an  unusual  manner,  it  is  obvious 
that  the  movements  of  the  eyeballs  and  these  corresponding 
parts  of  the  two  retinas  are  so  related,  the  one  to  the  other,  that 
the  former  bring  the  images  of  objects  to  fall  on  the  latter. 

We  can  easily  determine  which  are  the  corresponding  parts 
of  the  two  retinas  by  tracing  out  the  paths  of  the  rays  of  light 
falling  on  the  two  retinas,  §  528.  As  we  have  said,  when  we 
look  at  an  object  with  one  eye  the  visual  axis  of  that  eye  is 
directed  to  the  object,  and  when  we  use  two  eyes  the  visual 
axes  of  the  two  eyes  converge  at  the  object,  the  eyeballs  moving 
accordingly.     The  corresponding  points  of  the  two  retinas  are 


Chap,  hi.] 


SIGHT. 


943 


those  on  which  the  two  images  of  the  object  fall  when  the  visual 
axes  converge  at  the  object.  Thus  in  Fig.  158  if  vl  from  X  to 
X  and  X  to  x'  be  the  two  visual  axes,  a:,  x'  being  the  centres  of 
the  foveee  centrales  of  the  two  eyes,  then,  the  object  XYZ  being 
seen  single,  the  point  y  on  the  one  retina  will  '  correspond  '  to 
or  be  '  identical '  with  the  point  y'  on  the  other,  and  the  point  z 
in  the  one  to  the  point  z'  in  the  other. 


m  h 


m  m 

Fig.  158.    Diagram  illustrating  Corresponding  Points. 

L  the  left,  B  the  right  eye,  n.  nodal  point,  o.  optic  nerve,  x.  fovea,  x'y'z'  are 
points  in  the  right  eye  corresponding  to  the  points  xyz  in  the  left  eye.  v.  I.  vis- 
ual axis.  The  two  figures  below  are  projections  of  L  the  left  and  B  the  right 
retina.  /.  fovea,  o.  blind  spot.  It  will  be  seen  that  a  and  c  on  the  temporal  side 
of  L  corresponds  to  a'  and  c'  on  the  nasal  side  of  B.  v.  m.  h.  m.  lines  of  sepa- 
ration. 

When  the  whole  area  of  the  retina  in  each  eye  which  we 
use  for  binocular  vision  is  explored  in  this  way  we  find,  as 
follows  geometrically  from  the  paths  of  the  rays  of  light,  that 
the  upper  half  of  one  retina  corresponds  to  the  upper  half  of 
the  other,  the  lower  half  to  the  lower  half,  the  right  side  to  the 


944  MOVEMENTS   OF  THE  EYEBALL.        [Book  in. 

right  side,  and  the  left  side  to  the  left  side.  But  when  we  turn 
to  the  structure  of  the  retina  we  find  that  the  left  or  nasal  side 
of  the  right  eye,  since  it  contains  the  entrance  of  the  optic 
nerve,  is  comparable  with,  not  the  left,  but  the  right  or  nasal 
side  of  the  left  eye,  and  in  like  manner  the  right  or  temporal  side 
of  the  right  eye  is  comparable  with  the  left  or  temporal  side  of 
the  left  eye.  Hence,  considered  in  relation  to  the  structure 
of  the  retina,  the  corresponding  points  appear  to  be  reversed 
from  side  to  side,  though  not  from  top  to  bottom.  While 
the  upper  half  of  the  retina  of  the  left  eye  corresponds  to  the 
upper  half  of  the  retina  of  the  right  eye,  and  the  lower  to 
the  lower,  the  nasal  side  of  the  left  eye  corresponds  with  the 
temporal  side  of  the  right,  and  the  temporal  of  the  left  with 
the  nasal  side  of  the  right. 

It  will  be  observed  that  in  each  eye  a  vertical  plane  through 
the  visual  axis  (v.  I.  in  Fig.  158)  cuts  the  retina  in  a  vertical 
line  V.  m.,  which  divides  the  retina  into  two  lateral,  temporal 
and  nasal,  halves,  each  temporal  and  each  nasal  half  correspond- 
ing with  the  nasal  and  temporal  half  respectively  of  the  other 
eye.  When  the  visual  axes  of  the  two  eyes  are  parallel,  the 
two  vertical  planes  in  question  are  parallel  to  the  median  plane 
and  to  each  other.  Further,  a  horizontal  plane  drawn  through 
the  visual  axis  at  right  angles  to  the  above  vertical  plane  cuts 
the  retina  in  a  horizontal  line  h.  m. ;  and  this  also  divides  the 
retina  into  two  halves,  an  upper  and  lower  half,  the  upper  and 
the  lower  halves  of  both  retinas  being  corresponding.  These 
two  lines,  each  of  which  may  be  considered  as  a  series  of 
corresponding  points,  are  sometimes  spoken  of  as  lines  of 
separation. 

The  blending  of  the  two  sensations  into  one  occurs,  we 
repeat,  only  when  the  two  images  of  an  object  fall  on  corre- 
sponding points  of  the  two  retinas.  Hence  it  is  obvious  that  in 
single  vision  with  two  eyes  the  ordinary  movements  of  the  eye- 
balls must  be  such  as  to  bring  the  visual  axes  to  converge  at 
the  object  looked  at  so  that  the  two  images  may  fall  on  corre- 
sponding points.  When  the  visual  axes  do  not  so  converge,  and 
when  therefore  the  images  do  not  fall  on  corresponding  points, 
the  two  sensations  are  not  blended  into  one  perception,  and 
vision  becomes  double.  It  is  therefore  important  to  study  in 
some  detail  the  movements  of  the  eyeballs,  by  means  of  which, 
in  ordinary  vision,  the  relative  positions  of  the  two  retinas  are 
so  carefully  adjusted  that  we  habitually  see  objects  single  not 
double. 

§  590.  The  movements  of  the  Eyeball.  As  we  have  said,  the 
movements  of  the  eyeball  are  movements  of  rotation  round  an 
immobile  centre,  the  centre  of  rotation ;  but  these  movements 
are  limited  in  a  particular  way,  and  it  is  necessary  to  pay  atten- 
tion to  their  characters  and  limitation. 


Chap,  hi.]  SIGHT.  945 

One  position  of  the  eyeball,  for  reasons  which  we  shall  see 
presently,  is  called  the  primary  position,  and  it  will  be  desirable 
to  start  from  this  position.  Though  its  exact  determination 
requires  special  precautions  it  may  be  described  as  that  which  is 
assumed  when,  with  the  head  erect  and  vertical,  we  look  straight 
forwards  to  the  distant  horizon  ;  the  visual  axes  of  the  two  eyes 
are  then  parallel  to  each  other  and  to  the  median  plane. 

Let  us  now  suppose  three  axes  drawn  through  the  centre 
of  rotation,  in  the  three  planes  of  space :  —  one,  the  visual  axis 
itself,  which  we  may  call  the  longitudinal  axis ;  another  at 
right  angles  to  this  and  horizontal,  the  horizontal  axis ;  and  a 
third  also  at  right  angles,  but  vertical,  the  vertical  axis.  Cor- 
responding to  these  three  axes  we  have  three  main  possible 
movements  of  rotation.  The  eyeball  might  be  rotated  round  the 
vertical  axis  so  that  the  visual  axis  moved  from  side  to  side.  It 
might  be  rotated  round  the  horizontal  axis  so  that  the  visual  axis 
moved  up  and  down.  And  lastly,  it  might  be  rotated  round  the 
longitudinal  axis,  the  visual  axis  itself  remaining  motionless  and 
the  pupil  turning  round  like  a  wheel. 

Now  we  can  easily  carry  out  by  an  exercise  of  the  will  the 
first  and  second  of  these  movements.  We  can  easily  move  the 
eyes  up  and  down,  rotating  them  on  the  horizontal  axis,  as  when 
we  look  up  to  the  heavens  or  down  to  the  ground.  We  can  also 
move  the  eyes  from  side  to  side,  rotating  them  round  the  vertical 
axis,  as  when  we  look  to  the  right  or  to  the  left.  We  can  move 
the  two  eyes  sideways  together  in  the  same  direction  keeping  the 
visual  axes  parallel,  or  we  may  move  them  laterally  in  opposite 
directions,  as  when  the  visual  axes  being  parallel  we  make  them 
converge,  or  when  convergent  bring  them  back  to  or  towards 
parallelism.  And  we  can  combine  rotation  round  the  horizontal 
axis  with  rotation  round  the  vertical  axis,  and  so  give  oblique 
movements  to  the  eyeball.  We  can  do  all  this  by  an  exercise  of 
the  will,  but  we  cannot  by  any  voluntary  effort  carr}^  out  the 
third  kind  of  movement,  we  cannot  rotate  the  eyeball  round  the 
visual  axis,  we  cannot  twist  the  eye  in  a  swivel  movement  round 
its  longitudinal  axis.  There  are  certain  movements  of  the  eye 
in  which  such  a  swivel  rotation,  if  we  may  so  call  it,  does  to  a 
certain  extent  take  place,  and  when  we  induce  these  movements 
we  do  bring  about  such  a  swivel  rotation ;  but  we  cannot  bring 
about  swivel  rotation  by  itself,  we  can  only  effect  it  as  part  of 
the  particular  movements  in  question. 

And  there  is  a  reason  why  we  are  thus  limited  as  to  our 
power  of  moving  the  eyeball.  In  both  rotation  round  the  hori- 
zontal axis,  and  rotation  round  the  vertical  axis  and  in  all  the 
various  combinations  of  these  two  movements  which  are  possible, 
the  two  "  lines  of  separation  "  (§  589)  on  both  the  retinas  keep 
their  places  ;  there  is  no  dislocation  of  the  corresponding  regions 
of  the  two  retinas.    Obviously  the  two  retinal  circles  in  the  lower 

60 


946  LISTING'S   LAW.  [Book  in. 

part  of  Fig.  158  could  be  rotated  round  the  vertical  or  round  the 
horizontal  axis  or  round  any  intermediate  oblique  axis  without 
the  two  images  of  an  external  object  ceasing  to  fall  on  corre- 
sponding parts.  But  if  the  retinal  circles  were  twirled  round 
their  respective  visual  axes,  the  lines  of  separation,  v.  m.  and 
h.  w.,  would  rotate  in  a  clock-hand  fashion,  and  if  the  move- 
ments of  the  two  eyes  were  unequal  or  in  opposite  directions, 
a  dislocation  of  corresponding  parts  would  ensue,  and  vision 
would  become  double.  The  limitation  to  the  movements  of 
the  eyeball  so  as  to  avoid  a  swivel  rotation  is  in  the  interests 
of  binocular  vision. 

§  591.  Not  only  do  we  find  ourselves  thus  limited  in  our 
power  when  we  attempt  by  a  direct  effort  of  our  will  to  execute 
particular  movements  of  the  eyeball,  but  a  similar  limitation 
obtains  in  the  natural  movements  of  the  eye  in  vision.  The 
various  movements  of  the  eyeballs  which  we  carry  out  when 
we  are  looking  at  things  conform  to  a  general  law,  which  is 
known  as  "  Listing's  Law,"  and  which  may  be  described  as 
follows. 

We  stated  a  little  while  back  that  the  "  primary  position " 
of  the  eyeball  is  one  in  which  the  visual  axis  lies  parallel  to  the 
median  plane  and  is  directed  to  the  distant  horizon.  When  the 
eyeball  is  changed  from  this  primary  position  into  any  other 
position,  all  of  which  may  be  called  secondary  positions,  the 
change  is  effected  without  any  swivel  rotation  round  the  visual 
axis  itself ;  the  visual  axis  may  be  directed  up  and  down,  or 
from  side  to  side  or  in  any  intermediate  oblique  manner  with- 
out any  such  swivel  rotation  taking  place.  In  other  words  the 
movements  by  which  the  eyeball  is  brought  from  the  primary 
position  into  any  of  the  secondary  positions  are,  in  all  cases, 
movements  of  rotation  round  the  horizontal  axis,  or  round  the 
vertical  axis,  or  round  an  axis,  which  though  oblique,  being 
neither  horizontal  nor  vertical,  lies  in  the  same  plane  that  they 
do;  that  is  to  say  every  movement  from  the  primary  to  a  secon- 
dary position  is  a  movement  of  rotation  round  an  axis  lying  in 
a  plane  which  passing  through  the  centre  of  rotation  is  vertical 
to  the  visual  axis. 

The  experimental  proof  of  "  Listing's  law  "  may  be  obtained 
by  the  help  of  negative  images  (§  584)  in  the  following  manner. 
Let  the  eye  be  directed  to  a  grey  wall  or  board  which,  otherwise 
of  uniform  appearance,  is  marked  by  parallel  vertical  and  hori- 
zontal lines,  placed  at  some  little  distance  from  each  other  so  as 
to  give  a  pattern  of  squares.  At  one  of  the  intersections,  which 
is  to  be  used  as  the  fixed  point  of  vision,  place  two  narrow  strips 
of  red  paper  in  the  form  of  a  cross,  one  vertical  coinciding  with 
the  vertical  line  and  the  other  horizontal  coinciding  with  the 
horizontal  line.  Having  brought  the  eye  carefully  into  the  pri- 
mary position  stare  at  the  red  cross  until  on  turning  the  eye 


Chap,  hi.]  SIGHT.  947 

away  a  green  negative  image  is  produced.  If  now  the  vision  be 
directed  from  the  fixed  point  either  up  or  down  along  the  verti- 
cal line  of  the  pattern  on  the  wall,  or  from  side  to  side  along 
the  horizontal  line,  it  will  be  found  that  the  cross  of  the  nega- 
tive image  coincides  in  turn  with  each  of  the  crosses  of  the 
pattern  on  the  wall,  the  horizontal  limb  coinciding  with  a  horizon- 
tal line  and  the  vertical  limb  with  a  vertical  line.  This  shews 
that  during  the  up  and  down  and  during  the  side  to  side  move- 
ment, during  the  rotation  of  the  eyeball  round  its  horizontal  or 
round  its  vertical  axis,  no  swivel  rotation  has  taken  place,  for 
otherwise  the  negative  image  would  have  been  turned  round, 
and  its  cross  would  make  an  angle  with  the  image  of  the  cross 
on  the  wall.  If  the  pattern  on  the  wall  be  changed  so  that  the 
lines  while  still  at  right  angles  to  each  other  are  oblique,  not 
vertical  and  horizontal  (this  is  most  conveniently  done  by  using 
not  a  wall  but  a  large  board  and  turning  the  board  round),  and 
the  observation  be  repeated  except  that  the  eye  is  turned  not 
vertically  or  horizontally  but  obliquely  so  as  to  follow  the  lines 
of  the  pattern,  it  will  still  be  found  that  the  cross  of  the  nega- 
tive image  coincides  with  the  cross  of  the  pattern,  and  that 
whatever  be  the  angle  round  which  the  board  has  been  turned. 
This  shews  that  Listing's  law  holds  good  not  only  for  up  and 
down  and  side  to  side  movements  but  also  for  oblique  move- 
ments, for  movements  of  rotation  round  an  axis  which  whatever 
its  obliquity  lies  in  a  plane  at  right  angles  to  the  visual  axis. 

In  the  ordinary  movements  of  the  eye  then,  a  swivel  rota- 
tion round  the  visual  axis  does  not  take  place ;  and  this  limi- 
tation, since  it  holds  good  for  the  two  eyes  used  together,  as 
well  as  for  one  eye  used  by  itself,  serves  to  secure  single  vision 
with  two  eyes  inasmuch  as  it  avoids  changes  which  might  cause 
the  images  of  external  objects  to  fall  on  the  parts  of  the  two 
retinas  which  were  not  "  corresponding  parts."  In  certain  move- 
ments of  the  eyes,  however,  a  certain  amount  of  swivel  rotation 
does  take  place.  This  is  especially  seen  in  somewhat  unusual 
movements.  For  instance  when  the  head  is  turned  down  to  the 
shoulder,  or  again  when  in  directing  vision  to  any  object,  the  head 
is  moved  from  side  to  side,  the  eyes  do  not  move  with  the  head ; 
they  appear  to  remain  stationary,  very  much  as  the  needle  of 
a  ship's  compass  remains  stationary  when  the  head  of  the  "ship 
is  turned.  The  change  in  the  position  of  the  visual  axes  to 
which  the  movement  of  the  head  would  naturally  give  rise  is 
met  by  compensating  movements  of  the  eyeballs ;  Avere  it  not 
so,  steadiness  of  vision  would  be  impossible ;  and  these  compen- 
sating movements  are  found,  on  careful  examination,  to  include 
a  certain  amount  of  swivel  rotation  round  the  visual  axes.  In 
certain  other  more  usual  movements  some  amount  of  such  a 
swivel  rotation  is  also  present ;  and  indeed,  though  so  long  as 
the  visual  axes  remain  parallel,  movement  in  any  direction  may 


948  THE  OCULAE  MUSCLES.  [Book  iii. 

take  place  without  any  such  rotation,  a  slight  amount  does 
intervene  during  convergence  of  the  visual  axes,  as  when  we 
turn  our  eyes  from  a  distant  to  a  near  object.  On  careful 
examination,  however,  it  appears  that  such  an  amount  of  swivel 
rotation  as  does  take  place  is  after  all  for  the  purpose  of  secur- 
ing the  end  that  corresponding  parts  of  the  two  retinas  should 
be  affected  by  the  same  external  object ;  and,  though  we  cannot 
here  enter  more  fully  into  the  subject,  we  may  say  that  not  only 
the  more  general  movements  of  the  eye  which  obey  Listing's 
law,  but  also  those  which  form  an  exception  to  it,  appear  to  be 
carried  out  in  the  interests  of  binocular  vision.  We  may  now 
turn  to  the  study  of  the  ocular  muscles,  by  the  carefully  coordi- 
nated contractions  of  which  the  various  movements,  on  which 
we  have  dwelt,  are  brought  about. 

§  592.  The  muscles  of  the  eyeball  or  ocular  muscles.  The 
eyeball  is  moved  by  six  muscles,  four  of  which  are  straight, 
musculi  recti,  inferior,  superior,  internus  or  medialis  and  externus 
or  lateralis,  and  two  oblique,  musculi  obliqui,  inferior  and  supe- 
rior. The  four  straight  muscles,  taking  origin  from  the  back 
of  the  orbit  around  the  sphenoidal  fissure  and  the  entrance  of 
the  optic  nerve,  are  directed,  as  their  name  indicates,  straight 
forward,  (the  superior  rectus,  however,  having  a  peculiar  bend,) 
and  are  inserted  in  positions  corresponding  to  their  several 
names  into  the  sclerotic,  behind  the  cornea,  the  bundles  of 
fibres  of  the  tendons  being  interwoven  with  those  of  the  scle- 
rotic. The  tendon  of  the  internal  rectus  on  the  median  or 
nasal  side  of  the  eyeball  is  the  broadest  of  the  four ;  that  of  the 
superior  rectus  on  the  upper  surface  being  somewhat  narrower, 
and  those  of  the  inferior  rectus  on  the  under  surface  and  of  the 
external  rectus  on  the  lateral  or  temporal  side,  still  narrower 
(Fig.  159).  The  insertion  of  the  superior  rectus  lies  nearer  to 
that  of  the  external  rectus  than  to  that  of  the  internal  rectus  ; 
its  position  therefore  is  not  exactly  median,  indeed  for  two- 
thirds  of  its  width  it  lies  in  the  upper  lateral  quadrant  of  the 
sclerotic  ring.  The  insertions  of  the  external  and  of  the 
internal  rectus  are  both  median.  The  insertion  of  the  in- 
ternal rectus  is  the  one  closest  to,  and  that  of  the  superior 
rectus  the  one  farthest  away  from  the  cornea,  and  the  latter 
slants  so  as  to  be  nearer  the  cornea  at  its  median  than  at  its 
lateral  end. 

The  superior  oblique  muscle,  or  trochlear  or  pathetic  muscle, 
taking  origin  from  the  back  of  the  orbit  near  the  origin  of  the 
straight  muscles  and  running  forward  internal  to  the  superior 
rectus,  ends  in  a  tendon,  which  changing  its  direction  by  means 
of  a  pulley  (trochlea'),  and  passing  beneath  the  superior  rectus 
is  inserted  into  the  sclerotic  in  the  upper  region  of  the  bulb 
towards  its  hind  part.  The  line  of  insertion  of  the  tendon 
(Fig.  159)  runs  obliquely  from  the  temporal  towards  the  nasal 


Chap,  hi.] 


SIGHT. 


949 


side,  its  mid-point  lying  not  far  from  the  vertical  meridian  of  the 
eyeball. 

The  inferior  oblique  muscle  arises  from  the  front  of  the  floor 
of  the  orbit  on  the  nasal  side ;  it  is  directed  at  first  backwards 
to  the  temporal  side,  underneath  the  inferior  rectus,  between 
that  and  the  floor  of  the  orbit,  and  then  passing  upwards  and 
backwards  is  inserted  into  the  sclerotic  underneath  the  external 


LEFT       EYE 


FROM  TEMPORAL  SIDE 
Sup.R 

^^P-0      Ext.TR 


FROM    ABOVE 


Irif.O 


Sup.O 


fnf.R 


Ext.R 


Inf.O 


Fig.   159.     The    Left    eye    seen   from   A,    the    temporal   side.     B,  from 

ABOVE,    SHEWING    THE    INSERTIONS    OF    THE    OcULAR    MdSCLES,       (JeSSOp.) 


rectus  in  the  hind  temporal  part  of  the  ball.  The  line  of  inser- 
tion (Fig.  159)  is  also  an  oblique  one  like  that  of  the  superior 
ftblique  but  it  is  placed  somewhat  farther  past  it ;  its  hind  end  lies 
not  far  from  the  entrance  of  the  optic  nerve  and  it  runs  thence 
forwards  and  downwards. 

§  593.  The  manner  in  which  these  muscles  are  thus  sever- 
ally attached  to  the  e3''eball  suggests  that  in  contracting  they 
would  move  the  eyeball  in  the  following  ways.  Taking  changes 
in  the  direction  of  the  visual  axis  as  indicating  the  nature  of 
each  movement  we  should  expect  that  the  superior  rectus  would 
turn  the  visual  axis  upwards,  the  inferior  rectus  downwards, 
the  external  rectus  outwards  towards  the  temporal  side,  and  the 
internal  rectus  inwards  towards  the  nasal  side.  The  inferior 
oblique,  its  insertion  being  on  the  hind  and  lateral  part  of  the 
eyeball,  and  the  direction  of  the  muscle  being  downwards, 
would  in  contracting  turn  the  visual  axis  upwards,  wliile  the 
superior  oblique  having  a  somewhat  similar  insertion  but  acting 
in  an  opposite  direction  would  turn  the  visual  axis  downwards. 
Both  muscles  however  in  tlius  raising  or  lowering  the  visual 
axis  would,  owing  to  the  oblique  direction  of  their  insertions 
at  the  same  time,  turn  it  to  the  temporal  side ;  the  movement 
as  the  names  of  the  muscles  suggest,  would  be  an  oblique 
one. 


950 


ACTION  OF   OCULAE   MUSCLES.         [Book  in. 


The  six  muscles  therefore  Avould  seem  to  act  as  three  pairs, 
the  superior  and  inferior  rectus,  the  internal  and  external  rec- 
tus, and  the  inferior  and  superior  oblique,  each  pair  rotating 
the  eyeball  round  a  particular  axis.  Calculations  based  on  a 
careful  study  of  the  attachments  and  directions  of  the  several 
muscles,  and  the  results  of  actual  observations,  shew  that  this 
is  so,  and  that  the  movements  carried  out  by  the  several  pairs 
may  be  more  accurately  described  as  follows. 

The  superior  rectus  and  the  inferior  rectus  (see  Fig.  160) 
rotate  the  eye  round  a  horizontal  axis,  which  may  be  described 


obi  sup 


r  Slip 


Fig.  160.    Diagram  to  illustrate  the  actions  of  the  Muscles  of  the  Eye. 

■  The  eye  represented  is  the  left  eye  seen  from  above.  The  thick  lines  shew, 
by  means  of  the  arrows,  the  direction  in  which  the  several  muscles  pull,  the 
beginning  of  each  line  also  indicating  the  attachment  of  the  muscle.  The  dotted 
lines  indicate  the  axis  of  rotation  of  the  superior  and  inferior  rectus  and  of  the 
oblique  muscles.  The  axis  of  rotation  of  the  internal  and  external  rectus  being 
perpendicular  to  the  plane  of  the  paper  cannot  be  shewn,  v  x  represents  the 
visual  axis  and  h  x  a,  line  at  right  angles  to  it.     (After  Fick.) 


as  one  directed  from  the  root  of  the  nose  to  the  temple ;  it  is 
therefore  not  a  line  at  right  angles  with  the  visual  axis  but  one 
making  an  acute  angle  (20°)  with  such  a  line.  The  superior 
and  inferior  oblique  rotate  the  eye  round  a  horizontal  axis 
which  may  be  described  as  one  directed  from  the  centre  of  the 
eyeball  to  the  occiput ;  it  again  is  not  a  line  at  right  angles  to 
the  visual  axis,  but  makes  an  angle,  with  such  a  line,  larger 
(60°)  than  the  similar  angle  made  by  the  inferior  and  superior 
rectus,  and  turned  in  a  different  direction.  The  internal  rectus 
and  external  rectus  rotate  the  eyeball  round  a  vertical  axis  pass- 


Chap,  hi.]  SIGHT.  951 

ing  through  the  centre  of  rotation  of  the  eyeball  parallel  to  the 
median  plane  of  the  head  when  the  head  is  vertical :  this  there- 
fore is  at  right  angles  to  the  visual  axis,  and  so  differs  from  the 
other  two. 

When  we  compare  the  movements  thus  effected  by  these 
several  pairs  of  muscles  with  the  movements  which  we  described 
above  (§  590)  as  the  ordinary  movements  of  the  eye,  namely 
movements  of  rotation  round  a  vertical  and  round  a  horizontal 
axis  both  at  right  angles  to  the  visual  axis,  we  see  that  it  is  only 
the  movements  round  the  vertical  axis  which  can  be  carried  out 
by  one  pair  of  muscles  acting  alone,  the  particular  pair  being 
the  internal  and  external  rectus.  Neither  the  horizontal  axis  of 
rotation  of  the  inferior  and  the  superior  rectus,  nor  that  of  the 
oblique  muscles,  is  placed  exactly  at  right  angles  to  the  visual 
axis;  each  of  them  makes  an  oblique  angle  with  that  axis. 
Hence  when  in  carrying  out  the  ordinary  movements  of  the  eye 
we  rotate  the  eyeball  round  the  horizontal  axis,  we  do  not  em- 
ploy either  of  these  pairs  of  muscles  alone,  but  combine  them, 
making  use  of  one  muscle  of  one  pair  with  one  of  the  other. 
The  superior  and  inferior  rectus  in  moving  the  visual  axis  up 
and  down  also  turn  it  somewhat  inwards,  to  the  nasal  side ;  but 
this  is  corrected  if  the  oblique  muscles  act  at  the  same  time  ; 
and  it  is  found  that  the  rectus  superior  acting  with  the  inferior 
oblique  moves  the  visual  axis  directly  upwards,  and  the  rectus 
inferior  acting  with  the  superior  oblique  directly  downwards  in 
a  vertical  direction ;  that  is  to  say  the  two  combinations  rotate 
the  eyeball  round  a  horizontal  axis  at  right  angles  to  the  visual 
axis. 

Hence  there  are  only  two  movements  of  the  eyeball  which 
we  can  carry  out  by  the  help  of  one  muscle  alone,  namely  that 
in  which  we  simply  turn  the  visual  axis  to  the  nasal  side,  em- 
plojdng  the  internal  rectus,  and  that  in  which  we  turn  it  to  the 
temporal  side,  employing  the  external  rectus,  the  visual  axis  in 
both  cases  remaining  in  the  same  plane,  the  visual  plane.  In 
order  to  raise  or  lower  the  visual  axis  in  the  same  vertical  plane, 
without  lateral  movement,  we  must  use  two  muscles ;  and  if  we 
wish  to  execute  an  oblique  movement  combining  an  up  and  down 
with  a  side  to  side  movement  of  the  visual  axis  we  must  employ 
three  of  the  ocular  muscles.  These  several  movements,  with  the 
muscles  concerned,  may  be  stated  as  follows,  the  movement  in 
each  case  being  described  with  reference  to  changes  in  the  direc- 
tion of  the  visual  axis. 


^■s 


en  o 


^         To  nasal  side.  Internal  rectus. 

^  I  I  To  temporal  side.  External  rectus. 

■^         Upwards.  Superior  rectus  and  inferior  oblique. 

Downwards.  Inferior  rectus  and  superior  oblique. 


a  I 


M 

0) 

+3 

I—* 

rO 

> 
o 

O 

s 

952   COOEDINATION  OF  OCULAR  MOVEMENTS.  [Book  hi. 

Upwards  and  to  Superior  rectus,  internal  rectus  and 

nasal  side.  inferior  oblique. 

Downwards  and  to  Inferior  rectus,  internal  rectus  and 

nasal  side.  superior  oblique. 

Upwards  and  to  Superior  rectus,  external  rectus  and 

temporal  side.  inferior  oblique. 

Downwards  and  to  Inferior  rectus,  external  rectus  and 

temporal  side.  superior  oblique. 

The  fact  that  in  our  ordinary  movements  of  the  eye  we  do 
thus  combine  the  actions  of  muscles,  and  the  advantages  of  such 
a  combination  are  further  shewn  in  connection  with  that  swivel 
rotation  of  the  eye  round  the  visual  axis  itself,  which,  as  we 
have  seen,  is  wholly  avoided  in  many  of  our  movements  and 
which  we  cannot  carry  out  by  a  direct  effort  of  the  will.  The 
superior  rectus  acting  by  itself,  owing  to  the  position  of  its 
insertion  in  reference  to  the  direction  of  the  fibres,  not  only 
turns  the  visual  axis  inwards  while  directing  it  upwards,  but 
also  to  a  slight  extent  rotates  the  eye  round  the  visual  axis , 
and  the  inferior  rectus  as  well  as  both  the  oblique  muscles  in 
like  manner  tend  in  contracting  to  give  the  eyeball  such  a 
swivel  rotation.  This  tendency  of  the  superior  rectus  like  its 
tendency  to  turn  the  visual  axis  inwards  is  counteracted  by  the 
inferior  oblique,  the  swivel  rotation  of  the  latter  being  contrary 
in  direction  to  that  of  the  former ;  and  the  like  tendency  of  the 
inferior  rectus  is  in  like  manner  counteracted  by  the  superior 
oblique.  Thus  the  movements,  in  carrying  out  which  these 
muscles  are  combined,  are  rendered  free  from  the  swivel  rotation 
element.  On  the  other  hand  this  tendency  of  the  muscles  in 
question  is  utilized  in  the  particular  movements  in  which  the 
swivel  rotation  does  take  place. 

§  594.  The  coordination  of  the  movements  of  the  eyes.  The 
external  rectus  is  governed  by  the  sixth  nerve,  nervus  abducens, 
the  nucleus  of  which  lies  in  the  floor  of  the  fourth  ventricle  in 
a  position  indicated  by  the  eminentia  teres.  The  superior 
oblique  muscle  is  governed  by  the  fourth  nerve,  nervus  troch- 
learis,  the  nucleus  of  which  lies  in  the  floor  of  the  aqueduct, 
in  the  region  of  the  posterior  corpus  quadrigeminum.  All  the 
other  ocular  muscles  are  governed  by  the  third  nerve,  the 
nucleus  of  which  lies  in  the  floor  of  the  aqueduct  in  the  region 
of  the  anterior  corpus  quadrigeminum  ;  as  we  have  said  (§  539), 
the  fibres  of  the  third  nerve  going  to  these  ocular  muscles  seem 
to  be  more  especially  connected  with  the  hind  part  of  the 
nucleus. 

From  what  has  been  said  above  it  is  obvious  that,  even  in 
the  movements  of  one  eye,  a  coordination  of  the  motor  nervous 
impulses  must  in  most  cases  take  place.  When  we  turn  the 
visual  axis  outwards  the  motor  impulses  are,  it  is  true,  confined 


Chap,  hi.]  SIGHT.  953 

to  the  sixth  nerve,  reaching  the  external  rectus,  and  when  we 
turn  it  inwards  are  confined  to  the  third  nerve,  reaching  the 
internal  rectus  ;  but  in  all  other  movements  motor  impulses 
must  descend  to  at  least  two  muscles  along  different  nerve- 
branches,  and  in  many  cases  must  start  from  two  or  even  all 
three  of  the  cranial  nuclei  just  mentioned.  Even  in  movements 
of  one  eye  there  must  be,  in  most  cases,  more  or  less  coordina- 
tion of  actual  motor  impulses,  in  order  to  secure  due  efficiency 
of  the  movement;  by  actual  motor  impulses  we  mean  impulses 
leading  to  the  contraction  of  muscular  fibres,  irrespective  of 
any  influences  which  may  at  the  same  time  be  brought  to  bear 
on  antagonistic  muscles,  in  order  to  facilitate  or  qualify  the 
movement. 

But  if  this  is  true  in  the  case  of  one  eye,  much  moi-e  is  it 
true  when  we  use  both  eyes  in  binocular  vision. 

Two  facts  about  binocular  vision  strike  our  attention.  The 
one  is  that,  as  may  be  seen  by  watching  the  movements  of  any 
person's  eyes,  the  two  eyes  move  together.  If  the  right  eye 
moves  to  the  right,  so  does  also  the  left,  and,  if  the  object 
looked  at  be  a  distant  one,  exactly  to  the  same  extent;  if  the 
right  eye  looks  up,  the  left  eye  looks  up  also ;  and  so  with 
regard  to  other  movements.  Very  few  persons  are  able  by  a 
direct  effort  of  the  will  to  move  one  eye  independently  of  the 
other;  though  by  some  the  power  has  been  acquired.  We  shall 
refer  immediately  to  particular  movements  in  which  one  eye 
only  is  moved,  while  the  other  remains  motionless.  The  other 
salient  fact  is  that  the  movements  of  the  two  eyes  are  limited  in 
certain  ways.  As  we  have  seen  one  of  the  simplest  ocular 
movements  is  the  side  to  side  movement  of  the  visual  axis,  and 
one  of  the  commonest  binocular  movements  is  the  convergence 
of  the  visual  axes,  as  when  we  turn  our  eyes  from  something 
far  off  to  something  near,  or  converselj^  the  change  from  con- 
siderable convergence  to  less  convergence  as  when  we  turn  our 
eyes  from  something  near  to  something  farther  off.  In  a  large 
number  of  instances  this  change  to  convergence  from  parallelism, 
or  this  increase  or  decrease  of  convergence  takes  place  without 
any  change  in  the  visual  plane,  without  any  raising  or  lowering 
of  the  visual  axes;  in  such  instances  the  movement  is  carried 
out  in  convergence  by  the  two  internal  rectus  muscles,  or  in 
decrease  of  convergence  by  the  two  external  rectus  muscles; 
and  the  only  coordination  necessary  is  one  which  secures  that 
the  muscle  of  one  eye  should  work  in  harmony  with  tlie  muscle 
of  the  other  eye.  But  even  this  relatively  simple  movement  is 
limited  in  a  very  marked  way.  We  can  bring  the  visual  axes 
of  the  two  ej-es  from  a  condition  of  parallelism  to  one  of  almost 
any  degree  of  convergence,  but  we  cannot,  without  artificial 
assistance,  bring  them  from  a  condition  of  ^parallelism  to  one  of 
divergence.     The  stereoscope  will  enable  us  to  create  such  a 


954    COOEDINATION  OF  OCULAR  MOVEMENTS.   [Book  hi. 

divergence.  If  in  a  stereoscope  the  distance  between  the  pic- 
tures be  increased  very  gradually  so  as  carefully  to  maintain 
the  impression  of  a  single  object,  the  visual  axes  may  be  brought 
to  diverge ,  and  the  subject  of  the  experiment  may  himself  be 
made  aware  of  the  divergence,  by  the  sudden  removal  of  the 
instrument  from  his  eyes ;  his  vision  of  external  objects  is  for 
a  moment  double,  but  for  a  moment  only.  This  experiment 
shews  the  reason  of  the  limitation  of  which  we  are  speaking. 
So  long  as  the  visual  axes  are  parallel  or  appropriately  con- 
vergent the  images  of  external  objects  fall  on  corresponding 
parts  of  the  two  retinas,  and  single  vision  results;  when  the 
visual  axes  are  carried  beyond  parallelism,  the  images  on  the 
two  retinas  are  not  on  corresponding  parts  and  vision  is  double. 
Thus,  as  regards  convergence  or  divergence  of  the  visual  axes, 
the  movements  of  the  two  eyes  are  governed  by  the  principle 
that  the  will  can  of  itself  only  carry  out  those  movements  which 
are  consistent  with  images  of  external  objects  falling  on  corre- 
sponding parts  of  the  two  retinas.  There  is  an  exception  to  this 
in  the  case  of  extreme  convergence ;  we  can  as  in  squinting 
make  the  visual  axes  converge  too  much,  and  in  consequence  by 
a  simple  effort  of  the  will  can  obtain  double  vision  ;  but  this  is 
probably  in  order  to  leave  a  margin  which  shall  secure  our  being 
able  to  use  to  the  utmost  our  accommodation  mechanism  for 
near  objects ;  otherwise  the  rule  holds  good.  Not  only  so,  but 
as  the  above  experiment  also  shews,  when  by  artificial  assist- 
ance, which  is  in  itself  directed  towards  securing  single  vision 
with  the  two  eyes,  we  obtain  divergence  of  the  visual  axes, 
immediately  that  the  assistance  is  done  away  with  the  axes 
return,  by  an  involuntary  movement,  to  parallelism ;  the  double 
vision  occurring  at  the  moment  of  removal  of  the  instrument 
rapidly  gives  way  to  normal  single  vision.  Other  illustrations 
of  the  same  principle  may  be  met  with.  For  instance,  if  a  dis- 
tant object  be  looked  at  with  both  eyes,  but  with  a  prism  held 
horizontally  before  one  eye,  and  if  the  image  of  the  object  be 
kept  carefully  single  while  the  prism  is  turned  very  slowly 
from  the  horizontal  to  the  vertical  position,  then  on  suddenly 
removing  the  prism  a  double  image  is  for  a  moment  seen ;  this 
shews  that  the  eye  before  which  the  prism  was  placed  had 
moved  in  disaccordance  with  the  other.  The  double  image, 
however,  immediately  after  the  removal  of  the  prism,  becomes 
single  on  account  of  the  eyes  coming  into  accordance. 

When  we  examine  all  the  various  movements  of  the  eyes 
which  we  are  capable  of  making  by  a  direct  effort  of  the  will, 
we  find  that  they  are  all  of  such  a  kind  that  through  them  the 
two  images  of  an  external  object  are  brought  upon  correspond- 
ing parts  of  the  two  retinas  ;  conversely  the  movements  which 
could  be  effected  by  the  contractions  of  this  or  that  ocular  mus- 
cle, but  the  effect  of  which  would  be  to  bring  the  two  images 


Chap,  hi.]  SIGHT.  955 

on  to  parts  of  the  retina  which  do  not  correspond,  are  the  move- 
ments which  our  unassisted  will  cannot  carry  out. 

In  an  earlier  part  of  the  work  (§  478)  we  insisted  at  some 
length  on  the  important  share  taken  by  sensations,  or  at  least 
by  afferent  impulses,  in  the  coordination  of  motor  impulses;  and 
the  movements  of  the  eye  illustrate  this  in  a  very  marked  de- 
gree. All  the  various  movements  of  the  eye  are  dependent  on 
visual  sensations.  The  issue  of  each  efferent  motor  volitional 
impulse  is  dependent  on  afferent  visual  impulses.  In  order  to 
move  our  eyes,  we  must  either  look  at  or  for  an  object,  when 
we  wish  to  converge  our  axes,  we  look  at  some  near  object  real 
or  imaginary,  and  the  convergence  of  the  axes  is  usually  accom- 
panied by  all  the  conditions  of  near  vision,  such  as  increased 
accommodation  and  constriction  of  the  pupil.  And  so  with  other 
ocular  movements.  Above  all,  the  careful  selection  of  this  or 
that  ocular  muscle,  the  extent  to  which  it  is  to  be  thrown  into 
contraction,  its  accompaniment  by  the  contraction  of  other  ocular 
muscles  and  the  due  coordination  of  all  the  several  contractions 
—  all  these  things  are  so  determined  by  visual  sensations  that 
the  two  images  of  each  object  looked  at  fall  on  corresponding 
parts  of  the  two  retinas. 

A  little  reflection  will  shew  how  large  an  amount  of  coordi- 
nation must  thus  take  place  in  daily  life,  how  in  the  various 
movements  of  the  eye  there  must  be,  so  to  speak,  the  most  deli- 
cate picking  and  choosing  of  the  muscular  instruments.  When 
we  look  at  an  object  to  the  right,  since  we  thereby  turn  the 
right  eye  to  the  temporal  side,  and  the  left  eye  to  the  nasal  side, 
we  throw  into  action  the  external  rectus  of  the  right  eye  and 
the  internal  rectus  of  the  left;  and  similarly  when  we  look  to  the 
left  we  use  the  external  rectus  of  the  left  and  the  internal  rectus 
of  the  right  eye.  On  the  other  hand  when  we  look  at  a  near  ob- 
ject, and  therefore  converge  the  visual  axes,  we  use  the  internal 
rectus  of  both  eyes ;  and  when  we  look  at  a  distant  object,  and 
bring  the  axes  from  convergence  towards  parallelism,  we  use  the 
external  rectus  of  both  eyes.  Or  to  take  another  instance.  Sup- 
pose the  eyes,  to  start  with,  directed  for  the  far  distance,  and  that 
it  is  desired  to  direct  attention  to  a  nearer  point  lying  in  the  vis- 
ual line  of  the  right  eye.  In  this  case  no  movement  of  the  right 
eye  is  required ;  all  that  is  necessary  is  for  the  left  eye  to  be  turned 
to  the  right,  that  is,  for  the  internal  rectus  of  the  left  eye  to  be 
thrown  into  action.  But  in  ordinary  movements  the  contrac- 
tion of  this  muscle  is  always  associated  with  either  the  external 
rectus  of  the  right  eye,  as  when  both  eyes  are  turned  to  the 
right,  or  the  internal  rectus  of  that  eye,  as  in  convergence ;  the 
muscle  is  quite  unaccustomed  to  act  alone.  This  would  lead 
us  to  suppose  that  in  the  case  in  question  the  contraction  of  the 
internal  rectus  of  the  left  eye  is  accompanied  by  a  contraction 
of  both  the  external  and  the  internal  rectus  of  the  right  eye. 


956   COORDINATION  OF  OCULAR  MOVEMENTS.    [Book  hi. 

keeping  that  eye  in  lateral  equilibrium.  And  the  peculiar 
oscillating  movements  seen  in  the  right  eye,  as  well  as  the 
sense  of  effort  in  the  right  eye  which  is  felt  by  the  person,  sup- 
port this  idea.  We  need  not  multiply  these  instances ;  it  must 
be  sufficiently  obvious  that  a  very  large  amount  of  coordination 
takes  place  in  the  daily  use  of  our  eyes. 

§  595.  Such  a  coordination  involves  the  existence  of  what, 
to  continue  the  use  of  a  term  which  we  have  previously  used, 
we  may  call  a  coordinating  nervous  mechanism.  The  coor- 
dinated efferent  impulses  issue  from  one  or  more  of  the  nuclei 
of  the  three  cranial  nerves  concerned,  namely  the  sixth,  the 
fourth,  and  the  third.  The  afferent  visual  impulses  taking  part 
in  the  coordination,  we  have  in  an  earlier  part  of  this  book 
(§  496)  traced  to  the  primary  visual  centres,  and  thence  to  the 
occipital  cortex.  The  volitional  impulses  themselves  are  we 
have  seen  (§  484)  connected  in  some  way  or  other  with  an  area 
of  the  cortex  lying  in  the  monkey  in  the  frontal  lobe,  in  the 
neighbourhood  and  in  front  of  the  precentral  fissure  (Figs.  122, 
123)  and  probably  in  man  occupying  a  corresponding  position. 
How  are  these  three  factors  of  the  whole  nervous  action  brought 
to  bear  the  one  on  the  other  ?  When  it  is  remembered  how 
complex  and  delicately  balanced  are  the  movements  in  question, 
probably  the  most  intricate  and  the  most  delicately  balanced  of 
all  the  movements  of  the  body,,  it  will  readily  be  understood 
how  difficult  is  the  answer  to  such  a  question.  Stimulation  of 
the  cortical  area  for  movements  of  the  eyes  leads  as  might  be  ex- 
pected to  bilateral  movements,  to  movements  of  both  eyes.  The 
most  common  effect  of  stimulating  the  cortical  area  is  a  lateral 
movement  of  both  eyes  in  the  same  direction  towards  the  oppo- 
site side,  a  conjugate  lateral  deviation  of  both  visual  axes  towards 
the  opposite  side.  For  instance  when  the  cortical  area  of  the  left 
hemisphere  is  stimulated,  the  visual  axes  of  both  eyes  are  turned 
to  the  right,  the  external  rectus  of  the  right  eye  and  the  internal 
rectus  of  the  left  eye  being  thrown  into  contraction  by  impulses 
passing  down  the  right  sixth  nerve  and  left  third  nerve  ;  the 
efferent  impulses  therefore  cross  in  the  case  of  one  nerve  but 
not  in  the  case  of  the  other.  Similarly,  when  the  right  hemis- 
phere is  stimulated,  impulses  pass  down  the  right  third  nerve 
and  left  sixth  nerve. 

Though  these  lateral  movements  are  those  most  easily  pro- 
duced, other  movements,  conjugate  raising  or  lowering  of  both 
eyeballs,  oblique  movements,  and  even  movements  of  convergence 
have  been  obtained.  And,  were  our  means  of  stimulation  ade- 
quately discriminating,  it  would  probably  be  found  that  each  of 
the  several  ocular  movements  might  be  called  forth  by  stimu- 
lating the  appropriate  cortical  focus.  Like  movements  of  the 
eyeballs  may  also  be  obtained  by  stimulating  not  the  frontal 
motor  region,  but  the  occipital  region  (§  498).     These  latter 


Chap,  hi.] 


SIGHT. 


957 


movements  are  not  due  to  the  frontal  motor  area  being  in- 
directly thrown  into  action,  since  they  appear  even  after  this  has 
been  removed ;  they  are  obviously  brought  about  by  a  separate 
mechanism.  The  action  of  the  cortex,  moreover,  appears  not  to 
be  limited  to  producing  contractions  in  these  ocular  muscles; 
it  may  take  on  the  character  of  inhibition.  If  the  third  and 
fourth  nerves  be  divided  on  one  side,  so  as  to  leave  the  rectus 
externus  of  that  eye  alone  available,  not  only  is  the  opposite  eye 
moved  outwards,  upon  stimulation  of  the  cortex  on  that  side, 
but  the  eye  of  the  same  side  follows  it  to  a  certain  extent ;  that 
is  to  say  the  stimulation  of  the  cortex,  while  it  leads  to  contrac- 
tion in  the  opposite  rectus  externus,  inhibits  a  tonic  contraction 
of  the  rectus  externus  of  the  eye  of  the  same  side  and  so 
permits  that  eye  to  move  in  the  same  direction  as  its  fellow. 
This  is  a  further  indication  of  the  complexity  of  the  coor- 
dination of  these  ocular  muscles.  This  coordination  is  not 
effected  or  not  wholly  effected  in  the  cortex,  since  coordinate 
movements  may  be  produced  by  stimulating  the  fibres  leading 
from  the  cortex.  Possibly  some  at  least  of  the  coordination  is 
effected  by  help  of  the  anterior  corpora  quadrigemina,  since 
coordinate  ocular  movements  may  be  obtained  by  directly 
stimulating  these  bodies.  The  tract  of  fibres  known  as  the  pos- 
terior longitudinal  bundles,  which  seems  to  serve  as  a  tie  uniting 
the  several  ocular  nuclei,  probably  also  plays  a  part  in  the 
matter. 

The  Horopter. 

§  596.    When  we  look  at  any  object  we  direct  to  it  the  visual 
axes,  so  that  when  the  retinal  image  of  the  object  is  small,  the 


Fig.  161.    Diagram  illustrating  a  simplk  Horopter. 

"When  the  visual  axes  converge  at  C,  the  images  a  a  oi  any  point  A  on  the 
circle  drawn  through  C  and  the  nodal  points  k  i,  will  fall  on  corresponding  points. 

'  corresponding '  parts  of  the  two  retinas,  on  which  the  two 
images  of  the  object  fall,  lie  in  their  respective  fovea)  centrales. 


958  THE  HOROPTER.  [Book  iii. 

But  while  we  are  looking  at  the  particular  object  the  images  of 
other  objects  surrounding  it  fall  on  the  retina  surrounding  the 
fovea,  and  thus  go  to  form  what  is  called  indirect  vision.  And 
it  is  obviously  of  advantage  that  other  images,  besides  that  of 
the  object  to  which  we  are  specially  directing  our  attention, 
should  fall  on  '  corresponding '  parts  in  the  two  eyes.  Were  it 
not  so,  while  our  vision  of  the  particular  object  would  be  single, 
our  vision  of  all  its  surroundings  would  be  double ;  and  this,  at 
least  in  certain  cases,  would  be  confusing.  For,  even  when  we 
are  concentrating  our  attention  on  a  particular  object,  we  are 
still  conscious  of  its  surroundings,  and  besides,  our  appreciation 
of  any  image  falling  on  the  fovea  is  influenced  by  impressions 
Avhich  we  are  at  the  same  time  receiving  from  other  parts  of  the 
retina. 

Now  for  any  given  position  of  the  eyes  there  exists  in  the  field 
of  sight  a  certain  line  or  surface  of  such  a  kind  that  the  images 
of  the  points  in  it  all  fall  on  corresponding  points  of  the  retina. 
A  line  or  surface  having  this  property  is  called  a  Horopter.  The 
horopter  is  in  fact  the  aggregate  of  all  those  points  in  space 
which,  in  any  given  position  of  the  eyes,  are  projected  on  to  cor- 
responding points  of  the  retina ;  hence  its  determination  in  any 
particular  case  is  simply  a  matter  of  geometrical  calculation.  In 
some  instances  it  becomes  a  very  complicated  figure.  The  case 
whose  features  are  most  easily  grasped  is  that  of  a  circle  drawn 
in  the  plane  of  the  two  visual  axes  through  the  point  of  the  con- 
vergence of  the  axes  and  the  nodal  points  of  the  two  eyes  such 
as  is  shewn  in  Fig.  161.  It  is  obvious  from  geometrical  rela- 
tions that  the  two  images  of  any  point  in  such  circle,  the  rays 
from  which  can  enter  the  two  pupils  and  fall  on  the  two  retinas, 
will  fall  on  corresponding  points  of  the  two  retinas.  When  we 
study  the  various  horopters  of  the  several  positions  which  the 
two  eyes  can  take  up,  we  find  that  the  characters  of  the  horoj)- 
ter  are  adapted  to  the  needs  of  our  daily  life.  Thus  in  the  posi- 
tion assumed  by  the  two  eyes  when  we  stand  upright  and  look 
at  the  distant  horizon  the  horopter  is  (approximately,  for  nor- 
mal emmetropic  eyes)  a  plane  drawn  through  our  feet,  that  is 
to  say,  is  the  ground  on  which  we  stand ;  the  advantage  of 
this  is  obvious. 

Nevertheless,  in  most  positions  of  the  eyes  a  large  number  of 
the  images  which  make  up  the  binocular  visual  field,  do  not  lie 
on  any  horopter,  do  not  fall  on  corresponding  points,  and  give 
rise  not  to  one  sensation  only  but  to  two  sensations  differing  to 
a  certain  extent  from  each  other.  A  great  deal  of  what  we  see 
is  seen  double  by  us,  we  receive  from  many  objects  two  unequal 
impressions  ;  but  the  inequality  chiefly  serves  to  give  an  appear- 
ance of  "  solidity  "  to  the  objects,  to  assist  in  our  judgment  of 
solidity.  To  the  consideration  of  these  and  other  visual  judg- 
ments as  well  as  of  some  other  psychological  features  of  vision 
we  must  now  turn. 


SEC.  10.     ON"   SOIME  FEATURES   OF 
VISUAL  PERCEPTIONS  AND  ON  VISUAL  JUDGMENTS. 

§  597.  V^e  may  now  turn  our  attention  to  some  of  those 
differences  between  the  features  of  external  objects  and  our 
perception  of  them  which  are  more  distinctly  of  psychological 
origin ;  but  since  the  purpose  of  this  work  is  physiological  and 
not  psychological  we  must  be  content  to  treat  them  ver}*  briefly. 

Taking  first  of  all  the  general  features  of  the  field  of  vision, 
we  find  psychical  processes  entering  largel}'  even  into  these.  As 
we  have  incidentally  seen,  the  sensations  which  an  object  excites 
are  very  different  according  as  the  object  is  in  the  central  or  in 
the  peripheral  region  of  the  field  of  sight.  Two  parts  of  the 
object  sufficiently  far  apart  to  give  rise  to  two  sensations  in  the 
former  case  may  give  rise  to  one  sensation  only  in  the  latter 
case ;  and  the  colour  sensations  excited  by  the  same  object  may 
be  widely  different  in  the  two  cases.  If  we  picture  to  ourselves 
the  group  of  sensations  excited  by  the  image  of  an  object,  such  as 
a  flower,  when  the  image  falls  on  the  fovea,  and  compare  that 
group  with  the  group  of  sensations  excited  by  the  same  flower 
when  the  image  of  it  falls  on  the  periphery  of  the  retina,  sup- 
posing the  comparison  to  be  made  before  the  sensations  are 
moulded  into  psychical  perceptions,  the  two  groups  would 
appear  to  belong  to  very  unlike  objects.  Moreover,  when  we 
use  both  eyes,  the  images  of  some  of  the  objects  in  the  field  of 
sight  are  falling  on  both  retinas,  while  others  are  falling  on  one 
retina  only,  and  of  those  which  fall  on  both  retinas,  some  lie  on 
corresponding  points,  so  that  the  sensations  of  the  two  eyes  are 
blended,  while  others,  not  lying  in  the  horopter,  give  rise  to 
sensations  in  one  eye  different  from  those  in  the  other.  Could 
we  become  aware  of  the  crude  sensations  which  go  to  make  up 
our  field  of  vision,  they  would  appear  as  a  heterogeneous  med- 
ley. But  in  the  field  of  vision  of  which  we  are  actually  aware, 
that  in  which  the  crude  sensations  have  by  psychical  operations 
been  moulded  into  perceptions,  we  do  not  recognize  the  various 
discrepancies  of  which  we  are  speaking;  the  field  of  vision  is 
homogeneous.  When  we  look  at  a  landscape  we  are  not  aware 
that  objects  on  the   far  left  or  far  right  hand  are  producing 

tl59 


960  VISUAL  PEECEPTIONS.  [Book  in. 

sensations  in  a  way  very  different  from  that  in  which  objects 
directly  in  the  line  of  vision  are  producing  sensations ;  it  is 
only  by  special  analysis  that  we  become  acquainted  with  the 
properties  of  the  peripheral  retina.  In  actual  vision  the  activi- 
ties of  the  central  retina  by  virtue  of  psychical  processes  dom- 
inate those  of  the  periphery.  Conversely  though,  as  we  have 
said,  when  we  wish  to  see  anything  very  distinctly  we  habitually 
make  use  of  the  central  retina ;  yet  nevertheless  in  ordinary 
vision,  at  the  same  time  that  we  are  thus  making  use  of  the 
central  retina  we  are  also  receiving  impressions  from  the  whole 
of  the  rest  of  the  retina  within  the  field  of  vision,  and  these 
more  or  less  peripheral  impressions  influence  to  a  certain  extent 
the  psychical  effect  of  the  central  sensations.  Our  perception 
of  an  object,  such  as  a  flower,  is  not  the  same  when  we  look  at 
it  as  part  of  a  landscape,  making  use  of  the  whole  field  of  vision, 
as  when  we  look  at  it  through  a  tube  or  otherwise  in  such  a 
way  as  to  exclude  peripheral  vision ;  the  flower  in  the  latter  case 
seems  much  more  brilliant,  and  more  highly  coloured.  Some 
of  the  effect  in  this  case  may  be  physiological  and  due  to  retinal 
events,  but  the  greater  part  is  psychical.  The  influence  of 
psychical  processes  is  probably  also  illustrated  by  the  experi- 
ence that,  if  on  turning  our  back  on  a  landscape,  we  bend  the 
body  so  as  to  get  a  view  of  the  landscape  backwards  between  the 
legs,  all  the  objects  seem  to  have  an  unusually  brilliant  colouring. 
A  striking  difference  between  the  objective  field  of  sight 
and  the  subjective  field  of  vision  is  illustrated  by  the  fact  that, 
though,  as  we  have  seen,  that  part  of  the  retina  which  corre- 
sponds to  the  entrance  of  the  optic  nerve  is  quite  insensible  to 
light,  we  are  conscious  of  no  corresponding  blank  in  the  field 
of  vision.  When  in  looking  at  a  page  of  print  we  so  direct  the 
visual  axis  that  some  of  the  print  must  fall  on  the  blind  spot, 
no  gap  in  the  print  is  perceived ;  we  have  to  take  special  meas- 
ures (§  573)  to  discover  the  existence  of  the  spot.  We  could 
not  expect  to  see  a  black  patch,  because  what  we  call  black  is 
the  absence  of  the  sensation  of  light  from  structures  which  are 
sensitive  to  light ;  we  must  have  visual  organs  to  see  black. 
But  there  are  no  visual  organs  in  the  blind  spot,  and  conse- 
quently we  are  in  no  way  at  all  affected  by  the  rays  of  light 
which  fall  on  it.  By  psychical  operations  we  "fill  up,"  as  it  is 
said,  the  vacancy  caused  by  the  blind  spot,  so  that  there  is  in 
our  subjective  field  of  vision  no  gap  corresponding  to  the  gap 
in  the  retinal  image ;  we  treat  the  sensations  coming  from  two 
points  of  the  retina  lying  on  opposite  margins  of  the  blind  spot 
as  if  they  were  sensations  excited  in  two  points  lying  close 
together,  thus  preserving  the  continuity  of  the  field  of  vision 
between  them.  Concerning  the  particular  psychical  actions 
by  which  this  is  carried  out,  and  concerning  the  special  effects 
which  are  produced  when  an  object  in  the  field  of  sight  passes 


Chap,  hi.]  SIGHT.  961 

into  the  region  of  the  blind  spot  there  has  been  much  discus- 
sion ;  but  into  this  we  cannot  enter  here. 

In  ordinary  vision,  the  existence  of  the  blind  spot  is  of  little 
moment.  Since  it  lies  outside  the  region  of  distinct  vision,  and 
since  moreover  in  each  movement  of  the  eye  the  image  of  a 
fresh  part  of  the  external  world  falls  upon  it,  the  errors  to 
which  it  may  lead  are  not  serious  even  when  we  use  one  eye 
only.  The  deficiency  is  further  remedied  by  the  use  of  two 
eyes,  since,  the  two  blind  spots  being  each  on  the  nasal  side, 
the  image  of  an  object  will  not  fall  on  both  blind  spots  at  the 
same  time.  Other  smaller  or  accidental  imperfections  in  one 
or  both  eyes  are  similarly  remedied  b}'^  the  use  of  two  eyes. 

§  598.  Turning  now  to  the  psychical  processes  connected 
with  the  perception  of  particular  objects,  we  find  these  to  be 
very  complex.  Some  of  them  relate  to  the  very  formation  of 
the  perception  out  of  the  sensations  which  the  object  excites, 
and  are  often  of  such  a  kind  that  the  perceptions  which  they 
influence  so  distinctly  fail  to  correspond  with  the  actual  objects 
that  the  lack  of  correspondence  can  in  many  cases  be  demon- 
strated :  such  erroneous  perceptions  are  often  spoken  of  as  "  illu- 
sions." In  otlier  cases  the  psychical  processes  relate  to  a  further 
mental  action  by  which  we  form  judgments  as  to  the  features  of 
external  objects.  It  is  not  easy  however  always  to  draw  a  line 
between  a  'visual  judgment,'  such  as  that  involved  in  forming 
a  conclusion  as  to  the  size  of  an  external  object,  and  what  may 
be  called  a  mere  "  modified  perception,"  as  when  a  line  appears 
to  us  shorter  or  longer  than  it  really  is.  We  may  be  content 
here  to  treat  them  all  together. 

The  complexity  of  the  psychical  processes  in  question  comes 
about  in  various  waj^s.  On  the  one  hand  the  characters  of  a 
perception  are  determined  not  alone  by  the  sensations  which 
actually  give  rise  to  it  but  also  b}^  the  psychical  conditions  re- 
maining as  the  effect  of  former  like  sensations.  In  the  forma- 
tion of  perceptions  and  judgments,  suggestions  and  associations 
play  their  part;  so  that  each  perception,  while  it  adds  to,  is 
also  in  part  the  result  of  our  'experience.'  A  simple  illustra- 
tion of  this  is  seen  in  some  of  the  effects  of  colour.  Blue 
colours  as  we  have  seen  predominate  in  a  dim  light  such  as 
that  of  evening,  of  moonlight  or  of  winter,  whereas  reds  and 
yellows  are  marked  in  a  bright  light  such  as  that  of  full  sun- 
shine, or  of  a  summer's  day.  Hence,  when  a  landscape  is 
viewed  through  a  3'ellow  glass,  the  yellow  hue  suggests  to  the 
mind  bright  sunlight  and  summer  weather,  although  the  actual 
illumination  which  reaches  the  eye  is  diminished  by  the  glass. 
Conversely  when  the  same  landscape  is  viewed  through  a  blue 
glass  the  idea  of  moonlight  or  winter  is  suggested.  And  many 
other  instances  miglit  be  given  in  which  the  appreciation  of  the 
present  is  moulded  by  the  experience  of  the  past. 

61 


962  JUDGMENT   OF   SIZE.  [Book  in. 

On  the  other  hand  the  visual  perception  or  visual  judgment 
is  not  formed  exclusivel}^  out  of  the  visual  sensations  which  are 
excited  hy  the  image  of  an  object  falling  on  one  or  on  both  e^^es 
in  a  given  position.  In  looking  at  an  object,  a  movement  of  one 
or  both  eyes  often  takes  place,  and  the  perception  of  the  object  or 
a  judgment  concerning  the  object  is  formed  out  of  the  two  (or 
more)  sensations  excited  by  the  same  object  in  different  posi- 
tions of  the  eyes.  And  here  other  factors  enter  into  the  pro- 
cess, namely  sensations  other  than  visual  sensations,  sensations 
connected  with  the  contractions  of  the  muscles  of  the  eye, 
affections  of  what  is  known  as  "the  muscular  sense."  These 
come  into  play  even  when  we  use  one  eye  only,  but  are  espe- 
cially potent  when  we  use  both  eyes  in  binocular  vision ;  a  large 
number  of  our  visual  judgments  are  determined  by  the  muscular 
sensations  derived  from  the  movements  of  the  eyes  through 
which  Ave  look  at  the  object  whose  features  we  are  judging. 

Other  influences  also,  such  for  instance  as  sensations  of 
touch,  take  part  in  the  psychical  processes  in  question.  The 
mere  visual  sensations  which  external  objects  excite,  the  imme- 
diate and  direct  effects  of  the  visual  impulses,  form  after  all  but 
a  small  part  of  what  we  call  our  vision.  Such  sensations  and 
otlier  like  sensations  derived  through  other  senses  are  to  us  but 
symbols  of  things,  upon  which  the  mind  puts  its  own  interpre- 
tation. But  into  these  matters  we  cannot  enter  here.  We  must 
confine  ourselves  to  certain  common  facts  concerning  perceptions, 
illusions  and  visual  judgments,  and  more  especially  to  those  which 
relate  to  the  size  and  distance  of  external  objects  and  to  the 
characters  of  form  which  are  indicated  by  the  word  "solidity." 

§  599.  Appreciation  of  Appare?it  Size.  The  foundation  of 
our  judgment  of  the  size  of  any  object  is  the  size  of  the  retinal 
image  of  the  object.  We  can  distinguish  a  sensation  involving 
a  large  retinal  area  from  one  involving  a  small  area,  and  in  the 
region  of  distinct  vision  can  appreciate  even  small  differences ; 
this  is  of  course  only  an  exercise  of  the  power  of  localization. 
We  have  seen  however  that,  even  in  the  case  of  a  simple  and 
single  sensation  such  as  that  of  a  white  patch  on  a  black 
ground,  the  sensation  does  not  correspond  exactly  to  the  ob- 
jective stimulation  of  the  retina;  the  white  patch  through 
irradiation  §  582  appears  larger  than  it  really  is.  When  we 
come  to  deal  with  more  complex  groups  of  sensations  we  find 
that  over  and  above  any  such  physiological  modifications  of  the 
sensations,  the  psychical  processes  mentioned  above  affect  our 
perceptions  and  judgments  of  size,  often  giving  rise  to  illusions. 

If  a  line  such  as  AC,  Fig. 

•    •    A    o    0    o  •      162,   be    divided    into    two 

-p,       _  equal  parts  AB,  B  (7,  and  AB 

be  divided  by  distinct  marks 

into  several  parts,  as  is  shewn  in  the  figure,  while  BO  he,  left 


Chap,  hi.] 


SIGHT. 


963 


entire,  the  distance  AB  will  always  appear  greater  than  CB. 
The  retinal  images  of  the  spaces  from  A  to  B  and  from  B  to  C 
are  equal  and  the  corresponding  primary  visual  sensations  are 
also  equal,  but  the  mental  appreciation  of  A  B  in  interfered  with 
by  the  concurrent  sensations  of  the  several  intervening  dots  and 
intervals,  and  this  leads  to  a  mental  exaggeration  of  the  interval 
between  A  and  B.     So  also,  if  two  equal  squares  (Fig.  163)  be 

A  B 


Fig.  163. 

marked,  one  with  horizontal  and  the  other  with  vertical  alter- 
nate dark  and  light  bands,  the  former  will  appear  higher,  and 
the  latter  broader,  than  it  really  is.  Hence  short  persons  often 
affect  dresses  horizontally  striped  in  order  to  increase  their  ap- 
parent height,  and  very  stout  persons  avoid  longitudinal  stripes. 
Again,  when  a  short  person  is  placed  side  by  side  with  a  tall 
person,  the  former  appears  shorter  and  the  latter  taller  than  each 


J  S  ?  5 


Fig.  164. 


really  is.  By  reason  of  somewhat  similar  psychical  processes 
two  perfectly  parallel  lines  or  ban(U,  each  of  wliich  is  crossed 
by   slanting   parallel   short    lines    (Fig.  104),  will    appear    not 


964  JUDGMENT  OF  DISTANCE.  [Book  hi. 

parallel,  but  diverging  or  converging  according  to  the  direction 
of  the  cross-lines ;  the  direction  of  the  cross-lines  affects  our 
perception  of  the  distance  between  the  parallel  lines. 

§  600.  Judgment  of  Distance  and  Actual  Size.  The  size  of 
the  retinal  image  gives  us  by  itself  a  measure  not  of  the  real 
size  but  only  of  the  apparent  size  of  the  object.  The  size  of 
the  retinal  image  will  depend  on  the  distance  of  the  object  and 
on  the  dioptric  arrangements  of  the  eye;  with  the  same  dioptric 
arrangements  it  will  depend  on  the  angle  subtended  by  the 
diameter  of  the  object,  and  this  may  be  the  same  for  a  small 
object  near  as  for  a  large  object  far  off.  In  order  to  form  a 
judgment  as  to  the  actual  size  of  an  object,  we  must  adjust  our 
perception  of  the  apparent  size  by  means  of  a  judgment  of  the 
distance  at  which  the  object  is  placed ;  and  here  the  great  use 
of  two  eyes  comes  in. 

Even  with  one  eye  we  can,  to  a  certain  extent,  form  a  judg- 
ment not  only  as  to  the  position  of  the  object  in  a  plane  at 
right  angles  to  our  visual  axis,  but  also  as  to  its  distance  from 
us  along  the  visual  axis.  If  the  object  is  near  to  us,  we  have 
to  accommodate  for  near  vision;  if  far  from  us,  to  relax  our 
accommodation  mechanism  so  that  the  eye  becomes  adjusted 
for  distance.  The  muscular  sense  of  this  effort  enables  us  to 
form  a  judgment  whether  the  object  is  far  or  near.  Seeing  the 
narrow  range  of  our  accommodation,  and  the  slight  muscular 
effort  which  it  entails,  all  monocular  judgments  of  distance 
must  be  subject  to  much  error.  Every  one  who  has  tried  to 
thread  a  needle  or  to  pour  out  a  glass  of  wine  without  using 
both  eyes,  knows  such  errors. 

When,  on  the  other  hand,  we  use  two  eyes,  we  have  still 
the  variations  in  accommodation,  and  in  addition  have  all  the 
assistance  which  arises  from  the  muscular  effort  of  so  directing 
the  two  eyes  on  the  object  that  single  vision  shall  result. 
When  the  object  is  near,  we  converge  our  visual  axes;  Avhen 
distant,  we  bring  them  back  towards  parallelism.  This  neces- 
sary contraction  of  the  ocular  muscles  affords  a  muscular  sense, 
by  the  help  of  which  we  form  a  judgment  as  to  the  distance  of 
the  object.  We  can  judge  of  the  distance  of  a  vertical  line 
more  easily  than  of  a  horizontal  line,  because  we  can  converge 
our  vision  more  easily  upon  the  former;  this  is  seen  in  attempt- 
ing a  'high  jump'  over  a  horizontal  cord,  the  judgment  of  the 
distance  of  the  cord  is  facilitated  by  hanging  a  vertical  cord  or 
tape  to  it.  Conversely,  when  by  any  means  the  convergence 
which  is  necessary  to  bring  the  object  into  single  vision  is 
lessened,  the  object  seems  to  become  more  distant;  when  the 
convergence  is  increased,  the  object  seems  to  move  towards  us; 
this  may  be  seen  in  the  stereoscope. 

The  judgment  of  size  is,  as  we  said  above,  closely  connected 
with  that  of  distance.     The  real  size  of  the  object  can  be  inferred 


Chap,  hi.]  SIGHT.  965 

from  the  apparent  size,  that  is  to  sa}'  from  the  size  of  the  retinal 
image,  only  when  the  distance  of  the  object  from  the  eye  is 
known.  Thus  when  an  object  gives  rise  to  a  retinal  image  of  a 
certain  size,  that  is  to  say  has  a  certain  apparent  size,  we  esti- 
mate the  distance  from  us  of  the  object  giving  rise  to  the  image, 
and  upon  that  come  to  a  conclusion  as  to  its  real  size.  Con- 
versely, when  we  see  an  object,  of  whose  real  size  we  are 
otherwise  aware,  or  are  led  to  think  we  are  aware,  our  judgment 
of  its  distance  is  influenced  by  its  apparent  size.  Thus  when 
part  of  our  field  of  vision  is  occupied  by  the  image  of  a  man, 
knowing  otherwise  the  ordinary  size  of  a  man,  we  infer,  if  the 
image  be  very  small,  that  the  man  is  far  off.  The  reason  of  the 
image  being  small  may  be  because  the  man  is  far  off,  in  which 
case  our  judgment  is  correct;  it  may  be,  liowever,  because  the 
image  has  been  lessened  by  artificial  dioptric  means,  as  when 
the  man  is  looked  at  through  an  inverted  telescope,  in  which 
case  our  judgment  becomes  an  illusion.  So  also  a  picture  on  a 
magic  lantern  screen  when  gradually  enlaj-ged  seems  to  come 
forward,  when  gradually  diminished  seems  to  recede.  In  these 
cases  the  influence  which  the  absence  of  an}^  muscular  sense  of 
binocular  adjustment  or  monocular  accommodation  ought  to 
bring  to  bear  on  our  judgment,  is  thwarted  by  the  more  direct 
influence  of  the  association  between  size  and  distance.  An 
instructive  illusion  of  a  similar  kind  is  produced  by  developing 
in  the  eye  a  strong  negative  image  (§  584)  and  projecting  the 
image  on  to  a  screen  which  is  made  to  move  backwards  and 
forwards,  or  is  alternately  inclined  at  various  angles;  the  nega- 
tive image  appears  to  change  in  size  and  shape,  although  it  is 
absolutely  subjective  in  nature  and  wholly  independent  of  the 
movements  of  the  screen. 

The  complex  reaction  on  each  other  of  judgments  as  to  dis- 
tance and  size  is  illustrated  by  the  experience  that  an  object  such 
as  a  person  looks  unnaturally  large  when  seen  in  a  fog;  being 
seen  indistinctly,  he  is  judged  to  be  farther  off  than  he  really  is, 
and  so  appears  larger  than  he  naturally  would  do  at  the  distance 
at  which  he  is  supposed  to  be ;  and  we  are  similarly  influenced 
by  the  greater  or  less  brightness  or  saturation  of  colours.  Con- 
versely, distant  mountains  when  seen  distinctly  in  a  clear  atmos- 
phere appear  small,  because  on  account  of  their  distinctness  they 
are  judged  to  be  nearer  than  they  really  are.  The  indistinct- 
ness of  the  image  of  the  moon  or  sun  when  seen  on  the  horizon, 
similarly  contributes  to  its  appearing  larger  than  when  seen  in 
the  zenith ;  our  judgment  however  is  probably  in  this  case  also 
due  to  our  being  better  able  to  compare  the  moon  or  sun  with 
terrestrial  objects.  We  seem  moreover  in  this  matter  to  be 
especially  influenced  by  our  conception  (which  is  itself  an  illus- 
tration of  tlie  subject  we  have  in  hand)  that  the  vault  of  the 
heavens  is  flatter  than  it  really  is ;  the  zenith  appears  to  be  less 


966  JUDGMENT   OF   SOLIDITY.  [Book  in. 

distant  than  the  horizon ;  a  geometric  construction  will  shew 
that  a  body  of  the  same  size  placed  at  different  parts  of  the  real 
(spherictil)  vault  will  ap]3ear  greater  near  the  horizon  than  near 
the  zenith  of  the  flatter,  apparent  vault.  An  amusing  illustra- 
tion of  visual  judgments  may  be  obtained  by  asking  a  number  of 
persons  in  succession  what  they  regard  as  the  size  of  the  moon 
in  mid  heavens.  Even  making  allowance  for  dioptric  differ- 
ences in  individual  eyes  the  size  of  the  retinal  image  of  the 
moon  must  be  about  the  same  in  all  eyes.  And  j^et  while  some 
persons  will  be  found  ready  to  compare  the  moon  in  mid  heavens 
with  a  three-penny  piece,  others  will  liken  it  to  a  cart-wheel ; 
and  others  will  make  intermediate  comparisons. 

§  601.  Judgment  of  Solidity.  When  we  look  at  a  small 
circle  all  parts  of  the  circle  are  at  the  same  distance  from  us,  all 
parts  are  equally  distinct  at  the  same  time,  whether  we  look  at  it 
with  one  eye  or  with  two  eyes.  When,  on  the  other  hand,  we 
look  at  a  sphere,  the  various  parts  of  which  are  at  different  dis- 
tances from  us,  a  sense  of  the  accommodation,  but  much  more  a 
sense  of  the  binocular  adjustment,  of  the  greater  or  less  conver- 
gence of  the  two  eyes,  required  to  make  the  various  parts  suc- 
cessively distinct,  makes  us  aware  that  the  various  parts  of  the 
sphere  are  unequally  distant ;  and  from  that  we  form  a  judgment 
of  its  solidity.  As  with  distance  of  objects,  so  with  solidity, 
which  is  at  bottom  a  matter  of  distance  of  the  parts  of  an  object, 
we  can  form  a  judgment  with  one  eye  alone ;  but  our  ideas 
become  much  more  exact  and  trustworthy  when  two  eyes  are 
used.  We  are  further  much  assisted  by  the  effects  produced  by 
the  reflection  of  light  from  the  various  surfaces  of  a  solid  object, 
and  the  shadows  cast  by  its  raised  parts  ;  so  much  so,  that  raised 
surfaces  may  be  made  to  appear  depressed,  or  vice  versa,  and  flat 
surfaces  either  raised  or  depressed,  by  appropriate  arrangements 
of  shadings  and  shadow. 

Binocular  vision,  moreover,  affords  us  a  means  of  judging  of 
the  solidity  of  objects,  inasmuch  as  the  image  of  any  solid  ob- 
ject which  falls  on  to  the  right  eye  cannot  be  exactly  like  that 
which  falls  on  the  left,  though  both  are  combined  in  the  single 
perception  of  the  two  eyes.  Thus,  when  we  look  at  a  truncated 
pyramid  placed  in  the  middle  line  before  us,  the  image  which 
falls  on  the  right  eye  is  of  the  kind  represented  in  Fig.  165  R, 
while  that  which  falls  on  the  left  eye  has  the  form  of  Fig.  165 
L ;  yet  the  perception  gained  from  the  two  images  together  cor- 
responds to  the  form  of  which  Fig.  165  B  is  the  projection. 
Whenever  we  thus  combine  in  one  perception  two  dissimilar 
images,  one  of  the  one,  and  the  other  of  the  other  eye,  we  judge 
that  the  object  giving  rise  to  the  images  is  solid. 

This  is  the  simple  principle  of  the  stereoscope,  in  which  two 
slightly  dissimilar  pictures,  such  as  would  correspond  to  the 
vision  of  each  eye  separately,  are,  by  means  of  reflecting  mir- 


Chap,  hi.] 


SIGHT. 


967 


rors,  as  in  Wheatstone's  original  instrument,  or  by  prisms,  as  in 
the  form  introduced  by  Brewster,  made  to  cast  images  on  corre- 
sponding parts  of  the  two  retinas  so  as  to  produce  a  single  per- 
ception. Though  each  picture  is  a  surface  of  two  dimensions 
only,  the  resulting  perception  is  the  same  as  if  a  single  object, 
or  group  of  objects,  of  three  dimensions  had  been  looked  at. 

It  might  be  supposed  that  the  judgment  of  solidity  which 
arises  when  two  dissimilar  images  are  thus  combined  in  one 
perception,  was  due  to  the  fact  that  all  parts  of  the  two  images 


/ 

\ 

/ 

\ 

y 

B 

x 

\ 

/ 

\ 

/ 

\, 

Fig.   165. 

cannot  fall  on  corresponding  parts  of  the  two  retinas  at  the  same 
time,  and  that  therefore  the  combination  of  the  two  needs  some 
movement  of  the  eyes.  Thus,  if  we  superimpose  R  on  L  (Fig. 
165),  it  is  evident  that  when  the  bases  coincide  the  truncated 
ajDices  will  not,  and  vice  versa;  hence,  Avhen  the  bases  fall  on 
corresponding  parts,  the  apices  will  not  be  combined  into  one 
image,  and  vice  versa;  in  order  that  both  may  be  combined, 
there  must  be  a  slight  rapid  movement  of  the  eyes  from  the  one 
to  the  other.  That,  however,  no  such  movement  is  necessary 
for  each  particular  case  is  shewn  by  the  fact  that  solid  objects 
appear  as  such  when  illuminated  by  an  electric  spark,  the  dura- 
tion of  which  is  too  short  to  permit  of  any  movements  of  the 
eyes.  If  the  flash  occurred  at  the  moment  that  the  eyes  were 
binocularly  adjusted  for  the  bases  of  the  p3'ramids,  the  two  sum- 
mits not  falling  on  exactly  corresponding  parts  would  give  rise 
to  the  perceptions  of  two  summits,  and  the  whole  object  ought 
to  appear  confused.  That  it  does  not,  but,  on  the  contrary, 
appears  a  single  solid,  must  be  the  result  of  psychical  opera- 
tions, resulting  in  what  Ave  have  called  a  judgment. 

As  we  have  seen,  in  any  one  position  of  the  two  eyes,  only  a 
small  portion  of  the  field  of  sight  lies  in  the  horopter  and  falls 
on  corresponding  points  of  the  two  retinas.  Most  of  the  objects 
in  a  scene  on  which  we  look  give  rise  to  dissimilar  images  in 
the  two  eyes ;  and  we  attribute  solidity  to  them  by  reason  on 
the  one  hand  of  the  movements  of  the  eyes,  and  on  the  other 
hand  of  the  psychical  processes  just  mentioned.  Conversely  the 
same  processes  wliich  thus  give  rise  to  apparent  solidity  assist 
us  in  forming  judgments  of  distance. 

v^  602.  If  the  images  of  two  surfaces,  one  black  and  the 
other  white,  are  made  to  fall  on  corresponding  parts  of  the  eye, 


968  STRUGGLE   OF  THE   TWO   FIELDS.      [Book  hi. 

so  as  to  be  united  into  a  single  perception,  the  result  is  not 
always  a  mixture  of  the  two  impressions,  that  is  a  grey,  but,  in 
many  cases,  a  sensation  similar  to  that  produced  when  a  polished 
surface,  such  as  plumbago,  is  looked  at:  the  surface  appears 
brilliant,  is  said  to  have  a  "  lustre."  The  reason  probably  is 
because  when  we  look  at  a  polished  surface  the  amount  of 
reflected  light  which  falls  upon  the  retina  is  generally  different 
in  the  two  eyes ;  and  hence  we  associate  an  unequal  stimulation 
of  the  two  retinas  with  the  idea  of  a  polished  lustrous  surface. 
We  may  in  this  connection  refer  to  what  is  known  as  "  the 
struggle  of  the  two  fields  of  vision,"  though  the  matter  is  one  of 
sensations  and  not  of  judgments  or  intricate  psychical  processes. 
When  the  impressions  of  two  colours  are  united  in  binocular 
vision,  the  result  is  in  most  cases  not  a  mixture  of  the  two 
colours,  as  when  the  same  two  impressions  are  brought  to  bear 
together  at  the  same  time  on  a  single  retina,  but  a  struggle 
between  the  two  colours,  now  one,  and  now  the  other,  becoming 
prominent,  intermediate  tints  however  being  frequently  passed 
through.  This  may  arise  from  the  difficulty  of  accommodating 
at  the  same  time  for  the  two  different  colours  (§  548) ;  both 
eyes  will  be  accommodated  at  the  same  time  and  to  the  same 
degree,  but  if  two  eyes,  one  of  which  is  looking  at  red,  and  the 
other  at  blue,  be  at  one  moment  both  accommodated  for  red  rays, 
the  red  sensation  will  overpower  the  blue,  while  if  at  another 
moment  they  are  both  accommodated  for  blue,  the  blue  will 
prevail.  It  may  be  however  that  the  tendency  to  rhythmic 
action,  so  manifest  in  activity  of  other  simpler  forms  of  living 
matter  makes  its  appearance  also  in  the  cerebral  changes  involved 
in  binocular  vision. 


SEC.  11.   THE  nutritio:n'  of  the  eye. 

§  603.  The  main  blood-vessels  of  the  eye  are,  the  arteria 
centralis  supplying  the  retina,  and  the  (posterior)  ciliary  arteries 
supplying  the  choroid,  ciliary  processes  and  iris,  the  vessels 
going  to  the  choroid  being  called  the  short  ciliary  arteries,  and 
those  reaching  forward  to  the  ciliary  processes  and  iris,  the  long- 
ciliary  arteries.  From  the  arteria  centralis  retinee  the  blood  is 
returned  by  the  vena  centralis,  while  the  vena?  vorticosse  of  the 
(posterior)  ciliary  veins  gather  up  the  blood  of  both  the  long 
and  the  short  (posterior)  ciliary  arteries.  These  two  systems 
communicate  to  some  extent  with  each  other  by  anastomoses 
at  the  entrance  of  the  optic  nerve,  but  on  the  whole  are  inde- 
pendent. 

In  addition  to  the  above,  the  anterior  ciliary  arteries  pass  to 
the  eyeball  with  each  of  the  four  straight  ocular  muscles,  sup- 
plying the  front  part  of  the  sclerotic  as  well  as  the  edge  of  the 
cornea,  and  sending  through  the  sclerotic  '  perforating  '  arteries 
to  end  in  the  iris,  ciliary  processes,  and  front  part  of  the  cho- 
roid, and  so  join  the  system  of  the  posterior  ciliary  arteries. 
Corresponding  to  these  anterior  ciliary  arteries  are  veins  which 
make  their  way  back  to  the  ocular  muscles,  and  the  roots 
of  which  ai"e  especially  connected  with  the  circular  canal  of 
Schlemm.  Further,  the  edge  of  the  cornea  is  in  addition 
supplied  by  conjunctival  blood-vessels. 

The  blood-supply  of  the  various  parts  of  the  eye  is  therefore 
somewhat  as  follows.  Tlie  inner  layei's  of  the  retina  are  sup- 
plied in  a  direct  manner  by  the  arteria  centralis  retina?,  but  the 
outer  layers  together  with  the  pigment  epithelium  in  an  indirect 
manner  by  the  close  set  choroidal  network  "  choriocapillaris  " 
of  the  posterior  ciliary  arteries.  The  choroid  proper,  that  })art 
which  serves  as  an  investment  to  the  retina  and  specialized 
pigment  e[)ithelium,  is  supplied  by  the  shoi't  (posterior)  ciliary 
arteries ;  but  the  front  ciliary  part  of  tlie  choroid,  together  Avith 
the  ciliary  processes  and  iris,  receives  blood  from  the  long  (pos- 
terior) ciliary  arteries,  and  also  from  the  anterior  ciliary  arteries. 
The  cornea  is  supplied  by  the  conjunctival  as  well  as  by  the 

♦JG'J 


970  BLOOD-VESSELS    OF   THE   EYE.         [Book  iii. 

anterior  ciliary  arteries,  the  blood-vessels  as  we  have  said  extend- 
ing a  short  distance  only  within  the  circle  of  the  corneal  cir- 
cumference, while  the  scanty  supply  of  the  sclerotic  is  furnished 
in  the  front  part  of  the  eyeball  by  the  anterior  and  in  the  hind 
part  by  the  posterior  ciliary  arteries. 

The  nutritive  supply  of  the  lens,  with  its  capsule,  and  of  the 
vitreous  humour  is  an  indirect  one,  by  means  of  lymph ;  the 
anterior  surface  of  the  former  is  bathed  by  the  aqueous  humour; 
the  lymph  streams  in  the  vitreous  humour,  of  which  we  shall 
speak  immediately,  furnish  that  substance  with  the  scanty  nour- 
ishment it  needs,  and  sweep  by  the  posterior  surface  of  the 
lens. 

§  604.  In  speaking  of  the  movements  of  the  pupil  we  re- 
ferred to  vaso-motor  changes  in  the  eye.  So  far  as  our  present 
information  goes,  we  have  evidence  chiefly  of  vaso-constrictor 
fibres  wliich  passing  from  the  sympathetic  to  the  ciliary  ganglion 
(§  536)  reach  the  posterior  ciliary  arteries  by  the  short  ciliary 
nerves ;  but  there  are  facts  which  seem  to  shew  that  the  fifth 
nerve  supplies  vaso-dilator  fibres  through  the  ophthalmic  branch. 

The  separate  distribution  of  the  short  ciliary  arteries  to  the 
hinder  part  of  the  choroid  investment  which  is  busy  with  the 
nourishment  of  the  retina  and  which  takes  little  or  no  share 
in  the  movements  of  accommodation,  and  of  the  long  ciliary 
arteries  to  the  front  part  of  the  investment  which,  as  iris,  ciliary 
processes  and  muscle,  and  front  part  of  the  choroid  itself,  is 
concerned  in  the  movements  of  the  pupil  and  of  accommodation, 
suggests  that  a  corresponding  separate  distribution  of  vaso-motor 
nerves  also  exists ;  but  we  have  no  exact  experimental  evidence 
of  this. 

We  saw  in  speaking  of  the  brain  (§  522)  that  clear  evidence 
of  the  cerebral  vessels  being  subject  to  vaso-motor  influences 
was  wanting;  and  in  this  respect  once  more  the  retina  behaves 
like  a  part  of  the  brain.  Though  by  help  of  the  ophthalmoscope 
changes  of  calibre  in  the  retinal  vessels  can  easily  be  observed, 
we  have  as  yet  no  decisive  proof  that  such  changes  can  be 
brought  about  by  vaso-motor  nerves  acting  directly  on  the  arteria 
centralis  retinae.  The  changes  which  are  observed  seem  to  be 
determined  not  by  the  greater  or  less  contraction  of  the  mus- 
cular coat  of  the  retinal  vessels  themselves,  but  by  the  pressure 
to  which  the  blood  in  the  vessels  is  subjected,  and  that  may  be 
varied  by  many  extraneous  causes. 

The  Lymphatics  of  the  Eye. 

§  605.  Though  the  lymph  in  the  large  serous  cavities  may  be 
considered  to  play  a  mechanical  part  inasmuch  as  it  facilitates 
the  movements  of  the  viscera,  and  though  in  such  a  tissue  as  the 
skin,  the  lymph  in  the  cavities  and  vessels  of  the  dermis  may 


Chap,  hi.]  SIGHT.  971 

similarly  perform  a  mechanical  task  in  assisting  to  give  at  once 
tirmiiess  find  suppleness  to  the  skin,  yet  over  tlie  body  at  large 
the  function  of  the  Ijanph  is  preeminently  a  nutritive  one,  and 
its  mechanical  duties  are  insignificant.  As  regards  the  eye  the 
case  is  different.  The  eyeball  is  broadly  speaking  a  shell  filled 
with  fluid,  the  aqueous  and  vitreous  humours  ;  and  for  the  vari- 
ous functions  of  the  eye  it  is  necessary  that  this  shell  should  be 
filled  to  a  certain  extent,  should  be  tense  to  a  certain  degree, 
not  more  and  not  less;  and  this  fulness,  this  tension,  "intraocular 
tension,"  which  is  considerable,  probably  much  higher  than  the 
ordinary  pressure  in  the  lymph-spaces  of  the  body  at  large,  is 
provided  by  the  lymj)hatic  arrangements.  If  the  retina  were 
not  adequately  supported  by  the  vitreous  humour,  if  it  could 
flap  about  or  in  any  way  alter  its  curvature,  the  dioptric  arrange- 
ments of  the  eye  would  be  upset ;  if  the  vitreous  humour  at  one 
time  shrank,  at  another  expanded,  the  movements  of  accommo- 
dation could  not  be  carried  on  ;  if  the  aqueous  humour  were  now 
abundant,  now  scanty,  the  movements  of  the  pupil  would  become 
irregular  and  uncertain ;  and  if  the  whole  globe  were  so  flabby 
as  to  give  way  under  the  pull  of  each  ocular  muscle,  the  deli- 
cate movements  of  the  eyeball  on  which  we  lately  dwelt  would 
become  impossible.  Hence  the  lymphatics  of  the  eye  have  a 
double  importance,  inasmuch  they  not  only,  as  elsewhere,  assist 
in  maintaining  the  due  nutrition  of  the  several  tissues,  but  also 
in  a  mechanical  Avay  help  to  make  the  eye  an  adequate  dioptric 
instrument.  In  accordance  with  this  double  duty  we  find  a 
special  lymph  apparatus  added  to  the  more  general  lymphatic 
arrangements  such  as  exist  elsewhere. 

As  belonging  to  the  more  general  arrangements  we  may 
note  the  following.  The  lymph-spaces  of  the  cornea  pass  at 
the  margin  of  the  cornea  into  the  lymphatic  vessels  of  the  con- 
junctiva. The  scanty  lymph-spaces  of  the  sclerotic  pass  at  the 
extreme  front  into  the  conjunctival  lymphatics,  but  elsewhere 
are  continuous  either  on  the  inner  surface  with  the  pericho- 
roidal lymi)h-spaces,  or  on  the  outer  surface  and  that  more 
freely,  with  the  large  lymphatic  Tenonian  cavity.  Tenon's 
capsule  is  a  loose  thin  investment  of  connective  tissue  lying 
between  the  sclerotic  and  the  ocular  muscles  and  forming 
sheaths  round  the  tendons  of  the  latter.  Between  the  looser 
capsule  and  the  denser  sclerotic  is  a  large  irregular  h-mphatic 
cavity  bearing  the  above  name.  The  perivascular  and  otlier 
lymph-spaces  of  the  choroid  join  the  perichoroidal  spaces,  which 
in  turn  communicate  with  the  Tenonian  cavity  by  lymph-spaces 
or  lymphatics  accompanying  the  ciliary  veins  and  to  some 
extent  the  ciliary  arteries  as  these  pierce  the  sclerotic.  The 
Tenonian  cavity  itself  joins  a  large  lymphatic  cavity  surround- 
ing the  optic  nerve,  '  the  supravaginal '  cavity,  whence  the 
lymph  is  carried  away  by  the  ordinary  lymphatics  of  the  orbit. 


972  THE  AQUEOUS   HUMOUR.  [Boox  iir. 

The  perivascular  and  other  lymph-spaces  of  the  retina  are 
in  connection  with  the  lymph-spaces  of  the  optic  nerve,  which 
in  turn  join  the  subarachnoid  space  of  that  nerve,  and  this  is 
continuous  with  the  corresponding  space  in  the  brain.  There 
appear  to  be  also  paths  uniting  these  lymphatics  of  the  retina 
and  optic  nerves  with  the  perichoroidal  spaces  and  Tenonian 
cavity,  and  so  with  the  external  lymphatic  system. 

§  606.  In  the  special  lymph  apparatus  the  ciliary  processes, 
the  iris,  the  aqueous  humour  and  the  vitreous  humour  are  con- 
cerned. 

The  aqueous  humour.  We  have  more  than  once  spoken  of 
the  anterior  chamber  as  a  lymphatic  cavity ;  nevertheless  the 
aqueous  humour  contained  in  it  differs  greatly  from  ordinary 
lymph.  Not  only  does  it  contain  much  more  water,  the  total 
solids  being  not  much  more  than  1  p.c.  (1-3  p.c.)  but  also  the 
relative  proportion  of  the  solids  between  themselves  is  different 
from  that  of  lymph,  and  special  substances  are  present  in  it. 
The  proteids  are  particularly  scanty,  not  more  than  about  -1 
p.c.  ;  these  are  serum-albumin,  globulin,  and  apparently  fibrino- 
gen. Inorganic  salts  are  present  in  about  the  same  proportion 
as  in  blood  and  lymph,  viz.  -8  p.c. ;  and  these,  chiefly  sodium 
chloride,  with  an  unusual  proportion  (-4  p.c.)  of  so-called 
extractives,  furnish  nearly  all  the  solid  matter.  Among  these 
extractives  is  a  substance  which  reduces  cupric  solutions  but 
which  is  not  a  sugar,  though  its  exact  nature  is  as  yet  un- 
known ;  urea  and  sarcolactic  acid  (in  some  combination)  are 
also  said  to  be,  at  least  often,  present.  The  reaction  is  neutral 
or  faintly  alkaline. 

Like  the  '  serous  fluid '  in  the  large  serous  cavities  and  the 
cerebro-spinal  fluid  in  the  cavities  of  the  central  nervous  system, 
the  aqueous  humour  comes  and  goes ;  the  particular  fluid 
which  at  any  given  moment  is  present  in  the  eye  has  not 
always  been  there  ;  some  of  the  fluid  is  continually  passing 
away  and  fresh  fluid  continually  arriving.  If  fluid  be  with- 
drawn from  the  anterior  chamber  by  puncture  of  the  cornea, 
the  chamber  is  soon  refilled ;  indeed,  under  certain  circum- 
stances, a  considerable  quantity  of  fluid  may  be  drained  away 
from  the  chamber,  fresh  fluid  taking  the  place  of  that  which 
escapes.  And,  though  under  normal  conditions  the  quantity 
of  aqueous  humour  is  fairly  constant,  the  fluid  may  be  in  excess 
or  may  be  deficient,  and  the  one  phase  may  pass  into  the  other. 
The  question  therefore  arises.  Whence  comes  the  fluid  and 
whither  does  it  go  ? 

The  characters  of  aqueous  humour  just  given  shew  that  in 
many  respects  it  resembles  cerebro-spinal  fluid  though  differing 
in  several  features.  That  fluid,  we  have  seen  reason  to 
believe,  is  in  part  at  all  events  furnished  by  the  choroid  plex- 
uses, by  a  process  which  presents  some  analogies  with  the  act 


Chap,  hi.]  SIGHT.  973 

of  secretion.  And  the  resemblance  between  the  ciliary  processes 
and  the  choroid  plexuses,  for  both  are  vascular  folds  of  pia 
mater  covered  with  epithelium  derived  from  the  lining  of  the 
primitive  medullary  canal,  suggests  that  the  former  furnish  the 
aqueous  humour  in  some  such  way  as  the  latter  furnish  the  cere- 
bro-spinal  fluid.  There  is  a.  certain  amount  of  experimental 
evidence  in  favour  of  this  view,  for  when  such  a  substance  as 
fluorescin,  which  can  be  detected  by  the  greenish  tinge  which  it 
gives  to  the  fluids  and  tissues,  is  injected  into  the  body,  into 
the  subcutaneous  connective  tissue  or  peritoneal  cavity  for 
instance,  not  only  does  it  speedily  appear  in  the  aqueous 
humour,  but  the  ciliary  processes  are  said  to  be  the  parts  of  the 
eye  in  which  its  presence  may  be  first  detected.  It  may  be 
urged  that,  unlike  the  epithelium  covering  the  choroid  plexuses, 
the  pars  ciliaris  retinee  bears  no  distinctive  histological  indica- 
tions of  secretory  activit}^ ;  but,  as  we  shall  presently  have  occa- 
sion to  point  out,  a  wholly  analogous  layer  of  epithelium,  that 
lining  the  cavities  of  the  internal  ear,  though  possessing  no 
marked  secretory  features,  certain!}'  furnishes,  by  an  act  very 
similar  to  secretion,  a  more  or  less  lymph-like  fluid,  the  so-called 
endo-lymph.  The  phrase  '  secretion '  however  must  not  be 
strained.  The  somewhat  specialized  loose  stroma  of  both  the 
ciliary  processes  and  iris  undoubtedly  contains  in  its  meshes  a 
large  quantity  of  what  we  may  suppose  to  be  ordinar}^  l^^mph ; 
and  what  is  intended  by  the  above  view  is  that  while  some  of 
this  lymph  may  pass  by  the  perichoroidal  spaces  and  so  away  as 
ordinary  lymph,  a  much  larger  proportion  passes  on  to  the  free 
surfaces  abutting  on  the  posterior  and  anterior  chambers,  and  in 
so  passing  becomes  modified  in  nature. 

The  fluid  thus  furnished  by  the  ciliary  processes  makes  its 
way,  in  the  first  place,  into  the  posterior  cliamber ;  but  though 
the  iris,  as  we  have  seen  (v^  535)  lies  close  on  the  lens,  thereis 
undoubtedly  a  communication  between  the  two  chambers  suffi- 
ciently free  to  allow  fluid  to  pass  readily  from  one  to  the  other 
and  so  to  fill  the  anterior  chamber  from  the  posterior.  It  is 
difficult  to  suppose  that  some  of  the  lymph  with  Avhich  the 
sponge-like  stroma  of  the  iris  is  laden,  does  not  find  its  way 
direct  through  tlie  anterior  surface  of  the  iris  into  the  anterior 
chamber ;  and  such  a  transit  would  probably  be  assisted  by  the 
continual  clianges  of  the  pupil.  On  the  other  hand  the  extent 
of  surface  furnished  by  the  ciliary  processes,  Avhich  moreover 
also  have  the  advantages  of  movement  in  each  act  of  accommo- 
dation, is  very  large  compared  with  that  of  the  iris;  hence  we 
may  probably  with  confidence  conclude  tliat  the  greater  part 
of  the  aqueous  humour  is  furnished  by  the  ciliary  processes, 
though  the  iris  may  contribute.  We  may  add  that  probably 
the  iridic  contribution  differs  in  nature  from  the  rest,  since  the 
epithelium  which  the  fluid  has  to  traverse  is  a  thin  layer  of  flat 
epithelioid  plates. 


974  LYMPHATICS   OF   THE   EYE.  [Book  in. 

The  answer  to  the  question,  How  does  the  aqueous  humour 
leave  the  anterior  chamber?  presents  perhaps  less  difficulties. 
The  anterior  chamber  at  the  'iridic  angle'  communicates  freely 
with  the  spaces  of  Fontana,  and  these  with  the  canal  of 
Schlemm,  Avhich  in  turn  is  in  direct  connection  with  the  radi- 
cles of  the  anterior  ciliary  veins.  Since  the  ciliarj^  muscle  pulls 
on  the  tissue  surrounding  the  canal  of  Schlemm  it  is  possible, 
or  even  probable  that  the  movements  of  accommodation  help 
alternately  to  close  and  open  the  canal,  and  thus  to  pump 
its  contents  into  the  veins ;  by  this  means  the  exit  of  fluid 
from  the  anterior  chamber  is  rendered  less  dependent  on  the 
relative  pressures  of  the  blood  in  the  vein  and  of  the  fluid  in 
the  anterior  chamber.  By  this  channel  the  aqueous  humour 
gains  a  ready,  relatively  direct,  and  short  access  to  the  blood- 
stream. And  clinical  experience  shews  that  ii  this  way  be 
blocked  an  accumulation  of  aqueous  humour  results. 

We  may  conclude  then  that  the  aqueous  humour  is  a  reser- 
voir intercalated  in  a  stream  of  a  peculiar  fluid  which  is  passing 
from  the  ciliary  processes  through  the  small  posterior  and  larger 
anterior  chamber,  the  spaces  of  Fontana  and  the  canal  of 
Schlemm  into  the  venous  system.  This  reservoir  on  the  one 
hand  serves  a  mechanical  purpose  in  preserving  the  natural 
form  of  the  eye  and  in  affording  an  adequate  fluid  bed  for  the 
movements  of  the  iris,  and  on  the  other  hand,  by  bringing  new 
food  material  and  carrying  away  waste  products,  enables  the 
lens  to  carry  out  the  slow  and  scanty  metabolism  necessary  for 
its  life. 

§  607.  For  mechanical  purposes  the  due  condition  of  the 
vitreous  humour  is  perhaps  even  more  important  than  that 
of  the  aqueous  humour.  We  have  already  called  attention 
to  the  fact  that  the  vitreous  humour  in  spite  of  its  being 
originally  a  plug  of  mesoblastic  tissue,  in  adult  life  closely 
resembles  the  aqueous  humour  in  its  chemical  features ;  and 
indeed  it  is  practically  an  attenuated  mesoblastic  sponge  through 
which  is  continually  streaming,  though  at  a  low  rate,  a  fluid 
identical  with  or  exceeding  like  to  the  aqueous  humour. 
Through  the  optic  disc  the  fluid  of  the  vitreous  humour  has 
access  to  the  lymph-spaces  of  the  optic  nerve ;  material  injected 
into  the  pial  sheath  of  the  optic  nerve  finds  its  wa}'  through  the 
optic  disc  into  the  vitreous  humour  passing  along  a  '  central 
canal,'  'hyaloid  canal,'  which  remains  after  the  disappearance  of 
the  prolongation  of  the  arteria  centralis  retinse  (§  525).  And 
probaljly  some  of  the  fluid  of  the  vitreous  humour  finds  its  way 
by  this  path  into  the  subarachnoid  space. 

But  the  greater  part  of  the  fluid  of  the  vitreous  humour 
seems  to  belong  to  the  same  system  as  the  aqueous  humour. 
Fluids  pass  readily  in  some  way  or  other  through  the  suspensory 
ligament ;  fluid  injected  into  the  vitreous  humour  finds  its  way 


Chap,  hi.]  SIGIiT.  975 

into  the  anterior  cliamber,  and  a  block  at  the  iridic  ang-le  leads 
to  undue  distension,  not  of  the  anterior  and  posterior  chambers 
only,  but  of  the  whole  globe  of  the  eye ;  the  pressures  of  the 
aqueous  and  vitreous  humour  are  the  same  and  vary  similaily 
and  concurrently.  We  have  no  satisfactory  evidence  that  any 
large  amount  of  fluid  passes  direct  from  the  choroid  through  the 
retina,  past  the  internal  limiting  and  hyaloid  membranes  into 
the  vitreous  humour;  as  far  as  we  know  the  whole  of  the  lymph 
of  the  retina  is  carried  away  by  the  optic  nerve  in  the  manner 
mentioned  above ;  and  we  must  therefore  conclude  that  the 
region  of  the  zonule  of  Zinn  serves  as  the  door  both  for  the 
entrance  and  exit  of  fluid,  the  circulation  through  the  vitreous 
humour  between  its  indistinct  concentric  lamellce  being  secured 
by  diffusion  assisted  by  the  movements  of  the  eyeball. 

Tliis  important  flow  of  what  we  may  call  modified  Ij'mph  like 
that  of  the  more  ordinary  lympli  in  other  parts  of  the  body,  is 
determined  in  the  first  instance  by  the  blood  flow,  and  we  may 
apply  to  the  eye  the  remarks  which  were  made  when  (§  244) 
we  treated  generally  of  the  relations  of  lymph  to  blood-sujDply. 
Broadly  speaking  the  intraocular  pressure  rises  and  falls  with 
the  general  blood-pressure;  the  dim  cornea  and  sunk  eye  that 
betoken  the  approaching  end  are  due  to  the  fall  of  blood-pres- 
sure which  accompanies  death.  A  local  fall,  preceded  by  a 
transient  rise,  may  be  brought  about  by  stimulation  of  the  cervi- 
cal sympathetic,  and  a  local  rise  by  stimulation  of  the  ophthalmic 
branch  of  the  fifth  nerve,  stimulation  of  the  third  nerve  having 
apparently  little  effect  in  either  direction.  We  may  add  that, 
tempting  as  the  view  may  seem  that  the  lymph  arrangements 
of  the  eye  are  under  the  direct  control  of  the  nervous  system, 
we  have  no  evidence  that  such  is  the  case. 

Concerning  the  influence  of  the  nervous  system  on  the  gen- 
eral nutrition  of  the  eye,  and  the  disorders  which  follow  upon 
section  or  injury  to  the  fifth  nerve  we  have  already,  in  an 
earlier  part  of  the  work  (§  439),  said  all  that  at  present  we 
have  to  say. 


SEC.  12.     THE  PROTECTIVE  MECHANISMS  OF  THE  EYE. 

§  608.  The  eye  is  protected  by  the  two  eyelids,  each  of 
which  is  strengthened  and  rendered  firm  by  a  curved  plate  of 
dense  connective  tissue  called  the  tarsus  (or  incorrectly  the 
tarsal  cartilage),  which  is  larger  in  the  upper  than  in  the  lower 
ej^elid.  Elevation  of  the  upper  eyelid  assisted  by  some  depres- 
sion of  the  lower  eyelid  is  spoken  of  as  "  opening  the  eye  "  ; 
depression  of  the  upper  eyelid  assisted  by  elevation  of  the  lower 
eyelid  is  spoken  of  as  "shutting  the  eye."  The  latter  move- 
ment is  brought  about  by  the  contraction  of  the  orbicularis  oeuli, 
a  muscle  of  circularly  disposed  striated  fibres  placed  beneath  the 
skin  of  each  eyelid  and  stretching  also  over  the  adjoining  bony 
orbit.  The  muscle  is  governed  by  a  branch  of  the  seventh, 
facial  nerve,  and  may  be  thrown  into  action  as  part  of  a  reflex 
act  or  of  a  voluntary  effort.  When  the  facial  nerve  becomes 
incapable,  through  injury  or  disease,  of  carrying  motor  impulses, 
the  eye  cannot  be  shut  and  remains  widely  open.  There  are 
some  reasons  however  for  thinking  that  the  motor  fibres  for 
the  orbicularis,  though  forming  part  of  the  facial  nerve  outside 
the  brain,  take  origin  within  the  brain,  not  from  the  facial 
nucleus  but  from  the  hind  end  of  the  third,  oculo-motor  nucleus. 
In  the  reflex  contraction  of  the  orbicularis,  known  as  '  winking  ' 
or  'blinking,'  which  is  so  familiar  as  an  almost  typical  reflex 
movement,  but  which  in  the  waking  hours  is  repeated  so  regu- 
larly, twice  a  minute  or  so,  as  to  take  on  almost  the  characters 
of  a  rhythmic  automatic  act,  the  exciting  afferent  impulses  are 
carried  along  the  fibres  of  the  fifth  nerve  distributed  to  the 
cornea  and  conjunctiva,  and  probably,  but  not  certainly,  pass 
some  way  down  the  ascending  root  of  that  nerve. 

The  eye  is  opened  mainly  by  the  raising  of  the  upper  eyelid 
through  the  contraction  of  the  levator  palpehr(x.  superioris.  This 
muscle,  taking  origin  from  the  back  of  the  orbit  in  company 
with  the  ocular  muscles,  is  inserted  into  the  upper  surface  of 
the  tarsus  of  the  upper  eyelid,  beneath  the  orbicularis.  It  is 
governed  by  a  branch  of  the  third  nerve ;  hence  injury  or  dis- 
ease of  this  nerve  is  frequently  the  cause  of  a  drooping  of  the 
upper  eyelid  and  an  inability  to  open  the  eye  fully. 

976 


Chap,  in.]  SIGHT.  977 

A  portion  of  the  tendon  of  the  levator  palpebrse  closely- 
united  with  an  extension  of  the  tendon  of  the  superior  rectus  is 
inserted  into  the  hinder  part  of  the  upper  eyelid,  where  the  con- 
junctiva lining  it  is  about  to  be  reflected  over  the  ej-eball ;  and 
a  similar  extension  of  the  inferior  rectus  is  similarly  inserted 
into  the  lower  eyelid.  Hence  a  contraction  of  the  superior 
rectus,  while  elevating  the  visual  axis,  at  the  same  time  raises 
somewhat  the  upper  eyelid ;  and  in  like  manner  the  inferior  rec- 
tus, while  depressing  the  visual  axis,  lowers  the  lower  eyelid. 

Between  the  main  tendon  of  the  levator  palpebrse  and  the 
tendinous  slip  just  mentioned  lies  a  small  bundle  of  plain,  un- 
striated  muscular  fibres,  which  starting  from  the  levator,  ends 
in  the  hind  border  of  the  tarsus ;  it  is  sometimes  spoken  of  as 
the  middle  insertion  of  the  levator.  A  similar  bundle  of  plain 
muscular  fibres  connects  the  insertion  of  the  inferior  rectus  with 
the  tarsus  of  the  lower  eyelid.  These  two  small  plain  unstriated 
muscles  appear  to  be  governed  by  nervous  filaments  proceeding 
from  the  cervical  sympathetic,  stimulation  of  the  cervical  sj'm- 
pathetic  leading  to  contraction  of  these  muscles  and  so  to  a 
partial  opening  of  the  eye,  and  section  of  the  same  nerve  pre- 
venting their  being  thrown  into  contraction  and  so  contributing 
to  closure  of  the  ej'elids.  In  some  of  the  lower  animals  this 
closure  of  the  eye  upon  section  and  opening  upon  stimulation 
of  the  cervical  sympathetic  is  very  distinct.  In  those  animals 
which  possess  a  third  eyelid  this  is  retracted  by  stimulation  and 
comes  forward  upon  section  of  the  cervical  sympathetic. 

Stimulation  of  the  cervical  sj-mpathetic  also  causes  some 
protrusion  and  section  causes  recession  of  the  whole  eyeball ; 
this  is  seen  at  times  in  man  in  disease. 

§  609.  The  conjunctiva  which  lines  the  ocular  surface  of 
the  e^'elids  and  is  reflected  from  them  over  the  ej^eball,  the  line 
along  which  reflection  takes  place  being  s2:)oken  of  as  the  fornix 
conjunctivce^  consists  like  the  skin  of  the  body  of  which  it  is  a 
continuation,  of  an  epithelium  or  epidermis  resting  on  a  dermis 
of  connective  tissue.  It  differs  from  the  skin  in  the  dermis 
being  delicate  and  in  the  epidermis  being  thin  with  a  tendency 
for  the  constituent  cells  to  become  columnar ;  hence  it  is  some- 
times spoken  of  as  a  "mucous  membrane."  On  the  ocular 
surface  of  the  eyelids  the  conjunctiva  is  thrown  into  irregular 
ridges  or  imperfect  and  fused  papilhe,  giving  rise  to  a  satiny 
appearance  ;  here  the  epithelium  consists  of  several  layers  of 
cells,  the  uppermost  of  Avhicli  are  flattened.  Over  the  fornix, 
the  epithelium  consists  of  two  or  three  layers  only,  the  cells 
in  the  uppermost  layer  being  cubical  or  columnar  ;  over  tlie  bulb 
the  epithelium  consists  also  of  a  few  la3'ers  only,  the  upper  cells 
being  somewhat  flattened  and  the  dermis  being  thrown  up  into 
scattered  papilla). 

Imbedded  in  the  tarsus,  stretching  from  the  hind  border  to 

62 


978  TEAES.  [Book  hi. 

tlie  free  edge  of  tlie  lid  lies,  in  each  eyelid,  a  row  of  thirty 
or  fewer  largely  developed  sebaceous  glands  tlie  Meibomian 
glands.  Sebaceous  glands  are  also  attached  to  the  follicles 
of  the  eyelashes,  and  into  the  ducts  of  some  of  these  open  the 
glands  of  Moll,  which  have  the  structure  of  a  sweat  gland. 
Small  mucous  glands  are  moreover  found  in  the  conjunctiva 
especially  in  the  neighbourhood  of  the  fornix. 

These  several  glands  contribute  to  keep  the  surface  of  the 
eye  and  eyelids  moist ;  but  this  is  chiefly  effected  by  the  secre- 
tion of  the  lachrymal  gland  which  is  placed  above  the  upper 
eyelid  in  the  lateral  region  of  the  orbit,  and  which,  imperfectly 
divided  by  an  extension  of  the  tendon  of  the  levator  palpebra- 
rum into  two  masses,  discharges  its  secretion  by  several  ducts 
opening  along  the  fornix  conjunctivse.  Under  ordinary  circum- 
stances the  fluid  thus  secreted  is  carried  away  through  the 
punctum  lachrymale  of  the  upper  and  of  the  lower  eyelid,  at 
the  inner  angle  of  the  eye,  into  the  lachrymal  canaliculi,  and  so 
into  the  lachrymal  sac,  and  finally  into  the  cavity  of  the  nose. 
"When  the  secretion  becomes  too  abundant  to  escape  in  this  way 
it  overflows  on  to  the  cheeks  in  the  form  of  tears. 

The  structure  of  the  lachrymal  gland  is  in  its  main  features 
identical  with  that  of  an  albuminous  salivary  gland,  or  with  that 
of  the  parotid,  save  that  the  epithelium  of  the  ducts  is  never 
striated.  In  some  animals  a  somewhat  peculiar  gland,  the  Har- 
derian  gland,  lies  in  the  inner  (median)  region  of  the  orbit; 
this  varies  in  structure  in  different  animals,  being  in  some  a 
sebaceous  gland  united  with  a  gland  similar  in  structure  to  the 
lachrymal  gland. 

If  a  quantity  of  tears  be  collected,  they  are  found  to  form  a 
clear  faintly  alkaline  fluid,  in  many  respects  like  saliva,  con- 
taining about  1  p.c.  of  solids,  of  which  a  small  part  is  pro- 
teid  in  nature.  Among  the  salts  present  sodium  chloride  is 
conspicuous. 

The  nervous  mechanism  of  the  secretion  of  tears,  in  many 
respects,  resembles  that  of  the  secretion  of  saliva.  A  flow  is 
usually  brought  about  either  in  a  reflex  manner  by  stimuli 
applied  to  the  conjunctiva,  the  nasal  mucous  membrane,  the 
tongue,  and  the  interior  of  the  mouth,  or  more  directly  by 
emotions.  Powerful  stimulation  of  the  retina  by  light  will  also 
cause  a  flow,  as  will  electrical  or  other  stimulation  of  any  of  the 
cranial  or  upper  spinal  afferent  nerves.  Venous  congestion  of  the 
head  is  also  said  to  cause  a  flow.  The  efferent  nerves  are 
the  lachrymal  and  orbital  branches  of  the  fifth  iierve,  especially 
the  former,  stimulation  of  these  causing  a  copious  flow.  It  is 
said  that  stimulation  of  the  cervical  sympathetic  will  also  cause 
a  somewhat  scanty  flow  of  turbid  tears,  but  on  this  point  all 
observers  are  not  agreed. 

The  chief  use  of  the  act  of  blinking  is  to  keep  the  surface  of 


Chap,  hi.]  SIGHT.  979 

the  cornea  moist,  and  so  transparent;  if  the  cornea  be  kept  un- 
covered for  a  few  minutes  its  dried  surface  soon  becomes  dim. 
But  besides  this,  blinking-  undoubtedly  favours  the  passage  of 
tears  through  the  lachrymal  canaliculi  into  the  lachrymal  sac, 
and  hence  when  the  orbicularis  is  paralyzed  tears  do  not  pass  so 
readily  as  usual  into  the  nose  ;  but  the  exact  mechanism  by  which 
this  is  effected  has  been  much  disputed.  According  to  some 
authors,  the  contraction  of  the  orbicularis  presses  the  fluid  on- 
wards out  of  the  canals,  which,  upon  the  relaxation  of  the 
orbicularis,  dilate  and  receive  a  fresh  quantit}'.  Others  main- 
tain that  a  special  arrangement  of  muscular  fibres  keeps  the 
canals  open  even  during  the  closing  of  the  lids,  so  that  the 
pressure  of  the  contraction  of  the  orbicularis  is  able  to  have  full 
effect  in  driving  the  tears  through  the  canals. 


CHAPTER  IV. 
HEARING. 


SEC.  1.  ON  THE  GENEEAL  STRUCTUEE  OF  THE  EAE, 
AND  ON  THE  STEUCTUEE  AND  FUNCTIONS  OF  THE 
SUBSIDIAEY   AUDITOEY  APPAEATUS. 

§  610.  We  have  seen  that  the  eye  consists  on  the  one  hand 
of  the  special  modified  epithelium,  the  retina,  so  constituted  that 
light  falling  upon  it  gives  rise  to  visual  impulses  in  the  optic 
nerve  and  thus  to  visual  sensations  in  the  brain,  and  on  the 
other  hand  of  a  special  dioptric  mechanism,  into  the  construc- 
tion of  which  several  tissues  enter  and  which  is  so  arranged  as 
to  cause  the  rays  of  light  to  fall  in  a  proper  manner  on  the 
retina.  In  the  ear  we  meet  with  a  somewhat  similar  arrange- 
ment ;  we  may  recognize  on  the  one  hand  a  specially  modified 
epithelium,  which  we  may  call  the  auditorii_  ep_iilidivMi,  so  con- 
stituted that  the  vibrations  of  matter,  the  rapidly  alternating 
variations  of  pressure,  which  we  call  "  waves  of  sound,"  gener- 
ate in  the  auditory  nerve  connected  with  it,  auditory  impulses, 
developed  in  the  brain  into  auditory  sensations,  and  on  the  other 
hand  an  acoustic  apparatus  so  arranged  that  waves  of  sound  are 
conducted  iiraTproper  manner  to  the  auditory  epithelium.  But 
while,  as  we  have  seen,  the  optic  nerve  conveys,  so  far  as  Ave 
know,  visual  impulses  only,  we  have  reason  to  think  (§  478) 
that  some  fibres  at  least  of  the  auditory  nerve  convey  impulses 
which' do  not  give  rise  to  auditory  sensations,  but  enter  in  a 
peculiar  manner  into^he  mechanism  of  coordinated  movements. 

The  retina  as  we  have  seen  is  developed  out  of  the  optic 
vesicle,  and  the  subsidiary  dioptric  mechanism  is  built  up  around 
the  optic  vesicle  ;  and  in  a  somewhat  similar  way  the  auditory 
epithelium  is  developed  into  an  otic  vesicle,  and  the  subsidiary 
acoustic  apparatus  is  built  up  armmcPthe  otic  vesicle.  The 
otic  vesicle,  like  the  optic  vesicle,  is  Ihied  by  jin_ej)ithelium 
of  epiblastic_  origin,  but  is  not  like  that  vesseT  budded  off  from 
the   medullary  canal.     It  takes  origin  in  an  involution  of  the 

980 


Chap,  iv.]  HEARING.  981 

skin  covering  the  head ;  for  a  time  the  epithelium  of  the  vesicle 
is  continuous  with  the  epidermis  of  the  skin,  and  wholly  uncon- 
nected with  the  developing  brain ;  later  on  the  epithelial  invo- 
lution separates  from  the  skin,  becomes  a  closed  independent 
vesicle,  and  makes  connections  with  the  brain  through  the  audi- 
tory nerve  growing  out  to  meet  it.  The  otic  vesicle  therefore  is  | 
not  like  the  optic  vesicle  a  part  of  tlie  brain,  and  we  find  accord- 
ingly the  structure  of  the  auditory  epithelium  much  more  simple 
than  that  of  the  retina ;  it  corresponds  to  a  part  only  of  the 
retina,  to  the  more  external  layers  of  the  retina,  not  to  all  of 
them. 

We  have  seen  that  the  optic  fibres  are  connected  with  a  part 
only  of  the  optic  vesicle,  with  the  anterior  wall  only  of  the  re- 
tinal cup  and  not  with  tlie  whole  of  this  ;  the  part  of  the  anterior 
wall  which  forms  the  pars  ciliaris  retinte  and  the  whole  of  the 
posterior  wall  make  no  connections  with  the  optic  fibres  and 
remain  in  the  form  of  a  relatively  simple  epithelium.  The  con- 
nection of  the  auditory  nerve  with  the  walls  of  the  otic  vesicle 
is  still  more  partial ;  the  ji£i'\:£Lfibi'fiS-becom€-conH-eGt.edr with  tlie. 
epithelium  in  a  few  limited  areas..  It  is  only  in  these  areas  that 
the  epithelium  lining  the  otic  vesicle  becomes  differentiated 
into  the  special  auditory  epithelium ;  elsewhere  it  possesses 
relatively  simple  characters. 

The  cavity  of  the  optic  vesicle  is,  as  we  have  seen,  soon 
obliterated  by  the  coming  together  of  the  anterior  and  posterior 
walls.  The  cavity  of  the  otic  vesicle  is  permanent,  growing 
with  the  growth  of  the  organ  and  becoming  filled  with  a  peculiar 
fluid  secreted  by  the  walls,  called  endohjmj^h.  The  vesicle  as  it 
grows  soon  loses  its  early  simple,  more  or  less  spherical  form 
and  assumes  a  most  complicated  shape,  becoming  divided  into 
the  parts  known  as  the  utricle  with, the.  se«iioi4^cular  canals,  the 
saccule,  and  the  canalis_cochle_aris ;  of  these  we  sliall  speak  in 
detaiilater  on. 

§  611.  While  the  vesicle  is  assuming  this  complicated  shape, 
the  mesoblastic  tissue  .investing ->it  undergoes  a  differentiation. 
The  tissue  immediately  in  contact  with  the  epithelium  becomes 
connective  tissue  serving  as  a  dermis  to  support  the  epithelium, 
so  that  the  vesicle  becomes  a  (complicated)  sac  with  membranous 
walls  lined  with  epithelium  specially  modified  into  auditory 
e])itiielium  at  particular  places,  at  which  places  and  at  which 
places  alone,  the  auditory  nerve  makes  connections  with  the 
walls. 

The  outer  portion  of  the  mesoblastic  tissue  is  converted  into 
bone  of  a  somewhat  dense  character,  and  thus  furnishes  a  bony 
shell  or  envelope  enclosing  and  to  a  large  extent  following  the 
contour  of  the  complicated  membranous  sac.  Between  the 
outer  bony  envelope  and  the  inner  membranous  sac  is  developed 
a  large  irregular  lymphatic  space  which  (Fig.  166)  follows  to 


982  GENERAL   STRUCTURE   OF  EAR.       [Book  hi. 

a  great  extent  the  contour  of  the  sac,  but  is  broken  up  by  broad 
adhesions  of  the  membranous  sac  to  the  periosteum  lining  the 
bony  envelope  or  by  narrower  bridles  of  connective  tissue  cross- 
ing the  space ;  some  of  these  form  beds  for  the  branches  of  the 
auditory  nerve  on  their  way  to  the  auditory  epithelium.     The 


Fig.  166.    Diagram  to  illustrate  the  general  Stkuctuke  of  the  Ear. 

(After  Schwalbe.) 

The  figure  is  purely  diagrammatic,  intended  only  to  shew  in  one  view  all  the 
several  important  parts  in  relation  to  each  other ;  such  a  view  is  in  the  actual  ear 
impossible. 

m.e.  the  external  meatus  or  auditory  passage,  in  the  outer  part  where  the 
walls  are  cartilaginous,  m'.e'.  the  same  in  the  inner  part  where  the  wails  are 
osseous. 

T.C.  the  tympanic  cavity,  t.m.  the  tympanic  membrane,  m.  malleus,  i. 
incus,  St.  stapes,  attached  to  the  fenestra  ovalis.  f.r.  fenestra  rotunda.  E.t. 
Eustachian  tube. 

U.  the  utricle,  with  the  perilymph  space  around.  One  semicircular  canal  with 
its  ampulla  is  shewn,  with  the  bony  core  of  the  hoop.  8.  Saccule,  s.e.  saccu- 
lus  endolymphaticus  lying  within  the  cranial  cavity,  and  connected  by  the  ductus 
endolymphaticus  with  both  saccule  and  utricle,  chl.  the  canalis  cochlearis,  con- 
nected with  the  saccule  by  the  canalis  reuniens,  and  surrounded  by  its  perilymph 
space,  scala  vestibuli,  and  scala  tympani,  the  latter  ending  at  the  fenestra  rotunda, 
the  former  continuous  with  the  perilymph  space  of  the  vestibule  around  the 
utricle  and  saccule  ;  the  cochlea  is  shewn  diagrammatically  as  a  simple  curve, 
the  scala  vestibuli  and  scala  tympani  being  continuous  at  tlie  top. 

N.  aud.  the  auditory  nerve  shewing  the  three  main  divisions  of  the  trunk. 

fluid  in  this  space,  which  is  lymph  and  which  has  access  to 
the  lymphatics  of  neighbouring  parts,  receives  the  special  name 
of  'perili[m2jli.  A  portion  of  the  sac,  with  its  surrounding  peri- 
lymph space  and  bony  envelope,  undergoes  a  development 
differing  materially  from  that  of  the  rest  of  the  sac,  and  is 
known  as  the  cochlea.  The  bony  envelope  surrounding  the 
parts  of  the  membranous  sac  known  as  the  utricle  and  saccule 
does  not  follow  closely  the  contour  of  those  parts  but  remains 
an  undivided  part  called  the  vestibule  (Fig.  167) ;  the  parts  of 


Chap,  iv.] 


HEAEIXG. 


983 


the  membranous  sac  called  the  semicircular  canals  are  however 
followed  somewhat  closely  by  the  bony  envelope.  The  whole 
bony  envelope  may  be  dissected  out  from  the  spongy  bone  sur- 
rounding it,  and  may  be  obtained  as  a  separate  mass  (Fig.  167), 


,s.s,c 


ssc 


psc 


B 


.  mi 


Fig.  167.     The  Bony  Labyrinth.     Left  Ear.     (Schwalbe.) 

A.  seen  from  the  outside.    B.  seen  from  the  median  side.     Both  magnified  twice. 

Vb.  vestibule.  Chi.  cochlea.  ChV.  the  beginning  of  the  first  turn  of  the 
cochlea.  F.o.  fenestra  ovalis,  f.r.  fenestra  rotunda,  s.s.c.  superior,  p.s.c.  pos- 
terior, h.s.c.  horizontal  semicircular  canals,  m.i.  meatus  auditorius  internus, 
canal  for  the  auditory  nerve.  VIL  opening  of  the  canal  containing  the  seventh 
nerve. 

knoAvn  by  the  name  of  the  labyrinth.,  S^^^_^.^m  Idhyfinth  to  distin- 
guish it  from  the  memhrcmous  labyrmmwhiGh.  lies  Avithin  it, 
separated  from  it  by  the  perilymph  space.  The  bon_v  labyrinth 
consists  of  cochlea,  vestibule  and  semicircular  canals,  but  the 
part  of  the  membranous  labyrinth  corresponding  to  the  vestibule 
is  divided  into  utricle  and  saccule.  The  auditory  nerve  pierces 
the  bony  labyrinth  at  the  so-called  rneatus  auditoi'ius  internus 
(Fig.  167  m.i.^  on  its  way  to  be  distributed  to  the  walls  of  flTe 
membranous  sac. 

All  these  structures,  lying  at  first  not  far  beneath  the  skin 
and  forming  together  the  '  internal  ear,'  as  they  grow  come  into 
close  connection  with  a  passage  on  the  side  of  the  head  leading 
from  the  exterior  into  the  pharynx  and  known  as  the  "  first"  or 
"hyomandibular  visceral  cleft."  By  a  series  of  changes,  which 
we  need  not  describe  here,  and  indeed  about  which  there  is 
some  divergence  of  opinion,  this  simple  primitive  passage  is 
replaced  in  the  adult  by  two  passages  separated  from  each  other 
by  a  partition  known  as  the  niernhrana  ti/mjyani  (Figs.  166  t.m., 
168),  or  tympanic  membrane^  On  the  outer  side  of  the  mem- 
brane lies  a  tubular  channel,  the  ^xt^rnal  ai^ffi^Qri/  mp.atjis  (Figs. 
166  ?n.e.,  ??i'.e'.,  169  w.c),  lined  by  skin,  and  opening  on  to  the 
exterior  by  an  orifice  guarded  with  the  "pinna"  or  "auricle." 
On   the   inner   side    of  the    membrane    lies    the    drum-sliaped 


984 


GENEEAL   STEUCTURE   OE  EAE.       [Book  in. 


tj£nr£anic_e^nt2i  (Fig.  166  T.C),  often  called  the  " middle  ear," 
which  through  the  tubular  JEustackian  tube  (Figs7T[B6  J^.t.^ 
169  E.t.'),  opens  into  the  pharynx,  and  which  is  lined  through- 
out by  mucous  membrane  continuous  with  that  of  the  pharynx. 


mbr 


Fig.  168.    The  Membrana  Ttmpani.    (After  Schwalbe.)    (Magnified  four  times.) 

The  membrane  is  seen  from  the  external  meatus  and  the  handle  of  the  mal- 
leus, mhr,  is  represented  as  shining  through,  m.f.  the  membrana  flaccida,  the 
folds  of  which  are  represented  radiating  from  p.h.,  the  projection  outwards 
caused  by  the  end  of  the  short  process  of  the  malleus,  u.  the  umbo  of  the  mem- 
brane, to  which  is  attached  the  end  of  the  handle  of  the  malleus.  The  figure 
shews  diagrammatically,  the  radial  and  circular  fibres  of  the  membrane. 

The  '  internal  ear  '  forms  the  mesial  side  of  the  more  or  less 
flattened  and  drum-shaped  tympanic  cavity  opposite  to  the  outer 
side  which  is  to  a  large  extent  formed  by  the  tympanic  mem- 
brane ;  and  at  two  places  the  osseous  tissue  of  the  bony 
envelope  of  the  internal  ear  is  wanting,  the  gaps  giving  rise 


Fig.  169.     Diagram  to  illustrate  the  relations  of  Auditory  Passage, 
Tympanum  and  Eustachian  Tube.     (After  Schwalbe.) 

The  figure  represents  a  section  not  quite  horizontal,  being  inclined  down- 
wards in  front ;  the  right-hand  edge  of  the  page  may  be  taken  to  represent  the 
median  plane  of  the  head. 

m.e.  external  meatus,  T.  the  tympanic  cavity.  E.t,  the  Eustachian  tube. 
A.  is  placed  in  the  antrum  mastoideum.  m.t.  indicates  the  attachment  of  the 
tympanic  membrane. 

a.h.  the  axis  of  the  external  meatus,  c.  6.  d.  that  of  the  Eustachian  tube. 
dd'.  shews  the  curved  axis  of  the  antrum. 


Chap,  iv.] 


HEAEIXG. 


985 


to  what  in  the  dried  skull  ap[)L'ar  as  two  foramina,  but  in  the 
fresh  state  are  two  membranous  fenestrse.  One  of  these,  oval 
in  shape,  called  the  f&:uestr a  ovalis_  (Figs.  166,  170,  171 /.o.),  lies 
between  the  tympanic  cavity  on  the  outside  and  that  part  of 
the  perilymph  space  which  surrounds  the  division  of  the  mem- 


FiG.  170.     Frontal  (transverse  vertical)  section  through  the 
Tympanum.     (Left  Ear.)     (Schwalbe.) 

The  fii^Tire,  partly  diagrammatic,  is  magnified  twice,  and  shews  the  front  part 
of  the  tympanum  as  seen  from  beliind  ;  the  incus  has  been  removed,  the  artic- 
ular surface  on  the  head  of  the  malleus  being  indicated. 

mt.  The  membrana  tymjiani.  mf.  membrana  fliaccida.  mhr.  handle  of  the 
malleus,    p.h.  short  process  of  the  malleus. 

Ig.e.  external  ligament,  Ig.s.  the  superior  ligament  of  the  malleus. 

TT.  The  bony  projection  from  which  the  tendon  of  the  tensor  tympani 
passes  to  the  malleus,  f.o.  the  fenestra  ovalis.  v.  the  front  part  of  the  vesti- 
bule,    c.  the  beginning  of  the  first  (basal)  turn  of  the  cochlea. 


Fig.  171. 


Diagram  of  the  median  Wall  of  the  Tvmpanum  of  the  Left 

Ear.     Magnified  twice.     (After  Schwalbe.) 


1.  The  tympanic,  2.  the  epitympanic  region  ;  below  the  reference  figure  is 
seen  the  gentle  prominence  due  to  the  ampulhe.  ^.  the  antrum  mastoideum,  the 
line  ee  marking  its  limits.  E.t.  the  Eustachian  tube.  T.  T.  the  groove  for  the 
tensor  tympani.  f.n.  the  depression  of  the  fenestra  ovalis,  the  fenestra  itself 
being  shaded,  f.r.  the  depression  leading  to  the  fenestra  rotunda  ;  above,  and 
obliquely  to  the  left  of  this,  lies  the  projection  caused  by  the  base  of  the  cochlea, 
St.  the  prominence  for  the  stapedius,  with  the  orifice  for  the  exit  of  the  tendon. 
VII.  the  course  of  the  facial  nerve.  The  tympanum  proper  lies  within  the  let- 
ters a.  b.  d.e. 


986 


THE  AUDITOEY  OSSICLES. 


[Book  hi. 


branous  labjrintli  known  as  the  utricle  on  the  inside  ;  in  the 
dried  bony  Labyrinth  (Fig.  167  F.o.^  it  appears  as  a  hole  in 
the  vestibule.  The  other,  round  in  shape,  called  the  fenestra 
rotunda  (Figs.  166,  171 /.r.),  lies  between  the  tympanic  cavity 
and  a  part  of  the  perilymph  space  which  enters  into  the  con- 
struction of  the  cochlea ;  as  v/e  shall  see,  the  perilymph  space 
of  the  cochlea  may  be  regarded  as  a  peculiar  tubular  prolonga- 
tion of  that  of  the  vestibule,  and  the  membrane  of  the  fenestra 
rotunda  closes  as  it  were  the  end  of  this  prolongation. 

Certain  bones  of  the  skull,  converted  by  striking  develop- 
mental changes  into  a  jointed  chain  of  minute  bones,  the 
auditory  ossicles  (Figs.  166  m.  ^.  St.,  172),  are  by  processes  of 
growth  thrust  into  the  tympanic  cavity  in  such  a  way  that 
they  eventually  seem  to  lie  wholly  in  the  cavity,  and  to  form 


Pig.  172.    The  Auditory  Ossicles.     (After  Schwalbe  and  Helmholtz. ) 
Magnified  four  times. 

A.  The  malleus,  cp.  the  head  (caput) .  *  the  articulating  surface  for  the  incus. 
t.  tootli  locking  with  tooth  of  incus.  Ig.  is  placed  opposite  the  attachment  of 
the  ligaments,  jy.f.  processus  gracilis  or  Folianus,  represented  as  short,  p.b.  pro- 
cessus brevis.     m.6r.  handle  (manubrium). 

B.  The  incus.  *  surface  articulating  with  malleus,  t.  tooth  locking  with  tooth 
of  malleus,    p'.b'.  processus  brevis.    p'.Z'.  processus  longus. 

B'.  The  lower  end  of  the  processus  longus  seen  sideway ;  o.  its  expanded 
termination  or  os  orbiculare. 

C.  The  stapes,     c.  the  head.    /.  the  foot-plate. 

D.  The  three  ossicles  in  connection.  M.  malleus,  I.  incus,  st.  stapes ;  the 
other  letters  as  above. 


a  bridge  across  the  cavity  between  the  tympanic  membrane  on 
the  outer  side,  and  the  fenestra  ovalis  on  the  mesial  side  (Fig. 
173).  The  ossicles  are  three  in  number;  to  the  tympanic  mem- 
brane is  attached  the  malleus;  this  is  joined  to  the  ins2JiS,  which 
in  turn  is  joined  to  the  stapes,  the  end  of  which  is  attached  to 
the  fenestra  ovalis. 

§  612.     The  affections  of  consciousness,  which  we  call  sen- 
sations of  sound,  are  the  result  of  auditory  impulses  reaching 


Chap,  iv.]  HEARING.  987 

certain  parts  of  the  brain  along  the  auditory  nerve  ;  and  these 
auditory  impulses  are  generated  through  vibrations,  or  rhyth- 
mically repeated  variations  of  pressure  which  we  call,  ^  waves  of 
SQiuid,'  in  some  way  or  other  acting  upon  the  terminations  of 
the  auditory  fibres  in  the  auditory  epithelium.  The  waves 
of  sound  gain  access  to  the  epithelium  by  means  of  the  peri- 


Fig.  173.     The  Ossicles  in  PosiTio>f.    Magnified  four  times.    (After  Hensen.) 

The  figure  represents  a  section  tlirough  tympanum  in  the  line  of  the  long  axis 
of  the  malleus  and  incus  ;  the  short  process  of  the  inctis  p'b'  has  been  cut  through. 

T.C.  The  tympanic  cavity.  7?i6r.  handle  of  malleus.  ?(.  umbo.  j^. 5.  short 
process  of  the  malleus  shewn  in  dotted  outline  as  pushing  outwards  the  mem- 
brana  flaccida.  T.T.  the  attachment  of  the  tendon  of  the  tensor  tympani.  Ig. 
the  attachment  of  the  external  ligament  of  the  malleus,  h/.s.  the  superior  liga- 
ment of  the  malleus,  t.t.  the  teeth  of  the  incus.  j;7'.  the  long  process,  shaft,  of 
the  incus.     St.  the  stapes. 

lymph,  passing  probably  iji^^sorrm  par±a  .  rljie'^tly  through  the 
dermis  of  the  membranous  sac  to  the  overlying  epithelium, 
and  being  iii^other  javt^  tiaxumiittod  to  ■■  the-oiKJolympji  from 
the  perilymph  across  the  membranous  walls,  and  acting  on  the 
epithelium  through  the  endolymph. 

Waves-of  sound  may  be  and  to  a  certain  extent  are  conducted 
in  a  direct  manner  to  the  perilymph,  through  the  tissues,  espe- 
cially the  harder  bony  tissues,  of  the  head,  reaching  the  perilymph 
across  its  bony  envelope.  The  yast_jnajonty. however  of  the 
waves  of  sound  which  fall  upon  the  head  travel  through  the 
medium  of  the  air,  and  in  order  to  reach  the  perilymph  have  to 
pass  from  a  gaseous  medium,  the  air,  into  the  solid  and  liquid 
media  of  the  head.  Now  the  vibrations  of  particles  constituting 
waves  of  sound  are  not  readily  communicated  from  a  gaseous 
to  a  liquid  or  solid  medium ;  special  conditions  are  required 
to  effect  this.      The  transference  of  sound  from  the  air  to  the 


988  CONDUCTION  THEOUGH  TYMPANUM.     [Book  in. 

perilj^myh  is  attended  with  considerable  dijfficulty ;  and  the 
parts  of  the  ear  which  we  have  spoken  of  above  as  constituting 
the  middle  and  outer  ear,  serve  as  an  acoustic  apparatus  for 
facilitating  this  transference  and  thus  bringing  the  aerial  waves 
to  act  on  the  auditory  epithelium,  the  action  of  the  apparatus 
being  somewhat  as  follows. 

Waves  of  sound  falling  on  the  side  of  the  head  reach  the  tym- 
panic membrane  by  the  external  meatus,  and  throw  that  mem- 
brane into  vibrations.  These  vibrations  are  transmitted  through 
the  chain  of  ossicles  to  the  membrane  of  the  fenestra  ovalis  and 
so  to  the  perilymph  lying  on  its  far  side ;  sweeping  over  the 
perilymph  in  its  continuous  cavity  the  waves  eventually  break 
upon  the  fenestra  rotunda,  having  on  their  way  affected  the 
auditory  epithelium.  We  have  first  to  inquire  how  this  subsid- 
iary acoustic  apparatus  performs  its  work. 

The  conduction  of  sound  through  the  Tympanum. 

§  613.  The  chain  of  ossicles,  jointed  together,  attached  to 
the  tympanic  membrane  at  one  end,  and  to  the  fenestra  ovalis 
at  the  other,  and  secured  by  ligaments,  may  be  regarded  as  a 
lever.  Observations  and  experiments  shew  that  the  end  of  the 
short  process  of  the  incus  serves  as-  the  fulcrum,  the  power 
being  applied  at  the  umbo  in  which  the  handle  of  the  malleus 
ends,  and  the  eifect  being  brought  to  bear  on  the  end  of  the 
long  process  of  the  incus  attached  to  the  stapes.     In  thus  acting 


'ge 
Fig.  174.    The  Ligaments  of  the  Ossicles.     (After  Hensen.) 

The  figure  represents  a  nearly  horizontal  section  of  the  tympanum,  carried 
through  the  heads  of  the  malleus  and  incus. 

M.  malleus.    I.  incus,     t.  articular  tooth  of  incus.  Ig.a.  anterior  and  Ig.e.  ex- 
ternal ligament  of  the  malleus.     Ig.inc.  ligament  of  the  incus. 

The  line  ax  represents  the  axis  of  rotation  of  the  two  ossicles. 

as  a  lever  the  heads  of  the  malleus  and  incus  rotate  round  a 
horizontal  line  drawn  through  them  in  the  direction  of  the  line 
ax  in  Fig.  174.  Such  a  lever  may  be  represented  by  the  line 
xx'  in  Fig.  175.  Careful  measurements  shew  that  the  whole 
length  of  the  line  from  F  the  fulcrum  to  P  where  the  power  is 
applied,  is  about  9-5  mm.,  while  the  length  from  F  to  TF,  where 
the  effect  is  brought  to  bear,  is  about  6' 3  mm.     Hence  when 


Chap,  iv.] 


HEARING. 


989 


the  tympanic  membrane  is  driven  inwards,  the  corresponding 
inward  movement  of  the  stapes  in  the  fenestra  is  as  far  as 
extent  is  concerned  only  about  two-thirds  of  that  of  the  tj-m- 
panic  membrane.  By  the  principle  of  the  lever,  however,  the 
amount  of  pressure  exerted  by  the  movement  of  the  stapes,  the 


Fig.  175.     The  Malleus  and  Incus,  in  position.     (Helmholtz.) 

M.  The  malleus,  c,  the  head,  mhr,  the  handle,  p.f,  processes  Folianus. 
T.  T,  the  tendon  of  the  tensor  tyiupani. 

/.  The  incus,  p'b',  the  short  process,  p'Z'.  the  long  process,  t.  tooth  locking 
with  the  malleus. 

The  line  XX  represents  the  lever  formed  by  the  two  ossicles,  with,  F,  the 
fulcrum  at  the  attachment  of  the  short  process  of  the  incus,  P,  the  point  where 
the  power  is  applied  at  the  end  of  the  handle  of  the  malleus,  \V,  the  point  where 
the  effect  is  produced  at  the  os  orbiculare  of  the  incus. 


force  of  the  movement,  is  one  and  a  half  times  greater  than 
the  force  expended  in  producing  the  movement  of  the  tympanic 
membrane.  The  arrangement  of  the  lever  of  ossicles  therefore 
is  such  as  to  convert  a  relatively  large  movement  into  a  smaller 
movement  of  greater  intensity;  the  benefit  of  such  a  conversion 
is  obvious. 

§  614.  The  conduction  of  sound  from  the  external  air  to  the 
labyrinth  takes  place  by  means  of  the  tympanic  membrane  and 
the  chain  of  ossicles  acting  as  a  lever  in  the  manner  just  described. 

Stretched  membranes  have  the  property  of  being  readily 
thrown  into  vibrations  by  aerial  waves  of  sound,  and  of  trans- 
mitting the  vibrations  to  bodies  in  contact  with  themselves. 
The  tympanic  membrane  is  a  stretched  membrane  which,  by  its 
size,  nature  and  conformation  is  specially  adapted  to  take  up 
and  transmit  a  great  variety  of  vibrations.  Sound  is  a  vibration 
of  the  particles  of  matter,  a  series  of  movements  of  the  particles 
from  and  to  a  fixed  point.  In  air  and  other  gases  tlie  move- 
ments of  the  particles  lead  to  alternating  condensation  and 
rarefaction  of  the  medium,  the  sound  is  propagated  as  waves  of 
alternating  condensation  and  rarefaction,  which  since  tlie  to- 
and-fro  movement  of  the  particles  is  in  the  same  direction  as 


990  CONDUCTION   THEOUGH  TYT^IPANUM.    [Book  hi. 

tliat  in  which  the  undulations  are  travelling,  are  spoken  of  as 
'longitudinal'  Avaves.  In  liquids  the  transmission  of  sound  also 
takes  place  by  longitudinal  waves  of  alternating  condensation 
and  rarefaction,  and  sound  may  travel  through  solids  in  the 
same  way.  But  solids  in  the  form  of  membranes  or  plates, 
strings,  and  rods  may  also  give  rise  to  sounds  by  being  thrown 
into  bodily  vibrations,  a  rod  for  instance  bending  alternately 
to-and-fro  in  rapid  succession.  In  such  a  case  the  particles  of 
the  rod  move  sensibly  in  a  direction  transverse  to  the  long  axis 
of  the  rod;  and  the  vibrations  of  this  kind,  thus  giving  rise  to 
sounds,  are  spoken  of  as  "transversal"  vibrations.  It  will  be 
understood  that  a  rod,  membrane,  plate  or  string,  may  also  be 
the  subject  of  longitudinal  vibrations;  but  the  sound  given  out 
by  such  longitudinal  vibrations  differs  from  that  given  out  by 
transversal  vibrations  of  the  same  body.  A  rod,  string,  or 
membrane  thrown  into  sufficiently  rapid  and  strong  transversal 
vibrations,  will  communicate  its  vibrations  to  the  surrounding 
air,  and  so  give  forth  a  sound,  which  will  travel  through  the  air 
in  the  form  of  waves  of  longitudinal  vibrations.  Conversely, 
sound  travelling  through  the  air  in  waves  of  longitudinal  vibra- 
tions, and  striking  upon  a  rod,  string  or  membrane,  may  throw 
it  into  transversal  vibrations.  And  this  is  what  takes  place  in 
the  ear.  Aerial  waves  of  sound,  in  the  form  of  longitudinal 
vibrations,  alternating  condensations  and  rarefactions,  of  the 
air,  travelling  along  the  meatus,  fall  upon  the  tympanic  mem- 
brane, and  throw  it  into  transversal  vibrations;  the  membrane 
bends  bodily  inwards  and  outwards  in  time  with  the  condensa- 
tions and  rarefactions  of  the  air  in  the  meatus  on  its  outer 
surface. 

The  vibrations  of  a  rod,  a  tuning-fork  for  example,  are  com- 
paratively simple  in  character ;  and  we  find,  correspondingly, 
that  a  tuning-fork  is  very  limited  in  its  power  of  '  taking  up ' 
sounds  from  the  air,  of  being  thrown  into  vibrations  by  sounds 
falling  upon  it ;  it  will  only  take  up  from  the  air  the  particular 
sounds,  the  particular  tones  as  we  shall  presently  call  them,  which 
it  itself  gives  forth  when  thrown  into  vibrations  by  being  struck. 
The  vibrations  of  a  membrane  are  much  more  complex  ;  and  for 
this  reason  a  membrane  takes  up  much  more  readily  a  variety 
of  different  sounds  reaching  it  through  the  air.  Still  every 
membrane  has  its  fundamental  tone  or  tones,  as  they  are  called, 
those  which  it  naturally  gives  forth  when  thrown  into  vibra- 
tions ;  and  it  takes  up  these  from  the  air  much  more  readily 
than  any  other  sounds.  It  is  a  feature  of  the  tympanic  membrane 
that  it  takes  up,  without  any  marked  distinction,  a  very  great 
variety  of  sounds  within  a  very  large  range.  It  probably  has 
a  fundamental  tone  of  its  own,  but  this  is  kept  in  the  back- 
ground ;  it  is  not  prominent,  and  does  not  materially  influence 
our  hearing.     Were  it  otherwise,  were  the  tym|)anic  membrane 


Chap,  iv.]  HEARING.  991 

thrown  into  vibration  much  more  readily  by  a  particular  sound 
than  by  any  others,  that  sound  would  be  dominant  in  all  our 
hearing- ;  and  unless,  as  in  vision,  psychical  experience  inter- 
vened to  correct  the  mere  sensation,  we  should  be  misled  in  our 
judgments  as  to  what  was  taking  place  around  us. 

This  general  usefulness  of  the  tympanic  membrane  is  securedJ 
partly  by  features  proper  to  itself,  partly  by  the  fact  that  it  is| 
' damped '  by  the  attachment  to  it  of  the  chain  of  ossicles.) 
Without  attempting  to  enter  into  a  discussion  of  a  matter  which 
is  in  many  ways  complex,  we  may  say  that  the  following  feat- 
ures contribute  to  make  the  tympanic  membrane  sensitive  to  a 
large  variety  of  sounds.  In  the  first  place  its  dimensions  are 
relatively  smaU.  In  the  second  place  the  material  of  which  it 
is  composed  is  peculiar,  so  that  it  is  in  a  special  way  unyielding 
and  rigid;  it  retains  its  form  when  cut  away  from  its  bony 
attachments  by  a  circular  incision,  and  the  malleus,  including 
its  handle,  may  be  removed  from  it  without  distorting  it.  In 
the  third  place,  its  remarkable  form,  that  of  a  shallow  funnel 
with  sides  gently  convex  towards  the  meatus,  lias  a  marked 
effect  upon  its  capabilities  of  vibration.  The  chain  of  ossicles, 
attached  at  its  far  end,  to  the  membrane  of  the  fenestra  ovalis 
has  a  '  damping '  effect,  similar  to  that,  familiar  to  every  one,  of 
lessening  or  stopping  the  sound  of  a  vibrating  empty  wine-glass 
or  tumbler  by  pressing  the  finger  on  it,  and  this  'damping' 
while  it  diminishes  to  a  certain  extent  all  the  vibrations  of  the 
membrane  is  especially  effective  in  the  case  of  excessive  vibra- 
tions, such  as  those  which  might  be  produced  by  the  sound 
which  is  the  fundamental  note  of  the  membrane. 

§  615.  The  vibrations  thus  set  going  in  the  tj-mpanic  mem- 
brane are  transmitted  from  it  to  the  chain  of  ossicles.  The 
transmission  might  take  place  in  two  ways.  In  the  first  place 
the  vibrations,  the  alternate  bendings  inwards  and  outwards  of 
the  membrane,  might,  by  carrying  with  it  the  attached  handle 
of  the  malleus,  work  the  chain  of  ossicles  as  a  lever,  in  the 
manner  described  in  §  613,  so  that  each  inward  flexion  of  the 
tympanic  membrane  led  to  the  membrane  of  the  fenestra  ovalis 
pushing  the  peril3'-m])h  of  the  labyrinth  inwards,  while  tlie  return 
outwards  again  of  the  one  led  to  a  like  return  of  the  other. 
In  the  second  place  the  transversal  vibrations  of  the  tympanic 
membrane  might  set  up  longitudinal  vibrations  in  the  substance 
of  the  malleus,  which  would  travel  as  longitudinal  vibrations 
through  the  chain,  and  so  reach  the  perilym])h.  In  the  one 
case  the  whole  chain  of  ossicles  swings  to  and  fi'o,  in  the  other 
case  the  sound  is  propagated  by  molecular  movement.  That 
the  ossicles  do  move  en  masse  has  been  proved  by  recording 
their  movements  in  the  usual  graphic  method.  A  very  light  \ 
style  attached  to  the  end  of  the  incus  or  to  the  stapes  is  made  to 
write  on  a  travelling  surface  ;  when  the  tympanic  membrane  is 


992         CONDUCTION   THROUGH   TYMPANUM.     [Book  iii. 

thrown  into  vibrations  by  a  sound,  the  curves  described  by  the 
style  indicate  that  the  chain  of  bones  moves  with  every  vibra- 
tion of  the  membrane.  On  the  other  hand,  the  comparatively 
loose  attachments  of  the  several  ossicles  is  an  obstacle  to  the 
molecular  transmission  of  sonorous  vibrations  through  them. 
Moreover,  sonorous  vibrations  can  only  be  transmitted  to  or  pass 
along  such  bodies  as  either  are  very  long  compared  to  the  length 
of  the  sound-waves,  or,  as  in  the  case  of  membranes  and  strings, 
have  one  dimension  very  much  smaller  than  the  others.  Now 
the  bones  in  question  are  not  only  not  especially  thin  in  any 
one  dimension,  but  are  in  all  their  dimensions  exceedingly  small 
compared  with  the  wave-lengths  of  the  vibrations  of  even  the 
shrillest  sounds  we  are  capable  of  hearing ;  hence  they  must  be 
useless  for  the  molecular  propagation  of  vibrations.  We  may 
conclude  then  that  when  waves  of  sound  throw  the  tympanic 
membrane  into  vibrations,  each  inward  excursion  of  the  mem- 
brane is  followed  by  a  corresponding  impulse  given  by  the  foot 
of  the  stapes  to  the  perilymph.  As  we  have  seen  the  space 
through  which  the  end  of  the  incus  moves  is  less  than  that 
through  which  the  handle  of  the  malleus  moves,  and  the  move- 
ments of  the  stapes  are  in  addition  restricted  "by  the  manner  of 
its  attachment  to  the  rim  of  the  fenestra  ovalis ;  but  the  energy 
with  which  the  end  of  the  incus  and  hence  the  stapes  moves  is 
proportionately  increased,  so  that  we  might  almost  speak  of  the 
gentle  swingings  of  the  tympanic  membrane  being  converted 
into  smart  taps  on  the  perilymph  of  the  labyrinth. 

The  impulses  thus  given  to  the  perilymph  at  the  fenestra 
ovalis  travel  along  the  intricate  passages  of  the  perilymph 
spaces,  and  finally  break  upon  the  fenestra  rotunda ;  if  the 
membrane  which  closes~tIiis~orifice  be  watched  it  may  be  ob- 
served to  pulsate  in  sequence  with  the  pulsations  of  the  fenestra 
ovalis.  During  their  passage  these  impulses  are  communicated 
to  the  endolymph  and  in  some  way  or  other  affect  the  endings 
of  the  auditory  nerve.  How  they  do  this  we  shall  presently 
study ;  but  we  may  here  call  attention  to  the  fact  that  the 
waves  of  sound  which  fall  on  the  tympanic  membrane  are  for 
the  most  part  not  simple  in  character  but  complex,  and  in  many 
cases  exceedingly  so.  This  complexity  is  carried  on  into  the 
vibrations  of  the  tympanic  membrane  and  so  into  the  impulses 
given  to  the  perilymph;  the  waves  which  sweep  past  the  end- 
ings of  the  auditory  nerve  are,  so  to  speak,  reproductions  of  the 
complex  aerial  waves  passing  down  the  meatus. 

§  616.  By  far  the  greater  number  of  sounds  which  we  hear 
reach  the  tympanic  membrane  by  passing  through  the  air  down 
the  meatus.  One  great  use  of  the  long  external  jjassage  is  prob- 
ably tQ_4)XQt£ilt  the  delicate  tympanlc~membrane  from  the  acci- 
dents to  which  it  would  be  subject  were  it  freely  exposed  on  the 
surface  of  the  body ;  but  it  has  also  a  use  in  transmitting  to  the 


Chap,  iv.]  HEARING.  993 

tympanic  membrane  sounds  travelling  to  the  ear  in  certaiu  (lii;ec- 
tions  ingi:e--i:£at|ilj  than  those  coming  in  other  directions.  The 
constriction  of  the  meatus  at  the  junction  of  the  outer  and  mid- 
dle third  serves  as  a  sort  of  diaphragm  by  which  waves  of  sound 
travelling  too  much  out  of  the  line  of  the  meatus  are  turned 
back.  The  external  ear,  auricle,  or  pinna  has  also  probably  a 
similar  effect,  reflecting  into  the  meatus'waves  which  fall  upon 
it  in  a  particular  direction  or  waves  of  a  particular  kind.  But 
of  these  uses,  which  are  of  more  importance  in  some  animals 
than  in  man,  we  shall  speak  again  in  considering  the  manner  in 
which  we  recognize  the  directions  of  sounds. 

Sounds  however  may  reach  the  ear  by  paths  other  than  the 
meatus.  If  a  tuning-fork  be  struck  and  then  held  near  the  ear 
it  will  after  a  while  cease  to  be  heard,  the  sound  dies  away ;  but 
the  sound  is  heard  again  if  the  handle  of  the  fork  be  placed 
between  the  teeth ;  and  when  the  sound  again  dies  away,  it 
may  be  revived  by  gently  closing  the  external  meatus,  care 
being  taken  not  to  cause  compression  of  the  air  within.  When 
the  tuning-fork  is  held  between  the  teeth  its  vibrations  are 
transmitted,  through  the  bones  of  the  head  to  the  tymj)anic 
membrane,  which  thus  set  in  motion  acts  in  the  same  way  as 
when  it  is  set  in  motion  through  the  air  of  the  meatus.  That 
the  vibrations  which  thus  reach  the  internal  ear  are,  for  the 
mos_t  part  at  least,  conducted  through  the  tympanum,  and  not 
brought  to  bear  on  the  perilymph  directly  thTOugh  the  bony 
walls  of  the  labyrinth  is  not  only  indicated  by  the  effect  just 
mentioned  of  closing  the  meatus,  for  this  could  have  no  influ- 
ence on  the  labyrinth  itself,  but  may  be  also  proved  by  experi- 
ment. If  a  style  be  attached  to  the  stapes  laid  bare  in  the  skull, 
the  vibrations  of  a  tuning-fork  brought  int-o  contact  with  the 
skull,  will  lead  to  corresponding  movements  of  the  style. 

Not  only  may  vibrations  be  transmitted  from  the  skull  to 
the  tympanic  membrane,  but  also  conversely  the  vibrations  of 
the  membrane,  brought  about  in  the  usual  way  through  the 
meatus,  may  be  transmitted  to  the  bones  of  the  skull.  If  a  long 
tube  introduced  into  one  meatus  be  spoken  or  sung  into,  the 
sounds  may  be  heard  by  help  of  a  stethoscope  placed  over 
various  parts  of  the  head.  They  are  heard  best  perhaps  at  the 
opposite  meatus ;  the  vibrations  of  the  bones  of  the  skull  set 
going  by  one  tympanic  membrane  throw  the  other  tympanic 
membrane  also  into  vibrations. 

§  617.  Two  muscles  act  upon  the  auditory  apparatus  of  the 
tympanum  ;  one,  the  tensor  tympanic  acts  upon  the  malleus  and 
hence  upon  the  tympanic  membrane,  the  other,  the  stapedius, 
acts  upon  the  stapes. 

The  t£Jis£iiLtj;iipani  (Fig.  176)  is  a  slender  muscle,  lying  in 
a  groove  above  the  bony  canal  of  the  Eustachian  tube,  and  having 
very  much  the  direction  of  that  tube.     The  tendon  in  which  it 

63 


994 


MUSCLES   OF  TYMPANUM. 


[Book  hi. 


ends,  turns  round,  almost  at  right  angles  to  the  line  of  the 
muscle,  over  a  bony  prominence  at  the  end  of  the  groove,  and 
passing  athwart  the  cavity  of  the  tympanum  from  the  median 
side  outwards  (Fig.  170  T.T.^  is  attached  to  the  upper  part  of 
the  handle  of  the  malleus. 


ch.t 


Fig.  176.    Diagram  of  the  outer  wall  of  the  Tympanum  as  seen  from 
THE  mesial  side.     Magnified  twice.     (After  Schwalbe.) 

m.t  membranatympani.  ?n&.  handle  of  Jftlie  malleus.  /.  the  incus.  E.t. 
Eustachian  tube.  T.  T.  tensor  tympani,  the  tendon  of  which  is  seen  attached 
to  the  upper  part  of  the  handle  of  the  malleus.  Ig.a.  the  anterior  and  lg.s.  the 
superior  ligament  of  the  malleus,  ch.t.  the  chorda  tympani  nerve  traversing  the 
tympanic  cavity. 


The  effect  of  the  contraction  of  the  muscle  is  to  pull  the 
handle  of  the  malleus  and  so  the  tympanic  membrane  inwards 
towards  the  median  side.  Even  in  a  quiescent  state  it  may  be 
of  use  in  keeping  up  a  certain  amount  of  tension  and  in  pre- 
venting the  tympanic  membrane  being  pushed  out  too  far. 
When  it  contracts  it  certainly  renders  the  tympanic  membrane 
more  tense  ;  hence  it  has  been  supposed  on  the  one  hand  to  act 
as  a  damper  lessening  the  amount  of  vibration  of  the  membrane 
in  the  case  of  too  powerful  sounds,  and  on  the  other  hand  to  ac- 
commodate the  apparatus  to  the  sounds  falling  upon  it  since  the 
more  tense  membrane  is  more  readily  thrown  into  vibrations  by 
higher  notes  and  is  less  sensitive  to  lower  notes.  It  has  been 
urged  that  it  is  readily  thrown  into  contraction  at  the  commence- 
ment of  a  sound,  especially  of  a  noise,  and  returns  to  rest  during 
the  continuance  of  a  prolonged  musical  note,  the  contraction 
being  a  simple  contraction  or  twitch,  rather  than  a  continued 
tetanic  contraction ;  it  is  suggested  that  this  may  serve  to  tune 
the  membrane  as  it  were  for  the  sound  which  follows.  Efferent 
impulses  reach  it  through  fibres  of  the  fifth  nerve  from  the  otic 
ganglion,  and  its  activity  is  regulated  by  reflex  action,  vibrations 


CiiAr.  IV.] 


HEARING. 


995 


of  the  tympanic  membrane  starting  the  afferent  impulses.  In 
some  persons  the  muscle  seems  to  be  partly  under  the  dominion 
of  tliej^ll,  since  a  peculiar  crackling  noise  which  these  persons 
can  produce  at  pleasure  appears  to  be  caused  by  contraction  of 
the  tensor  tympani. 

The  stapedius  is  a  small  muscle  imbedded  in  the  bone  of  the 
median  wall  of  the  tympanum,  the  tendon  issuing  by  a  hole 
close  to  the  fenestra  ovalis  (Fig.  171  jSt.}  and  being  inserted  into 
the  head  of  the  stapes  (Fig.  177  ST^.  It  is  supposed  to  regu- 
late the  movements  of  the  stapes,  and  especially  to  prevent  the 
foot-plate  being  driven  too  far  into  the  fenestra  ovalis  during 


Fig.  177.     The  Stapes  in  Position.     Much  magnified.     (Schwalbe.) 


1.  The  end  of  the  shaft  of  the  incus.  2.  Its  expansion  or  os  orbiculare. 
2'.  The  articular  cartilage  of  the  same.  3.  The  capitulum  of  the  stapes.  3'.  Its 
articular  cartilage.  4.  The  hoops  of  the  stapes.  5.  The  foot-plate  of  the  stapes. 
5'.   Its  articular  cartilage.     6.   The  membrane  of  the  fenestra  ovalis. 

ST.  The  tendon  of  the  stapedius  muscle  attached  to  the  capitulum  of  the 
stapes. 


large  or  sudden  movements  of  the  tympanic  membrane.  Con- 
tractions of  the  muscle  pull  the  front  part  of  the  stapedial  foot- 
plate towards  the  tympanum,  the  hind  part  being  thereby 
pressed  somewhat  into  the  labyrinth  and  the  whole  mem- 
branous ring  round  the  foot  being  rendered  more  tense ;  but 
the  total  effect  is  to  diminish  the  pressure  in  the  labyrinth. 
It  perhaps  may  be  regarded  as  the  antagonist  of  the  tensor 
tympani.     It  is  governed  by  fibres  from  the  facial  nerve. 

§  618.  The  cavity  of  the  tympanum  is,  as  we  have  seen, 
continuous  with  the  Eustachian  tube.  The  walls  of  the  tube  in 
the  first  third  of  its  length  adjoining  the  tympanum  are  osseous, 
but  in  the  remaining  two-thirds  are  cartilaginous  and  mem- 
branous. The  tube,  whose  lumen  is  of  varying  diameter  and 
special  shape,  passes  obliquely  forwards,  downwards,  and  to- 
wards the  median  line  (Fig.  171,  £.t.')  to  open  at  the  side 
of  the    upper  part   of   the   pharynx.     The   mucous  membrane 


996  THE  EUSTACHIAN   TUBE.  [Book  hi. 

lining  the  tube  consists  of  a  ciliated  epithelium  resting  on  a 
dermis  rich  in  reticular  and  adenoid  tissue,  and  bearing  glands. 
The  action  of  the  cilia  is  such  that  the  movement  which  they 
effect  is  directed  from  the  tympanum  to  the  pharynx.  The 
mucous  membrane  lining  the  tympanum  is  a  continuation  of 
that  lining  the  tube  and,  like  that,  ciliated  except  over  the 
tympanic  membrane,  the  chain  of  ossicles,  and  probably  some 
other  parts ;  in  these  situations  the  epithelium  consists  of  a 
single  layer  of  flat  non-ciliated  cells,  and  a  similar  epithelium 
lines  the  antrum  and  mastoid  cells  which  continue  the  cavity  of 
the  tympanum  backwards  and  upwards. 

One  u^se^  of  the  Eustachian  tube  is  to  carry  down  to  the 
pharynx  the  flrtid,  TioTfnally  very  small  in  amount,  Avhich  is 
secreted  by  the  mucous  lining  of  the  tympanum,  but  a  far  more 
important  use  is  that  of  placing  the  air  in  the  tympanum  in 
communication  with  that  in  the  pharynx  and  so  with  the 
external  air,  by  which  means  the  pressure  on  the  two  sides  of 
the  tympanic  membrane  is  equalized.  If  as  happens  sometimes 
the  tube  is  definitely  closed,  the  absorption  of  the  gases  in  the 
air  at  first  present  in  the  tympanum  diminishes  the  pressure  on 
the  inner  side  of  the  tympanic  membrane,  and  so  interferes  with 
the  vibrations  of  the  membrane.  Moreover  it  is  desirable  that 
general  changes  of  pressure  in  the  external  atmosphere  should 
be  rapidly  followed  by  corresponding  changes  in  the  pressure 
within  the  tympanum,  since  the  tympanic  membrane  would  not 
vibrate  normally  if  any  marked  difference  of  pressure  on  the 
two  sides  were  brought  about ;  and  this  would  result  if  the  way 
from  the  tympanum  to  the  external  air  through  the  tube  were 
blocked. 

The  lumen  of  the  tube  has  in  its  lower  part  the  form  of  an 
obliquely  vertical  slit,  the  sides  touching  or  nearly  so ;  and 
much  dispute  has  taken  place  as  to  whether  the  tube  is  nor- 
mally closed.. or  open.  It  is  undoubtedly  opened  during  the  act 
Iof  swallowing,  and  during  the  act,  by  the  action  of  certain  mus- 
cles of  the  palate,  air  is  forced  up  into  the  tympanum.  It  may 
be  opened  also  by  a  forced  inspiration  or  a  forced  expiration 
when  the  nose  and  mouth  are  kept  closed ;  in  the  former  case 
the  pressure  of  the  air  in  the  tympanum  is  diminished,  in  the 
latter  case  increased.  Although  under  normal  circumstances 
the  lumen  is  so  far  patent  as  to  allow  the  escape  of  the  fluid 
driven  by  the  cilia,  the  evidence  goes  to  shew  that  it  is  prac- 
cally  closed ;  sounds  for  instance  generated  in  the  pharynx  do 
not  throw  the  tympanic  membrane  into  vibrations  in  such  a 
way  as  they  would  do  if  the  tube  were  thoroughly  open. 
Apparently  the  occasional  opening,  such  as  that  effected  by 
swallowing,  is  sufficient  to  keep  the  pressure  within  the  tym- 
\  panum  at  its  proper  level.  When  the  general  pressure  of  the 
\   external  atmosphere  is  rapidly  increased  or  diminished,  tempo- 


Chap,  iv.]  HEARING.  997 

rary  deafness,  especially  to  low  notes,  frequently  ensues,  in  con- 
sequence of  the  pressure  within  the  tympanum  not  following 
the  changes  of  the  pressure  without.  This  however  is  soon 
remedied  by  the  act  of  swallowing,  which  opens  the  tube  and 
thus  equalizes  the  pressure. 

An  abnormal  permanency  in  the  closure  of  the  tube  is  recog- 
nized as  a  cause  of  deafness,  and  may  be  remedied  by  catheter- 
ism  of  the  tube,  that  is  to  say,  opening  up  the  tube  by  passing 
an  instrument  into  it  from  the  pharynx. 


SEC.  2.     ON  AUDITOEY  SENSATIONS. 

§  619.  The  vibrations  which  we  call  sound  are  transmitted 
as  we  have  seen  to  the  perilymph  through  the  fenestra  ovalis, 
by  means  of  the  tympanic  membrane  and  chain  of  ossicles. 
The  vibrations  of  the  perilymph  in  some  way  or  other,  by  help 
of  the  auditory  epithelium,  give  rise  in  the  fibres  of  the  audi- 
tory nerve  to  auditory  impulses,  and  these  reaching  the  brain 
are  developed  into  auditory  sensations.  Before  we  attempt  to 
consider  how  the  vibrations  of  the  perilymph  thus  give  rise  to 
auditory  impulses  it  will  be  convenient  to  adopt  the  plan  which 
we  pursued  in  the  case  of  vision,  and  to  deal  first  with  some 
of  the  leading  characters  of  auditory  sensations  such  as  can  be 
ascertained  by  psychological  methods. 

We  readily  recognize  two  classes  of  sensations ;  the  objective 
causes  of  the  one  class  we  speak  of  as  noises,  those  of  the  other 
class  as  musical  sounds.  When  we  inquire  into  the  physical 
features  of  the  two  classes  we  find  that  the  vibrations  which 
constitute  a  musical  sound  are  repeated  at  regular  intervals, 
and  thus  possess  a  marked  periodicity  or  rhythm.  When  no 
marked  periodicity  is  present  in  the  vibrations,  when  the  repeti- 
tion of  the  several  vibrations  is  irregular,  the  sensation  produced 
is  that  of  a  noise.  There  is  however  no  abrupt  line  between 
the  two.  Between  a  pure  and  simple  musical  sound  produced 
by  a  series  of  vibrations  each  of  which  has  exactly  the  same 
period,  and  a  harsh  noise  in  which  no  consecutive  vibrations  are 
alike,  there  are  numerous  intermediate  stages.  Much  irregu- 
larity may  present  itself  in  a  series  of  sounds  called  music,  and 
in  some  of  the  roughest  noises  the  regular  repetition  of  one  or 
more  vibrations  may  be  easily  recognized.  Still  it  will  be  desir- 
able to  consider  the  two  classes  as  distinct,  and  it  will  be  con- 
venient to  deal  first  with  musical  sounds. 

§  620.  The  sensations  which  are  produced  by  musical  sounds 
possess  three  marked  characters.  In  the  first  place  our  audi- 
tory sensations,  like  our  other  sensations,  may  be  more  or  less 
intense  ;  and  the  character  in  a  musical  sound  which  corresponds 
to  the  intensity  of  the  sensation  we  call  loudness.  This  is  deter- 
mined by  the  amplitude  of   the  vibrations,  by  the  amount  of 

998 


Chap,  iv.]  HEARING.  999 

energy  which  is  expended  in  producing  the  vibratory  move- 
ments ;  the  greater  the  disturbance  of  the  air  (or  other  medium) 
the  louder  the  sound.  Using  the  term  '  wave  '  to  denote  the 
characters  of  the  vibrations,  the  loudness  of  a  sound  is  indicated 
by  the  height  of  the  wave. 

In  the  second  place  we  recognize  a  character  which  we  call 
pitch.  This  is  determined  by  the  frequenjj^"  of  repetition  of  the 
vibrations,  by  the  time  taken  up  by  each  vibration ;  the  greater 
the  number  of  consecutive  vibrations  which  fall  upon  the  ear  in 
a  second,  the  shorter  the  time  of  each  vibration,  the  higher  the 
pitch.  Hence  the  pitch  of  a  sound  is  indicated  by  ihp.  J^nr/tji  of 
the  wave,  a  low  note  having  a  long,  a  high  note  a  short  wave- 
length. We  are  able  to  distinguish  a  whole  series  of  musical 
sounds  of  different  pitch,  from  the  lowest  to  the  highest  audible 
note. 

In  the  third  place,  we  distinguish  musical  sounds  by  what  is 
usually  called  their  quality  (timbre) ;  the  same  note  sounded  on 
a  piano  and  on  a  violin  produces  very  different  sensations,  even 
though  the  two  instruments  give  rise  to  vibrations  having  the 
same  period  of  repetition.  This  arises  from  the  fact  that  the 
musical  sounds  generated  by  most  musical  instruments  are  not 
simple  but  compound  vibration;  the  instrument  sets  going  in 
the  surrounding  air  not  one  series  only  of  vibrations  of  one 
wave-length,  but  several  series  of  different  wave-lengths ;  as  we 
shall  see  however,  the  several  vibrations  travel  through  the  air, 
not  as  a  group  of  waves  but  as  one  compound  wave.  When  the 
note  C  in  the  bass  clef  is  struck  on  the  piano,  and  we  analyze 
tlie  total  sound,  we  find  that  it  can  be  resolved  partly  into  a 
series  of  vibrations  with  a  period  characteristic  of  the  pure  tone 
of  C  of  the  bass  clef,  and  partly  into  other  series  of  vibrations 
with  periods  characteristic  of  the  C  in  the  octave  above  (middle 
C),  of  the  G  above  that,  of  the  C  of  the  next  octave,  and  of  the 
E  above  that.  And  the  sensation  which  we  associate  with  the 
sound  of  the  C  in  the  bass  clef  on  the  piano  is  determined  by 
the  characters  of  the  complex  vibration  arising  out  of  these 
several  constitutent  simple  vibrations.  Almost  all  musical 
sounds  are  thus  composed  of  what  is  called  di  funilamental  tone 
accompanied  by  a  number  of  partial  tones.  When  a  violin 
string  gives  out  a  musical  note,  the  fundamental  tone  is  j^i'O- 
duced  by  the  string  vibrating  along  its  whole  length,  the  par- 
tial tones  by  the  string  vibrating  at  the  same  time  in  segments 
or  definite  parts  of  the  whole  length  ;  and  so  with  other  instru- 
ments;  hence  the  name  'partial.'  Since  these  partial  tones 
have  a  higher  pitch  than  the  fundamental  tone  they  are  fre- 
quently spoken  of  as  '  partial  uppertones  or  overtones  '  or  sim[)ly 
as  '  oyertQlies.'  The  partial  tones  varj-  in  number  and  I'clative 
prominence  m  different  instruments  and  thus  give  rise  to  a  dif- 
ference in  the  sensation  caused  by  the  whole  sound.     Hence 


1000  AUDITORY   SENSATIONS.  [Book  in. 

wliile  a  ',toiig '  is  a  single  series  of  simple  vibrations,  a  '  note ' 
may  be  and  generally  is  a  number  of  series  of  different  vTBra- 
Itions  occurring  together.  While  the  fundamental  tone  deter- 
Imines  the  pitch  of  a  note,  the  quality  of  the  note  is  determined 
jby  the  number  and  relative  prominence  of  the  partial  tones. 

§  621.  In  much  the  same  way  that  rays  of  light  of  more 
than  or  of  less  than  a  certain  wave-length  are  incapable  of  ex- 
citing the  retina,  our  vision  being  limited  to  the  range  of  the 
visible  spectrum,  waves  of  sound  of  more  than  or  of  less  than 
a  certain  wave-length  are  unable  to  affect  the  ear  so  as  to  pro- 
duce a  sensation  of  sound.  Vibrations  having  a  recurrence 
bek)w  about  30  a  second  are  unable  to  produce  a  sensation  of 
sound;  the  note  of  the  16-feet  organ  pipe,  33  viljrations  a  sec- 
ond, gives  us  the  sensation  of  a  droning  sound ;  a  tone  of  40 
vibrations  is  quite  distinct.  Some  authors  however  place  the 
limit  at  24  or  even  15  a  second.  If  waves  of  long  wave-length 
are  powerful  enough  we  may  feel  them  by  the  sense  of  touch, 
though  not  by  that  of  hearing.  What  we  have  just  said  applies 
to  vibrations  which  are  simple,  such  as  give  rise  to  a  pure  tone ; 
if  the  fundamental  tone  is  accompanied  by  partial  tones  we 
may  hear  one  or  other  of  these,  and  are  thus  apt  to  say  we  hear 
the  former  when  in  reality  we  only  hear  the  latter.  As  regards 
the  limit  of  high  notes,  it  is  g^ssiblejbo^ hear  a  note  caused  by 
vjbrations  as  rapid  as  40,000  a  second;  at  least  some  persons 
have  this  power,  though  the  limiLior^most  persons,  is  far  lower, 
flJ2£lnt  1fi,00().  Some  persons  hear  low  sounds  more  easily  than 
high  ones,  and  vice  versa.  This  may  be  so  pronounced  as  to 
justify  the  subjects  being  spoken  of  as  deaf  to  low  or  high  tones 
respectively,  a  condition  which  may  be  compared  in  a  general 
way  to  color  blindness.  The  range  in  different  animals  differs 
very  widely,  the  high  notes  of  the  instrument  known  as  Galton's 
whistle,  though  inaudible  to  man,  are  distinctly  heard  by  some 
other  animals,  for  instance,  cats. 

The  limitations  which  are  thus  imposed  on  our  hearing  do 
not  wholly  correspond  to  the  limitations  of  our  vision.  In  the 
latter  case  the  limits  are  fixed  wholly  by  the  capacities  of  the 
retina  and  cerebral  centres ;  radiant  rays  of  longer  wave-length 
than  the  extreme  visible  red  are  able  to  get  access  to  the  retina 
through  the  dioptric  apparatus  though  they  are  unable  to  excite 
visual  impulses,  or  at  least  such  visual  impulses  as  can  affect 
consciousness.  In  the  case  of  hearing,  though  the  auditory 
epithelium  is  probable,  like  the  retinal  structures,  limited  in  its 
1  powers,  narrower  limits  are  fixed  by  the  subsidiary  acoustic 
apparatus ;  the  tympanic  membrane,  extensive  as  is  its  range 
compared  with  that  of  most  artificial  membranes,  cannot  respond 
to  all  vibrations  ;  and  hence  its  powers  fix  the  limits  of  hearing. 
The  reason  why  we  appreciate  high  notes  more  readily  than 
low  ones  is  probably  to  be  referred  to    the   tympanum  rather 


Chap,  iv.]  HEAEING.  1001 

than  to  the  auditory  epithelium.      And  the   condition    of   the 
tympanal  apparatus  as  affected  b}'  disease  will  determine  the 
relative  appreciation  of  low  or  high  tones  ;  in  certain  states  of  i 
the  tympanum  the  ear  becomes  unusually  sensitive  to  high  notes  ;j 
an  instance  of  this  is  seen  in  the  paralysis  of  the   stapedius 
muscle  due  to  injury  or  disease  of  the  seventh  nerve.  ' 

§  622.  We  dwelt,  in  speaking  of  vision,  on  our  power  of 
appreciating  differences  of  brightness  or  luminosity  ;  we  have 
a  similar  power  of  appreciating  differences  in  loudness ;  and 
that  relation  between  differences  in  the  intensity  of  the  stimulus 
and  differences  in  the  intensity  of  the  sensation,  which  we  spoke 
of  as  Weber's  law  (§  550),  holds  good  for  hearing  as  well  as 
for  vision. 

The  power  of  distinguishing  difference  of  pitch,  the  power 
of  recognizing-  the  difference  between  two  notes  of  different 
pitch,  and  the  appreciation  of  the  qualities  of  various  musical 
sounds  which  is  built  up  on  this,  may  in  a  general  way  be  com- 
pared to  acuteness  of  colour  Aasion.  It  is,  however,  veiy  differ- 
ent from  that  in  many  respects,  and  varies  much  more  widely 
than  does  that.  As  is  well  known  the  difference  in  this  power 
between  different  individuals,  according  as  they  have  or  have 
not  a  '  musical  ear,'  is  very  great.  Some  persons,  even  though 
fairly  sensitive  to  differences  of  loudness,  are  unable  to  distin- 
guish two  notes  differing  considerably  in  wave-length.  On  the 
other  hand  a  well-trained  ear  can  distinguish  the  difference  of  a 
sinoie  or  even  of  a  half  vibration  a  second,  and  that  through  a 
long  range  of  notes.  As  might  be  expected  the  power  of  ap- 
preciating difference  of  pitch  is  not  the  same  for  all  audible 
notes.  The  range  of  an  ordinary  appreciation  of  tones  lies 
between  40  and  4000  vibrations  a  second,  i.e.  between  the 
lowest  bass  C  (Cj  33  vibrations)  and  the  highest  treble  C  (C^ 
4224  vibrations)  of  the  piano ;  tones  above  and  below  these, 
even  though  audible,  are  distinguished  from  each  other  with 
great  difficulty.  The  power  of  recognizing,  and  being  able  to 
name,  a  note  when  heard,  is  an  extension  of  and  based  upon 
this  power  of  recognizing  differences  of  pitch,  though  not  by 
itself  exactly  the  same  thing. 

§  623.  We  said,  in  speaking  of  vision  (§  551),  that,  prob- 
ably, several  undulations  falling  in  succession  upon  the  retina 
were  necessary  for  the  development  of  a  visual  sensation.  In 
like  manner,  in  order  that  a  distinct  sensation  of  a  musical 
sound  may  be  developed,  several,  or  at  least  more  than  one 
wave  of  sound  must  fall  on  the  ear.  The  various  observers 
are  not  agreed  as  to  the  lower  limit  of  the  number  of  vibra- 
tions necessary  in  order  that  the  affection  of  consciousness 
may  take  the  form  of  a  definite  nuisical  sound :  some  place 
it  at  five,  others  higher,  while  it  lias  been  asserted  that  two 
vibrations  are  sufficient.      When  the  vibrations  are  thus  lim- 


1002  AUDITOKY   SENSATIONS.  [Book  iii. 

ited  in  number,  the  sound,  even  though  it  is  recognized  as  a 
musical  sound,  is  not  clearly  appreciated ;  its  pitch  is  not  dis- 
tinctly recognized.  In  such  a  case  the  recognition  may  be 
made  more  full  and  certain  by  increasing  the  number  of  vibra- 
tions ;  in  order  that  we  may  appreciate  the  pitch  of  a  sound  the 
ear  must  receive  a  larger  number  of  vibrations  than  are  neces- 
sary merely  to  enable  us  to  recognize  that  the  sound  is  a  definite 
one.  Conversely  even  when  the  vibrations  are  too  few  to  give 
rise  to  a  sensation  of  a  definite  tone,  consciousness  is  not  wholly 
unaffected,  an  auditory  sensation  is  produced,  though  it  cannot 
be  called  one  of  tone.  These  facts  indicate  the  complex  nature 
of  the  nervous  processes  which  form  the  basis  of  auditory  sen- 
sations ;  we  might  say  this  of  sensations  in  general,  for  similar 
results  are  observed  in  the  case  of  all  sensations. 

§  624.  As  we  said  above  (§  619)  noises  are  not  sharply 
defined  from  musical  sounds,  they  differ  only  in  being  more  com- 
plex and  less  regular  ;  and  what  has  just  been  said  in  respect  to 
musical  sounds,  holds  good  to  a  large  extent  for  noises.  We 
readily  distinguish,  in  noises,  difference  of  loudness ;  we  may 
also  in  many  cases  recognize  a  dominance  of  pitch,  due  to  the 
fact  that  among  the  multifarious  vibrations  certain  groups  of 
vibrations  are  repeated  periodically ;  we  distinguish  a  rumbling 
noise  in  which  vibrations  of  slow  recurrence  are  prominent  from 
a  harsh  shrill  noise  in  which  rapid  vibrations  are  similarly 
prominent ;  we  also  recognize  qualities  in  noises,  we  distinguish 
one  noise  from  the  other  by  the  characters  of  the  predominant 
constituent  vibrations.  Owing  to  the  fact  to  which  we  just  now 
referred,  that  in  a  musical  sound  the  effect  on  consciousness  is  a 
summation  of  the  individual  effects  of  the  several  vibrations,  we 
are  more  sensitive  to  a  musical  sound  of  not  too  short  duration, 
than  to  a  noise  involving  an  equal  expenditure  of  energy.  On 
the  other  hand  the  limit  of  the  number  of  movements  necessary 
to  give  rise  to  a  sensation  of  noise  is  less  than  that  required  for 
a  musical  sound;  a  few  vibrations  insufficient  in  number  to  give 
rise  to  the  sensation  of  a  tone  are  able  to  give  rise  to  an  auditory 
sensation  which  we  may  call  a  noise,  and  probably  one  movement 
of  the  tympanic  membrane  might  if  ample  enough  give  rise  to 
such  an  auditory  sensation.  Moreover  owing  to  the  very  irregu- 
larity of  a  noise,  to  the  varied  character  of  the  constituent  molec- 
ular movements,  we  have  a  very  great  range  in  distinguishing 
various  noises ;  persons  who  have  great  difficulty  in  detecting 
different  notes  can  often  readily  recognize  differences  in  noises. 

§  625.  In  treating  of  vision  we  dwelt  at  some  length  on  the 
phenomena  of  exhaustion  which  make  their  appearance  when  the 
stimulus  is  continued.  These  occur  in  hearing  also,  and  indeed 
are  indicated  by  such  common  phrases  as  "  a  deafening  noise  ;  " 
but  they  are  liot  so  prominent  as  in  vision,  and  do  not  so  dis- 
tinctly serve  as  thel)asis''fo"r  tKeoretTcal^discussions.     They  are 


Chap,  iv.]  HEARIXG.  1003 

best  studied  by  means  of  musical  sounds,  since  with  these  owing 
to  their  very  nature  the  stimuhation  is  more  uniform  than  with 
noises.  With  almost  any  note,  the  sensation  diminishes  and 
finally  disappears  if  the  sound  be  maintained  long  enough ;  but  i 
the  exhaustion  comes  on  iiioje  i^idIyLJU!ifeli-feig4i.  than  with  low 
notes,  especially  with  very  high  ones.  If  a  sounding  tuning- 
fork  be  held  up  to  one  ear,  and  then,  just  as  the  sound  becomes 
inaudible  be  transferred  to  the  other  ear,  the  sound  may  be  dis- 
tinctly heard ;  the  fresh  untired  sensory  apparatus  of  the  one 
side  is  sensitive  to  the  vibrations  which  the  tired  apparatus  of 
the  other  side  can  no  longer  feel.  Or,  if  the  tuning-fork  which 
the  tired  ear  can  no  longer  hear,  be  replaced  by  one  vibrating 
at  the  moment  so  far  as  can  be  arranged  with  the  same  intensity 
as  it,  but  of  distinctly  different  pitch,  this  will  be  heard ;  the 
first  tuning-fork  only  tired  certain  parts  of  the  sensory  apj)ara- 
tus,  those  affected  by  vibrations  of  a  certain  period  characteristic 
of  the  pitch  of  that  tuning-fork,  but  left  untired  the  parts  of  the 
sensory  apparatus  responding  to  the  vibration  of  other  periods, 
such  as  those  of  the  second  tuning-fork.  Again,  the  quality  of 
a  note  struck  on  a  musical  instrument  depends  as  we  have  seen 
on  the  presence  of  partial  tones,  having  certain  relations  to  the 
fundamental  tone.  Now,  if  immediately  before  striking  a  note 
on  an  instrument,  choosing  especially  an  instrument  whose  notes 
are  '  rich '  by  virtue  of  the  number  or  prominence  of  the  partial 
tones,  we  cause  one  of  the  partial  tones  of  the  note  to  be  sounded 
powerfully  in  the  ear,  the  note  when  subsequently  struck  does 
not  possess  its  full  quality  ;  it  appears  '  thin  '  or  '  poor.'  This 
is  because  the  previous  sounding  of  the  partial  tone  has  tired 
the  particular  part  of  the  auditory  apparatus  with  which  we 
hear  the  partial  tone,  and  in  the  whole  sensation  of  the  subse- 
quent full  note  the  constituent  sensation  corresponding  to  that 
particular  partial  tone  is  absent  or  at  least  is  below  its  normal 
intensity.  Thus  we  have  in  auditory  sensations  something 
analogous  to  the  "  negative  image  "  of  visual  sensations. 

We  do  not  in  hearing  experience  a  sensation  analogous  to 
the  visual  sensation  of  white  light,  a  simultaneous  stimulation 
of  the  apparatus  by  vibrations  of  all  kinds,  and  cannot  therefore 
experience  an  auditory  sensation  corresponding  to  the  visual 
sensation  of  black ;  the  nearest  approach  perhaps  to  such  a  psy- 
chological condition  is  that  in  which  we  are  placed  upon  the 
sudden  cessation  of  powerful  and  varied  music ;  at  such  times 
we  seem  to  be  the  subject  of  a  "  silence  which  can  be  heard." 

§  626.  As  in  the  case  of  visual  sensations,  so  likewise  in 
tlie  case  of  auditory  sensations  the  duration  of  the  sensation  is 
longat-^hfwk  that  of  the  action  of  the  stimulus,  the  auditory 
sensation  lasts  after  the  waves  of  sound  have  ceased  to  fall  upon 
the  ear.  Hence  when  two  sensations  follow  each  other  witliin 
a  sufficiently  short  intervat7~^iey^^"\fused  into  one.     Since  a 


1004  AUDITORY   SENSATIONS.  [Book  in. 

membrane,  thrown  into  vibrations  by  a  passing  sound  may  con- 
tinue to  vibrate  after  the  sound  has  ceased,  we  might  perhaps 
expect  that  tliis  would  be  the  case  with  the  tympanic  membrane, 
and  that  hence  the  interval  of  fusion  would  be  longer  in  the  case 
of  hearing  than  in  that  of  vision,  for  in  the  latter  case  we  have 
no  corresponding  behaviour  of  any  part  of  the  dioptric  apparatus. 
But  we  have  seen  (§  614)  that  the  acoustic  arrangements  of  the 
tympanum  very  rapidly  damp  the  tympanic  membrane;  and,  as 
a  matter  of  fact,  the  interval  in  question  is  decidedly  shorter  in 
hearing  than  in  vision.  Visual  sensations  separated  by  less 
than  jIq  sec.  become  fused  (§  552);  but  auditory  sensations 
separated  by  not  more  than  ^^  sec.  may  remain  distinct;  if 
two  seconds  pendulums  be  set  swinging  not  quite  in  accord 
with  each  other  and  made  to  tick,  the  tick  of  the  one  can  be 
distinguished  from  that  of  the  other  even  when  they  differ  in 
time  by  not  more  than  y^  sec. 

§  627.  When^two  notes  are  sounded  at  the  samaJiine  the 
two  sound  waves  (we  may  suppose  the  notes  to  be  pure  ones, 
consisting  of  a  fundamental  tone  only  without  partial  tones)  do 
not  travel  as  two  separate  waves,  but  are  compounded  as  we 
have  already  said,  into  a  single  wave,  the  characters  of  which 
will  depend  on  the  relative  characters  of  the  two  constituents. 
If  the  two  notes  have  the  same  period,  that  is  to  say  are  iden- 
ticaX,  the  effect^  will  be  simply  an  increase  in  amplitude ;  the 
compound  wave  will  have  its  crests  higher,  and  its  troughs 
deeper  than  those  of  either  of  the  single  waves,  but  will  other- 
wise be  like  both  of  them.  If  two  tuning-forks  of  exactly  the 
same  pitch  be  struck,  the  sensation  which  we  experience  is  the 
same  as  that  which  we  experience  from  either  of  them  alone, 
only  more  intense ;  the  sound  is  louder. 

If,  however,  the  two  tuning-forks  are  not  of  the  same  pitch, 
but  so  related  that  the  period  of  vibration  of  the  one  is  not  an 
exact  multiple  of  that  of  the  other,  the  sensation  which  we 
experience  when  the  two  sound  together  has  certain  marked 
features.  We  hear  a  sound  which  is  the  effect  on  our  ear  of 
the  compound  wave  formed  out  of  the  two  waves ;  but  the 
sound  is  not  uniform  in  intensity.  As  we  listen  the  sound  is 
heard  now  to  grow  louder  and  then  to  grow  fainter  or  even  to 
die  away,  but  soon  to  revive  again,  and  once  more  to  fall  s,wa,j, 
thus  rising  and  falling  at  regular  intervals,  the  rhythmic  change 
being  either  from  sound  to  actual  silence  or  from  a  louder  sound 
to  a  fainter  one.  Such  variations  of  intensity  are  due  to  the  fact 
that,  owing  to  the  difference  of  pitch,  the  vibratory  impulses  of 
the  two  sounds  do  not  exactly  correspond  in  time.  Since  the 
vibration  period,  the  time  during  which  a  particle  is  making  an 
excursion,  moving  a  certain  distance  in  one  direction  and  then 
returning,  is  shorter  in  one  sound  than  in  the  other,  it  is  obvious 
that  the  vibrations  belonging  to  one  sound  will  so  to  speak  get 


Chap,  ir.]  HEARING.  1005 

ahead  of  those  belonging  to  the  other;  hence  a  time  will  come 
when,  while  the  impulse  of  one  sound  is  tending  to  drive  a 
particle  in  one  direction,  sa}'  forwards,  the  impulse  of  the  other 
sound  is  tending  to  drive  the  same  particle  in  the  other  direc- 
tion, backwards.  The  result  is  that  the  particle  will  not  move, 
or  will  not  move  so  much  as  if  it  were  subject  to  one  impulse 
only,  still  less  to  both  impulses  acting  in  the  same  direction; 
the  vibrations  of  the  particle  mil  be  stopped  or  lessened,  and 
the  sensation  of  sound  to  which  its  vibrations  are  giving  rise 
will  be  wanting  or  diminished ;  the  one  sound  has  more  or  less 
completely  neutralized  or  "interfered"  with  the  other,  the  crest 
of  the  wave  of  one  sound  has  more  or  less  coincided  with  the 
trough  of  the  wave  of  the  other  sound.  Conversely  at  another 
time,  the  two  impulses  will  be  acting  in  tlie  same  direction  on 
the  same  particle,  the  movements  of  the  particle  will  be  inten- 
sified, and  the  sound  will  be  augmented.  And  the  one  condition 
will  pass  gradually  into  the  other.  The  repetitions  of  inci'eased 
intensity  thus  brought  about  are  spoken  of  as  begj^ 

The  length  of  the  interval  at  which  the  beats  recur  will 
depend  on  the  difference  of  period  of  the  two  sounds  in  relation 
to  the  actual  period  or  pitch  of  each.  It  may  be  stated  gener- 
ally that  the  number  of  beats  in  a  second  is  equal  to  the  differ- 
ence between  the  number  of  vibrations  per  second  of  the  two 
sounds ;  thus  two  very  low  pitched  tuning-forks,  vibrating 
respectively  at  64  and  72  a  second,  will  give  8  beats  a  second, 
and  two  very  high  pitched  tuning-forks,  vibrating  respectively 
at  4224  and  4752  a  second,  will  give  528  beats  a  second;  but  in 
this  respect  there  are  complications  which  we  cannot  consider 
here. 

Beats  are  produced  when  the  periods  of  the  coincident  sounds 
are  not  exact  multiples  of  each  other.  \Yii6n jLhe-^riods  are_ 
exact  mplti^Us^-xiD  beats^ogcui' ;  two  tuning-forks,  for  instance, 
the  period  of  one  of  which  is  exactly  double  that  of  the  other, 
give  rise  to  no  beats  when  sounding  together;  and  so  in  other 
instances. 

By  beats  then  a  continuous  musical  sound  may  be  broken 
up  into  a  series  of  discontinuous  sounds.  When  the  beats  are 
repeated  a  few  times  only  in  a  second  the  discontinuous  sounds 
give  rise  to  discontinuous  sensations;  we  hear  the  separate  beats. 
But  if  the  beats  are  repeated  sufficiently  lapidly  the  successive 
sensations  are  fused  in  one,  we  cease  to  hear  ilie  beats  as  such, 
though  we  have  other  evideifce  that  the  beats  emuluue  to  be 
produced.  Just  as  a  series  of  simple  vibrations  when  repeated 
sufficiently  rapidly,  say  40  times  a  second,  gives  rise,  b}'  summa- 
tion, to  a  single  musical  sound,  to  a  tone,  so  a  series  of  groups  of 
vibrations,  each  group  corresponding  to  the  interval  between  two 
beats,  gives  rise  when  the  groups  follow  each  other  rapidly,  by 
a  similar  summation,  to  a  continuous  sensation.     And,  though 


1006  BEATS.  [Book  hi. 

the  matter  is  one  which  has  been  much  disputed,  the  evidence 
seems  to  shew  that  the  continuous  sensation  thus  produced  is  a 
musical  sound,  a  tone,  which  has  been  called  a  "  beat£taiie^' 
whose  pitch  is  determined  by  the  number  of  beats  repeated  in  a 
second. 

The  rapidity  however  with  which  beats  must  be  repeated  in 
order  to  give  rise  to  a  continuous  sensation,  is  different  from 
that  with  which  single  vibrations  must  be  repeated  in  order  to 
give  rise  to  a  musical  sound.     Beats  repeated  30  or  40  times  a 

(second  are  readily  distinguished  as  such;  it  is  not  until  they 
reach  a  rapidity  of  repetition  of  about  132  a  second  that  they 
cease  to  be  distinctly  recognized.  Before_theyjdi§apgear  or  as 
they  disappear,  at  the  time  when  they  can  no  longer  be  recog- 
nized as  separate  beats,  but  have  not  as  yet  become  fused  into 
a  completely  continuous  sensation,  they  give  to  the  sound 
which  they  accompany  a  peculiar  quality,  a  particular  rough- 
ness and  harshness.  This  quality  if  excessive  is  disagreeable 
to  the  ear ;  we  speak  of  it  as  di^&onance. 

From  what  has  been  said  it  is  obvious  that  when  a  piece  of 
music  is  played  on  an  instrument  and  still  more  when  it  is 
played,  as  in  a  concert,  on  several  instruments  of  different 
kinds,  the  disturbance  in  the  air,  and  the  consequent  vibrations 
of  the  tympanic  membrane  and  of  the  perilymph,  are  in  the 
highest  degree  complex.  If  the  disturbance  has  certain 
characters,  the  sound  gives  us  pleasure,  if  other  characters, 
we  regard  the  sound  as  disagreeable ;  and  it  is  found  that  the 
disagreeable  features  of  music  are  associated  with  the  presence 
of  beats,  and  still  more  with  the  presence  of  that  ill-defined 
roughness  which,  as  we  said  just  now,  is  the  characteristic  of 
beats  when,  through  rapidity  of  repetition,  they  are  about  to 
disappear.  At  the  same  time  there  are  reasons  for  thinking 
that  it  is  the  prominence  rather  than  the  mere  presence  of  this 
element  which  offends  the  ear,  that  the_  eleroentj^s^ajrece^^ 
ingredient  of  effective  music,  and  that  even^the  very  quality 
of  a  musical  sound  is  dependent  in  part  on  a  certain  minute 
admixture  of  vibrations  disagreeing  in  period  with  the  funda- 
mental tone  and  with  the  regular  partial  tones.  But  this  is  a 
matter  into  which  we  cannot  enter  here ;  we  have  referred  to 
it  because  it  illustrates  the  extreme  complexity  of  the  processes 
which  underlie  our  sensations  of  sound. 


SEC.    3.     ON   THE   DEVELOPMENT   OF   AUDITOEY 
IMPULSES. 


§  628.  We  may  now  turn  for  a  little  while  to  the  obscure 
question,  How  the  vibrations  of  the  perilymph  give  rise  to  audi- 
tory impulses  and  so  to  auditory  sensations. 

In  speaking  of  the  ossicles  (§  615)  we  gave  reasons  for 
thinking  that  the  vibrations  of  the  tympanic  membrane  are 
carried  onward  by  the  chain  of  ossicles  swinging  as  a  whole, 
and  not  conveyed  through  the  chain  from  molecule  to  molecule. 
A  similar  argument  may  be  applied  to  the  perilymph.  The 
dimensions  of  the  whole  labyrinth  compared  with  the  length  of 
the  waves  of  sound  are  so  minute  that  molecular  vibrations 
may  be  neglected.  Moreover  the  walls  of  the  labyrinth  may, 
as  a  whole,  be  regarded  as  absolutely  rigid  so  that,  the  peri- 
lymph being  incompressible,  each  blow  given  at  the  fenestra 
ovalis  is  transmitted  instantaneously  through  the  whole  mass 
of  perilymph  ;  the  fluid  driven  in  by  the  inward  thrust  of  the 
stapes  has  to  find  room  for  itself  elsewhere,  and  that  room  is 
furnished  by  the  outward  bulge  of  the  membrane  of  the  fenes- 
tra rotunda,  for  we  may  neglect  other  means  of  escape  such  as 
the  lymph  spaces  around  the  endolymphatic  duct,  the  nerves 
and  the  blood  vessels.  Hence  at  each  movement  of  the  stapes 
the  whole  mass  of  the  perilymph  swings  bodil}-,  the  membrane 
of  fenestra  rotunda  moving  outwards  and  inwards  at  the  same 
instant  that  the  stapes  moves  inwards  and  outwards  ;  and  each 
such  mass-vibration  of  the  perilymph  repeats  the  characters  of 
the  vibration  of  the  ossicles  and  tympanic  membrane,  of  which 
it  is  the  continuation. 

As  they  sweep  over  the  vestibule,  these  vibrations  are  com- 
municated through  the  Avails  of  the  enclosed  membranous  laby- 
rinth to  the  endolymph.  The  vibrations  of  the  endolymph,  or 
of  the  Avails  themselves,  affect  in  some  Avay  or  other  the  audi- 
tory epithelium  of  the  three  cristse  and  the  tAA'o  maculte. 

The  A'ibrations  also  travel  from  the  vestibule  into  the  scala 
vestibuli  of  the  cochlea,  ascending  the  spiral  from  below  up- 
wards.    As  they  ascend  they  are  transmitted  across  the  mem- 

1007 


1008 


OKIGIN   OF  AUDITORY   IMPULSES.     [Book  iii. 


^Eajife^f  Reissner,  the  endolympli  of  the  canalis  cochlearis,  and 
the  basilar  membrane  to  the  scala  tjanpani,  and  so  reach  the  fenes- 
tra rotunda.  The  bulk  of  the  vibrations  ascending  the  scala 
vestibuli  thus  reach  the  scala  tj^mpani  by  crossing  the  canalis 
cochlearis,  and  in  so  crossing  affect  in  some  way  or  other 
the  auditory  epithelium  of  the  organ  of  Corti^  it  is  probably 
only  a  remnant  which  at  the  summit  of  the  spiral  passes  di- 
rectly from  the  one  scala  to  the  other.  The  features  of  the 
basilar  membrane  point  to  its  being  readily  thrown  into  vibra- 
tions, and  we  may  conclude  that  the  vibrations  started  at  the 
fenestra  ovalis  and  transmitted  from  the  scala  vestibuli  to  the 
scala  tympani  throw  the  basilar  membrane  into  corresponding 
vibrations.  By  the  vibrations  of  the  basilar  membrane,  or  by  a 
more  direct  action  of  the  vibrations  of  the  endolymph,  the  au- 
ditory epithelium  is  so  affected  as  to  give  rise  to  auditory  im- 
pulses. 

We  now  come  upon  matters  of  no  little  difficulty.    We  have 
seen  reason  to  think  that  the  eighth  nerve  serves  as  the  chan- 


naud 


i.sp  mb  fg  sp 


nah       r.as     chl'    chl  chP 

Fig.  178.     The  Membkanods  Labyrinth  (of  the  Right  Eak)  as  seen 

FROM    ABOVE,   MAGNIFIED    SIX    TIMES.        (After  RetziuS.) 

The  bony  envelope  has  been  wholly  removed  from  the  vestibular  division, 
but  only  in  part  broken  through  in  the  cochlear  division. 

cM  the  cochlea,  cliV  the  first  part  of  the  basal  whorl,  chV  the  summit.  To 
the  right,  where  the  bony  wall  has  been  broken  through,  are  seen  :  I.sp  the 
spiral  lamina,  m.h  the  basilar  membrane,  Ig.sp  the  spiral  ligament. 

n  aucl  the  auditory  nerve,  lying  alongside  of  which  is  seen  VII,  the  seventh, 
facial  nerve. 

m.s  macula  of  the  saccule,  m.w  macula  of  the  utricle,  cr.p  the  crista  of  the 
posterior  semicircular  canal,  with  r.a.p  the  branch  of  the  auditory  nerve  dis- 
tributed to  it,  cr.s  crista  of  the  superior  canal  with  r.a.s  its  nerve,  a.h  ampulla 
and  cr.h  crista  of  the  horizontal  canal,  with  r.ah  its  nerve. 


X  the  conjoined  posterior  and  superior  canals. 
with  c.u.s  its  junction  with  the  utricle. 


d.e  ductus  endolymphaticus, 


Chap,  iv.] 


HEARING. 


1009 


nel  for  impulses  other  than  auditory  impulses,  for  impulses 
which  take  part  in  the  development  of,  and  in  the  maintenance 
of,  the  sense  of  equilibrium.  We  have  further  seen  reason  to 
think  that  the  whole  of  the  vestibular  division  of  the  nerve,  the 
part  which  is  connected  with  the  maculee  of  the  utricle  and  sac- 
cule (Figs.  178, 179)  as  well  as  the  part  which  is  connected  with 
the  cristas  of  the  semicircular  canals,  acts  in  this  way.  But  there 
is  no  reason  to  think  that,  in  the  higher  animals  possessing  a 


A^ 


i^s.a 


Cock- 


Fig.  179.     The  Membranous  Labyrinth  and  the  Endings  of  the  Auditory 

Nerve. 

The  figure  is  wholly  diagrammatic,  and  is  introduced  as  giving  a  simpler  view 
of  the  essential  parts  of  Fig.  178 ;  it  should  be  used  only  in  conjunction  with  that. 

TJ.  utricle.  ;S^.  saccule.  A.  S.  C.  Superior  (or  anterior) ,  P.  S.  C.  posterior, 
H.S.C.  horizontal,  semicircular  canals. 

Coch.  The  canalis  cochlearis  represented  as  a  tube  partially  unrolled,  c. 
canalis  reuniens,  joining  the  saccule  with  the  canalis  cochlearis.  a. v.  ductus 
endolymphaticus,  shewing  its  origin  from  both  saccule  and  utricle,  and  its 
dilated  blind  end,  the  saccus  endolymphaticus. 

A.N.  The  auditory  nerve  ending  in  the  cristfe  of  the  ampullae,  in  the  maculse 
of  the  utricle  and  saccule,  and  along  the  whole  length  of  the  canalis  cochlearis. 
The  branch  of  the  vestibular  division  of  the  nerve  ending  in  the  saccule  remains 
in  close  contact  with  the  cochlear  division,  longer  than  does  the  rest  of  the  vesti- 
bular division  ending  in  the  utricle  and  ampullte  (the  branch  to  the  posterior 
canal  shou.ld  have  been  represented  as  lying  in  contact  with  that  to  the  saccule). 

well-developed  cochlea,  the  cochlear  division  of  the  nerve,  dis- 
tributed solely  to  the  cochlea,  has  any  such  function ;  this  divis- 
ion of  the  nerve  seems  to  carry  auditory  impulses  only.  We 
may  therefore  in  the  first  instance  confine  ourselves  to  the  coch- 
lea exclusively.  Now  in  the  cochlea  the  connection  of  the 
fibres  of  the  auditory  nerve  seems  to  be  exclusively  limited  to 
the  hair-cells,  inner  and  outer ;  and  we  may  conclude  that  these 
hair-cells  are  in  some  way  or  other  concerned  in  the  develop- 
ment of  auditory  impulses.  This  view  is  supported  by  the  anal- 
ogy of  vision ;  for  we  have  seen  reason  to  think  that  visual 
impulses  begin  in  the  rods  and  cones,  which  like  the  hair-cells  are 

64 


1010       FUNCTIONS  OF  THE  OEGAN  OF  CORTI.     [Book  hi. 

modified  epithelium  cells,  and  we  shall  presently  find  that  modi- 
fied epithelium  cells  also  play  an  essential  or  important  part  in 
the  development  of  sensations  other  than  those  of  vision  and 
hearing.  We  may  conclude  that  the  vibrations  of  the  peri- 
lymph in  some  way  or  other  bring  about  such  changes  in  the 
hair-cells  as  to  give  rise  to  auditory  impulses.  What  is  the 
exact  nature   of   these  changes  and  what  is  the  exact  way  in 


Fig.  180.     Diagram  of  a  transverse  section  of  a  Whorl  of  the  Cochlea. 

Sc.v.    Scala  Vestibuli.     Sc.  T.  Scala  Tympani.     Cxhl.  Canalis  coclilearis. 

n.aud.  auditory  nerve.  Gg.sp.  Spiral  ganglion.  Lam.sp.  Lamina  spiralis. 
lb.  limbus.  L.v.  labium  vestibulare.  Lt.  labium  tympani.  m.B.  membrane  of 
Eeissner.  Lg.sp.  spiral  ligament.  Str.v.  stria  vascularis.  Org.G.  organ  of 
Corti.  m.&.  l)asilar  membrane,  t.l.  lymphatic  epithelioid  lining  of  the  basilar 
membrane  on  the  tympanic  side.     m.t.  tectorial  membrane. 


Chap,  iy.] 


HEARING. 


1011 


which  they  are  brought  about?  To  these  questions  we  can  at 
present  give  no  satisfactory  answer  at  all.  Any  attempt  to 
answer  them  leads  us  at  once  into  speculations.  The  rod-like 
appendages  of  the. hair-cells,  the  so-called  hairs  (Figs.  181,  182) 
are  too  short  and  uniform,  to  permit  us  to  suppose  that  they  are 
vibrating  organs  responding  b}'  their  vibrations  to  the  vibrations 
of  the  perilymph  and  so  bringing  those  vibrations  to  bear  on  the 
substance  of  the  hair-cells.  The  vibrations  find  their  way,  so  to 
speak,  to  the  hair-cells  in  some  other  way.  The  membrana  tec- 
toria  (Figs.  180, 181,  m.t.^  has  the  aspect  of  an  organ  serving  to 


oaucf  '^P'l    tspn 


Ospn    CD 


'e-sp 


Fig.  181.     Diagram  of  the  Organ  op  Corti.     (After  Retzius.) 

i.r.  inner  rod  of  Corti,  o.r.  outer  rod  of  Corti. 

i.h.e.  inner  hair-cells,  n.c.  the  group  of  nuclei  beneath  it.  o.h.c.  outer  hair- 
cell,  or  cell  of  Corti,  of  the  first  row,  c.D.  its  twin  cell  of  Deiters  ;  four  rows  of 
these  twin  cells  are  shewn. 

n.aud.  the  auditory  nerve  perforating  the  tympanic  lip  l.t,  and  lost  to  view 
among  the  nuclei  beneath  the  inner  hair-cell,  i.sp.n.  the  inner  spiral  strand  of 
nerve-fibrillte.  t.sp.n.  the  spiral  strand  of  the  tunnel,  o.sp.n.  the  outer  spiral 
strand  belonging  to  the  first  row  of  outer  hair-cells  ;  the  three  succeeding  spiral 
strands  belonging  to  the  three  other  rows  are  also  shewn.  Nerve-fibrilliB  are 
shewn  stretching  radially  across  the  tunnel. 

H.c.  Hensen's  cells,  Cl.c.  Claudius  cells,  m.h.  basilar  membrane,  tl.  lym- 
phatic epithelioid  lining  of  the  basilar  membrane,  on  the  side  towards  the  scala 
tyuipani.  hj.sp.  spiral  ligament,  c'.  cells  lining  the  spiral  groove,  overhung  by 
l.v.  the  vestibular  lip.  m.t.  the  tectorial  membrane  ;  a  fragment  of  it  is  seen 
torn  from  the  rest  and  adherent  to  the  organ  of  Corti  just  outside  the  outermost 
row  of  outer  hair-cells. 


'  damp  '  the  vibrations  of  the  basilar  membrane,  and  the  hairs  of 
the  hair-cells  may  perhaps  rather  serve  the  purpose  of  bringing 
their  damping  action  to  bear  directly  on  the  substance  of  the 
hair-cells  ;  for  the  membrane  in  question  comes  down  into  direct 
contact  with  them.  We  may  further  suppose  that  in  the  de- 
velopment of  auditory  impulses,  the  peculiar  rods  of  Corti  (Fig. 
182)  play  some  special  part.  But  concerning  all  these  matters 
we  can  at  present  do  hardly  more  than  make  guesses,  and  those 
unprofitable  ones. 


1012      FUi^^CTIONS  OF  THE  ORGAN  OF  CORTI.    [Book  hi. 


One  point  deserves  mention.  We  saw  reason  to  think  (§  573) 
that  visual  impulses  cannot  be  generated  in  the  optic  fibres 
otherwise  than  through  the  intervention  of  the  retinal  struc- 
tures ;  in  the  absence  of  the  retina  an  animal  is  wholly  blind. 
In  pigeons,  however,  from  which  the  labyrinths  of  both  ears  have 
been  entirely  removed,  a  certain  apparent  power  of  response  to 
sounds  has  been  observed;  the  animals  still  seemed  to  hear. 
And  it  has  been  contended  that  such  cases  are  instances  of  the 
mere  fibres  of  the  auditory  nerve  apart  from  their  special  termi- 
nations being  sensitive  to  the  vibrations  of  sound ;  it  is  suggested 


mb 

Fig.  182.     Diagram  of  the  constituents  of  the  Organ  of  Cokti.     (After 

Retzius.) 

A.  Inner  hair-cells.     A',  the  head  seen  from  above. 

B.  Inner,  B'.  outer  rod  of  Corti,  ph.  (in  each)  phalangar  process. 

C.  The  twin  outer  hair-cell.  C.c.  cell  of  Corti,  h.  its  auditory  hairs,  n.  its 
nucleus,  x,  Hensen's  body.  D.c.  cell  of  Deiters,  n'.  its  nucleus,  ph.p.  its  pha- 
langar process,  fil.  the  cuticular  filament,  m.b.  basilar  membrane,  m.r.  reticulate 
membrane. 

C.  The  head  of  the  cell  of  Corti  as  seen  from  above. 

D.  The  organ  of  Corti  seen  from  above,  i.h.c.  the  heads  of  the  inner  hair- 
cells,  ir.h.  the  head  and  phalangar  process  of  the  inner  rod.  o.r.li.  the  head 
of  the  outer  rod,  •withph.p.  its  phalangar  process,  covered  to  the  left  hand  by  the 
inner  rods,  but  uncovered  to  the  right,  o.h.c.  the  heads  of  the  cells  of  Corti 
supported  by  the  rings  of  the  reticulate  membrane,  ph.  one  of  the  phalangse  of 
the  reticulate  membrane. 


Chap,  iv.]  HEAEING.  1013 

that  the  fibres  are  directly  stimulated  by  the  vibrations  passing 
through  the  bone,  in  canals  of  which  the  fibres  lie.  Such  a  con- 
clusion presents  great  difficulties ;  we  shall  have  to  refer  to  it 
again  later  on. 

^  629.  Leaving  this  view  for  tlie  present  on  one  side,  and 
assuming  that  the  waves  of  sound  are  converted  into  auditory 
impulses  by  means  of  the  hair-cells,  we  may  now  turn  to  another 
question,  also  one  of  great  difficulty.  How  do  the  different 
vibrations  which  determine  the  nature  of  different  sounds  so 
differently  affect  the  hair-cells  as  to  give  rise  to  sensations  of 
corresponding  difference?  A  complex  sound,  consisting  of  vi- 
brations of  more  than  one  period,  travels  as  we  have  said,  not  as 
a  group  of  discrete  waves,  each  corresponding  to  a  vibration  of 
a  particular. period,  but  as  a  complex  wave  in  which  the  simple 
waves  are  compounded  into  one;  and  the  vibrations  of  the  tym- 
panic membrane,  followed  by  the  vibrations  of  the  perilymph, 
have  the  same  composite  character.  When  for  instance  a  note 
is  sung,  or  sounded  on  a  musical  instrument,  the  air  in  the  ex- 
ternal auditory  passage  is  not  the  subject  of  one  set  of  waves 
corresponding  to  the  fundamental  tone,  and  of  other  sets  corre- 
sponding to  the  several  partial  tones,  but  vibrates  in  the  pattern 
of  one  composite  wave ;  the  tympanic  membrane  executes  one 
complex  vibration,  and  a  corresponding  single  complex  vibra- 
tion excites  the  auditory  epithelium.  And  this  holds  good  not 
for  a  single  sound  onl}^  but  for  a  mixture  of  sounds.  We  can 
in  a  clumsy  way  take  a  graphic  record  of  the  vibrations  of  a 
dead  tympanic  membrane,  hy  attaching  a  marker  to  the  stapes  ; 
could  we  take  an  adequate  record  of  the  movements  of  the 
living  tympanum  of  one  of  the  audience  at  a  concert,  we 
should  obtain  a  curve,  a  phonogram,  which  though  a  single 
curve  only  would  be  on  the  one  hand  a  record  of  the  multi- 
tudinous vibrations  of  the  concert,  and  on  the  other  hand  a 
picture  of  the  actual  blows  with  which  the  perilymph  had 
struck  the  auditory  epithelium. 

Now,  whatever  be  the  exact  nature  of  the  process  by  which 
the  vibrations  of  the  perilymph  give  rise  to  auditory  impulses, 
we  may  consider  it  as  probable  that,  in  giving  rise  to  those 
impulses,  the  complex  vibration  is  analyzed  again  into  its  con- 
stituent simple  vibrations,  tliat  the  vibrations  start  afresh  so  to 
speak  in  the  auditor}^  epitlielium,  marshalled  in  the  same  array 
as  that  in  which  they  started  from  the  sounding  instruments, 
as  if  the  auditory  epithelium  itself  constituted  the  band  playing 
the  music.  And  indeed  that  something  of  this  kind  does  take 
place  is  indicated  by  the  fact  that  an  adequately  sensitive  ear 
can  in  a  musical  sound  detect  one  or  more  of  the  partial  tones 
as  distinct  from  the  fundamental  tone,  or  still  more  easily  can 
in  a  mixed  concert  detect  the  several  notes  of  the  several  instru- 
ments, though  as  we  have  just  said  in  the  movements  of  the 


1014      FUNCTIONS  OF  THE  ORGAN  OF  CORTI.    [Book  hi. 

tympanic  membrane,  all  the  constituent  factors  are  merged  into 
one  complex  sweep.  We  may  conclude  then  that  we  possess 
some  means  of  analyzing  the  composite  waves  of  sound  which 
sweep  through  the  perilymph,  and  of  sorting  out  their  constitu- 
ent vibrations. 

There  is  at  hand  a  simple  and  easy  physical  method  of  ana- 
lyzing composite  sounds.  If  a  person  standing  before  an  open 
pianoforte,  the  loud  pedal  being  held  down,  sings  out  any  note, 
it  will  be  observed  that  a  number  of  the  strings  of  the  pianoforte 
will  be  thrown  into  vibration,  and  on  examination  it  will  be  found 
that  those  strings  which  are  thus  set  going  correspond  in  pitch 
to  the  fundamental  tone  and  to  the  several  partial  tones  of  the 
note  sung.  The  note  sung  reaches  the  strings  as  a  complex 
wave,  but  the  strings  are  able  to  analyze  the  wave  into  its  con- 
stituent vibrations,  each  string  taking  up  those  vibrations  and 
those  vibrations  only  which  belong  to  the  tone  given  forth  by 
itself  when  struck.  If  we  suppose  that  each  terminal  fibril  or 
each  group  of  fibrils  of  the  auditory  nerve  is  connected  with  a 
terminal  organ  so  far  like  a  pianoforte-string  that  it  will  readily 
vibrate  in  response  to  a  series  of  vibrating  impulses  of  a  given 
period  and  to  none  other,  and  that  we  possess  a  number  of  such 
terminal  organs  sufficient  for  the  analysis  of  all  the  sounds  which 
we  can  analyze,  and  that  each  terminal  organ  so  affected  by  par- 
ticular vibrations  gives  rise  to  a  sensory  impulse  and  thus  sup- 
plies the  basis  for  a  sensation  of  a  distinct  character  —  if  we 
suppose  these  organs  to  exist,  our  appreciation  of  sounds  is  in 
part  explained. 

When  the  rods  of  Corti  were  first  discovered,  it  was  thought 
that  they  were  specially  connected  with  the  nerve  fibres,  and 
served  mechanically  to  stimulate  the  fibrils  passing  along  their 
limbs,  by  striking  them  after  the  fashion  of  minute  hammers. 
Since  these  rods,  to  whose  striking  resemblance  to  the  keys  of  a 
pianoforte  we  have  already  called  attention,  are  arranged  in  a 
long  series  the  members  of  which  vary  regularly  in  the  length 
and  in  the  span  of  their  arch,  from  the  bottom  to  the  top  of  the 
spiral,  it  was  supposed  that  each  pair  would  vibrate  in  response 
to  a  particular  tone,  and  hence  that  the  whole  series  served  for 
the  analysis  of  sound. 

But  this  view  proved  untenable.     Whatever  purpose  they 
I  serve,  the  rods  of  Corti  produce  their  effect,  not  by  acting  di- 
j  rectly  on  nerve  fibrils,  but  by  contributing  in  some  way  or  other 
I  to  the  play  of  the  hair-cells ;  and,  whatever  be  the  way  in  which 
they  intervene,  they  do  not  vary  in  length  and  arrangement 
along  the  spiral  to  such  an  extent  as  the  above  view  demands. 
Moreover,  they  are  wholly  absent  froni  the  rudimentary  cochlea 
of  birds,  though  these  creatures  very  clearly  can  appreciate  mu- 
sical sounds.     This  last  fact  proves  indubitably  that  the  rods  in 
question  are  not  absolutely  essential  for  the  recognition  of  tones. 


Chap,  iv.]  HEARIXG.  1015 

since  it  is  in  the  highest  degree  improbable  that  birds  are  able 
to  recognize  tones  in  some  manner  absolutely  different  from 
that  employed  by  mammals. 

In  the  face  of  these  difficulties  it  has  been  suggested  that  the 
basilar  membrane,  which  is  present  in  birds  as  well  as  in  mam- 
mals, and  which,  being  tense  radially  but  loose  longitudinally, 
i.e.  along  the  spiral  of  the  cochlea,  may  be  considered  as  con- 
sisting of  a  number  of  parallel  radial  strings,  each  capable  of 
independent  vibrations,  is  the  sought-for  organ  of  analysis ;  for 
it  may  be  shewn  mathematicallj^  that  a  membrane  so  stretched 
in  one  direction  only  is  capable  of  vibrating  in  such  a  manner. 
And  the  radial  dimensions  of  the  basilar  membrane  increasing 
as  they  do  upwards  from  the  bottom  of  the  spiral  to  Tiear  tHe 
top  give  a  mucli  greater  range  of  difference  than  do  the  rods  of 
Corti.  According  to  this  view,  when  a  composite  vibration 
sweeps  along  the  cocldea  it  throws  into  sympathetic  vibrations 
those  small  portions  and  those  portions  only  of  the  basilar  mem- 
brane, the  vibrations  of  which  correspond  to  the  single  vibra- 
tions of  which  the  composite  vibration  is  made  up ;  and  the 
vibrations  in  turn  so  affect  the  overlying  structures,  that  audi- 
tory impulses  are  generated  in  particular  groups  of  fibrils  of  the 
auditory  nerve.  These  auditory  impulses  reaching  the  brain 
give  rise  to  a  corresponding  sensation  of  a  particular  sound. 

But  the  dimensions  of  even  the  basilar  membrane  do  not 
seem  wholly  adequate  for  the  purpose  ;  since  the  latest  meas- 
urements shew  that  in  man  its  range  is  very  limited.  If  we 
take  the  whole  width  of  the  membrane,  the  range  is  from  '21  mm. 
at  the  base  to  -36  mm.  at  the  top,  though  if  we  take  the  specially 
moditied  part  reaching  from  the  outer  feet  of  the  rods  to  the 
spiral  ligament,  we  get  a  wider  range,  namely,  from  -075  mm. 
at  the  base  to  426  mm.  at  the  top.  On  the  other  hand  the  esti- 
mated number  of  radial  fibres  of  the  membrane  is  very  large, 
24,000  ;  and  even  if  we  suppose  that  several  fibres  always  vibrate 
together,  this  would  still  leave  some  thousands  of  groups  of 
strings,  each  group  acting  as  an  analyzer. 

In  the  present  state  of  our  knowledge  the  whole  matter  must 
be  left  as  uncertain.  Even  if  the  basilar  membrane  acts  in  some 
such  way  as  suggested,  the  other  structures  in  the  auditory  epithe- 
lium present  problems  as  yet  insoluble.  The  true  function  of  the 
rods  of  Corti  and  of  tlie  reticulate  membrane  of  wliich  these  form 
a  part,  of  the  cells  of  Deiters,  of  the  inner  hair-cells  as  distin- 
guished from  the  outer  hair-cells,  as  well  as  the  reason  there  are 
four  rows  of  the  latter  (whereby  probably  the  effect  of  the  vibra- 
tions of  a  group  of  the  basilar  fibres  is  increased)  and  only  one 
of  the  former,  all  these  are  as  yet  merely  questions  which  can- 
not be  answered. 

§  630.  Even  admitting  that,  in  some  way  or  another,  sets 
of  vibrations  or,  to  use  a  more  general  term,  sets  of  molecular 


1016  FUNCTIONS   OF  THE  VESTIBULE.       [Book  in. 

movements  are  started  in  the  auditory  epithelium,  in  more  or 
less  complete  correspondence  with,  the  sets  of  vibrations  which 
originate  from  the  musical  instrument  or  other  sounding  body, 
and  admitting  further  that  each  set  of  such  molecular  move- 
ments in  the  auditory  epithelium  starts  a  particular  nervous 
impulse  in  a  fibril  or  in  a  set  of  fibrils  of  the  auditory 
nerve,  we  are  very  far  from  having  solved  the  problem  of 
hearing. 

It  must  be  borne  in  mind  that  making  the  fullest  allowance 
for  the  assistance  afforded  us  by  the  organ  of  Corti,  the  appre- 
ciation of  any  sound  is  ultimately  a  psychical  act.  The  analysis 
of  the  vibrations  by  help  of  the  basilar  membrane  or  otherwise 
is  simply  preliminary  to  a  synthesis  of  the  auditory  impulses  so 
generated  into  a  complex  sensation.  We  do  not  receive  a  dis- 
tinct series  of  specific  auditory  impulses  resulting  in  a  specific 
sensation  for  every  possible  variation  in  the  wave-length  of  sono- 
rous vibrations  any  more  than  we  receive  a  distinct  series  of 
specific  visual  impulses  for  every  possible  wave-length  of  lumi- 
nous vibrations.  In  each  case  we  have  probably  a  number  of 
primary  sensations,  from  the  various  mingling  of  which,  in  dif- 
ferent proportions,  our  varied  complex  sensations  arise ;  but 
there  is  this  difference  between  the  eye  and  the  ear  that  whereas 
in  the  former  the  number  of  primary  sensations  appears  to  be 
limited  to  three  or  at  least  to  six,  in  the  latter  the  number  is 
probably  Yevj  large ;  what  the  exact  number  is  has  not  at  pres- 
ent been  even  suggested.  Our  appreciation  of  a  sound  is  at  bot- 
tom an  appreciation  of  the  combined  effect  produced  by  the 
relative  intensities  to  which  the  primary  auditory  sensations 
are,  with  the  help  of  the  organ  of  Corti,  excited  by  the  sound. 
The  appreciation  and  the  subjective  analysis  of  sounds  is  ulti- 
mately a  psychical  process ;  and  though  there  are  individual 
differences  in  the  structural  finish  and  physical  capabilities  of 
the  auditory  epithelium  as  of  other  parts  of  the  ear,  the  differ- 
ences in  the  psychical  or  at  least  cerebral  powers  of  individuals 
are  far  greater ;  and  when  we  speak  of  a  musical  ear  we  reall}?- 
mean  a  musical  mind  or  a  musical  brain. 

§  631.  If  the  organ  of  Corti,  as  appears  from  the  above, 
affords  the  means  by  which  we  appreciate  tones,  it  is  evident 
that  by  it  also  we  must  be  able  to  estimate  loudness,  for  the 
quality  of  a  musical  sound  is  dependent  on  the  intensity,  as 
well  as  on  the  pitch,  of  the  partial  tones  in  relation  to  the 
fundamental  tone  and  to  each  other.  Further,  as  we  said  above 
the  distinction  between  noise  and  music  is  a  quantitative  and 
fluctuating  one ;  indeed  the  tendency  of  inquiry  seems  to  shew 
that  the  quality  or  timbre  of  a  sound,  and  it  is  this  which  so 
largely  contributes  to  the  value  of  a  sound  as  an  element  of 
music,  is  in  part  dependent  on  vibrations,  which  being  irregular, 
that  is,  having  no  exact  arithmetical  relation  to  the  fundamental 


Chap,  iv.]  HEAEING.  1017 

tone,  may  be  spoken  of  as  noise.  But,  if  noise  is  only  confused 
music,  and  music  more  or  less  orderly  noise,  the  cochlea  must 
be  a  means  of  appreciating  noises  as  well  as  musical  sounds. 

We  may  therefore  reject  the  view  which  has  been  put  forward 
by  some  that  while  by  the  cochlea  we  appreciate  musical  sounds, 
our  knowledge  of  noises  is  gained  by  auditory  impulses  reaching 
us  through  the  vestibule. 

Are  we  to  conclude  then  that  the  vestibule  has  nothing  to 
do  with  hearing,  is  concerned  only  with  equilibrium  ?  A  certain 
support  is  given  to  this  view  by  cases  in  man  where  deafness 
seems  to  have  been  due  to  disease  couiined-tQ—the  cochlea ;  and 
in  a,nima,1s_deafness  is  said  to  have  been  produced  by  diviiiiion  ef 
_the  cochlear  nerve,  the  vestibulai\u.erve-being-left4«taot.  More- 
over animals  possessing  a  cochlea  certainly  continue  to  hear  and 
to  hear  well  after  division  of  both  vestibular  nerves  ;  but  this  is 
not  a  valid  argument  against  no  auditory  impulses  at  all  pass- 
ing along  this  nerve,  since  the  cochlea  is  obviously  adequate  by 
itself  for  ordinary  hearing,  and  the  loss  of  the  vestibule  might 
simply  entail  in  the  character  of  the  sense  changes  too  fine  to 
be  readily  recognized  in  a  dumb  animal. 

On  the  other  hand  vertebrates,  lower  in  the  scale  than  birds 
and  reptiles,  namely,  fishes,  though  they  have  a  well-developed 
vestibular  labyrinth,  possess  either.. no  cochlea  at  .alL  or.,the 
merestjbrace  of  one,  and  yet  undoubtedly  are  the  subject  of 
auditory  sensations,  in  some  cases  of  acute  sensations.  The 
evidence  that  fishes  hear  seems  irresistible,  they  are  said  to 
respond  to  musical  sounds  ;  and  yet  those  who  hold  the  views 
just  explained  are  driven  to  maintain  either  that  fishes  do 
not  hear  in  the  true  sense  of  the  word  but  only  feel  vibra- 
tions, or  that  they  hear  by  means  of  an  insignificant  fragment 
of  their  relatively  large  vestibule.  The  structure  of  the  pis- 
cine and  amphibian  vestibular  auditory  epithelium  is  in  the 
main  putting  aside  smaller  matters,  such  as  the  length  of 
the  auditory  hairs,  the  size  and  abundance  of  otoliths  and  oto- 
conia and  the  like,  so  identical  with  that  of  birds  and  reptiles 
and  of  mammals,  that  it  is  impossible  to  resist  the  conclusion 
that  it  serves  the  same  purpose  in  all  the  several  classes.  In 
birds  and  reptiles  the  short  rudimentary  nearly  straight  tubular 
cochlea  possesses  a  short  basilar  membrane,  an  auditory  epithe- 
lium in  which  a  distinction  of  outer  and  inner  hair-cells  is  fore- 
shadowed, and  a  tectorial  membrane.  But  if  we  are  to  suppose 
that  these  creatures  receive  auditory  impulses  exclusively  from 
the  cochlea,  and  none  at  all  from  the  vestibule,  it  is  a  matter  of 
wonder  that  the  cochlea  of  the,  for  the  most  part,  dumb  croco- 
dile should  appear  almost  as  highly  developed  as  that  of  the 
vocal  bird.  Or  again,  if  the  bird  and  reptile  already  possessing 
a  cochlea  still  derive  auditory  sensations  by  means  of  the  vesti- 
bule, we  may  conclude  that  maunuals  also  do  the  same. 


1018  FUNCTIONS    OF   THE   VESTIBULE.      [Book  hi. 

The  changes  in  the  endolymph  which  give  rise  to  the  impulses 
affecting  equilibrium,  namely  a  simple  change  in  the  amount  of 
pressure  or  a  simple  shifting  of  position,  a  simple  flowing,  are 
so  different  from  the  changes,  the  rapid  repeated  vibrations  pro- 
duced by  sound,  that  it  seems  permissible  to  conceive  of  the 
cristse  and  maculae  reacting  differently  towards  the  two  agencies, 
and  so  giving  rise  to  impulses  of  different  natures,  one  auditory 
and  the  other  not.  The  value  of  the  hairs  of  the  cristae  and 
ampullae  as  vibrating  organs  have  probably  been  exaggerated ; 
among  other  things  the  medium  in  which  they  move,  the  some- 
what viscid  endolymph,  is  unfavourable  to  vibrations ;  and  the 
otoliths  and  otoconia,  if  they  have  any  relations  to  vibrations, 
probably  serve  as  '  dampers.'  Still  the  hairs  probably  do  vibrate 
as  the  endolymph  vibrates ;  and  we  may  imagine  that  the  changes 
in  the  hair-cells  and  hence  in  the  nerve  fibres  and  hence  in  the 
brain  are  different  when  the  hairs  are  thrown  into  series  of 
vibrations  from  what  they  are  when  the  hairs  are  gently  pressed 
or  gently  moved.  Further  we  may  reflect  that  in  ourselves  the 
sensations  gained  by  the  cochlea  are  so  dominant,  that  we  may 
be  at  the  same  time  receiving  sensations  through  the  vestibule 
without  being  aware  that  we  are  doing  so  ;  these  latter  may, 
further,  be  of  a  different  nature  from  the  former,  and  the  vesti- 
bular hearing  of  a  fish  may  be  something  very  different  from 
our,  mainly,  cochlear  hearing.  At  any  rate  we  may  hesitate 
to  accept  the  view  that  no  auditory  impulses  travel  along  the 
vestibular  nerve.  But  if  we  do  thus  hear  by  means  so  to  speak 
of  a  double  organ,  then  the  origin  and  nature  and  effects  of 
auditory  impulses  must  be  still  more  complex  and  difficult 
than  appears  from  the  study  of  the  cochlea  alone,  perplexing 
as  they  even  then  seem. 

The  difficulties  attending  an  adequate  conception  of  the 
nature  and  origin  of  auditory  impulses  are  further  increased  by 
the  following  observation.  Two  tuning-forks,  not  quite  in  uni- 
son, produce  ' beats'  when  they  are  sounding  together;  the  beats 
are  due  to  the  influence  exerted  by  one  set  of  waves  on  the  other 
set  (§  627).  But  it  is  stated  that,  if  the  two  forks  be  listened  to, 
one  with  one  ear  and  the  other  with  the  other,  precautions  being 
taken  so  that  the  vibrations  reaching  the  one  auditory  nerve  by 
the  one  ear,  cannot,  by  conduction  through  the  bones  of  the 
head  or  otherwise,  also  gain  access  to  the  other  auditory  nerve, 
the  beats  are  still  heard.  This  observation,  unless  there  be 
some  hidden  fallacy  in  it,  seems  to  shew  that  the  beats  arise  in 
the  brain  itself,  that  the  impulses  travelling  along  the  auditory 
nerve  so  far  resemble  in  their  nature  and  character,  the  waves 
of  sound  giving  rise  to  them,  that  the  two  sets  of  impulses 
along  the  two  nerves  meeting  in  the  brain  give  rise  to  beats, 
just  as  do  the  two  sets  of  waves  meeting  in  the  air.  If  we  fur- 
ther couple  with  this  conclusion  the  view  referred  to  above 


Chap,  iv.]  HEAEING.  1019 

(§  628),  that  waves  of  sound  falling  on  the  auditory  fibres 
themselves  may  give  rise  to  auditory  sensations,  we  arrive  at 
the  conception  that  the  auditory  impulses  are  mere  copies  so 
to  speak  of  the  physical  sound  waves,  a  conception  which,  if 
substantiated,  would  necessitate  a  revisal  of  all  our  views  con- 
cerning nervous  impulses. 


SEC.  4.    ON  AUDITORY  PEECEPTIONS  AND 
JUDGMENTS. 

§  632.  In  spite  of  the  many  and  striking  differences  between 
the  two  senses,  it  is  possible  to  draw  several  parallels  between 
auditory  and  visual  sensations.  When  we  are  the  subject  of  a 
visual  sensation  we  refer  the  cause  not  to  changes  taking  place 
in  the  retina,  but  to  some  luminous  object  in  the  external  world. 
So  also,  when  we  are  the  subject  of  an  auditory  sensation  we  re- 
fer the  cause  not  to  changes  taking  place  in  the  internal  ear,  but 
to  some  sounding  body  outside  the  ear  and  in  the  vast  majority 
of  cases  to  some  sounding  body  outside  ourselves.  We  do  not 
simply  feel  auditory  sensations,  we  perceive  sounds,  cf.  §  581. 

We  have  seen  that  in  the  case  of  the  eye,  visual  sensations, 
excited  by  events  taking  place  in  the  visual  apparatus  itself, 
may  be  confounded  with  sensations  excited  by  objects  in  the 
external  world,  and  much  the  same  happens  with  the  ear  also. 
The  tympanic  membrane  for  instance  may  be  thrown  into  vibra- 
tions not  by  waves  of  sound,  but  by  objects  coming  mechanically 
into  contact  with  it;  particles  of  the  dried  secretion  of  the  ex- 
ternal auditory  passage,  the  'wax  of  the  ear,'  playing  on  the 
tympanic  membrane,  may  give  rise  to  auditory  sensations,  a 
'  buzzing '  or  '  singing  in  the  ears,'  which  we  cannot  by  the  mere 
psychological  examination  of  the  sensations  themselves  distin- 
guish from  auditory  sensations  excited  in  the  ordinary  way  by 
sonorous  vibrations  reaching  us  from  some  sounding  body  at  a 
distance.  And  in  a  general  way,  we  may  speak  of  entotic  phe- 
nomena,  corresponding  to  the  entoptic  phenomena  on  which  we 
dwelt  (§  549)  in  speaking  of  vision. 

Auditory  sensations  moreover  may  arise,  in  the  complete 
quiescence  of  the  tympanic  apparatus  and  perilymph,  as  the 
result  of  changes  either  in  the  auditory  epithelium  or  in  the 
central  auditory  nervous  apparatus.  We  may  be  subject  to 
ttuditorj^  phantoms  or  Jiallumnaiiojns,  corresponding  to  ocular 
phantoms  or  hallucinations,  and  like  them  often  misleading  or 
distressing.  Few  persons,  moreover,  can  listen  to  exciting 
music  or  can  hear  impressive  cries  without  experiencing  "re- 
current" auditory  sensations. 

1020 


Chap,  iv.]  HEAEIXG.  1021 

§  633.  In  one  important  respect  the  parallel  between  hearing 
and  sig'ht  fails.  When  we  see  an  object,  the  rays  of  light  com- 
ing from  the  object  excite  a  particular  part  of  the  retinal  expan- 
sion ;  and  our  appreciation  of  the  position  wdiich  that  object 
holds  in  space  is  based  on  our  power  of  "localizing"  retinal 
changes.  The  terminal  expansion  of  the  auditory  nerve  how- 
ever has  no  such  definite  relations  to  the  positions  in  space  of 
objects  from  whence  sounds  are  proceeding ;  we  have  no  evi- 
dence that  any  particular  part  either  of  the  organ  of  Corti  or 
of  the  maculae  is  alone  or  specially  affected  by  sounds  coming 
from  a  particular  quarter  -,  and  the  evidence  that  sounds  affect 
the  three  cristas  differently  according  to  the  direction  of  the 
sound  is  at  least  doubtful.  Hence  we  possess  no  "  auditory 
field"  which  can  be  directly  compared  with  the  "visual  field;" 
and  our  conclusions  as  to  the  direction  in  which  the  sounds 
which  reach  our  ears  have  travelled,  our  judgments  as  to  the 
position  in  space  of  bodies  exciting  auditory  sensations  are 
formed  in  an  indirect  manner. 

The  vast  majorit}^  of  the  sounds  which  we  hear  reach  the 
auditory  epithelium  by  way  of  the  tympanic  membrane  and  chain 
of  ossicles ;  even  the  sounds  which  are  conducted  to  the  ear 
through  the  bones  and  hard  parts  of  the  head  pass  to  a  large 
extent  by  this  way  (§  616)  ;  in  normal  hearing  the  auditory  sen- 
sations Avhich  are  generated  by  vibrations  transmitted  directly 
through  the  bony  walls  of  the  labyrinth  to  the  perilymph  are 
probably  insignificant.  Now  it  is  only  in  relation  to  these  latter 
that  the  disposition  in  space  of  the  three  semicircular  canals 
can  possibly  have  any  meaning ;  the  vibrations  reaching  the  peri- 
lymph by  way  of  the  tympanic  membrane,  whatever  their  orig- 
inal direction,  have  all  the  same  direction  when  they  enter  at 
the  fenestra  ovalis,  and  fall  in  the  same  way  upon  the  three 
semicircular  canals.  We  may  therefore  conclude  that  the  posi- 
tion in  space  of  the  three  canals  in  question  has  nothing  to  do 
with  our  ordinary  judgments  as  to  the  direction  of  sounds.  In 
forming  those  judgments  we  are  assisted  mainl}-  by  two  things. 

In  the  first  place  a  peculiar  character  of  the  outwardness 
which  we  attribute  to  our  usual  auditory  sensations,  that  by 
which  we  judge  the  sound  to  arise  not  only  outside  the  in- 
ternal ear  but  outside  our  whole  body,  seems,  in  some  way, 
largely  dependent  on  the  vibrations  which  cause  the  sensation 
having  travelled  along  the  external  auditory  passage.  If  the 
two  passages  be  filled  with  fluid  the  hearer  refers  the  sounds 
which  he  hears,  in  spite  of  their  starting  at  some  distance  off, 
not  to  the  external  world  outside  himself,  but  to  the  inside  of 
his  own  head;  the  sounds  appear  to  him  to  come,  not  it  may  be 
remarked  from  the  internal  ear  or  any  part  of  it,  but  from  the 
roof  of  the  mouth,  or  the  top  of  the  skull  or  the  back  of  the 
head.     So  also  if  the   ear-pieces  of  a  binaural  stethoscope  be 


1022  AUDITOEY  PEECEPTIONS.  [Book  hi. 

pushed  well  up  into  the  auditory  passages,  the  sounds  heard 
through  the  instrument  seem  to  come  from  the  roof  of  the  ob- 
server's own  mouth. 

The  difference  between  such  an  abnormal  mode  of  hearing 

and  ordinary  hearing  does  not  lie  in  the  fact  that  in  the  former 

case  the  tympanic  membrane  is  not  used  at  all ;  for  even  when 

the  external  passage  is  filled  with  fluid,  a  layer  of  air  which 

always  adheres  to  the  tympanic  membrane  permits  at  least  a 

certain  amount  of  vibration  of  that  membrane  ;  and  on  the  other 

hand  when  the  sound  is  actually  generated  in  the  roof  of  the 

mouth,  and  rightly  judged  to  be  generated  there,  the  tympanic 

membrane  by  its  vibrations  conducts  the  greater  part  of  the 

1  sound  to  the  internal  ear.     How  it  is  that  the  passage  of  the 

/vibrations  through  the  external  passage  imparts  to  the  sensation 

'  this   attribute   of    outwardness   is   not   clear.      Indeed   certain 

sounds  may  be  made  to  lose  this  particular  outwardness,  though 

the  external  passage  be  still  employed.     If  two  musical  sounds 

lof  the  same  pitch  be  listened  to  with  the  two  ears  separately 

by   means   of   two   telephones,  the   sound   will,  under   certain 

:  conditions,  appear  to  originate  somewhere  in  the  head  of  the 

observer. 

§  634.  In  the  second  place  our  appreciation  of  the  particular 
quarter  from  which  a  sound,  recognized  by  help  of  the  external 
passage  to  be  of  outward  origin,  has  travelled  is  dependent  on 
our  using  two  ears.  As  our  ordinary  vision  is  largely  binocular, 
so  our  ordinary  hearing  is,  to  a  still  larger  extent,  binaural.  In 
the  case  of  the  ear  there  are  no  sharp  limitations  to  the  range  of 
the  organ  of  either  side;  through  the  medium  of  the  air  and 
external  auditory  passage  or  of  the  hard  parts  of  the  head  a 
sound  which  affects  one  ear  affects  to  a  certain  extent  the  other 
ear  also  ;  hence  all  our  hearing  is,  under  ordinary  circumstances, 
binaural.  And  in  some  such  v/ay  as  two  visual  sensations 
excited  in  "  corresponding  parts  "  of  the  two  retinas  are  fused 
into  one,  so  every  sound  which  reaches  us  is  heard  not  as  two 
sounds,  one  by  one  ear  and  the  other  by  the  other,  but  as  one 
sound  by  the  two  ears  together. 

When  the  sounding  body  is  on  one  side  of  the  head,  say  the 
right  side,  the  sensations  excited  through  the  right  internal  ear 
are  more  powerful  than  those  excited  through  the  left  internal 
ear;  we  are  not  distinctly  conscious  of  the  difference  between 
the  two  sensations,  the  combined  effect  is  a  single  sensation; 
but  the  difference  does  affect  our  consciousness  in  a  certain 
way,  and  that  affection  of  consciousness  serves  as  a  basis  for  the 
judgment  that  the  sounding  body  is  somewhere  on  our  right 
hand.  Hence  we  are  able  to  judge  the  lateral  much  more 
,  readily  than  the  fore  and  aft  position  of  a  sounding  body.  If  a 
I  tuning-fork  be  held  in  the  median  vertical  plane  over  the  head, 
the  eyes  being  shut,  though  it  is  easy  to  recognize  it  as  being  in 


Chap,  iv.]  HEARING.  1023 

the  median  plane,  it  is  very  difficult  to  say  what  is  its  position  ' 
in  that  plane,  i.e.  whether  it  is  more  towards  tlie  front  or  back 
of  the  head.     Hence  also  a  man  who  is  absolutely  deaf  of  one 
ear  has  great  difficulty  in  recognizing  the  direction  of  sounds.       \ 

Further,  when  we  desire  to  judge  particularly  as  to  the 
direction  of  a  sound,  we  listen  to  it,  and  in  the  act  move  the 
head  into  the  position  in  which  we  hear  the  sound  most  dis-j 
tinctly.  In  this  way  the  movements  of  the  head  in  hearing 
play  a  part  somewhat  analogous  to  the  movements  of  the  eyes 
in  vision. 

Even  in  the  case  of  ourselves,  and  still  more  in  the  case  of 
some  animals,  the  form  of  the  external  ear  favours  the  entrance 
into  the  meatus,  and  hence  the  access  to  the  tympanic  mem- 
brane, of  sounds  travelling  in  a  particular  direction;  this  also 
assists  our  judgment  of  the  direction  of  sounds.  Hence,  by 
tying  back  the  ears  and  affixing  artificial  ears,  differing  in  shape 
or  position  from  the  natural  ones,  we  may  make  false  judgments 
in  this  matter. 

Moreover,  in  forming  a  judgment  as  to  the  direction  of 
sounds  we  appear  to  be  guided  by  something  more  than  the 
mere  relative  intensity  of  the  sounds  falling  on  the  two  ears. 
When  a  complex^  sound  emanates  from  a  body  on  one  side  of  us, 
the  constituent  vibrations  do  not  travel  equally  and  uniformly 
over  and  around  the  head  ;  some  are  refracted  more  than  others, 
so  that  they  do  not,  reach,  the  two -eai;s_  .eq.ually  ;  and  besides 
when  they  reach  them  are  not  equally  reflected  by  the  two 
pinnse.  In  this  way  partial  tones  of  different  pitch,  and  this 
applies  especially  to  high  tones,  reach  the  two  tympanic  mem- 
branes in  unequal  intensities,  and  the  sound  of  which  they  form 
part  appears  as  heard  by  the  one  ear  of  a  quality  slightly  differ- 
ent from  that  heard  by  the  other  ear ;  this  difference  of  quality, 
like  the  difference  in  mere  intensit}^  of  the  sound  as  a  whole, 
serves  as  a  basis  for  recognizing  the  direction  of  the  sound. 
Such  a  difference  will  be  more  marked  in  the  complex  sounds 
which  we  call  noises  than  in  purer  and  more  simple  musical 
sounds ;  and,  as  a  matter  of  fact,  our  appreciation  of  direction  \ 
is  more  accurate  in  the  case  of  noises  than  of  musical  sounds.  ) 
An  exception  to  this  rule  is  met  with  in  the  case  of  the  human 
voice,  the  direction  of  which,  though  it  is  as  a  whole  a  musical 
sound,  can  be  judged  better  than  even  that  of  a  noise  ;  but 
noises  enter  largely  into  the  human  voice,  and  besides  we  are 
much  more  practised  in  relation  to  it  than  in  relation  to  any 
other  kind  of  sound.  All  our  judgments  of  the  direction  of 
sounds  are  however  at  the  best  imperfect. 

§  635.  Our  judgment  of  the  distance  of  so\iii.ds  is  even  still 
more  limited.  A  ^soundrwhose  characters  we  know  appears  to 
us  near  when  it  is  loud,  and  far  off  when  it  is  faint.  A  blind- 
fold person  will  be  unable  to  distinguish  between  the  difference 


1024  JUDGING  DIEECTION   OF   SOUNDS.     [Book  hi. 

of  intensity  produced  on  the  one  hand  by  a  tuning-fork  being 
held  before  him,  first  with  the  broad  edge  of  the  fork  toward 
him  and  then  with  the  narrow  edge,  and  the  difference  on  the 
other  hand  caused  by  the  removal  of  the  tuning-fork  to  a  distance. 
And  our  judgments  in  this  respect  may  be  false,  as  is  seen  in  the 
effects  produced  by  the  ventriloquist.  We  can  on  the  whole 
better  appreciate  the  distance  of  noises  than  of  musical  sounds, 
differences  of  quality  as  well  as  of  intensity  playing  the  same 
part  in  the  judgment  of  distance  as  of  direction ;  when  a  sound 
becomes  distant  the  intensity  of  the  fundamental  tone  dimin- 
ishes more  rapidly  than  do  those  of  the  higher  partial  tones, 
and  hence  the  quality  of  the  sound  is  affected. 


CHAPTER   V. 

TASTE   AND   SMELL. 

SECTION  1.     OLFACTOEY  SENSATIONS. 

§  636.  Particles  of  odoriferous  matters  present  in  the  in- 
spired air,  passing  through  the  lower  nasal  chambers,  diffuse 
into  the  upper  nasal  chambers,  and  falling  on  the  olfactory 
epithelium  produce  sensory  impulses  which,  ascending  to  the 
brain,  give  rise  to  sensations  of  smell.  If  we  assume  that  the 
rod  cells,  and  not  the  cylinder  cells,  are  the  special  functional 
elements,  we  may  suppose  that  the  sensory  impulses  are  orig- 
inated by  the  contact  of  the  odoriferous  particles  with  the 
free  endings  of  the  rod  cells ;  but  they  may  be  due  to  contact 
with  the  cylinder  cells ;  in  either  case  we  are  wholly  in  the  dark 
as  to  the  manner  in  which  the  contact  of  the  particles  with  the 
cells  brings  about  the  molecular  changes  constituting  a  nervous 
impulse.  We  cannot  even  say  whether  we  ought  to  speak  of 
the  first  step  by  which  the  contact  of  the  particle  begins  the 
series  of  changes  as  a  chemical  or  as  a  physical  process. 

In  nearly  all  cases  the  odoriferous  particles  are  conveyed  to 
the  membrane  in  a  gaseous  medium,  namely,  the  atmosphere ; 
but  before  they  can  gain  access  to  the  cells  they  must  become 
dissolved  or  at  least  suspended  in  fluid ;  for  the  whole  olfactory 
membrane  is  kept  moist  by  a  layer  of  fluid,  secreted  by  the 
glands,  and  the  odoriferous  particles  must  pass  into  this  layer 
of  fluid  before  they  can  gain  access  to  the  cells.  Indeed,  the 
proper  condition  of  this  layer  of  fluid  is  one  of  the  essential 
conditions  of  the  exercise  of  the  sense.  If  on  the  one  hand  the 
membrane  be  too  dry,  or  if  on  the  other  hand  the  secretion  be 
too  abundant  or  altered  in  quality,  the  power  of  smelling  is 
diminished  or  even  wholly  sus[)ended.  It  is  a  matter  of  com- 
mon experience  that  a  nasal  catarrh  interferes  with  smell. 
When  the  nostril  is  filled  with  rose-water  the  odour  of  roses  is 
not  perceived;  and  simply  filling  the  nostrils  with  distilled 
water  suspends  for  a  time  all  smell,  the  sense  gradually  returu- 
t35  lOlio 


1026  SMELL.  [Book  hi. 

ing  after  the  water  has  been  removed;  the  water  apparently 
acts  injuriously  on  the  delicate  olfactory  cells.  If  instead  of 
using  rose  water,  the  rose  scent  be  dissolved  in  "  normal  saline 
solution"  which  (§  14)  more  closely  resembles  the  natural 
secretion,  the  cells  can  perform  their  function,  and  the  scent  is 
perceived.  The  glands  of  the  olfactory  membrane  form  an 
important  subsidiary  apparatus  for  the  development  of  olfactory 
sensations. 

The  other  subsidiary  apparatus  of  smell  is  exceedingly 
meagre.  By  the  forced  nasal  inspiration,  called  sniffing,  we 
draw  air  so  forcibly  through  the  nostrils  that  currents  pass  up 
into  the  upper  as  well  as  the  lower  nasal  chambers ;  and  thus  a 
more  complete  contact  of  the  odoriferous  particles  with  the 
olfactory  membrane  than  that  supplied  by  mere  diffusion  is 
provided  for. 

§  637.  We  have  every  reason  to  think  that  any  stimulus 
applied  to  the  olfactory  cells  will  produce  the  sensation  of  smell ; 
but  the  proof  of  this  is  not  absolutely  clear ;  and  we  have  no 
definite  evidence  as  to  what  is  the  result  of  directly  stimulating 
the  fibres  of  the  olfactory  nerve.  The  olfactory  membrane  how- 
ever is  certainly  the  only  part  of  the  body  in  which  odours  as 
such  can  give  rise  to  any  sensations :  and  the  sensations  to 
which  they  give  rise  are  always  those  of  smell.  The  mucous 
membrane  of  the  nose  is  however  also  an  instrument  for  the 
development  of  afferent  imjDulses  other  than  the  specific  olfac- 
tory ones.  Chemical  stimulation  of  the  nasal  mucous  mem- 
brane by  pungent  substances  such  as  ammonia  gives  rise  to  a 
sensation  distinct  from  that  of  smell,  a  sensation  which  does  not 
afford  us  the  same  information  concerning  the  chemical  nature 
of  the  stimulus,  as  does  a  real  olfactory  sensation,  and  which 
is  much  more  allied  to  the  sensations  produced  by  chemical 
stimulation  of  other  surfaces  sensitive  to  chemical  action.  This 
sensation  moreover  seems  to  be  developed  both  in  the  non- 
olfactory,  and  in  the  olfactory  regions  of  the  nasal  mucous 
membrane ;  and  it  is  probable  that  these  two  kinds  of  sensa- 
tions, the  one  produced  by  odours,  the  other  by  pungent  sub- 
stances, thus  arising  in  the  olfactory  membrane  are  conveyed 
by  different  nerves,  the  former  by  the  olfactory,  the  latter  by 
the  fifth  nerve. 

Each  substance  that  we  smell  causes  a  specific  sensation,  and 
we  are  not  only  able  to  recognize  a  multitude  of  distinct  odours, 
but  also  in  certain  cases  to  distinguish  individual  odours  in  a 
mixed  smell.  And  though  we  may  recognize  certain  odours  as 
more  like  to  each  other  than  to  other  odours,  or  can  even  make 
a  rough  classification  of  odours,  we  cannot,  as  we  can  in  the  case 
of  visual  colour  sensations,  reduce  our  multifarious  olfactorj'' 
sensations  to  a  smaller  number  of  primary  sensations  mixed  in 
various  proportions.     Nor  have  we  at  present  any  satisfactory 


Chap,  v.]  TASTE    AND   SMELL.  1027 

guide  to  connect  the  characters  of  an  olfactory  sensation  with 
the  chemical  constitution  of  the  body  giving  rise  to  it. 

The  sensation  takes  some  time  to  develope  after  the  contact 
of  the  stimulus  with  the  olfactory  membrane,  and  may  last 
very  long.  When  the  stimulus  is  repeated  the  sensation  very 
soon  dies  out:  the  sensory  terminal  organs  speedily  become 
exhausted.  The  larger,  apparently,  the  surface  of  olfactory 
membrane  employed,  the  more  intense  the  sensation ;  animals 
with  acute  scent  have  a  proportionately  large  area  of  olfactory 
membrane.  The  greater  the  quantity  of  odoriferous  material 
brought  to  the  membrane,  the  more  intense  the  sensation  up  to 
a  certain  limit;  and  an  olfactometer  for  measuring  olfactory 
sensations  has  been  constructed,  the  measurements  being  given 
by  the  size  of  the  superficial  area,  impregnated  with  an  odorif- 
erous substance,  over  which  the  air  must  pass  in  order  to  give 
rise  to  a  distinct  sensation.  The  limit  of  increase  of  sensation 
however  is  soon  reached,  a  minute  quantity  producing  the 
maximum  of  sensation  and  further  increase  giving  rise  to  ex- 
haustion. The  minimum  quantity  of  material  required  to  pro- 
duce an  olfactory  sensation  may  be  in  some  cases,  as  in  that  of 
musk,  almost  immeasurably  small. 

In  ordinary  circumstances  odoriferous  particles  reach  both 
nostrils,  and  we  receive  two  sets  of  olfactory  nervous  impulses, 
one  along  each  olfactory  bulb.  These  however  are  fused  into 
one  sensation  ;  our  olfactory  sensations  are  almost  exclusively 
binasal.  When  two  different  odours  are  presented  separately 
to  tlie  two  nostrils,  by  means  of  two  tubes  for  instance,  the 
effect  is  not  always  the  same.  Sometimes  an  oscillation  of  sen- 
sation similar  to  that  spoken  of  in  binocular  vision  (§  602) 
takes  place.  At  other  times,  the  particular  result  depending  on 
the  nature  of  the  odours,  one  sensation  only  is  felt,  the  one 
sensation  wholly  destroys  the  other.  And  we  may  infer  from 
this  that  when,  as  frequently  happens,  in  a  mixture  of  odours 
we  can  only  recognize  one  dominant  odour,  the  suppression  of 
the  missing  sensations  is  not  due  to  the  chemical  action  of  one 
odour  upon  another,  or  to  the  one  odour  preventing  the  other 
from  acting  on  the  olfactory  cells ;  but  from  a  central  cerebral 
obliteration  of  all  the  sensations  l)ut  one. 

§  638.  As  in  the  cases  of  the  previous  senses,  we  project 
our  olfactory  sensations  into  the  external  world  ,  the  smell  ap- 
pears to  be  not  in  our  nose,  but  somewhere  outside  us.  We 
can  judge  of  the  position  of  the  odour  however  even  less  defi- 
nitely than  we  can  of  that  of  a  sound.  Our  chief  guide  seems 
to  be  that  we  by  turning  the  head  ascertain  in  wliich  direction 
we  experience  the  strongest  sensations. 

The  sense  of  smell  seems  to  play  a  far  more  important  part  in 
the  lives  of  the  lower  animals  than  it  does  in  our  own  life  ;  and 
what  we  now  possess  is  probably  the  mere  remnant  of  a  once 


1028  SMELL.  [Book  hi. 

powerful  meclianism.  We  may  perhaps  connect  with  this  on 
the  one  hand  the  fact  that,  even  in  ourselves,  the  olfactory  fibres 
have  allotted  to  them  what  is  virtually  a  whole  segment  of  the 
brain,  namely  the  olfactory  lobe,  and  on  the  other  hand  the  fact 
that  olfactory  sensations  seem  to  have  an  unusually  direct  path 
to  the  inner  working  of  the  central  nervous  system.  Mental 
associations  cluster  more  strongly  round  sensations  of  smell  than 
round  almost  any  other  impressions  we  receive  from  without. 
And  powerful  reflex  effects  are  very  frequent,  many  people  faint- 
ing in  consequence  of  the  contact  of  a  few  odorous  particles 
with  their  olfactory  cells. 

The  assertion  that  the  olfactory  nerve  is  the  nerve  of  smell 
has  been  disputed.  Cases  have  been  recorded  of  persons  who 
appeared  to  have  possessed  the  sense  of  smell,  and  yet  in  whom 
the  olfactory  lobes  were  found  after  death  to  be  absent.  Direct 
experiments  on  animals  however  shew  that  loss  of  the  olfactory 
lobes  entails  loss  of  smell.  On  the  other  hand,  it  is  stated  that 
section  or  injury  of  the  fifth  nerve  causes  a  loss  of  smell  though 
the  olfactory  nerve  remains  intact ;  but  in  these  cases  it  has  not 
been  shewn  that  the  olfactory  membrane  remains  intact,  and  it 
is  quite  possible  that,  as  in  the  case  of  the  eye,  changes  may 
take  place  in  the  nasal  membrane  as  the  result  of  the  injury  to 
the  fifth  nerve,  sufficient  to  prevent  its  performing  its  usual 
functions. 


SEC.  2.     GUSTATOKY  SENSATIONS. 

§  639.  The  word  taste  is  frequently  used  when  the  word 
smell  ought  to  be  employed.  We  sj^eak  of  '  tasting '  odoriferous 
substances,  such  as  an  onion,  a  wine,  a  savoury  dish,  and  the 
like,  when  in  reality  we  only  smell  them  as  we  hold  them  in 
our  mouth ;  this  is  proved  by  the  fact  that  the  so-called  taste  of 
these  things  is  lost  when  the  nose  is  held,  or  the  nasal  mem- 
brane rendered  inert  by  a  catarrh.  If  the  nose  be  held  and  the 
eyes  shut,  it  is  very  difficult  to  distinguish  in  eating  between 
an  ajjple,  an  onion  and  a  potato ;  the  three  may  be  recognized 
by  their  texture,  but  not  by  their  "  taste."  Most  of  what  we 
call  '  flavours '  appeal  in  reality  to  the  sense  of  smell  not  to  that 
of  taste. 

We  also  experience  by  means  of  the  surfaces  with  which  we 
taste  sensations  other  than  those  of  taste.  We  feel  b}^  means  of 
the  mucous  membrane  of  the  mouth  sensations  of  the  same  kind 
as  those  whicli  we  feel  by  means  of  the  skin,  and  which  we  shall 
study  presently  as  tactile  sensations  or  sensations  of  pressure, 
sensations  of  heat  and  of  cold ;  indeed  the  tactile  sensations  of 
the  tip  of  the  tongue  are  remarkabl}^  acute.  We  also  experi- 
ence by  means  of  the  mouth  sensations  of  pain  and  other  more 
or  less  indefinite  sensations  which  we  shall  presently  speak  of 
as  phases  of  "  general  "  or  "  common  sensibility;"  and  in  this 
respect  the  mucous  membrane  of  the  mouth  is  much  more  sensi- 
tive than  the  skin  towards  chemical  substances ;  an  acid  for 
instance  or  other  corrosive  liquid,  in  such  a  concentration  as 
when  apjilied  to  the  skin  produces  a  sensation  not  essentially 
different  from  that  of  mere  contact  with  an  innocuous  liquid, 
may  when  applied  to  the  mouth  produce  a  very  jiainful  sensa- 
tion. Again,  when  the  interrupted  current  is  applied  to  the 
tongue  we  not  only  feel  the  contact  of  the  electrodes  but  expe- 
rience a  peculiar  sensation  which  is  probably  due  to  the  contrac- 
tions excited  b}-  the  current  in  the  muscular  fibres  of  the  tongue; 
we  say  we  "•  feel  the  current." 

i^  640.  There  are  however  certain  sensations  quite  distinct 
from  those  just  mentioned  and  quite  independent  of  smell  which 

1029 


1030  TASTE   SENSATIONS.  [Book  iii. 

■we  experience  when  various  substances  are  placed  in  tlie  mouth; 
and  these,  which  are  the  gustatory  sensations  proper,  may  be 
broadly  classified  into  '  bitter,'  'sweet,'  'acid'  or  'sour,'  and  'salt,' 
to  which  perhaps  should  be  added  'metallic '  and  'alkaline.'  The 
sensation  of  bitterness,  such  as  that  produced  by  quinine,  and 
the  sensation  of  sweetness,  such  as  that  produced  by  sugar,  are 
very  definite  and  specific  sensations ;  they  appear  to  be  of  an 
order  different  from  those  of  acidity  or  sourness  and  of  saltness ; 
indeed  an  acid  '  taste  '  is  apt  to  merge  into  an  affection  of  gen- 
eral sensibility  mentioned  above.  The  characters  '  metallic ' 
and  '  alkaline  '  should  perhaps  be  regarded  as  qualifying  one  or 
other  of  the  other  sensations  rather  than  as  being  independent 
sensations. 

In  the  ordinary  course  of  things  these  sensations  are  excited 
by  the  contact  of  specific  sapid  substances  with  the  mucous 
membrane  of  the  mouth,  the  substances  acting  in  some  \\a.j  or 
other,  by  virtue  of  their  chemical  constitution,  on  the  endings 
of  the  gustatory  fibres.  When  we  taste  quinine,  the  particles  of 
the  quinine,  we  must  suppose,  set  up  chemical  changes  in  the 
cells  of  the  taste-buds  or  in  other  parts  of  the  epithelium,  and 
by  means  of  those  changes  gustatory  impulses  are  started.  But 
mechanical  or  electrical  stimuli,  in  the  absence  of  sapid  sub- 
stances, will  give  rise  to  gustatory  sensations.  When  the  tongue 
is  smartly  tapped,  in  addition  to  the  sensation  of  touch  or  the 
more  or  less  painful  sensation  which  may  be  produced,  a  sensa- 
tion, which  we  must  call  a  sensation  of  taste,  is  developed  and 
often  lasts  for  some  considerable  time.  If  a  constant  current 
be  applied  to  the  tongue,  sensations  of  taste  are  developed  at 
the  two  electrodes,  that  at  the  anode  differing  from  that  at  the 
kathode,  and  the  exact  nature  of  each  being  dependent  upon 
the  region  of  the  mouth  stimulated.  It  is  probable  that  in  this 
case  electrolysis  either  of  the  fluids  covering  the  epithelium  or 
of  the  substance  of  the  epithelial  cells  themselves  generates 
bodies  which  act  as  chemical  stimuli ;  and  it  is  possible  that  the 
mechanical  disturbance  of  the  cells,  when  the  tongue  is  tapped, 
also  sets  free  chemical  stimuli.  But  sensations  of  taste  may  be 
provoked  by  an  interrupted  induced  current,  so  feeble  as  not  to 
be  felt  as  an  electric  current,  and  so  arranged  that  the  make  and 
break  shocks  are  equalized ;  in  this  case  there  can  be  little  or  no 
electrolysis,  and  we  may  infer  that  the  current  acts  in  some  way 
or  another  on  the  specific  nerve  endings.  It  is  somewhat  singu- 
lar that  heat  when  applied  to  the  tongue  appears  not  to  produce 
any  sensations  of  taste. 

As  we  shall  presently  see,  the  nerve  fibres  concerned  in  taste 
belong  either  to  the  fifth  nerve  or  to  the  glossopharyngeal  nerve 
or  to  both  nerves.  We  saw  in  dealing  with  vision  that  the  evi- 
dence as  to  whether  direct  stimulation  of  the  optic  fibies  without 
the  intervention  of  the  retinal  structures  could  produce  visual 


Chap,  v.]  TASTE   AXD   SMELL.  1031 

sensations  was  uncertain.  We  liave  no  satisfactory  evidence 
whatever  that  direct  stimulation  of  the  gustatory  fibres  along 
their  course  in  either  the  above  two  nerves  will  produce  sensa- 
tions of  taste.  As  far  as  the  sense  of  taste  is  concerned  we 
have  no  adequate  evidence  that  specific  gustatory  impulses  can 
be  developed  in  the  gustatory  fibres  apart  from  changes  in  the 
nerve  endings.  But  the  evidence  is  negative  only;  and  the 
case  is  one  not  suited  for  experiment,  since  both  nerves  along 
their  whole  course  are  mixed  nerves  containing  other  afferent 
fibres  than  those  of  taste. 

§  641.  It  is  essential  for  the  development  of  taste,  that  the 
substance  to  be  tasted  should  be  dissolved;  hence,  the  value  of 
the  glands,  which  are  especially  abundant  in  the  neighbourhood 
of  tiie  taste-buds.  The  effect  is  also  increased  by  friction  ;  and 
the  tongue  and  lips  may  be  regarded  as  a  subsidiary  apparatus 
which  by  their  movements  assist  in  bringing  the  sapid  sub- 
stances into  contact  with  the  mucous  membrane  of  the  mouth. 
A  substance  may  give  rise  to  hardly  any  sensation  of  taste  when 
simply  placed  on  the  extended  tongue,  and  yet  excite  very  dis- 
tinct sensations  when  rubbed  between  the  tongue  and  the  soft 
palate ;  indeed  we  generally  make  use  of  this  movement  known 
as  "  smacking  the  lips,"  when  we  desire  to  obtain  strong  taste 
sensations.  In  this  act  however  we  not  only  make  use  of  the 
most  sensitive  surfaces  and  call  in  the  aid  of  friction,  but  we 
also  increase  the  sensation  by  employing  a  large  area  of  sensi- 
tive surface ;  for  the  larger  the  surface  the  more  intense  is  the 
sensation. 

The  sensation  takes  some  time  to  develope,  and  endures  for 
a  long  time,  though  this  may  be  in  part  due  to  the  stimulus 
remaining  in  contact  Avith  the  terminal  organs. 

A  temperature  of  about  40°  is  the  one  most  favourable  for 
the  production  of  the  sensation.  At  temperatures  much  above 
or  below  this,  taste  is  much  impaired.  A  weak  solution  of  qui- 
nine readily  tasted  at  the  normal  temperature  of  the  mouth  is 
not  tasted  if,  immediately  before,  very  cold  or  very  hot  water  be 
held  in  the  mouth  for  a  little  while. 

We  may  experience  at  the  same  time  coincident  taste  sensa- 
tions of  different  kinds,  such  for  instance  as  one  of  bitterness 
with  one  of  saltness ;  but  in  some  cases  one  sensation  interferes 
with  the  other,  as  for  instance  bitterness  and  sweetness.  A 
taste  sensation  following  upon  a  previous  sensation  of  a  differ- 
ent kind,  is  frequently  influenced  by  its  predecessor,  being 
sometimes  augmented,  sometimes  inhibited. 

Though  we  can  hardly  be  said  to  project  our  sensations  of 
taste  into  the  external  world,  as  we  do  those  of  sight,  hearing 
and  smell,  we  assign  to  them  no  subjective  localization.  When 
we  place  quinine  in  our  mouth,  the  resulting  sensation  of  taste 
gives  us  no  information  as  to  where  the  quinine  is,  though  we 


1032  TASTE   SENSATIONS.  [Book  m. 

may  learn  that  by  concomitant  general  sensations  arising  in  the 
buccal  mucous  membrane.  And  it  must  be  remembered  that 
all  our  gustatory  sensations  are  always  accompanied  by  tactile 
or  other  sensations ;  we  do  not,  as  in  the  case  of  smell,  experi- 
ence the  specific  sensation  alone  and  apart  by  itself.  And  not 
infrequently,  as  when  substances  at  once  sapid  and  pungent  are 
placed  in  the  mouth,  the  general  sensation  of  pungency  over- 
comes and  hides  the  specific  gustatory  sensation.  In  the  case 
of  acids,  it  is  often  difficult  to  distinguish  between  the  acid 
taste  and  the  more  general  effect  of  the  acid  on  the  common 
sensibility  of  the  buccal  membrane  of  which  we  spoke  above 
§  639. 

Though  we  possess  a  gustatory  apparatus  with  separate 
nerves  on  each  side  of  the  mouth  all  our  sensations  are  single. 
Nor  can  we  distinguish  a  pure  gustatory  sensation  developed  on 
one  side  only  from  one  developed  on  both  sides,  if  the  two  are 
equally  intense. 

As  in  the  case  of  the  senses  previously  dealt  with  we  may 
experience  subjective  gustatory  sensations,  sensations  of  central 
origin  due  to  changes  in  the  central  sensory  organs  (§  502) ; 
and  these,  though  originated  not  by  gustatory  impulses  but  by 
other  events,  may  seem  to  us  identical  with  those  set  up  in  an 
ordinary  way  by  gustatory  impulses  reaching  the  centre  along 
the  gustatory  fibres. 

§  642.  Sensations  of  taste  are  not  originated,  either  by 
sapid  substances  or  otherwise,  equally  in  all  parts  of  the  lining 
membrane  of  the  mouth.  The  part  in  which  they  are  best 
developed,  and  always  developed  if  developed  at  all,  is  the  back 
of  the  tongue,  in  the  neighbourhood  of  the  circumvallate  papillae. 
They  are  also  developed  at  the  tip  and  along  the  sides  of  the 
tongue,  but  to  a  variable  extent  in  different  individuals ;  some 
persons  have  very  acute  and  distinct  taste  sensations  in  these 
parts,  others  little  or  none  at  all.  On  the  dorsal  surface  of  the 
middle  of  the  tongue  very  feeble  taste  sensations,  if  any  at  all, 
are  developed ;  they  are  always  wholly  absent  from  the  under- 
surface  of  the  tongue.  Some  taste  sensations  are  also  developed 
in  the  soft  palate  and  front  surface  of  the  palatine  arches ;  but 
these  again  vary  much  in  distinctness  in  different  individuals. 
In  the  cases  recorded  in  which  taste  remained  after  the  entire 
extirpation  of  the  tongue  including  the  circumvallate  papillse, 
the  sensations  seem  to  have  been  chiefly  developed  in  the  soft 
palate.  There  is  also  some  evidence  that  taste  sensations  may 
be  developed  on  the  hinder  surface  of  the  epiglottis. 

In  individuals  who  receive  sensations  from  all  or  several  of 
the  various  parts  above  mentioned,  it  commonly  happens  that 
bitter  things  are  most  readily  appreciated  at  the  back  of  the 
tongue  and  sweet  things  at  the  tip ;  and  this  distribution  may 
perhaps  be  considered  as  the  normal  one ;  but  individual  varia- 


Chap,  v.]  TASTE   AND   SMELL.  1033 

tions  in  tliis  respect  are  met  with ;  many  persons  taste  both 
bitter  and  sweet  things  best  at  the  back  of  the  tongue  ;  and 
some  persons  taste  bitter  things  quite  distinctly  at  the  tip. 
The  salt  taste  is  said  to  prevail  at  the  tip  and  the  acid  taste  at 
the  sides  of  the  tongue ;  but  many  persons  experience  acid  and 
salt  tastes  in  those  regions  and  those  regions  only  in  which  they 
experience  bitter  and  sweet  tastes. 

We  have  already  said  that  bitter  and  sweet  tastes  seem  to  be 
on  a  different  footing  from  acid  and  salt  tastes ;  and  we  have  a 
certain  amount  of  evidence  that  the  two  former  sensations  are 
brought  about  by  means  of  terminal  organs  different  from  those 
by  means  of  which  the  two  latter  are  brought  about.  If  some 
of  the  leaves  of  a  plant  which  grows  in  India  and  is  called 
G-ymnema  sylvestre,  be  chewed,  or  if  the  mouth  be  washed  with 
a  decoction  of  the  leaves,  for  some  little  time  afterwards  bitter 
and  sweet  tastes  are  lost,  neither  quinine  nor  sugar  exciting  the 
usual  sensations,  though  acid  and  salt  tastes  remain  unaffected. 
We  may  interpret  this  result  as  indicating  that  the  drug  in  some 
way  or  other  '  paralyzes,'  that  is  to  sa}- ,  suspends  the  action  of, 
the  terminal  organs,  whatever  they  may  be,  by  means  of  which 
bitter  and  sweet  tastes  are  developed,  but  leaves  untouched 
those  by  which  other  gustatory  sensations  are  developed.  The 
action  of  the  same  drug  supports  the  further  conclusion  that 
the  terminal  organs  of  bitter  tastes  are  different  from  those  of 
sweet  tastes ;  since  by  using  an  adequately  weak  dose  of  the 
drug  the  sweet  taste  may  be  abolished  while  the  bitter  taste 
remains  distinct. 

Indeed  it  is  probable  that  the  distribution  of  the  several 
kinds  of  tastes  over  different  regions  of  the  mouth,  which  we 
mentioned  above,  is  dependent  on  the  distribution  of  different 
kinds  of  terminal  organs ;  it  is  probable  that  we  experience 
bitter  tastes  by  means  of  the  back  of  the  tongue  because  the 
terminal  organs  of  the  bitter  taste  are  limited  to,  or  at  least 
most  abundant  in,  the  back  of  the  tongue,  those  of  the  sweet 
taste  by  the  front  of  the  tongue  because  the  terminal  organs 
of  the  sweet  taste  are  more  abundant  there ;  and  so  on.  If 
a  small  quantity  of  a  particular  bromine  derivative  of  the  sub- 
stance which  from  its  remarkably  sweet  taste  has  been  called 
'saccharine,'  be  placed  carefully  on  the  tip  of  the  tongue,  a 
sweet  taste  is  developed ;  but  if  the  same  substance  be  carefully 
placed  on  the  back  of  the  tongue  the  result  is  not  a  sweet  but 
a  bitter  taste.  At  least  this  is  the  result  in  the  case  of  those 
individuals  who  taste  bitter  at  the  back  of,  and  sweet  at  the 
tip  of,  th«  tongue.  From  this  we  may  infer  that,  in  such 
tongues,  the  speciiic  terminal  organs  of  the  sweet  taste  are 
more  or  less  completely  limited  to  the  front,  and  those  of  the 
bitter  taste  to  the  back  of  the  tongue,  both  sets  of  terminal 
organs  being  of  such  a  nature  that  Avhile  quinine  affects  the  one 


1034  TASTE   SENSATIONS.  [Book  iii. 

only  and  sugar  the  other  only,  the  substance  of  which  we  are 
speaking  is  able  to  affect  both  of  them.  In  a  somewhat  similar 
way  certain  salts,  magnesium  sulphate  for  instance,  when  applied 
to  the  back  of  the  tongue  excite  a  bitterish  taste,  but  when 
applied  to  the  tip  of  the  tongue  excite  an  acid  or  a  sweetish  acid 
taste. 

We  said  a  little  while  back  that  a  weak  interrupted  current, 
so  applied  as  to  produce  little  or  no  electrolytic  effect,  was  able 
to  develope  sensations  of  taste,  varying  in  kind  according  to  the 
region  of  the  tongue  stimulated.  When  the  electrodes  are 
applied  to  the  tip  of  the  tongue,  the  more  usual  result  is  that 
though  an  acid  taste  is  the  most  prominent,  a  mixed  gustatory 
sensation  is  developed,  in  which  a  sweet  taste  may  be  often 
recognized  as  a  constituent.  In  like  manner  a  bitter  constituent 
may  be  recognized  in  the  sensation  developed  when  the  elec- 
trodes are  placed  at  the  back  of  the  tongue.  If  the  tongue  be 
previously  subjected  to  the  influence  of  G-ymnema,  the  taste 
at  the  tip  is  free  from  all  sweetness  and  that  at  the  back  free 
from  all  bitterness,  the  sensations  which  are  then  experienced 
being  variously  described  as  simply  "  metallic,"  or  "  salt,"  or 
"  acid."  From  this  result  we  may  draw  the  important  infer- 
ence that  the  interrupted  current  developes  a  bitter  and  a 
sweet  taste  by  acting  in  some  way  or  other  directly  on  the 
specific  terminal  organs  of  the  two  respective  tastes,  very  much 
in  the  same  manner  as  do  bitter  and  sweet  things. 

We  have  already  said  that  when  an  acid,  especially  a  some- 
what strong  acid,  is  placed  on  the  tongue  or  in  the  mouth,  the 
pure  gustatory  acid  sensation  is  apt  to  be  confused  with  the 
sensation  of  pungency,  the  affection  of  general  sensibility  which 
the  acid  also  brings  about  and  which  speedily  merges  into  pain. 
These  two  sensations  may  be  differentiated  by  means  of  cocaine. 
If  the  tongue  be  painted  with  a  weak  solution  of  cocaine,  the 
general  sensibility,  the  groundwork  so  to  speak  of  pain,  is  abol- 
ished, while  the  pure  gustatory  sensations  are  at  first  hardly 
affected  at  all ;  a  relatively  strong  acid  which  previously  made 
the  tongue  "smart"  so  that  real  gustatory  sensations  were  ob- 
scured, now  developes  a  pure  'rich'  acid  taste  alone.  It  is 
moreover  said  that  cocaine  applied  to  the  tongue  in  increasing 
strength  of  solution  abolishes  the  several  classes  of  sensations 
in  the  following  order :  general  sensibility  and  pain,  bitter  taste, 
sweet  taste,  salt  taste,  acid  taste,  tactile  sensations. 

Taking  all  these  facts,  and  others  which  we  might  bring 
forward,  into  consideration,  we  are  led  to  the  conclusion  that 
the  development  of  the  several  kinds  of  gustatory  sensations 
depends  on  the  presence  of  specific  terminal  organs  in  the  sur- 
faces by  means  of  which  we  taste.  There  appear  to  be  distinct 
terminal  organs  for  bitter  tastes,  for  sweet  tastes,  for  acid  tastes, 
for  salt  tastes,  and  possibly  for  other  tastes,  all  differing  from 


Chap,  v.]  TASTE   AND   SMELL.  1035 

the  terminal  organs  for  tactile  sensations,  and  from  the  structures 
whatever  they  maybe  which  are  concerned  in  general  sensibility. 
Further,  we  have  a  certain  amount  of  evidence  that  these  ter- 
minal organs  are  at  least  chiefly  jjresent  in  the  fungiform  and 
circumvallate  papillse.  By  careful  manipulation  it  is  possible, 
under  a  lens,  by  means  of  a  flnely  pointed  brush  to  limit  the 
application  of  a  minute  drop  of  a  sapid  liquid,  such  as  syrup, 
solution  of  quinine  and  the  like  to  a  single  papilla,  and  to 
appreciate  the  sensation  thus  caused  before  the  material  has  had 
time  to  spread  by  diffusion.  When  this  is  done,  it  is  found  that 
taste  sensations  are  readily  produced  if  the  sapid  substance 
be  applied  to  a  papilla,  but  not  at  all  or  less  readily  if  it  be 
applied  between  the  papillse.  Further,  some  papillse  are  found 
especially  sensitive  to  sweet,  or  to  bitter  or  to  acid  substances, 
or  to  two  of  these  to  the  exclusion  of  the  other.  And  somcAvhat 
similar  results  are  obtained  by  a  limited  application  of  the  electric 
current.  Since  the  taste-buds  are  especially  developed  on  these 
circumvallate  and  fungiform  papillse,  we  ma}"  infer  that  the  name 
taste-bud  has  been  wisely  chosen.  But  the  development  of  taste 
sensations,  including  bitter  sensations,  at  the  tip  of  tlie  tongue, 
from  which  taste-buds  are  said  to  be  absent,  presents  a  difficulty. 
Unless  we  suppose  that  taste-buds,  though  often  absent  from 
the  tip  of  the  tongue,  are  present  in  those  cases  in  which  sensations 
are  developed,  we  must  conclude  that  gustator}"  sensations  may 
originate  by  the  help  of  some  kind  of  nerve  ending  other  than 
that  of  taste-buds.  It  might  be  suggested  that  bitter  and  sweet 
tastes  are  developed  by  means  of  taste-buds  and  acid  and  salt 
tastes  by  means  of  other  endings ;  but  there  is  no  satisfactor}^ 
evidence  of  this. 

§  643.  The  question  which  nerve  or  nerves  subserve  taste 
and  what  is  the  course  of  the  gustatory  fibres  is  one  which  pre- 
sents great  difficulties.  The  front  surface  of  the  tongue  is  sup- 
plied by  the  lingual  or  gustatory  branch  of  the  fifth  nerve,  the 
hind  surface  by  the  glossopharyngeal  nerve,  which  nerve  also 
supplies  the  soft  palate,  though  a  branch  (palatine)  of  the  fifth 
nerve  goes  there  also.  Tlie  nerves  traced  to  the  taste-buds  in  the 
papillse  foliatse  and  circumvallatse  belong  to  the  glossopharjni- 
geal  nerve,  and  it  can  hardly  be  doubted  that  gustator}"  fibres 
run  in  the  branches  of  that  nerve  which  go  to  the  back  of  the 
tongue.  On  the  other  hand  in  the  cases  in  wliich  sensations 
are  distinctly  developed  in  the  tip  of  the  tongue  we  must  infer 
that  gustatory  fibres  run  in  the  lingual  branch  of  the  fifth,  since 
no  glossopharyngeal  fibres  are  distributed  to  this  part  of  the 
tongue. 

But  it  by  no  means  follows  from  this  that  gustatory  fibres 
pass  straight  both  up  the  trunk  of  the  glossopharyngeal  nerve, 
and  up  the  trunk  of  the  fifth  nerve  to  their  respective  nuclei  in 
the  bulb. 


1036  TASTE   SEI^SATIOXS.  [Book  in. 

On  the  one  hand  there  is  a  good  deal  of  evidence  to  shew  a 
connection  between  sensations  of  taste  and  the  chorda  tympani 
nerve.  Cases  have  occurred  in  wliich  disease  of  the  ear,  involv- 
ing destruction  of  the  chorda  tympani  within  the  tympanum, 
has  been  followed  by  loss  of  taste  in  the  tongue  on  the  same 
side ;  and  stimulation  of  the  chorda  tympani  within  the  tym- 
panum has  been  known  to  give  rise  to  sensations  of  taste. 
Neither  of  these  results  is  conclusive.  The  chorda  tympani 
contains  afferent  fibres  which  have  a  remarkable  effect  on  the 
nutritive  processes  of  the  tongue,  and  the  loss  of  taste  due  to 
destruction  of  the  chorda  might  be  due  to  disordered  nutrition 
of  the  tongue,  and  so  be  analogous  to  the  loss  of  smell  which 
may  follow  injury  of  the  fifth  nerve.  Again,  where  stimulation 
of  the  chorda  within  the  tympanum  produces  sensations  of 
taste,  these  may  be  due  to  efferent  impulses  producing  changes 
in  the  tongue,  which  in  turn  give  rise  to  sensations  of  taste 
reaching  the  brain  by  other  channels  than  the  chorda  itself ;  we 
have  no  satisfactory  evidence  that  direct  stimulation  of  the 
central  stump  of  a  divided  chorda  will  give  rise  to  sensations 
of  taste.  The  connection  between  the  chorda  and  taste,  how- 
ever, may  be  of  a  more  real  kind. 

On  the  other  hand  we  must  bear  in  mind  how  varied  and 
complex  are  the  junctions  in  the  skull  between  the  fifth  nerve, 
the  seventh  nerve,  and  the  glossopharyngeal  nerve,  by  way  of 
the  Vidian  nerve,  the  petrosal  nerves,  the  tympanic  plexus, 
Jacobson's  nerve,  and  the  otic  and  sphenopalatine  ganglia. 
And  it  seems  possible  to  suppose  that  fibres  leaving  the  brain 
by  the  fifth  nerve  might  find  their  way  not  directly  to  the  lin- 
gual branch  but  by  a  roundabout  way  through  the  chorda  tym- 
pani, and  that  at  the  same  time  other  fibres  from  the  same  iifth 
nerve  might  ultimately  join  the  glossopharyngeal  nerve  and 
reach  the  back  of  the  tongue  by  that  nerve.  There  are  no  cases 
•on  record  in  which  disease  of  the  glossopharyngeal  nerve  within 
the  cranial  cavity  has  led  to  distinct  loss  of  taste ;  but  cases 
have  been  recorded  in  which  disease  of  the  fifth  nerve  within 
the  cranial  cavity,  and  so  far  as  could  be  ascertained  limited  to 
the  fifth  nerve,  has  led  to  an  entire  loss  of  taste  over  the  whole 
of  one  side  of  the  tongue,  both  back  and  tip.  Such  cases  lead 
to  the  at  least  provisional  conclusion  that  the  gustatory  fibres 
are  fibres  belonging  to  the  fifth,  though  they  may  reach  the 
tongue  partly  by  way  of  the  glossopharyngeal,  partly  by  way  of 
the  chorda  tympani. 


CHAPTER  VI. 

ON  CUTANEOUS   AND   SOME   OTHER   SENSATIONS. 

« 

SEC.  1.   THE  GENERAL  FEATURES  OF  CUTANEOUS 

SENSATIONS. 

§  644.  The  sensations  which  we  experience  by  means  of 
the  skin  and  cutaneous  nerves  appear,  in  the  first  instance,  to  be 
of  at  least  three  kinds.  In  the  first  place,  all  bodies,  whatever 
their  chemical  or  physical  nature,  be  they  gaseous,  liquid  or  solid, 
when  brought  into  contact  with  the  skin,  when  made  to  exert 
mechanical  pressure  on  the  skin,  produce  sensations  of  a  certain 
kind;  these  sensations,  whose  characters  depend  mainly  on  the 
amount  of  pressure  exerted  and  on  the  region  and  area  of  the 
skin  pressed  upon,  may  be  conveniently  spoken  of  as  tactile 
sensations  or  sensations  of  touch  jproiier.  In  the  second  place, 
when  either  by  actual  contact  with,  or  by  the  mere  proximity 
of  hot  or  cold  bodies,  of  whatever  nature,  the  temperature  of  an 
area  of  the  skin  is  changed  with  sufficient  rapidity,  we  ex- 
perience sensations  of  a  kind  different  from  the  tactile  sensations 
just  mentioned;  these  we  may  speak  of  as  sensations  of  tem- 
perature^ sensations  of  heat  and  cold.  In  the  third  place,  when 
too  violent  a  pressure  is  exerted  on  the  skin,  or  when  the 
changes  of  temperature  are  excessive,  or  when  certain  changes 
giving  rise  neither  to  tactile  nor  to  temperate  sensations  are 
produced,  or  take  place  in  the  skin,  we  experience  sensations 
which  we  call  t^ensations  of  pain.  This  third  kind  of  sensation 
stands,  in  many  respects,  apart  from  the  other  two,  and  it  will 
be  convenient  to  study  sensations  of  pain  by  themselves.  Sen- 
sations of  touch  proper  and  of  heat  and  cold  are  much  more 
akin  and  may  be  treated  of  together. 

Tactile  Sensations  or  Sensations  of  Pressure. 

§  645.  Many  of  the  characters  of  tactile  sensations  are  of 
the  same  order  as  those  of  visual  sensations,  which  we  studied 
somewhat  fully,  and  indeed  similar  characters  may  be  more  or 

1037 


1038  01^   CUTANEOUS   AND  [Book  in. 

less  distiuctly  recognized  in  all  sensations.  The  amount,  that 
is  to  say  the  intensity  of  the  sensation,  varies  with  tlie  amount 
of  the  stimulus,  with  the  amount  of  pressure  brought  to  bear  on  a 
given  area  of  the  skin.  Taking  the  same  spot  of  skin,  the  tip  of 
the  forefinger  for  instance,  we  can  experimentally  ascertain  the 
minimum  of  pressure  of  which  we  can  become  conscious,  such  for 
example  as  that  exerted  by  a  minute  fragment  of  some  light  body, 
pith  or  wool,  falling  through  a  certain  small  height.  Starting  from 
this  minimum  and  increasing  the  pressure,  we  find  the  sensation 
also  to  increase  up  to  a  certain  limit ;  and  Weber's  law  (§  550) 
holds  good  for  tactile  sensations,  indeed  may  be  more  easily  veri- 
fied in  their  case  than  perhaps  in  the  case  of  other  sensations. 

When  two  sensations  follow  each  other  in  tlie  same  spot  of 
skin  at  a  sufficiently  short  interval  they  are  fused  into  one ; 
thus,  if  the  finger  be  brought  to  bear  lightly  on  the  edge  of  a 
rotating  card  cut  into  a  series  of  teeth,  the  teeth  cease  to  be  felt 
as  such  when  they  follow  each  other  at  a  rapidity  of  about 
1500  in  a  second.  The  vibrations  of  a  cord  cease  to  be  appre- 
ciable by  touch  when  they  reach  the  same  rapidity. 

When  two  sensations  are  generated  at  the  same  time  at  two 
points  of  the  skin  too  close  together  they  become  fused  into  one ; 
but  to  this  feature,  which  is  of  a  different  nature  from  the  pre- 
ceding, we  shall  return  presently. 

Tlie  sensa,tion  caused  by  pressure  is  at  its  maximum  soon 
after  its  beginning,  and  thenceforward  diminishes.  The  more 
suddenly  the  pressure  is  increased,  the  greater  the  sensation ; 
and  if  the  increase  be  sufficiently  gradual,  even  very  great 
pressure  may  be  applied  without  giving  rise  to  any  sensation. 
A  sensation  in  any  spot  is  increased  when  the  surrounding  areas 
of  skin  are  not  subject  to  pressure  at  the  same  time.  Tlius  if 
the  finger  be  dipped  into  mercury  the  pressure  of  the  mercury 
will  be  felt  more  at  the  surface  of  the  fluid  adjoining  the  skin 
which  is  not  in  contact  with  the  mercury,  than  in  the  parts  of 
the  skin  wholly  covered  with  the  mercury ;  and  if  the  finger  be 
drawn  up  and  down,  the  sensation  caused  will  be  that  of  a  ring 
moving  along  the  finger.  This  effect  may  be  compared  with 
those  of  'contrast'  in  visual  sensations  (§  583). 

All  parts  of  the  skin  are  not  equally  sensitive  to  pressure ; 
the  minimum  of  pressure  which  can  be  felt  or  the  smallest 
difference  of  pressure  which  can  be  appreciated  differs  very 
much  at  different  parts  of  the  skin.  Measured  in  this  way, 
tactile  sensations  are  much  more  acute  on  the  palmar  surface  of 
the  finger,  or  on  the  forehead,  than  on  the  arm  or  on  the  sole  of  the 
foot  or  on  the  back.  In  making  these  determinations  all  mus- 
cular movements  should  be  avoided  in  order  to  eliminate  the 
muscular  senses  of  which  we  shall  speak  later  on ;  and  the  area 
stimulated  should  be  as  small  and  the  contact  as  uniform  as 
possible. 


CHAr.  VI.]  SOME   OTHER   SENSATIONS.  1039 

111  a  similar  manner  small  consecutive  variations  of  pressure, 
as  in  counting  a  pulse,  are  more  readily  appreciated  by  certain 
parts  of  the  skin,  such  as  the  tip  of  the  finger,  than  by  others. 
In  all  cases  variations  of  pressure  are  more  easily  distinguished 
when  they  are  successive  than  when  they  are  simultaneous. 

§  646.  Tlie  localization  of  tactile  sensations.  When  anything 
touches  a  spot  of  our  skin,  we  not  only  experience  a  '  pressure 
sensation '  of  greater  or  less  intensity  according  to  the  amount 
of  pressure  exerted  and  the  particular  region  of  skin  pressed 
upon,  we  are  also  at  the  same  time  aware  that  the  sensation  has 
been  started  in  that  spot,  that  the  spot  in  question  and  not 
another  has  been  touched.  When  we  are  touched  on  the  finger 
or  on  the  back  we  refer  the  sensations  to  the  finger  or  to  the 
back  respectively,  and  when  we  are  touched  at  two  places  on 
the  same  finger  at  the  same  time  we  refer  the  sensations  to  two 
parts  of  the  finger.  We  localize  oui'  touch  sensations  with  refer- 
ence to  the  surface  of  our  body  after  the  same  fashion  that  we 
localize  our  visual  sensations  with  reference  to  the  external 
world.  Our  whole  skin  serves  us  as  a  'field  of  touch'  anal- 
ogous to  the  '  visual  field '  of  the  eye  ;  and  as  when  experiencing 
a  visual  sensation,  Ave  refer  it  to  its  presumed  cause  and 
say  we  perceive  a  light  in  some  part  or  other  of  the  field  of 
sight,  so  when  we  experience  a  tactile  sensation  we  say  we 
perceive  that  something  has  touched  this  or  that  part  of  our 
skin ;  the  tactile  sensation  has  become  a  tactile  perception. 
As  the  accuracy  of  our  visual  perceptions  is  largely  dependent 
on  the  smallness  of  the  retinal  interval  which  must  separate  two 
simultaneous  retinal  stimulations  in  order  that  these  shall  give 
rise  to  two  separate  sensations,  vision  being  most  distinct  in 
the  fovea  centralis  where  this  interval  is  smallest,  so  also  the 
accuracy  of  our  tactile  perceptions  is  dependent  on  the  smallness 
of  a  like  cutaneous  interval.  Where,  as  in  the  tip  of  the  finger, 
the  interval  is  small,  contact  with  even  a  small  area  of  surface 
may  give  rise  to  several  simultaneous  but  distinct  sensations, 
each  of  which  we  localize  ;  and  we  thus  obtain  by  means  of  one 
contact  several  perceptions  affording  a  considerable  amount  of 
information  concerning  the  nature  of  the  surface.  Where,  as 
in  the  skin  of  the  back,  the  interval  is  great,  contact  with  even 
a  large  area  of  surface  may  give  rise  to  one  sensation,  which  we 
do  not  resolve  into  its  components,  all  the  several  sensory  im- 
pulses from  the  skin  fusing  into  one  common  sensation  ;  we 
only  localize  this  one  sensation,  we  have  only  one  perception  of 
something  touching  that  part  of  our  back,  and  the  information 
which  we  thus  acquire  concerning  the  nature  of  the  surface  in 
contact  with  the  skin  is  limited. 

As  the  above  remark  indicates,  the  interval  in  question 
varies  very  widely  in  different  parts  of  the  surface  of  the  body ; 
our  power  of  localization  is  much  finer  in  certain  parts  than  in 


1040  ON   CUTANEOUS   AND  [Book  in. 

others.  Moreover  the  distribution  of  the  fineness  of  localization 
is  not  identical  with  that  of  the  mere  appreciation  of  pressure  ; 
some  parts  may  be  very  sensitive  and  yet  possess  imperfect 
localization.  The  magnitude  of  the  interval  of  space  which 
must  separate  two  simultaneous  stimulations  of  the  skin  in  order 
that  the  two  consequent  sets  of  impulses  may  give  rise  to  two 
distinct  sensations  may  be  conveniently  determined  for  different 
regions  of  the  skin  by  measuring  the  distance  at  which  two 
points  (preferably  blunted)  of  a  pair  of  compasses  must  be  held 
apart,  so  that  when  the  two  points  are  in  contact  with  the  skin, 
the  two  consequent  sensations  can  be  localized  with  sufficient 
accuracy  to  be  referred  to  two  points  of  the  body,  and  not 
confounded  together  as  one. 

The  following  tabular  statement  of  results  thus  obtained  may 
be  taken  as  shewing  in  a  general  way  the  relative  power  of  locali- 
zation in  the  more  important  regions  of  the  surface  of  the  skin. 

Tip  of  tongue 

Palm  of  terminal  phalanx  of  finger 
Palm  of  second  „  „ 

Tip  of  nose 

White  part  of  lips     

Back  of  second  phalanx  of  finger 

Skin  over  malar  bone 

Back  of  hand 

Forearm 

Sternum 

Back  o         , 

As  a  general  rule  it  may  be  said  that  the  more  mobile  parts, 
or  those  which  execute  the  widest  movements,  or  execute  move- 
ments most  easily  and  frequently,  such  as  the  hands  and  lips, 
are  those  by  which  we  can  thus  discriminate  sensations  most 
readily.  The  lighter  the  pressure  used  to  give  rise  to  the  sensa- 
tions, provided  that  the  impulses  generated  are  adequate  to  excite 
distinctly  appreciable  sensations,  the  more  easily  are  two  sensa- 
tions distinguished;  thus  two  compass  points  which,  when 
touching  the  skin  lightly,  appear  as  two,  may,  when  firmly 
pressed,  give  rise  to  one  sensation  only.  The  distinction  be- 
tween the  sensations  is  obscured  by  neighbouring  sensations 
arising  at  the  same  time.  Thus  two  points  readily  distin- 
guished as  two  when  the  skin  is  under  ordinary  conditions, 
are  confused  into  one  when  brought  to  bear  inside  a  ring  of 
heavy  metal  pressing  on  the  skin. 

It  need  hardly  be  said  that  these  tactile  perceptions,  like  all 
other  perceptions,  are  increased  by  exercise.  We  may  speak  of 
our  '  field  of  touch,'  as  being  composed  of  tactile  areas  or  units, 
in  the  same  way  that  we  spoke  (§  557)  of  our  field  of  vision  as 


1-1 

mm, 

2-2 

4-4 

6-Q 

8-8 

11-1 

15-4 

29-8 

39-6 

44-0 

66-0 

Chap,  yi.]  SOME   OTHER   SENSATIOKS.  1041 

being  composed  of  visual  areas  or  units ;  but  all  tliat  was  there 
said  concerning  tke  subjective  nature  of  the  limits  of  visual  areas, 
applies  equally  well,  mutatis  mutandis,  to  tactile  areas.  When 
two  points  of  the  compasses  are  felt  as  two  distinct  sensations, 
it  is  not  necessary  that  two,  and  only  two,  nerve-fibres  should  be 
stimulated,  or,  putting  the  matter  more  generally,  that  two  or 
only  two  discrete  sets  of  sensory  impulses,  should  travel  along 
separate  paths  to  separate  cerebral  centres.  All  that  is  necessary 
is  that  the  two  cerebral  sensation-areas  should  not  be  too  com- 
pletely fused  together.  The  improvement  by  exercise  of  the 
sense  of  touch  must  be  explained  not  by  an  increased  develop- 
ment of  the  terminal  oi'gans,  not  by  a  growth  of  new  nerve- 
fibres  in  the  skin,  but  by  a  more  exact  limitation  of  the 
sensational  areas  in  the  brain,  as  for  example  by  the  develop- 
ment of  a  resistance  which  limits  the  radiation  taking  place 
from  the  centres  of  the  several  areas. 

Sensations  of  Heat  and   Cold. 

§  647.  When  we  bring  intp  contact  with,  or  even  into  the 
immediate  neighbourhood  of  a  spot  of  skin,  a  body  distinctly 
liotter  than  is  the  skin  at  the  spot  for  the  time  being,  we  ex- 
perience a  special  sensation ;  we  feel  something  in  the  skin  that 
was  not  there  before,  but  that  something  is  wholly  unlike  the 
effect  of  pressure,  and  we  call  the  sensation  a  sensation  of  heat. 
The  sensation  is  obviously  due  to  the  rise  in  the  temperature  of 
skin  which  is  the  direct  effect  of  the  contact  with  or  the  nearness 
of  the  hot  body.  Our  skin  has  a  certain  temperature  which  varies 
from  time  to  time,  according  to  circumstances,  and  is  not  the 
same  in  all  regions  of  the  skin  at  the  same  time.  A  given  spot 
of  skin  at  a  given  time  will  have  a  certain  temperature  ;  that 
temperature  does  not  give  rise  to  a  distinct  sensation  tliough  its 
effects  may  enter  into  what  we  may  call  general  sensibility  ;  we 
may  not  be  directly  conscious,  for  instance,  that  the  forehead 
has  one  temperature  and  the  hand  another,  though  the  two  tem- 
peratures may  differ  widely.  It  appears  then  that  we  are  only 
conscious  of  a  cutaneous  sensation  of  heat  when  the  tempera- 
ture of  a  region  of  the  skin  which  has  previously  been  fairly 
constant  is  raised  ;  we  may  add  suddenly  raised,  for  in  sensa- 
tions of  heat  as  of  pressure  the  stimulus  must  act  Avith  a  certain 
rapidity  in  order  to  produce  a  distinct  effect  on  consciousness. 

If  the  body  brought  into  contact  with  or  near  to  the  skin, 
instead  of  being  distinctly  hotter  is  distinctly  colder  than  the 
skin  we  also  experience  a  special  sensation,  a  sensation  of  cold ; 
and  this  sensation  differs  in  kind  not  onl}'  from  that  of  pressure, 
but  also  from  that  of  heat.  We  might  expect  perhaps  that 
since  cold  only  differs  from  heat  in  degree,  both  being  degrees 
of  temperature,  that  the  sensations  of  heat  and  cold  would  also 

6G 


1042  OIT   CUTANEOUS   AND  [Book  iii. 

be  alike,  diffeiing  only  in  degree ;  but  when  we  appeal  to  our 
consciousness  we  recognize  that  they  differ  in  kind.  So  long 
as  sensations  of  heat  and  cold  remain  sensations  of  heat  and 
cold,  they  appear  to  us  not  as  merely  different  phases  of  the 
same  thing  but  as  quite  unlike  ;  when  the  exciting  heat  or  cold 
is  excessive  we  perhaps  may  fail  to  distinguish  between  the  two, 
but  that  is  because  both  are  lost  in  the  sensation  of  pain.  It 
appears  then  that  we  are  conscious  of  a  specific  sensation  of 
cold  when  the  temperature  of  a  region  of  the  skin  which  has 
previously  been  fairly  constant  is  with  sufficient  rapidity  lowered. 
To  how  large  an  extent  we  are,  under  ordinary  circumstances, 
unconscious  of  the  actual  temperature  of  the  skin  and  how 
sensitive  we  are  to  even  slight  changes  of  temperature  may  be 
illustrated  by  using  one  region  of  the  skin  as  a  stimulus  of  heat 
or  cold  for  another.  At  a  time,  for  instance,  when  we  are  not 
directly  conscious  of  the  hand  being  either  colder  or  hotter  than 
the  forehead,  by  putting  the  one  up  to  the  other  we  may  experi- 
ence a  distinct  sensation  telling  us  that  the  hand  decidedly 
differs  in  temperature  from  the  forehead ;  we  feel  at  once  that 
one  is  warmer  or  colder  than  the  other,  though  it  may  take 
some  little  time  to  recognize  which  is  the  warmer  or  the  colder. 

§  648.  These  sensations  of  heat  and  cold  behave  very  much 
in  the  same  way  as  sensations  of  pressure.  We  have  already 
said  that  the  change  of  temperature  like  the  change  of  pressure 
must  be  effected  with  a  certain  rapidity  in  order  to  produce  a 
distinct  sensation,  and  in  general  the  more  gradual  the  change 
the  less  intense  is  the  sensation. 

As  might  be  expected  from  the  fact  that  it  takes  a  longer 
time  to  produce  a  change  of  temperature  than  to  exert  pressure, 
the  sensation  of  either  heat  or  cold  is  somewhat  slowly  devel- 
oped and  lasts  some  considerable  time ;  hence  consecutive  sensa- 
tions readily  fuse  into  one. 

Since  it  is  the  changed  temperature  and  not  the  particular 
temperature  arrived  at  which  is  the  basis  of  the  sensation,  a 
hot  body  or  a  cold  body  gives  rise  to  a  sensation  only  at  the 
first  contact  or  approach  and  for  some  little  time  afterwards, 
the  effect  diminishing  from  the  very  moment  that  the  change 
has  been  established.  Hence  a  hot  body  or  a  cold  body  ap- 
plied to  the  skin,  even  when  kept  itself  at  a  constant  tempera- 
ture and  not  cooled  or  heated  by  contact  with  the  cooler  or 
warmer  skin,  ceases  after  a  while  to  be  felt  as  hot  or  cold.  For 
this  reason  the  repeated  dipping  of  the  hand  into  hot  or  cold 
water  produces  a  greater  sensation  than  when  the  hand  is 
allowed  to  remain  all  the  time  in  the  water,  though  in  the 
latter  case  the   temperature  of  the  skin  is  most  affected. 

The  effects  of  contrast  are  obvious  in  sensations  of  heat 
and  cold  as  in  those  of  pressure ;  when  the  hand  is  dipped  in 
hot  water  the  sensation  is  most  intense  at  the  ring  where  the 
hand  emersfes  from  the  surface  of  the  water. 


Chap,  vi.]  SOME   OTHER   SEXSATIONS.  1043 

We  can  with  some  accuracy  distinguish  small  differences  of 
temperature,  especially  those  lying  near  the  normal  tempera- 
ture of  the  skin.  In  this  respect  these  sensations  follow 
Weber's  law,  though  apparently  slight  differences  of  cold  are 
more  easily  recognized  than  those  of  slight  heat.  The  range 
of  the  greatest  sensitiveness  seems  to  lie  between  27°  and  33°. 

The  regions  of  the  skin  most  sensitive  to  variations  in 
temperature  are  not  identical  with  those  most  sensitive  to  varia- 
tions in  pressure.  Thus  the  cheeks,  eyelids,  temj^les  and  lips, 
are  more  sensitive  than  the  hands.  The  least  sensitive  parts 
are  the  legs,  and  front  and  back  of  the  trunk ;  to  this  matter 
however  we  shall  return. 

As  with  pressure  sensations  so  also  with  sensations  of  heat 
and  cold,  two  sensations  excited  at  a  certain  distance  apart  may 
or  may  not  be  fused  into  one,  the  distance  necessary  for  the 
sei)aration  of  the  sensations  varying  in  different  regions  of  the 
bod}^,  and  being,  as  might  be  expected  from  the  ease  with  w^hich 
heat  and  cold  are  conducted,  much  greater  than  in  the  case  of 
pressure  sensations.  We  also  '  localize  '  the  sensations  of  heat  and 
cold ;  we  can  recognize  which  region  of  the  skin  is  being  heated 
or  cooled  ;  and  thus  tliese  sensations  also  enter  into  our  percep- 
tions of  the  external  world. 

§  649.  We  have  treated  of  the  sensations  of  touch  and  of 
heat  and  cold  as  cutaneous  sensations ;  but  they  are  not  con- 
fined to  the  skin  commonly  so  called.  We  experience  the 
same  sensations  in  varying  degree  by  help  of  the  lining  of  the 
mouth  and  pharynx,  which  is  called  a  mucous  membrane ;  and 
they  may  also  be  traced  for  a  short  distance  up  the  rectum  beyond 
the  margin  of  tlie  skin  proper.  But  in  both  these  situations, 
the  lining  membrane  is  by  origin  and  in  structure  epiblastic, 
that  is  to  say  cutaneous,  and  in  possessing  cutaneous  functions 
shews  its  real  nature.  These  functions  are  most  marked  at  the 
beginning  of  the  passages,  the  tip  of  the  tongue  being  ver}^ 
sensitive  to  touch  and  heat  and  cold,  with  a  w^ell-developed 
power  of  localization  ;  they  are  very  rapidly  lost  in  the  rectum, 
and  more  gradually  disappear  at  the  lower  part  of  the  pharynx 
and  in  the  oesophagus ;  a  fluid  which  in  the  mouth  is  felt  dis- 
tinctly as  hot  gives  rise  to  a  sensation  of  pain  not  of  heat  when 
it  is  swallowed,  and  a  cold  or  warm  drink  is  only  felt  as  cold 
or  warm  when  swallowed  in  quantity  sufficient  to  affect  by  con- 
duction the  abdominal  skin.  The  maintenance  of  these  cutane- 
ous functions  in  the  initial  ]")arts  of  the  alimentary  canal,  which 
are  under  the  dominion  of  the  will,  is,  like  the  sense  of  taste,  a 
safeguard  against  the  introduction  into  the  canal  of  noxious 
substances ;  in  the  subsequent  parts,  no  longer  subject  to  the 
will,  an}'-  warning  which  such  sensations  might  give  would  be 
too  late  and  useless. 


SEC.  2.     ON  PAINFUL   AND   SOME   OTHER  KINDS   OF 

SENSATION. 


§  650.  When  excessive  pressure  is  exerted  on  tlie  skin,  or 
when  the  change  of  temperature  passes  certain  limits,  the  sensa- 
tion which  is  excited  ceases  to  be  recognized  as  one  either  of 
touch  or  of  temperature  and  takes  on  characters  of  its  own;  we 
then  call  it  a  sensation  of  pain.  In  this  respect  the  skin  as  a 
sensory  organ  appears  at  first  sight  to  differ  from  the  other 
organs  of  sense  which  we  have  studied.  We  have  no  evidence 
that  simple  stimulation  of  the  retina,  however  excessive,  will 
give  rise  to  pain,  meaning  by  pain  the  kind  of  sensation  we  feel 
when  the  skin  is  cut  or  burnt.  We  often  speak  it  is  true,  espe- 
cially in  cases  of  disease  of  the  eye,  of  exposure  to  light  causing 
pain,  but  the  pain  in  such  cases  is  felt  through  the  eyeball,  not 
through  the  retina  and  optic  nerve;  the  pain  is  not  an  excessive 
development  of  visual  sensations,  it  is  a  phase  of  that  sensibility 
which  the  subsidiary  structures  of  the  eye  share,  in  common  as 
we  shall  see  presently,  with  not  only  the  skin  but  nearly  all 
other  structures  of  the  body.  In  like  manner  we  have  no  evi- 
dence that  an  auditory  or  an  olfactory  or  a  gustatory  sensation 
can,  through  mere  intensity,  become  converted  into  a  sensation 
of  pain,  though  we  may,  in  the  act  of  hearing,  smelling  or 
tasting,  receive  sensations  of  pain  from  the  ear,  nose  or  mouth. 
We  often  of  course  apply  the  word  'painful'  to  a  sound,  or  a 
group  of  visual  sensations,  or  a  smell  or  a  taste;  but  that  is  in 
the  sense  of  being  exceedingly  disagreeable,  and  has  reference 
to  our  classification  of  the  complex  psychical  effects  of  all  our 
sensations  into  those  which  are  pleasurable  and  those  which  are 
painful.  Without  entering  into  any  psychological  analysis,  we 
may  assume  that  the  pain  which  we  feel  when  the  finger  is  cut 
is  a  wholly  different  thing  from  the  pain  which  is  given  to  a 
most  delicately  musical  ear  by  even  the  most  horrible  discord; 
and  in  what  follows  we  shall  use  the  word  pain  in  the  first  of 
these  two  meanings. 

§  651.  The  above  considerations  suggest  that  in  the  case  of 
the  skin  as  in  the  cases  of  the  other  organs  of  special  sense,  a 

1044 


Chap,  vi.]         OK   CUTANEOUS   SENSATIONS.  1045 

sensation  of  pain  is  not  simply  an  exaggeration  of  a  sensation 
of  pressure  or  of  a  sensation  of  temperature,  but  is  a  separate 
sensation,  developed  in  a  different  way  in  the  skin,  a  sensation 
which  may  override  and  so  seem  to  replace  the  sensation  of 
pressure  or  temperature  developed  at  the  same  time,  but  which 
must  not  be  confounded  with  it.  And  this  view  derives  support 
from  the  fact  that  events  taking  place  in  many  other  parts  of 
the  body,  from  which  we  experience  sensations  neither  of  touch 
nor  of  temperature,  may  under  favourable  circumstances  give 
rise  to  pain  in  varying  degree.  When,  for  instance,  a  tendon  is 
laid  bare  contact  with  a  body  will  not  produce  tactile  sensations, 
heating  or  cooling  will  not  produce  temperature  sensations ;  one 
cannot  by  means  of  the  tendon  as  one  can  by  means  of  the  skin 
perceive  that  a  rough  or  smooth  body,  that  a  hot  or  cold  body, 
has  been  brought  to  act  upon  it.  Indeed  in  respect  to  all  struc- 
tures other  than  the  skin  and  nerves,  to  such  structures  namely 
as  muscles,  tendons,  ligaments,  bones,  and  the  viscera  generally, 
there  is  a  large  amount  of  experimental  and  clinical  evidence 
shewing-  that,  so  long  as  these  are  in  a  normal  condition,  experi- 
mental stimulation  of  them  does  not  give  rise  to  any  distinct 
change  of  consciousness;  a  muscle  or  a  tendon,  the  intestine, 
the  liver  or  the  heart  may  be  handled,  pinched,  cut  or  cauterized 
without  any  pain  or  indeed  any  sensation  at  all  being  felt  or  any 
signs  given  of  consciousness  being  affected.  Nevertheless  when 
the  parts  are  in  an  abnormal  condition  even  slight  stimulation 
may  produce  a  very  marked  effect  on  consciousness.  If,  for 
instance,  a  tefidon  becomes  inflamed,  any  movement  causing  a 
change  in  the  tendon,  especially  one  putting  the  tendon  on  the 
stretch,  will  affect  consciousness  and  give  rise  to  a  sensation. 
But  the  sensation  is  one  of  pain  and  not  of  any  other  kind. 
Moreover  we  simply  'feel'  the  pain,  we  do  not  'perceive'  the 
cause  of  it;  because  we  feel  the  pain  we  infer  that  something 
has  caused  it,  but  we  cannot  from  tlie  nature  of  the  pain  itself 
decide  whether  that  something  is  a  stretching  of  the  tendon,'the 
contact  of  a  hard  or  soft  body,  the  approach  of  some  hot  or  cold 
body,  the  application  of  some  chemical  substance,  the  passage 
of  an  electric  current,  or  intrinsic  events  taking  place  in  the 
tendon  itself  as  the  result  of  physiological  changes.  And  so  in 
other  instances;  there  is  hardly  a  part  of  the  body  changes  in 
which  may  not,  under  certain  circumstances,  give  rise  to  sensa- 
tions of  pain.  We  can  to  a  variable  extent,  in  a  more  or  less 
ill-delined  manner,  localize  the  sensation;  we  can  distinguish  a 
pain  in  the  foot  from  one  in  the  leg,  a  pain  in  the  tliumb  from 
one  in  a  linger ;  we  may  occasionally  lix  the  pain  in  a  very  small 
limited  area,  though  especially  if  the  sensation  be  intense,  the 
pain  radiates  and  its  localization  becomes  obscure.  And  we 
may  here  remark  that  when  we  thus  localize  a  pain  arising  in 
the  structures  of  which  we  are  speaking,  we  refer  the  pain  not 


1046  ON   CUTANEOUS   AND  [Book  in. 

to  the  structures  themselves  but  to  neighbouring  parts  and 
especially  to  the  skin ;  the  intense  pain,  for  instance,  of  "  renal 
colic,"  caused  by  the  impact  of  a  calculus  in  the  ureter  is  referred 
by  us  not  to  the  ureter  itself  but  to  adjoining  parts,  to  the  cor- 
responding somatic  segment ;  and  so  in  other  instances.  We 
can  also  recognize  certain  characters  in  different  pains,  beyond 
that  of  the  mere  degree  of  intensity ;  we  speak  of  pains  as  being 
burning,  aching,  gnawing,  cutting,  throbbing  and  the  like.  But 
in  all  cases  the  pain  remains  a  mere  sensation ;  when  it  comes, 
all  we  can  say  is  that  we  feel  in- a  particular  region  of  the  body 
a  pain  of  a  certain  intensity  and  having  a  certain  character.  We 
infer  that  something  is  wrong,  but  the  pain  in  no  way  tells  us 
what  the  wrong  is ;  we  may  call  the  pain  a  burning  one  because 
it  is  more  or  less  like  the  pain  which  we'  feel  when  the  skin  is 
burnt ;  but  in  the  vast  majority  of  cases  heat  has  nothing  what- 
ever to  do  with  pains  of  a  burning  character ;  and  so  with  other 
kinds  of  pain,  the  character  of  the  pain  does  not  in  itself  tell  us 
anything  about  its  cause. 

Are  we  then  to  regard  pain  as  a  sensation  of  a  kind  by  itself, 
the  very  threshold  of  which,  the  very  least  amount  of  which  that 
can  in  any  way  affect  our  consciousness,  must  be  regarded  as 
already  pain  ?'  In  attempting  to  answer  this  question  the  follow- 
ing considerations  deserve  attention. 

We  are  in  a  certain  obscure  way  aware  of  what  we  may  call 
the  general  condition  of  our  body.  To  put  an  extreme  case,  if 
the  whole  of  our  abdominal  viscera  were  removed  we  should  be 
aware  of  the  loss.  We  should  be  aware  of  this  through  more 
ways  tlian  one.  The  tactile  sensations  from  the  abdominal  skin 
would  be  in  such  a  case  different  from  the  normal,  and  moreover 
the  muscular  sense  of  the  abdominal  walls  and  of  all  the  muscles 
whose  actions  bear  on  the  abdomen,  would  make  us  aware  of  the 
void.  But  beyond  all  these  indirect  ways,  it  is  probable  that 
we  should  in  a  more  or  less  obscure  manner  be  directly  conscious 
of  the  loss.  It  is  probable  that  sensory  impulses,  not  of  the 
character  of  pain,  are  continually,  or  from  time  to  time,  passing 
upwards  from  the  abdominal  viscera  to  the  central  nervous  sys- 
tem. These  do  not  affect  our  consciousness  in  such  a  distinct 
manner  as  to  enable  us  to  examine  them  psychologically  in  the 
same  way  that  we  are  able  to  examine  special  sensations  such  as 
those  of  sight,  or  even  sensations  of  pain ;  they  are  even  less 
well  defined  than  those  of  the  muscular  sense  ;  nevertheless  they 
do  enter,  though  obscurely,  into  our  consciousness,  so  that  we 
become  aware  of  any  great  change  in  them,  and  they  have 
been  spoken  of  under  the  title  of  "common"  or  "general  sensi- 
bility." In  discussing  the  manner  in  which  the  manifold  coor- 
dinate movements  of  the  body  were  carried  out  we  saw  reasons 
for  thinking  that  the  central  processes  of  the  nervous  sys- 
tem were  largely  determined  by  varied  afferent  impulses  which 


CiiAP.  VI.]  SOME   OTHER   SENSATIONS.  1047 

produced  their  effects  without  giving  rise  to  any  sharp  and 
decided  change  of  consciousness ;  many  of  these  are  probably 
afferent  impulses  of  the  common  sensibility  of  which  we  are 
now  speaking. 

If  we  suppose  that  the  skin  in  common  with  the  other  tissues 
of  the  body  possesses  this  common  sensibility,  and  if  we  further 
suppose  that  in  the  skin  as  elsewhere,  these  afferent  impulses 
when  developed,  as  is  the  case  under  normal  circumstances,  to  a 
slight  extent  only  are  not  distinctly  recognized  by  consciousness, 
and  that  when  they  do  assume  such  a  magnitude  or  intensity  as 
to  break  in  upon  consciousness  the  change  of  consciousness 
which  they  produce  is  of  the  kind  which  we  call  pain,  we  reach 
a  conclusion  which  is  also  supported  by  other  considerations. 
On  the  one  hand  such  a  view  is  in  accord  with  the  conclusion 
that  cutaneous  sensations  of  pain  are  wholly  distinct  from  and 
developed  in  a  wholly  different  way  from  sensations  of  touch 
and  temperature ;  and,  as  we  shall  .see,  to  this  conclusion  we 
are  led  by  several  different  arguments.  On  the  other  hand  it 
relieves  us  from  the  following  difficulty.  It  may  happen  to  a 
man  to  suffer  pain  in  a  particular  region  or  tissue  of  the  body, 
once  only  in  the  course  of  his  lifetime  or  possibly  not  even  once  ; 
nay,  we  may  suppose  that  in  this  or  that  region  or  tissue  pain  is 
felt  once  only  in  one  individual  among  a  large  number  of  per- 
sons. If  we  suppose  that  pain  is  not  as  suggested  above  an 
excessive  phase  of  something  which  is  continually  going  on  in 
a  lower  phase,  but  a  something  by  itself  quite  distinct  from  all 
other  sensations,  we  are  driven  to  conclude,  since  such  a  sensa- 
tion must  have  a  special  mechanism,  including  special  afferent 
nerve  fibres  to  carry  it  out,  that  in  the  case  in  question  such  a 
mechanism  of  pain  has  been  preserved  intact  but  unused  through 
whole  generations  in  order  that  it  may  once  in  a  while  come 
into  use  ;  which  is  in  the  highest  degree  improbable.  This  diffi- 
culty disappear-!  if  we  suppose  that  the  constantly  smouldering 
embers  of  common  sensibility  may  be  at  any  moment  fanned  into 
the  flame  of  pain. 

We  may  conclude  then  that  the  skin  in  common  with  other 
tissues  possesses  common  sensibility,  and  that  when  this  is  ex- 
cited in  excess,  so  as  to  distinctly  affect  consciousness,  we  call 
it  pain.  We  thus  experience  through  the  skin  three  kinds  of 
sensations,  those  of  touch,  of  temperature,  and  of  common  sensi- 
bility, but  the  two  former  only  are  developed  by  further  psychical 
processes  into  perceptions ;  it  is  by  them  alone  that  we  obtain 
through  the  skin  knowledge  of  external  objects. 

§  652.  There  is  another  consideration  to  be  taken  into  view. 
The  agents  which  applied  to  the  skin  produce  pain,  act  violently 
on  the  skin,  in  many  cases  injuring  the  epidermis  and  affecting 
the  dei-mis.  Moreover  if  the  epidermis  be  removetl,  and  the 
stimulus,  mechanical,  thermal  or  chemical,  be  applied  to  the 


1048  0^   CUTANEOUS  AND  [Book  in. 

dermis  or  to  the  nerves  running  in  it,  we  still  experience 
sensations  of  pain,  though  no  longer  those  of  touch  and  tem- 
perature; when  a  sharp  or  hot  body  is  made  to  touch,  not 
the  intact  skin  but  a  wound,  we  suffer  pain,  but  do  not  rec- 
ognize the  sharpness  or  the  heat  which  is  causing  the  pain. 
This  suggests  that  the  special  sensations  of  touch  and  tem- 
perature are  brought  about  by  special,  epithelial  structures 
serving  as  the  differentiated  ends  of  nerve  fibres,  but  that  com- 
mon sensibility  and  pain  need  no  such  special  endings ;  this 
however  opens  up  questions  which  we  must  consider  separately 
by  themselves. 

§  653.  Hunger  and  thirst.  We  may  introduce  here  the  few 
words  that  we  have  to  say  concerning  two  affections  of  con- 
sciousness, which  may  perhaps  be  considered  as  kinds  of  sensa- 
tion, namely,  hunger  and  thirst. 

We  refer  our  feelings  of  thirst  to,  or  at  least  we  associate 
them  with,  a  particular  condition  of  the  mucous  membrane  of 
the  mouth,  especially  of  the  soft  palate.  When  the  mucous 
membrane  of  this  region  becomes  drier  than  normal,  as  for  in- 
stance by  being  exposed  to  too  great  an  evaporation,  we  feel 
'thirsty,'  and  the  feeling  is  at  once  removed  by  adequately 
moistening  the  membrane.  Under  ordinary  circumstances  how- 
ever the  condition  of  thirst  is  brought  about,  not  bj  anything 
bearing  specially  or  exclusively  on  the  mucous  membrane  of  the 
soft  palate  or  even  of  the  whole  mouth,  but  by  the  diminution 
of  the  water  present  in  the  body  either  through  restriction  of 
the  intake,  or  through  excess  of  the  output  in  the  secretions, 
such  as  that  of  sweat,  or  through  both  together.  This  is  often 
spoken  of  as  diminution  of  the  water  of  the  blood ;  but  most 
probably  the  specific  gravity  of  the  blood  is  kept  constant  by 
the  withdrawal  of  water  from  the  lymph,  so  that  the  loss  falls 
on  the  latter  fluid.  Such  a  diminution  of  the  water  of  the  body 
ma}^  be  brought  about  by  circumstances  such  as  excessive  sweat- 
ing which  in  themselves  do  not  cause  special  drjaiess  of  the 
mucous  membrane  of  the  soft  palate ;  this  part  then  under- 
goes a  loss  of  water  in  common  with  the  other  tissues,  but 
not  in  a  special  degree.  Nevertheless  thirst  thus  brought  about 
may  be  temporarily  assuaged  by  simple  moistening  of  the  soft 
palate.  From  this  we  may  infer  that  the  sensation  of  thirst  is 
brought  about  by  afferent  sensory  impulses  started  in  the 
mucous  membrane  of  the  soft  palate  by  a  deficiency  of  water  in 
that  membrane,  perhaps  by  a  drain  on  the  lymph  spaces  of  that 
membrane. 

We  are  in  the  habit  of  assuaging  thirst  by  drinking  water, 
or  watery  fluids,  and  in  doing  so  produce  both  a  direct  local 
effect  on  the  palate  and  a  general  indirect  effect  on  the  body. 
In  the  absence  of  the  local  effect,  the  indirect  effect  is  slow  in  com- 
ing and  needs  a  large  quantity  of  fluid  ;  when  in  cases  of  gastric 


Chap,  yi.]  SOME   OTHER   SENSATIOXS.  1049 

fistula  water  is  introduced  into  the  stomach  through  the  fistulous 
opening,  large  c^uantities  may  be  given  before  thirst  is  assuaged. 
The  sensation  of  hunger  is  in  a  somewhat  similar  manner  re- 
ferred to,  or  associated  with,  the  condition  of  the  gastric  mucous 
membrane.  We  feel  hungry  when  the  stomach  is  empty.  But 
even  more  distinctly  than  in  the  case  of  thirst  the  main  cause  of 
the  sensation  seems  to  be  a  general  condition  of  the  body,  namely, 
that  produced  by  the  products  of  digestion  ceasing  to  be  thrown 
into  the  blood.  The  sensation  is  not  due  to  the  mere  emptiness 
of  the  stomach,  tliough  the  emptiness  of  the  stomach  is  one  of 
the  results  of  the  abstinence  from  food  ,  for  the  feeling  of  hunger 
may  disappear  though  the  stomach  may  remain  empty,  if  ade- 
quate nourishment  be  conveyed  in  other  ways,  as  by  injection 
into  the  bowels;  conversely  even  we  ourselves  may  under  ab- 
normal conditions  feel  hungry  on  a  full  stomach,  and  in  some 
animals,  herbivora,  the  stomach  is  always  more  or  less  full. 
The  sensation  however  docs  seem  to  be  in  some  way  specially 
connected  with  the  condition  of  the  gastric  walls,  much  in  the 
same  way  that  thirst  is  specially  connected  with  the  palate ;  the 
products  of  digestion  have  a  much  greater  power  in  appeasing 
hunger  when  they  act  locall}^  and  directly  on  the  gastric  mem- 
brane than  when  they  are  simply  brought  to  bear  on  the  body 
at  large,  and  a  small  quantity  of  food  will  immediately  satisfy 
hunger  when  introduced  into  the  stomach,  though  it  will  have 
no  effect  when  introduced  otherwise.  Moreover  our  own  con- 
sciousness clearly  connects  the  sensation  in  some  way  or  other 
with  the  stomach. 

As  to  what  is  the  particular  change  in  the  gastric  membrane 
which  thus  gives  rise  or  assists  in  giving  rise  to  the  sensation  we 
know  little  or  nothing ;  indigestible  substances  such  as  cannot 
be  properly  called  food  when  taken  into  the  stomach  at  least 
temporarily  remove  the  sensation.  And  we  have  little  or  no 
knowledge  as  to  the  particular  nerves  wdiich  serve  as  the  paths 
for  the  afferent  impulses  which  we  may  suppose  to  be  generated 
in  the  gastric  membrane.  Division  of  the  vagus  nerve  on  both 
sides  is  said  to  have  no  effect  on  hunger,  from  this  we  may  con- 
clude that  the  impulses  do  not  pass  up  this  nerve,  though  it  ap- 
pears to  be  the  sensory  nerve  of  the  stomach.  But  we  have  no 
evidence  that  the  impulses  pass  along  the  sympathetic  nerves. 

Allied  somewhat  to  liunger  is  the  peculiar  feeling  which  we 
may  perhaps  also  speak  of  as  a  sensation,  known  as  nausea,  the 
precursor  of  vomiting  and  brought  about  like  vomiting  by  a 
variety  of  events.  We  have  little  or  no  knowledge  of  it  viewed 
as  a  sensation. 

The  affection  of  consciousness  which  is  produced  by  the  form 
of  cutaneous  stimulation  known  as  "  tickling "  is  of  a  peculiar 
character,  differing  from  tactile  sensations.  Indeed  it  is  prob- 
ably undesirable  to  speak  of  it  or  of  other  like  psychical  effects 


1050  ON   CUTANEOUS   SENSATIONS.  [Book  in. 

of  cutaneous  stimulation  as  a  sensation,  since  it  seems  to  be  not 
the  direct  effect  of  the  sensory  cutaneous  impulses,  which  are 
probably  ordinary  tactile  impulses,  but  rather  the  effect  on  con- 
sciousness of  changes  in  the  central  nervous  system  brought 
about  by  those  sensory  impulses. 


SEC.  3.  ON  THE  MODE  OF  DEVELOPMENT  OF 
CUTANEOUS  SENSATIONS. 

§  654.  Our  studies  so  far  point  to  the  conclusion  that  sensa- 
tions of  touch  and  temperature  stand  on  the  same  footing  as 
visual,  auditory  and  other  special  sensations ,  and  it  will  be  profit- 
able now  to  compare  in  some  detail  the  former  with  the  latter. 
In  doing  so  we  may,  in  order  to  make  the  matter  more  simple, 
confine  ourselves  in  the  first  instance  to  sensations  of  touch 
proper,  that  is  to  sensations  of  mere  contact  and  pressure,  dis- 
cussing later  on  the  relations  of  these  to  sensations  of  heat  and 
cold. 

In  studying  vision  we  came  to  the  conclusion  that  the  undula- 
tions of  the  ether  so  affect  the  rods  and  cones  and  other  retinal 
structures  as  to  give  rise  to  visual  impulses,  and  tliat  these  visual 
impulses,  travelling  up  the  fibres  of  the  optic  nerve  to  the  visual 
centres,  gave  rise  by  means  of  those  centres  to  the  affections  of 
consciousness  which  we  call  visual  sensations  ;  we  may  leave 
aside  in  the  present  instance  all  reference  to  the  complexity  of 
the  visual  centres. 

We  obtained  absolute  proof  that  the  only  way  in  which  light 
can  give  rise  to  visual  impulses  in  the  optic  fibres  is  by  acting  on 
the  retinal  structures.  Since  the  optic  fibres  are  the  only  nerve 
fibres  in  direct  connection  with  the  retinal  structures  visual 
impulses  can  be  carried  by  them  alone.  As  we  pointed  out  we 
know  absolutely  nothing  about  the  nature  of  visual  impulses 
themselves  ;  our  conclusions  concerning  the  various  characters 
and  kinds  of  visual  impulses  are  simply  deductions  from  the  psy- 
cliological  examination  of  our  sensations:  our  objective  know- 
ledge" of  them  is  linfited  to  the  fact  that  when  light  falls  on  a 
functionally  active  retina  an  electric  change  is  developed  in  the 
optic  fibres.  As  we  mentioned  in  §  553  the  statement  that  stim- 
ulation of  the  optic  fibres  themselves,  as  when  the  optic  nerve  is 
cut  Avith  a  knife,  gives  rise  to  visual  sensations,  has  led  to  the 
adoption  of  the  view  that  any  impulse  passing  along  the  optic 
fibres,  however  started,  whether  by  the  action  of  light  on  the 
retina,  or  by  direct  stimulation  of  the  fibres  themselves,  gives  rise 

1051 


1052  ON   CUTANEOUS  AND  [Book  iii. 

to  a  visual  sensation  and  must  therefore  be  regarded  as  a  visual 
impulse.  This  view,  under  the  title  of  the  doctrine  of  "  the  spe- 
cific energy  of  nerves,"  has  been  extended  to  the  nerves  of  the 
other  special  senses  and  indeed  to  nerves  in  general.  This  doc- 
trine teaches  that,  owing  either  to  the  constitution  of  the  central 
ending  of  a  sensory  fibre  or  to  that  combined  with  the  nature  of 
the  fibre  itself  (the  view  may  also  be  adapted  to  motor  fibres), 
whatever  impulses  are  generated  in  the  fibre  can  give  rise  to  those 
events  only  which  are  specific  to  that  central  ending,  impulses 
of  all  kinds  along  an  optic  fibre  giving  rise  to  visual  sensations, 
impulses  of  all  kinds  along  an  auditory  fibre  giving  rise  to  audi- 
tory sensations,  and  so  on.  Hence  under  this  view  the  purpose 
of  the  specific  terminal  organ  is  simply  to  allow  the  specific 
stimulus  of  the  sense,  light  in  the  case  of  the  retina,  to  develope 
impulses  in  the  specific  nerve,  a  result  which,  in  the  absence  of 
the  terminal  organ,  it  is  powerless  to  achieve.  We  saw  however 
(§  553)  that  according  to  some  observers  direct  stimulation  of 
the  optic  fibres,  as  when  the  nerve  is  cut,  does  not  produce  visual 
sensations,  and  therefore  does  not  give  rise  to  visual  impulses ; 
so  far  as  can  be  ascertained  such  a  stimulation  of  the  fibres 
appears  to  produce  no  effect  at  all  on  the  central  nervous  system. 
In  arguing  from  this  result  we  must  remember  that  the  optic  nerve 
is  not  a  true  nerve  and  that  its  fibres  are  not  comparable  with  the 
fibres  of  a  true  nerve ;  the  optic  nerve  is  a  part  of  the  brain,  and 
its  fibres  are  analogous  to  the  internuncial  fibres  of  the  white  mat- 
ter of  the  central  nervous  system,  concerning  which  as  we  saw 
(§  510)  the  view  has  been  urged  that  they  are  not  capable  of 
being  stimulated  directly.  Neglecting  however  this  view  which 
is  at  best  very  doubtful,  we  are  led,  if  we  accept  the  insensitive- 
ness  of  the  optic  nerve  to  direct  stimulation  as  true,  to  modify 
the  doctrine  of  the  specific  energy  of  nerves  in  the  following  way. 
We  must  suppose  that  the  visual  centres  are  so  constituted  that 
they  are  stirred  up  to  the  development  of  visual  sensations  by  the 
advent  only  of  those  kind  of  impulses  which  are  started  by  means 
of  the  terminal  organ.  Since  electric  changes  are  developed  in 
the  optic  fibres  as  in  other  nerve  fibres  when  the  optic  fibres  are 
directly  stimulated,  we  may  infer  that  direct  stimulation  does  lead 
to  nervous  impulses ;  and  we  may  further  infer  that  these  reach 
the  visual  centres  but  are  unable  to  develope  visual  sensations 
because  they  are  not  true  visual  impulses  such  as  are  generated 
by  help  of  the  terminal  organs. 

The  facts  which  we  mentioned  in  speaking  of  hearing  (§  628) 
as  seeming  to  shew  that  the  fibres  of  the  auditory  nerve  in  the 
absence  of  the  labyrinth  may  be  directly  stimulated  by  sound,  if 
we  accept  them  as  valid,  afford  a  strong  support  to  the  simpler 
conception  of  the  specific  energy  of  nerve  fibres.  On  the  other 
hand  the  modified  view  is  supported,  though  the  support  is  of 
a  negative  kind  only,  by  the  behaviour  of  the  other  organs  of 


Chap,  vi.]  SOME   OTHER   SENSATIONS.  1053 

special  sense.  We  have  no  satisfactory  experimental  or  other 
evidence  that  stimulation  of  the  olfactory  fibres  otherwise  than 
through  the  terminal  organs  will  give  rise  to  olfactor}^  sensations. 
We  have  evidence  that  stimulation  of  the  centres  by  various 
means  will  give  rise  to  the  specific  sensations,  but  not  that  stim- 
ulation of  the  fibres  of  the  nerves  themselves  will.  The  brandies 
of  the  glossopharyngeal  and  fifth  nerves  distributed  to  the  organs 
of  taste  are,  unlike  the  above,  mixed  nerves,  and  when  they  are 
stimulated  sensations  other  than  specific  taste  sensations  are  also 
developed,  and  the  former  might  obscure  the  latter;  still  the 
evidence  so  far  as  it  goes  supports  the  view  that  stimulation  of 
gustator}^  fibres  otherwise  than  through  their  terminal  organs 
does  not  lead  to  the  development  of  gustatory  sensations.  In 
the  case  of  touch  the  evidence  is  perhaps  still  stronger.  We 
must  in  any  case  suppose  that  each  cutaneous  nerve  distributed 
to  a  given  area  of.  skin  contains  fibres  which  subserve  the  sense 
of  touch  exercised  by  that  area,  and  which  pass  from  the  terminal 
organs  in  that  area,  whatever  their  nature,  to  the  parts  of  the 
central  nervous  system,  whatever  they  may  be  (§  505),  which  act 
as  centres  of  touch  sensations.  If  these  fibres  when  directly  stim- 
ulated, apart  from  their  terminal  organs,  necessarily  give  rise  to 
touch  sensations,  stimulation  of  the  nerve  itself  while  running  in 
the  subcutaneous  tissue  should  give  rise  to  touch  sensations.  But 
experience  shews,  as  we  said  a  little  while  ago,  that  this  is  not  the 
case.  Whenever  the  nerve  fibres  themselves  are  directl}-  stimu- 
lated, as  for  instance  when  the  epidermis  is  removed  from  the 
skin  or  when  a  nerve  is  laid  bare,  then  however  the}'  be  stimu- 
lated, be  the  stimulus  weak  or  strong,  if  consciousness  be  affected 
at  all,  the  affection  takes  on  the  form  of  pain  ;  psychological 
examination  of  the  subjective  result  discloses  nothing  that  can 
be  called  a  sensation  of  touch.  A  familiar  instance  of  the  dif- 
ference between  the  effects  of  stimulating  a  nerve  trunk,  and 
those  of  stimulating  the  cutaneous  terminal  organs  of  special 
sense,  is  seen  in  the  effect  of  dipping  the  elbow  into  a  freezing 
mixture.  The  cold  affects  the  skin  of  the  elbow  and  gives  rise  to 
sensations  of  cold  in  that  part ;  but  the  cold,  if  intense  enougli, 
also  affects  the  underlying  trunk  of  the  ulnar  nerve,  and  by 
direct  stimulation  of  the  fibres  in  the  trunk  developes  sensory 
impulses  ;  these  impulses  however  are  those  not  of  sensations  of 
cold,  but  of  pain  ;  and  the  pain,  in  accordance  with  a  principle 
to  which  we  shall  presently  call  attention,  is  referred  to  the  ter- 
minal distribution  of  the  ulnar  nerve  on  the  ulnar  side  of  the  hand 
and  arm.  In  speaking  above  (§  651)  of  pain  we  said  that  exces- 
sive pressure  or  excessive  heat  or  excessive  cold  applied  to  the 
skin,  overrides  or  annuls  pressure  and  temperature  sensations 
and  gives  rise  to  mere  sensations  of  pain ;  and  it  might  be  urged 
that  when  a  nerve  is  directly  stimulated  the  specific  sensations 
of  touch  and  temperature  are  similarly  annulled.     Rut  in  the 


1054  ON  CUTANEOUS   AND  [Book  m. 

case  of  the  skin  an  excessive  or  violent  stimulation  is  necessary 
to  produce  this  effect,  whereas  a  nerve  may  be  directly  stimulated 
by  so  slight  a  stimulus  as  to  give  rise  to  hardly  more  than  dis- 
comfort without  distinct  pressure  or  temperature  sensations  being 
felt ;  and  we  can  hardly  suppose  that  in  such  a  case  these  are 
present  but  are  annulled  by  an  amount  of  pain  so  slight  as  that 
which  is  produced.  Thus  making  every  allowance  for  the  sug- 
gestion that  sensations  of  pain  may  override  and  obscure  con- 
comitant sensations  of  touch  and  temperature,  Ave  seem  driven 
to  the  conclusion  that  the  latter  sensations  can  only  be  developed 
by  help  of  special  terminal  organs,  and  that  a  stimulation  of  the 
nerve  fibres  themselves  if  it  produces  any  effect  at  all  on  con- 
sciousness gives  rise  to  pain,  and  to  pain  alone. 

We  are  in  this  way  led  to  conceive  of  the  skin  as  provided 
on  the  one  hand  with  specific  fibres  ending  in  specific  terminal 
organs  and  serving  for  sensations  of  touch  and  temperature, 
and  on  the  other  hand  with  fibres  of  common  sensibility  having 
no  such  specific  terminal  organs,  the  two  kinds  of  fibres  being 
mixed  together  in  the  common  cutaneous  nerve.  These  fibres 
moreover  have  not  only  different  peripheral  but  also  different 
central  endings,  and  during  at  least  some  part  of  their  course 
run  in  different  tracts  or  in  a  different  manner  in  the  central 
nervous  system ;  for  as  we  saw  in  treating  of  the  central 
nervous  system  (§  508)  cases  of  disease  of  the  central  nervous 
system  have  been  recorded  in  which  over  certain  cutaneous 
areas  sensations  of  touch  had  been  lost,  while  common  sensi- 
bility and  sensations  of  pain  remained,  or  vice  versa.  We  may 
add  that  the  difference  between  the  central  paths  or  endings  of 
the  nerves  of  touch  and  those  of  pain  is  further  shewn  by  the 
fact  that  in  certain  nervous  diseases  (tabes)  when  the  skin  is 
pricked  with  a  pin,  the  contact  of  the  pin  may  be  felt  as  mere 
touch  for  so  long  a  time  as  one  or  two  seconds  before  pain  is  felt; 
the  diseased  condition  enormously  delays  the  transmission  of  the 
impulses  of  pain  but  has  not  so  much  effect  on  those  of  touch. 

§  655.  We  may  go  a  step  further;  there  is  a  certain  amount 
of  evidence  that  the  terminal  organs  and  fibres  concerned  in 
touch  proper,  in  sensations  of  pressure,  are  different  and  separate 
from  those  concerned  in  sensations  of  heat  and  cold.  In  the 
first  place  the  general  topographical  distribution  over  the  sur- 
face of  the  body  of  sensitiveness  to  pressure  is  different  from 
that  of  sensitiveness  to  temperature.  A  familiar  instance  of 
this  is  seen  in  bringing  the  palm  of  the  hand  to  touch  the  fore- 
head. In  the  former  the  sense  of  touch  is  highly  developed,  in 
the  latter  the  sense  of  temperature ;  hence  with  the  forehead 
we  feel  that  the  hand  is  warm  or  cold,  with  the  hand  we  feel 
that  the  forehead  is  rough  or  smooth ;  at  least  these  two  feel- 
ings respectively  preponderate,  the  one  in  the  one  part,  the  other 
in  the  other.     In  the  second  place,  if  the  stimulation  of   the 


Chap,  vi.]  SOME    OTHER    SENSATIONS.  lOoo 

skill  be  confined  to  extremely  minute  areas,  if  the  pressure,  or 
the  change  of  temperature  be  brought  to  bear  as  much  as  pos- 
sible on  a  mere  point  of  the  skin,  it  is  found  that  some  points 
of  the  skin  are  sensitive  to  pressure  but  not  to  change  of 
temperature,  while  others  again  are  sensitive  to  change  of 
temperature  but  not  to  pressure.  If  a  blunt  pointed  but  other- 
Avise  fine  needle  be  used  to  exert  pressure,  a  little  exploration 
will  ascertain  that  at  some  points  the  amount  of  pressure  can 
readily  be  recognized,  the  sense  of  touch  is  acute,  while  at  other 
points,  and  these  may  be  quite  near  the  others,  the  amount  of 
pressure  cannot  be  recognized,  and  indeed  no  sensation  is 
experienced  until  the  pressure  is  excessive  and  then  the  sensa- 
tion felt  is  not  one  of  touch  proper  but  of  pain.  Similarly  if 
heat  or  cold  be  applied  by  means  of  a  metal  tube  or  rod  nar- 
rowed to  a  fine  point,  it  will  be  found  that  some  points  of  the 
skin  are  very  sensitive  to  changes  of  temperature,  while  other 
points  are  insensitive  to  temperature,  the  application  of  heat  or 
cold  giving  rise  to  pain  onl}^  and  not  to  specific  sensations  of 
heat  or  cold.  Further,  the  points  of  the  skin  which  are  sensitive 
to  pressure  are  those  which  are  not  sensitive  to  heat  or  cold,  and 
vice  versa.  Such  results  as  these  are  only  intelligible  on  the 
supposition  that  the  terminal  organs  for  pressure  are  different 
from  those  for  heat  and  cold  and  differently  distributed  over 
the  surface  of  the  skin. 

§  656.  The  punctiform  method  of  exploring  the  sensitiveness 
of  the  skin  has  further  led  to  a  result  which  is  unexpected  and 
indeed  presents  difficulties.  Heat  and  cold  in  themselves  differ 
only  in  degree ;  they  are  positive  and  negative  phases  of  the 
same  thing.  We  should  therefore  naturally  expect  that  the  same 
terminal  organs  would  be  employed  for  sensations  both  of  heat 
and  of  cold,  and  that  the  same  points  of  the  skin  would  be  alike 
sensitive  both  to  heat  and  to  cold.  But  the  results  of  experi- 
mentation by  the  method  in  question  contradict  this  expectation. 
It  is  found  that  some  points  are  sensitive  to  heat,  that  is  to  say 
a  sensation  is  developed  when  the  temperature  of  the  point  of 
the  skin  is  raised  above  what  it  happens  to  be  at  the  time  of 
experimenting,  but  are  not  sensitive  to  cold,  that  is  to  say  no 
sensations  are  develoi:)ed  when  the  temperature  of  the  point  of 
the  skin  is  lowered  below  what  it  happens  to  be  at  the  time 
of  experimenting;  and  other  points  may  similarly  be  found  to 
be  sensitive  to  cold  but  not  to  heat.  Moreover  this  result  is 
in  accord  with  results  gained  otherwise.  If  the  arm  or  leg  be 
"sent  to  sleep  "  by  pressure  on  the  brachial  or  sciatic  nerves  the 
skin  will  be  found  at  a  certain  stage  to  be  little  sensitive  to 
warmth  though  distinctly  sensitive  to  cold.  So  also  the  whole  sur- 
face of  the  glans  penis,  in  contrast  to  the  prepuce,  is  very  sliglitly 
sensitive  to  cold,  but  distinctly  sensitive  to  warmth.  ^Moreover 
cases  of  disease  of  the  central  nervous  system  have  been  recorded 


1056  OX  CUTANEOUS   AND  [Book  in. 

in  whicli  the  skin  of  a  limb  was  sensitive  to  warmth,  that  is  to 
degrees  of  temperature  above  that  of  the  limb,  but  insensitive 
to  cold.  It  may  be  remarked  that  in  these  cases,  as  in  that  of 
the  limb  "  gone  to  sleep,"  the  sensations  of  touch  proper  and  of 
cold  seem  to  run  together  and  sensations  of  pain  and  of  heat 
also  to  run  together. 

It  seems  probable  then  from  these  considerations  that  we 
possess  three  sets  of  terminal  organs  and  three  sets  of  fibres, 
one  for  pressure,  a  second  for  heat  and  a  third  for  cold.  It 
must  be  borne  in  mind  however  that  the  three  sensations  are 
not  wholly  independent,  since  sensations  of  pressure  are  modi- 
fied if  changes  in  temperature  be  taking  place  at  the  same  time 
in  the  same  spot  of  skin.  Thus  a  penny  cooled  down  nearly 
to  zero  and  placed  on  the  forehead  will  be  judged  by  most 
people  to  be  as  heavy  or  even  heavier  than  two  pennies  of  the 
temperature  of  the  forehead  itself,  that  is  to  say  the  sensation 
of  pressure  is  increased  by  a  concomitant  sensation  of  cold ;  and 
a  similar  modification  of  the  sensation  of  pressure  is  also  often 
observed  when  the  object  pressing  is  not  colder  but  warmer  than 
the  skin  pressed  on.  A  similar  effect  seems  to  be  shewn  in  certain 
cases  of  disease  of  the  central  nervous  system  in  which  it  has 
been  recorded  that  a  hot  body  such  as  a  heated  spoon  was  felt 
when  brought  in  contact  with  the  skin,  though  the  same  spoon 
applied  at  the  temperature  of  the  skin  itself  produced  no  sensa- 
tion at  all,  and  the  heated  spoon  was  recognized  not  as  a  hot 
body,  but  simply  as  something  touching  the  skin.  The  exact 
explanation  of  these  facts  is  not  very  clear,  but  it  may  perhaps 
be  argued  that  the  effect  is  brought  about  amid  the  central  proc- 
esses through  which  the  sensations  are  developed  and  does  not 
shew  that  the  sensations  have  common  terminal  organs. 

§  657.  In  attempting  to  understand  the  nature  of  the  periphe- 
ral events  through  which  the  sensory  impulses  giving  rise  to 
sensations  of  pressure  of  heat  and  of  cold  are  developed  two  or 
or  three  matters  must  be  borne  in  mind.  In  the  first  place,  as 
we  have  already  said,  though  the  skin  has  a  temperature  of  its 
own,  we  are  not  directly  conscious  of  that,  or  at  all  events  are 
not  distinctly  conscious  of  it  in  the  same  way  that  we  become 
conscious  of  any  sudden  change  in  that  temperature  ,*  nor  indeed 
are  we,  except  in  extreme  cases,  distinctly  conscious  that  the  tem- 
perature of  one  region  differs  from  that  of  another,  or  that  the 
temperature  of  the  same  region  gradually  varies  from  time  to 
time.  It  would  seem  as  if  the  development  of  a  clear  and  dis- 
tinct sensation  was  largely  dependent  on  the  contrast  as  to  tem- 
perature between  an  area  of  the  skin  and  surrounding  areas ; 
and  indeed  we  have  already  pointed  out  the  marked  effects  of 
contrast.  The  same  applies  to  pressure ;  we  are  not,  at  least 
distinctly  and  directly,  conscious  of  the  uniform  pressure  of  the 
atmosphere  over  the  whole  surface  of  the  body,  when  we  stand 


Chap,  vi.]  SOME    OTHER    SENSATIONS.  1057 

naked  in  still  air.  We  are  not  however  justified  in  assuming 
that  under  the  above  circumstances  nothing  whatever  is  taking 
place  in  the  sensory  nerves  of  the  skin,  that  when  we  feel  a  sensa- 
tion the  change  in  the  sensory  apparatus  (using  that  phrase  in 
its  widest  sense  to  include  both  peripheral  and  central  parts) 
is  one  from  absolute  quiescence  to  activity ;  it  is  not  impossible, 
and  some  facts  indeed  seem  to  suggest,  that  even  when  we  feel 
no  distinct  cutaneous  sensations,  afferent  impulses  still  continue 
to  stream  onwards  from  the  periphery  to  the  central  nervous 
system,  supplying  as  it  were  a  groundwork  of  nervous  events 
which  enter  largely  in  various  ways  into  the  conduct  of  the 
whole  body,  but  which  do  not  distinctly  affect  consciousness. 
If  this  be  so,  we  may  infer  that  the  affection  of  consciousness  which 
we  call  a  sensation  is  the  immediate  effect  of  an  adequately 
large  change  in  this  groundwork,  rather  than  of  a  set  of  quite 
new  isolated  impulses  passing  straight  up  from  the  peripheral 
organ  to  the  "seat  of  consciousness." 

In  the  second  place  when  we  do  experience  sensations  of 
temperature  the  sensation  is  caused  not  by  the  mere  change  of 
temperature  but  by  the  altered  condition  of  the  skin  which 
results  from  that  chang^e.  When  an  area  of  the  skin  havinof 
a  normal  temperature  is  brought  in  contact  with  a  cold  body, 
the  skin  undergoes  a  change  fi'om  a  normal  to  a  lower  tempera- 
ture, and  we  experience  a  sensation  of  cold.  Now,  if  it  Avere 
only  the  change  from  a  normal  to  a  lower  temperature  which 
gave  rise  to  the  sensation,  though  the  sensation  might  and  prob- 
ably would  last  much  longer  than  the  change  itself,  it  could  not 
be  prolonged  by  the  mere  maintenance  of  the  lower  temperature 
when  once  the  change  had  been  established.  But  experience 
shews  that  it  is ;  we  still  feel  a  sensation  of  cold,  at  a  time  when 
the  contact  of  the  cold  body  is  not  producing  any  further  lower- 
ing of  temperature  and  at  most  is  only  maintaining  the  lower 
temperature  already  brought  about.  Nay,  more,  the  sensation  of 
cold  continues  after  the  cooling  body  has  been  removed,  at  the 
time  when  the  skin  is  returning  to  its  normal  temperature,  that 
is  to  say  is  undergoing  the  very  opposite  change  of  temperature, 
namely,  one  from  cold  to  heat.  And  the  same  considerations 
apply  to  sensations  of  heat. 

§  658.  We  may  conclude  then  that  when  the  application  of 
cold  or  of  heat  to  the  skin  causes  a  sensation  of  cold,  the  cold 
or  heat  produces  a  condition  in  the  material  of  the  skin,  which 
condition  starts  nervous  impulses  in  the  afferent  nerves  of  cold 
and  heat  sensations.  Since  the  application  of  cold  or  of  heat  to 
the  nerve  fibres  underlying  the  skin  does  not  produce  a  sensation 
of  cold  or  heat,  but  only  a  sensation  of  pain,  we  may  further 
conclude  that  the  material  whose  condition  starts  the  sensation 
is  placed  in  the  skin  itself,  in  the  epidermis  or  in  the  imme- 
diately underlying  dermis.     Since  we  experience  sensations  of 

G7 


1058  ON   CUTANEOUS   SENSATIONS.  [Book  iii. 

cold  and  heat  in  regions  of  the  skin,  not  only  free  from  touch 
corpuscles  but  also  free  from  any  dermic  terminal  organs  as  yet 
known,  the  "  points  "  of  the  skin  determined  experimentally  to 
be  points  of  cold  and  heat  sensations,  having  been  repeatedly 
found  when  extirpated  to  be  free  from  all  such  dermic  organs, 
we  may,  though  with  less  certainty,  still  further  infer  that  the 
material  exists  somewhere  in  the  epidermis.  We  may  add  that 
sensations  of  temperature  may  be  felt  in  the  cornea,  from  which 
all  dermic  terminal  organs  seem  certainly  to  be  absent.  And 
our  knowledge  that  the  nerve  fibres  end  as  fine  fibrillse  between 
and  among  the  cells  of  the  Malpighian  layer  brings  us  to  the 
final  conclusion  that  the  material  of  wiiich  we  are  speaking  is 
to  be  sought  for  either  in  the  fine  nerve  fibrillse  themselves,  or, 
as  seems  more  likely,  in  some  or  other  of  the  cells  of  the  Mal- 
pighian layer  specially  connected  with  those  fibrillse. 

Beyond  this  we  cannot  go  ;  and  even  admitting  thus  much, 
it  is  difficult  to  understand  how,  if  the  change  be  one  from  a 
higher  to  a  lower  temperature,  the  lower  temperature,  vv^hatever 
may  have  been  the  exact  degree  of  the  higher  temperature, 
should  in  giving  rise  to  sensations  of  cold  affect  one  set  of  fibres 
only,  or  how  the  higher  temperature  should  similarly  affect 
another  set  of  fibres  only ;  but  we  must  leave  the  matter  here. 

The  considerations  which  have  just  been  brought  forward  in 
relation  to  sensations  of  heat  and  cold,  may  also  be  applied  to 
sensations  of  pressure ;  with  regard  to  them  also  we  are  driven 
to  the  conclusion  that  they  take  origin  in  the  lower  layer  of  the 
epidermis  through  some  condition  brought  about  by  the  pres- 
sure. We  can  appreciate  pressure  by  the  cornea,  from  which 
as  we  have  said  dermic  organs  are  absent.  If  the  'points  of 
skin'  in  various  parts  of  the  body,  determined  experimentally 
to  be  points  of  pressure  sensation,  be  extirpated  and  examined 
it  is  found  that  dermic  organs  are  not  necessarily  present ; 
indeed  such  points  of  pressure  sensations  do  not  differ  essen- 
tially in  structure  from  points  of  heat  or  cold  sensations,  though 
some  slight  difference  in  the  manner  of  distribution  of  the 
dermic  nerve  filaments  has  been  described. 

We  are  thus  brought  to  the  conclusion  that  the  so-called 
touch  corpuscles  are  in  no  way  essential  to  touch.  At  the  same 
time  their  remarkable  prominence  in  those  parts  of  the  skin  in 
which  touch  is  most  sensitive  would  seem  to  shew  that,  even  if 
not  necessary,  they  are  in  some  way  adjuvant  to  pressure  sensa- 
tions. But  what  that  aid  may  be  is  at  present  a  mere  matter 
of  speculation ;  and  we  are  perhaps  still  more  in  the  dark  as  to 
functions  of  the  end-bulbs  and  of  the  Pacinian  bodies. 


SEC.  4.   THE  MUSCULAE  SENSE. 

§  659.  Before  we  go  on  to  deal  with  some  of  the  psychical 
aspects  of  cutaneous  sensations  it  will  be  desirable  to  speak  of 
certain  sensations  accompanying  and  belonging  to  the  movements 
of  the  body  which  are  carried  out  by  means  of  tlie  skeletal 
muscles ;  for  these  sensations,  often  spoken  of  as  constituting  a 
"  muscular  sense,"  are  in  many  ways  related  to  or  mixed  up 
with  cutaneous  sensations. 

When  we  examine  our  own  consciousness  we  find  that  we 
are  aware  of  the  position  not  only  of  the  whole  body  (this  matter 
we  discussed  some  time  back),  lout  also  of  the  several  parts  of 
our  body.  In  this  we  are  under  ordinary  circumstances  assisted 
by  sight ;  but  sight  is  not  necessary.  If  for  instance,  with  the 
eyes  shut,  we  place  the  arm  in  any  attitude,  we  are  aware  of  the 
attitude  and  can  describe,  or  by  movements  of  the  other  arm  im- 
itate with  considerable  accurac}^  the  details  of  the  attitude,  the 
relative  positions  of  the  upper  arm,  forearm,  hand,  fingers  and 
the  like.  If  we  change  the  attitude  by  moving  the  arm  or  part  of 
the  arm  we  can,  though  the  eyes  be  still  shut,  tell  the  amount 
and  characters  of  the  change. 

Again,  when  we  examine  our  own  consciousness  we  find 
that  we  possess  a  measure  of  the  amount  of  resistance  to  our 
movements  which  we  from  time  to  time  meet  with.  When  we 
come  into  contact  with  an  external  object  we  are  conscious  not 
only  of  the  pressure  exerted  by  the  object  on  our  skin,  but  also 
of  the  pressure  which  we  exert  on  the  object ;  we  can  appreciate 
the  amount  of  effort  which  we  make  to  produce  by  pressure  an 
effect  upon  the  object.  A  similar  api)reciation  of  our  own  efforts 
assists  us  largely  in  forming  a  judgment  as  to  the  weight  of  an 
object.  If  we  place  the  hand  and  arm  flat  on  a  table,  we  can 
estimate  the  pressure  exeiled  by  a  body  resting  on  the  palm  of 
the  hand,  and  so  come  to  a  conclusion  as  to  its  weight;  in 
this  case  we  are  conscious  only  of  the  pressure  exerted  by  the 
body  on  our  skin.  If  however  we  hold  the  body  in  the  hand, 
we  not  only  feel  the  pressure  of  the  body,  but  we  are  also  aware 
of  the  exertion  required  to  support  and  lift  it.  And  we  find  by 
experience  that  when  we  trust  to  this  appreciation  of  the  amount 

1059 


1060  ON  CUTANEOUS   AND  [Book  in. 

of  effort  needed  to  lift  an  object  as  Avell  as  to  sensations  of 
pressure,  we  can  form  mucli  more  accurate  judgments  con- 
cerning the  weight  of  the  object  than  when  we  rely  on  sensations 
of  pressure  alone.  When  we  want  to  tell  how  heavy  a  thing  is, 
we  are  not  in  the  habit  of  allowing  it  simply  to  press  on  the  hand 
laid  flat  on  a  table  or  otherwise  at  rest ;  we  hold  it  in  our  hand 
and  lift  it  up  and  down. 

The  above  instances  deal  with  three  things-  which  it  might 
be  desirable  to  keep  separate,  namely,  '  position,'  '  movement ' 
and  '  effort ; '  it  might  seem  desirable  to  speak  of  "  a  sense  of 
position,"  "  a  sense  of  movement,"  and  "  a  sense  of  effort." 
But,  if  we  leave  out  of  consideration  the  problems  connected 
with  our  appreciation  of  the  position  of  the  head,  which  as  we 
have  seen  seems  especially  dependent  on  afferent  impulses 
passing  up  the  auditory  (vestibular)  nerve,  we  may  say  that 
the  position  of  the  various  parts  of  our  body  is  so  closely 
dependent  on  movement,  that  is  on  the  contraction  of  skeletal 
muscles,  some  muscle  or  other  playing  its  part  in  almost  every 
position  and  every  change  of  position,  that  in  the  discussion  on 
which  we  are  now  entering  it  will  be  hardly  profitable  to  dis- 
tinguish between  the  two  ;  and  we  may  use  the  term  "  muscular 
sense "  to  denote  our  appreciation  both  of  movement  and  of 
position  resulting  from  movement. 

§  660.  There  are  more  valid  reasons  for  distinguishing 
between  our  appreciation  of  an  effort  and  our  appreciation  of 
the  movement  which  is  the  result  of  that  effort.  For  the  view 
has  been  put  forward  and  supported  by  argument  that  when 
we  make  a  muscular  effort,  we  are  directly  conscious  of  the 
nervous  processes  of  the  central  nervous  system  underlying  the 
effort,  that  the  changes  in  the  central  nervous  system  involved 
in  initiating  and  executing  a  movement  of  the  body  so  affect 
our  consciousness  that  we  have  a  sense  of  the  nervous  effort 
itself,  of  the  innervation  as  it  has  been  called ;  and  it  is  urged 
that  the  condition  of  the  central  nervous  system  through  which 
we  appreciate  the  nature  and  magnitude  of  the  effort  is  thus  the 
direct  effect  of  central  changes,  and  not  the  outcome  of  afferent 
impulses  proceeding  from  the  part  moved. 

Whether  it  be  the  case  or  not  that  consciousness  is  thus 
directly  affected  by  changes  in  the  central  nervous  system,  such 
for  instance  as  those  taking  place  in  the  motor  cortical  area  or 
in  the  pyramidal  tract,  the  evidence  goes  to  shew  that  any  such 
affection  has,  at  most,  very  little  share  in  that  appreciation  of 
our  movements  which  is  generally  called  "the  muscular  sense." 
Not  only  is  our  appreciation  of  passive  movements  very  similar 
to  our  appreciation  of  active  movements  (we  are  as  well  aware 
of  an  attitude  in  which  our  arm  has  been  placed  by  others  as 
of  one  in  which  we  have  placed  it  ourselves),  but  also  if  a 
muscular  contraction  be  brought  about  not  by  any  action  at  all 


Chap,  vi.]  SOME   OTHER   SENSATIONS.  1061 

of  the  central  nervous  system,  but  by  the  direct  electric  or  other 
stimulation  of  the  muscles  or  motor  nerves,  the  muscular  sense 
of  the  movement  which  results  differs  little  from  that  of  a  like 
voluntary  movement.  If  for  instance,  while  our  eyes  are  shut, 
the  wrist  be  bent  by  direct  stimulation  of  the  flexor  muscles,  we 
are  aware  of  the  movement  and  can  appreciate  its  character  and 
amount ;  we  can  even  use  such  an  artificial  movement  to  judge 
of  weight  and  resistance.  It  is  indeed  urged  that  our  judgment 
under  such  conditions  is  less  secure  than  when  the  movement  is 
a  voluntary  one  ;  and  from  this  it  is  argued  that  our  judgment 
is  at  least  assisted  by  our  appreciation  of  the  central  changes  by 
a  "  sense  of  the  effort "  as  distinguished  from  a  muscular  sense 
of  peripheral  origin ;  but  even  this  is  disputed.  We  may  con- 
clude that  our  appreciation  of  our  movements  and  muscular 
efforts  is  largely,  if  not  wholly,  dependent  on  what  ma}^  be 
called  a  muscular  sense  which  is  the  outcome  of  afferent  im- 
pulses proceeding  from  the  periphery  and  started  in  the  parts 
concerned  in  the  movement. 

§  661.  Coming  next  to  the  questions,  What  is  the  exact 
nature  of  these  afferent  impulses  ?  In  what  tissues  are  they 
started,  and  along  what  paths  do  they  travel?  we  find  the 
answers  beset  with  considerable  difficulties.  Every  movement 
of  the  body,  even  a  simple  one,  is  in  reality  a  complex  affair, 
and  the  carrying  it  out  involves  changes  in  several  tissues.  In 
the  first  place  there  are  changes  in  one  or  more  muscles,  changes, 
of  contraction  in  active  movements,  of  extension  and  relaxation 
in  passive  movements.  In  the  second  place  there  are  changes  in 
the  skin  which  during  a  movement  is  in  one  spot  stretched, 
in  another  relaxed  or  folded  ;  and  in  movements  of  locomotion 
the  pressure  of  the  foot  on  the  ground  is  continually  changing. 
In  the  third  place,  by  far  the  majority  of  movements  affect  a 
joint,  and  hence  involve  changes  in  the  relations  of  the  articu- 
lar surface,  in  the  capsule  and  ligaments  and  in  the  tendons. 
All  these  are  possible  sources  of  afferent  impulses. 

Now  we  know  that  the  skin  is  a  source  of  afferent  impulses 
and  so  of  sensations,  namely,  the  sensations  of  pressure,  of 
temperature  and  of  pain ;  and  we  may  fairly  suppose  that 
stretching  or  slackening  the  skin  gives  rise  to  impulses  either 
analogous  to  those  caused  by  the  pressure  of  an  external  object 
or,  it  may  be,  of  a  nature  more  akin  to  those  which  belong  to 
general  sensibility.  Hence  it  is  possible  that  these  do  at  least 
contribute,  under  normal  circumstances,  to  what  as  a  whole  we 
call  the  muscular  sense. 

Indeed  it  is  maintained  b}-  some  that  these  cutaneous  im- 
pulses furnish  the  whole  basis  of  what  is  called  the  muscular 
sense,  the  name  on  this  view  being  of  course  erroneous.  In 
attempting  to  judge  of  such  a  view  we  may  appeal  on  the  one 
hand  to  our  own  consciousness,  and  on  the  other  hand  to  the 


1062  ON"   CUTANEOUS   AND  [Book  iii. 

phenomena  of  incoordinate  movements.  In  a  previous  part 
of  this  work,  we  chvelt  upon  the  importance  of  afferent 
impulses  as  factors  in  the  coordination  of  movements.  We 
have  had  occasion  repeatedly  to  insist  that  all  the  movements 
of  the  body,  a  large  number  of  those  which  are  involuntary  as 
well  as  all  those  which  are  voluntary,  are  guided  by  afferent 
impulses,  and  that  in  the  absence  of  these  afferent  impulses  the 
movements  are  apt  to  become  uncertain  and  imperfect,  or  even 
to  fail  altogether.  We  need  not  here  repeat  what  we  have  pre- 
viously urged;  it  is  sufficient  for  our  present  purpose  to  say 
that  conspicuous  among  these  afferent  impulses  are  those  which 
form  the  groundwork  of  the  muscular  sense ;  at  times  the}'"  may 
do  their  work  without  directly  affecting  consciousness  but  at 
other  times  they  bring  about  a  distinct  affection  of  conscious- 
ness, and  it  is  this  affection  of  consciousness  which  is  more  prop- 
erly called  the  muscular  sense. 

Now,  on  the  one  hand,  we  find  upon  examination  that 
coordination  of  movements  is  not  distinctly  affected  by  the 
diminution  of  cutaneous  sensations,  but  may  be  maintained  in 
the  absence  of  cutaneous  sensations  and  indeed  in  the  absence 
of  the  skin.  Thus  frogs  are  said  to  be  able  to  execute  their 
ordinary  movements  without  signs  of  incoordination  after  the 
whole  skin  has  been  removed.  Cases  of  nervous  diseases  have 
been  recorded  in  which,  if  not  complete  absence  of,  at  least 
great  failure  in,  cutaneous  sensations,  has  not  been  accompanied 
by  any  decided  loss  of  coordination.  And  if  we  appeal  to  our  own 
consciousness  we  do  not  find  the  muscular  sense  notably  dimin- 
ished by  temporary  anaesthesia  of  the  skin ;  if,  for  instance,  the 
skin  of  the  arm  be  rendered  for  a  while  anaesthetic,  we  do  not 
find  any  marked  change  in  our  power  of  judging  weights  or 
resistance,  or  in  appreciating,  with  the  eyes  shut,  the  position 
of  the  limb. 

On  the  other  hand  we  find  recorded  cases  of  nervous  dis- 
eases in  which  loss  of  coordination,  and  loss  of  the  muscular 
sense,  as  indicated  by  the  difficulty  or  inability  to  judge  weights 
and  resistance  and  to  recognize  with  the  eyes  shut  the  position 
of  the  limbs  or  other  parts  of  the  body,  have  occurred  without 
notable  loss  of  cutaneous  sensations.  This  is  often  strikingly 
shewn  in  cases  of  the  disease  or  group  of  diseases  known  as 
''  tabes  dorsalis,"  often  spoken  of  from  one  of  its  prominent 
symptoms  as,  "locomotor  ataxy,"  the  conspicuous  pathological 
condition  of  which  is  a  structural  change  in  the  posterior  col- 
umns of  the  lower  part  of  the  cord.  In  certain  stages  of  this 
disease  the  patient  may  retain  good  cutaneous  sensations,  he 
may  experience  tactile,  temperature  and  painful  sensations  in 
the  skin  of  his  legs,  for  instance,  and  possess  adequate  muscu- 
lar strength  in  his  legs,  and  yet,  from  want  of  coordination,  be 
unable  to  move  them  properly  unless  he  be  assisted  by  sight. 


Chap,  vi.]  SOME   OTHER    SENSATIONS.  1063 

So  long  as  his  eyes  are  open  he  may  be  able  to  stand  and  Avalk, 
but  if  his  eyes  are  shut  he  often  falls,  and  when  he  moves, 
moves  with  a  staggering  uncertain  gait ;  he  fears,  in  the  dark, 
to  go  up  or  down  stairs  even  though  he  knows  them  well. 
When  a  direct  appeal  is  made  to  his  consciousness  he  appears 
to  possess  little  or  no  muscular  sense ;  he  is  unaware,  so  long 
as  his  eyes  are  shut,  of  the  position  of  the  limbs  affected  by  the 
disease,  and  if  the  arms  are  affected  is  unable  properly  to  judge 
weights.  These  cases  of  ''  tabes  "  are  very  varied  in  their  symp- 
toms, which  indeed  alter  as  the  disease  advances.  Concerning 
them  and  similar  phenomena  presented  by  other  allied  nervous 
diseases  there  has  been  much  discussion ;  but  the  evidence 
afforded  by  them,  sujoported  as  it  is  to  a  certain  extent  by 
experimental  results,  is  sti'ongly  in  favour  of  the  view  that  the 
afferent  impulses  which  determine  coordination  and  which  go 
to  make  up  what  we  are  now  calling  the  muscular  sense  are 
other  than  those  started  in  the  skin. 

We  may  therefore  dismiss  cutaneous  sensations  as  not  being 
essential  factors  of  the  sense. 

§  662.  There  remain  on  the  one  hand  the  muscles,  with 
which  we  may  in  the  first  instance  include  the  belonging  ten- 
dons, and  on  the  other  hand  the  joints  with  their  belonging  liga- 
ments ;  the  afferent  impulses  under  discussion  must  come  from 
one  or  other  or  both  of  these.  We  cannot  by  an  appeal  to  our 
own  consciousness  localize  the  muscular  sense  so  as  to  lodge 
it  exclusively  either  in  the  one  or  the  other  of  these  parts  and 
must  trust  to  indirect  indications.  On  the  one  hand  there  seems 
to  be  a  close  connection  between  the  muscular  sense  and  the 
'sense  of  fatigue;'  and  the  latter  appears  to  be  determined 
by  the  condition  of  the  muscles.  Again,  in  many  of  our  move- 
ments we  employ  a  part  only  of  a  muscle,  and  it  is  difficult 
to  suppose  that  the  afferent  impulses  which  guide  us  in  using 
that  part  only,  depend  alone  on  the  effect  which  the  partial 
use  of  the  muscle  produces  on  the  joints  or  other  parts.  On 
the  other  hand,  when  we  have  a  muscular  sense  of  the  move- 
ments of  the  fingers,  we  can  hardly  suppose  that  the  sense  is 
afforded  by  impulses  coming  exclusively  from  the  muscles 
moving  the  fingers,  distant  as  these  often  are  from  the  joints 
which  they  move.  And,  again,  the  movements  of  which  we  are 
most  distinctly  sensible,  are  especially  the  movements  affecting 
joints ;  indeed  we  have  some  difficulty  in  appreciating  the 
amount  and  character  of  a  movement  not  necessarily  involv- 
ing a  joint  such  as  one  caused  by  contractions  of  the  orbicu- 
lar muscle  of  the  mouth  or  of  the  eye,  even  though  in  these 
cases  we  are  assisted  by  cutaneous  sensations. 

We  have  evidence  both  that  the  joints  and  that  the  mus- 
cles can  supply  the  necessary  afferent  impulses.  The  joints  are 
well  supplied  with  afferent  nerve  fibres,  and  undoubtedly  give 


1064  ON   CUTANEOUS   AND  [Book  iii. 

rise  to  afferent  impulses.  On  the  other  hand  afferent  impulses 
may  proceed  from  muscles.  When,  for  instance,  a  nerve  twig 
going  to  a  muscle  is  stimulated,  centripetall}^,  after  division, 
reflex  movements  result;  if  the  stimulus  is  weak  the  move- 
ment is  confined  to  the  muscle  itself  (we  are  supposing  that 
other  nerve  twigs  going  to  the  muscle  are  left  intact) ;  if 
the  stimulus  is  strong,  the  movement  spreads  to  neighbouring 
muscles.  Again,  the  phenomena  often  spoken  of  as  '  muscle 
reflexes '  such  as  the  '  kn^e-jerk '  and  the  like  (§  515)  are  all 
so  many  proofs  of  afferent  impulses  passing  up  from  the  mus- 
cles. In  speaking  of  the  knee-jerk,  we  called  attention  to  the 
influence  exerted  upon  the  movement  of  the  muscle  employed, 
by  afferent  impulses  proceeding  from  the  antagonistic  muscles ; 
and  instances  might  be  multiplied  of  the  action  and  '  tone ' 
of  one  muscle  being  modified  by  afferent  impulses  pass- 
ing up  from  its  antagonist  to  the  nervous  centre.  And 
undoubtedly  muscles  are  well  supplied  with  afferent  fibres. 
When  the  anterior  roots,  fibres  from  which  supply  a  given 
muscle,  are  cut,  a  very  large  number  of  the  nerve  fibres  present 
in  the  muscle  remain  undegenerated ;  these,  which  end  partly  in 
the  tendon  by  a  plexiform  arrangement  of  fibrils  terminating  in 
minute  end-bulbs  known  as  the  organ  of  Golgi,  but  partly  and 
indeed  largely  in  a  somewhat  similar  manner,  in  connection  with 
the  muscular  fibres  themselves,  seem  to  be  undoubtedl}^  afferent 
fibres.  Hence  we  have  anatomical  support  for  the  view  that 
the  afferent  impulses  of  the  muscular  sense  ma}^  come  from  the 
muscles  and  their  tendons  no  less  than  from  the  joints.  In  at- 
tempting further  to  distinguish  between  the  actual  muscular 
fibres  themselves  and  the  tendons  as  the  source  of  these  im- 
pulses while  admitting  that  the  tendon  form  part  of  the 
source,  we  may  conclude  that  the  above-mentioned  termina- 
tions of  afferent  fibres  among  the  muscular  fibres  themselves 
indicate  that  these  also  form  another  part;  and  this  view  is 
supported  by  the  connection,  also  mentioned  above,  of  fatigue 
with  the  muscular  sense. 

Against  the  view  that  the  afferent  impulses  of  the  muscular 
sense  come  from  the  muscular  fibres  themselves  has  been  urged 
the  fact  that  these,  tested  experimentally,  possess  in  a  normal 
condition  a  very  feeble  general  sensibility;  when  a  muscle  is 
cut  or  pinched  comparatively  little  or,  according  to  some 
observers,  no  pain  is  felt;  it  is  only  under  abnormal  circum- 
stances, as  when  a  muscle  is  inflamed,  that  direct  stimulation 
of  this  kind  causes  pain ;  and  the  pain  which  we  feel  in 
cramp  is  similarly  the  product  of  an  abnormal  condition,  for 
even  an  extremely  violent  muscular  effort  does  not  cause 
us  actual  pain.  This  argument  however  is  not  valid,  for 
not  only  may  it  equally  well  be  applied  to  the  other  set  of 
tissues,  tendons,  ligaments  and  the  like,  which  in  a  normal  con- 


Chap,  vi.]  SOME   OTHER   SENSATIONS.  1065 

clition  possess  a  similarly  feeble  general  sensibility,  but  it  sup- 
poses that  the  muscular  sense  is  merely  a  development  of  gen- 
eral sensibility  not  a  special  sense,  like  that  of  touch.  We 
have  no  positive  reasons  for  this  supposition,  and  arguments 
based  on  the  analogy  of  the  skin  oppose  it.  We  have  seen  rea- 
son to  regard  the  cutaneous  sensations  of  pressure  and  tempera- 
ture as  wholly  distinct  from  those  of  general  sensibility,  that  is 
to  say  of  pain  ;  and  we  may  conclude  that  the  muscular  sense  is 
similarly  a  special  sense,  similarly  distinct  from  affections  of 
common  sensibility  in  either  muscular  fibres  or  their  connective 
tissue  appendages. 

We  ought  therefore  probably  to  conclude  that  the  muscular 
sense  though  based  in  part  on  impulses  derived  from  the  mus- 
cles, and  further  from  the  muscular  fibres  themselves  as  w^ell  as 
from  the  tendons,  is  also,  and  possibly  to  a  large  extent,  based  on 
impulses  derived  from  the  joints,  though  we  cannot  as  yet  assign 
accurately  the  relative  share.  If  this  be  so  the  '  muscular ' 
sense  is  not  a  wholly  appropriate  term ;  but  it  would  be  unde- 
sirable, at  present  at  least,  to  attempt  to  replace  it  by  a  new  one. 

This  muscular  sense,  using  the  term  in  its  broad  meaning, 
enters  largely  into  our  life.  By  it  we  are  not  only  enabled  to 
coordinate  and  execute  adequately  the  various  movements  which 
we  make,  but  through  it  we  derive  much  of  our  knowledge  of 
the  external  world.  Through  it  we  are  also  conscious  of  the 
varying  condition  of  the  several  parts  of  our  body  even  when 
the  muscles  are  at  rest;  the  tired  and  especially  the  paralyzed 
limb  is  said  to  '  feel  heavy.'  In  this  way  the  state  of  our  mus- 
cles and  other  tissues  largely  determines  our  general  feeling  of 
health  and  vigour,  of  weariness,  ill  health  and  feebleness. 

The  fact  that  the  Pacinian  bodies  are  found  around  joints 
has  led  to  the  suggestion  that  these  serve  as  the  terminal  organs 
of  the  muscular  sense  ;  but  especially  bearing  in  mind  what  has 
just  been  said,  the  argument  which  we  used  against  considering 
the  touch  corpuscles  as  the  terminal  organs  of  touch  may,  with 
perhaps  still  greater  force  be  applied  against  regarding  the 
Pacinian  bodies  as  the  terminal  organs  of  the  muscular  sense. 


SEC.  5.     ON  TACTILE  PERCEPTIONS  AND  JUDGMENTS. 

§  663.  As  a  means  of  gaining  knowledge  of  external 
things  the  sense  of  touch  ranks  next  in  importance  to  that 
of  sight.  Auditory  sensations  enter  largely  and  in  several  ways 
into  our  life ;  they  serve  as  an  important  means  of  communica- 
tion; together  with  smell  and  taste  they  afford  pleasure  and 
guide  our  acts ;  but,  as  regards  our  direct  knowledge  of  tlie  ex- 
ternal world,  we  learn  by  means  of  them  very  little  compared 
with  what  we  learn  by  sight  and  touch.  To  a  certain  extent 
we  make  use  of  touch  by  itself ;  we  bring  the  surface  of  a  body 
into  contact  with  some  region  of  the  skin  such  as  the  finger, 
and  by  the  several  sensations  which  we  receive  either  from 
several  points  of  that  region  at  the  same  time,  or  from  one  or 
more  points  in  succession,  we  learn  certain  characters  of  the 
surface,  whether  for  instance  it  is  rough  or  smooth.  We  thus 
also  ascertain  whether  the  body  be  hot  or  cold ;  and  we  may, 
within  certain  limits,  form  a  judgment  of  the  size  of  the  surface 
by  simply  estimating  the  size  of  the  area  of  our  skin  with  which 
the  body  can  be  in  contact  at  the  same  time. 

But  though  we  may  and  do  thus  base  conclusions  on  tactile 
perceptions  alone,  we  most  frequently  employ  touch  in  associa- 
tion with  sisfht  on  the  one  hand  and  with  the  muscular  sense  on 
the  other. 

The  ties  indeed  between  touch  and  the  muscular  sense  are 
many  and  close.  When  we  explore  the  nature  of  a  body  by  touch 
we  press  the  skin,  of  the  finger  for  instance,  on. the  body  ;  and  we 
do  that  not  merely  in  order  to  determine  to  what  extent  the  tac- 
tile sensation  is  increased  by  the  increase  of  pressure,  but  also 
and  indeed  chiefly  to  ascertain  the  amount  of  resistance  to  pres- 
sure which  is  offered  by  the  body.  But  that  resistance,  through 
which  chiefl.y  we  judge  whether  the  body  be  soft  or  hard,  is 
appreciated  not  by  the  tactile  but  by  the  muscular  sense. 

Or  again,  placing  the  finger  on  the  surface  of  a  body,  and 
moving  the  finger  over  the  surface  in  such  a  way  that  the  con- 
tact, as  judged  by  the  pure  tactile  sensation,  remains  the  same, 
we  find  that  in  one  case  the  movement  has  been  continued  in 
the  same  plane,  whereupon  we  judge  the  surface  to  be  flat,  that 

1066 


Chap,  vi.]         OX   CUTANEOUS   SEXSATIOXS.  10C7 

in  another  case  the  finger  has  been  gradually  carried  out  of 
the  plane,  whereupon  we  judge  the  surface  to  be  curved,  and 
that  in  the  third  case  the  movement  of  the  finger  has  been 
irregular,  whereupon  we  judge  that  the  surface  is  irregular; 
and  so  on.  In  each  case  we  estimate  the  movement  by  the  mus- 
cular sense,  and  thus  by  a  combination  of  muscular  sense  and 
of  touch  we  form  a  judgment  of  the  conformation  of  external 
bodies.  In  the  same  way,  and  indeed  as  part  of  the  same  pro- 
cess, by  a  combination  of  the  muscular  sense  and  of  touch  we 
estimate  the  size  of  external  objects.  By  a  like  double  act  we 
estimate  the  position  in  space  in  relation  to  our  body  of  such 
objects  as  are  within  our  reach,  such  as  can  be  touched  either 
directly  by  one  of  our  limbs  or  indirectly  by  help  of  a  stick  or 
otherwise.  So  closely  bound  together  are  the  muscular  sense 
and  the  sense  of  touch  proper,  that  in  common  language  we 
speak  of  learning  this  or  that  by  touch,  when  we  really  employ 
both  senses. 

§  664.  No  less  close  are  the  ties  between  sight  and  touch ; 
indeed  a  very  large  part  of  our  psychical  life  is  built  up  on  the 
association  of  visual  and  tactile  sensations.  There  is  no  part 
of  the  external  world,  including  our  own  bodies,  Avhich  we  can 
explore  by  touch,  which  we  cannot,  either  directly  or  by  optical 
aids  such  as  mirrors,  also  explore  by  vision ;  and  our  concep- 
tions of  the  nature  of  all  such  things  is  the  outcome  of  a  com- 
bination of  the  two  senses,  or  rather  bearing  in  mind  what  has 
just  been  said,  of  the  three  senses,  sight,  touch,  and  the  muscu- 
lar sense.  It  is  relatively  easy  to  recognize  blindfold,  by  touch 
alone,  the  characters  of  objects  with  which  Ave  are  already  pre- 
viously familiar  by  help  of  vision  ;  but  it  is  ver}^  difficult  to 
form  by  touch  alone  an  accurate  judgment  of  the  form  and  size 
of  objects  which  we  have  never  seen.  Were  we  limited  to 
sight  alone,  we  should  form  one  set  of  conceptions  of  the  Avorld, 
were  we  limited  to  touch  we  should  form  another ;  and  the  two 
sets  would  be  different. 

In  the  conceptions  which  we  form  in  actual  life  the  two  are 
combined.  The  congenitally  blind  are  limited  to  one  set  only ; 
and,  when,  as  has  happened  in  cases  of  congenital  cataract, 
those  who  have  been  blind  from  birth  are  restored  to  vision 
after  they  have  grown  up  and  have  accumulated  a  store  of  tac- 
tile conceptions,  they  fail  at  first  to  connect  their  new  visual 
sensations  with  their  old  tactile  experience.  The  stories  of  the 
first  experiences  in  vision  of  such  persons,  as  that  for  instance 
of  the  man  who  had  to  feel  a  cat  in  order  to  connect  the  visual 
image  with  his  previous  tactile  image,  and  having  carefully  felt 
it  all  over  said  "  Now,  Puss !  I  shall  know  you  again,"  illus- 
trate the  close  dependence  on  each  other  of  visual  and  tactile 
normal  perceptions.  This  is  also  indicated  by  the  zeal  with 
which  in  former  days  the  question  was  discussed  whether  a  man 


1068  ON   CUTANEOUS   AND  [Book  hi. 

who  had  been  born  blind  and  restored  to  sight  in  adult  life, 
could  recognize  at  first  sight  and  by  sight  alone  a  cube,  a 
square,  and  a  sphere.  It  is  perhaps  especially  in  relation  to 
size  and  space,  that  the  two  senses  work  together. 

There  are  no  converse  cases  of  persons  who,  born  without 
touch,  and  trusting  to  sight  alone  have,  in  later  life,  had  touch 
restored  to  them  ;  but  there  are  many  things  within  our  vision, 
which  are  beyond  our  touch  at  the  moment  and  some  which  we 
can  never  touch  at  any  time ;  our  conceptions  of  these  latter 
are  more  or  less  uncertain,  and  the  direct  visual  sensations  have 
to  be  strengthened  or  corrected  not  by  mere  sensations  but  by 
intellectual  efforts  and  reasoning.  A  group  of  visual  sensa- 
tions, constituting  a  visual  image,  may  have  an  ordinary  objec- 
tive cause,  but  may  be  an  ocular  illusion;  and  the  test  which 
we  at  once  apply  to  determine  this  is  that  of  touch ;  the  ordi- 
nary idea  of  a  '  ghost '  is  that  of  a  something  which  we  can  see 
but  cannot  touch,  which  excites  visual  sensations  but  affords  no 
tactile  sensations.  Conversely  a  touch  by  something  invisible, 
a  touch  as  of  a  body  which  we  ought  to  be  able  to  see  but  can- 
not, we  also  recognize  as  unreal.  The  concordance  of  touch 
and  vision  affords  in  fact  to  a  large  extent  the  standard  by 
which  we  judge  of  the  reality  of  tilings. 

§  665.  The  last  remark  naturally  leads  to  the  statement 
that  as  in  the  case  of  the  other  sensations,  so  in  the  case  of  the 
several  cutaneous  sensations,  we  may  have  sensations  Avhich  are 
not  due  to  their  ordinary  objective  causes. 

We  have  seen  that  visual  sensations  may  arise  from  changes 
in  the  retina  started  not  by  light  but  by  other  agents,  mechani- 
cal  and  others;  and  the  question  presents  itself.  Can  touch 
proper,  the  sensation  of  pressure,  be  excited  otherwise  than  by 
pressure  and  sensations  of  temperature  by  changes  in  the  skin 
other  than  those  of  temperature  ?  No  very  definite  answer  can 
be  given  to  this  question,  though  the  case  quoted  above  (§  656) 
in  which  a  heated  spoon  applied  to  the  skin  produced  a  sensa- 
tion not  of  heat  but  of  contact,  points  perhaps  to  the  affirma- 
tive, as  does  also  the  fact  that  electric  currents  applied  to  the 
skin  may  produce  sensations,  pricking  sensations,  which  if  not 
identical  with,  may  at  least  be  confused  with  those  of  pressure. 

Cutaneous  sensations  of  all  kinds  may  however  be  of  central 
origin,  may  be  due  to  changes  in  the  central  nervous  system 
quite  independent  of  all  events  in  the  skin,  and  may  yet  be 
referred  to  this  or  that  region  of  the  skin  and  to  the  objective 
cause  which  ordinarily  gives  rise  to  the  sensation.  Painful 
sensations  indeed  may  rise  from  changes  not  only  in  the  central 
organs  but  at  any  part  of  the  whole  length  of  the  nerve,  all 
being  referred  to  the  cutaneous  terminations  of  the  nerves  on 
which  the  cause  of  pain  is  usually  brought  to  bear.  Tactile 
and  temperature  sensations  as  we  have  said  cannot  originate  in 


Chap,  vi.]  SOME   OTHER   SEXSATIONS.  1069 

changes  in  the  nerves  themselves,  but  they  may  arise  through 
changes  in  the  central  organs  ;  we  may  be  subject  to  tactile 
phantoms  comparable  to  ocular  phantoms.  Compared  with  vis- 
ual sensations  however  our  tactile  sensations  are  so  to  speak 
fragmentar}^  A  momentary  exposure  of  the  retina  may  fill  the 
mind  with  a  complex  visual  image,  full  of  the  most  varied  inci- 
dent ,  but  the  tactile  impressions  which  we  can  receive  at  any 
one  moment  are  few  and  simple.  Hence  our  tactile  phantoms 
are  also  simple ;  we  ma}^  fancy  that  some  invisible  garment  has 
swept  past  us,  or  that  a  scorching  flame  has  passed  near  us,  we 
may  feel  that  the  hand  or  that  the  head  is  swollen  and  large,  and 
we  may  experience  an  imaginary  pain  in  every  region  of  the 
skin  in  turn ;  but  the  most  that  we  can  thus  feel  is  simple  com- 
pared with  the  possible  complexity  of  an  ocular  or  even  an  audi- 
tory phantom. 

§  666.  Like  other  sensations  our  tactile  sensations  while 
they  sometimes  give  us  trustworthy  information  of  the  exter- 
nal world  at  other  times  may  give  rise  to  illusions.  This  is 
well  illustrated  by  the  so-called  experiment  of  Aristotle.  It 
is  impossible  in  an  ordinary  position  of  the  fingers  to  bring  the 
radial  side  of  the  middle  finger  and  the  ulnar  side  of  the  ring 
finger  to  bear  at  the  same  time  on  a  small  object  such  as  a  marble. 
Hence  when  with  the  ej^es  shut  we  cross  one  finger  over  the 
other,  and  place  a  marble  between  them  so  that  it  touches  the 
radial  side  of  the  one  and  the  ulnar  side  of  the  other,  we  recog- 
nize that  the  object  is  such  as  could  not  under  ordinary  condi- 
tions be  touched  at  the  same  time  by  these  two  portions  of  our 
skin,  and  therefore  judge  that  we  are  touching  not  one  but  two 
marbles.  Upon  repetition  however  we  are  able  to  correct  our 
judgment  and  the  illusion  disappears. 


CHAPTER  VII. 
ON   SOME   SPECIAL   MUSCULAR   MECHANISMS. 

SEC.  1.     THE   VOICE. 

§  667.  If  a  small  mirror,  warmed  in  order  to  avoid  the  con- 
densation of  moisture  upon  it,  be  placed  in  an  appropriate  slant- 
ing position,  namely,  at  about  an  angle  of  45°  with  the  horizon, 
in  the  back  of  the  pharynx  with  its  upper  margin  resting  against 
the  base  of  the  uvula  and  be  adequately  illuminated,  a  view  of 
the  interior  of  the  larynx  may  be  obtained.  Such  a  mirror  with 
its  various  appurtenances  is  called  a  laryngoscope.  The  details 
of  the  view  thus  gained  will  of  course  vary  with  the  exact  posi- 
tion and  inclination  of  the  mirror,  but  the  following  may  be 
taken  as  the  average  appearance  (Fig.  183). 

In  front  (reversed  of  course  in  the  mirror  image)  will  be 
seen  the  edge  of  the  back  of  the  tongue  (i),  and  immediately 
in  front  of  this  the  top  of  the  epiglottis  (e).  These  parts  will 
of  necessity  appear  much  foreshortened,  and  peeping  out  from 
underneath  the  top  edge  of  the  epiglottis  may  be  seen  the  swell- 
ing at  its  base  known  as  the  "  tubercle  "  or  "  cushion  of  the 
epiglottis  "  (e').  The  curved  sides  of  the  epiglottis  will  be 
seen  sweeping  away  to  the  right  and  to  the  left,  and  emerging 
from  near  the  end  of  each  will  be  visible  the  ary-epiglottic  fold 
(ar.ep/.)  on  which  are  obvious  first  the  round  swelling  due 
to  the  cartilage  of  Wrisberg  Qw}  and  next  that  due  to  the  car- 
tilage of  Santorini  (s).  If  at  the  time  when  the  view  is  being 
taken,  the  voice  is  being  uttered  and  especially  if  a  high  note  is 
being  given  (Fig.  187  A}  the  two  cartilages  of  Santorini  are  in 
close  apposition,  and  the  mucous  membrane  between  is  folded 
up.  If  no  voice  is  being  uttered  and  especially  if  a  deep  inspi- 
ration be  taken  (Fig.  187  B  and  (7),  the  cartilages  of  Santorini 
are  far  apart  and  the  mucous  membrane  between  them  appears 
as  a  ridge  completing  at  the  hind  part  the  rim  of  the  aperture 
to  the  larynx ;  there  may  also  be  seen  on  each  side  lying  imme- 
diately to  the  median  side  of  the  prominence  of  the  cartilage 

1070 


Chap,  vii.]      SPECIAL   MUSCULAE   MECHANISMS.         1071 


S   phpv 


Fig.  183.     Diagram  of  a  Laryngoscopic  view  of  the   Lartnx  (magnified 

twice). 

L.  the  base  of  the  tongue,  e.  the  epiglottis,  seen  foreshortened  with  e'  its 
cushion,  ar.ep.f.  tlie  ary-ejDiglottic  folds.  TF.  the  Capituluin  Wrisbergi,  S.  Cap- 
ituluni  Santoriui ;  the  mucous  membrane  between  the  arytenoids  is  stretched 
straight,  the  notch  being  merely  indicated,  c.v.  vocal  cords,  c.v.s.  ventricular 
bands,  v. I.  the  opening  into  the  ventricle  of  the  larynx  seen  between  them.  The 
former,  bounding  the  widely  open  glottis  of  more  or  less  ti'iangTilar  form,  through 
which  a  view  of  the  trachea  (^Tr.)  is  obtained  are  seen  to  end  in  the  processus 
vocales  (p.v.). 

On  each  side  of  the  larynx  is  seen  s.p.  the  pyriform  recess,  ph.  the  hind  wall 
of  the  phlaryux.     l.f.  the  median  glosso-epiglottic  fold. 


of  Santorini  a  .shallower  prominence  clue  to  the  top  of  the  aryte- 
noid itself,  shewn  at  a  in  Fig.  187  B.  Between  the  two  phases 
of  complete  apposition  and  of  the  widest  separation  of  the  tuber- 
cles of  Santorini,  intermediate  phases  may  from  time  to  time  be 
seen,  such  as  those  shewn  in  Fig.  183,  Fig.  187  B. 

These  several  structures  define  the  superior  aperture  of  the 
larynx:  which  in  the  laryngoscopic  view,  owing  to  the  foreshort- 
ening, is  not  seen  as  it  is  in  a  dissection  (Figs.  184,  185,  186) 
namely  as  a  slanting  orifice  with  a  long  fore  and  aft  diameter 
but  appears  as  a  rhomboidal  space  with  the  transverse  diameter 
generally  the  longer  one.  If  no  voice  is  being  uttered,  and  the 
breathing  be  gentle  and  quiet,  the  glottis  may  be  seen  within 
this  aperture  as  a  slit,  more  or  less  in  the  form  of  an  elongated 
isosceles  triangle  with  the  apex  dipping  beneath  the  cnshion  of 
the  epiglottis,  the  sides  formed  by  the  vocal  cords,  anil  the  base 
by  the  arytenoids  with  the  membrane  between  them.  Jn  a  favour- 
able view  (Fig.  183)  the  vocal  cords  (v.c.)  ma}'  be  seen  to  be 
attached  to  the  processus  vocales  and  the  distinction  between 
the  membranous  and  cartilaginous  glottis  observed.  On  the 
outside  of  each  vocal  cord,  separated  from  it  by  the  mouth  of 


1072 


THE  VOICE. 


[Book  hi. 


the  corresponding  ventricle  of  the  larynx  and  reaching  to  the 
side  of  the  laryngeal  aperture,  may  be  seen  the  ventricular  band 
(c.v.s.).  By  their  white  colour  the  vocal  cords  present  a  strong 
contrast  to  the  rest  of  the  larynx. 


^.}^ 


-  ar.e.f 


..m.a.t 


.Cri 


Fig.   184. 


Fig.  185. 


Fig.  184.     Diagram  of  the  Superior  Aperture  or  the  Larynx. 

The  oesophagus  and  pharynx  are  supposed  to  be  laid  open  from  behind. 

e.  the  epiglottis  with  e'  its  cushion,  ar.e.f.  the  ary-epiglottic  fold,  on  which 
are  seen  the  swellings  or  "capitula"  caused  (W)  by  the  cartilage  of  AVrisberg 
and  (S)  by  the  cartilage  of  Santorini.  i.  the  notch  or  incisura  in  the  mucous 
fold  uniting  transversely  the  two  arytenoid  cartilages. 

1.  (placed  in  the  middle  line  of  the  base  of  the  tongue)  the  median  and  (2) 
the  lateral  glosso-epiglottic  folds,  the  latter  forming  the  boundary  of  the  depres- 
sion (3)  called  the  vallecula.  4.  the  pharyngo-epiglottic  fold.  5.  the  pharyngo- 
laryngeal  or  pyriform  recess. 


Fig.  185.     Diagram  of  the  Larynx  in  vertical  section. 

e.  the  epiglottis.  I.  the  base  of  the  tongue.  Hy.  hyoid  bone.  Th.  thjToid 
cartilage  ;  Cri.  cricoid  cartilage  ;   Tr.  tracheal  cartilage  ;  all  cut  across. 

W.  the  swelling  due  to  the  cartilage  of  Wrisberg  and  S.  that  due  to  the  car- 
tilage of  Santorini ;  from  these  eminences  folds  descend  towards  the  processus 
vocalis  of  the  arytenoid,  c.v.  the  true,  and  c.v.s.  the  false  vocal  cord  or  "  ventri- 
cular band,"  with  the  mouth  of  the  ventricle  of  the  larynx  i;.  between  them, 
m.a.t.  the  transverse  arytenoid  muscle  cut  across.  P  is  placed  in  the  cavity  of 
the  pharynx. 


Chap,  vii.]      SPECIAL   MUSCULAR   MECHANISMS.         1073 

If  the  voice,  and  especially  if  a  high  note,  be  uttered  the 
view  changes  (Fig.  187  A)  ,  besides  an  alteration  in  the  form  of 
the  laryngeal  aperture,  the  vocal  cords  are  seen  to  be  brought 
close  together  and  nearly  parallel  so  that  the  glottis  becomes  a 
mere  slit.  If  no  voice  is  being  uttered  and  a  deep  inspiration 
be  taken  changes  of  another  kind  may  be  observed  (Fig.  187  6')  ; 
the  glottis  becomes  a  wide  aperture  with  the  form  of  a  truncated 


Fjg.  186.     Diagram  of  the  Larynx  in  vertical  transverse  section. 

Hy.  hyoid  bone.  TTi.  thyroid  cartilage;  Cn.  cricoid  cartilage;  m.th.h. 
thyro-hyoid  membrane,  all  cut  across. 

e.  epiglottis,  e'  its  cushion,  c.v.s.  ventricular  bands,  c.r.  vocal  cords,  with  v. 
the  ventricles  of  the  larynx  between  them.     T.  the  trachea. 

A.  the  internal  thyro-arytenoid  muscle,  cut  across  ;  it  is  seen  to  form  the  bulk 
of  the  wedge-shaped  projection  of  which  the  vocal  cord  is  the  extreme  edge.  B. 
the  external  thyro-arytenoid,  cut  across. 


rhomboid,  the  obtuse  angle  on  each  side  marking  the  attach- 
ment of  the  vocal  cord  to  the  processus  vocalis  :  through  this 
wide  opening  the  tracheal  rings  are  clearly  visible,  and  indeed 
with  an  appropriate  position  of  the  mirror  the  bifurcation  of  the 
trachea  into  the  bronchi  may  under  favourable  circumstances  be 
observed.  When  changes  in  the  voice  or  in  the  breathing  are 
being  made,  the  white  glistening  vocal  cords  may  be  seen  to 
come  together  or  to  go  apart  like  the  blades  of  a  pair  of  scissors. 

68 


1074  THE   VOICE.  [Book  iii. 

§  668.  Laryngoscopic  observation  then  teaches  that  the 
larynx  is  used  not  only  for  the  utterance  of  voice,  for  phona- 
tion,  but  also  for  breathing ;  and  indeed  in  speaking  of  respira- 
tion we  called  attention  to  this  ;  but  the  former  is  its  more 
important  use  and  we  may  chiefly  dwell  on  this,  referring  in- 
cidentally to  the  respiratory  functions. 

In  order  that  the  membranous  edges  of  an  aperture  may  be 
readily  thi'own  into  sonorous  vibrations  by  a  blast  of  air,  the 
edges  should  be  brought  near  together  and  the  aperture  reduced 
to  a  mere  slit.  Hence  the  fundamental  condition  for  the  forma- 
tion of  the  voice,  and  indeed  speaking  generally  of  voices  of  all 
kinds,  is  the  approximation  and  consequent  more  or  less  paral- 
lelism of  the  vocal  cords. 

In  the  voice,  as  in  other  sounds  (cf.  §  620),  we  distinguish 
three  fundamental  features :  (1)  Loudness.  This  depends  on 
the  strength  of  the  expirator}^  blast.  (2)  Pitch.  This  depends 
on  the  rapidity  of  the  vibrations,  and  this  we  may  in  a  broad 
way  consider  as  determined  on  the  one  hand  by  the  length  and 
on  the  other  hand  by  the  tension  of  the  vocal  cords.  What  we 
may  call  the  natural  length  of  the  vocal  cords  is  constant,  or 
varies  only  with  age  *,  and  the  influence  of  this  factor  bears  on 
the  general  range  of  the  voice,  not  on  the  particular  note  given 
out  at  any  one  time.  The  tension  of  the  vocal  cords  on  the 
contrary  is  very  variable,  and  the  pitch  of  any  particular  note 
uttered  depends  in  the  main  on  this ;  hence  great  importance 
attaches  to  the  mechanisms  by  which  changes  in  the  tension  of 
the  vocal  cords  are  brought  about.  But,  as  we  shall  see,  the 
problems  connected  with  the  compass  of  a  voice  and  with  changes 
of  pitch  are  very  complex  ;  in  considering  these  things  we  have 
to  do  with  much  more  than  mere  variations  in  the  tension  of  the 
vocal  cords  along  the  whole  of  what  we  have  called  their  natural 
length.  These  matters  however  we  shall  deal  with  later  on, 
and  may  for  the  present  consider  tension  as  the  main  factor  of 
changes  in  pitch.  (3)  Quality.  This,  as  we  have  seen  (§  620), 
depends  on  the  number  and  character  of  the  partial  tones  accom- 
pan3dng  any  fundamental  note  sounded,  and  is  determined  by  a 
variety  of  circumstances,  chief  among  which  are,  on  the  one 
hand  the  form,  thickness  and  other  physical  qualities  of  the 
cords,  and  on  the  other  hand,  the  disposition  of  the  resonance 
chamber,  or  parts  of  the  respiratory  passage  other  than  the 
glottis  itself. 

We  may  confine  ourselves  in  the  first  instance  to  the  condi- 
tions which  determine  the  mere  utterance  of  the  voice  and  to  the 
mechanisms  which  affect  the  tension  of  the  vocal  cords,  and  hence 
the  pitch  of  the  voice.  The  problems  therefore  which  we  have 
to  attack  are,  first,  By  what  means  are  the  cords  brought  near  to 
each  other  or  drawn  asunder  as  occasion  demands?  and  secondly. 
By  what  means  is  the  tension  of  the  cords  made  to  vary  ?     We 


Chap,  vii.]      SPECIAL   MUSCULAR   MECHANIS:MS.  1075 

may  speak  of  these  two  actions  as  narrowing  or  widening  of  the 
glottis,  adduction  or  abduction  of  the  edges  of  the  glottis,  and 
tightening  or  relaxation  of  the  vocal  cords.  We  may  first  dwell 
on  the  muscular  aspects  of  the  mechanisms  by  which  these  results 
are  brought  about,  taking  the  nervous  factors  into  consideration 
afterwards.  The  change  of  form  of  the  glottis  is  best  understood 
when  it  is  borne  in  mind  that  each  arytenoid  cartilage  is,  when 
seen  in  horizontal  section  (Fig.  187),  somewhat  of  the  form  of 
a  triangle,  Avith  a  median,  an  external,  and  a  posterior  side,  the 
processus  vocalis  being  j^laced  in  the  anterior  angle  at  the  junc- 
tion of  the  median  and  external  sides.  When  the  cartilages  are 
so  placed  that  the  processus  vocales  are  approximated  to  each 
other  and  the  internal  surfaces  of  the  cartilages  nearly  parallel, 


Fig.  187.     The  Larynx  as  seen  by  means  of  the  Laryngoscope  in  different 
CONDITIONS  OF  THE  Glottis.     (From  Quain's  Anatomy,  after  Czermak. ) 

A  while  singing  a  high  note  ;  B  in  quiet  breathing ;  C  during  a  deep  inspira- 
tion. The  corresponding  diagrammatic  figures  ^4',  B\  C,  illustrate  the  clianges 
in  position  of  the  arytenoid  cartilages,  and  the  form  of  the  rima  vocalis  and 
rima  respiratoria  in  the  above  three  conditions. 

I  the  base  of  the  tongue  ;  e  the  upper  free  part  of  the  epiglottis  ;  e'  the 
tubercle  or  cushion  of  the  epiglottis  ;  'ph.  part  of  the  anterior  wall  of  the  pharynx 
behind  the  larynx  ;  lo  swelling  in  the  aryteno-epiglottidean  fold  caused  by  the 
cartilage  of  Wrisberg ;  .s  swelling  caused  by  the  cartilage  of  Santorini ;  a  the 
simimit  of  the  arj'tenoid  cartilage ;  cv  the  vocal  cords ;  cvs  the  ventricular  bands ; 
tr  the  trachea  with  its  rings  ;  h  the  two  bronchi  at  their  commencement. 


1076 


THE  VOICE. 


[Book  hi. 


the  glottis' is  narrowed  (Fig.  187  A'}.  When  on  the  contrary 
the  cartilages  are  wheeled  round  on  the  pivots  of  their  articula- 
tions, so  that  the  processus  vocales  diverge,  and  the  internal  sur- 
faces of  the  cartilages  form  an  angle  with  each  other,  the  glottis 
is  widened  (Fig.  187  B',  C'}.  Moreover  the  two  cartilages  may 
to  a  certain  extent  be  bodily  drawn  together,  or  dragged  apart, 
the  two  hind  angles,  between  the  median  and  posterior  sides, 
being  now  close  together,  now  apart. 

§  669.  The  muscles  of  the  larynx  though  small,  are  numer- 
ous and  complicated,  and  are  so  disposed  in  respect  to  their 
origins  and  insertions  and  to  the  sweep  of  their  fibres,  that  the 
effect  of  the  contraction  of  one  muscle  will  depend  upon  whether 
or  no  and  how  far  other  muscles  are  thrown  into  contraction  at 
the  same  time ;  moreover  in  the  case  of  some  of  the  muscles 
at  least  the  effect  is  different  according  as  the  whole  muscle  or 
a  part  only  contracts. 

The  first  muscle  to  which  we  may  call  attention  is  the  trans- 
verse arytenoid  (M.  arytenoideus  posticus  s.  transversus)  (Fig. 
188).     This  is  a  relatively  thick  muscle  covering  the  hind  sur- 


m.cr.ar.p 


Fig.  188.     Diagram   of   the  Transverse   and  Oblique  Arytenoid   and  of 
THE  Posterior  Crico-arytenoid  Muscles. 

A.  shews  the  three  muscles  in  position  in  reference  to  the  aperture  of  the 
larynx  ;  B.  shews  the  attachments  of  the  transverse  arytenoid  and  posterior  crico- 
arytenoid. 

m.ar.t.  transverse  arytenoid  muscle,  m.cri.ar.p.  posterior  crico-arytenoid 
muscle,  m.ar.o.  oblique  arytenoid  muscle.  Cri.  cricoid  cartilage.  Ary.  ary- 
tenoid cartilage,  p.m.  processus  muscularis  of  arytenoid  cartilage.  W-  promi- 
nence of  cartilage  of  Wrisberg.  8.  prominence  of  cartilage  of  Santorini  (in  B,  it 
marks  the  cartilage  itself),    m.cr.th.p.  is  the  small  posterior  crico-thyroid  muscle. 


Chap,  vii.]      SPECIAL   MUSCULAR  MECHANISMS.         1077 

faces  of  both  arytenoid  cartilages ;  the  fibres  starting  from  the 
outer  edge  of  one  cartilage  run  transversely  across  to  the  outer 
edge  of  the  other  cartilage,  and  the  belly  of  the  muscle  occupies 
the  concave  hind  surfaces  of  the  two  cartilages  together  with  the 
intervening  space.  The  effect  of  the  contraction  of  this  muscle 
is  to  bring  the  two  cartilages  closer  together  and  so  to  narrow 
the  glottis;  indeed  if  in  an  animal  it  be  divided,  the  glottis 
remains  widely  open  behind.  It  is  an  important  closer  of  the 
glottis,  adductor  of  the  vocal  cords.  When  it  is  not  contracting 
the  cartilages  come  apart  through  the  elastic  reaction  of  their 
connections. 

Most  important  is  a  mass  of  muscular  fibres,  which  starting 
from  the  lower  part  of  the  reentering  angle  of  the  thyroid  pass 
horizontally  but  inclined  somewhat  upwards  to  the  arytenoids  at 
about  the  level  of  the  vocal  cords.  The  whole  mass  is  described 
as  forming  two  muscles.  The  outer  or  lateral  part  ending  in  the 
outer  edge  of  the  arytenoid  and  upper  part  of  its  processus  mus- 
cularis  is  called  the  external  thyro-arytenoid  (M.  thyro-aryte- 
noideus  externus)  (Figs.  189,  m.th.ar.e.  186  B.^  The  direction 
of  the  muscle  as  a  whole  is  horizontally  backwards,  though  in- 
clined outwards  and  upwards,  but  the  constituent  individual 
bundles   run   in   various    ways  and  some  even  pass  vertically 


pm  Oe     mart 

Fig.  189.     Diagram  to  illustrate  the  Thtro-arttenoid  Muscles. 

The  figure  represents  a  transverse  section  of  the  Larynx  through  the  bases  of 
the  arytenoid  cartilages. 

Anj.  arytenoid  cartilage,  p.m.  processus  muscularis.  p.v.  processus  vocalis. 
7%.  thyroid  cartilage,     c.v.  vocal  cords.     Oe  is  placed  in  the  oesophagus. 

m.th.ar.i.  internal  thyro-arytenoid  muscle,  m.th.ar.e.  external  thyro-aryte- 
noid muscle,  m.th.ar.ep.  part  of  the  thyro-ary-epiglottic  muscle  cut  more  or  less 
transversely. 

into  the  ventricular  bands.  To  the  inner  or  median  side 
of  this  external  muscle,  between  it  and  the  corresponding 
vocal  chord,  lies  the  inner  muscle  which  running  from  tlie 
reentering  angle  of  tlie  thyroid  to  the  processus  vocalis 
and  outer  surface  of  the  arytenoid  forms  a  wedge-sliaped 
mass,   the   thin   edge  of  which  is  covered  by  the  actual  vocal 


1078  THE   VOICE.  [Book  in. 

cord.  It  is  called  the  internal  thyro-arytenoid  (M.  thyro-ary- 
tenoideus  internus  s.  M.  vocalis)  (Figs.  189,  190,  m.th.ar.i. 
186  A)  and  has  by  some  authors  been  subdivided  into  a  median 
and  lateral  division.     The  general  direction  of  the  muscle  is 


l.cr.  ar. 


Fig.  190.     The  Internal  Thtro-akytenoid  Mtiscle. 

The  left  halves  of  the  thyroid  and  cricoid  have  been  removed  so  as  to  shew  the 
right  arytenoid  in  position. 

Th.  thyroid.  Cri.  cricoid.  Ary.  arytenoid.  S.  cartilage  of  Santorini. 
l.cr.ar.  the  crico-arytenoid  ligament,  m.th.ar.i.  the  internal  thyro-arytenoid 
muscle,  vyith  c.v.  the  vocal  cord. 

horizontally  backwards,  but,  as  in  the  external  muscle,  the  con- 
stituent bundles  run  in  various  directions  and  some  are  said 
to  end  or  begin  in  the  vocal  cord  itself.  One  most  important 
action  of  these  two  muscles  is  undoubtedly  to  bring  the  ary- 
tenoids nearer  to  the  thyroid  and  so  to  slacken  the  vocal  cords ; 
but  they  produce  other  effects,  and  their  contractions,  especially 
those  of  the  external  muscle,  help  under  circumstances  to  bring 
the  vocal  cords  together  and  so  to  narrow  the  glottis.  They  also, 
as  we  shall  see,  produce  changes  in  the  form  and  thickness  of 
the  cords. 

Of  less  importance  than  any  of  the  above  is  a  small  muscle 
which  starting  from  the  processus  muscularis  of  one  arytenoid 
passes  (Fig.  188  A,  m.ar.o.')  obliquely  upwards  towards  the 
summit  of  the  other  arytenoid,  crossing  its  fellow  obliquely  at 
the  back  of  the  transverse  arytenoid  muscle,  which  it  thus  par- 
tially covers ;  some  of  the  fibres  seem  to  end  in  the  cartilage  of 
Santorini  but  most  of  them  are  continued  to  the  thyroid,  the 
ary-epiglottic  fold,  and  the  base  of  the  epiglottis.  It  is  called 
the  oblique  arytenoid  (M.  arytenoideus  obliquus)  or  it  may  be 
regarded  as  part  of  a  flat,  irregular  muscle,  the  thyro-ary-epi- 
glottic  muscle  (Fig.  186  m.th.ar.ep.').     Its  action  is  to  approx- 


Chap,  vii.]       SPECIAL   MUSCULAR   MECHANISMS.         1079 

imate  the  two  arytenoids  and  so  to  help  in  closing  the  glottis. 
It,  with  the  transverse  arytenoid  and  the  external  thyro-aryte- 
noid  muscles,  may  be  looked  upon  as  forming  together  a  sort  of 
sphincter  of  the  larynx  ;  their  combined  contractions  certainly 
tend  to  close  the  glottis. 

A  relatively  large  and  very  important  muscle  is  the  posterior 
crico-arytenoid  (M.  crico-arytenoideus  posticus)  (Fig.  188 
m.cri.ar.p.').  This,  starting  from  the  lower  part  of  the  hind  sur- 
face of  the  cricoid  near  to  the  median  line,  passes  obliquely  up- 
wards to  be  inserted  into  the  outer  edge  of  the  arytenoid  just 
below  the  insertion  of  the  transverse  arytenoid  muscle,  at  the 
upper  part  of  the  processus  muscularis.  Its  chief  action  is  by 
wheeling  the  outer  corners  of  the  arytenoids  backwards  to  throw 
the  processus  vocalis  outwards  and  so  to  widen  the  glottis ;  it  is 
in  this  way  a  special,  we  may  perhaps  say  the  only,  dilator  of  the 
glottis,  or  abductor  of  the  cords ;  but  it  is  maintained  that  it 
has  other  actions. 

The  above  muscle  acting  as  a  dilator  meets  its  antagonist  in 
the  lateral  crico-arytenoid  (M.  crico-arytenoideus  lateralis  s.  ante- 
rior)  (Fig.   191  m.cr.ar.l.^,  which   taking  origin  from  a  large 


m.cpar.I. 


-i-c.r 


m.cr.th.o 


Fig.   191.  Fic.  192. 

Fig.  191.     The  Lateral  Ckico-arytenoiu  Muscle. 

Ary.  arytenoid,  p. v.  processus  vocalis.  p.m.  processus  muscularis.  Cri. 
cricoid.  1.  surface  for  articulation  of  lower  cornu  of  thyroid,  m.cr.ar.  I.  the 
lateral  crico-arytenoid  muscle. 

Fig.  192.     The  Crico-thyroid  Muscle. 

Th.  thyroid;  c.i.  its  inferior  cornu.  Cri.  cricoid;  m.cr.th.r.  the  straight 
part,  m.cr.th.o.  the  oblique  part  of  the  crico-thyroid  muscle,  m.cr.th.  crico-thy- 
roid membrane. 


portion  of  the  upper  border  of  the  cricoid  cartilage  in  its  lateral 
parts  in  front  of  the  thyro-cricoid  articulation,  passes  upwards 
and  backwards  to  be  inserted  into  the  processus  muscularis  and 
outer  side  of  the  arytenoid  in  front  of  and  below  the  insertion 


1080  THE  VOICE.  [Book  iii. 

of  the  posterior  crico-arytenoid.  Its  main  action  is  to  wheel 
the  outer  corner  of  the  arytenoid  forwards  and  inwards  and 
thus,  by  converging  the  processus  vocales^  to  adduct  the  cords 
and  to  narrow  the  glottis ;  and  it  has  been  urged  that  it  may  in 
this  action  be  assisted  not  antagonized  by  a  part  of  the  preceding 
muscle. 

The  last  muscle  to  which  we  need  call  attention,  and  which 
in  some  respects  stands  apart  from  the  rest,  is  the  crico-thyroid 
(M.  crico-thyroideus  anticus).  This  (Fig.  192  er.th.')  starts 
from  the  front  lateral  surface  of  the  cricoid,  near  its  lower  bor- 
der, and  passing  obliquely  backwards  and  upwards  is  inserted 
into  the  lower  edge  and  inner  lateral  surface  of  the  thyroid.  It 
is  sometimes  subdivided  into  a  front  part  (^cr.th.r.')  the  fibres 
of  which  run  more  directly  upwards  (M.  cr.  thy.  rectus)  and  a 
lateral  part  (^c7\th.o.')  the  fibres  of  which  run  in  a  more  oblique 
direction  (M.  cr.  thy.  obliquus).  The  action  of  the  muscle  is  a 
somewhat  complicated  one,  but  the  effect  of  its  contractions  as 
a  whole  is,  if  the  thyroid  be  regarded  as  the  more  moveable 
of  the  two  cartilages,  to  pull  the  thyroid  downwards  and  for- 
wards over  the  front  part  of  the  cricoid,  or,  if  the  thyroid  be 
supposed  to  be  the  more  fixed,  to  rotate  the  cricoid  on  its  tran- 
verse  axis,  pulling  upwards  the  front  part  and  tilting  down- 
wards the  hind  part  on  which  the  arytenoids  sit;  the  latter 
is  probably  its  real  action.  Upon  either  view,  its  contractions 
increase  the  distance  between  the  reentering  angle  of  the 
thyroid  and  the  processus  vocalis  and  so  stretch  the  vocal  cord ; 
it  is  in  fact  the  main  tightener  of  the  vocal  cords. 

There  are  other  small  muscles  in  the  larynx  as  well  as  mus- 
cles connecting  the  larynx  with  surrounding  parts  ;  but  it  is  not 
necessary  for  us  to  dwell  on  them  here.  Meanwhile  it  is  ob- 
vious from  what  we  have  said  that  narrowing  or  widening  the 
glottis,  and  slackening  or  tightening  the  vocal  cords,  are 
brought  about  by  the  above  muscles  acting  somewhat  as 
follows. 

§  670.  Narrowing  of  the  glottis  ;  adduction  of  the  vocal  cords. 
The  glottis  is  narrowed  by  the  combined  contraction  of  the  three 
muscles  which  we  spoke  of  above  as  forming  a  sort  of  sphincter 
for  the  larynx,  namely,  the  transverse  arytenoid,  the  oblique  ary- 
tenoid and  the  (external)  thyro-arytenoid.  These  produce  their 
effect  chiefly  by  bringing  the  two  cartilages  near  to  each  other 
in  the  middle  line,  and  in  this  action  the  transverse  arytenoid 
muscle  is  the  most  potent.  Hence  this  muscle  may  be  regarded 
as  the  most  effective  of  the  constrictors  of  the  glottis. 

The  glottis  is  also  narrowed  by  the  lateral  crico-arytenoid, 
but  this  produces  its  effect  by  rotation  of  the  arytenoid  carti- 
lages ;  it  pulls  the  processus  muscularis  forwards  and  so  throws 
the  processus  vocalis  inwards. 

Widening  of  the  glottis  ;  abduction  of  the  vocal  cords.     The 


Chap,  vii.]      SPECIAL   MUSCULAE   MECHANISMS.        1081 

chief  if  not  the  only  agent  for  the  widening  of  the  glottis  is 
the  posterior  crico-arytenoid.  This,  pulling  the  outer  edge  of 
the  arytenoid  backwards,  throws  the  processus  vocales  out- 
wards, and  so  abducts  the  vocal  cords.  It  has  been  argued  that 
the  transverse  arytenoid  acting  alone  or  in  concert  with  the  above, 
or  at  least  in  the  absence  of  any  contraction  of  the  other  mem- 
bers of  the  sphincter  group,  would  also  wheel  the  outer  edge  of 
the  arytenoid  in  the  same  way  and  so  also  abduct  the  vocal 
cords ;  but  the  evidence  seems  to  be  against  this  view. 

Tightening  of  the  vocal  cords.  This  is  especially  effected  by 
one  muscle  on  each  side,  namely  by  the  crico-thja'oid  which, 
by  bringing  the  thyroid  and  the  front  part  of  the  cricoid  nearer 
to  each  other,  increases  the  distance  between  the  thyroid  and  the 
arytenoids  when  the  latter  are  fixed.  Supposing  the  transverse 
arytenoid  and  posterior  crico-arytenoid  to  fix  the  ar}i;enoids, 
the  direct  effect  of  the  contraction  of  the  crico-thyroid  is  to 
tighten  the  vocal  cords.  There  is  besides  a  special  action  of  the 
internal  thyro-arytenoid  by  Avhich  this  muscle  becomes,  in  con- 
trast to  the  external  thyro-arytenoid,  a  tightener  of  the  cord ;  of 
this  action  we  shall  speak  later  on. 

Slackening  of  the  vocal  cords.  This  is  effected  by  the  whole 
sphincter  group  just  mentioned,  but  more  especially  by  the  ex- 
ternal thja'o-arytenoid  and  to  a  certain  extent  by  the  internal 
thyro-arytenoid;  these  acting  alone,  produce  an  effect  the  re- 
verse of  that  of  the  crico-thyroid,  bringing  the  arytenoid 
cartilages  nearer  to  the  thyroid  cartilage,  and  so  shortening 
the  distance  between  the  processus  vocales  and  that  body. 

These  several  acts,  however,  the  widening  or  narrowing  of 
the  glottis,  the  tightening  or  slackening  of  the  vocal  cords,  are 
only  the  gross  acts,  so  to  speak,  of  the  movements  of  the  larjnix. 
When  a  voice  of  any  kind  has  to  be  uttered  the  cords  must 
be  approximated  and  to  a  certain  extent  tightened ;  and  for 
the  carrying  out  of  even  these  gross  acts  not  one  muscle  only 
but  more  than  one,  and  often  several  are  brought  into  play ;  the 
movements  which  give  rise  to  any  kind  of  voice  are  combined 
and  coordinated  movements.  But,  as  we  shall  see  presently, 
when  this  or  that  particular  kind  of  voice  is  being  uttered  or 
when  changes  in  the  voice  are  being  effected,  the  alDove  words, 
widening,  tightening  and  the  like,  very  imperfectly  describe 
what  is  taking  place  in  the  larynx ;  changes  of  a  very  complex 
nature  are  brought  about,  and  for  these  the  greatest  nicety  of 
combination  is  necessary. 

§  671.  We  may  noAv  turn  to  the  nervous  mechanisms  of  the 
larynx.  Fibres  of  the  superior  laryngeal  branch  of  the  vagus 
nerve  are  distributed  to  the  mucous  membrane  of  the  larynx, 
and  serve  as  the  afferent  channels  by  which  impulses  from  the 
exquisitely  sensitive  surface  pass  upwards  to  the  central  ner- 
vous system. 


1082  THE   VOICE.  [Book  in. 

The  same  superior  laryngeal  nerve  also  contains  motor  fibres 
for  the  crico-thyroid  muscle ;  and  in  this  respect  this  muscle, 
the  chief  tensor  of  the  vocal  cords,  stands  apart  from  all  the 
rest  of  the  muscles  of  the  larynx,  for  these  are  all  supplied  by 
the  recurrent  laryngeal  branch.  These  motor  fibres,  both  of  the 
superior  and  of  the  recurrent  laryngeal  nerves,  though  running 
in  the  trunk  of  the  vagus  are  generally  believed  to  belong  not  to 
the  vagus  proper  but  to  the  spinal  accessory  nerve,  and  to  the 
division  of  that  nerve  which  we  have  called  the  bulbar  accessory 
nerve ;  but  on  this  point  opinions  are  not  agreed. 

There  are  some  reasons  for  thinking  that  the  superior  laryn- 
geal contains  afferent  fibres  not  only  for  the  crico-thyroid  but 
also  for  at  least  some  of  the  muscles  whose  motor  fibres  come 
from  the  recurrent  laryngeal;  and  it  has  been  suggested  that 
these  afferent  fibres  of  the  superior  laryngeal  convey  the  afferent 
impulses  of  the  '  muscular  sense  ; '  but  this  needs  further  inves- 
tigation. 

In  dealing  with  the  nervous  mechanism  we  must  now  dis- 
tinguish between  the  larynx  as  a  part  of  the  mechanism  of 
breathing  and  as  an  organ  of  voice.  During  breathing  the 
glottis  is  open,  and  at  least  during  at  all  deep  or  laboured 
breathing  undergoes  as  we  have  previously  said  an  increased 
widening  during  inspiration  followed  by  narrowing  during  the 
succeeding  expiration.  In  many  animals  this  rhythmic  respira- 
tory movement  is  very  marked;  but  careful  laryngoscopic  obser- 
vations shew  that  in  man  during  quite  quiet  breathing  there  is 
no  appreciable  change  in  the  width  of  the  glottis. 

Much  difference  of  opinion  has  been  expressed  as  to  whether 
the  width  of  the  glottis  thus  permanently  maintained  during 
quiet  breathing  is  identical  with  that  assumed  after  death.  But 
careful  laryngoscopic  measurements  shew  that  during  life  the 
glottis  is  distinctly  wider  than  after  death ;  the  average  width 
during  quiet  breathing,  is  in  man  about  14  mm.,  in  woman  about 
12  mm. ;  after  death  in  man  5  mm.,  in  woman  4  mm.  This 
points  to  a  continued  '  tonic '  contraction  of  some  or  other  of  the 
dilators  of  the  glottis  ;  and  the  muscle  especially  concerned  in 
this  action  appears  to  be  the  posterior  crico-arytenoid.  Whether 
this  tonic  dilator  action,  whose  centre  lies  in  the  bulb,  close  to  or 
forming  part  of  the  general  respiratory  centre,  is  automatic  in 
nature  or  maintained  in  a  reflex  manner  by  afferent  impulses, 
we  need  not  stay  now  to  discuss ;  nor  need  we  dwell  on  the 
question  whether  the  widened  glottis  is  the  result  merely  of  the 
action  of  the  dilator  muscle,  or  whether  it  is  the  balance  of  a 
struggle  between  antagonistic  muscles ;  though  analogy  would 
perhaps  lead  us  to  expect  the  latter  to  be  the  case,  the  evidence 
appears  to  be  in  favour  of  the  former  view. 

The  rhythmic  alternation  of  widening  and  of  narrowing  ob- 
served in  laboured  breathing  is,  through  the  activity  of  the  bul- 


Chap,  vii.]     SPECIAL   MUSCULAR   MECHANISMS.  1083 

bar  nervous  mechanism,  brought  about  by  the  various  muscles 
spoken  of  above,  the  sphincter  group  being  especially  used  for 
narrowing.  When  occasion  requires,  a  powerful  action  of  this 
group  leads,  as  in  the  first  step  of  a  '  cough,'  to  complete 
closure  of  the  glottis ;  and  further  security  in  the  act  is  ob- 
tained by  the  narrowing  of  the  vestibule  or  space  above  the 
vocal  cords,  the  ventricular  bands  being  brought  together  by  the 
thyro-ary-epiglottic  assisted  by  other  muscles. 

Both  the  continued  patency  and  the  rhythmic  changes  are 
carried  out  by  means  of  the  recurrent  laryngeal  nerves.  When 
in  a  living  animal  both  these  nerves  are  divided,  the  glottis  be- 
comes narrowed,  assuming  what  may  be  considered  its  natural 
dimensions,  namely,  those  proper  to  it  after  death,  when  all  mus- 
cular contractions  have  ceased.  Owing  to  the  narrowing  the 
entrance  and  exit  of  air  into  and  out  of  the  lungs  is  less  easy 
than  before,  and  a  certain  amount  of  dyspnoea,  especially  ob- 
vious if  the  breathing  be  hurried,  may  result ;  but  the  extent  to 
which  this  occurs  differs  much  in  different  kinds  of  animals  and 
indeed  in  different  individuals.  It  need  hardly  be  said  that 
when  both  the  recurrent  nerves  are  divided  the  rhythmic  widen- 
ing and  narrowing  wholly  cease,  the  glottis  remaining  immobile  ; 
the  voice  also  is  lost.  When  the  nerve  is  divided  on  one  side 
only,  the  glottis  becomes  deformed ;  when  an  attempt  to  utter 
voice  is  made,  the  vocal  cord  on  that  side  remains  farther  away 
from  the  middle  line  than  its  fellow,  owing  to  the  failure  of  the 
adductor  muscles  on  that  side,  and  no  voice  is  produced,  since 
the  approximation  and  parallelism  of  the  vocal  cords  can  no 
longer  be  effected.  On  the  other  hand  during  a  deep  inspira- 
tion the  glottis  is  deformed  by  the  vocal  cord  on  that  side  being 
nearer  the  middle  line  than  its  fellow,  owing  to  the  failure  of  the 
posterior  crico-arytenoid  on  that  side.  When  the  peripheral  por- 
tion of  one  recurrent  nerve  is  stimulated,  the  vocal  cord  of  the 
same  side  is  approximated  to  the  middle  line ;  when  both  nerves 
are  stimulated,  the  vocal  cords  are  brought  together  and  the  glot- 
tis is  narrowed ;  though  the  nerve  is  distributed  to  both  dilating 
and  constricting,  to  abductor  and  adductor  muscles,  the  latter 
overcome  the  former  when  the  nerve  is  artificially  stimulated. 
But  this  is  true  only  when  the  stimulus  is  adequately  strong ;  if 
the  stimulus  be  weak,  the  abductors  alone  are  thrown  into  con- 
traction and  the  glottis  is  widened.  We  ma}-,  in  this  connec- 
tion, add  the  remark  that  ether  paralyzes  the  adductors  before 
the  abductors,  and  has  this  effect,  even  after  division  of  both 
recurrent  nerves ;  the  more  general  respiratory  function  of  the 
larynx,  the  maintenance  of  a  wide  passage  by  means  of  the 
abductors,  is  preserved,  while  the  more  special  function  of  pho- 
nation,  the  narrowing  of  the  glottis  by  the  adductors,  is  lost. 
A  like  differentiation  of  the  two  functions  is  shewn,  in  a  re- 
verse way,  by  the  clinical  experience  that  while  functional  ner- 


1084  THE   VOICE.  [Book  hi. 

vous  disorders,  such  as  hysteria,  are  marked  by  failure  of  the 
adductors  alone,  the  characteristic  loss  of  voice  being  due  to 
this,  the  first  effect  of  structural  changes  in  the  bulb  or  other 
parts  of  the  nervous  mechanism  is  to  bring  about  failure  of 
the  abductors ;  indeed  the  condition  of  the  larynx  as  shewn 
by  the  laryngoscope  may  be  used  as  an  aid  to  the  diagnosis 
of   commencing  organic  disease. 

§  673.  When  the  larynx  is  used  for  voice  the  recurrent 
laryngeals  are  brought  into  play  in  order  to  produce  the  essen- 
tial condition  of  voice,  the  approximation  of  the  vocal  cords. 
The  vocal  cords  having  been  adequately  approximated,  low 
notes  may  be  uttered  without  any  further  change  in  the  larynx ; 
in  their  natural  position  of  rest  the  vocal  cords  are  sufficiently 
tense  to  permit  their  being  thrown  into  vibrations  when  brought 
near  enough  together  and  subjected  to  a  sufficient  blast.  In 
order  however  to  utter  notes  at  all  high,  the  tension  of  the 
cords  must  be  increased ;  and  this  as  we  have  said  is  brought 
about  chiefly  by  means  of  the  superior  laryngeal  nerves  and  the 
crico-thyroid  muscles.  Hence  when  these  nerves  are  divided  or 
fail  through  disease,  high  notes  can  no  longer  be  uttered ;  and 
the  division  or  failure  of  the  nerve  even  on  one  side  only  will 
bring  about  this  result. 

When  in  using  the  voice  a  change  has  to  be  made  from  a 
higher  to  a  lower  note,  while  the  action  of  the  crico-thyroid 
ceases  or  is  lowered,  that  of  the  antagonistic  thyro-arytenoid 
comes  into  play,  and  the  recurrent  laryngeal  nerve  is  again 
used. 

§  673.  Utterance  of  the  voice  is  a  conspicuously  voluntary 
act  and  in  the  vast  majority  of  cases  an  eminently  skilled  act. 
Hence  we  find,  as  we  have  already  (§  484 — 488)  seen,  an  area  in 
the  motor  region  of  the  cerebral  cortex  devoted  to  phonation. 
This  in  the  monkey  (Fig.  123)  lies  at  the  lowest  part  of  the  as- 
cending frontal  convolution  wedged  in  between  the  sylvian 
fissure  and  the  lower  end  of  the  precentral  fissure ;  in  man  as 
we  have  seen  (Fig.  136)  the  more  highly  developed  area  for 
'  speech '  is  situate  at  the  posterior  end  of  the  third  frontal 
convolution,  having  as  we  have  also  seen  an  importance  on 
the  left  side  of  the  brain  which  it  does  not  possess  on  the 
right. 

Stimulation  of  the  area  in  question  in  the  monkey  or  of  the 
corresponding  area  in  the  dog  leads  to  adduction  of  the  cords 
and  closure  of  the  glottis,  the  resulting  movement  being  bi- 
lateral. As  in  the  case  of  other  areas,  the  effect  is  more 
pure,  the  laryngeal  movement  is  less  mixed  with  other  move- 
ments, when  the  stimulation  is  strictly  limited  to  a  certain 
part  of  the  whole  area,  in  this  case  to  the  lower  part.  But 
stimulation  of  the  cortex  near  the  pure  centre  for  phonation 
leads  to  an  acceleration  in  the  rhythm  of  and  exaggeration  of 


Chap,  vii.]      SPECIAL   MUSCULAR   MECHANISMS.  1085 

the  laryngeal  respiratory  movements,  as  indeed  of  the  respiratory 
movements  as  a  whole  ;  though  the  respiratory  laryngeal  move- 
ments are  in  the  main  worked  by  a  bulbar  mechanism,  they  can 
be  influenced  by  cortical  changes. 

As  in  the  case  of  the  other  cortical  motor  areas,  the  path 
from  the  cortical  area  for  phonation  to  the  muscles  whose  ac- 
tions it  governs  runs  in  the  pyramidal  tract  through  the  in- 
ternal capsule.  Moreover  in  the  bulb  there  appears  to  be  a 
subordinate  nervous  mechanism,  with  which  the  impulses  or 
influences  descending  the  pyramidal  tract  make  connection  be- 
fore they  issue  as  coordinate  motor  impulses  along  the  laryngeal 
nerves ;  and  indeed  by  local  electrical  stimulation  of  the  bulb, 
in  the  floor  of  the  fourth  ventricle,  adduction  or  abduction  of 
the  cords  may  be  brought  about.  The  bulbar  mechanism  for 
abduction  is  placed  higher  up  than  that  for  adduction,  and 
stimulation  of  either  side  produces  in  both  cases  bilateral  move- 
ments. 

§  674.  So  far  we  have  mainly  spoken  of  the  voice  as  the 
result  of  two  gross  acts,  the  narrowing  of  the  glottis  and  the 
tightening  of  the  cords.  We  must  now  say  a  few  words  on 
some  other  changes  in  the  larynx,  especially  in  reference  to  the 
various  qualities  and  kinds  of  voice.  Many  of  the  features  of 
the  voice  are  conferred  upon  it  by  means  of  the  parts  of  the 
respiratory  passage  above  or  below  the  vocal  cords,  by  what  we 
have  spoken  of  as  the  resonance  chamber  or  tube ;  these  we 
shall  deal  with  in  treating  of  'speech,'  and  may  here  confine 
ourselves,  in  the  main,  to  changes  in  the  larynx  itself.  It 
should  be  noted  however  that  whenever  voice  is  uttered  the 
larynx  is  more  or  less  firmly  fixed  by  the  extrinsic  laryngeal 
muscles,  such  as  the  thyro-hyoid,  sterno-thyroid,  pharyngeal 
muscles  and  others.  The  exact  position  in  which  it  is  fixed 
will  depend  on  the  pitch  of  the  notes  which  are  uttered ;  it  is 
raised  for  high  notes  and  lowered  for  low  ones,  and  may  be  fixed 
either  above  or  below  or  at  the  natural  position  of  rest. 

We  are  accustomed  to  classify  voices  according  to  the  range 
of  pitch  within  which  the  voice  can  sing  truly  and  with  ease, 
and  we  thus  distinguish,  in  ascending  scale,  such  voices  as  bass, 
barytone  and  tenor  in  the  male,  alto,  mezzo-soprano  and  soprano 
in  the  female.  Could  we  consider  the  vocal  cord  as  a  membra- 
nous edge,  possessing  a  form  and  nature  which  was  constant  or 
varied  only  with  age,  so  that  the  rapidity  of  its  vibrations,  and 
hence  the  pitch  of  the  voice,  depended  solely  upon  its  length, 
fixed  by  the  growth  of  the  individual,  and  upon  its  varying  ten- 
sion, determined  by  muscular  contraction,  the  result  being 
influenced  by  the  var3-ing  width  of  the  glottis,  the  structural 
basis  of  the  distinction  between  the  several  kinds  of  voice  would 
be  simple  enough ;  the  bass  and  the  contralto  voices  would  have 
long  vocal  cords,  and  the  other  voices   in   each  sex  would  be 


1086  THE  VOICE.  [Book  iii. 

in  ascending  scale  successively  shorter.  The  vocal  cord,  how- 
ever, is  not  of  such  a  permanent  nature ;  it  undergoes  under 
the  influence  of  muscular  contraction  changes  other  than  those 
of  tension  affecting  its  whole  length ;  its  form  may  be  altered 
and  the  positions  or  attitudes  which  it  may  assume  cannot  be 
described  as  simply  those  of  greater  or  less  distance  from  its 
fellow  along  its  whole  length.  It  is  in  section  as  we  have  said 
wedge-shaped ;  and  the  projecting  angle  of  the  wedge  may  be 
an  open  broad  one,  or  a  narrow  acute  one ;  the  vibrating  cord 
may  be  thick  or  thin ;  and  its  vibrations  will  vary  accordingly. 

The  change  from  thick  to  thin  is  apparently  brought  about 
by  muscular  contraction;  it  has  been  suggested  that  a  partial 
contraction  of  some  of  the  fibres  of  the  thyro-arytenoid  muscle, 
external  and  also  internal,  more  particularly  of  the  bundles 
which  take  a  more  or  less  vertical  direction,  produce  the  result ; 
but  the  exact  mechanism  is  by  no  means  clear,  though  special 
examination  of  the  larynx  shews  that  such  a  change  of  thick- 
ness may  take  place. 

Again,  there  are  reasons  for  thinking  that  contraction  of  the 
internal  thyro-arytenoid  muscle  as  a  whole  affects  the  form  and 
the  physical  condition  of  the  vocal  cord,  of  which  it  furnishes 
so  to  speak  the  body ;  the  strand  of  elastic  fibres  which  forms 
the  surface  of  the  cord  lies  upon  the  muscle  somewhat  after  the 
fashion  of  a  fascia,  and  when  the  muscle,  which  in  a  state  of 
rest  is  somewhat  curved  with  the  concavity  towards  the  glottis, 
thickens  and  shortens  in  its  contraction,  carrying  with  it  the 
overlying  layer  of  elastic  fibres,  it  brings  the  whole  cord  into  a 
different  form  and  different  physical  condition ;  and  this  must 
affect  the  character  of  the  vibrations.  Again,  it  is  maintained 
that  some  of  the  fibres  of  the  internal  thyro-arytenoid  running 
forward  from  the  processus  vocalis  and  outer  surface  of  the 
arytenoid  are  inserted  into  the  layer  of  elastic  fibres  at  varying 
distances  from  the  thyroid.  If  some  of  these  bundles  were  to 
contract  by  themselves  they  might  render  the  front  part  of  the 
cord  tense  and  the  hind  part  relativel}""  lax,  or  might  modify  in 
particular  parts  that  general  tension  of  the  whole  cord  which 
was  being  effected  by  the  crico-thyroid. 

Further,  the  closure  of  the  glottis,  the  adduction  of  the 
cords  may  take  place  in  different  ways,  according  as  this  or  that 
muscle  or  part  of  a  muscle  is  being  especially  used.  While  the 
vocal  cords  are  being  sufficientl}^  approximated  to  allow  the  expira- 
tory blast  to  throw  them  into  vibrations  the  cartilaginous  glottis 
may  remain  fairly  open,  or  may  be  nearly  or  quite  closed ; 
and  each  of  these  conditions  must  affect  the  voice  in  a  different 
way.  Again,  we  have  seen  that  the  two  vocal  cords  are  close 
together  at  their  insertion  into  the  thyroid,  and  diverge  from  the 
middle  line  on  each  side  so  that  the  membranous  glottis,  when 
the  larynx  is  at  rest,  is  an  isosceles  triangle.     We  might  infer 


Chap,  vii.]      SPECIAL   MUSCULAR   MECHANISMS.         1087 

from  this  that  when  the  cords  are  adducted,  the  glottis  must 
always  remain  an  isosceles  triangle  with  the  angle  at  the  ajjex, 
next  to  the  thyroid,  becoming  more  and  more  acute  as  adduction 
proceeds,  and  that  the  parts  of  the  cords  in  front,  nearer  the 
thyroid,  must  come  into  actual  contact  before  the  parts  behind, 
nearer  the  processus  vocales,  do.  But  the  laryngoscope  shews 
that  the  form  of  the  membranous  glottis  is  very  varied ;  it  may 
be  open  behind  and  closed  in  front,  or  closed  both  behind  and  in 
front  and  open,  even  widely  so,  in  the  middle,  or  may  be  along 
almost  its  whole  length  a  slit  with  parallel  sides,  and  in  that 
case  either  very  narrow,  a  mere  linear  cleft,  or  of  appreciable 
width.  And  though  the  exact  mechanisms  are  obscure,  we  can- 
not doubt  but  that  these  several  phases  result  from  special 
muscular  contractions. 

§  675.  We  might  dwell  on  other  changes  which  may  by  help 
of  the  laryngoscope  be  observed  in  the  larynx  during  the  pro- 
duction of  the  voice,  all  shewing  that  muscular  contractions 
may  produce  complex  and  varied  changes  in  the  larynx  besides 
simple  adduction  or  abduction  and  general  tension  or  slackening 
of  the  vocal  cords ;  but  we  have  said  enough  for  our  present 
purpose,  which  is  to  insist  that  in  the  production  of  voice  the 
mere  dimensions  of  the  larynx,  and  we  might  add  other  natural 
inborn  features,  serve  but  as  the  playground  for  muscular  skill ; 
it  is  the  latter  much  more  than  the  former  which  determines  the 
characters  and  the  powers  of  the  voice.  A  laryngoscopist,  even 
the  most  experienced,  would  probably  hesitate  from  a  mere  in- 
spection of  the  larynx  to  predicate  the  nature  of  the  singing 
voice.  He  could  not  even  predicate  the  possession  of  a  singing 
voice  of  any  kind.  Of  two  larynges,  provided  they  were  both 
of  normal  structure,  he  would  be  unable  to  say  which  belonged 
to  the  man  who  could  and  which  to  the  man  who  could  not  sing ; 
for  the  power  to  sing  is  determined  not  by  the  build  of  the 
larynx  but  by  the  possession  of  an  adequate  nervous  mechanism 
through  which  finely  appreciated  auditory  impulses  are  enabled 
so  to  guide  the  impulses  of  the  will  that  these  find  their  way  with 
sureness  and  precision  to  the  appropriate  muscular  bundles. 
And  what  is  true  of  the  difference  between  singing  and  not 
singing  at  all  is  in  a  similar  way  true  of  the  difference  between 
singing  low  and  singing  high,  as  well  as  of  the  difference 
between  singing  superbly  and  singing  indifferently  well.  The 
physiological  difference  between  a  bass  voice  and  a  tenor  voice, 
between  a  contralto  and  a  soprano  probably  lies  not  so  much  in 
the  mere  natural  length  of  the  vocal  cords  as  in  the  constitution 
of  the  nervous  and  muscular  mechanism ;  experience  shews  that 
cords  of  the  same  natural  length  may  in  one  individual  be  the 
instrument  of  a  bass,  in  another  of  a  tenor  voice,  or  in  one  indi- 
vidual of  a  contralto,  in  another  of  a  soprano  voice.  Again, 
though  the  "  magnificent  organ '"  of  a  distinguished  artist  may 


1088  THE  VOICE.  [Book  iii. 

have  certain  inborn  qualities  which  lighten  the  labours  of  the 
nervous  mechanism,  it  is  the  latter  which  is  the  real  basis  of 
the  artist's  fame ;  the  former  may  be  so  slight  or  so  abstruse  as 
to  escape  observation,  and  a  larynx,  the  notes  of  which  have 
charmed  the  world,  may  yield  through  the  laryngeal  mirror  a 
picture  of  the  most  commonplace  kind. 

That  the  build  of  the  larynx  is  thus  wholly  subordinate  to 
the  nervous  mechanism  is  further  illustrated  by  the  fact  that 
the  same  larynx  may  and  indeed  does  produce  different  kinds 
of  voice.  The  difference  in  the  kind  of  voice  which  may  be 
brought  about  by  the  nervous  system  working  the  same  larynx 
in  different  ways  is  strikingly  shewn  by  comparing  what  is 
called  the  chest  voice  and  the  head  voice.  In  the  former, 
which  deals  with  relatively  low  notes,  the  sounds  are  full  and 
strong,  and  the  lower  resonance  chamber  which  is  supplied  by 
the  trachea,  bronchi  and  indeed  by  the  whole  chest,  is  thrown 
into  powerful  and  palpable  vibrations ;  hence  the  name  '  chest 
voice.'  The  latter,  which  deals  with  relatively  high  notes,  is 
thin  and  poor,  being  deficient  in  partial  tones,  is  not  accom- 
panied by  the  same  conspicuous  vibrations  of  the  chest  but  is 
accompanied  by  vibrations  of  the  head ;  hence  the  name  '  head 
voice.' 

It  is  obvious  that  the  dispositions  of  the  larynx  must  be  very 
different  in  the  two  voices ;  but  what  the  differences  exactly 
are  has  been  and  still  is  a  matter  of  controversy,  and  indeed 
extended  laryngoscopic  observation  leads  to  the  conclusion  that 
the  change  from  the  one  voice  to  the  other  is  not  effected  in 
precisely  the  same  way  by  all  larynges.  The  evidence  however 
seems  to  shew  that  in  the  chest  voice  the  vocal  cords  are  rela- 
tively broad  and  thick,  and  that  the  membranous  glottis  is 
open  along  its  whole  length.  The  cords  will  of  course  vary  as 
to  their  tension  through  the  range  of  the  voice,  being  more 
tense  with  the  higher  notes,  and  the  width  of  the  glottis  is  not 
always  the  same ;  but  it  is  probable  that  throughout  the  voice 
the  cords,  in  producing  the  fundamental  tone  of  any  note  sung, 
vibrate  along  their  whole  length,  and  also  through  their  whole 
breadth,  the  partial  tones  being  due  of  course,  as  in  other 
musical  instruments,  to  vibrations  of  segments.  In  order  to 
throw  into  vibration  along  their  whole  length  such  relatively 
broad  and  thick  cords  a  powerful  blast  of  air  is  needed,  and 
hence  the  transmission  of  the  vibrations  downwards  to  the 
chest. 

When  the  same  larynx  shifts  to  the  head  voice  the  vocal 
cords  appear  to  become  narrow,  thin  and  always  distinctly 
tense.  In  some  cases  the  membranous  glottis  is  closed  before 
and  behind,  so  that  the  cords  are  free  to  vibrate  in  their  middle 
portion  only,  and  here  the  slit  is  sometimes  a  relatively  wide 
elliptical  space ;    in  other   cases   the  glottis  seems  to  be  open 


Chap,  vii.]      SPECIAL   IMUSCULAR  MECHANISMS.         1089 

along  its  whole  extent,  thongli  reduced  to  a  mere  linear  slit; 
but  it  is  probable  that  in  all  cases  the  cords  vibrate  along  a  part 
only  and  not  along  the  whole  of  their  length.  In  order  to 
throw  into  adequate  vibrations  the  thin  edges  now  presented,  a 
less  powerful  blast  is  required,  and  the  vibrations  are  no  longer 
felt  in  the  chest,  though  they  are  transmitted  through  the 
pharyngeal  passages  to  the  head. 

As  subsidiary  conditions  we  may  mention  that  in  the  chest 
voice  the  superior  aperture  of  the  larynx  is  widely  open,  the 
transverse  diameter  being  perhaps  especially  long,  while  in  the 
head  voice  the  aperture  is  constricted,  at  times  remarkably  so. 
In  the  chest  voice  the  epiglottis  is  usually  depressed  so  as  to 
hide  from  sight,  in  the  laryngoscopic  view,  the  front  part  of  the 
cords,  while  in  the  head  voice  it  is  usually  raised,  but  many 
variations  in  the  attitude  of  the  epiglottis  may  be  observed.  In 
the  head  voice  the  cartilaginous  glottis  seems  always  to  be  com- 
pletely closed,  whereas  in  the  chest  voice  it  is  found  in  some 
cases  to  be  closed,  in  other  cases  to  be  more  or  less  open. 

Making  all  allowance  for  discordance  of  opinion  as  to  what 
are  the  exact  conditions  of  each  kind  of  voice,  and  admitting 
the  imperfection  of  our  knowledge  as  to  both  the  purjjose  and 
the  mode  of  production  of  many  of  the  differences  observed, 
we  may  at  least  draw  the  conclusion  that  in  the  case  of  each 
kind  of  voice  a  certain  general  disposition  of  the  mechanism  is 
made,  that  a  certain  '  setting '  of  the  machine  takes  place,  by 
which  the  quality  of  the  voice  is  determined,  and  that  the 
machine  thus  set  is  played  upon  so  as  to  produce  a  series  of 
notes  differing  in  pitch  but  all  retaining  the  same  particular 
quality.  The  setting  of  the  machine  in  the  chest  voice  is  such 
that  the  notes  produced  by  it  are  lower  notes  reaching  up  to  a 
certain  pitch  only,  the  setting  not  being  adapted  for  higher 
notes,  and  conversely  the  setting  of  the  head  voice  allows  of 
the  production  of  high  notes  only,  being  incompatible  with  the 
production  of  low  notes. 

It  may  be  urged  that  the  setting  for  the  chest  voice  is  really 
the  natural  disposition  of  the  larynx  and  that  of  the  head  voice 
a  strange  and  artificial  condition  into  which  the  larynx  is 
forced  (and  indeed  the  latter  is  in  certain  cases  called  a  "fal- 
setto "  voice,  which  term  however  has  a  technical  meaning  not 
always  coincident  with  head  voice)  ;  but  a  closer  examination 
of  voices  shews  that  there  is  in  reality  no  one  natural  condi- 
tion of  the  larjmx,  and  that  there  are  other  dispositions  or  set- 
tings of  the  lar^mx  besides  those  of  the  chest  voice  and  the 
head  voice,  these  being  so  to  speak  extreme  cases. 

When  a  singer  sings  a  series  of  notes  in  an  ascending  scale 
it  will  be  observed  that  beginning  with  the  lowest  notes  the 
voice  during  a  certain  range  remains  through  all  the  notes  of 
the  same  quality,  differing  in  pitch  only,  but  that  at  or  about  a 

69 


1090  THE   VOICE.  [Book  in. 

certain  note,  the  voice  in  passing  from  one  note  to  the  next 
above  is  not  merely  raised  in  pitch  but  absolutely  altered  in 
quality,  and  further  maintains  the  new  features  in  the  succeed- 
ing higher  notes.  This  sudden  change  is  spoken  of  as  the 
'  break '  in  the  voice,  and  a  range  of  notes  of  differing  pitch  but 
of  the  same  quality  which  is  thus  se|)arated  by  a  '  break '  from 
another  range  of  notes  of  a  different  quality  is  called  a  'register.' 
Larj-ngoscopic  observations,  especially  recent  ones  in  which  pho- 
tographic aid  has  been  called  in,  shew  that  during  a  register 
there  is  a  particular  'setting'  of  the  larynx,  which  is  maintained 
throughout  the  whole  register,  the  chief  change  observable  being 
an  increasing  tension  of  the  cords  as  the  notes  rise  in  pitch,  and 
that  at  the  break  there  is  a  sudden  shifting  of  the  setting,  the 
new  setting  being  maintained  during  the  ensuing  register 
with  changes  which  as  before  are  chiefly  directed  to  a  tension 
of  the  cords  increasing  with  the  rising  notes. 

In  most  voices  the  ear  may  recognize  two  such  breaks,  separa- 
ting three  registers,  lower,  middle,  and  upper,  the  lower  and 
upper  being  usually  the  chest  and  head  voice  described  above. 
But  some  voices  are  marked  by  three  breaks  separating  four 
registers,  the  differences  being  distinctly  recognizable  by  the  ear, 
and  there  ai'e  some  reasons  for  thinking  that  a  break,  that  is  a 
change  in  the  setting  of  the  larnyx,  may  take  place  without 
being  evident  to  the  ear,  though  visible  by  the  help  of  the  laryn- 
goscope. We  may  add  one  part  of  the  training  of  a  singing  voice 
consists  in  rendering  the  break,  the  transition  from  one  register 
to  another,  as  little  obvious  to  the  ear  as  possible. 

It  would  be  beyond  the  scope  of  this  work  to  enter  upon  the 
details  of  the  several  registers  of  the  different  kinds  of  voices, 
beyond  the  little  we  have  said  touching  the  chest  voice  and  head 
voice ;  these  are  matters  of  great  difficulty  subject  to  much  con- 
troversy, and  indeed,  as  we  have  already  said,  observations  tend 
to  shew  that  exactly  the  same  disposition  does  not  obtain  for  the 
same  register  in  all  larynges ;  it  seems  probable  that  two  larynges 
may  gain  the  same  end  by  two  different  manoeuvres,  may 
produce  the  same  kind  of  voice  by  different  dispositions  of  the 
larynx.  In  any  case  the  subject  is  one  of  extreme  complexity,  and 
we  have  ventured  to  dwell  on  it,  even  so  long  as  we  have,  because 
it  affords  a  striking  illustration  of  what  we  have  more  than  once 
insisted  upon,  the  complicated  character  so  often  belonging  to 
the  muscular  contractions  by  which  the  animal  bod}^  gains  its 
ends,  and  the  delicately  adjusted  coordination  of  efferent  nervous 
impulses  needed  to  secure  for  the  effort  a  complete  success.  We 
have  so  repeatedly,  in  previous  parts  of  this  work,  insisted  on 
the  importance  of  afferent  impulses  to  the  coordination  of  com- 
plex movements  that  it  is  hardly  necessary  here  to  do  more  than 
to  point  out  that  the  connection  of  the  use  of  the  voice  with 
auditory  sensations  affords  striking  instances  of  the  truth  of 


Chap,  vii.]      SPECIAL   MUSCULAR   MECHAE'ISMS.         1091 

what  we  have  insisted  upon.  Auditory  sensations  are  at  least 
as  important  for  the  proper  management  of  the  voice  as  are 
visual  sensations  for  the  movements  of  the  eyes,  and  more  im- 
portant than  are  visual  sensations  for  the  movements  of  the  body 
and  limbs.  Indeed  they  are  in  a  way  essential  to  the  very  utter- 
ance of  the  voice;  the  dumbness  which  is  so  conspicuous  a  con- 
comitant of  conofenital  deafness  is  in  most  cases  due  not  to 
deficiency  in  the  muscular  apparatus  or  even  m  the  nervous 
mechanism  on  what  we  may  call  its  motor  side,  but  to  the  lack 
of  afferent  impulses  from  the  auditory  nerve.  And  in  popular 
language  we  recognize  this  dependence  of  the  management  of 
the  laryngeal  muscles  on  auditory  sensations  when  we  talk  of 
such  or  such  one,  who  is  delicient  in  this  respect,  as  "  having  no 
ear." 

§  676.  The  ventricles  of  the  larynx  appear  to  be  useful  in 
allowing  the  vocal  cords  sufficient  room  for  their  vibrations ; 
they  also  supply  a  secretion  by  which  the  vocal  cords  are  kept 
adequately  moist.  The  purpose  of  the  ventricular  bands  is  not 
exactly  known  ;  it  has  been  suggested  that  they  may  at  times 
exert  a  damping  action  by  being  brought  down  to  touch  the 
vocal  cords ;  but  this  is  A'ery  doubtful.  The  epiglottis,  the 
position  of  which  as  we  have  seen  varies  in  different  kinds  of 
voice,  has  also  an  influence  on  the  character  of  the  voice;  and 
further  influences  Avhich  we  shall  consider  under  'speech'  are 
exerted  by  tlie  form  of  the  pharynx  and  the  mouth. 

§  677.  At  the  age  of  puberty  a  rapid  development  of  the 
laiynx  takes  place,  leading  to  a  change  in  the  range  of  the  voice. 
The  peculiar  harshness  of  the  voice  when  it  is  thus  '  breaking ' 
seems  to  be  due  to  a  temporary  congested  and  swollen  condition 
of  the  mucous  membrane  of  the  vocal  cords  accompanjdng  the 
active  growth  of  the  whole  larj'nx.  The  change  in  the  mucous 
membrane  may  come  on  quite  suddenly,  the  voice  '  breaking ' 
for  instance  in  the  course  of  a  nio-ht. 


SEC.   2.     SPEECH. 

§  678.  All  sounds  as  we  have  seen  (§  619)  may  be  divided 
into  musical  sounds,  in  which  the  vibrations  are  regular,  and 
noises,  in  which  the  vibrations  are  irregular ;  but  we  have  also 
seen  that  the  distinction  between  the  two  is  not  a  sharp  one. 
The  vibrations  into  which  the  air  in  the  larynx  is  thrown  by  the 
vibrations  of  the  vocal  cords  in  ordinary  voice  are  on  the  whole 
regular ;  the  sound  so  produced  is  a  musical  sound.  The  vibra- 
tions of  the  glottis  may  however  vary  as  to  the  degree  of  their 
regularity ;  and  under  certain  circumstances  they  may  be  so 
irregular  that  the  sound  becomes  an  undeniable  noise ;  as  for 
instance  in  the  sound  which  we  call  a  '  cry '  or  a  '  shriek.' 

The  sounds  produced  in  the  larynx  like  other  musical  sounds 
consist  of  partial  tones  added  to  a  fundamental  tone,  and  are  in 
many  cases  very  rich  in  partial  tones.  By  modifying  the  shape 
of  the  passage  leading  through  the  pharyngeal,  the  buccal,  and 
to  a  certain  extent  the  nasal  cavities,  to  the  opening  of  the 
mouth,  which  we  have  spoken  of  as  a  resonance  tube  or  cham- 
ber, and  which,  for  reasons  which  we  shall  see,  we  may  now  call 
the  vowel  ehamher,  we  are  able  to  render  loud  and  prominent  one 
or  other  of  the  partial  tones  of  a  sound  which  is  produced  by 
the  larynx  and  thus  to  affect  its  quality  as  it  leaves  the  mouth. 

We  are  also  able,  quite  independently  of  the  larynx  (and 
indeed  independently  of  breathing),  to  create  sounds  by  means 
of  parts  of  tlie  mouth  or  other  portions  of  the  vowel  chamber. 
These  are  for  the  most  part  noises  but,  as  for  instance  in  whis- 
tling, may  be  musical  sounds. 

In  speech  we  make  use  on  the  one  hand  of  laryngeal  sounds, 
more  or  less  modified  in  quality  by  the  vowel  chamber,  and  on 
the  other  hand  of  sounds  generated  in  various  parts  of  that 
chamber;  our  speech  in  fact  consists  of  a  basis  of  musical  sounds 
with  an  addition  of  noises. 

§  679.  One  great  feature  of  speech  is  that  it  is  "  articulate  ;  " 
it  consists  of  syllables  jointed  together,  the  parts  of  speech  which 
we  call  words  being  formed  of  two  or  more  syllables,  or  at  times 
of  one  only.  In  the  great  majority  of  syllables  we  recognize  two 
kinds  of  sounds  which  we  call  vowels  and  consonants.     Though  it 

1092 


Chap,  yii.]         SOME   SPECIAL  MECHANISMS.  1093 

is  easy  to  say  which  is  a  vowel  and  which  is  a  consonant,  it  is 
difficult  to  frame  a  definition  which  shall  be  free  from  all  objec- 
tions. It  has  been  said  that  vowels  are  formed  by  the  voice, 
that  is  by  the  vibrations  of  the  vocal  cords  (hence  the  name 
vowel,  voealis)^  and  consonants  by  the  mouth,  lips  or  other  parts 
of  the  chamber  above  the  larynx ;  but  as  we  shall  see,  on  the 
one  hand  the  vowels,  as  indeed  the  name  which  we  have  adopted 
for  the  chamber  indicates,  are  formed  by  help  of  that  chamber, 
and  on  the  other  hand  many  consonants  are  formed  by  help  of 
the  voice.  The  word  'consonant'  expresses  the  view  that  what 
we  call  consonants  are  always  sounded  with  some  vowel  or  other 
and  cannot  be  sounded  alone  by  themselves ;  but  several  conso- 
nants can  be  so  sounded ;  hence  the  name  is  inappropriate.  We 
may  make  the  distinction  that  whereas  in  a  vowel  the  form 
assumed  by  the  resonance  tube  merely  modifies  the  sound  pro- 
duced by  the  larynx,  in  a  consonant  a  change  of  form  in  the 
same  tube  creates  a  noise  which  may  exist  by  itself  or  may 
mingle  with  the  sound  produced  by  the  larynx  ;  but  this  is  not 
exact,  since  as  we  shall  see  such  a  consonant  as  i)/may  be  used 
(as  for  instance  in  'bottom,'  in  which  though  we  write  we  do 
not  sound  the  second  o)  in  such  a  way  that  the  form  of  the 
mouth  only  modifies  a  laryngeal  sound,  and  the  utterance  may 
be  continued  indefinitely,  like  that  of  a  vowel.  Indeed  we 
employ  J/and  certain  other  consonants  in  two  ways;  we  use  31 
as  a  true  consonant  in  company  with  a  vowel  as  in  '  my '  or,  as 
in  the  above  instance,  we  may  use  it  by  itself,  it  alone  forming 
a  syllable.  In  this  latter  function  J/ may  conveniently  be  called 
a  sonant,  the  sounds  of  speech  being  divided  into  '  sonants '  and 
true  '  consonants.'  We  may  however  leave  these  definitions  and 
turn  at  once  to  the  mode  of  formation  of  the  several  vowels 
and  consonants,  or  rather  to  the  more  common  of  these,  since 
each  language  has  its  own  vowels  and  sounds,  and  while  some 
are  common  to  all,  some  are  special  to  a  few,  or  even  to  one. 
We  may  merely  remark  that  in  speech  the  A'owels  bear  the  brunt 
of  the  work,  they  carry  the  '  accent,'  and  the  consonants  are,  so 
to  speak,  built  upon  them  as  on  a  foundation.  Some  consonants 
(sonants)  however  may  be  used  like  vowels  to  carry  accent. 

§  680.  Vowels.  With  the  utterance,  either  in  singing  or 
speaking,  of  each  vowel  the  vowel  chamljer  is  moulded  into  a 
particular  shape.  Taking  the  most  common  vowels  U,  O,  A,  E,  I 
pronounced  in  tlie  way  in  which  most  nations  pronounce  them 
and  so  corresponding  to  our  oo,  o,  bi'oad  a  {ah},  e  as  in  bet,  and 
ee,  we  find  the  following. 

In  U  the  vowel  chamber  is  large  with  a  narrow  opening  at 
the  mouth.  The  larynx  is  depressed,  or  at  least  not  raised  above 
the  position  of  rest,  the  tongue  is  flattened,  especially  in  front, 
and  the  lips  are  protruded  so  as  to  reduce  the  mouth  to  a  narrow 
round  opening.     The  form  of  the  vowel  chamber,  with  a  wide 


1094  SPEECH.  [Book  hi. 

pharyngeal  and  a  narrow  buccal  orifice,  may  be  compared  to  that 
of  a  round  flask  without  a  neck.  In  A,  the  mouth  is  opened 
wide,  the  larynx  is  somewhat  raised,  the  tongue  flattened  and  at 
the  same  time  driven  somewhat  backwards  towards  the  hind  wall 
of  the  pharynx,  so  that  the  entrance  from  the  pharynx  to  the 
larjmx  is  narrowed.  The  vowel  chamber  thus  assumes  a  form 
which  may  be  compared  to  that  of  a  funnel,  the  wide  end  being 
at  the  mouth  and  the  narrow  end  at  the  larynx.  In  O  the  shape 
is  intermediate  between  that  of  U  and  that  of  A,  the  exact  shape 
depending  on  the  kind  of  O  which  is  being  uttered. 

In  I  the  shape  is  very  different.  The  larynx  is  raised  and 
the  tongue  is  carried  forwards  and  upwards  in  such  a  way  that 
it  touches  the  teeth  and  the  hard  palate  at  the  sides  and  nearly 
so  in  the  middle  leaving  only  a  narrow  canal  in  the  middle  line. 
At  the  same  time  the  lips  instead  of  being  protruded  are  drawn 
back  and  the  soft  palate  is  raised  high  up.  In  this  way  there  is 
developed  above  the  larynx  a  relatively  large  pharyngeal  space 
which  communicates  with  the  exterior  by  a  narrow  canal ;  the 
form  of  the  vowel  chamber  may  now  be  compared  to  that  of  a 
round  flask  with  a  long  narrow  neck.  In  E,  and  the  other  vowels 
between  A  and  I,  the  shape  of  the  resonance  is  correspondingly 
intermediate ;  in  passing  from  A  to  I,  the  tongue  is  brought  for- 
wards and  upwards,  the  buccal  orifice  narrowed  and  the  larynx 
raised. 

In  each  of  the  above  cases  what  we  have  called  the  vowel 
chamber  acts  as  a  resonance  chamber ;  that  is  to  say  in  each 
case,  owing  to  the  shape  of  the  cavity  (in  relation  to  the  nature 
of  its  walls),  the  air  in  the  chamber  is  more  readily  thrown  into 
vibrations  by  certain  tones  than  by  others,  and  when  a  sound 
containing  those  particular  tones  is  sounded  into  the  chamber, 
those  particular  tones  are  reinforced  and  rendered  loud  and 
prominent.  The  shape  of  the  vowel  chamber  in  uttering  U  is 
such  that  the  cavity  acts  as  a  resonator  towards  a  particular  tone, 
namely,  the  bass  /,  or  more  probably  the  bass  5,  and  while  the 
laryngeal  sound  with  its  fundamental  and  partial  tones  is  j)ass- 
ing  through  it,  reinforces  and  renders  loud  the  tone  5,  occurring 
as  a  constituent  of  the  whole  sound.  And  similarly  with  the 
other  vowels.  In  fact  vowel  sounds  are  musical  sounds  in  which 
a  particular  constituent  tone  is  reinforced  and  rendered  loud  out 
of  proportion  to  the  other  tones ;  in  the  case  of  some  vowels  two 
tones  are  so  reinforced.  The  tone  thus  reinforced  is  generally  a 
partial  tone,  but  may  be  the  fundamental  tone.  When  the  vowel 
is  sung  or  spoken  in  notes  of  different  pitch  the  particular  par- 
tial tone  which  is  reinforced  will  occupy  different  positions  in 
the  series  of  partial  tones ;  it  may  be  the  first,  second,  third  or 
other  partial  tone  according  to  the  pitch  of  the  fundamental 
tone. 

That  the  vowel  chamber  does  act  in  this  way  as  a  resonator 


Chap,  vii.]         SOME    SPECIAL    MECHANISMS.  1095 

for  a  particular  tone  is  shewn  by  moulding  the  cavity  into  the 
proper  form  for  uttering  a  particular  vowel,  and  bringing  before 
the  mouth  a  series  of  sounding  tuning-forks  of  different  pitch ; 
it  will  be  found  that  it  is  the  sound  of  one  tuning-fork  and  one 
in  particular  which  is  reinforced  and  made  louder,  namely,  the 
one  whose  pitch  corresponds  to  the  fundamental  tone  of  the  par- 
ticular vowel  cavity ;  in  the  case  of  the  vowel  U  for  instance  it 
will  be  the  tuning-fork  having  the  pitch  h.  On  the  other  liand 
that  what  we  recognize  as  vowel  sounds  do  result  from  the  rein- 
forcement in  a  musical  sound  of  a  particular  tone  or  of  particular 
tones  may  be  shewn  by  setting  into  vibration  a  series  of  tuning- 
forks  of  different  pitch,  in  imitation  of  a  musical  sound  with  its 
constituent  tones,  and  then  in  turn  reinforcing  the  sound  of  par- 
ticular tuning-forks  by  the  help  of  artificial  resonators.  When 
this  is  done  the  reinforcement  of  the  appropriate  tone  gives  rise 
to  a  vowel  sound,  the  reinforcement  of  h  giving  rise  to  U  and  so 
on.  The  curves  moreover  described  by  the  vowel  sounds  in  the 
phonograph,  in  which  the  vibrations  of  the  air  transmitted  to  a. 
thin  plate  or  membrane  are  made  to  write  on  a  recording  surface, 
are  in  form  such  as  would  be  described  by  sounds  in  which 
particular  constituent  tones  were  reinforced  in  the  manner 
described.  Again,  as  we  said  in  dealing  with  henring  (§  629), 
when  a  note  is  sung  into  the  open  piano,  the  particular  strings 
of  the  piano  corresponding  to  the  constituent  tones  of  the  note 
sung  are  thrown  into  sympathetic  vibration  ;  in  the  sound  thus 
returned  by  the  piano  the  nature  of  the  vowel  on  whicli  the  note 
was  sung  may  be  recognized  ;  the  string  corresponding  to  the 
characteristic  tone  of  the  vowel  is  thrown  into  appropriately 
strong  vibrations. 

The  nature  of  the  vowel  sounds  is  especially  w^ell  illustrated 
by  the  kind  of  speech  which  we  call  whispering.  In  this,  in  con- 
trast to  audible  speech,  no  musical  sounds  are  generated  by  the 
vocal  cords.  A  laryngeal  sound  is  generated  but  it  is  a  noise,  not 
a  musical  sound,  and  is  caused  by  the  friction  of  the  air  as  it 
passes  through  the  glottis,  which  assumes  a  peculiar  form,  the 
processus  vocales  projecting  inwaids  towards  each  other,  leaving 
the  cartilaginous  glottis  as  well  as  the  greater  part  of  the  mem- 
branous glottis  more  or  less  open.  This  noise,  like  the  musical 
sound  of  audible  speech,  is  modified  by  the  parts  of  the  mouth 
and  pharynx,  and  in  it  we  may  recognize  vowels  and  conso- 
nants. The  noise  of  the  whisper,  though  weak,  contains  multi- 
farious vibrations,  contains  among  other  and  irregular  vibrations, 
the  regular  vibrations  corresponding  to  the  several  vowels. 
When  we  whisper  a  vowel,  we  '  set '  the  vowel  cliamber  so  that 
it  may  reinforce  the  set  of  vibrations  of  the  particular  jiitch 
characteristic  of  the  vowel ;  and  a  well-trained  ear  may  recog- 
nize in  the  whispered  vowel  the  dominant  tone. 

A  vowel  then  is  essentially  a  musical  sound   of  a    special 


1096  SPEECH.  [Book  hi. 

quality,  due  to  the  dominance  of  a  particular  tone,  originating 
in  the  glottis  and  reinforced  by  the  conformation  of  the  vowel 
chamber.  This  view  at  least,  as  expounded  above,  is  the  one 
generall}^  held  ;  but  recent  observations  based  on  graphic  records 
have,  it  is  right  to  say,  thrown  some  doubt  on  the  matter;  these 
we  cannot  discuss  here.  There  are  moreover  many  subsidiary 
questions  connected  with  the  formation  of  vowels  into  which  we 
cannot  enter  here.  We  will  merely  add  that  in  uttering  a  true 
diphthong,  the  conformation  of  the  vowel  chamber  proper  to  the 
initial  vowel  is  changed  rapidly  but  gradually  and  without  any 
obvious  break  into  that  proper  to  the  final  vowel. 

§  681.  Consonants.  These  as  we  have  already  said  in  some 
cases  so  far  resemble  vowels  in  that  they  are  modifications  of 
the  voice,  that  is  to  say  of  laryngeal  sounds,  caused  by  the  dis- 
position of  the  parts  of  the  vowel  chamber,  the  disposition  how- 
ever being  in  all  such  cases  different  from  that  giving  rise  to  a 
vowel,  and  moreover  the  very  assumption  of  the  disposition  tak- 
ing part  in  the  generation  of  the  whole  sound.  Such  consonants 
however  are  relatively  few,  the  great  majority  of  consonants  are 
caused  by  changes  which  set  up  vibrations  in  some  part  or  other 
of  the  vowel  chamber  or  in  the  larynx  itself ;  they  are  noises 
which  may  or  may  not  be  accompanied  by  voice,  the  vibrations 
producing  a  different  consonant  according  as  they  are  or  are  not 
accompanied  by  voice.  Such  vibrations  may  be  set  up  in  several 
ways.  In  the  case  of  many  consonants  vibrations  are  set  up  by 
the  passage  being  closed  and  then  suddenly  opened  at  a  particu- 
lar part ;  such  consonants  are  spoken  of  as  explosives.  In  the 
case  of  these  consonants  the  noise  which  is  their  essential  part 
cannot  be  prolonged,  it  is  momentary  in  duration,  though  when 
it  is  accompanied  by  voice  the  latter  may  continue  for  some 
time.  In  the  case  of  other  consonants  the  noise  is  caused  by  the 
rush  of  air  through  a  narrow  space  and  the  consonants  may  be 
described  as  '  frictional ' ;  or  the  noise  may  be  produced  by  the 
vibratory  movements  of  particular  parts.  Both  these  kinds  of 
consonants  can  naturally  be  prolonged  for  an  indefinite  time, 
and  have  been  called  continuous  consonants. 

On  the  other  hand  the  characters  of  a  consonant  are  also 
dependent  on  the  particular  part  of  the  passage  at  which  they 
are  generated;  and  this  also  may  be  used  as  a  means  of  classifi- 
cation. Some  consonants  are  produced  at  the  lips  by  the  move- 
ment or  position  of  the  lips  in  reference  to  each  other  or  to 
the  teeth;  these  are  called  labial  and  labio-dental  consonants. 
Others  again  are  produced  by  the  movement  or  position  of  the 
tongue  in  reference  to  the  teeth  or  to  the  teeth  and  hard  palate ; 
these  are  called  dental.  Yet  others  are  produced  by  the  move- 
ment of  the  back  of  the  tongue  in  relation  to  the  soft  palate  and 
pharynx  or  fauces ;  these  are  called  guttural  consonants,  the 
name  being  also  applied  to  certain  consonants  which  are  essen- 


Chap,  vii.]         SOME   SPECIAL  MECHANISMS.  1097 

tially  noises  generated  in  the  larynx  itself.  These  names  are  use- 
ful for  a  general  broad  classification  ;  the  term  dental  however  is 
used  to  include  consonants  which  are  formed  by  the  tongue  in 
relation  to,  not  the  teeth,  but  the  front  part  of  the  hard  palate; 
hence  it  is  to  that  extent  open  to  objection.  There  are  also 
other  classifications  into  which  we  cannot  enter  here. 

§  682.  When  the  various  languages  of  the  Avorld  are  exam- 
ined the  number  of  consonants,  that  is  to  say  of  sounds  used  in 
speech  and  having  the  characters  on  which  we  are  dwelling,  is 
found  to  be  very  large ;  and  concerning  the  nature  and  mode 
of  formation  of  many  of  them  much  discussion  has  taken  place. 
We  must  content  ourselves  here  with  very  briefly  indicating  the 
chief  facts  concerning  the  mode  of  formation  of  the  most  impor- 
tant and  common. 

The  group  of  consonants  represented  by  M,  N,  NG,  are  very 
closely  allied  to  vowels.  In  each  of  these  as  in  a  vowel  the 
larynx  is  thrown  into  vibrations ;  but  instead  of  the  vibrations 
passing  out  by  the  mouth  through  a  passage  which  has  assumed 
a  form  belonging  to  this  or  that  vowel,  the  passage  to  the  mouth 
is  closed,  and  the  vibrations  find  their  way  out  through  the  nasal 
cavity  which  acts  as  a  resonance  chamber.  When  a  vowel  is 
sounded  the  soft  palate  either  completely  shuts  off  the  nasal 
cavity  from  the  vowel  chamber,  or  at  least  offers  such  resistance 
that  an  insignificant  proportion  of  the  expiratory  blast  passes 
into  the  nasal  cavities.  A  vowel  may  be  sung  powerfully,  and 
yet  a  flame  exposed  to  the  nostrils  only  will  shew  no  movements ; 
in  the  case  at  least  of  some  vowels  however,  a  piece  of  cold  pol- 
ished steel  will  become  dim,  shewing  that  some  air  is  passing 
through  the  nostrils.  When  the  communications  between  the 
nasal  and  pharyngeal  cavities  are  sufficiently  free,  and  the  other 
conditions  are  favourable  for  the  nasal  cavities  to  act  as  a  reso- 
nance chamber,  the  vowel  sounds  are  apt  to  take  on  a  nasal  char- 
acter; and  this  occurs  more  readily  when  the  vowel  is  said  than 
when  it  is  sung.  In  the  group  of  consonants  in  question  the 
nasal  cavities  become  all  important,  the  passage  through  the 
mouth  being  blocked.  In  M  the  passage  is  closed  by  shutting 
the  lips,  in  N  b}'-  the  application  of  the  tongue  to  the  front  of 
the  hard  palate  and  upper  teeth,  in  NG  by  the  application  of  the 
tongue  to  the  soft  palate. 

While  in  the  above  group  no  new  vibrations  are  added  to  the 
laryngeal  vibrations,  in  the  ordinary  L  which  like  them  is  based 
on  laryngeal  sounds,  new  vibrations,  constituting  a  noise,  are 
added.  The  passage  is  not  completely,  only  partially  closed ;  the 
front  of  the  tongue  is  pressed  against  the  liard  palate  in  such  a 
way  that  the  passage  is  blocked  in  the  middle  but  the  air  escapes 
through  narrow  channels  on  each  side.  It  is  the  noise  caused  by 
the  rush  of  air  through  these  narrow  spaces  which  added  to  the 
voice  produces  the  sound  we  distinguish  as  L.     In  certain  forms 


1098  SPEECH.  [Book  hi. 

of  L,  for  instance  in  the  Welsh  lU  the  noise  is  not  accompanied 
by  hiryngeal  vibrations. 

R  is  also  allied  to  the  above  in  so  far  as  it  too  needs  voice, 
and  is  based  on  laryngeal  vibrations.  But  these  vibrations  are 
modified  in  a  special  way ;  they  are  rendered  intermittent  by 
vibratory  movements  started  in  some  part  of  the  passage,  there 
being  different  kinds  of  R,  according  as  the  interruption  takes 
place  at  the  tongue,  or  at  the  fauces.  The  common  R  is  produced 
by  the  vibrations  of  the  point  of  the  tongue  raised  against  the 
front  of  the  hard  palate,  and  the  guttural  R  by  the  vibrations  of 
the  uvula  against  the  root  of  the  tongue.  In  the  feeble  English 
R  there  appear  to  be  no  vibrations ;  the  vowel  chamber  is  simply 
narrowed  in  front  by  the  tip  of  the  tongue. 

It  will  be  observed  that  L  and  the  common  R  resemble  each 
other  to  a  considerable  extent  in  the  position  of  the  tongue  ;  the 
chief  difference  is  that  in  L  the  tongue  is  not  itself  the  subject 
of  muscular  movement,  and  the  vibrations  are  produced  by  the 
friction  of  the  expiratory  blast  through  the  narrow  channel, 
whereas  in  R  the  vibratory  interruptions  are  produced  by  the 
movements  of  the  tongue.  If  in  pronouncing  L  the  tongue  is 
suddenly  set  in  movement,  or  in  pronouncing  R  the  tongue  is 
suddenly  arrested  in  its  movements  while  in  approximation  to 
the  palate,  the  one  consonant  is  changed  into  the  other ;  and,  as 
is  well  known,  certain  persons,  for  instance  the  Chinese,  are  apt 
to  use  the  one  instead  of  the  other. 

The  explosives  differ  according  to  the  part  where  the  inter- 
ruption takes  place  ;  and  in  each  kind  of  interruption  the  sound 
is  different  and  receives  a  different  name  according  as  voice  is 
used  or  no.  P  is  uttered  when  the  lips,  being  first  closed  and  an 
expiratory  blast  driven  against  them,  are  suddenly  opened.  Dur- 
ing this  act  no  voice  is  uttered.  If  voice  is  uttered  the  P  becomes 
B.  These  are  labial  explosives.  In  T,  the  interruption  which  is 
suddenly  removed  is  caused  by  the  application  of  the  tongue  to 
the  front  part  of  the  hard  palate  in  the  case  of  the  English,  to 
the  teeth  in  the  case  of  most  other  languages ;  it  is  called  a 
dental  explosive,  dental  being  used  in  the  wide  meaning  stated 
above.  With  T,  there  is  no  voice ;  if  voice  be  added  the  sound 
becomes  D.  Since  P  differs  from  B,  and  T  from  D,  only  in  the 
absence  or  presence  of  voice,  the  removal  or  addition  of  voice 
will  at  once  convert  in  each  case  the  one  consonant  into  the 
other ;  and  by  certain  nations  P  and  B  are  used  the  one  for  the 
other,  as  are  also  T  and  D. 

It  will  be  observed  that  B  and  D,  both  with  voice,  have  cer- 
tain relations  to  j\I  and  N  respectively.  In  B  and  j\I  the  action 
is  labial,  in  D  and  N  dental,  voice  being  present  in  all;  the  dif- 
ference is  that  in  IVI  and  N,  the  action  consists  in  the  establish- 
ment of  an  obstruction  in  the  buccal  passage,  in  B  and  D  in 
the  sudden  removal  of  an  obstruction.     If  there  be,  as  in  nasal 


Chap,  vii.]         S0:ME   SPECIAL  MECHANISMS.  1099 

catarrh,  an  adequate  obstruction  to  the  exit  thronq-h  the  nasal 
passages  of  the  expiratory  blast  which  creates  the  sound,  it 
becomes  difficult  if  not  impossible  to  establish  tlie  obstruction 
in  the  mouth,  since  in  that  case  there  is  no  exit  at  all  for  the 
expiratory  blast.  Hence  in  nasal  catarrh  there  is  a  tendency  for 
the  effort  to  pronounce  M  to  result  in  B,  and  that  to  pronounce 
N  in  D ;  '  name  '  becomes  '  dabe.' 

If  the  tongue  be  brought  to  the  back  instead  of  to  the  front 
of  the  hard  palate  the  consonant  K  (hard  C)  is  uttered ;  if  voice 
be  added  the  sound  is  G  (hard).  These  are  guttural  explosives. 
Allied  to  them  is  the  brief  sound  Avhicli  in  certain  cases  inaugu- 
rates a  vowel  and  which,  due  to  the  sudden  openimg  of  the  closed 
glottis,  is  immediately  followed  by  the  vibrations  of  the  cords  and 
so  by  the  true  vowel  sound.  This  is  the  spiritus  lenis  as  distin- 
guished from  H  or  the  spiritus  aspei\  of  which,  formed  as  it  is  in 
a  different  manner,  we  shall  speak  directly. 

Certain  other  consonants  are  continuous,  and  like  L  are 
formed  by  the  rush  of  air  through  a  constriction  formed  some- 
where in  the  passage  ;  they  are  frictional  in  origin.  They  differ 
however  from  the  ordinary  L  in  that  they  are  not  always  accom- 
panied by  voice;  like  the  explosives  they  may  be  uttered  without 
any  vibrations  of  the  vocal  cords,  and  when  these  do  accompany 
the  frictional  sound  the  consonant  is  altered  in  its  characters  and 
receives  another  name.  As  in  the  case  of  the  explosives,  in  form- 
ing the  different  members  of  the  group,  the  vibrations  giving  rise 
to  the  sound  are  started  in  different  parts  of  the  passage,  at  the 
lips,  at  the  teeth  or  hard  palate,  or  at  tlie  fauces. 

When  the  constriction  is  caused  by  the  lip  being  brought  into 
contact  with  the  teeth  (and  generalh^  the  lower  lip  and  upper 
teeth  are  used),  so  as  to  reduce  the  outlet  to  a  narrow  space,  the 
vibrations  started  at  the  constriction  give  rise  to  F,  when  no 
voice  is  uttered  at  the  same  time.  If  voice  be  also  uttered  the 
F  becomes  V.  If  the  teeth  take  no  part  in  the  constriction,  and 
this  be  made  exclusively  by  the  two  lips,  the  vowel  chamber  at  the 
same  time  assuming  the  shape  proper  to  tlie  vowel  U,  the  sound 
if  voice  be  uttered  is  W,  the  English  W  and  the  allied  French 
ou  (in  oioi)  may  be  regarded  as  the  vowel  U  (in  two  different 
forms)  turned  into  consonants.  The  sound  which  is  formed  by 
the  two  lips  alone,  in  the  absence  of  voice  is  the  English  (North 
Country)   Wh. 

When  the  constriction  is  formed  between  the  tongue  and  the 
teeth  in  such  a  way  that  the  tip  of  the  tongue  protrudes  between 
the  partiall}^  open  rows  of  teeth  the  sound  is  called  Th:  a  'liard' 
Th  as  in  'thin '  if  without  voice,  a  'soft'  Th  as  in  'this'  if  with 
voice.  The  effect  of  this  manoeuvre  does  not  differ  greatly  from 
that  of  forming  F,  and  certain  persons  in  attempting  to  }u-o- 
nounce  a  liard  Th  give  utterance  to  F,  as  in  the  cockney  'nuflin.' 

When  the  constriction  takes  place  between  the  tongue  and 


1100  SPEECH.  [Book  hi. 

the  teetli  in  sucn  a  way  that  a  narrow  channel  is  formed 
between  the  upper  incisors  and  the  tip  of  the  tongue  curved 
into  a  groove  the  sound  is  called  S  (soft  C)  if  without  voice, 
and  Z  if  with  voice.  If  the  constriction  be  formed  a  little  far- 
ther back  behind  the  front  teeth  by  the  approximation  of  the 
tongue  to  the  front  of  the  hard  palate,  the  sound  uttered  with- 
out voice  is  /Sh ;  if  voice  be  added  the  sound  becomes  the  French 
y,  which  we  represent  by  z  as  in  '  azure  'or  by  ^  as  in  '  badger.' 

If  instead  of  being  formed  by  the  teeth  the  constriction  be 
carried  farther  back  from  the  region  of  the  teeth  and  hard 
palate  to  that  of  the  soft  palate  and  fauces,  a  guttural  aspirate 
is  formed.  Without  voice  this  is  the  hard  Oh  as  in  the  Scotch 
'  loch,'  with  voice  the  soft  Ch. 

Y  appears  to  be  the  vowel  /  (ee)  used  as  a  consonant,  much 
in  the  same  way  that,  as  stated  above,  W  is  U  used  as  a  conso- 
nant. 

Lastly,  a  consonantal  sound  may  be  formed  by  the  glottis 
itself  sujDplying  a  constriction  but  in  such  a  way  that  the  vocal 
cords  are  not  thrown  into  musical  vibrations.  When  in  utter- 
ing a  vowel  we  begin  with  the  glottis  not  closed  as  in  the 
spiritus  lenis  but  open,  and  send  through  the  glottis  an  expira- 
tory blast  which  creates  irregular  vibrations  by  friction  before 
the  cords  are  brought  into  a  proper  position  for  their  regular 
vibrations,  the  result  is  the  aspirate  H,  the  spiritus  asper.  The 
particularly  powerful  H  of  Arabic  is  produced  by  bringing  the 
processus  vocales  into  contact  with  or  near  to  each  other  but  so 
as  to  leave  the  cartilaginous  glottis  widely  open  and  the  mem- 
branous glottis  more  or  less  open;  at  the  same  time  the  ven- 
tricular bands  are  approximated,  and  the  superior  aperture  of 
the  larynx  is  forcibly  constricted.  The  expiratory  blast  driven 
through  the  series  of  irregular  passages  gives  rise  to  the  irregu- 
lar vibrations  which  constitute  the  sound,  the  vocal  cords  being 
motionless  or  at  least  not  giving  rise  to  the  regular  vibrations  of 
voice. 

We  have  seen  that  in  whispering  no  true  voice  is  uttered, 
no  regular  vibrations  are  generated  in  the  vocal  cords,  though 
the  passage  of  the  air  through  the  glottis  produces  vibrations 
which  serve  as  the  basis  of  the  whisper,  being  modified  by  the 
vowel  cavity  so  as  to  form  vowels.  To  these  vibrations  may 
be  added  the  vibrations  of  the  consonants,  so  that  a  whisper 
becomes  complete  though  feeble  speech.  Since  the  irregular 
glottic  vibrations  of  a  whisper  are  very  weak  compared  with  the 
relatively  powerful  true  vocal  vibrations,  the  distinction  between 
consonants  with  voice  and  without  voice  is  in  a  whisper  largely 
obscured;  it  is  difficult  for  instance  in  a  whisper  to  distinguish 
between  P  and  B. 


SEC.  3.     ON  SOME  LOCOMOTOR  MECHANISMS. 

§  683.  The  skeletal  muscles  are  for  the  most  part  arranged 
to  act  on  the  bones  and  cartilages  as  on  levei-s,  examples  of  the 
first  kind  of  lever  being  rare,  and  those  of  the  third  kind,  where 
the  power  is  applied  nearer  to  the  fulcrum  than  is  the  weight, 
being  more  common  than  the  second.  This  arises  from  the  fact 
that  the  movements  of  the  body  are  chiefly  directed  to  moving 
comparatively  light  weights  through  a  great  distance,  or  through 
a  certain  distance  with  great  precision,  rather  than  to  moving 
heavy  weights  through  a  short  distance.  The  fulcrum  is  gen- 
erally supplied  by  a  (perfect  or  imperfect)  joint,  and  one  end 
of  the  acting  muscle  is  made  fast  by  being  attached  either  to  a 
fixed  point,  or  to  some  point  rendered  fixed  for  the  time  being 
by  the  contraction  of  other  muscles. 

There  are  indeed  few  movements  of  the  body  in  which  one 
muscle  only  is  concerned ;  in  the  majority  of  cases  several  mus- 
cles act  together  in  concert ;  the  movements  of  the  larynx  which 
we  have  just  studied  afford  a  striking  illustration  of  this.  The 
relations  of  the  muscles  which  thus  act  together  are  many  and 
varied.  When  one  muscle  is  contracting,  the  contractions  of 
another  muscle  or  of  other  muscles  may,  as  just  stated,  serve  to 
secure  a  fixed  point,  or  may  enforce  the  effect  of  the  first  mus- 
cle, or,  and  this  is  perhaps  the  most  common  case,  may  give  a 
special  direction  to  the  action,  the  movement  effected  being 
the  resultant  of  the  forces  employed  in  combination.  Many 
muscles  are,  either  partially  or  wholly,  antagonistic  in  action  to 
each  other,  such  for  instance  as  the  flexors  and  extensors,  and 
such  muscles  as  those  of  the  face,  which  act  bilaterally  in  oppo- 
site directions  on  parts  placed  in  the  middle  line ;  and  the  rela- 
tions of  these  antagonistic  muscles  seem  to  be  specially  com- 
plex. When  a  muscle  contracts  it  is,  as  we  saw  in  treating  of 
nerve  and  muscle,  of  advantage  tliat  the  muscle  should  at  the 
moment  of  contraction  be  already  "on  the  stretch  ;  "  this  is  pro- 
vided by  the  anatomical  disposition  of  the  parts  assisted  proba- 
bly, as  we  saw  (§  470),  by  skeletal  tone,  but  is  also  further 
secured  by  the  action  of  its  antagonists,  which  moreover,  after 

1101 


1102  WALKING.  [Book  in. 

the  muscle  has  contracted,  assist  its  return  to  its  proper  position 
of  rest.  When  a  point  has  to  be  fixed  the  two  sets  of  muscles 
which  act  as  antagonists  on  the  point  are  both  thrown  into  con- 
traction, in  proportion  to  their  relative  effect.  If  the  action  of 
one  muscle  (or  set  of  muscles)  is  to  be  dominant  its  antagonist 
may  take  no  part  in  the  action,  being  neither  contracted  nor 
relapsed ;  but  there  are  reasons  for  thinking  that,  in  many  cases 
at  all  events,  the  action  of  one  muscle  though  remaining  domi- 
nant is  tempered  and  guarded  so  to  speak  by  a  concomitant 
feebler  action  of  its  antagonist ;  on  the  other  hand  we  have  evi- 
dence as  in  the  case  of  the  ocular  muscles  (§  595)  that  it  may 
be  assisted  by  an  inhibition,  a  relaxation  of  the  antagonist. 
These  several  phases  are  governed  by  the  nervous  sj^stem,  and 
the  behaviour  of  antagonistic  muscles  and  groups  of  muscles 
affords  many  instances  of  what  we  have  so  often  insisted  upon, 
namely,  that  nearly  all  the  various  movements  of  our  body  are 
coordinate  movements,  and  that  in  many  cases  the  coordination 
is  extremely  complex. 

§  684.  The  erect  posture,  in  which  the  weight  of  the  body 
is  borne  by  the  plantar  arches,  is  the  result  of  a  series  of  con- 
tractions of  the  muscles  of  the  trunk  and  legs,  having  for  their 
object  the  keeping  the  body  in  such  a  position  that  the  line  of 
gravity  falls  within  the  area  of  the  feet.  That  this  does  require 
muscular  exertion  is  shewn  by  the  facts  that  a  person  when 
standing  perfectly  at  rest  in  a  completely  balanced  position 
falls  when  he  becomes  unconscious,  and  that  a  dead  body  can- 
not be  set  on  its  feet.  The  line  of  gravity  of  the  head  falls  in 
front  of  the  occipital  articulation,  as  is  shewn  by  the  nodding 
of  the  head  in  sleep.  The  centre  of  gravity  of  the  combined 
head  and  trunk  lies  at  about  the  level  of  the  ensiform  cartilage, 
in  front  of  the  tenth  thoracic  vertebra,  and  the  line  of  gravity 
drawn  from  it  passes  behind  a  line  joining  the  centres  of  the 
two  hip-joints,  so  that  the  erect  body  would  fall  backward 
were  it  not  for  the  action  of  the  muscles  passing  from  the 
thighs  to  the  pelvis  assisted  by  the  anterior  ligaments  of  the 
hip-joints.  The  line  of  gravity  of  the  combined  head,  trunk 
and  thighs  falls  moreover  a  little  behind  the  knee-joints,  so  that 
some,  though  little,  muscular  exertion  is  required  to  prevent 
the  knees  from  being  bent.  Lastly,  the  line  of  gravity  of  the 
whole  body  passes  in  front  of  the  line  drawn  between  the  tAvo 
ankle-joints,  the  centre  of  gravity  of  the  whole  body  being 
placed  at  the  end  of  the  sacrum ;  hence  some  exertion  of  the 
muscles  of  the  calves  is  required  to  prevent  the  body  falling 
forwards. 

§  685.  In  wallcmg  advantage  is  taken  of  this  forward  posi- 
tion of  the  centre  of  gravity,  and  the  tendency  to  fall  forwards 
is  utilized  to  swing  each  leg  in  turn  forwards  after  the  fashion 
of  a  pendulum.     In  each  step  there  is  a  moment  at  which  the 


Chap,  vii.]  SOME   SPECIAL   MECHAmSMS.  1103 

body  is  resting  verticall}^  on  one  leg,  say  the  right,  while  the 
otlier  is  inclined  obliquely  behind.  The  two  legs  and  the  plane 
of  the  PTOund  form  a  riCTht-ansfled  trianijle,  of  which  the  left  lesf 
is  the  hj'pothenuse,  the  riglit  angle  being  between  the  right 
leg  and  the  ground.  At  a  certain  moment  the  foot  of  the  right 
leg  will  be  fiat  on  the  ground  and  the  line  of  gravity  will  pass 
through  its  heel.  But  the  centre  of  gravity  is  moving  for- 
Avards:  even  if  there  had  been  no  previous  steps,  and  so  no 
momentum,  the  body  and  with  it  the  centre  of  gravity,  unless 
prevented  by  muscular  effort,  would  have  fallen  forward;  we 
may  therefore  speak  of  the  line  of  gravity  as  travelling  for- 
Avards ;  it  passes  from  the  heel  to  the  toe  (of  the  right  foot). 
If  the  body  were  simply  falling  forwards  the  centre  of  the  hip- 
joint  would  move  downwards  as  well  as  forwards,  describing  a 
circle  with  the  leg  as  a  radius.  But  at  the  moment  of  which 
we  are  speaking  the  (right)  leg  is  somewhat  flexed,  both  at  the 
ankle  and  still  more  at  the  knee.  And,  as  the  line  of  gravity 
is  travelling  forward  from  the  heel  to  the  toe,  the  active  part 
of  the  performance  intervenes.  The  foot  is  raised  from  the 
ground  from  the  heel  forwards,  until  it  is  only  the  ball  of  the 
great  toe  which  is  resting  on  the  ground,  and  the  whole  leg  is, 
by  muscular  effort,  straightened.  In  this  act  the  right  leg  acts 
as  a  lever,  the  ball  of  the  great  toe  serving  as  a  fulcrum ;  and 
the  effect  of  the  act  is  to  prevent  the  centre  of  gravit}^,  or  the 
hip-joint,  from  moving  downwards,  and  to  carry  it  forwards 
only  in  more  nearly  a  straight  line.  In  thus  carrying  the  hips 
(and  body)  forward  the  leg  has  changed  its  position ;  from 
being  vertical  and  flexed  with  the  whole  sole  resting  on  the 
ground,  it  has  become  inclined  forwards  obliquely,  extended 
straight,  with  the  toes  only  resting  on  the  ground.  It  has 
assumed  the  same  posture  as  that  of  the  left  leg  at  the  moment 
at  which  we  started. 

Even  at  that  moment  the  left  leg  was  behind  the  line  of 
gravity,  and  unless  it  moved  would  become  more  and  more  so  as 
the  changes  in  the  right  leg  went  on ;  hence  if  left  to  itself  it 
would  swing  forward  much  as  a  pendulum  which  had  been 
raised  up  would  swing  forward  when  let  go.  And  during  the 
changes  in  the  right  leg  which  we  have  just  described  the  left 
does  swing  forward,  its  movement  being  chiefly  determined  like 
that  of  a  pendulum  by  gravity,  though  it  may  be  assisted  by 
direct  muscular  effort,  and  is  certainly  so  guided,  being  for 
instance  slightly-  flexed  during  the  transit.  It  swings  forward 
in  front  of  the  line  of  gravity  and  is  thus  brought  to  the 
ground,  the  toes  in  proper  walking  making  contact  first  and 
the  heel  later,  though  many  people  who  wear  shoes  bring  the 
heel  down  at  least  as  soon  as  the  toes.  It  swings  we  say  in 
front  of  the  line  of  gravity ;  but  that  line  of  gravit}'  is  travel- 
ling forwards,  so  that  in  a  very  short  time  the  body  is  resting 


1104 


WALKING. 


[Book  hi. 


vertically  on  the  left  leg,  with  the  line  of  gravity  falling  at  the 
left  heel.  That  is  to  say,  the  left  leg  has  now  assumed  the  posi- 
tion which  the  right  leg  had  when  we  began ;  meanwhile,  as  we 
have  seen,  the  right  leg  has  assumed  the  former  position  of  the 
left  leg ;  the  step  is  completed,  and  the  movements  of  the  next 
step  merely  repeat  those  of  the  one  which  we  have  described. 

It  is  obvious  from  the  above  that  in  walking  there  are  in 
each  step  periods  when  both  feet  are  touching  the  ground,  and 
periods  when  one  or  the  other  foot  is  raised  from  the  ground, 
but  there  is  no  period  when  both  feet  are  off  the  ground.  This 
is  shewn  in  the  diagram,  Fig.  193,  which  represents  two  steps. 


R 


a 


g 


Fig.  193.     Diagram   to   illustrate   the   contact   of  the   Eeet  with  the 
Ground  in  Walking. 

i?,  the  right  foot.  L,  the  left  foot.  In  each  case  the  curved  line  represents 
the  time  when  the  foot  is  not  in  contact  with  the  ground,  and  the  straight  line 
when  it  is  in  contact. 

During  a — 5,  the  left  leg  (l)  leaves  the  ground  as  indicated 
by  the  curving  of  the  line.  During  h — c  both  feet  are  on  the 
ground.  During  c — d  the  right  leg  (r)  is  above  but  the  left 
(l)  is  still  on  the  ground.  During  d — e  both  are  on  the 
ground  and  the  double  step  is  completed,  the  next  step  begin- 
ning again  at  e  with  the  left  leg  leaving  the  ground. 

We  have  said  that  the  centre  of  gravity  is  in  walking  pre- 
vented from  moving  downwards  as  well  as  forwards,  as  it 
Avould  do  in  the  act  of  falling  forwards.  It  does  not  however 
describe  a  straight  line  forwards,  it,  and  with  it  the  top  of  the 
head,  rises  and  falls  at  each  step  of  each  leg,  and  hence  de- 
scribes a  series  of  consecutive  curves  not  unlike  the  line  of 
flight  of  many  birds. 

Since  in  standing  on  both  feet  the  line  of  gravity  falls 
between  the  two  feet,  a  lateral  displacement  of  the  centre  of 
gravity  is  necessary  in  order  to  balance  the  body  on  one  foot. 
Hence  in  walking  the  centre  of  gravity  describes  not  only  a 
series  of  vertical,  but  also  a  series  of  horizontal  curves,  inas- 


Chap,  vii.]         SOME   SPECIAL   IMECHANISMS. 


1105 


much  as  at  each  step  the  line  of  gravity  is  made  to  fall  alter- 
nately on  each  standing  foot.  While  the  left  leg  is  swinging, 
the  line  of  gravity  falls  within  the  area  of  the  right  foot,  and  the 
centre  of  gravity  is  on  the  right  side  of  the  pelvis.  As  the  left 
foot  becomes  the  standing  foot,  the  centre  of  gravity  is  shifted 
to  the  left  side  of  the  pelvis.  The  actual  curve  described  by 
the  centre  of  gravity  is  therefore  a  somewhat  complicated  one, 
being  composed  of  vertical  and  horizontal  factors. 

The  natural  step  is  the  one  which  is  determined  by  the 
length  of  the  swinging  leg,  since  this  acts  as  a  pendulum ;  and 
hence  the  step  of  a  long-legged  person  is  naturally  longer  than 
that  of  a  person  with  short  legs.  The  length  of  the  step  how- 
ever may  be  diminished  or  increased  by  a  direct  muscular 
effort,  as  when  a  line  of  soldiers  keep  step  in  spite  of  their  hav- 
ing legs  of  different  lengths.  Such  a  mode  of  marching  must 
obviously  be  fatiguing,  inasmuch  as  it  involves  an  unnecessary 
expenditure  of  energy. 

In  slow  walking,  which  Fig.  193  may  be  taken  to  illustrate, 
there  is  an  appreciable  time  during  which,  while  one  foot  is 
already  in  position  to  serve  as  a  fulcrum,  the  other,  swinging, 
foot  has  not  yet  left  the  ground.  In  fast  walking  this  period  is 
so  much  reduced,  that  one  foot  leaves  the  ground  the  moment 
the  other  touches  it;  hence  there  is  practically  no  period  during 
which  both  feet  are  on  the  ground  together;  this  might  be 
shewn  by  omitting  b — c  and  d — e  in  Fig.  193. 


R 


Fig.  194.     Diagkam  to  illustrate  Running. 

L,  the  line  of  contact  with  the  ground  of  the  left,  B  of  the  right  foot ;  in 
each  case  the  curved  portion  of  the  line  represents  the  time  during  which  the 
foot  leaves  the  ground. 

When  the  body  is  swung  forward  on  the  one  foot  acting  as 
a  fulcrum  with  such  energy  that  this  foot  leaves  the  ground 
before  the  other,  swinging,  foot  has  reached  the  ground,  as 
shewn  in  Fig.  194,  there  being  an  interval,  b — c,  d — e  in  the 
figure,  during  which  neither  foot  is  on  the  ground,  the  person 
is  said  to  be  runniiu/,  not  walking. 

In  jumping  this  propulsion  of  the  body  takes  place  on  botli 
feet  at  the  same  time  ;  in  hopping  it  is  effected  on  one  foot  only. 

70 


BOOK  IV. 


THE  TISSUES  AND  MECHANISMS  OF 
REPRODUCTION. 


EEPEODUCTIOX. 

§  686.  Many  of  the  individual  constituent  parts  of  an 
animal  body  are  capable  of  reproduction,  i.e.  they  can  give  rise 
to  parts  like  themselves;  or  they  are  capable  of  regeneration, 
i.e.  their  places  can  be  taken  by  new  parts  more  or  less  closely 
resembling  themselves.  The  elementary  tissues  undergo  dur- 
ing life  a  very  large  amount  of  regeneration.  Thus  the  old 
epithelium  scales  which  fall  away  from  the  surface  of  the  body 
are  succeeded  by  new  scales  from  the  underlying  layers  of  the 
epidermis ;  old  blood-corpuscles  give  place  to  new  ones  ;  worn- 
out  muscles,  or  those  which  have  failed  from  disease,  are  re- 
newed by  the  accession  of  fresh  fibres ;  divided  nerves  grow 
again  ;  broken  bones  are  united  ;  connective  tissue  seems  to  dis- 
appear and  appear  almost  Avitliout  limit;  new  secreting  cells 
take  the  place  of  the  old  ones  which  are  cast  off;  in  fact,  with 
the  exception  of  some  cases,  such  as  cartilage,  and  these  doubt- 
ful exceptions,  all  those  fundamental  tissues  of  the  body  which 
do  not  form  part  of  highly  differentiated  organs  are,  within 
limits  fixed  more  by  bulk  than  by  anything  else,  capable  of 
regeneration.  To  that  regeneration  by  substitution  of  mole- 
cules, which  is  the  basis  of  all  life,  is  added  a  regeneration  by 
substitution  of  mass. 

In  the  higher  animals  regeneration  of  whole  organs  and 
members,  even  of  those  whose  continued  functional  activity  is 
not  essential  to  the  well-being  of  the  body,  is  never  witnessed, 
though  it  may  be  seen  in  the  lower  animals ;  the  digits  of  a 
newt  may  be  restored  by  growth,  but  not  those  of  a  man.  And 
the  repair  which  follows  even  partial  destruction  of  highly 
differentiated  organs,  such  as  the  retina,  is  in  the  higher 
animals  very  imperfect. 

In  the  higlier  animals  the  reproduction  of  the  wliole  in- 
dividual can  be  effected  in  no  other  way  than  by  the  process  of 
sexual  generation,  through  which  the  female  representative 
element  or  ovum  is,  under  the  influence  of  the  male  representa- 
tive or  spermatozoon,  developed  into  an  adult  individual. 

We  do  not  purpose  to  enter  here  into  any  of  the  morpholog- 

1109 


1110  EEPKODUCTIOK  [Book  iv. 

ical  problems  connected  with  the  series  of  changes  through 
which  the  ovum  becomes  the  adult  being ;  or  into  the  obscure 
biological  inquiry  as  to  how  the  simple,  all  but  structureless 
ovum  contains  within  itself,  in  potentiality,  all  its  future  devel- 
opments, and  as  to  what  is  the  essential  nature  of  the  male 
action.  These  problems  and  questions,  which  are  fully  dis- 
cussed in  other  works,  do  not  properly  enter  into  a  work  on 
physiology,  except  under  the  view  that  all  biological  problems 
are,  when  pushed  far  enough,  physiological  problems.  We 
shall  limit  ourselves  to  a  brief  survey  of  the  more  important 
physiological  phenomena  attendant  on  the  impregnation  of  the 
ovum,  and  on  the  nutrition  and  birth  of  the  embryo,  incident- 
ally calling  attention  to  some  of  the  leading  structural  features 
of  the  parts  concerned. 


CHAPTER  I. 
IMPREGNATION. 

SEC.  1.     MENSTEUATION. 

§  687.  Feom  puberty,  which  may  be  said  to  occur  at  from 
IB  to  17  years  of  age,  to  the  climacteric,  which  may  be  said 
to  arrive  at  from  45  to  50  years  of  age,  the  exact  time  in  each 
case  varying  considerably  and  being  apparently  determined  by 
various  conditions,  the  human  female  is  subjected  monthly  to  a 
discharge  from  the  vagina  known  as  the  '  menses,'  '  catamenia,' 
and  by  many  other  terms.  The  discharge  is  the  result  of 
chang-es  in  the  lininsr  membrane  of  the  uterus.  A  similar 
change  in  the  uterus  occurs  in  the  lower  animals,  being  repeated 
at  intervals  differing  in  length  in  different  animals,  and  is 
usually  accompanied  by  sexual  excitement  and  changes  in  the 
external  genital  organs ;  the  phenomena  are  then  spoken  of  by 
such  names  as  '  heat,'  '  rut,'  &c. 

The  female  human,  and  other,  is  also  subject  to  another 
recurring  process,  the  escape  of  an  ovum  from  its  Graafian 
follicle  in  the  ovary,  a  process  spoken  of  as  "ovulation."  The 
changes  of  the  uterus  during  menstruation  have  the  appearance 
of  being  a  preparation  for  the  reception  of  a  discharged  ovum 
so  that  the  latter  if  fertilized  may  be  developed  into  an  embryo ; 
and  there  are  many  reasons  for  thinking  that  the  acts  of  ovula- 
tion and  menstruation  are  coincident  with  a  causal  connection 
between  the  two.  But  this  cannot  be  regarded  as  definitely 
proved,  and  many  observers  maintain  that  menstruation  may 
and  does  occur  without  any  discharge  of  an  ovum,  and  con- 
versely that  an  ovum  may  be  discharged,  as  in  copulation,  quite 
apart  from  menstruation. 

The  discharge  of  an  ovum,  whether  at  menstruation,  or  at 
another  time  appears  to  take  place  as  follows.  The  whole 
ovary  at  this  time  becomes  congested,  the  blood  vessels  being  so 
dilated  and  filled  with  blood  that  we  may  almost  speak  of  the 

1111 


1112  MENSTRUATION.  [Book  iv. 

condition  as  one  of  erection ;  and  the  ripe  follicle,  whose  ovum 
is  about  to  escape,  bulges  from  its  surface.  The  most  projecting 
portion  of  the  wall  of  the  follicle,  which  has  previously  become 
excessively"  thin,  is  now  ruptured,  apparently  by  the  mere  dis- 
tension of  the  cavity,  and  the  ovum,  now  lying  close  under  the 
projecting  surface  of  the  follicle,  escapes,  invested  by  some 
of  the  cells  of  the  discus  proligerus,  into  the  Fallopian  tube. 
Much  discussion  has  taken  place  as  to  how  the  entrance  of  the 
ovum  into  the  Fallopian  tube  is  secured.  It  is  probable  that 
under  ordinary  circumstances  the  ovary  is  embraced  by  the 
trumpet-shaped  fimbriated  mouth  of  the  Fallopian  tube,  and  the 
contact  is  probably  rendered  more  complete  by  the  turgid  and 
congested  condition  of  both  organs  ;  it  is  possible  that  the  plain 
muscular  fibres  present  in  the  mouth  of  the  tube  may  assist, 
and  indeed  gliding  movements  of  the  mouth  of  the  tube  over 
the  ovary  have  been  observed  in  animals.  It  has,  however,  been 
asserted  that  the  turgescence  of  the  tube  does  not  occur  until 
after  the  ovum  has  become  safely  lodged  in  the  tube,  and  it 
is  argued  that  the  ovum  is  carried  in  the  proper  direction  by  cur- 
rents set  up  by  the  action  of  the  ciliated  epithelium  lining  the 
tube,  currents  whose  direction  and  strength  seem,  as  shewn  by 
experiment,  to  be  adequate  to  carry  into  the  uterus  particles 
present  in  the  peritoneal  fluid ;  and  the  groove  in  the  ciliated 
surface  of  the  ovarian  fimbria  especially  connected  with  the 
ovary,  suggests  itself  as  the  natural  path  for  the  ovum. 
Arrived  in  the  tube,  the  ovum  travels  downwards  very  slowly, 
by  the  action  probably  of  the  cilia  lining  the  tube,  though  possi- 
bly its  progress  may  occasionally  be  assisted  by  the  peristaltic 
contractions  of  the  muscular  walls.  The  stay  of  the  ovum 
in  the  Fallopian  tube  may  extend  to  several  days ;  the  channel, 
as  we  have  seen,  is  a  narrow  one,  especially  at  the  entrance  into 
the  uterus.  The  escape  of  the  ovum  is  followed  by  changes  in 
the  follicle  and  rest  of  the  ovary,  leading  to  the  formation  of  a 
corpus  luteum. 

Concerning  the  exact  nature  of  the  changes  in  the  uterus 
which  lead  to  the  menstrual  flow  there  has  been  and  is  much  dis- 
cussion. The  opportunities  for  an  exact  histological  investigation 
of  a  human  menstruating  uterus  are  rare ;  but  from  what  has 
been  observed  under  the  most  favourable  circumstances,  aided 
by  the  results  of  the  study  of  the  monkey  which  is  subject  to  a 
periodic  change  exceedingly  like  human  menstruation,  we  may 
conclude  that  the  changes  which  occur  are  somewhat  as  follows. 
In  the  first  stage  of  the  period  not  only  does  the  whole  uterus, 
the  cervix  excepted,  become  congested,  the  blood  vessels 
being  distended,  and  the  mucous  membrane  especially  red,  thick 
and  swollen,  but  a  new  growth  takes  place  in  the  mucous  mem- 
brane at  least  in  its  more  superficial  layers.  This  growth  affects 
not  only  the  epithelial  but  also  the  connective  tissue,  stroma. 


CiiAp.  I.]  FEMALE   OKGANS.  1113 

elements,  and  the  blood  vessels  become  enlarged,  more  or  less 
irregular  spaces  filled  with  blood  being  developed  close  under 
the  surface.  In  this  way  a  modified  superficial  portion  of  the 
mucous  membrane  becomes  differentiated  from  the  underlying 
portion  ;  and  from  this  modified  portion,  which  may  be  compared 
to  tlie  decidua  of  pregnancy  of  which  we  shall  presently  speak, 
a  certain  amount  of  haemorrhage  into  the  cavity  of  the  uterus 
early  takes  place.  But  the  blood  thus  escaping  does  not  form 
the  main  menstrual  flow.  The  growth  of  the  modified  mucous 
membrane  is  immediately  followed  by  its  rapid  degeneration, 
and  the  part  so  degenerated  is  cast  off,  laying  bare  the  deeper, 
less-changed  layers  of  the  mucous  membrane,  from  the  torn  and 
open  irregular  blood  spaces  of  which  a  more  copious  flow  of 
blood  takes  place.  It  is  this  freer  escape  of  blood  which,  mixed 
with  the  detritus,  or  even  with  conspicuous  pieces  of  the  shed 
membrane  and  containing  many  cells  resembling  leucocytes, 
constitute  the  menstrual  flow ;  as  the  hsemorrhage  diminishes, 
these  constituents  other  than  actual  blood  become  more  promi- 
nent, and  the  discharge  becomes  less  and  less  coloured.  The 
degeneration  and  shedding  is  in  turn  followed  by  new  growth 
from  the  deeper  parts  of  the  mucous  membrane  left  behind, 
whereby  the  normal  mucous  membrane  is  restored  in  its  entirety. 
The  amount  of  change  which  takes  place  probably  differs  in 
different  individuals ;  in  some  cases  possibly  the  amount  of 
proliferation  and  subsequent  degeneration  is  relatively  slight, 
the  haemorrhage  being  more  comparable  to  that  from  any  con- 
gested mucous  membrane,  such  as  nasal  haemorrhage ;  in  other 
cases  again,  according  to  some  observers,  the  whole  thickness 
of  the  mucous  membrane  may  be  removed,  the  muscular  coat 
beneath  being  laid  bare.  The  blood  as  it  passes  through  the 
vagina  becomes  somewhat  altered,  probably  by  the  influence 
of  the  other  constituents  of  the  discharge,  and  wdien  scanty 
coagulates  but  slightly ;  when  the  flow  however  is  considerable, 
distinct  clots  may  make  their  appearance. 


SEC.  2.     THE   MALE   OEGANS. 

§  688.  The  tail  of  a  spermatozoon  may  be  regarded  as  a 
single  cilium,  the  movements  of  which  are  of  an  undulatory 
character,  the  waves  travelling  from  the  middle  piece  to  the  end 
of  the  tail ;  and  the  statements  previously  made  (§  86)  concern- 
ing ciliary  action  may  be  applied  generally  to  the  movement  of  a 
spermatozoon.  The  motion  is  apparently  not  a  very  rapid  one, 
for  it  has  been  calculated  that  a  half  vibration  takes  at  least  a 
quarter  of  a  second.  It  has  also  been  calculated  that  a  sperma- 
tozoon progresses  at  the  rate  of  about  2  or  3  mm.  a  minute. 

When  discharged  semen  is  left  to  itself  the  movements 
continue  for  some  (24  or  48)  hours,  but  they  appear  to  last 
much  longer  in  the  female  passages.  Spermatozoa  have  been 
observed  in  movement  when  removed  from  the  neck  of  the 
living  human  uterus  5  or  even  7  days  after  coitus ;  and  in  some 
of  the  lower  animals  the  duration  of  vitality  may  be  enormously 
long.  Making  all  allowance  for  any  possible  direct  nutrition 
of  the  living  substance  of  the  spermatozoon  by  means  of  the 
fluid  of  the  semen,  we  must  conclude  that  the  energy  of  the 
movement  is  derived  from  the  expenditure  of  what  we  may 
venture  to  call  the  contractile  material  stored  up  in  the  middle 
piece  and  tail  of  the  organism  at  its  formation ;  the  material  of 
the  head  we  may  suppose  to  be  devoted  entirely  to  the  work  of 
impregnation.  So  small  a  store  must  be  soon  exhausted ;  hence 
it  is  difficult  to  suppose  that  vigorous  movements  can  be 
continued  for  very  long  periods ;  and  probably  the  activity  of 
the  spermatozoa  is  largely  dependent  on  the  circumstances  by 
which  it  is  surrounded ;  it  may  remain  motionless  in  one 
medium,  and  become  active  when  the  medium  is  changed. 
The  spermatozoon  is  probably  quiescent  so  long  as  it  remains  in 
the  seminal  tubes,  but  we  have  no  exact  inforniation  as  to 
whether  or  no  movements  begin  in  the  epididymis  and  vas 
deferens  without  exposure  to  air;  and  it  is  possible  that  after 
coitus  the  beginning  and  maintenance  of  its  vigorous  move- 
ments may  largely  depend  on  the  condition  of  the  secretions  in 
the  vagina  and  uterus.     In  this  connection  it  may  be  noted  that 

1114 


Chap,  i.]  MALE   ORGANS.  1115 

the  movements  of  a  spermatozoon,  like  ciliary  movements,  are 
favoured  by  fluids  having  a  weak  alkaline  reaction,  whereas 
almost  any  degree  of  acidity  (unless  used  to  neutralize  exces- 
sive alkalinity)  arrests  them ;  and  the  mucous  secretion  of  the 
uterus  while  it  is  alkaline  at  the  neck  of  the  uterus  becomes 
acid  as  it  passes  down  the  vagina.  Hence  it  might  be  inferred 
that  those  spermatozoa  only  which  rapidly  find  their  way  into 
the  OS  uteri  manifest  vigorous  movements ;  but  it  would  be  dan- 
gerous to  lay  too  great  stress  on  this. 

§  689.  The  semen  contains  a  relatively  large  quantity  of 
solid  matter,  and  this  in  turn  is  to  a  great  extent  furnished  by 
the  spermatozoa ;  indeed  the  spermatozoa  form  so  large  a  por- 
tion of  the  semen  that  the  chemical  substances  present  in  the 
former  are  dominant  in  the  latter.  The  head  of  a  spermatozoon 
appears  to  be  largely  composed  of  the  body  or  group  of  bodies 
known  as  nuclein  or  nucleo-albumin,  a  result  which  supplies 
chemical  evidence  of  the  nuclear  nature  of  the  spermatozoan 
head;  and  nuclein  forms  a  considerable  portion  of  the  solid 
matter  of  the  whole  semen.  Lecithin  is  also  present  in  the 
semen  in  considerable  quantity  ;  otherwise  the  chemical  features 
of  the  secretion,  which  are  as  yet  imperfectly  known,  present  no 
special  interest.  The  crystals  found  in  dry  semen  are  not  as  was 
once  thought  of  a  proteid  nature  but  are  compound  phosphates 
containing  an  organic  base.  As  discharged  in  coitus  the  semen 
proper  from  the  testicle  is  mixed  with  the  prostatic  and  other 
secretions. 

From  the  testicle  itself  various  forms  of  proteid  of  the 
globulin  class  have  been  extracted ;  and  glycogen  is  not  unfre- 
quently  present. 

The  cavity  of  the  vesicida  seminalis  serves  as  a  temporary 
receptacle  for  the  semen,  though  some  secretion,  and  in  some 
animals  a  decided  quantity,  takes  place  from  its  interior.  In 
certain  animals  the  secretion  clots,  and  then  appears  to  contain 
a  substance  identical  with  or  allied  to  fibrinogen ;  in  these  ani- 
mals the  clot  which  is  thus  formed  by  the  mixture  of  the  male 
secretion  with  the  bloody  secretion  of  the  rutting  female  helps 
to  secure  the  retention  of  the  former  within  the  female  passages. 
The  secretion  of  the  prostate  presents  no  special  features,  exce[)t 
that  it  is  apt  to  contain  peculiar  concentric  corpuscles ;  but  tlie 
fact  that  the  prostate  remains  undeveloped  in  castrated  animals 
suggests  that  the  secretion  plays  some  part  in  coitus.  The 
glands  of  Cowper  afford  a  thick  mucous  secretion. 

§  690.  Erectile  Tissue.  The  erectile  tissue  of  the  corpora 
cavernosa  and  corpus  spongiosum  consists  of  an  irregular  laby- 
rinth formed  by  trabecular  composed  of  connective  tissue  with 
abundant  elastic  elements  mixed  up  with  a  large  but  variable 
amount  of  plain  muscular  tissue.  The  spaces  of  the  trabecuhe 
are  lined  by  spindle-shaped  epitheloid  plates,  resting  in  some 


1116  ACCESSORY   MALE   ORGANS.  [Book  iv. 

cases  on  a  layer  of  plain  muscular  fibres,  and  are  venous  sinuses, 
into  whicli  blood  finds  its  way  chiefly  through  the  terminal 
capillaries  of  the  numerous  arteries  lying  in  the  trabeculse  but 
also  in  some  cases  by  minute  arteries  opening  directly  into  the 
spaces;  from  tlie  sinus  the  blood  finds  its  way  out  into  smaller 
regular  veins.  In  the  corpora  cavernosa,  and  to  a  less  extent  in 
the  corpus  spongiosum,  the  small  arteries  in  the  trabeculse  are 
extremely  twisted  up  and  looped,  bulging  into  the  venous  sinuses 
as  arterial  coils,  the  so-called  'helicine  arteries.'  When  the 
arteries  supplying  these  masses  of  erectile  tissue,  namely,  the 
branches  of  the  pudic  arteries  and  dorsal  artery  of  the  penis,  are 
constricted,  and  when  the  plain  muscular  fibres  of  the  trabeculse 
are  in  a  state  of  contraction,  whereby  the  venous  spaces  are 
largely  closed,  the  greater  part  of  tlie  blood  flowing  through 
the  arteries  finds  its  way  by  ordinary  capillaries  into  the  efferent 
veins,  little  blood  passes  into  the  venous  sinuses,  and  the  whole 
tissue  is  relatively  small  in  bulk.  When  on  the  other  hand  the 
arteries  are  dilated  and  in  addition  the  muscular  bundles  of  the 
trabeculse  are  relaxed,  a  large  quantity  of  blood  passes  into 
the  venous  sinuses,  these  become  greatly  distended  with  blood ; 
the  whole  mass  of  erectile  tissue  becomes  turgid,  and  in  propor- 
tion to  the  resisting  nature  of  the  outer  envelope,  as  is  especially 
seen  in  the  corpora  cavernosa,  hard  and  rigid. 

§  691.  In  the  dog  and  cat,  fibres  from  the  anterior  roots  of 
the  second  and  first,  or  sometimes  from  the  third,  sacral  nerves 
form  the  nervi  erigentes,  which  passing  to  the  pelvic  plexus  are 
distributed  to  the  penis  and  to  other  organs  ;  in  the  monkey 
the  fibres  are  supplied  by  the  seventh  lumbar  and  first  sacral, 
sometimes  also  by  the  second  sacral  nerves.  They  receive  this 
name  because  stimulation  of  them  leads  to  erection  of  the 
penis ;  and  this  results  from  a  vaso-dilator  action  on  the  arteries 
supplying  the  erectile  tissue.  Erection  of  the  penis  is  hence  to 
a  large  extent  a  vaso-dilator  effect.  But  not  wholly  so ;  the 
entrance  of  the  blood  from  the  dilated  arteries  into  the  venous 
sinuses  is  facilitated  by  the  relaxation-  of  the  muscular  bundles 
in  the  trabeculse,  whose  contraction  would  offer  an  obstacle  to 
the  spaces  becoming  filled.  Further  the  filling  of  the  venous 
sinuses  tends  of  itself  to  compress  the  large  longitudinal  veins 
running  in  the  centre  of  the  corpora  cavernosa  and  thus  to 
increase  the  distension  already  begun  ;  moreover  contractions  of 
the  striated  muscles,  the  transversus  perinaei,  and  the  hulho- 
cavernosus,  between  the  bundles  of  which  the  veins  pass,  also 
tend  to  check  the  outflow  and  so  to  increase  the  erection.  In 
the  dog  even  powerful  stimulation  of  the  nervi  erigentes  will 
not  produce  complete  erection  ;  the  factors  just  mentioned  are 
absent,  and  the  blood,  though  it  more  or  less  fills  the  venous 
sinuses,  flows  freely  away  by  the  veins. 

The  dilating  action  of  the  nervi  erigentes  and  the  nervous 


Chap,  i.]  MALE   OEGAKS.  1117 

impulses  leading  to  the  subsidiary  acts  in  erection  may  be  set 
going  as  part  of  a  reflex  action,  by  stimulation  of  the  glans  penis. 
Of  such  a  reflex  act  the  centre  lies  in  the  lumbar  spinal  cord 
and  erection,  with  emission  of  semen,  has  been  witnessed  in  a 
dog  after  division  of  the  spinal  cord  in  the  thoracic  region.  But 
erection  also  takes  place  as  the  result  of  emotions,  in  which 
case  we  may  suppose  that  impulses  descending  from  the  brain 
affect  the  lumbar  centre  in  a  direct  manner ;  and  indeed  erection 
has  been  experimentally  brought  about  by  stimulation  of  certain 
parts  of  the  brain. 

The  antagonistic  act,  namely,  constriction  of  the  blood  ves- 
sels and  retraction  of  the  penis  may,  in  the  cat,  be  brought 
about  by  stimulation  of  fibres  coming  from  the  upper  lumbar 
(and  possibly  the  lower  thoracic)  region,  and  reaching  their 
destination  by  way  of  the  sympathetic. 

§  692.  The  emission  of  semen,  for  which  act  erection  is 
preparatory,  is  carried  out  by  a  succession  of  agencies.  The 
epididymis  with  its  coni  vasculosi  may  be  regarded  as  a  reser- 
voir filled  by  the  secretory  activity  of  the  seminal  tubes ;  hence 
its  relatively  enormous  length.  It  is  possible  that  the  act  may 
begin  with  an  increase  of  secretory  activity  on  the  part  of  the 
seminal  tubes,  bearing  perhaps  especially  on  the  fluid  parts  of 
the  semen,  by  which  the  epididymis  becomes  overfilled;  we 
have  no  positive  evidence  of  this.  Nor  have  we  evidence  of  any 
pressure,  either  intrinsic  by  means  of  the  plain  muscular  fibres 
which  are  said  to  occur  scantily  in  the  septa  of  the  testis,  or 
extrinsic  through  the  cremaster  or  other  muscles,  being  brought 
to  bear  on  the  contents  of  the  seminal  tubes.  Hence  we  may 
conclude  provisionally  that  the  act  begins  with  a  propulsion  of 
the  contents  of  the  distended  epididymis  by  means  of  peristaltic 
contractions  of  the  muscular  walls  of  that  tube.  In  any  case 
the  flow  of  fluid  having  reached  the  vas  deferens,  is  carried 
along  that  tube  by  the  peristaltic  contractions  of  its  mucli 
stouter  and  much  more  muscular  walls.  In  the  monkey  stimu- 
lation of  the  anterior  roots  of  the  second  and  third  lumbar 
nerves  leads  to  a  powerful  contraction  of  the  vas  deferens, 
sweeping  down  it  in  a  single  wave. 

One  effect,  possibly  a  chief  effect,  of  the  flow  along  the  vas 
deferens  is  to  fill  and  distend  the  vesiculre  seminales ;  or  we 
may  suppose  that  preparatory  feeble  contractions  of  the  epididy- 
mis fill  and  distend  both  the  vas  deferens  and  the  vesicuhi; 
seminales,  and  that  the  act  really  begins  with  a  more  powerful  con- 
traction of  both  these  distended  organs  by  which  their  contents 
are  rapidly  ejected  into  the  prostatic  urethra ;  at  the  same  time 
contractions  of  the  muscular  fibres  of  the  prostate  discharge 
the  secretion  of  that  gland  into  the  urethral  canal.  So  far 
plain  muscular  fibres  only  are  brought  into  play ;  but  the  act 
is  completed  by  the  aid  of  striated  muscles,  namely,  by  forcible 


1118  ACCESSOEY  MALE   OEGANS.  [Book  iv. 

contractions  of  the  levator  ani,  of  the  constrictor  urethrse 
including  the  external  sphincter  of  Henle,  of  the  ischio-cavernous 
muscle,  which  starting  from  the  ischium  on  each  side  embraces 
the  root  of  the  penis,  and  of  the  bulbo-cavernosus  muscle  (or 
ejaculator  urinse)  which  starting  from  the  perinseum  embraces 
the  beginning  of  the  urethra  and  corpus  spongiosum.  A 
contraction  begins  in  the  external  sphincter  ani,  extends  to  the 
levator  ani  and  then  passes  to  the  other  muscles,  progressing 
in  a  wave-like  manner  from  behind  forwards,  and  is  repeated  in 
a  more  or  less  distinctly  rhythmic  manner  until  all  the  semen 
is  ejected  from  the  urethra. 

These  expulsive  contractions,  especially  the  last  named, 
appear  like  erection  to  be  carried  out  by  the  help  of  a  centre 
in  the  lumbar  region  of  the  cord,  and  for  them  afferent  impulses 
generated  in  the  sensitive  surface  of  the  glans  penis  are  more 
essential  than  for  simple  erection.  In  the  dog  stimulation  of  the 
internal  pudic  nerve  throws  the  whole  group  of  striated  muscles 
just  named  into  successive  contractions  as  described,  but  each 
muscle  may  be  made  to  contract  separately  by  stimulation  of  its 
own  individual  branch. 

The  semen  being  received  into  the  vagina,  the  walls  of 
which,  and  especially  the  external  appendages  of  which,  are  at 
the  time  in  a  state  of  turgescence  resembling  the  erection  of  the 
penis,  but  less  marked,  lies,  probably,  at  the  far  end  of  the 
vagina  in  a  pool  into  which  the  os  uteri  dips ;  and  it  is  possible 
that  contractions  of  the  round  ligaments  (which  contain  striated 
muscular  fibres)  by  tilting  the  cervix  backwards  assist  in  bring- 
ing the  OS  uteri  into  the  semen.  In  this  manner  the  spermato- 
zoa find  their  way  into  the  uterus  and  so  into  the  Fallopian 
tube,  where  (probably  in  its  upper  part)  they  come  in  contact 
with  the  ovum.  In  the  rabbit  spermatozoa  may  reach  the  ovary 
within  two  hours  after  coitus.  In  the  case  of  some  animals 
impregnation  may  take  place  at  the  ovary  itself.  The  passage 
of  the  spermatozoa  is  most  probably  effected  mainly  by  their 
own  vibratile  activity ;  but  in  some  animals  a  retrograde 
peristaltic  movement  travelling  from  the  uterus  along  the 
Fallopian  tubes  has  been  observed ;  this  might  assist  in  bring- 
ing the  seinen  to  the  ovum,  but  inasmuch  as  these  movements 
are  probably  parts  of  the  act  of  coitus  and  impregnation  may 
be  deferred  till  some  time  after  that  event,  no  great  stress  can 
be  laid  upon  them. 


CHAPTER   II. 

PREGNANCY  AND   BIRTH. 

SEC.  1.     THE  PLACENTA. 

§  693.  The  spermatozoa  travelling  up  the  female  passages 
come  in  contact  with  the  ovum.  Making  their  way  through  the 
cells  of  the  discus,  which  by  this  time  are  undergoing  degenera- 
tive changes,  and  piercing  the  zona  pellucida,  they  enter  the 
vitellus ;  it  is  stated  that  as  a  rule  one  spermatozoon  only 
actually  reaches  the  vitellus.  Here  the  tail,  which  by  its  vibratile 
activity  has  thus  brought  the  spermatozoon  to  its  destination, 
ceases  to  move  and  soon  disappears ;  but  the  head  (which  is  a 
prepared  and,  so  to  speak,  purified  nucleus,  a  male  pronucleus) 
unites  with  the  pronucleus  of  the  ovum  to  form  the  nucleus  of 
the  now  impregnated  ovum. 

As  the  result  of  this  action  of  the  spermatozoon  on  the 
ovum,  the  latter,  instead  of  d3dng  as  when  impregnation  fails, 
awakes  to  new  nutritive  activity.  It  undergoes  segmentation, 
the  one  cell  becomes  by  cell-division  a  mass  of  cells,  which, 
passing  through  a  series  of  remarkable  morphological  changes, 
into  the  details  of  which  we  cannot  enter  here,  developes  into 
an  embryo. 

§  694.  No  sooner,  however,  have  these  changes  begun  in 
the  ovum  than  correlative  changes,  brought  about  probably  by 
reflex  action,  but  at  present  most  obscure  in  their  causation, 
take  place  in  the  uterus.  The  mucous  membrane  of  this  organ, 
whether  the  coitus,  which  was  the  cause  of  the  impregnation, 
took  place  at  a  menstrual  period  or  at  some  time  in  the  interval, 
undergoes  changes  which  though  more  intense  are  at  first 
not  unlike  those  of  menstruation ;  it  becomes  congested,  and 
a  rapid  growth  takes  place,  characterized  by  a  proliferation  of 
the  epithelial  and  other  tissues.  Unlike  what  takes  place  in 
menstruation,  however,  this  new  growth  does  not  give  way  to 
ha3morrliage  and  immediate  decay ;  it  remains,  and  ma}^  be  dis- 

1119 


1120  THE  PLACENTA.  [Book  iv. 

tinguished  as  a  new  temporay  lining  to  the  uterus,  the  so- 
called  decidua.  Into  this  decidua  the  ovum,  on  its  descent  from 
the  Fallopian  tube,  in  which  it  has  already  undergone  some 
developmental  changes,  is  received ;  and  in  this  it  becomes 
embedded,  the  new  growth  closing  in  over  it.  Meanwhile  the 
rest  of  the  uterine  structures,  especially  the  muscular  tissue, 
become  also  much  enlarged ;  as  pregnancy  advances  a  large 
number  of  new  muscular  fibres  are  formed. 

As  the  ovum,  now  developing  into  the  embryo  and  its 
appendages,  continues  to  increase  in  size,  it  bulges  into  the  cavity 
of  the  uterus,  carrying  with  it  the  jDortion  of  the  decidua  which 
has  closed  over  it.  Henceforward,  accordingl}^,  a  distinction 
is  made  in  the  now  rapidly  developing  decidua  between  the 
decidua  rejiexa,  or  that  part  of  the  membrane  which  covers  the 
projecting  ovum,  and  the  decidua  vera,  or  the  rest  of  the  mem- 
brane lining  the  cavity  of  the  uterus,  the  two  being  continuous 
round  the  base  of  the  projecting  ovum.  That  part  of  the 
decidua  which  intervenes  between  the  ovum  and  the  nearest 
uterine  wall  is  spoken  of  as  the  decidua  serotina.  As  the 
embryo  with  its  appendages  continues  to  enlarge,  carrying 
with  it  the  decidua  reflexa,  the  latter  becomes  pushed  against 
the  decidua  vera,  gradually  obliterating  the  cavity  of  the  uterus, 
except  at  the  cervix ;  about  the  end  of  the  third  month,  in  the 
human  subject,  the  two  come  into  complete  contact  all  over,  and 
ultimately  the  distinction  between  them  is  lost. 

The  decidua  reflexa,  and  the  decidua  vera  in  general,  undergo 
various  changes,  but  these  are  of  subordinate  interest  compared 
with  those  which  take  place  in  that  part  of  the  decidua  vera 
which  is  called  the  decidua  serotina,  and  which  lead  to  a  special 
union  between  maternal  tissues  and  tissues  belonging  to  the 
growing  embryo,  a  union  which  gives  rise  to  the  structure 
known  as  the  placenta  or  '  after-birth.' 

§  695.  During  the  development  of  the  ovum  while  some 
of  the  cells,  arising  by  cell-division  from  the  primordial  cell, 
become  the  embryo  proper,  others  form  the  appendages  of  the 
embryo ;  to  the  latter  belongs  the  double  bag  which  encloses 
the  embryo,  and  which  consists  of  an  inner  bag,  the  trnie  amnion 
and  an  outer  bag,  t\v&  false  amnion.  The  latter  over  the  whole 
of  its  surface  is  in  contact  with  the  decidua,  and  developes  a 
number  of  branched  villi,  consisting,  like  the  rest  of  the  mem- 
brane, of  an  epithelium  (epiblast)  resting  on  a  dermic  (meso- 
blastic)  basis ;  these  villi  are  embedded  in  or  applied  to  the 
decidual  surface.  The  false  amnion,  bearing  villi,  often  called 
the  chorion,  is  at  first  devoid  of  blood  vessels  ;  but  a  diver- 
ticulum of  tbe  hinder  part  of  the  developing  alimentary  canal 
of  the  embryo,  called  the  allantois,  grows  out  rapidly  into 
the  space  (containing  fluid)  between  the  false  and  the  true 
amnion,  and  soon  applies  itself  to  the  former.     As  it  grows,  two 


Chap,  ii.]  PREGNANCY   AXD    BIRTH.  1121 

arteries,  continuations  of  the  primitive  aorta,  the  allantoic 
arteries,  subsequently  called  umbilical  arteries,  make  their 
appearance.  These  carry  the  blood  of  the  embryo  to  the  villi 
of  the  chorion ;  from  thence  it  is  returned  at  first  to  two  veins, 
but  ultimately  to  a  single  vein  running  in  company  with  the 
umbilical  arteries,  and  called  the  umbilical  vein. 

At  first  all  the  villi  over  the  whole  surface  of  the  chorion 
except  at  two  opposite  poles  are  thus  supplied  with  blood, 
but  ultimately  the  supply  is  restricted  to  that  part  of  the 
chorion  which  is  applied  to  the  decidua  serotina.  Here  the 
villi  become  developed  into  large  and  conspicuous  vascular 
tufts,  whereas  over  the  rest  of  the  chorion  the}^  soon  atrophy ; 
and  here  the  placenta  is  formed  through  a  number  of  complex 
changes,  the  details  of  which  have  been  and  still  are  the  subject 
of  much  discussion,  changes  by  which  the  whole  region,  stretch- 
ing from  the  basal  portion  of  the  uterine  glands,  or  even  from 
the  uterine  muscular  coat,  to  the  connective  tissue  which  carries 
the  capillary  loops  in  which  the  umbilical  arteries  end,  is  so 
altered  that  it  becomes  difficult  to  say  which  are  maternal, 
which  are  embryonic  elements,  which  structures  are  of  glandu- 
lar and  true  epithelial  origin,  which  of  connective  tissue  or 
epithelioid  origin. 

There  is  evidence  that  in  the  formation  of  the  placenta, 
the  hypertrophied  glandular  mucous  membrane,  having  done 
its  work  in  nourishing  by  secretory  activity  the  embr3'o  at  an 
early  stage,  is,  at  least  in  its  more  superficial  portions,  absorbed, 
eaten  as  it  were,  by  the  advancing  chorionic  vascular  tufts.  This 
is  introductory  to  the  special  vascular  arrangements  of  the 
placenta,  the  uterine  glands  making  way  for  the  system  of  blood 
sinuses ;  but  even  in  the  full-grown  placenta  we  may  recognize 
that  the  interchange  between  mother  and  foetus  is  effected  not 
in  a  wholl}^  mechanical  manner  by  the  mere  bringing  into 
close  juxtaposition  the  maternal  and  foetal  blood,  but  also  by  an 
activity  wdiich  we  may  venture  to  call  secretory  on  the  one 
hand  of  the  epithelium  covering  the  villi,  and  on  the  other 
hand  of  the  decidual  cells,  whatever  may  be  the  exact  origin 
and  nature  of  each  of  these  kinds  of  cell. 

As  the  nutrition  of  the  embryo  becomes  more  and  more 
concentrated  in  the  altered  decidua  serotina  or  placenta,  the 
decidua  vera  and  reflexa,  having  pla^^ed  their  part,  are  done 
away  with.  They  are  not,  however,  shed  abruptly  as  in  men- 
struation ;  they  are  returned  piecemeal  by  absorption  into  the 
maternal  system ;  they  atrophy  until  the  whole  reflexa  and  the 
superficial  part  of  the  vera  is  reduced  to  a  mere  membrane 
adherent  to  the  expanded  chorion,  while  the  basal  portion  of 
the  vera  remains  to  grow  up  after  the  birth  of  the  foetus  into 
a  normal  mucous  membrane. 

The  serotina  having  become  the   maternal   portion  of   the 

71 


1122  THE  PLACENTA.  [Book  iv. 

placenta  continues  its  functions  during  the  whole  of  the  intra- 
uterine life  of  the  embryo.  When  the  term  of  the  maternal 
nutrition  of  the  embryo  is  ended  and  birth  takes  place,  there 
is  a  sudden  disruption  of  tissue  along  the  line  of  the  decidual 
layer,  either  where  this  joins  the  muscular  coat,  the  whole 
mucous  coat  being  subsequently  renewed,  or  at  some  little 
distance  from  it,  the  '  basal  remnants '  of  the  glands  being 
left  to  grow  up  into  the  new  mucous  lining;  and  the  trans- 
formed serotina,  like  the  changed  mucous  membrane  of 
menstruation  but  even  more  suddenly  and  abruptly,  is  shed 
as  the  "after-birth."  With  the  placenta  there  are  also  shed 
the  so-called  '  membranes,'  that  is  to  say  the  amniotic  mem- 
branes together  with  the  membranous  remnants  of  the  vera 
and  reflexa,  which  have  become  adherent  to  and  fused  with 
these.  Hence  ultimately  the  whole  decidua,  the  whole  trans- 
formed mucous  membrane  of  the  pregnant  uterus,  like  the 
changed  mucous  membrane  of  menstruating  uterus  is,  though 
in  a  different  manner,  cast  off. 

We  may  add  that  the  form  and  structure  of  the  placenta 
and  the  mode  of  connection  between  the  mother  and  the 
embryo  differ  in  different  placental  animals ;  in  all  cases, 
however,  the  blood  of  the  chorionic  villi  of  the  embryo  are 
bathed  in  sinus-like  blood-spaces  of  the  mother.  In  all  cases 
too  there  is  a  development  around  the  villi  of  epithelial  struct- 
ures of  a  secretory  character ;  in  ruminant  animals  collections 
of  such  cells  form  what  is  called  '  uterine  milk.'  It  is  in  these 
cells  belonging  to  the  border  line  between  mother  and  infant, 
whether  they  are  of  maternal  or  of  embryonic  origin,  that  the 
glycogen,  which  is  so  often  present  in  the  placenta,  is  placed,  and 
the  presence  of  this  substance  may  be  taken  as  a  token  of  the 
metabolic  activity  of  these  cells. 


SEC.  2.     THE   KUTRITIOK    OF  THE   EMBRYO. 

§  696.  In  a  hen's  egg  a  very  small  part  only  of  the  whole 
egg,  namely,  a  minute  collection  of  cells  called  the  blastoderm, 
is  actually  developed  into  the  chick  and  its  appendages ;  by  far 
the  greater  part  of  the  mass  included  within  the  egg-shell, 
namely  the  '  yolk '  and  the  '  white,'  is  mere  nutritive  material. 
Through  the  porous  egg-shell  the  oxygen  of  the  air  has  adequate 
access  to  the  contents  within,  and  through  the  same  egg-shell 
carbonic  acid  can  escape.  The  yolk  and  the  white  supply  all  the 
food  needed  by  the  developing  chick  until  it  is  hatched,  and 
either  directly  or  indirectly  by  means  of  the  allantoic  vessels  the 
tissues  of  the  embryo  and  its  appendages  breathe  through  the 
shell. 

In  the  mammal  the  supply  of  yolk  is  insignificant;  almost 
from  the  first  the  developing  ovum  receives  nutritive  material 
from  the  mother.  Within  the  ovary  the  ovum  is  fed  by  the 
cells  of  the  Graafian  follicle ;  and  a  similar  mode  of  feeding  is 
continued  for  some  little  time  in  the  uterus.  The  repeated  cell 
division  of  the  ovum  produces  a  compact  mass  of  cells,  the 
'  mulberry  mass,'  and  this  in  turn  is  converted  into  the  '  blasto- 
dermic vesicle,'  which  consists  of  a  cellular  membrane  investing 
fluid  contents ;  during  this  conversion  a  considerable  increase  in 
the  total  bulk  of  the  ovum  takes  place,  water  and  nutritive 
material  passing  into  the  ovum  from  the  mother,  probably  from 
the  cells  lining  the  Fallopian  tube.  Received  witliin  the  uterus 
and  covered  up  b}^  the  decidua,  the  developing  embryo  is  sup- 
plied with  food  and  oxygen  by  the  cells  of  the  uterine  mucous 
membrane  with  which  it  lies  in  contact,  very  much  in  the  same 
way  that  the  growing  ovum  was  supplied  by  the  cells  of  the 
Graafian  follicle ;  and  the  same  uterine  cells  carry  away  the 
scanty  waste  matters  of  the  embrj^o's  nutritive  activity. 

The  amount  of  food  which  the  embryo  needs  and  receives  is 
at  first  small  but  continually  and  rapidly  increases  ;  the  amount 
of  oxygen  which  the  embryo  needs  is  at  first  insignificant,  but 
the  need  of  oxygen  also  increases  continually  and  rapidl}-,  though 
especially  during  the  early  stages  it  is  limited  by  the  fact  that 
the  processes  going  on  in  the    embryonic  tissues  are  largely 

1123 


1124  THE  NUTEITIOX   OF   THE  E^SIBRYO.       [Book  iv. 

synthetic,  directed  to  the  building  up  of  the  tissues,  and  such 
processes  consume  very  little  oxygen  compared  with  the  pro- 
cesses leading  to  expenditure  of  energy  in  movement  and  heat. 
Hence  the  simple  method  of  nutrition  and  respiration  by  means 
of  the  direct  contact  of  the  cells  of  the  uterine  mucous  mem- 
brane is  exchanged  for  the  special  vascular  mechanism  of  the 
placenta,  by  which  the  embrj^o  lives  upon  and  breathes  through 
the  uterine  blood  of  the  mother.  From  an  early  period  up 
to  birth  the  placental  circulation  is  the  chief,  we  may  almost 
say  the  only  means  by  which  the  embryo  breathes  and  is  fed ; 
but  the  details  of  the  placental  events  are  changing  during  the 
whole  of  this  time.  The  embryo,  all  the  while  increasing  in 
bulk,  passes  through  phase  after  phase ;  the  structural  features 
of  one  day  give  way  to  those  of  the  next,  its  morphological 
history  being  as  it  were  a  series  of  dissolving  views ;  and  each 
new  structural  phase  entails  new  functional  events  both  in 
the  embryo  itself  and  in  the  placenta.  This  is  perhaps  especiallj- 
seen  in  the  earlier  stages  at  a  time  when  the  placental  circula- 
tion has  been  established  in  its  main  outlines,  but  in  the  embryo 
most  of  the  future  organs  are  still  in  a  shadowy  inchoate  condi- 
tion. At  this  epoch,  of  the  total  bulk  of  blood  coursing 
from  the  embryo  towards  the  tissues  of  the  mother  and  back 
again,  the  greater  part  is  at  any  one  moment  to  be  found  in  the 
placenta  and  only  a  small  part  in  the  tissues  of  the  embryo  itself  ; 
later  on  the  blood  is  equall}^  divided  between  the  placenta  and 
the  embryo ;  and  still  later  the  embrj^o  has  the  larger  share,  and 
it  is  the  smaller  part  wliich  is  at  any  one  moment  flowing 
through  the  chorionic  villi  of  the  placenta.  There  can  be  no 
doubt  that  in  the  earlier  phase  the  influences  which  the  placental 
structures  exert  on  the  foBtal  blood  are  in  many  ways  different 
from  those  which  are  exerted  later  on.  We  find  that  during  the 
earlier  phases  the  cellular  placental  elements  are  correspondingly 
prominent,  indicating  that  much  labour  of  the  kind  for  which  cells 
are  necessarj^is  being  then  carried  on,  whereas  in  the  later  stages 
the  placental  mechanism  approaches  though  it  never  quite 
reaches  the  more  mechanical  conditions  of  a  simple  membrane 
separating  the  foetal  and  maternal  blood.  We  cannot  enter  at  all 
fully  here  into  the  successive  phases  ;  we  must  confine  ourselves 
chiefly  to  the  main  features  of  what  is  going  on  during  the  latter 
months  of  gestation  when  the  placental  circulation  is  in  full  swing. 
§  697.  At  this  time  the  somewhat  rapid  strokes  of  the  foetal 
heart  drive  the  foetal  blood  through  the  umbilical  arteries  to  the 
capillaries  of  the  chorionic  villi,  from  whence  it  is  returned 
by  the  umbilical  vein.  From  experiments  on  lambs  and  other 
animals  it  would  appear  that  the  blood  pressure  in  the  umbilical 
artery  is  moderately  high  (-40  to  80  mm.  Hg.)  and  that  in  the 
umbilical  vein  very  considerable  (15  to  30  mm.  Hg.),  higher 
than  the  venous  pressure  in  the  mother  in  a  vein  of  correspond- 


Chap,  ii.]  PREGNANCY   AXD   BIRTH.  1125 

ing  size ;  the  difference  between  the  arterial  and  venous 
pressure  is  therefore  relatively  less  than  in  the  mother.  Cor- 
responding to  this  the  velocity  of  the  blood  flow  is  relatively 
low.  The  number  of  red  corpuscles  in  a  given  bulk  of  foetal 
blood,  which  was  of  course  at  first  very  scanty,  has  by  this  time 
much  increased,  but  as  a  rule  remains  up  to  the  end  less  than 
that  of  the  mother,  though  this  has  become  diminished  by  the 
pregnancy.  In  many  cases  no  marked  distinction  of  colour 
can  be  observed  between  the  blood  in  the  umbilical  arteries  and 
that  in  the  umbilical  vein,  but  such  difference  as  can  be  noted 
is  in  the  direction  of  the  blood  in  the  vein  beinsf  brigrhter  than 
that  in  the  arteries,  and  at  times  this  is  conspicuously  the  case. 
If,  for  instance,  the  fcetus  at  the  time  of  observation  happens 
to  make  prolonged  movements,  the  contrast  between  the  dark 
blood  of  the  umbilical  arteries  and  the  bright  blood  of  the 
umbilical  vein  may  become  striking.  An  examination  of  the 
gases  of  the  blood  shews  that  the  blood  in  the  vein  contains 
more  oxygen  and  less  carbonic  acid  than  that  of  the  arteries ; 
the  former  for  instance  has  been  found  to  contain  from  7  to  20 
p.c.  of  oxygen  and  40  p.c.  of  carbonic  acid,  the  latter  2  to  6  p.c. 
of  oxygen  and  46  p.c.  of  carbonic  acid.  Hence  the  blood  in  the 
umbilical  vein  is  essentially  arterial  blood,  and  that  in  the 
umbilical  arteries  essentially  venous  blood.  It  may  be  observed 
that  while  as  regards  the  amount  of  carbonic  acid  the  blood  of 
the  foetus  runs  parallel  to  that  of  the  mother,  the  arterial  blood 
of  the  foetus  (in  the  umbilical  vein)  contains  less  oxygen  than 
that  of  the  mother.  This  is  not  due  alone  to  the  relatively 
smaller  amount  of  hsemoglobin,  for  as  shewn  by  experiment 
the  haemoglobin  of  the  foetal  arterial  blood  is  far  from  being 
saturated  with  ox3^gen,  whereas  as  "we  have  seen  (§  286)  that  of 
the  adult  is,  or  very  nearly  so.  We  may  add  that  the  foetal 
blood  left  to  itself  uses  up  its  free  oxygen  rapidly,  very  much 
more  rapidly  than  does  adult  blood. 

The  maternal  blood  is  convej'ed  to  the  placental  sinuses  by 
arteries  which  open  directly  into  the  sinuses.  Hence,  though 
independently  of  any  influence  exerted  by  the  foetal  blood  the 
blood  returned  from  the  sinuses  by  the  uterine  veins  is  venous 
blood,  rendered  venous  by  the  maternal  tissues  themselves,  yet 
the  blood  in  the  sinus  to  which  the  capillaries  of  the  villi  are  ex- 
posed may  be  regarded  as  rather  arterial  than  venous,  and  in 
any  case  contains  more  oxygen  and  less  carbonic  acid  than  does 
the  foetal  blood  arriving  by  the  umbilical  arteries.  Seeing  that 
the  relatively  narrow  uterine  arteries  open  out  suddenl}^  in  the 
wide  placental  sinuses  the  flow  in  the  latter  must  be  slow ;  the 
flow  in  the  foetal  vessels  is  also  as  we  have  seen  not  rapid  ;  hence 
ample  time  is  given  for  the  interchange  of  gases.  The  change 
which  is  thus  effected  is  probably  carried  out  by  diffusion,  the 
amount  of  change  being  determined  by  the  relative  percentages 


1126  THE  NUTEITIO^   OF   THE  EMBRYO.      [Book  iv. 

of  the  srases  in  the  maternal  and  foetal  blood.  At  least  we  have 
no  more  evidence  in  the  case  of  this  placental  respiration  than 
we  had  in  the  case  of  the  pulmonary  respiration  that  the  inter- 
change is  in  any  way  assisted  by  cellular  activity  of  a  secretory 
kind.  The  placental  respiration  of  the  mammal  seems  in  fact 
exactly  to  repeat  the  branchial  respiration  of  the  fish ;  in  the 
former  the  foetus  breathes  by  means  of  the  maternal  blood 
in  the  same  way  that  in  the  latter  the  fish  breathes  by  means  of 
the  water  in  which  it  lives. 

It  follows  from  the  above  that  the  foetus  may  be  asphyxiated 
in  two  ways :  on  the  one  hand  by  interference  with  the  access  of 
foetal  blood  to  the  placenta,  as  when  the  cord  is  tied,  and  on  the 
other  hand  by  the  maternal  circulation  being  arrested,  or  by  the 
maternal  blood  being  wanting  in  oxygen.  When  the  mother 
is  asphyxiated  the  foetus  is  asphyxiated  too,  the  oxygen  passing 
from  the  foetal  blood  to  that  of  the  mother.  In  such  a  case, 
owing  to  the  more  imperious  demands  of  the  maternal  blood, 
the  store  of  oxygen  in  the  foetal  blood  is  sooner  exhausted 
and  asphyxia  is  more  rapidly  developed  than  in  the  case  when 
the  cause  lies  in  the  foetus,  not  in  the  mother,  and  the  oxygen 
simply  disappears  from  the  foetal  blood  as  it  is  slowly  used  up 
by  the  foetal  tissues ;  for  the  rate  of  foetal  oxidation  though  it 
increases  continually  during  the  intra-uterine  life,  especially  in 
the  later  stages,  is  slow  compared  to  what  it  becomes  some 
time  after  birth. 

§  698.  The  foetus  not  only  breathes  but  also  feeds  and 
probably  excretes  by  means  of  the  placenta  ;  the  blood  returning 
by  the  umbilical  vein  is  not  only  richer  in  oxygen  and  poorer  in 
carbonic  acid  but  also  richer  in  nutritive  material  and  poorer 
in  waste  products  than  the  blood  of  the  umbilical  arteries. 
In  dealing  however  with  the  nutrition  of  the  embryo  we  must 
bear  in  mind  a  special  condition  under  which  the  embryo  lives. 
As  we  have  said  the  embryo  proper  becomes  at  an  early  date 
invested  with  the  double  membranous  bag  of  the  amnion, 
consisting  of  the  inner  amnion  and  outer  (false)  amnion. 
Between  the  two  there  is  at  first  a  space,  into  which  as  we  have 
seen  the  allantois  grows  in  order  to  become  the  placenta ;  but,  as 
the  fluid,  which  from  the  first  is  present  within  the  inner  bag, 
increases  in  amount,  without  any  corresponding  increase  in  the 
fluid  between  the  inner  and  outer  bag,  the  (true)  amnion  in  its 
expansion  after  the  formation  of  the  placenta  reaches  and  unites 
with  the  false  amnion  which  by  this  time  is  known  as  the 
chorion.  The  whole  interior  of  the  uterus  is  lined,  next  to  the 
decidua,  by  a  membrane  apparently  simple  but  composed  of 
united  amnion  and  chorion,  and  within  this,  surrounding  and 
supporting  the  embryo,  lies  the  amniotic  fluid,  which  at  first 
scanty  rapidly  increases  in  amount  until  in  the  later  stages  of 
pregnancy  it  may  amount  to  800  c.c.  or  even  much  more. 


Chap,  ii.]  PREGNANCY   AND  BIRTH.  1127 

In  the  roof  of  the  uterus,  in  the  region  of  the  placenta,  the 
amniotic  fluid  is  in  close  proximity  not  only  to  the  branching 
umbilical  arteries  and  veins  of  the  foetus,  but  also  to  many 
of  the  maternal  blood  vessels,  being  separated  from  the  maternal 
blood  by  nothing  more  than  the  thin  wall  of  the  blood  vessel 
and  the  membrane  just  spoken  of.  The  fluid  is  also  over  the 
rest  of  the  internal  surface  of  the  uterus,  in  close  proximity  to 
the  blood  vessels  of  the  maternal  decidua,  and  indeed  in  the 
later  stages,  when  the  decidua  apart  from  the  placenta  has 
largely  retrograded,  to  the  blood  vessels  of  the  uterine  mucous 
membrane.  The  conditions  therefore  are  favourable  for  the 
transudation  of  material  from  the  blood  of  the  mother  into  the 
amniotic  cavity ;  and  we  have  experimental  evidence  that  not 
only  water  but  various  substances  may  pass  in  this  way  from 
the  one  to  the  other.  If  indigo-carmine  (§  336)  be  injected 
into  the  veins  of  the  mother,  none  passes  by  the  umbilical  vein 
into  the  tissues  of  the  foetus ;  these  remain  wholly  uncoloured. 
Yet  the  amniotic  fluid  becomes  deeply  tinged  with  the  pigment, 
which  obviously  must  have  passed  directly  from  the  mother 
into  the  amniotic  cavity.  Hence  we  may  conclude  that  though 
the  amniotic  fluid  is  at  first  derived  exclusively  from  the 
foetus,  and  during  the  whole  time  is  partly  derived  from  the 
same  source,  it  is  also,  and  especially  in  the  later  stages,  largely 
derived  by  direct  transudation  from  the  mother. 

Into  this  amniotic  space  the  passages  of  the  foetus,  the 
mouth,  anus,  &c.  open,  and  it  serves  as  we  shall  see  as  a  reposi- 
tory for  the  excretions  of  the  foetus.  Into  it  is  discharged 
such  urine  as  the  foetus  secretes,  into  it  are  shed  the  foetal 
epidermic  scales,  and  appendages  such  as  hairs,  and  into  it  may 
be  discharged  the  contents  of  the  alimentary  canal,  known 
as  the  meconium.  Now,  hairs,  epidermic  scales,  in  the  case 
of  hoofed  mammals  portions  of  shed  hoofs,  and  at  times 
meconium  have  been  found  in  the  foetal  stomach;  they  arrived 
there  by  the  foetus  swallowing  the  amniotic  fluid;  we  have 
other  evidence  that  the  foetus  in  the  uterus  may  execute 
swallowing  movements,  and  if  these  are  executed  they  must 
lead  to  swallowing  of  the  amniotic  fluid,  since  this  will  pass 
into  the  mouth  and  pharynx  whenever  the  mouth  is  opened.  If 
these  swallowing  movements  occur  frequently,  and  there  is 
some  evidence  that  they  do,  nutritive  material  contained  in  the 
fluid  and  derived  directly  from  the  mother,  might  thus  be  con- 
veyed to  the  fcBtus;  the  latter  might  be  nourished  by  means  of 
the  amniotic  fluid.  But,  even  making  all  allowance  for  any 
possible  nourishment  in  this  way,  we  may  probably  regard  it  as 
insignificant  comixired  with  that  whicli  is  carried  on  by  the 
placental  and  umbilical  vessels;  we  may  assume  that  the  food 
of  the  foetus  reaches  it  mainly  by  passing  from  the  maternal 
sinuses  into  the  capillaries  of  the  chorionic  villi. 


1128  THE  NUTRITION   OF  THE  EMBRYO.      [Book  iv. 

§  699.  Judging  from  analogy  we  may  conclude  that  tlie  food 
of  the  foetus  consists,  like  that  of  the  adult,  of  proteids,  fats, 
carbohydrates  and  salts  conveyed  in  water.  In  attempting  to 
understand  how  these  materials  pass  from  the  blood  of  the 
maternal  sinus  to  the  blood  of  the  foetal  villus,  we  have  to  face 
problems  of  the  same  kind  as  those  which  we  met  with  in  con- 
sidering absorption  from  the  alimentary  canal  (§  253). 

Here  as  there  diffusion  and  filtration  play  their  parts ;  but 
here  also  as  there  the  passage  of  material  does  not  follow  the 
laws  of  diffusion  and  filtration  which  regulate  the  passage  of 
material  through  non-living  membranes.  We  have  evidence 
that  diffusible  substances  pass  readily  from  mother  to  foetus 
and  from  foetus  to  mother.  When  sugar  is  injected  in  consid- 
erable quantity  into  the  vessels  of  the  mother,  it  is  found  in 
excess  in  the  tissues  of  the  foetus.  When  such  a  drug  or  poison 
as  atropin  is  injected  into  the  mother  it  passes  to  the  foetus, 
and  manifests  its  presence  there  by  dilation  of  the  pupils.  Not 
only  may  the  foetus  be  killed  by  injection  of  strychnine  into 
the  mother,  but  the  mother  may  be  killed  by  the  injection  of 
strychnine  carefully  restricted  to  the  foetus.  Again,  if  curare, 
which  is  inert  towards  the  foetus  at  least  up  to  a  certain  dose, 
be  injected  into  the  foetus,  the  mother  is  affected  by  the  drug, 
the  fact  that  the  drug  does  not  poison  the  foetus  assisting  in 
its  transmission  to  the  mother ;  this  result  is  especially  worthy 
of  notice  since  curare  has  a  very  low  diffusible  power.  The 
influence  of  diffusion  seems  to  be  further  illustrated  by  the 
fact  that  if  large  quantities  of  sugar  or  other  diffusible  sub- 
stance be  injected  into  the  blood  vessels  of  the  mother,  while 
the  thickened  plasma  of  the  maternal  blood  is  diluted  by  the 
entrance  of  water,  as  shewn  by  the  diminished  proportion  of 
red  corpuscles,  that  of  the  foetus  as  shewn  by  the  same  method 
undergoes  concentration ;  water  passes  from  the  foetal  blood  to 
meet  the  needs  of  the  maternal  blood. 

Nevertheless  that  in  the  passage  of  nutritive  material  from 
the  mother  to  the  foetus,  and  of  waste  products  from  the  fcetus 
to  the  mother,  we  have  to  deal  with  something  more  than  ordi- 
nary diffusion,  is  shewn  by  the  fact  that  the  specific  gravity  of 
the  foetal  blood  differs  from,  being  definitely  above,  that  of  the 
maternal  blood ;  if  diffusion  had  its  full  power  the  specific 
gravities  of  the  two  bloods  would  soon  become  equalized. 
Although  exact  information  concerning  the  matter  is  at  present 
very  limited  or  almost  wholly  wanting,  it  is  probable  that  the 
epithelium  cells  of  the  placenta,  either  those  of  the  villi  or  the 
'  decidual '  cells  or  both,  play  a  part  not  unlike  that  played 
by  the  epithelium  of  the  alimentary  canal  or  even  play  a  more 
important  part.  Whether  the  proteids  of  the  maternal  blood 
undergo  a  change  analogous  to  peptonification  in  passing  to  the 
foetus,  whether  the  mother  furnishes  fat  to  the  foetal  blood, 


Chap,  ii.]  PKEGNANCY   AND   BIRTH.  1129 

and  if  so  how,  —  to  these  and  other  questions  which  suggest 
themselves  no  very  satisfactoiy  answer  can  at  present  be  given. 
With  regard  to  fat,  leaning  on  the  analogy  of  the  conclusion 
at  which  (§  486)  we  arrived,  that  in  the  adult  the  fat  of  the 
food  is  probably  not  taken  up  by  the  tissues  as  fat  during  the 
nutrition  of  the  tissues  by  the  blood,  we  may  perhaps  suppose 
that  the  mother  does  not  supply  the  foetus  with  fat  as  such. 
We  have  already  referred  to  the  significant  presence  of  glyco- 
gen in  the  placenta ;  and  it  would  almost  seem  as  if  the  pla- 
centa exerted  at  one  and  the  same  time  on  the  material  passing 
from  the  mother  to  the  foetus  influences  comparable  not  only 
with  those  exerted  by  the  walls  of  the  alimentary  canal  but 
also  with  those  subsequently  exerted  by  the  hepatic  cells  on 
the  material  which  passes  by  way  of  the  portal  vein  from  the 
intestines  to  the  right  side  of  the  heart.  Again  the  very  phrase 
"  uterine  milk  "  suggests  that  the  placenta  epithelial  cells  exer- 
cise a  secretory  and  metabolic  influence  comparable  to  that  of 
the  mammary  gland.  But  how  far  these  analogies  are  false  or 
true  future  research  must  determine ;  and  putting  aside  for  a 
while  the  special  problems  thus  suggested  we  may,  in  a  broad 
way,  say  that  the  foetus  lives  on  the  blood  of  its  mother,  very 
much  in  the  same  way  that  all  the  tissues  of  any  animal  live  on 
the  blood  of  the  body  of  which  they  are  the  parts. 

§  700.  For  a  long  time  all  the  embryonic  tissues  are  '  pro- 
toplasmic '  in  character ;  that  is  to  say,  the  gradually  differen- 
tiating elements  of  the  several  tissues  remain  still  embedded  in 
undifferentiated  material ;  and  during  this  period  there  must 
be  a  general  similarity  in  the  metabolism  going  on  in  various 
parts  of  the  body.  As  differentiation  becomes  more  and  more 
marked,  it  obviously  would  be  an  economical  advantage  for 
partially  elaborated  material  to  be  stored  up  in  various  foetal 
tissues,  so  as  to  be  ready  for  immediate  use  when  a  demand 
arose  for  it,  rather  than  for  a  special  call  to  be  made  at  each 
occasion  upon  the  mother  for  comparatively  raw  material  need- 
ing subsequent  preparatory  changes.  Accordingly,  we  find  the 
tissues  of  the  foetus  at  a  very  early  period  loaded  with  glyco- 
gen. The  muscles  are  especially  rich  in  this  substance,  but  it 
occurs  in  other  tissues  as  well.  The  abundance  of  it  in  the 
former  may  be  explained  partly  by  the  fact  that  they  form  a 
very  large  proportion  of  the  total  mass  of  the  fcetal  body,  and 
partly  by  the  fact  that,  Avhile  during  the  presence  of  the  glyco- 
gen they  contain  much  undifferentiatecl  substance,  they  are 
exactly  the  organs  which  will  ultimately  undergo  a  large 
amount  of  differentiation,  and  therefore  need  a  large  amount 
of  material  for  the  metabolism  which  the  differentiation  entails. 
It  is  not  until  the  later  stages  of  intra-uterine  life,  at  about  the 
fifth  month,  when  it  is  largely  disappearing  from  the  muscles, 
that  the  glycogen  begins  to  be  deposited  in  the  liver.     By  this 


1130  THE  NUTEITION   OF  THE  EMBEYO.      [Book  iv. 

time  histological  differentiation  has  advanced  largely,  and  the 
use  of  the  glycogen  to  the  economy  has  become  that  to  which 
it  is  put  in  the  ordinary  life  of  the  animal ;  hence  we  find  it 
deposited  in  the  usual  place.  We  do  not  know  how  much  car- 
bohydrate material  finds  its  way  into  the  umbilical  vein ;  and 
we  cannot  therefore  state  what  is  the  source  of  the  foetal  glyco- 
gen ;  but  it  is  at  least  possible,  not  to  say  probable,  that  it 
arises,  in  part  at  all  events,  from  a  splitting  up  of  proteid 
material  in  the  foetal  body. 

§  701.  Concerning  the  rise  and  development  of  the  func- 
tional activities  of  the  embryo,  our  knowledge  is  almost  a  blank. 
We  know  scarcely  anything  about  the  various  steps  by  which 
the  primary  fundamental  qualities  of  the  living  matter  of  the 
ovum  are  differentiated  into  the  complex  phenomena  which  we 
have  attempted  in  this  book  to  expound.  We  can  hardly  state 
more  than  that  while  muscular  contractility  becomes  early 
developed,  and  the  heart  probably,  as  in  the  chick,  beats  even 
before  the  blood-corpuscles  are  formed,  movements  of  the  foetus 
are  in  the  human  subject  first  felt  about  the  sixteenth  week ; 
they  probably  occur  before  but  are  not  easily  recognized,  while 
from  that  time  onward  they  increase  and  subsequently  become 
very  marked.  They  are  often  spoken  of  as  reflex  in  character, 
and  some  of  them  are  undoubtedly  of  this  nature.  When  the 
uterus  of  a  pregnant  animal  is  prematurely  opened,  various 
reflex  movements  of  the  foetus  may  be  excited  by  appropriate 
stimulation,  different  kinds  of  animals  differing  in  this  respect 
as  they  do  with  regard  to  the  powers  of  the  new-born  animals. 
Such  reflex  movements  may  be  witnessed  before  the  placental 
circulation  has  been  interrupted,  but  they  are  increased  if  the 
foetus  be  made  to  breathe.  We  have  already  referred  to  swal- 
lowing movements  ;  and  may  add  that  an  immature  fcetal  ani- 
mal may  be  made  to  bite  by  introducing  the  finger  into  its 
mouth.  Some  of  these  normal  intra-uterine  movements  appear 
however  to  be  not  reflex  but  automatic  if  not  voluntary  in 
nature.  Movements  of  the  limbs,  apparently  automatic,  have 
been  observed  in  foetuses  in  which  the  brain  has  not  been 
developed.  We  may  add  that  in  the  human  subject  the  occur- 
rence of  intra-uterine  convulsions  is  fully  acknowledged. 

§  702.  The  digestive  functions  are  naturally,  in  the  absence 
of  all  food  from  the  alimentary  canal,  in  abeyance.  Though 
pepsin  may  be  found  in  the  gastric  membrane  at  about  the 
fourth  month,  it  is  doubtful  whether  a  truly  peptic  gastric  juice 
is  secreted  during  intra-uterine  life ;  trypsim  appears  in  the 
pancreas  somewhat  later,  but  an  amylolytic  ferment  cannot  be 
obtained  from  that  organ  till  after  birth.  The  date  however 
at  Avhich  these  several  ferments  make  their  appearance  in  the 
embryo  appears  to  differ  in  different  animals.  The  excretory 
functions  of  the  liver  are  developed  early,  and  about  the  third 


Chap,  ii.]  PREGNANCY  AND  BIRTH.  1131 

month  bile-pigment  and  bile-salts  iind  tlieir  way  into  the  intes- 
tine. The  quantity  of  bile  secreted  during  intra-uterine  life 
accumulates  in  the  intestine  and  especially  in  the  rectum,  form- 
ing, together  with  material  secreted  by  the  walls  of  the  alimen- 
tary canal  and  some  desquamated  epithelium,  the  so-called 
meconium.  Human  meconium  is  found  to  contain  about  20 
p.c.  of  solids.  These  consist  of  a  considerable  quantity  of 
cholesterin  (-7  i^.c),  some  fatty  acids,  bile  salts  with  bile  pig- 
ments, both  largely  unaltered,  and  calcium  and  sodium  salts ; 
the  ash  is  rather  more  than  1  p.c.  Though  bile  contributes 
normally  to  form  the  meconium,  it  is  not  essential,  for  a  con- 
siderable quantity  has  been  found  in  the  foetus  in  cases  where 
the  liver  has  been  absent. 

The  distinct  formation  of  bile  is  an  indication  that  the  pro- 
ducts of  foetal  metabolism  are  no  longer  wholly  carried  off  by 
the  maternal  circidation ;  and  to  the  excretory  function  of  the 
liver  there  are  now  added  those  of  the  skin  and  kidney.  Since 
in  man,  and  in  many  other  animals,  such  substances  as  are 
secreted  by  the  kidney  find  their  way  at  an  early  date  into  the 
cavity  of  the  amnion,  tlie  determination  of  the  history  of  the 
renal  secretion  is  a  matter  of  difficulty,  for  as  we  have  seen 
the  amniotic  fluid  is  derived  in  part  at  least  directly  from  the 
mother,  and  substances  present  in  it  may  or  may  not  have  been 
discharged  into  it  by  the  foetus.  The  amniotic  fluid  varies  not 
only  in  quantity  but  also  in  specific  gravity  (1*002  to  1-086)  and 
in  comiDosition,  and  there  does  not  seem  to  be  any  definite  rela- 
tion between  its  specific  gravity  and  the  quantity  in  which  it 
occurs,  or  between  its  specific  gravity  and  the  size  or  age  of  the 
foetus.  It  maybe  said  to  contain  on  the  average  about  1-6  p.c. 
of  solid  matter,  of  which  about  '2  are  proteids,  "8  extractives 
and  -6  salts.  The  proteids  are  serum  albumin  and  probably 
paraglobulin,  mucin  or  a  mucin-like  body  being  also  present. 
Sugar  appears  to  be  sometimes  present,  sometimes  absent.  The 
most  important  constituent  is  perhaps  urea,  which  seems  to  be 
always  present.  Since  this  is  found  at  quite  an  early  stage, 
before  any  secretion  from  the  foetal  kidney  could  take  place,  it 
may  be  thus  considered  as  derived  from  the  mother  and  com- 
parable in  origin  to  the  urea  found  in  serous  fluids ;  but  since 
urine  containing  urea  is  found  in  the  fcetal  bladder  at  least  as 
early  as  the  seventh  month,  we  may  conclude  that  during  the 
later  stages  of  pregnancy,  and  possibly  much  earlier,  part  of 
the  urea  of  the  amniotic  fluid  comes  from  the  foetal  kidney.  In 
some  animals,  ex.  gr.  ruminants,  the  cavity  of  the  allantois  re- 
mains for  a  long  time  permanent  and  filled  with  fluid,  instead  of 
as  in  man  becoming  at  an  early  date  obliterated  in  its  distal 
portion.  In  these  animals  the  kidneys  discharge  their  secretion 
into  this  allantoic  sac,  and  in  the  contents  of  the  sac  is  found 
the  body  allied  to  urea,   allantoin,  so  called  from  its  having 


1132  THE  NUTRITION   OF   THE  EMBRYO.     [Book  iv. 

been  first  discovered  in  this  situation.  Traces  of  allantoin  liave 
also  been  found  in  human  amniotic  fluid,  which  result  suggests 
that  this  substance  is  at  any  early  stage  formed  by  the  kidney 
but  subsequently  gives  place  to  the  permanent  urea. 

There  is  no  evidence  that  any  sweat  is  secreted  by  the  foetus 
in  the  uterus ;  and  indeed  if  any  such  secretion  does  take  place 
this  can  only  be  for  the  discharge  of  solid  matter,  and  not  as  in 
the  adult  for  the  discharge  of  water ;  but  the  epidermic  scales 
are  undoubtedly  shed,  and  may  be  detected  in  the  amniotic 
fluid. 

§  703.  About  the  middle  of  intra-uterine  life,  when  the  f cetal 
circulation  is  in  full  development,  the  blood  flowing  along  the 
umbilical  vein  (see  Fig.  195)  is  chiefly  carried  by  the  ductus 
venosus  into  the  inferior  vena  cava  and  so  into  the  right  auricle. 
Thence  it  appears  to  be  directed  by  the  valve  of  Eustachius 
through  the  foramen  ovale  into  the  left  auricle,  passing  from 
which  into  the  left  ventricle  it  is  driven  into  the  aorta.  Part 
of  the  umbilical  blood,  however,  instead  of  passing  directly  to 
the  inferior  cava,  enters  with  the  blood  carried  by  the  portal 
vein  into  the  hepatic  circulation,  from  which  it  returns  to  the 
inferior  cava  by  the  hepatic  veins.  The  inferior  cava  also  con- 
tains blood  coming  from  the  lower  limbs  and  lower  trunk. 
Hence  the  blood  which  passing  from  the  right  auricle  into  the 
left  auricle  through  the  foramen  ovale  is  distributed  by  the  left 
ventricle  through  the  aortic  arch,  though  chiefly  blood  coming 
direct  from  the  placenta,  is  also  blood  which  on  its  way  from 
the  placenta  has  passed  through  the  liver  and  blood  derived 
from  the  tissues  of  the  lower  part  of  the  body  of  the  foetus. 
The  blood  descending  as  foetal  venous  blood  from  the  head  and 
limbs  by  the  superior  vena  cava  appears  not  to  mingle  largely 
with  that  of  the  inferior  vena  cava,  but  to  fall  into  the  right 
ventricle,  from  which  it  is  discharged  through  the  ductus  arteri- 
osus (Botalli)  into  the  aorta  below  the  arch,  whence  it  flows 
partly  to  the  lower  trunk  and  limbs,  but  chiefly  by  the  umbili- 
cal arteries  to  the  placenta.  A  small  quantity  only  of  the  con- 
tents of  the  right  ventricle  finds  its  way  into  the  lungs.  Now 
the  blood  which  comes  from  the  placenta  by  the  umbilical  vein 
direct  into  the  right  auricle  is,  as  far  as  the  respiration  of  the 
foetus  is  concerned,  arterial  blood ;  and  the  portion  of  umbilical 
blood  which  traverses  the  liver  probably  loses  at  this  epoch  very 
little  oxygen  during  its  transit  through  that  giand,  the  liver 
being  at  this  period  much  more  a  simple  excretory  than  an 
actively  metabolic  organ.  Hence  the  blood  of  the  inferior  vena 
cava,  though  mixed,  is  on  the  Avhole  arterial  blood ;  and  it  is 
this  blood  which  appears  to  be  sent  by  the  left  ventricle  through 
the  arch  of  the  aorta  into  the  carotid  and  subclavian  arteries. 
Thus  the  head  of  the  foetus  is  provided  with  blood  compara- 
tively rich  in  oxygen.     The  blood  descending  from  the  head 


Chap,  ii.] 


PREGNANCY  AND  BIRTH. 


1133 


and  upper  limbs  by  the  superior  vena  cava  is  distinctly  venous  ; 
and  this  passing-  from  the  right  ventricle  by  the  ductus  arterio- 
sus is  driven  along  the  descending  aorta,  and  together  with 
some  of  the  blood  passing  from  the  left  ventricle  round  the 


S.vc 


Fig.  105.     Diagram  to  illlstrate  the  Foetal  Circulation. 

It  will  be  understood  that  the  figure  is  purely  diagrammatic  and  constructed 
simply  to  shew  in  a  convenient  manner  the  general  course  taken  by  the  blood. 

The  winged  arrow  indicates  venous,  the  plain  arrow  arterial,  or,  in  parts, 
mixed  blood. 

UV.  The  umbilical  vein,  passing  in  part  to  the  liver  (indicated  in  outline), 
joined  by  blood  from  the  alimentary  canal  along  the  mesenteric,  becoming  the 
portal  vein  V.P.,  but  chiefly  flowing  on  by  the  ductus  venosus  D.  V.  (into  which 
fall  the  hepatic  veins  V.H.)  into  the  inferior  vena  cava,  /.  V.  C. 

This  chiefly  arterial  but  still  mixed  blood  passes  through  the  right  auricle 
B.A.,  the  foramen  ovale /.o.  to  the  left  auricle  L.A.,  thence  to  the  left  ventricle 
L.  V.  and  so  by  the  arch  of  the  aorta  Ao.  to  the  arteries  of  the  head  and  upper 
limbs. 

The  venous  blood  of  the  head  and  upper  limbs  passes  from  the  superior  vena 
cava  S.  V.  G.  through  the  right  auricle  to  the  right  ventricle  E.  V.  and  thence  by 
the  pulmonary  artery  P.A.  and  ductus  arteriosus  D.A.  to  the  descending  aorta, 
and  so  to  the  umbilical  arteries  U.A. 

aortic  arch  falls  into  the  umbilical  arteries  and  so  reaches  the 
placenta.  The  foetal  circulation  then  appears  to  be  so  arranged 
tliat,  while  the  most  distinctly  venous  blood  is  driven  by  the 
right  ventricle  back  to  the  placenta  to  be  arterialized,  the  most 


1134  THE  NUTRITION  OF  THE  EMBEYO.      [Book  iv. 

distinctly  arterial  (but  still  mixed)  blood  is  driven  by  the  left 
ventricle  to  the  cerebral  structures,  which,  we  may  conclude, 
have  more  need  of  oxygen  than  have  the  other  tissues.  Con- 
trarv  to  what  takes  place  afterwards,  the  work  of  the  right 
ventricle  is  in  the  foetus  greater  than  that  of  the  left ;  and,  ac- 
cordingly, that  greater  thickness  of  the  left  ventricular  walls, 
so  characteristic  of  the  adult,  does  not  become  marked  until 
close  upon  birth. 

§  704.  In  the  later  stages  of  pregnancy  the  mixture  of  the 
various  kinds  of  blood  in  the  right  auricle  increases  prepara- 
tory to  the  changes  taking  place  at  birth.  But  during  the 
whole  time  of  intra-uterine  life  the  amount  of  oxygen  in  the 
blood  passing  from  the  aortic  arch  to  the  brain  is  sufficient  to 
prevent  any  inspiratory  impulses  being  originated  in  the  bul- 
bar respiratory  centre.  This,  during  the  whole  period  elapsing 
between  the  date  of  its  structural  establishment,  or  rather  the 
consequent  full  development  of  its  irritability  and  the  epoch  of 
birth,  remains  dormant  ;  the  oxygen-supply  to  its  substance  is 
never  brought  so  low  as  to  set  going  the  respiratory  molecular 
explosions.  As  soon  however  as  the  intercourse  between  the 
maternal  and  umbilical  blood  is  interrupted  by  separation  of 
the  placenta  or  by  ligature  of  the  umbilical  cord,  or  when,  as  by 
the  death  of  the  mother,  the  umbilical  blood  ceases  to  be  replen- 
ished with  oxygen  by  the  maternal  blood,  or  when  in  any  other 
way  blood  of  sufficiently  arterial  quality  ceases  to  find  its  way 
by  the  left  ventricle  to  the  bulb,  the  supply  of  oxygen  in  the 
respiratory  centre  sinks,  and  when  the  fall  has  reached  a  certain 
point  an  impulse  of  inspiration  is  generated  and  the  foetus  for 
the  first  time  breathes.  This  action  of  the  respiratory  centre 
maybe  assisted  by  adjuvant  impulses  reaching  the  centre  along 
various  afferent  nerves,  such  as  those  started  by  exposure  of  the 
body  to  the  air,  or  to  cold  ;  but  these  are  subordinate,  not  essen- 
tial. A  retarded  first  breath  may  be  hurried  on  by  dashing 
water  on  the  face  of  the  new-born  infant  ;  but  so  long  as  the 
placental  circulation  is  intact,  stimulation,  even  varied  and 
strong,  of  the  foetal  skin,  though  it  may  give  rise  to  reflex 
movements  of  the  limbs  and  other  parts,  will  not  call  forth  a 
breath  ;  whereas,  on  the  other  hand,  upon  the  cessation  of  the 
placental  circulation,  the  foetus  may  make  its  first  respiratory 
movements  while  it  is  still  invested  with  the  intact  membranes 
and  thus  sheltered  from  the  air  and  indeed  from  all  external 
stimuli. 

§  705.  When  the  first  breath  is  taken,  as  under  normal  cir- 
cumstances it  is,  with  free  access  to  the  atmosphere,  and  the 
lungs  become  inflated  with  air  (we  dwelt  in  dealing  with  res- 
piration, §  257,  on  some  features  of  this  first  breathing),  the 
scanty  supply  of  blood  which  at  the  moment  was  passing  from 
the  right  ventricle  along  the  pulmonary  artery  returns  to  the 


Chap,  il]  PREGNANCY   AND  BIRTH.  1135 

left  auricle  brighter  and  richer  in  oxj^gen  than  ever  was  the 
foetal  blood  before.  With  the  diminution  of  resistance  in  the 
pulmonary  circulation  caused  by  the  expansion  of  the  thorax, 
a  larger  supply  of  blood  passes  into  the  pulmonary  artery  in- 
stead of  into  the  ductus  arteriosus,  and  this  derivation  of  the 
contents  of  the  right  ventricle  increasing  with  the  continued 
respiratory  movements,  the  current  through  the  latter  canal  at 
last  ceases  altogether,  and  its  channel  shortly  after  birth  be- 
comes obliterated.  The  obliteration  is  ultimately  secured  by 
proliferation  of  the  internal  coat,  in  which  even  before  birth 
the  sub-epithelial  layer  is  unusually  developed,  a  thrombus 
(§  33)  at  times  helping,  but  before  this  takes  place  the  closure 
seems  to  be  assisted  by  the  mechanical  arrangements  of  the 
parts.  Corresponding  to  the  greater  flow  into  the  pulmonary 
artery,  a  larger  and  larger  quantity  of  blood  returns  from  the 
pulmonary  veins  into  the  left  auricle.  At  the  same  time  the 
current  through  the  ductus  venosus  from  the  umbilical  vein 
having  ceased,  the  flow  from  the  inferior  cava  has  diminished  ; 
and  the  blood  of  the  right  auricle  finding  little  resistance  in  the 
direction  of  the  ventricle,  which  now  readily  discharges  its  con- 
tents into  the  pulmonary  artery,  but  finding  in  the  left  auricle, 
which  is  continually  being  filled  from  the  lungs,  an  obstacle 
to  its  passage  through  the  foramen  ovale,  ceases  to  take  that 
course.  Any  return  of  blood  from  the  now  vigorous  and  active 
left  auricle  into  the  right  auricle  is  prevented  by  the  valve 
which,  during  the  latter  stages  of  intra-uterine  life,  has  been 
growing  up  in  the  left  auricle  over  the  foramen  ovale.  At 
birth  the  edge  of  this  valve  is  to  a  certain  extent  free  so  that, 
in  case  of  an  emergency,  as  when  the  pulmonary  circulation  is 
obstructed,  a  direct  escape  of  blood  into  the  left  auricle  from 
the  overburdened  right  auricle  can  take  place.  Eventually,  in 
the  course  of  the  first  year,  adhesion  takes  place,  and  the  sepa- 
ration of  the  two  auricles  becomes  complete.  With  its  larger 
supply  of  blood  and  greater  work  the  left  ventricle  acquires 
the  greater  thickness  characteristic  of  it  during  life.  Thus  the 
foetal  circulation,  in  consequence  of  the  respiratory  movements 
to  which  its  interruption  gives  rise,  changes  its  course  into  that 
characteristic  of  the  adult. 


SEC.  3.     PARTURITION. 

§  706.  Owing  to  the  growth  of  the  fcetus,  and  also  to  the 
accumulation  of  the  amniotic  fluid,  the  uterus  towards  the 
end  of  pregnancy  has  become  much  distended  and  has  risen 
into  the  cavity  of  the  abdomen,  displacing  the  abdominal 
viscera.  The  expansion  of  the  uterus  during  pregnancy  is  a 
complex  process  in  which  the  mechanical  effects  of  the  in- 
creasing internal  pressure  are  mingled  with  those  of  growth. 
Though  the  uterine  walls  are,  as  we  have  said,  much  thickened 
by  the  addition  of  new  muscular  fibres  as  well  as  by  the 
increase  in  length,  breadth,  and  thickness  of  the  individual 
fibres,  and  also  enlarged  by  the  vascular  development,  they 
become  somewhat  thinned  again  towards  the  end  of  pregnancy 
by  reason  of  the  great  distension  of  the  cavity.  The  Fallopian 
tubes  and  the  round  ligaments  share  in  the  uterine  enlargement, 
in  so  far  as  their  muscular  tissue  is  increased  ;  but  the  mucous 
membrane  of  the  former  does  not  alter,  and  the  only  changes 
taking  place  in  the  ovary  are  those  concerned  with  the  corpus 
luteum  left  by  the  shed  ovum.  The  walls  of  the  vagina  are 
congested,  soft  and  hypertrophied.  Previous  to  labour  the 
foetus  occupies  in  the  womb  a  position  which  it  assumes  at  a 
quite  early  date,  namely,  one  in  which  the  head  is  directed 
downwards  towards  the  pelvis  ;  this  is  at  least  the  normal 
position,  though  deviations  from  it  not  infrequently  occur. 

From  an  early  date  waves  of  contraction,  at  times  rhythmical, 
sweep  over  the  enlarged  uterus  and  towards  the  end  of  preg- 
nancy become  more  marked.  As  a  rule  these  are  "  insensible  " 
contractions,  that  is  to  say  the  mother  is  not  conscious  of  them, 
though  at  times  they  may  be  distinctly  felt ;  and  in  all  cases 
they  are  temporary,  producing  no  permanent  effect  on  the  uterus 
or  its  contents.  Though,  as  shewn  by  the  cases  of  premature 
labour  and  abortion,  whether  occurring  from  natural  causes  or 
induced  artificially,  the  uterine  muscles  are  capable  at  even  an 
early  date  of  carrying  out  the  systematic  contractions  which 
lead  to  the  expulsion  of  the  f(Etus,  they  do  not  in  normal  partu- 
rition enter  upon  this  phase  of  activity  until  a  certain  time  has 

1136 


Chap,  ii.]  PKEGNANCY   AND   BIRTH.  1137 

been  run.  In  the  human  subject  the  period  of  gestation 
generally  lasts  from  275  to  280  days,  i.e.  about  40  weeks, 
the  general  custom  being  to  expect  parturition  at  about  280 
days  from  the  last  day  of  the  last  menstruation.  Seeing  how- 
ever that,  in  many  cases,  it  is  uncertain  whether  the  ovum 
which  developes  into  the  embryo  left  the  ovary  in  connection 
with  the  last  menstruation  or  with  the  first  one  missed  or 
during  the  intervening  weeks,  an  exact  determination  of  the 
duration  of  pregnancy  is  difficult  if  not  impossible. 

In  some  animals  the  period  of  gestation  is  longer,  in  others 
shorter  than  in  man,  being  in  the  mare  about  350  days,  in  the 
cow  about  280  days,  sheep  about  150  days,  dog  about  60  days, 
rabbit  about  30  days. 

Immediately  preceding  labour  a  secretion  of  mucus,  coming 
from  the  os  uteri  and  at  times  mixed  with  blood,  is  often  a 
sign  or  '  show '  that  the  efficient  uterine  contractions,  are  about 
to  begin. 

§  707.  The  onset  of  labour  is  marked  by  rhythmically 
repeated  contractions  of  the  uterus  which  most  distinctly  affect 
consciousness  and  are  recognized  as  "labour  pains."  The  first 
effect  of  these  is  the  openmg  up  or  widening  of  the  os  uteri 
constituting  the  "first  stage  of  labour."  The  contractions  may 
perhaps  be  spoken  of  as  "  peristaltic "  in  character,  but  the 
arrangement  of  the  bundles  of  muscular  fibres  in  the  body  of 
the  uterus  is  a  complex  one,  and  the  gross  effect  of  the  contrac- 
tions is  to  exert  pressure,  probably  of  a  fairly  uniform  kind,  on 
the  uterine  contents,  that  is  on  the  amniotic  fluid  or  "  waters  " 
enclosed  in  the  "  membranes  "  and  surrounding  the  foetus.  These 
membranes  are  the  amnion,  the  chorion  and  the  decidua,  the  first 
being  easily  separated  from  the  other  two  along  the  loose  con- 
nective tissue  joining  it  to  the  chorion,  and  thus  forming  an 
inner  and  outer  sheet  or  membrane.  Over  the  os  uteri  the 
decidua  consists  of  decidua  retlexa  only ;  and  here  the  mem- 
branes with  the  contained  fluid  act  as  a  hydraulic  plug  directing 
the  force  of  the  uterine  contractions  towards  expanding  the 
mouth.  As  labour  goes  on  a  special  character  of  the  uterine 
contractions  becomes  prominent.  In  the  contractions  of  which 
we  spoke  above  as  occurring  during  pregnancy  before  labour 
really  commences,  the  relaxation  of  each  muscular  fibre  follow- 
ing upon  a  contraction 'is  a  complete  one.  But  in  labour  the 
muscular  fibres  while  with  each  pain  they  contract  and  relax, 
are  all  the  while  becoming  permanently  and  progressively 
thicker  and  shorter.  This  change  by  which  the  uterine  wall 
becomes  progressively  thicker  and  more  compact  has  been 
spoken  of  under  the  not  very  desirable  term  "retraction," 
as  distinguished  from  "  contraction,"  but  appears  to  be  a  sort 
of  tonic  contraction  or  perhaps  rather  a  residue  of  contraction 
like  that  seen  in  skeletal  muscles  under  certain  conditions ; 

72 


1138  PARTURITION.  [Book  iv. 

at  each  recurring  "pain"  the  shortening  of  the  muscular 
fibres  starts  so  to  speak  from  this  more  permanent  shortening 
instead  of  from  complete  relaxation,  and  the  return  is  to  it  not 
to  complete  relaxation. 

This  more  permanent  tonic  contraction  or  "  retraction  "  does 
not  however  affect  the  whole  uterus ;  it  is,  broadly  speaking, 
confined  to  the  body  and  absent  from  the  cervix.  Indeed  in 
the  latter  region  all  contractions  are  wanting,  the  muscular 
fibres  appear  to  be  inhibited,  and  the  walls  yielding  to  the 
pressure  exerted  upon  them  become  thinner  instead  of  thicker ; 
as  the  pressure  increases  the  fibres  possibly  become  lamed 
or  paralyzed.  In  this  way  a  distinction  is  established  between 
an  "  upper  segment "  of  the  uterus  corresponding  to  the  body, 
the  walls  of  which  become  thicker  and  shorter  through  the 
continued  and  progressive  "  retraction,"  and  a  "lower  segment," 
corresponding  to  the  cervix  but  possibly  including  the  lower 
part  of  the  body,  the  w^alls  of  which  become  stretched  and 
thinner,  the  line  of  demarcation  between  the  two  segments 
being  often  called  "the  retraction  ring."  As  the  pressure 
in  the  body  of  the  uterus  continues  and  waxes  greater,  the 
mouth  becomes  wider  and  wider,  until  the  head  of  the  foetus 
begins  to  pass  through  it  into  the  vagina,  the  walls  of  which 
like  those  of  the  "lower  segment"  have  meanwhile  become 
stretched  and  thinner  ;  and  as  the  foetus  is  thus  leaving  the 
uterus  the  progressive  tonic  contraction  adapts  the  uterine 
walls  to  the  lessening  cavity.  Sometimes  the  membranes  are 
ruptured,  with  escape  of  the  "  waters,"  before  the  head  has  left 
the  uterus,  at  other  times  they  form  a  bulging  cushion  preceding 
and  making  way  for  the  foetus. 

When  the  os  uteri  has  become  fully  expanded  and  is  ready 
to  allow  the  head  of  the  foetus  to  pass  through  it  into  the  vagina, 
the  intrinsic  contractions  of  the  uterus  begin  to  be  assisted  by  an 
extrinsic  force,  by  contractions  of  the  abdominal  walls  which 
thus  exert  on  the  uterus  and  its  contents  a  pressure  very  similar 
to  that  exerted  on  the  rectum  in  defyecation  (§  225).  These 
contractions,  which  mark  the  onset  of  the  "second  stage"  of 
labour,  are  rhythmical  in  nature  like  those  of  the  uterus  itself, 
and  synchronous  with  them.  The  expulsive  power  of  the  uterus 
is  thus  greatly  increased,  and  the  head  of  the  foetus  followed 
by  the  rest  of  its  body  is  driven  through  the  vagina  and  then 
through  the  vulva,  these  playing  apparently  a  wholly  passive 
part  in  the  matter,  and  the  child  is  thus  literally  "  thrust  upon 
the  world." 

At  the  very  beginning  of  labour  there  takes  place  at  the 
internal  os  a  cleavage  of  the  decidua  vera  between  a  deeper  less 
altered  and  a  superficial  more  altered  layer,  so  that  the  latter, 
attached  to  the  chorion  and  thus  forming  part  of  the  "mem- 
branes," sejjarates  from  the  uterine  surface.     This  separation^ 


Chap,  ii.]  PEEGNANCY  AND  BIRTH.  1139 

the  begimiing  of  which  is  the  cause  of  the  '*  show  "  spoken  of 
above,  and  which  is  considered  to  be  a  mechanical  effect  of  the 
uterine  contractions  but  which  must  be  prepared  for  by  histo- 
logical changes,  during  the  early  stages  of  labour  extends  up- 
wards for  two  or  three  inches  only ;  but,  at  the  last,  it  is  carried 
right  through  the  "decidual  layer"  of  the  placenta.  Hence, 
after  the  expulsion  of  the  foetus,  the  uterus  contains  within  its 
cavity,  separated  from  and  now  foreign  to  itself,  the  placenta 
and  membranes,  the  latter  consisting  of  amnion,  chorion,  the 
whole  of  the  remains  of  the  decidua  refiexa  and  a  variable  part 
of  the  decidua  vera;  and,  under  normal  conditions,  these  are 
by  the  last  expulsive  efforts  ejected  with  or  immediately  after 
or  soon  after  the  child.  As  a  rule  the  membranes  are  ruptured 
and  the  amniotic  fluid  escapes  before  the  head  extrudes,  but  at 
times  the  child  is  born  still  surrounded  by  the  intact  mem- 
branes with  their  contained  fluid;  it  comes  into  the  world  in 
its  "caul." 

When  the  placenta  and  membranes  have  left  the  uterus 
(they  not  unfrequently  are  lodged  for  a  while  in  the  vagina), 
the  tonic  contraction  or  "  retraction  "  spoken  of  above,  which 
during  the  whole  of  laboui-  has  been  following  up  the  advance 
of  the  foetus,  and  progressively  lessening  the  uterine  cavity, 
continues  is  work  and  serves  an  important  purpose.  When  the 
last  pain  of  labour,  by  which  the  emptied  uterus  is  gathered  up 
into  a  small  hard  ball,  passes  away,  the  walls  under  normal 
conditions  do  not  wdiolly  relax,  a  permanent  tonic  contraction 
keeps  the  walls  thick  and  in  contact,  thus  closing  the  uterine 
cavity;  and  over  this  compact  and  closed  uterus  waves  of 
rhythmical  contraction,  the  "  after-pains,"  still  for  a  while  pass 
Avithout  altering  its  permanent  condition.  By  this  continued 
contraction  or  retraction,  not  only  the  open,  torn  ends  of  the 
vessels  of  the  decidua  but  all  the  vessels  throughout  the  thick- 
ness of  the  uterine  walls  are  so  compressed  that  all  extensive 
bleeding  is  prevented.  Should  this  continued  contraction  give 
away  to  relaxation,  hremorrhage  or  "  flooding  "  follows.  This 
retraction  or  tonic  contraction,  whatever  be  its  exact  nature, 
which  is  so  conspicuous  in  the  uterus  but  which  perhaps  may  be 
recognized  in  a  lesser  degree  as  mere  ordinary  tonic  contraction 
in  other  rhythmically  contracting  organs,  in  the  bladder,  in  the 
intestine,  and  even  in  the  heart,  ai3pears  to  serve  more  than  one 
purpose  in  the  Avork  of  the  uterus;  by  continually  lessening  the 
uterine  cavity  it  renders  more  efficient  during  labour  the  rhyth- 
mic uterine  "  pains,"  by  compressing  the  blood  vessels  during 
labour  it  gradually  shuts  off"  the  extraA^agant  blood  supply  now 
no  longer  needed,  and  by  continuing  that  compression  after 
labour  and  by  closing  the  uterine  cavity  it  prevents  hiemorrhage 
and  wards  off  the  evil  effects  which  the  free  entrance  into  the 
uterine  cavity  of  foreign  organisms  might  bring  about.     And 


1140  PAETURITION.  [Book  iv. 

probably  it  is  on  account  of  its  great  usefulness  that  this  peculiar 
form  of  muscular  activity  is  so  prominent  in  the  uterus. 

Even  before  labour  proliferation  of  the  epithelioid  cells  may 
be  observed  in  the  lining  membrane  of  the  uterine  vessels;  these 
are  rapidly  increased  after  labour  is  completed,  and  form  part  of 
the  healing  processes  which  follow.  The  tonic  contraction  of 
which  we  have  been  speaking  is  maintained  until  the  blood 
vessels  are  permanently  closed  by  these  nutritive  healing 
processes.  After  birth  the  muscular  elements  of  the  uterus 
dwindle,  many  of  the  fibres  undergoing  fatty  degeneration,  and 
thus  the  mucous  and  muscular  walls  are  gradually  brought 
back  to  their  natural  condition.  During  the  early  days  of  this 
process  of  involution  a  discharge,  the  lochia,  takes  place  from 
the  internal  surface  of  the  uterus. 

§  708.  The  whole  process  of  parturition  may  be  broadly 
considered  as  a  reflex  act,  the  nervous  centre  of  which  is  placed 
in  the  lumbar  cord.  In  a  dog,  whose  thoracic  cord  had  been 
completely  severed,  parturition  took  place  as  usual ;  and  the  fact 
that,  in  the  human  subject,  labour  will  progress  in  a  natural 
manner  while  the  patient  is  unconscious  from  the  administra- 
tion of  chloroform,  though  it  is  often  retarded  and  sometimes 
arrested,  shews  that  in  woman  also  the  contractions  both  of  the 
uterus  and  of  the  abdominal  muscles  are  involuntary,  however 
much  the  latter  may  be  assisted  by  direct  volitional  efforts. 

Observations  on  animals  shew  that  even  in  a  virgin  uterus 
and  in  one  which  is  not  enlarged  by  pregnancy  movements  can 
be  excited  in  a  reflex  manner  through  the  central  nervous 
system  and  may  occur  rhythmically  in  an  apparently  sponta- 
neous manner  ;  but  the  latter  are  often  absent  or  are  so  slight 
as  readily  to  escape  observation,  and  the  former  are  often  feeble 
and  excited  with  difficulty.  In  a  pregnant  animal  on  the  other 
hand,  especially  if  pregnancy  be  advanced,  powerful  rhythmic 
expulsive  movements  repeatedly  occur  in  the  apparent  absence 
of  all  extrinsic  stimuli  and  are  very  readily  provoked  by  the 
stimulation  of  various  afferent  nerves.  They  may  also  be 
induced  by  direct  stimulation  of  the  spinal  cord  at  any  part 
of  its  whole  length  as  well  as  of  various  regions  of  the  brain ; 
the  analogy  with  the  movements  of  the  urinary  bladder  leads 
us  to  suppose  that  the  impulses  thus  started  in  the  brain 
and  upper  part  of  the  spinal  cord  do  not  pass  directly  to  the 
uterus  but  throw  into  activity  the  reflex  centre  in  the  spinal 
cord.  Movements  of  the  uterus  are  readily  excited  when  the 
blood  ceases  to  be  duly  arterialized,  extrusion  of  the  foetus 
being  a  common  result  when  a  pregnant  animal  is  asphyxi- 
ated ;  and  though  the  venous  blood  may  act  in  part  as  a  direct 
stimulus  to  the  uterine  muscles  the  contractions  are  mainly  due 
to  the  blood  exciting  the  nervous  centre.  Drugs  such  as  ergot 
which  increase  uterine  contractions  probably  in  like  manner 


Chap.  II.]  PREGXANCY   AND   BIRTH.  1141 

produce  their  effect  chiefly  at  least  through  their  action  on  the 
nervous  centre.  The  ready  way  in  which  the  uterus  enlarged 
by  pregnancy  responds  by  reflex  contraction  to  the  stimulation 
of  various  afferent  nerves  is  illustrated  in  the  human  subject 
by  the  means  usually  adopted  to  secure  after  the  birth  of 
the  child  that  continued  contraction  by  which  hcemorrhage  is 
avoided.  Should  for  any  reason  such  a  contraction  fail  to  take 
place,  it  may  be  secured  by  applying  cold  or  pressure  to  the 
abdominal  walls  or  by  introducing  a  hand  or  some  foreign  body 
into  the  vagina,  or,  what  perhaps  best  illustrates  the  reflex 
nature  of  the  matter,  by  applying  the  child  to  the  nipple ;  in 
the  latter  case  the  relatively  feeble  afferent  impulses  generated 
in  the  mammary  nerves  by  the  sucking  of  the  child  are  especially 
potent  in  producing  by  reflex  action  contraction  of  the  uterine 
muscles. 

§  709.  The  nerves  of  the  uterus  reach  that  organ  chiefly 
along  the  broad  ligament  in  company  with  the  blood  vessels, 
are  partly  medullated,  partly  non-medullated,  and  are  derived 
from  the  pelvic  plexus  lying  between  the  rectum  and  the 
vagina.  The  pelvic  plexus,  on  which  as  also  on  the  nerves 
passing  to  the  uterus,  numerous  small  ganglia  are  scattered,  is 
a  continuation  on  each  side  of  the  body  of  the  medially  placed 
hypogastric  plexus,  but  it  is  joined  by  branches  coming  directly 
from  the  sacral  nerves.  In  the  lower  animals  (dog)  the  roots 
which  supply  fibres  to  the  uterus  are  on  the  one  hand  the  uj^per 
lumbar,  which  traverse  the  sympathetic  strand  known  as  the 
hypogastric  nerve,  and  on  the  other  hand  probably  the  first  and 
second  sacral.  In  the  human  subject  the  corresponding  roots  are 
probably  the  upper  lumbar  and  third,  fourth  and  second  sacral. 

Stimulation,  in  the  dog,  either  of  the  hypogastric  nerve  or  of 
the  sacral  nerves  produces  contractions  in  the  pregnant  uterus ; 
it  is  stated  that  the  mode  of  contraction  is  different  in  the  two 
cases,  in  the  latter  the  longitudinally  disposed  fibres,  in  the 
former  the  circularly  disposed  fibres  being  especially  thrown 
into  action.  Moreover,  it  is  said  that  while  the  fibres  passing 
by  the  hypogastric  nerve  are  vaso-constrictor  towards  the 
uterine  arteries,  those  passing  by  the  sacral  nerves  are  vaso- 
dilator. Other  observers  have  failed  to  obtain  any  such  differ- 
ence between  circular  and  longitudinal  contractions,  and  find 
that  in  some  animals  at  least,  while  contractions  of  the  uterus 
may  be  readily  brought  about  by  stimulation  of  the  sympathetic 
nerves  from  the  lumbar  region,  passing  through  the  hypogastric 
nerves,  contractions  cannot  with  certainty  be  obtained  by  stim- 
ulating the  sacral  nerves.  On  the  other  hand,  stimulation  of 
the  sacral  nerves,  of  the  second,  third,  and  sometimes  of  the 
first,  readily  produces  movements  of  the  vagina.  It  may  be 
added  that  stimulation  of  certain  areas  of  the  cerebral  cortex 
will  give  rise  to  movements  of  the  uterus  and  of  the  vao'ina. 


1142  PAETUEITION.  [Book  iv. 

§  710.  Thougli  under  normal  circumstances  efficient  uterine 
contractions  do  not  set  in  until  the  full  period  of  gestation  is 
completed,  yet  by  reason  of  changes  in  the  uterus  or  its  con- 
tents, occurring  from  natural  causes  or  induced  artificially,  the 
full  swing  of  movements  may,  at  almost  any  time,  though  at 
some  times  more  readily  than  at  others,  be  brought  about.  On 
the  other  hand  it  may  be  delayed  for  a  considerable  time  beyond 
the  proper  term.  We  may  be  said  to  be  in  the  dark  as  to 
why  the  uterus,  after  remaining  for  months  subject  only  to 
futile  contractions,  is  suddenly  thrown  into  powerful  and  effi- 
cient action,  and  within  it  may  be  a  few  hours  or  even  less  gets 
rid  of  the  burden  which  it  has  borne  with  such  tolerance  for 
so  long  a  time.  None  of  the  various  hypotheses  which  have 
been  put  forward  can  be  considered  as  satisfactory.  There  is 
no  evidence  for  the  view,  based  on  the  occurrence  of  contrac- 
tions in  consequence  of  an  asphyxiated  condition  of  the  blood, 
that  the  onset  of  labour  is  caused  by  a  gradual  diminution  of 
oxygen  or  accumulation  of  carbonic  acid  in  the  blood,  reaching 
at  last  to  a  climax.  Nor  are  there  sufficient  facts  to  connect 
parturition  with  any  condition  of  the  ovary  resembling  that 
accompanying  menstruation.  Nor  can  much  stress  be  laid  on 
the  supposition  that  the  real  exciting  cause  is  the  separation  of 
the  decidua  from  the  permanent  uterine  wall,  the  separation 
being  the  outcome  of  the  preceding  processes  of  growth,  since 
the  actual  separation  itself  seems  to  be  caused  by  the  initial  con- 
tractions of  labour,  and  the  histological  changes  which  precede 
it  are  only  one  set  of  changes  among  many  others  all  having 
their  goal  in  labour.  We  can  only  say  that  labour  is  the  cul- 
minating point  of  a  series  of  events,  and  must  come  sooner  or 
later,  though  its  immediate  advent  may  at  times  be  decided  by 
accident ;  but  it  would  not  be  profitable  to  discuss  this  question 
here. 

The  action  of  the  abdominal  muscles  in  parturition,  at  least 
so  much  as  takes  place  independently  of  the  will,  is,  in  contrast 
to  that  of  the  uterine  muscles,  obviously  a  reflex  act  of  a  more 
ordinary  kind  carried  out  by  means  of  the  spinal  cord ;  and  we 
may  suppose  that,  though  the  mere  contractions  of  the  uterus 
may  serve  as  a  possible  source,  the  necessary  stimulus  is  sup- 
plied by  the  pressure  of  the  foetus  in  the  vagina ;  in  support  of 
this  it  may  be  noted  that  the  action  becomes  much  intensified 
towards  the  end  of  labour  as  the  stress  and  strain  caused  by  the 
advancing  head  tell  more  and  more  on  the  external  skin. 

§  711.  Hence  as  we  have  said  the  whole  act  of  parturition 
may  with  reason  be  considered  as  a  reflex  one.  Whether  it  be 
wholly  a  reflex  or  in  a  certain  sense  an  automatic  one,  the  act 
can  readily  be  inhibited  by  other  contemporary  actions  of  the 
central  nervous  system.  Thus  emotions  very  frequently  become 
a  hindrance  to  the  progress  of  parturition ;  as  is  well  known, 


Chap.  II.]  PREGNANCY   AND   BIRTH.  1143 

the  entrance  into  the  bedroom  of  a  stranger  often  causes  for  a 
time  the  sudden  and  absolute  cessation  of  'labour'  pains,  which 
previously  may  have  been  even  violent.  Judging  from  the 
analogy  of  micturition,  we  may  suppose  that  this  inhibition  of 
uterine  contractions  is  brought  about  by  an  inhibition  of  the 
centre  in  the  lumbar  cord  leading  to  a  sudden  cessation  of  the 
augmentor  action  of  which  we  spoke  above  as  far  as  the  uterus 
itself  is  concerned,  and  in  a  more  direct  way  to  a  cessation  of 
the  contractions  of  the  abdominal  muscles.  Some  observations 
tend  to  shew  that  a  region  of  the  bulb  exerts  such  an  inhibitory 
influence  ;  but  the  matter  needs  fuller  investigation. 


CHAPTER   III. 
THE   PHASES   OF   LIFE. 

§  712.  The  child,  has  at  birtli,  on  an  average,  rather  less 
than  one-third,  the  maximum  length,  and.  about  one-twentieth 
the  maximum  weight,  to  which  in  future  years  it  will  attain. 

The  composition  of  the  body  of  the  new-born  babe,  as  com- 
pared, with  that  of  the  ad  alt,  will  be  seen  from  the  following 
table,  in  which  the  details  are  more  full  than  those  given  in 
§  413 ;  the  figures  in  brackets  are  more  recent  observations. 


Weight  of  organ  in  percentage 
of  Body-weight. 

We 

adu" 

with 

ba 

ight  of  organ  in 
It,  as  compared 

New-born  babe. 

Adult. 

HXatlj   Ui    llt/W-UUiiX 

be  taken  as  1. 

Eye     , 

•28 

•023 

1^7 

Brain 

Kidneys 

Skin 

14^34  (12-28) 

•88 
11-3 

2-37  ( 

•48 
6^3 

2^25) 

3^7    (3^34) 
12 
12 

Liver 

Heart 

Stomach  and 
Intestine 

4-39    (5^03) 
•89      (-73) 

}   2^53 

2^77  (3.05) 
•52    (-49) 

2^34 

13^6  (11^05) 
15     (12^1) 

20 

Lungs 

2^16 

2^01 

20 

Skeleton 

16^7 

15^35 

26 

Muscles,  &c. 

2^34 

4-31 

48 

Testicle 

•037 

•08 

60 

It  will  be  observed  that  the  brain  and  eyes  are,  relatively 
to  the  whole  body-weight,  very  much  larger  in  the  babe  than 
in  the  adult.  This  disproportion  is  a  very  marked  embryonic 
feature,  and  has  a  morphological  or  phylogenic,  as  well  as  a 
physiological  or  teleological,  significance.  Inasmuch  as  the 
smaller  body  has  relatively  the  larger  surface,  the  skin  is  natur- 
ally proportionately  greater  in  the  babe ;  but  the  same  dispro- 
portion is  observed  in  the  kidneys,  these  like  the  skin  increasing 

1144 


Chap,  hi.]  THE   PHASES   OF   LIFE.  1145 

in  weiglit  twelve  times  only  between  birth  and  full  growth, 
whereas  the  whole  body  increases  twenty  times.  The  heart 
and  the  liver  according  to  the  newer  observations  behave  very 
similarl}^  and  even  according  to  the  older  observations  lag  con- 
siderably behind  the  whole  frame,  whereas  the  lungs  and  the 
alimentary  canal  almost  exactly  keep  pace  with  it,  and  the 
skeletal  framework,  in  spite  of  its  being  specifically  lighter  in 
its  earlier  cartilaginous  condition,  maintains  throughout  life 
very  nearly  the  same  relative  weight.  The  muscles  on  the 
contrary  grow  more  than  twice  as  fast  as  the  whole  body ;  the 
great  increase  in  these  covers  the  relative  decrease  of  the  other 
parts,  and  it  is  largely  by  the  laying  on  of  flesh  and  fat  that 
the  babe  gains  the  bulk  of  the  man. 

§  713.  We  usually  measure  growth  by  taking  account  of 
two  sets  of  changes,  changes  of  stature  and  changes  of  weight ; 
and  we  may  study  both  these  changes  in  more  than  one  way. 

If  we  measure  the  height  at  intervals  we  may  plot  out  the 
curve  of  growth  of  stature  ,  and  when  w^e  do  this  we  find  that 
the  curve  rises  rapidly  at  first  but  afterwards  more  slowdy, 
shcAving  that  the  increment  is  decreasing,  and  at  about  the 
twenty-fifth  year  ceases  to  rise  at  all.  From  thence  to  about 
fifty  years  of  age  the  height  remains  stationary,  after  which 
there  may  be  a  decrease,  especially  in  extreme  old  age.  The 
curve  moreover  is  not  regular,  but  indicates  by  its  changes 
that  the  increment  of  height  in  a  given  time  is  now  smaller, 
now  greater. 

Tlie  curve  of  weight  is,  on  the  wdiole,  at  first  very  similar 
to  that  of  height,  rising  in  a  somewhat  similar  way  and  shew- 
ing similar  irregularities ;  but  instead  of  ceasing  to  rise  at 
about  the  twenty-fifth  year  it  continues  to  rise,  though  marked 
with  many  irregularities,  and  may  continue  to  do  so  until  about 
the  fortieth  year.  After  the  sixtieth  year  a  decline  of  variable 
extent  is  generally  witnessed.  It  should  be  noted  that  in  the 
first  few  days  of  life,  so  far  from  there  being  an  increase,  there 
is  an  actual  decrease  of  weight,  so  that,  even  on  the  seventh 
day  the  weight  still  continues  to  be  less  than  at  birth ;  and  a 
similar  post-natal  loss  of  weight  is  observed  in  animals.  If  we 
take  the  curve  of  growth  from  the  impregnation  of  the  ovum 
onwards  this  post-natal  loss  of  weight  will  appear  as  an  abrupt 
change  in  the  curve  due  to  the  so  to  speak  violent  act  of  birth. 
It  should  be  added  that  the  curves  both  of  height  and  weight 
exhibit  diiterences  dependent  on  sex,  circumstances,  race,  cli- 
mate and  the  like. 

We  may  also  study  the  progress  of  growth  by  measuring 
the  increment  of  growth  in  a  given  time,  in  a  year  for  instance, 
and  plotting  out  the  curve  of  the  yearly  increment.  When  we 
do  this  we  obtain  very  instructive  results.  We  find  that  the 
yearly  increment  decreases  very  rapidly  during  the  first  two 


1146  GKOWTH.  [Book  iv. 

or  three  years,  then  remains  nearly  stationary  or  even  rises,  and 
at  about  the  seventh  or  eighth  year  undergoes  a  marked  fall. 
This  fall,  however,  is  temporary  only ;  the  curve  soon  rises 
again  and  with  some  irregularities  attains  a  maximum  between 
the  twelfth  and  fifteenth  year,  from  which  point  onwards  it  falls 
rapidly  with  some  minor  irregularities.  These  marked  varia- 
tions in  the  increment  of  growth  which  are  obviously  connected 
with  and  preparatory  to  the  important  change  which  we  call 
puberty,  are  seen  in  the  curves  both  of  stature  and  of  weight, 
the  changes  in  weight  occurring  however  rather  later  than  those 
of  stature,  and  both  being  somewhat  different  in  boys  from  what 
they  are  in  girls.  Both  are  also  influenced  by  the  conditions  of 
life;  but  a  study  of  the  curves  of  growth  of  young  people  living 
under  various  surroundings,  while  it  teaches  the  great  impor- 
tance of  properly  administering  to  the  wants  of  youth,  at  the 
same  time  illustrates  the  recuperative  elasticitj^  of  the  bodily 
frame;  it  may  often  be  observed  that  the  ill  effects  of  adverse 
circumstances,  provided  they  be  not  too  great,  are  soon  recov- 
ered from  under  the  influence  of  a  happy  change ;  food  and 
comfort  will  turn  the  abnormal  fall  in  the  curve  of  growth  of  a 
starved  waif  into  a  sharp  rise. 

Lastly,  we  may  study  growth  by  observing  the  actual  rate  of 
growth,  by  measuring  the  magnitude  of  the  fraction  of  the  total 
weight  which  is  added  to  the  weight  in  a  given  time  ;  we  take 
weight  because  this  is  the  most  significant  element  of  growth. 
When  this  method  is  adopted,  an  examination  of  such  statistics 
as  are  available  with  regard  to  man,  confirmed  by  the  results  of 
careful  observations  on  young  animals,  tends  to  shew  that  the 
rate  diminishes  continually  from  birth  onwards,  the  diminution 
being  rapid  at  first  but  slower  afterwards,  and  being  broken  by 
various  irregularities.  In  other  words,  the  power  of  growth 
diminishes  continually  though  somewhat  irregularly  throughout 
life,  and  a  like  diminution  apparently  obtains  in  intra-uterine 
existence.  It  seems  as  if  the  impetus  of  growth  given  at  im- 
pregnation gradually  dies  out. 

§  714.  The  saliva  of  the  babe,  very  scanty  at  first  and  not 
abundant  until  teething  begins,  is  active  on  starch  though  less 
so  than  in  the  adult,  and  its  gastric  juice,  unlike  that  of  many 
new-born  animals,  has  good  peptic  powers,  and  its  pancreas 
good  tryptic  powers,  though  apparently  the  pancreatic  action 
on  starch  is  feeble.  From  this  we  may  infer  that  its  digestive 
processes  are  in  general  identical  with  that  of  the  adult  though 
ill  suited  for  any  large  amount  of  starch  in  the  food  ;  and  they 
are  feeble,  since  the  faeces  of  the  infant  contain  a  considerable 
quantity  of  undigested  food  (fat,  casein,  &c.),  as  well  as  un- 
altered bile-pigment,  and  undecomposed  bile-salts. 

The  heart  of  the  babe,  as  shewn  in  the  preceding  Table,  is, 
relatively  to  its  body-weight,  larger  than  the  adult,  and  the 


Chap,  hi.]  THE   PHASES    OF    LIFE.  1147 

frequency  of  the  heart-beat  much  greater,  viz.  about  130  or  140 
per  minute,  falling  to  about  110  in  the  second  year,  and  about 
90  in  the  tenth  year.  Corresponding  to  the  smaller  bulk  of  the 
body,  the  whole  circuit  of  the  blood  system  is  traversed  in  a 
shorter  time  than  in  the  adult  (12  seconds  as  against  22)  ;  and 
consequently  the  renewal  of  the  blood  in  the  tissues  is  ex- 
ceedingly rapid.  Relatively  to  the  body-weight  there  is  also 
considerably  more  blood  in  the  babe  than  in  the  adult.  The 
respiration  of  the  babe  is  quicker  than  that  of  the  adult,  being 
at  first  about  35  per  minute,  falling  to  28  in  the  second  year,  to 
26  in  the  fifth  year,  and  so  onwards.  The  respiratory  work, 
while  it  increases  absolutely  as  the  body  grows,  is,  relatively  to 
the  body-weight,  greatest  in  the  earlier  years.  It  is  worthy  of 
notice,  that  the  absorption  of  oxygen  is  said  to  be  during  these 
earlier  years  relatively  more  active  than  the  production  of  car- 
bonic acid  ;  that  is  to  say,  there  is  a  continued  accumulation  of 
capital  in  the  form  of  a  store  of  oxygen-holding  explosive  com- 
pounds (cf.  §  289).  This,  indeed,  is  the  strikimg  feature  of 
infant  metabolism.  It  is  a  metabolism  directed  largely  to  con- 
structive ends.  The  food  taken  represents,  undoubtedly,  so 
much  potential  energy  ;  but  before  that  energy  can  assume  a 
vital  mode,  the  food  must  be  converted  into  tissue ;  and,  in  such 
a  conversion,  morphological  and  molecular,  a  large  amount  of 
energy  must  be  expended.  The  metabolic  activities  of  the 
infant  are  more  pronounced  than  those  of  the  adult,  for  the 
sake,  not  so  much  of  energies  which  are  spent  on  the  world 
without,  as  of  energies  which  are  for  a  while  buried  in  the 
rapidly  increasing  mass  of  flesh.  Thus  the  infant  requires  over 
and  above  the  wants  of  the  man,  not  only  an  income  of  energy 
corresponding  to  the  energy  of  the  flesh  actually  laid  on,  but 
also  an  income  corresponding  to  the  energy  used  up  in  making 
that  living  sculptured  flesh  out  of  the  dead  amorphous  proteids, 
fats,  carbohydrates  and  salts,  which  serve  as  food.  Over  and 
above  this,  the  infant  needs  a  more  rapid  metabolism  to  keep  up 
the  normal  bodily  temperature.  This,  which  is  no  less,  indeed 
slightly  ('30)  higher,  than  that  of  the  adult,  requires  a  gre^iter 
expenditure,  inasmuch  as  the  infant  with  its  relatively  far  larger 
surface,  and  its  extremely  vascular  skin,  loses  heat  to  a  propor- 
tionately much  greater  degree  than  does  the  grown-up  man.  It 
is  a  matter  of  common  experience  that  children  are  more  affected 
by  cold  tlian  are  adults.  The  bodily  temperature  is  moreover 
less  stable  in  the  infant  than  in  the  adult,  and  departures  from 
the  normal  temperature  have  not  the  grave  significance  they 
have  in  the  adult. 

This  rapid  metabolism  is  however  not  manifest  immediately 
upon  birth.  During  the  flrst  few  days,  corresponding  to  the 
loss  of  weight  mentioned  above,  the  respiratory  activities  of  the 
tissues  are  feeble;  the  embryonic  habits  seem  as  yet  not  to  have 


1148  THE  NUTEITION   OF  THE  BABE.       [Book  iv. 

been  completely  thrown  off,  and,  as  was  stated  in  §  306,  new- 
born animals  bear  with  impunity  a  deprivation  of  oxygen,  which 
AYOuld  be  fatal  to  them  later  on  in  life. 

Associated  probably  with  these  constructive  labours  of  the 
growing  frame  is  the  prominence  of  the  lymphatic  system. 
Not  only  are  the  lymphatic  glands  largely  developed  and  more 
active  (as  is  probably  shewn  by  their  tendency  to  disease  in 
youth),  but  the  quantity  of  lymph  circulation  is  greater  than 
in  later  years.  Characteristic  of  youth  is  the  size  of  the  thymus 
body,  which  increases  up  to  the  second  year,  and  may  then  re- 
main for  a  while  stationary,  but  generally  before  puberty  has 
suffered  a  retrogressive  metamorphosis,  so  that  very  often  hardly 
a  vestige  of  it  remains  behind.  The  thyroid  body  is  also  rela- 
tively greater  in  the  babe  than  in  the  adult ;  the  spleen,  on  the 
other  hand,  relatively  to  the  body-weight  does  not  change 
greatly,  though  rather  smaller  in  the  adult.  As  we  have  already 
said  the  recuperative  power  of  infancy  and  early  youth  is  very 
marked. 

The  quantity  of  urine  passed,  though  scanty  in  the  first 
two  days,  rises  rapidly  at  the  end  of  the  first  week,  and  in 
youth  the  quantity  of  urine  passed  is,  relatively  to  the  body- 
weight,  larger  than  in  adult  life.  This  may  be,  at  least  in 
quite  early  life,  partly  due  to  the  more  liquid  nature  of  the 
food,  but  is  also  in  part  the  result  of  the  more  active  metabo- 
lism. For  not  only  is  the  quantity  of  urine  passed,  but  also 
the  amount  of  urea  and  of  some  other  urinary  constituents 
excreted,  relatively  to  the  body-weight,  greater  in  the  child 
than  in  the  adult.  The  presence  of  uric  acid,  of  oxalic  acid, 
and  according  to  some,  of  hippuric  acid  in  unusual  quantities 
is  a  frequent  characteristic  of  the  urine  of  children.  It  is 
stated  that  calcic  phosphates,  and  indeed  the  phosphates  gen- 
erally, are  deficient,  being  retained  in  the  body  for  the  building 
up  of  the  osseous  skeleton. 

§  715.  It  would  be  beyond  the  scope  of  this  work  to  enter 
into  the  psychical  condition  of  the  babe  or  the  child,  and  our 
knowledge  of  the  details  of  the  working  of  the  nervous  system 
in  infancy  is  too  meagre  to  permit  of  any  profitable  discussion. 
It  is  hardly  of  use  to  say  that  in  the  young  the  whole  nervous 
system  is  more  irritable  or  more  excitable  than  it  is  in  later 
years  ;  by  which  we  probably  to  a  great  extent  mean  that  it 
is  less  rigid,  less  marked  out  into  what,  in  preceding  portions 
of  this  work,  we  have  spoken  of  as  nervous  mechanisms.  In 
new-born  puppies  and  some  other  animals  stimulation  of  the 
various  cerebral  areas  does  not  give  rise  to  the  usual  localized 
movements  ;  these  do  not  appear  until  some  time  after  birth  ; 
but  in  this  respect  differences  are  observed  in  different  kinds  of 
animals  corresponding  to  the  well-known  differences  between 
different  kinds  of  animals  in  the  powers  possessed  at  birth  ; 


Chap,  iti.]  THE   PHASES   OF   LIFE.  1149 

the  human  babe  as  regards  the  latter  is  intermediate  between 
the  puppy  and  the  young  guinea-pig.  As  we  have  seen,  the 
fibres  of  the  various  tracts  in  the  central  nervous  system  ac- 
quire their  medulla  at  different  epochs  ;  there  is  experimental 
evidence  in  support  of  the  view,  otherwise  probable,  that  the 
assumption  of  functional  activity  follows  in  the  same  order  ; 
and  the  pyramidal  tract  is  as  we  have  seen  the  one  in  which 
the  fibres  are  very  late  in  acquiring  their  medulla.  It  has 
been  asserted  that  in  a  new-born  animal  stimulation  of  the 
vagus  produces  no  cardiac  inhibition  and  that  this  does  not 
appear  for  several  days  ;  other  observers  however  have  ob- 
tained positive  results  and  that  even  in  the  uterus  ;  probably 
in  this  respect  also  animals  differ.  In  the  human  infant  the 
sense  of  touch,  both  as  regards  pressure  and  temperature, 
appears  well  developed,  as  does  also  the  sense  of  taste,  and 
possibly,  though  this  is  disputed,  that  of  smell.  The  pupil 
(larger  in  the  infant  than  in  the  man)  acts  fully,  and  normal 
binocular  movements  of  the  eyes  have  been  observed  in  an 
infant  less  than  an  hour  old.  The  eye  is  from  the  outset 
fully  sensitive  to  light,  though  of  course  visual  perceptions  are 
imperfect.  Auditory  sensations  on  the  other  hand,  seem  to 
be  dull,  though  not  wholly  absent,  during  the  first  few  days  of 
life  ;  this  may  be  partly  at  least  due  to  absence  of  air  from  the 
tympanum  and  to  a  tumid  condition  of  the  t3^mj)anic  mucous 
membrane.  As  the  child  grows  up  his  senses  sharpen  with 
constant  exercise,  and  in  his  early  years  he  possesses  a  general 
acuteness  of  sight,  hearing,  and  touch,  which  frequently  be- 
comes blunted  as  his  psychical  life  becomes  fuller.  Children 
however  are  said  to  be  less  apt  at  distinguishing  colours  than 
in  sighting  objects  ;  but  it  does  not  appear  whether  this  arises 
from  a  want  of  perceptive  discrimination  or  from  their  being 
actually  less  sensitive  to  variations  in  hue.  A  characteristic 
of  the  nervous  system  in  childhood,  the  result  probably  of  the 
more  active  metabolism  of  the  body,  is  the  necessity  for  long  or 
frequent  and  deep  slumber. 

§  716.  Dentition  marks  the  first  epoch  of  the  new  life.  At 
about  seven  months  the  two  central  incisors  of  the  lower  jaw 
make  their  way  through  the  gum,  followed  immediately  by  the 
corresponding  teeth  in  the  upper  jaw.  The  lateral  incisors, 
first  of  the  lower  and  then  of  the  upper  jaw,  appear  at  about 
the  ninth  month,  the  first  molars  at  about  the  twelfth  month, 
the  canines  at  about  a  year  and  a  half,  and  the  temporary  den- 
tition is  completed  by  the  appearance  of  the  second  molars 
usually  before  the  end  of  the  second  year. 

About  the  sixth  year  the  permanent  dentition  commences 
by  the  appearance  of  the  first  permanent  molar  beyond  the 
second  temporary  molar  ;  in  the  seventh  year  the  central  per- 
manent  incisors  replace  their   temporary  representatives,  fol- 


1150  DENTITION.  [Book  iv. 

lowed  in  the  next  year  by  the  lateral  incisors.  In  the  ninth 
year  the  temporary  first  molars  are  replaced  by  the  first  bi- 
cuspids, and  in  the  tenth  year  the  second  temporary  molars  are 
similarly  replaced  by  the  second  bicuspids.  The  canines  are 
exchanged  about  the  eleventh  or  twelfth  year,  and  the  second 
permanent  molars  are  cut  about  the  twelfth  or  thirteenth 
year.  There  is  then  a  long  pause,  the  third  or  wisdom  tooth 
not  making  its  appearance  till  the  seventeenth,  or  even  twenty- 
fifth  year,  or  in  some  cases  not  appearing  at  all. 

§  717.  Shortly  after  the  conclusion  of  the  permanent  den- 
tition (the  wisdom  teeth  excepted)  the  occurrence  of  puberty 
marks  the  beginning  of  a  new  phase  of  life ;  and  the  difference 
between  the  sexes,  hitherto  merely  potential,  now  becomes  func- 
tional. In  both  sexes  the  maturation  of  the  generative  organs 
is  accompanied  by  the  well-known  changes  in  the  body  at  large  ; 
but  the  events  are  much  more  obvious  in  the  typical  female  than 
in  the  aberrant  male.  Though  in  the  boy,  the  breaking  of  the 
voice  and  the  rapid  growth  of  the  beard  which  accompany  the 
appearance  of  active  spermatozoa,  are  striking  features,  yet  they 
are  after  all  superficial ;  and  though,  as  we  have  seen  (§  713), 
the  curves  of  his  increasing  weight  and  height  undergo  before 
and  at  this  period,  characteristic  variations,  the  general  events 
of  his  economy  pursue  for  a  while  longer  an  unchanged  course  ; 
the  boy  does  not  become  a  man  till  some  years  after  puberty ; 
and  the  decline  of  his  functional  manhood  is  so  gradual  that 
frequently  it  ceases  only  when  disease  puts  an  end  to  a  ripe  old 
age.  With  the  occurrence  of  menstruation,  on  the  other  hand, 
at  from  thirteen  to  seventeen  years  of  age,  subsequent  to  the 
acceleration  of  growth  noted  above  §  713,  which  indeed  appears 
preparatory  to  it,  the  girl  almost  at  once  becomes  a  woman,  and 
her  functional  womanhood  ceases  suddenly  at  the  climacteric  in 
the  fifth  decennium.  During  the  whole  of  the  child-bearing 
period  her  organism  is  in  a  comparatively  stationary  condition. 
The  variations  in  the  yearly  increment  of  the  girl  before  puberty 
though  not  so  marked  are  more  complex  than  those  of  the 
boy,  and  she  reaches  the  maximum  of  yearly  increment  sooner 
than  does  he  ;  during  this  whole  period  indeed  she  precedes 
him  in  growth  and  she  has  nearly  reached  her  maximum,  while 
he  is  still  continuing  to  grow.  Her  curve  of  weight  from  the 
nineteenth  year  onward  to  the  climacteric,  remains  stationary, 
being  followed  subsequently  by  a  late  increase,  so  that  while 
the  man  reaches  his  maximum  of  weight  at  about  forty,  the 
woman  is  at  her  greatest  weight  about  fifty. 

Of  the  statical  differences  of  sex,  some,  such  as  the  formation 
of  the  pelvis,  and  the  costal  mechanism  of  respiration,  are  di- 
rectly connected  with  the  act  of  child-bearing,  while  others 
have  only  an  indirect  relation  to  that  duty;  and  indications  at 
least  of  nearly  all  the  characteristic  differences  are  seen  at  birth. 


Chap,  hi.]  THE   PHASES    OF  LIFE.  1151 

The  baby  boy  is  heavier  and  taller  than  the  baby  girl,  and  the 
maiden  of  five  breathes  with  her  ribs  in  the  same  way  as  does 
the  matron  of  forty.  The  woman  is  lighter  and  shorter  than 
the  man,  the  limits  in  the  case  of  the  former  being  from  1-444 
to  1-740  metres  of  height  and  from  39-8  to  93-8  kilos  of  weight, 
in  the  latter  from  1-467  to  1-890  of  height,  and  from  49-1  to 
98-5  kilos  of  weight.  The  muscular  system  and  skeleton  are 
both  absolutely  and  relatively  less  in  woman,  and  her  brain  is 
lighter  and  smaller  than  that  of  man,  being  about  1272  grammes 
to  1424.  Her  metabolism,  as  measured  by  the  respiratory  and 
urinary  excreta,  is  also  not  only  absolutely  but  relatively  to 
the  body-weight  less,  and  her  blood  is  not  only  less  in  quantit}' 
but  also  of  lighter  specific  gravity  and  contains  a  smaller  pro- 
portion of  fed  corpuscles.  Her  strength  is  to  that  of  man  as 
about  5  to  9,  and  the  relative  length  of  her  step  as  1000  to 
1157. 

§  718.  From  birth  onward  (and  indeed  from  early  intra- 
uterine life)  the  increment  of  growth  as  we  have  seen,  though 
undergoing  certain  variations,  continues  to  diminish.  At  last 
a  point  is  reached  at  which  the  curve  cuts  the  abscissa  line,  and 
the  increment  becomes  a  decrement.  After  the  culmination  of 
manhood  at  forty  and  of  womanhood  at  the  climacteric,  the 
prime  of  life  declines  into  old  age.  The  metabolic  activity  of 
the  bod}^,  which  at  first  was  sutficient  not  only  to  cover  the 
daily  waste  but  to  add  new  material,  later  on  is  able  only  to 
meet  the  daily  wants,  and  at  last  is  too  imperfect  even  to  sus- 
tain in  its  entirety  the  existing  frame.  Neither  as  regards 
vigour  and  functional  capacity,  nor  as  regards  weight  and 
bulk,  do  the  turning-points  of  the  several  tissues  and  organs 
coincide  either  with  each  other  or  with  that  of  the  body  at 
large.  We  have  already  seen  that  the  life  of  such  an  organ  as 
the  thymus  is  far  shorter  than  that  of  its  possessor.  The  eye 
is  in  its  dioptric  prime  in  childhood,  when  its  media  are  clearest 
and  its  muscular  mechanisms  most  mobile,  and  then  it  for  the 
most  part  serves  as  a  toy  ;  in  later  years,  when  it  could  be  of 
the  greatest  service  to  a  still  active  brain,  it  has  already  fallen 
into  a  clouded  and  rigid  old  age.  The  skeleton  reaches  its 
limit  very  nearly  at  the  same  time  as  the  whole  frame  reaches 
its  maximum  of  height,  the  coalescence  of  the  various  epiphyses 
being  pretty  well  completed  by  about  the  twenty-fifth  year. 
Similarly  the  muscular  system  in  its  increase  tallies  with  the 
weight  of  the  whole  body.  The  brain,  in  spite  of  the  increas- 
ing complexity  of  structure  and  function  to  which  it  continues 
to  attain  even  in  middle  life,  early  reaches  its  limit  of  bulk  and 
weight.  At  about  seven  years  of  age  it  attains  what  may  be 
considered  as  its  first  limit,  for  though  it  may  increase  some- 
what up  to  twenty,  thirty,  or  even  later  years,  its  progress  is 
much  more  slow  after  than  before  seven.     The  vascular  and 


1152  OLD  AGE.  [Book  iv. 

digestive  organs  as  a  whole  may  continue  to  increase  even  to 
a  very  late  period.  From  these  facts  it  is  obvious  that  though 
the  phenomena  of  old  age  are,  at  bottom,  the  result  of  the  indi- 
vidual decline  of  the  several  tissues,  they  owe  many  of  their  fea- 
tures to  the  disarrangement  of  the  whole  organism  produced  by 
the  premature  decay  or  disappearance  of  one  or  other  of  the 
constituent  bodily  factors.  Thus,  for  instance,  it  is  clear  that 
were  there  no  natural  intrinsic  limit  to  the  life  of  the  muscular 
and  nervous  systems,  they  would  nevertheless  come  to  an  end 
in  consequence  of  the  nutritive  disturbances  caused  by  the  loss 
of  the  teeth.  And  what  is  true  of  the  teeth  is  probably  true  of 
many  other  organs,  with  the  addition  that  these  cannot,  like  the 
teeth,  be  replaced  by  mechanical  contrivances.  Thus  the  term 
of  life  which  is  allotted  to  a  muscle  by  virtue  of  its  molecular 
constitution,  and  which  it  could  not  exceed  were  it  always 
placed  under  the  most  favourable  nutritive  conditions,  is,  in 
the  organism,  further  shortened  by  the  similar  life-terms  of 
other  tissues  ;  the  future  decline  of  the  brain  is  probably  in- 
volved in  the  early  decay  of  the  thymus. 

Two  changes  characteristic  of  old  age  are  the  so-called  cal- 
careous and  fatty  degenerations.  These  are  seen  in  a  com- 
pletely typical  form  in  cartilage,  as,  for  instance,  in  the  ribs ; 
here  the  cell-substance  of  the  cartilage  corpuscle  becomes  hardly 
more  than  an  envelope  of  fat  globules,  and  the  supple  matrix 
is  rendered  rigid  with  amorphous  deposits  of  calcic  phosphates 
and  carbonates,  which  are  at  the  same  time  the  signs  of  past 
and  the  cause  of  future  nutritive  decline.  And  what  is  obvious 
in  the  case  of  cartilage  is  more  or  less  evident  in  other  tissues. 
Everywhere  we  see  a  disposition  on  the  part  of  the  living  sub- 
stance of  the  tissue  to  fall  back  upon  the  easier  task  of  forming 
fat  rather  than  to  carry  on  the  more  arduous  duty  of  manu- 
facturing new  material  like  itself;  everywhere  almost  we  see 
a  tendency  to  the  replacement  of  a  structured  matrix  by  a 
deposit  of  amorphous  material.  In  no  part  of  the  system  is 
this  more  evident  than  in  the  arteries ;  one  common  feature  of 
old  age  is  the  conversion  by  such  a  change  of  the  supple  elastic 
tubes  into  rigid  channels,  whereby  the  supply  to  the  various 
tissues  of  nutritive  material  is  rendered  increasingly  more 
dithcult,  and  their  intrinsic  decay  proportionately  hurried. 

Of  the  various  tissues  of  the  body  the  muscular  and  ner- 
vous are  however  those  in  which  functional  decline,  if  not 
structural  decay,  becomes  soonest  apparent.  The  dynamic 
coefficient  of  the  skeletal  muscles  diminishes  rapidly  after 
thirty  or  forty  years  of  life,  and  a  similar  want  of  power  comes 
over  the  plain  muscular  fibres  also ;  the  heart,  though  it  may 
not  diminish,  or  even  may  still  increase  in  weight,  possesses 
less  and  less  force,  and  the  movements  of  the  intestine,  bladder, 
and  othei^  organs,  diminish  in  vigour.     In  the  nervous  system, 


Chap,  hi.]  THE   PHASES   OF   LIFE.  1153 

the  lines  of  resistance,  which,  as  we  have  seen,  help  to  map  out 
the  central  organs  into  mechanisms,  and  so  to  produce  its  mul- 
tifarious actions,  become  at  last  hindrances  to  the  passage  of 
nervous  impulses  in  any  direction,  while  at  the  same  time  the 
molecular  energy  of  the  impulses  themselves  becomes  less.  The 
eye  becomes  feeble,  not  only  from  cloudiness  of  the  media  and 
presbyopic  muscular  inability,  but  also  from  the  very  bluntness 
of  the  retina  ;  the  sensory  and  motor  impulses  pass  with 
increasing  slowness  to  and  from  the  central  nervous  system, 
and  the  brain  becomes  a  more  and  more  rigid  mass  of  nervous 
substance,  the  molecular  lines  of  which  rather  mark  the  history 
of  past  actions  than  serve  as  indications  of  present  potency. 
The  epithelial  glandular  elements  seem  to  be  those  whose 
powers  are  the  longest  preserved ;  and  hence  the  man  who  in 
the  prime  of  his  manhood  was  a  '  martyr  to  dyspepsia '  by 
reason  of  the  sensitiveness  of  gastric  nerves  and  the  reflex 
inhibitory  and  other  results  of  their  irritation,  in  his  later  years, 
when  his  nerves  are  blunted,  and  when  therefore  his  peptic  cells 
are  able  to  pursue  their  chemical  work  undisturbed  by  extrinsic 
nervous  worries,  eats  and  drinks  with  the  courage  and  success 
of  a  boy. 

§  7i9.  Within  the  range  of  a  lifetime  are  comprised  many 
periods  of  a  more  or  less  frequent  recurrence.  In  spite  of  the 
aids  of  a  complex  civilization,  all  tending  to  render  the  condi- 
tions of  his  life  more  and  more  equable,  man  still  shews  in  his 
economy  the  effects  of  the  seasons.  The  birth-rate  for  instance 
shews  an  increase  in  winter,  and  most  people  gain  weight  in 
Avinter  and  lose  weight  in  summer.  Careful  observations  of 
school  children  shew  that  these  increase  in  length  rapidly  in 
the  spring  but  hardly  at  all  in  the  autumn,  and  very  slowly  in 
the  winter,  Avhile  their  increase  in  weight  is  most  marked  in  the 
autumn,  being  very  slight  or  even  negative  in  the  spring,  and 
not  great  in  winter.  Some  of  these  apparent  effects  of  the 
season  are  the  direct  results  of  varying  temperature,  but  some 
probably  are  habits  acquired  by  descent,  and  in  some  again  the 
connection  is  a  very  indirect  or  possibly  not  a  real  one.  Within 
the  year,  an  approximately  monthly  period  is  manifested  in  the 
female  by  menstruation,  though  there  is  no  exact  evidence  of 
even  a  latent  similar  cycle  in  tlie  male.  The  i)henomena  of 
recurrent  diseases,  and  the  marked  critical  days  of  many  other 
maladies,  may  be  regarded  as  pointing  to  cycles  of  smaller 
duration  than  that  of  the  moon's  revolution,  save  in  the  cases 
in  which  the  recurrence  is  to  be  attributed  rather  to  periodical 
phases  in  the  disease-producing  germ  itself,  than  to  variations 
in  the  medium  of  the  disease. 

§  720.  Prominent  among  all  other  cyclical  events  is  the 
rhythmic  rise  and  fall  in  the  activities  of  the  central  nervous 
system ;  most  animals  possessing  a  well-developed  nervous  sys- 

73 


1154  SLEEP.  [Book  IV. 

tern,  must,' night  after  night,  or  day  after  day,  or  at  least  time 
after  time,  lay  them  down  to  sleep.  The  salient  feature  of 
sleep  is  the  cessation  or  extreme  lowering  of  the  psychical 
activity  of  the  brain  and  of  the  nervous  processes  which  serve 
as  the  basis  of  that  activity.  When  sleep  is  at  its  height,  the 
afferent  nervous  impulses  which  external  agents  set  going  in 
the  afferent  somatic  nerves  such  as  those  of  the  special  senses, 
are  no  longer  the  starting  points  of  complex  cerebral  processes ; 
not  only  do  they  fail  to  excite  consciousness  and  to  leave  their 
mark  on  memory,  but  they  may  be  unable  to  call  forth  even  a 
simple  reflex  movement.  And  yet  they  are  not  wholly  without 
effect;  for  though  a  set  of  feeble  afferent  impulses  may  pro- 
duce no  visible  reaction  and  leave  no  impression  on  the  mind  of 
the  sleeper,  yet  impulses  of  the  same  kind,  if  made  stronger  in 
proportion  to  the  depth  of  the  sleep,  may  be  followed  by  their 
wonted  cerebral  consequences,  and  may  thus  awake,  as  we  say, 
the  sleeper.  It  would  seem  as  if  the  afferent  impulses  met  in 
their  course  with  an  unwonted  resistance  to  their  progress,  as 
if  the  wheels  of  the  cerebral  machinery  worked  stiffly  so  that 
the  lesser  shocks  of  molecular  change  which  otherwise  would 
have  moved  them,  were  broken  and  wasted  upon  them.  Cor- 
responding to  this  block  or  lessened  inroad  of  afferent  impulses, 
the  outflow  of  efferent  impulses  is  stopped  or  largely  dimin- 
ished ;  the  body  gives  no  sign  of  the  working  of  a  conscious 
will,  the  eyelids  drop  and  the  head  nods,  and  the  various  actions 
by  which  the  erect  posture  is  maintained  are  let  go  for  lack  of 
the  governing  motor  impulses.  And  psychological  self-inquiry 
tells  us  that  in  comj^lete  sleep  this  absence  of  outward  signs  of 
cerebral  activity  has  its  fellow  in  the  absence  of  inward  marks  ; 
the  interval  between  falling  asleep  and  awakening  is  a  blank 
and  gap  in  the  history  of  the  mind. 

We  say  '  complete  sleep '  since  there  are  many  degrees  of 
sleep,  the  state  which  we  call  that  of  dreaming  being  one  of 
them ;  and  between  the  most  perfect  wide-awakefulness  and 
that  deepest  slumber  which  refuses  for  a  long  time  to  give  way 
before  even  the  strongest  stimuli,  no  clear  line  of  demarcation 
can  be  drawn.  When  we  fall  asleep  the  tie  between  'ourselves' 
and  the  external  world  is  not  suddenly  snapped,  we  do  not  by 
one  step  pass  from  consciousness  to  unconsciousness ;  and  the 
same  when  conversely  we  awake ;  as  the  world  vanishes  from 
us  or  comes  back  to  us,  the  afferent  impulses  of  sight,  of  sound 
and  of  other  kinds,  for  a  period  which  may  be  brief  but  always 
exists,  produce,  before  they  cease  or  begin  appreciably  to  affect 
us  at  all,  effects  in  ascending  or  descending  scale  which  we  call 
unreal.  And  the  outward  signs  of  sleep  may  vary  from  one  in 
which  volition  is  present  and  even  dominant,  to  one  in  which  even 
the  simplest  reflex  movements  of  the  skeletal  muscles  are  with 
difficulty  evoked,  and  the  maintenance  of  some  skeletal  tone 


Chap,  hi.]  THE   PHASES    OF   LIFE.  1155 

(§  470)  and  of  breathing  afford,  so  far  as  the  skeletal  muscles 
are  concerned,  almost  the  only  token  that  the  central  nervous 
system  is  alive.  But  we  cannot  enter  here  into  the  psychology 
of  sleep  and  dreams. 

Though  the  phenomena  of  sleep  are  largely  confined  to  the 
central  nervous  system  and  especially  to  the  cerebral  hemi- 
pheres,  the  whole  body  shares  in  the  condition.  The  pulse  and 
breathing  are  slower,  the  intestine,  the  bladder,  and  other  in- 
ternal muscular  mechanisms  are  more  or  less  at  rest,  and  the 
secreting  organs  are  less  active,  some  apparently  being  wholly 
quiescent ;  the  secretion  of  mucus  attending  a  nasal  catarrh  is 
largely  diminished  during  slumber,  and  the  sleeper  on  Avaking 
rubs  his  eyes  to  bring  back  to  his  conjunctiva  its  needed  moist- 
ure. The  output  of  carbonic  acid,  and  the  intake  of  oxygen, 
especially  the  former,  is  lessened ;  tlie  urine  is  less  abundant, 
and  the  urea  falls.  Indeed  the  whole  metabolism  and  the  de- 
pendent temperature  of  the  body  are  lowered ;  but  we  cannot 
say  at  present  how  far  these  are  the  indirect  results  of  the  con- 
dition of  the  nervous  system,  or  how  far  they  indicate  a  partial 
slumbering  of  the  several  tissues. 

Thoracic  respiration  is  said  to  become  more  prominent  than 
diaphragmatic  respiration  during  sleep,  and  a  rise  and  fall  of 
the  respiratory  movements,  resembling  if  not  identical  with  the 
Cheyne-Stokes  rhythm  of  respiration  (§  305),  is  frequently 
observed.  During  sleep  the  pupil  is  constricted,  during  deep 
sleep  exceedingly  so ;  and  dilation,  often  unaccompanied  by 
any  visible  movements  of  the  limbs  or  body,  takes  place  when 
any  sensitive  surface  is  stimulated ;  on  awaking  also  the  pupils 
dilate.  The  eyeballs  have  been  generally  described  as  being 
during  sleep  directed  upwards  and  converging,  or  according  to 
some  authors,  diverging  ;  but  others  maintain  that  in  true  sleep 
the  visual  axes  are  parallel  and  directed  to  the  far  distance. 
The  ej^es  of  children  have  been  described  as  continually  execut- 
ing during  sleep  movements,  often  irregular  and  unsymmetrical 
and  unaccompanied  by  changes  in  the  pupils.  The  contraction 
of  the  pupils  is  worthy  of  notice,  since  it  shews  that  the  condi- 
tion of  sleep  is  not  merely  the  simple  and  direct  result  of  the 
falling  away  of  afferent  impulses  ;  Avhen  the  eyes  are  closed  in 
slumber  the  pupils  ought,  since  the  retina  is  then  quiescent,  to 
dilate  ;  that  they  are  constricted,  tlie  more  so  tlie  deeper  the 
sleep,  shews  that  important  actions  in  the  brain,  probably  in 
the  middle  portions  of  the  brain,  are  taking  place. 

We  are  not  at  present  in  a  position  to  trace  out  the  events 
which  culminate  in  this  inactivity  of  the  cerebral  structures. 
The  analogies  between  ordinary  sleep  and  winter  sleep  or 
hibernation  are  probably  real ;  the  chief  difference  appears  to 
be  that  in  the  latter  tlie  diminished  activity  is  due  to  an  ex- 
trinsic cause,  cold,  and  in  the  former  to  intrmsic  causes,  to 


1156  SLEEP.  [Book  iv. 

changes  in  tlie  organism  itself ;  but  we  saw  in  treating  of 
hibernation,  that  intrinsic  changes  prepared  the  way  for  the 
action  of  external  cold.  It  has  been  urged  that  during  sleep 
the  brain  is  anEemic,  and  though  observations  have  jdelded  con- 
flicting results,  the  evidence  seems  to  be  in  favour  of  this  view ; 
but  even  if  this  anaemia  is  a  constant  accompaniment  of  sleep, 
it  must,  like  the  vascular  condition  of  a  gland  or  any  other 
active  organ,  be  regarded  as  an  effect,  or  at  least  as  a  subsidiary 
event,  rather  than  as  a  primary  cause.  Nor  can  the  view  which 
regards  sleep  as  the  result  of  a  change  in  the  mechanical  ar- 
rangements of  the  cranial  circulation,  such  as  either  a  retarda- 
tion or  acceleration  of  the  venous  outflow,  be  considered  as 
satisfactory.  The  essence  of  the  condition  is  rather  to  be  sought 
in  purely  molecular  changes,  though  we  cannot,  however,  at 
present  make  any  definite  statements  concerning  the  nature  of 
these  molecular  changes. 

The  phenomena  of  sleep  shew  very  clearly  to  how  large  an 
extent  an  apparent  automatism  is  the  ultimate  outcome  of  the 
effects  of  antecedent  stimulation.  When  we  wish  to  go  to  sleep 
we  withdraw  our  automatic  brain  as  much  as  possible  from  the 
influence  of  all  extrinsic  stimuli.  We  lie  down  in  order  to 
relieve  the  skeletal  muscles  and  indeed  the  heart  too  from  the 
labour  entailed  by  the  erect  posture;  we  put  off  the  tight  gar- 
ments which  continually  spur  the  skin;  we  empty  the  bladder 
to  avoid  the  stimulus  of  its  distension;  and  we  choose  for  sleep 
the  night  and  a  quiet  place,  drawing  the  curtains,  in  order 
that  our  eyes  may  be  withdrawn  from  light  and  our  ears  from 
sounds.  In  this  connection  may  be  quoted  the  interesting  case 
which  is  recorded  of  a  lad  whose  sensory  tie  with  the  external 
world  was,  from  a  complicated  ansesthesia,  limited  to  that 
afforded  by  a  single  eye  and  a  single  ear;  the  lad  could  be 
sent  to  sleep  at  will,  by  closing  the  eye  and  stopping  the  ear. 

§  721.  The  cycle  of  the  day  is  however  manifested  in  many 
other  ways  than  by  the  alternation  of  sleeping  and  waking, 
with  all  the  indirect  effects  of  these  two  conditions.  There  is 
a  diurnal  curve  of  temperature,  apparently  independent  of  all 
immediate  circumstances,  the  hereditary  impress  of  a  long  and 
ancient  sequence  of  days  and  nights.  Even  the  pulse,  so  sen- 
sitive to  all  bodily  changes,  shews,  running  through  all  the 
immediate  effects  of  the  changes  of  the  minute  and  the  hour, 
the  working  of  a  diurnal  influence  which  cannot  be  accounted 
for  by  waking  and  sleeping,  by  working  and  resting,  by  meals 
and  abstinence  between  meals.  And  the  same  may  be  said 
concerning  the  rhythm  of  respiration,  and  the  products  of  pul- 
monary, cutaneous  and  urinary  excretion.  There  seems  to  be 
a  daily  curve  of  bodily  metabolism,  which  is  not  the  product 
of  the  day's  events.  Within  the  day  we  have  the  narrower 
rhythm  of  the  respiratory  centre  with  the  accompanying  rise 


Chap,  hi.]  THE   PHASES   OF   LIFE.  1157 

and  fall  of  activity  in  the  vaso-motor  centres.  And  lastly, 
there  stands  out  the  fundamental  fact  of  all  bodily  pei'iodicity, 
that  alternation  of  the  heart's  systole  and  diastole  which  ceases 
only  at  death.  Though,  as  we  have  seen,  the  intermittent  flow 
in  the  arteries  is  toned  down  in  the  capillaries  to  an  apparently 
continuous  flow,  still  the  constantly  repeated  cycle  of  the  cardiac 
shuttle  must  leave  its  mark  throughout  the  whole  web  of  the 
body's  life.  Our  means  of  investigation  are,  however,  still  too 
gross  to  permit  us  to  track  out  its  influence. 


CHAPTER  IV. 
DEATH. 

§  722.  When  tlie  animal  kingdom  is  surveyed  from  a 
broad  standpoint,  it  becomes  obvious  that  the  ovum,  or  its  cor- 
relative the  spermatozoon,  is  the  goal  of  an  individual  existence; 
that  life  is  a  cycle  beginning  in  an  ovum  and  coming  round  to 
an  ovum  again.  The  greater  part  of  the  actions  which,  looking 
from  a  near  point  of  view  at  the  higher  animals  alone,  we  are 
apt  to  consider  as  eminently  the  purposes  for  which  animals 
come  into  existence,  when  viewed  from  the  distant  outlook 
whence  the  whole  living  world  is  surveyed,  fade  away  into  the 
likeness  of  the  mere  byplay  of  ovum-bearing  organisms.  The 
animal  body  is  in  reality  a  vehicle  for  ova;  and  after  the  life  of 
the  parent  has  become  potentially  renewed  in  the  offspring,  the 
body  remains  as  a  cast-off  envelope  whose  future  is  but  to  die. 

Were  the  animal  frame  not  the  complicated  machine  we  have 
seen  it  to  be,  death  might  come  as  a  simple  and  gradual  disso- 
lution, the  '  sans  everything '  being  the  last  stage  of  the  succes- 
sive loss  of  fundamental  powers.  As  it  is,  however,  death  is 
always  more  or  less  violent;  the  machine  comes  to  an  end  by 
reason  of  the  disorder  caused  by  the  breaking  down  of  one  of  its 
parts.  Life  ceases  not  because  the  molecular  powers  of  the  whole 
body  slacken  and  are  lost,  but  because  a  weakness  in  one  or  other 
part  of  the  machinery  throws  its  whole  working  out  of  gear. 

We  have  seen  that  the  central  factor  of  life  is  the  circulation 
of  the  blood,  but  we  have  also  seen  that  blood  is  not  only  use- 
less, but  injurious,  unless  it  be  duly  oxygenated ;  and  we  have 
further  seen  that  in  the  higher  animals  the  oxygenation  of  the 
blood  can  only  be  duly  affected  by  means  of  the  respiratory 
muscular  mechanism,  presided  over  by  the  respiratory  centre  in 
the  bulb.  Thus  the  life  of  a  complex  animal  is,  when  reduced 
to  a  simple  form,  composed  of  three  factors:  the  maintenance  of 
the  circulation,  the  access  of  air  to  the  hoemoglobin  of  the  blood, 
and  the  functional  activity  of  the  respiratory  centre  ;  and  death 
may  come  from  the  arrest  of  any  one  of  these  three.  As  it  has 
been  put,  death  may  begin  at  the  heart  or  at  the  lungs  or  at  the 

1158 


Chap,  iv.]  DEATH.  1159 

brain.  In  reality,  however,  when  we  jdusIi  the  analysis  further, 
the  central  fact  of  death  is  the  stoppage  of  the  heart,  and  the 
consequent  arrest  of  the  circulation ;  the  tissues  then  all  die, 
because  they  lose  their  internal  medium.  The  failure  of  the 
heart  may  arise  in  itself,  on  account  of  some  failure  in  its  ner- 
vous or  muscular  elements,  or  by  reason  of  some  mischief  affect- 
ing its  mechanical  working.  Or  its  stoppage  may  be  due  to 
some  fault  in  its  internal  medium,  such  for  instance  as  a  want 
of  oxygenation  of  the  blood,  which  in  turn  may  be  caused  by 
either  a  change  in  the  blood  itself,  as  in  carbonic  oxide  poison- 
ing, or  by  a  failure  in  the  mechanical  conditions  of  resi^iration, 
or  by  a  cessation  of  the  action  of  the  respiratory  centre.  The 
failure  of  this  centre,  and  indeed  that  of  the  heart  itself,  may 
be  caused  by  nervous  influences  proceeding  from  the  brain,  or 
at  least  brought  into  operation  by  means  of  the  central  nervous 
system  ;  it  may,  on  the  other  hand,  be  due  to  an  imperfect  state 
of  blood,  and  this  in  turn  may  arise  from  the  imperfect  or  per- 
verse action  of  various  secretory  or  other  tissues.  The  modes 
of  death  are  in  reality  as  numerous  as  are  the  possible  modifica- 
tions of  the  various  factors  of  life ;  but  they  all  end  in  a  stop- 
page of  the  circulation,  and  the  withdrawal  from  the  tissues  of 
their  internal  medium.  Hence  we  come  to  consider  the  death 
of  the  body  as  marked  by  the  cessation  of  the  heart's  beat  when- 
ever that  cessation  is  one  from  which  no  recovery  is  possible ;' 
and  by  this  we  are  enabled  to  fix  an  exact  time  at  which  we  say 
the  body  is  dead.  We  can,  however,  fix  no  such  exact  time  to 
the  death  of  the  individual  tissues.  They  are  not  mechanisms, 
and  their  death  is  a  gradual  loss  of  power.  In  the  case  of  the 
contractile  tissues,  we  have  apparently  in  rigor  mortis  a  fixed 
term,  by  which  we  can  mark  the  exact  time  of  their  death.  If 
we  admit  that  after  the  onset  of  rigor  mortis  recovery  of  irrita- 
bility is  impossible,  then  a  rigid  muscle  is  one  permanently  dead. 
In  the  case  of  the  other  tissues,  we  have  no  such  objective  sign, 
since  the  rigor  mortis  of  other  tissues  manifests  itself  cliiefly  by 
obscure  chemical  signs.  And  in  all  cases  it  is  obvious  that  the 
jDOssibility  of  recovery,  depending  as  it  does  on  the  skill  and 
knowledge  of  the  experimenter,  is  a  wholly  artificial  sign  of 
death.  Yet  we  can  draw  no  other  sharp  line  between  the  seem- 
ingly dead  tissue  whose  life  has  flickered  down  into  a  smoulder- 
ing ember  which  can  still  be  fanned  back  again  into  flame,  and 
the  handful  of  dust,  the  aggregate  of  chemical  substances  into 
which  the  decomposing  tissue  finally  crumbles. 

Moreover,  the  failure  of  the  heart  itself  is  at  bottom  loss  of 
irritability,  and  the  possibility  of  recovery  here  also  rests,  as  far 
as  is  known  at  present,  on  the  skill  and  knowledge  of  those  who 
attempt  to  recover.  So  that  after  all  the  signs  of  the  death  of 
the  whole  body  are  as  artificial  as  those  of  the  death  of  the  con- 
stituent tissues. 


INDEX. 


Aberration,  spherical,  871 ;  chromatic,  873 

Abscissa  line,  mode  of  measuring  curves 
on  (footnote),  18t) 

Absorption  from  alimentary  canal,  398, 
412 

Achroodextrin,  315 

Acid-albumen,  formation  of,  19,  88,  324 

Acid,  benzoic,  an  antecedent  of  hippuric 
acid,  539 ;  renal  action  on,  540 ;  butyric, 
etc.,  in  fat,  60G 

,  carbonic,  clotting  retarded  by  its 

presence,  21 ;  develojiment  of,  in  rigor 
mortis,  91,  92 ;  set  free  during  muscular 
contraction,  94,  637  ;  in  expired  and  in- 
spired air,  440 :  amount  of  daily  excre- 
tion of,  441,  624;  percentage  of,  in 
expired  air,  412,  466;  its  relations  in 
the  blood,  461 ;  causes  of  its  escape 
from  the  blood,  466;  its  exit  from  the 
lungs,  465;  in  the  blood,  eifect  of  ex- 
cess of,  487 ;  thrown  off  by  the  skin, 
552;  lessened  'output'  of,  in  sleep, 
1155 

,cholalic,587;  hippuric, chemical  com- 
position, 517;  how  formed  in  kidney, 
539 

,  hydrochloric,  in  gastric  juice,  352; 

lactic  in  the  blood,  48 

,  lactic,  isomeric  variations  of  (foot- 
note), 90;  its  effect  on  the  heart,  259; 
fermentation,  394 

,  uric,  chemical  composition,  516 ;  a? 

result  of  special  metabolism,  596 

Acids,  fatty,  361,  390;  in  milk,  613;  in 
sebum,  551 

,  organic,  in  urine,  518;  various,  in 

spleen-pulp,  583 

Action,  currents  of,  100,  102, 107, 108,  753 

.peristaltic,  in  plain  muscular  tissue, 

133  ;  of  the  stomach,  377  ;  of  intestine, 
383;  increased  in  asphyxia,  386;  of 
ureter,  532,  544;  of  bladder,  546 

,  reflex,  144-1411,  f)  IS :  purposive  natui'e 

of,  146;  of  spinal  cord,  696-710 

,  automatic,  146,  711 

Addison's  disease,  602 

1 


Adenoid  tissue  in  lymphatic  glands,  42; 
multiplication  of  leucocytes  in,  42,  43 

Adipose  tissue,  structure  of,  604 

After-birth,  formation  of,  1120 

After-images,  882 ;  negative  and  positive, 
934 

After-pains,  services  affected  by,  1139 

Age,  vascular  changes  due  to,  299;  old 
age,  phenomena  of,  1151 

Air,  tidal  and  stationary  in  lungs,  425 ; 
complementary,  supplemental,  and  re- 
sidual, 426;  expired  air,  changes  of, 
440,  443 

Albumin,  acid  and  alkali,  19,  20,  88 

Albumose,  clotting  prevented  by,  29;  dis- 
tinguished from  peptone,  325 

Alcohol,  changes  in  proteids  produced  by, 
24;  physiological  action  of,  304;  its 
probable  action  on  cardiac  tissues, 
306,  307 ;  use  in  diet,  662 

Alimentary  canal,  vaso-constrictor  nerves 
of,  279,  368;  structure,  311;  circular 
and  longitudinal  coats  of,  371,  372; 
spontaneous  movements  of,  383-385; 
nerve  supply  to  coats,  384,  385;  peri- 
staltic action,  383;  mutually  destruc- 
tive juices  of,  392 ;  absorption  from, 
398,  417 

Alkaloids,  vegetable,  their  kinship  to 
urea,  654 

Allantoin,  1132 

Allantois,  growth  of  the,  1120 

Altitudes,  high,  diminished  oxygen  press- 
ure in,  464 

Alvergniat's  gas-pump,  445 

Amblyopia,  789 

Ammonia,  mode  of  conversion  into  urea, 
595,  596 

Amnion,  true  and  false,  1120,  1126 

Amniotic  tluid,  1126  ;  function,  1127 ;  com- 
position, 1131 

AmoibfE,  characteristics  of,  3-7 

AnuBboid  movements  of  white  corpus- 
cles, 36,  40, 137,  2!)2 

Ainphibia,  double  vascular  supply  to 
kidney,  533 


101 


1162 


INDEX. 


Ampullae  of  the  semicircular  canals, 
732 

Amylolytic  action  of  saliva,  317 

Anabolic  changes  of  lining  substance,  39 

Anacrotic  pulse,  usually  pathological, 
230,  236 

Ansemia,  lessened  number  of  red  corpus- 
cles in,  33;  respiration  impeded  by, 
510 

Anelectronus,  113;  its  relation  to  irrita- 
bility, 117 

"  Animal  starch,"  574 

Animals,  cold-blooded,  temperature  of, 
641;  warm-blooded,  temperature  of, 
641 

Annulus  of  Vieussens,  247,  253-256 

Anode,  the,  57 

Aorta,  proportion  of  sectional  area  of 
capillaries  to  the,  151 ;  comparative 
blood-pressure  in,  193,  203-210,  236. 

Aortic  valves,  189,  207 

Aphasia,  connection  of,  with  cortical 
lesion,  766,  769 ;  iDhenomena  of,  770 

Apnoea,  how  produced,  489,  490 

Aqueous  humour,  972 

Area,  cortical  motor,  in  dog,  740;  in 
monkey,  744;  in  anthropoid  ape,  748; 
in  man,  764;  mode  of  action  in,  769; 
for  movements  of  the  eyes,  790;  for 
speech,  766,  1084;  for  movements  of 
larynx,  1084;  for  respiratory  move- 
ments, 1084 

,  cortical,  for  vision,  791 ;  for  smell, 

794 ;  for  cutaneous  sensations,  799 

,  diabetic,  575;  visual,  889;  tactile, 

1040 

Aristotle,  experiment  of,  1069 

Arterial  pressure,  155;  see  also  Blood, 
pressure  of ;  tracings  of,  158,  159,  168, 
169;  heart-beat  in  inverse  ratio  to, 
261;  as  affected  by  tonic  contraction, 
264;  by  quantity  of  blood,  297;  by 
exercise,  304;  vaso-motor  action  on, 
275,  276,  283 

scheme,  model  of,  167 ;  tone,  264 

Arterialization,  effect  of  deficient,  509 

Arteries,  effect  of  ligature  on,  28,  154; 
elasticity  and  contractility  of,  151,  166, 
245;  pulse  in,  153,  161,  180;  changes 
of  calibre  in,  221,  262;  supply  of  vaso- 
motor nerves  to,  262,  275;  effect  on 
blood-pressure  of  their  contractility, 
276;  as  affected  by  age,  299,  1152;  of 
the  brain,  827;  of  the  eye,  969;  renal, 
525;  renal,  in  amphibia,  533;  radial, 
273-276;  umbilical,  1124;  umbilical, 
venous  blood  in  the,  1125 ;  helicine,  in 
erectile  tissue,  1116 

Artificial  pulse,  tracings  of,  168,  223 

Arytenoid  cartilages,  1076 

Ash  of  muscle,  salts  in,  93;  of  nerves, 
salts  in,  106 


Asphyxia,  phenomena  of,  485,  491 ;  in- 
creased peristaltic  action  in,  386 

Astigmatism,  871 

Ataxy,  locomotor,  807 

Atrophy,  acute,  of  liver,  decrease  of  urea 
in,  594 

Atropin,  cardiac  inhibition  counteracted 
by,  257 ;  salivary  secretion  arrested  by, 
338,  351 ;  its  action  on  sweating,  556, 
557;  its  effect  on  the  pujjil,  867;  on 
accommodation,  868 

Auditory  sensations,  998 ;  impulses,  1007 ; 
auditory  epithelium,  structure,  1013; 
auditory  hairs,  1009 

Aura,  forms  of,  preceding  epileptiform 
attacks,  795 

Automatic  action,  146,  711 

Axis-cylinder  process,  in  nerve  cells  of 
spinal  cord,  143 

Babe,  the,  composition  of  as  compared 
with  adult,  1144;  digestive  processes 
of,  1146  ;  nervous  sj^stem  of  the,  1148 

Bacteria,  ingestion  of  by  white  corpus- 
cles, 44;  results  of  their  j)resence  in 
alimentary  canal,  361,  393 

Bacterium  photometricum,  its  reaction 
towards  light,  921 

Banting,  his  method  of  reducing  fatness, 
669 

Basis,  molecular,  of  chyle,  403 

Bat,  the  gastric  glands  of,  346 

Beats,  in  musical  sounds,  1005 

Beehive,  temperature  of,  641 

Bidder's  ganglia  in  heart  of  frog,  243 

Bile,  characters  and  composition,  354 ; 
secretion  of,  366,  367 ;  action  on  food, 
357;  on  fats,  392;  antagonistic  to 
peptic  action,  358,  393;  storage  of  in 
gall-bladder,  367;  resorption  of  under 
pressure,  370;  osmotic  passage  of  fat 
facilitated  by,  417 ;  relation  of  to  for- 
mation of  urea,  589;  foetal  secretion 
of,  1130 

Bile-acids,  their  formation,  586 

Bile  pigments,  36,  356,  584 ;  salts,  356 

Bilin,  its  composition,  357 

Bilirubin,  its  relations  with  haematin,  34, 
584 ;  composition  of,  356 ;  formation  of, 
586 

Biliverdin,  its  composition,  356 

Birth,  changes  of  the  lungs  at,  427; 
changes  of  circulation  at,  1134 

Bladder,  muscles  of,  545 

Blastodermic  vesicle,  1123 

Blind  spot  of  retina.  916 

Blindness,  total  and  partial,  788 

Blood,  the,  l.'J-50 ;  an  internal  medium  of 
interchange,  13,  149;  clotting  of,  15- 
30;  circumstances  affecting  clotting, 
20;  causes  of  clotting,  26;  its  relation 
to  vascular  walls,  27,  294;  corpuscles 


INDEX. 


1163 


(see  Corpuscles) ;  laky  blood,  how 
formed,  31 ,  450;  results  of  injection 
of,  538;  chemical  composition  of,  4G- 
48;  specific  gravity  of,  4G;  quantity 
and  distribution  of,  49,  50;  in  a  part, 
mode  of  measuring,  270;  results  of 
changes  in,  278  ;  rate  of  flow  in  ves- 
sels, 172,  177,  178;  its  dependence  on 
vaso-motor  action,  275;  amount  driven 
by  each  heart-beat,  153,  217 ;  time  oc- 
cupied by  one  circulation,  218;  quality 
of,  its  eifect  on  heart-beat,  258;  its 
effect  on  peripheral  resistance,  294; 
as  affected  by  exercise,  304;  by  defi- 
cient aeration,  487;  quality  of,  in  in- 
fant, 1147 

Blood,  the,  oedema  due  to  changes  in,  411 ; 
respiratory  changes  in,  443-461;  gases 
of,  447;  how  measured,  445;  entrance 
of  oxygen  into  by  diffusion,  424;  exit 
of  carbonic  acid  from,  4(j(j ;  relations 
of  oxygen  in,  447;  of  carbonic  acid  in, 
401 ;  of  nitrogen  in,  461 

,  venous,  colour  of,  455 ;  spectrum  of, 

455 ;  an  excitant  of  respiratory  centre, 
484 ;  its  slowing  effect  on  heart-beat, 
504;  in  umbilical  arteries,  1125 

,  arterial,  colour  of,  455;   constancy 

of  percentage  of  sugar  in,  572;  course 
of,  in  foetus,  1132;  circulation  of  (.see 
Circulation);  platelets,  44 ;  in  relation 
to  inrtammation,  292 

pressure,  arterial  and  venous  com- 
pared, 153-161,  166;  in  arteries,  153- 
161 ;  how  measured,  154  et  supra ;  in 
veins,  155,  159;  mode  of  registering, 
156;  in  capillaries,  160-163,  166;  phe- 
nomena of,  163;  its  relation  to  pe- 
ripheral resistance,  163-170;  artificial 
scheme  of,  167-170;  endocardiac,  191- 
197 ;  aortic  and  ventricular  compared, 
203-210,  236 ;  negative,  217 ;  as  affected 
by  cardiac  inhibition,  251 ;  by  stimula- 
tion of  depressor,  281 ;  of  sciatic,  282 ; 
by  action  of  drugs,  295;  by  changes  iu 
amount  of  blood,  2S)6,  297;  by  respira- 
tion, 498 

,  in  the  kidneys,  525,  528,  532;  in 

the  brain,  829;  in  umbilical  vein  and 
artery,  1124 

,  serum,  constituents  of,  19,  20 

,  supply,  its  influence  on  muscular 

irritability,  128 

,  vessels,  their  influence  on  fluidity 

of  blood,  27,  28 

Blushing,  its  cause,  286,  303 

Body,  the,  characteristics  of,  in  life  ami 
death,  1.2;  average  composition  of,  619 ; 
metabolic  processes  of,  559 ;  clianges 
during  starvation,  620;  potential  and 
actual  energy  of,  (532;  expenditure  of 
energy  by,  634 ;  sense  of  position  of,  729 


Bois-Reymond,  du,  his  'key,'  58;  on 
muscle-currents,  100,  101 

Bi-achial  plexus,  constrictor  and  dilator 
fibres  in,  270 

Brain,  the,  reaction  of,  833;  its  action  on 
spinal  reflexes,  708 

,  cerebral  hemispheres,  relation  of,  to 

crossed  side  of  body,  804;  results  of  re- 
moval of,  in  frog,  719;  I'esults  of  re- 
moval of,  in  birds,  723;  results  of 
removal  of,  iu  mammals,  725,  727 

,  cortex,    experimental    interference 

with,  740;  localization  of  function  in, 
741 ;  results  of  removal,  762 

,  grey  matter  of,  corpora  quadrige- 

mina,  anterior,  their  connection  with 
vision,  786, 814 ;  cor^jora  geniculata,  786 

,  fibres  of,  superior  peduncles  of  cere- 
bellum, 812 

,  optic  tracts,  784;   endings  of,   786, 

787;  splanchnic  functions,  816;  venous 
sinuses  of,  828;  circulation  in,  829; 
supply  of  blood  to,  831 ;  its  condition 
during  sleep,  1155 

Breaking  of  the  voice  at  puberty,  1091 

Breath,  the  first,  cause  of,  1134;  effect 
of,  1135 

Breathing,  normal  rate  of,  433;  male  and 
female,  differences  between,  434;  an 
involuntary  act,  472;  laboured,  nervous 
mechanism  of,  1082 

Bright's  disease,  cedema  of,  411 

Broca's  convolution,  766 

Brownian  movements  in  molecular  basis 
of  chyle,  403 

Buffy  coat  of  clotted  blood,  16 

Burdach,  column  of,  682 

Burdon-Sanderson,  his  stethometer,  431 

Calcium  salts,  their  presence  necessary 
in  clotting  of  blood,  26 

Calcic  lihosphate,  insolubility  of  curd 
dependent  on,  331;  in  milk,  613 

Calories,  combustion  of  food  expressed 
in,  633 

Calorimeters,  635 

Canals,  semicircular,  result  of  injury  to, 
730 

Capacity,  vital,  428 

Capillaries  described,  13;  their  permea- 
bility, 13,  150-152;  blood-interchanges 
effected  in,  13,  150;  structure  of,  150; 
proportion  of  sectional  area  of,  to  aorta, 
151,  152,  162;  measurements  of  blood- 
pressure  in,  160;  disappearance  of 
pulse  in,  161 ;  peripheral  resistance  in, 
160-l(i6;  calibre,  289;  plasmatic  layer 
in,  290 

Capillary  circulation,  normal  phenomena 
of,  161,289;  as  affected  by  inflamma- 
tion, 291 

Capsule,  Tenon's,  971 


1164 


INDEX. 


Capsules,  supra-renal,  histology  of,  601 
Carbo-hydrates  in  white  corpuscles,  40; 
in  muscle  substance,  92 ;  in  food-stuffs, 
312;    various  forms  of,  314;    in   food, 
presence  of  glycogen  dependent  on,  olJl, 
567;    formation  of  fat  from,  608;    as 
food,  potential  energy  of,  633 
Carbon  monoxide,  asphyxia  from,  494 
Carbonic  acid,  see  Acid,  carbonic 
Cardio-graphic  tracings,  201,  209 
Cardio-inhibitory  centre,  249,  252 
Cardiometer,  Roy  and  Adami's,  199 
Casein,  323;  precipitation  of,  330;  a  con- 
stituent of  milk,  612;  its  formation  in 
mammary  gland,  614 
Cartilage,   thyroid,   1077,   1078;    cricoid 
and   arytenoid,  1076,  1079;   of  Santo- 
rini,  1070;  of  Wrisberg,  1070,  1072 
Cells,  albuminous,  changes  in,  342;  car- 
diac muscle,  in  frog,  243;  in  mammal, 
244;    central,  of   gastric  glands,  352; 
ciliary,  134;  action  of  chloroform  on, 
136;  ciliary,  of  Claudius,  1011;  colum- 
nar fat,  absorbed   by,  417;    of  Corti, 
1012;    of    Deiters,   1015;    cylinder,  of 
auditory  epithelium,  1008;  of  olfactory 
mucous  membrane,  1026;   differentia- 
tion of,  during  development  of  ovum, 
6;  epithelial,  134;  epithelium,  135;  fat, 
604;    ganglionic,  143;   of  grey  matter, 
143;  hair,  inner  and  outer,  of  auditory 
epithelium,  1011,  1017;    hepatic,   con- 
tinuous activity  of,  369;   changes   in, 
566;  glycogen  in,  568  ;  action  on  hgemo- 
globin,  588;   of  mammary  gland,  610; 
mucous,    of     salivary     glands,     344; 
'loaded,'  345;    nerve,  of   spinal  cord, 
140-143;    of  central    nervous   system, 
143;    ovoid,    or    parietal,    of    gastric 
glands,  347;  of  pancreas,  341;   of  pa- 
rotid, 343 

of  Lieberkiihn,  422 

,  secreting,  series  of  events  in,  347, 

350,  352;    double  function  of  cells   of 
villi,  422 
Cell-action  in  absorption,  422 
Cell-substance,  milk  partly  formed  from, 

615 
Cellulose,  digestion  of,  in  large  intestine, 
396  ;  a  food-stuff  for  the  herbivora,  661 
Centres,  nervous,  cardio-inhibitory,  249; 
vaso-motor,   280,   284;    limits   of,   282, 
283 ;  pupil-constrictor,  860 
for  deglutition,  373,  376;  for  lacta- 
tion,  618;    for    micturition,    547;    for 
micturition,    inhibition     of,    707;     for 
movements  of  eyeball,  956;  for  parturi- 
tion,   1140;    for   phonation,    1084;    for 
vomiting,  379 

■ for  respiration,  473;  for  respiration, 

automatic     action     of,    476,     488;     as 
affected    by    blood-supply,    484,    509; 


activity  of,  increased  by  exercise,  488; 
for  sweating,  557 

,  possible,  for  thermal  changes,  647; 

trophic,  for  nutrition  of  nerves,  679; 
visual,  higher  and  lower,  793 

Cerebellar  tract,  689 

Cerebellum,  see  Brain,  cerebellum 

Cerebral  operations,  time  taken  by,  819 

Cerebria  in  nerve  tissue,  105 

Changes,  anabolic  and  katabolic  in  living 
substance,  39,  651 ;  nervous  regulation 
of,  654;  diurnal,  in  functions,  1156 

Chauveau  and  Lortet,  their  hcemata- 
chometer,  174,  178 

and  Marey,  their  mode  of  measuring 

endocardiac  pressure,  191,  192 

Chest,  exijausion  and  contraction  of,  dur- 
ing respiration,  426 

Chest-voice,  how  produced,  1089 

Cheyne-Stokes  respiration,  490,  1155 

Chiasma,  optic,  decussation  of  fibres  in, 
784 

Chloral,  its  effect  on  stimulation  of  de- 
pressor, 282 

Chlorides,  their  presence  in  serum,  47 

Chloroform,  its  effect  on  ciliary  action, 
136 

Cholesterin,  composition  of,  355;  a  con- 
stituent of  bile,  lb. ;  its  presence  in 
blood,  47;  in  red  corpuscles,  48;  in 
nerve  substance,  105 ;  in  gallstones,  ib. ; 
in  milk,  613 

Chondrin,  action  of  gastric  juice  on,  329 

Chorion,  the,  1120 

Choroid,  development  of,  837;  blood- 
supply  of,  969 

Chromogens,  their  presence  in  urine,  519 

Chyle,  characters  of,  402,  molecular  basis 
of,  403;  passage  of  fat  into,  413,  417; 
presence  of  sugar  in,  414;  absence  of 
peptone  in,  416;  elaboration  of,  in  the 
villus,  418 

Chyme,  how  formed,  388 

Cilia,  134 

Ciliary  movements,  52,  134;  circum- 
stances affecting,  134-136;  plexuses, 
aqueous  humour  furnished  by,  972 

Circulation  of  the  blood,  main  facts  of, 
153;  capillary,  162,  175,289;  hydraulic 
principles  of  the,  163, 164 ;  aids  to,  171 ; 
rate  of  flow,  172-178;  time  occupied  by 
circuit,  178,  22.') ;  constant  and  variable 
factors  of,  299;  as  affected  by  blood- 
supply,  301 ;  changes  in,  300;  causes  of 
irregularity  in,  301;  of  cessation  of, 
302;  placental,  1124;  early  foetal,  1124; 
late  foetal,  1132;  changes  of,  taking 
place  at  birth,  1135;  renal,  525 

Circus  movements,  735 

Clarke's  column  of  spinal  cord,  682 

Clotting  of  blood,  15-30;  retarded  by 
cold,  16;   by  addition  of  saline  solu- 


INDEX. 


11  Go 


tions,  IG,  22;  by  oil,  21;  by  carbonic 
acid  iu  the  blood,  21 ;  by  injection  of 
albumose,  29;  causes  of,  26;  in  the 
living  bodj',  2'J;  favoured  by  presence 
of  foreign  bodies,  21,  2!l,  45;  clotting 
of  fluids  iu  the  body  other  than  blood, 
23;  clotting  of  muscle  plasma  in  rigor 
mortis,  90;  clotting  of  lymph,  400 

Coagulation  of  proteids  by  heat,  18 

Cochlea  of  ear,  982 

Coitus,  behaviour  of  spermatozoa  after, 
1119 

Cold,  its  influence  on  clotting,  1(1,  21 ;  on 
irritability  of  muscle  and  nerve,  127, 
128;  on  vaso-constrictor  action,  304; 
on  skin  action,  541 ;  great,  lowering  of 
metabolism  bj%  650;  terminal  organs 
for  sensation  of,  1045 ;  sensations  of, 
due  to  changes  of  skin  temperature, 
1041 

Colostrum,  composition  of,  614 

Colour  sensations,  many  kinds  of,  891 ; 
mixing  of,  892,  894,  898;  characters  of, 
895  ;  Young-Helmholtz's  theory  of,  898  ; 
due  to  metabolic  changes,  901 ;  in  rela- 
tion to  intensity  of  stimulus,  913;  un- 
equal change  of,  under  diminishing 
light,  914 

Colour  vision,  variations  in,  906 

blindness,  906;  different  kinds  of, 

907  ;  dichromic  iu  nature,  907 ;  Young- 
Helmholtz's  theory  of,  908 ;  Bering's 
theory  of,  909;  absolute,  912 

Colours,  complementary,  89li;  primary, 
898 ;  of  arterial  and  venous  blood,  443- 
457 

Combustion  of  various  foods,  rates  of, 
compared,  633 

Commissure,  inferior  optic,  785 

Cones  of  retina,  approximate  dimensions 
of,  889 

Conjunctiva,  structure  of,  977 

Connective  tissue,  action  of  gastric  juice 
on,  330;  in  relation  to  lymphatic  ves- 
sels, 398 

Constant  current,  its  action,  110 ;  as  com- 
pared with  induction  shock,  118 

Consonants  and  vowels,  1093;  manner  of 
formation,  1096;  classification  of,  1097 

Constriction  of  arteries,  265;  of  pupil  a 
rertex  act,  860 

Constrictor  fibres,  270 

Contractile  tissues,  the,  51-138 

Contractilitj',  53 

Contraction  of  muscle,  movements  of 
body  due  to,  51 ;  simple  and  tetanic, 
55;  graphic  method  of  recording,  55; 
simple,  phenomena  of,  65;  tetanic,  75- 
80;  of  skeletal  muscles,  tetanic  in  char- 
acter, 79 ;  wave  of,  83 ;  optical  changes, 
85;  chemical  changes  due  to,  94;  ther- 
mal   changes    due    to,    95;     electrical 


changes  during,  104 ;  '  making  and 
breaking,'  110,  111;  influenced  by 
nature  of  stimulus,  119;  isometric  and 
isotonic,  ib. :  prolonged,  of  red  muscle, 
122;  as  influenced  by  load,  123;  idio- 
muscular,  125;  exhausting  effects  of 
the  products  of,  130;  of  plain  muscle, 
131-134;  spontaneous,  133;  tonic,  134; 
relation  of  to  amoeboid  movements, 
137,  138;  of  heart,  216;  features  of 
heart  contraction,  243,244;  of  villus, 
420 

Contractions,  peristaltic,  in  plain,  mus- 
cular tissue,  133;  of  ureter,  532,  544; 
of  bladder,  546;  of  the  stomach,  377; 
of  the  intestine,  383 

,  rhythmical,  of  spleen  tissue,  580;  of 

the  uterus  during  pregnancy,  1136; 
during  'labour,'  1137;  after  parturi- 
tion, 1139 

— -  of  the  abdominal  walls,  383,  1138 

Contrast  visual,  simultaneous,  932;  suc- 
cessive, 933 

Conus  medullaris,  689 

Convulsions,  auEemic,  how  produced,  492 

Coordination  of  movements,  machinery 
of,  in  birds,  730;  in  mammals,  731;  in 
man,  733;  parts  of  middle  brain  con- 
cerned in,  737 

of  ocular  movements,  952;  nervous 

mechanism  governing,  956 

Cord,  spinal,  139,675-718;  diagrammatic 
metamere  of,  140;  ganglia  of  the,  143; 
reflex  actions  manifested  by  the,  144; 
geueral  features,  681 ;  white  matter, 
structure,  693;  white  matter,  tracts  of, 
delimitation  of,  686;  neuroglia,  690; 
grey  matter,  691  ;  nature  of,  693; 
ascending  and  descending  degenera- 
tion, 68(5;  special  features  of  the  sev- 
eral regions,  689-695 ;  variation  in 
sectional  area  of  white  matter,  6'.'0;  of 
grey  matter,  691 ;  course  of  pyramidal 
tracts  in  the,  6SG  ;  cerebellar  tract,  689  ; 
reflex  actions  of,  144,  696-710;  com- 
plexity of,  700  ;  reflex  actions  of,  in 
man,  704;  inhibition  of,  707;  time  re- 
quired for,  709;  automatic  actions  of^ 
711-718;  action  in  disease,  717;  hypo- 
thetical segmentation  of,  777;  motor 
mechanisms  of,  806 ;  lymphatic  arrange- 
ments of,  824 

Cornea,  blood-supply  to,  969 

Cornu,  anterior,  of  cord,  681,  6S7 ;  con- 
nection of  elTercnt  fibres  in,  144 

Corpora  Arantii,  184 

geniculata,     786;     quadrigomina, 

connection   of   with   vision,  786;    con- 
siderations touching  the,  814;  corpora 
cavernosa,  1115,  1116 
Corpus  spongiosum,  1115 
Corpuscles  of  blood,  not  an  essential  part 


1166 


INDEX. 


of  clot,  16;  relations  of,  with  the 
plasma,  27 

Corpuscles  of  blood,  cartilage,  presence  of 
glycogen  in,  574  ;  colostrum,  614 

,  red  and  white,  relative  proportions, 

36;  composition,  37-48 

,  red,  microscopic    appearance,    31; 

structure,  32;  chemical  composition, 
32;  method  of  counting,  34;  their  life 
and  death,  35;  their  destruction  in 
the  liver,  ib. ;  as  oxygen  bearers,  32, 
34,  36 ;  formed  in  red  marrow  of  bones, 
35;  their  passage  through  the  capil- 
laries, 290;  diapedesis  of,  293;  iDropor- 
tion  of  in  foetal  blood,  1125 

,  white   (see   also  Leucocytes),  their 

connection  with  clotting,  29;  appear- 
ance and  structure  of,  36,  37 ;  composi- 
tion of,  37,  48 ;  type  of  all  living  tissue, 
39,  42 ;  amoeboid  movements,  40,  137, 
290,292;  origin,  42;  work,  44;  granu- 
lation in,  45;  behaviour  in  inflamma- 
tion, 292-294  ;  their  migration,  292 

Cortex,  see  Brain,  cortex 

Corti,  organ  of,  1008,  1016;  rods  of,  1011, 
1014 ;  inner  and  outer,  1011 ;  cells  of, 
1012 

Coughing,  512 

Cowper,  glands  of,  1115 

Cramp,  abolished  by  anelectrotonus,  117 

Crassamentum,  or  clot,  15 

Cream,  613 

Cretinism,  601 

Crico-arytenoid  muscle,  the  posterior, 
1079 

Crico-thyroid  muscle,  1079 

Cricoid  cartilage,  1079 

Croaking  of  frog,  connection  of,  with 
corpora  quadrigemina,  815 

Crying,  512 

Curd,  330;  curdling  of  milk,  phenomena 
of,  ib. 

Currents  of  action,  in  a  muscle,  102;  in 
a  nerve,  107 ;  as  started  by  excitation 
of  cortex,  748 

of  rest,  in  a  muscle,  98;  in  a  nerve, 

107;  in  electrotonus,  115 

,   electrical,   constant   and  induced, 

56,  57  ;  interrupted  or  Faradaic,  62, 63; 
electrotonic,  114 

Curves,  mode  of  measuring  (footnote), 
186 

Cutaneous  sensations,  see  Sensations, 
cutaneous 

Cyanogen  compounds,  their  relation  to 

urea,  516,  598 
Cycle,  cardiac,  described,  182,  212;  dura- 
tion of  phases,  214 

Death,  a  gradual  process,  1 ;  slow  clot- 
ting of  blood  after,  27  ;  of  blood  corpus- 
cles,  36,   44;   from   failure   of   heart's 


action,  301;  from  high  temperature, 
phenomena  of,  650;  phenomena  of, 
1158 

Decidua,  formation  of,  1120;  reflexa, 
1120;  absorption  of,  1121;  vera,  1120; 
absorption  of,  1121;  serotina,  1120; 
its  transformation  into  the  placenta, 
1120;  expulsion  of,  after  parturition, 
1139 

Decussation  of  the  pyramids,  758;  of 
optic  fibres,  784 

Defaecation,  how  effected,  381 

Degeneration  of  severed  nerve,  126;  of 
muscle  after  severance  of  nerve,  ib.; 
of  constrictor  prior  to  dilator  fibres  in 
severed  nerve,  270;  of  nerve  fibres  in 
mixed  nerve,  679 ;  calcareous  and  fatty, 
1152 

Deglutition,  how  effected,  372;  different 
stages  of,  374 ;  a  reflex  act,  375 ;  move- 
ments of  cEsophagus  in,  376 

Dentition,  temporary,  1149;  permanent, 
1149 

Depressor  nerve,  281 

Despretz  signal,  70,  71 

Dextrins,  characters  of,  315 

Dextrose,  reactions  of,  315;  appeara'nce 
of,  in  liver  after  death,  562 ;  as  food  of 
muscle,  652 

Diabetes,  natural  and  artificial,  575 

Diapedesis  of  red  corpuscles,  293 

Diaphragm,  method  of  recording  move- 
ments of,  431 ;  its  movements  during 
respiration,  434;  tetanus  of,  produced 
by  stimulation  of  vagus,  478 

Diastole  of  heart's  action,  182,  185 

Dicrotic  wave,  origin  of,  232-236 

Dicrotism  in  pulse  tracings,  230;  causes 
of,  232;  less  marked  in  rigid  arteries,  234 

Diet,  average,  633;  normal,  composition 
of,  658;  need  of  the  three  classes  of 
food-stuffs  in,  660  ;  value  of  alcohol  in, 
662 ;  vegetable,  physiological  value  of, 
664,  666;  modifications  of,  with  regard 
to  size  of  body,  667  ;  to  climate,  668 ;  to 
labour,  669;  to  mental  work,  671 

"  Differential  capacity,  extreme,"  428 

Differential  manometer  of  Hiirthle,  204, 
205 

Diffusible  substances,  absorption  of,  420 

Diffusion,  laws  of,  42'1 ;  passage  of  gases 
in  the  tissues  by,  424;  in  air  of  lungs, 
425 

Digestion,  tissues  and  mechanisms  of, 
311-313;  of  living  tissues,  353;  muscu- 
lar mechanisms  of,  371;  effects  on,  of 
presence  of  bacteria,  361,  393;  main 
products  of,  412;  course  taken  by  pro- 
ducts of,  413 

,  gastric,  321;  circumstances  affect- 
ing, 327;  gross  effect  of,  388;  time 
needed  for,  389 


INDEX. 


1167 


Digestion,  pancreatic,  3.")9;  salivary,  314; 
infantine,  114(5 

Digitalis,  physiological  action  of,  542 

Dioptric  mechanisms,  834 ;  apparatus, 
simple  form  of,  839;  imperfections  in, 
870 

"Discrimination  period,"  821 

Distance,  judgment  of,  yiJ4 

Distension  of  lungs  after  birth,  cause  of, 
427 

Diuretics,  their  mode  of  action,  542 

Diurnal  curves  of  functions,  lloG 

Division  of  labour,  physiological,  (! 

Dog,  pancreas  of,  341 ;  submaxillary 
gland  of,  338;  submaxillary  nerve, 
supply  to,  334;  succus  eutericus  of, 
3()3;  nerves  of  alimentary  canal,  384; 
saliva  of,  387  ;  composition  of  haemo- 
globin in  blood  of,  450;  cortical  motor 
area  in,  740 

Dropsy,  character  of  lymph  in,  401 

Drowning,  493 

Ductus  venosus,  foetal  blood  carried  to 
the  heart  by,  1132;  arteriosus,  foetal 
circulation  through  the,  1132;  oblitera- 
tion of,  after  birth,  1135 

Dudgeon's  sphygmograpli,  221 

Dura  mater  (of  the  eye),  839 

Dyspnoea,  at  high  altitudes,  405 ;  nature 
of,  485,  48();  cardiac,  50!';  its  effect  on 
the  kidney,  529;  sweating  caused  by, 
557 

Ear,  structure,  980-986 ;  embryonic  his- 
tory of,  980;  otic  vesicle,  980;  general 
relation  of  parts,  981 ;  cochlea,  982, 
983;  general  use  of  parts,  986;  tym- 
panum, conduction  of  sound  through, 
98!l;  muscles  of,  993;  auditory  ossicles, 
986 ;  Eustachian  tube,  995 

of  rabbit,  vaso-motor  control  of  cir- 
culation in,  203 

Egg-albumin,  coagulation  of,  323;  its 
conversion  into  acid  albumin,  324 

Elasticity,  diminished,  in  exhausted  mus- 
cles, 130;  of  arteries,  as  affecting  cir- 
culation, 164;  as  affecting  dicrotism, 
234;  of  lungs,  amount  of  pressure  ex- 
erted by,  427 

Electric  stimuli  described,  50;  electric 
changes  during  muscle  contraction,  97  ; 
in  ii  nerve  impulse,  107  ;  electric  si)ark, 
vision  by  illumination  of,  881;  electric 
currents,  their  development  by  retinal 
processes,  928 

Electrotonic  currents,  114 

Electrototuis,  features  of,  112 

Embryo  of  mammal,  undifferentiated 
protoplasm  in,  3o;  development  of  red 
corpuscles  in,  35;  glycogen  in  muscles 
of,  92,  573;  growth  of  embryo  of  mam- 
mal, 1120;  respiration  of,  1123;  nutri- 


tion of,  1123-1135 ;  supply  of  oxygen  to 
the,  1123;  development  of  adipose  tis- 
sue in,  006 
Emetics,  various,  action  of,  380 

Emission  of  semen,  1117:  the  striated 
muscles  concerned  in,  1118;  the  ner- 
vous centre  for,  1118 

Emotio!is,  as  affecting  respiration,  476; 
respiratory  mechanism  a  means  of  ex- 
pressing, 511 ;  their  effect  on  secretion 
of  urine,  543 ;  of  saliva,  335 ;  on  splanch- 
nic functions,  817 

Emulsion  of  fats,  action  of  bile  and  pan- 
creatic juice  on,  358,  391,  417 

End-plates  of  nerves,  probable  action  of 
urari  on,  55 

Endolj^mph  of  semicircular  canals,  in 
relation  to  coordinate  movements,  732; 
secretion  of,  in  otic  vesicle,  981 

Energy,  potential,  of  bodies,  living  and 
dead,  1;  set  free  by  breaking  down  of 
living  substance,  ib. ;  of  living  body 
expended  in  work,  2,  630;  of  dead 
body  shewn  as  heat,  3;  renewed  and 
set  free  by  different  tissues,  0;  energj' 
of  the  body,  income  of,  (532;  expendi- 
ture of,  634;  potential  energy  of  vari- 
ous diets,  632,  058 

Entoptic  phenomena,  874 

Epididymis,  action  of  in  emission,  1117 

Epiglottis,  the,  1073  ;  cushion  of  the,  1075 

Epithelium,  ciliated,  134 

cells,  their  action  in  absorption,  421 : 

renal,  secretion  by  the,  533 

Equilibrium,  nitrogenous,  026;  sense  of, 
729 

Erect  posture,  how  preserved,  1102 

Erectile  tissue,  structure  and  action  of, 
1115 

Erection  of  penis,  see  Penis 

Eructation,  composition  of  gases  of,  390 

Erythrodextrin,  315 

Eserin,  pupil-contraction  caused  by,  866 

Evaporation,  temperature  of  body  regu- 
lated by,  555 

Eupuoea,  485 

Eustachian  valve  in  adult  life,  182;  in 
foetal  life,  1132 

tube,  984,  996 

Excretin,  a  f;\ical  constituent,  397 

Excretion,  tissues  of,  8 

Exercise,  effect  of,  on  the  muscles,  128- 
130;  on  vascular  mechanism,  304,  305: 
on  respiration,  508  ;  on  the  secretion  of 
urea,  f)38;  on  the  production  of  heat, 
()44;  production  of  carbonic  acid  in- 
creased by,  637;  visual  discrimination 
increased  by,  890;  tactile  perceptions 
increased  by,  1040 

Exhaustion  of  muscle  and  nerve  tissue, 
125, 129, 130;  of  muscles,  129;  auditory 
effects  of,  1002 


1168 


INDEX. 


Expiration,  how  effected,  437 

Extractives,  various,  of  spleen  pulp,  583; 
of  the  thymus,  602 ;  their  value  in  diet, 
631,  662 

Eye,  the,  nature  of  movements  caused  by 
stimulation    of    cortex,    790;    general 
structure,   835 ;    development   of,   838 
changes  in  during  accommodation,  851 
sclerotic  coat,  837 ;  choroid  coat,  837 
lens,   changes,   of    curvature   of,   844 
humour  of  vitreous,  837,  974;  aqueous 
humour,  839,  972  ;  diagrammatic,  842 

,  accommodation  of,  846-853;  for  far 

and  near  objects,  846;  changes  during 
855;  mechanism  of,  854;  mechanism 
nervous,  868 ;  associated  movements  in 
869 ;  imperfections  of,  870 

,  constrictor  influences  on,  857-862 

dilator  influences  on,  863-866;  emme 
tropic,   849,    870;    myopic,     850,     870 
hypermetropic,  850,  870;    presbyopic, 
850 

,  pupil,  changes  of,  857-869 ;  constric- 
tion and  dilation  of,  857;  nerves  sup- 
plying, 859;  constriction  of,  a  reflex 
act,  860;  changes  in  through  action 
of  cervical  sympathetic,  862 ;  nature  of 
dilating  mechanism,  863;  action  of 
drugs  and  other  agencies  in,  866 

,  retina,  development  of,  835 ;  a  part 

of  the  brain,  837 ;  rods  and  cones,  1111 ; 
function  of,  925;  possible  differences 
between,  927;  rods,  presence  of  visual 
purple  in  the,  924 ;  pigment  epithelium, 
926 ;  stimulation  of,  by  other  agencies 
than  light,  884;  visual  areas  of,  889; 
intrinsic  light  of,  904 ;  colour  blindness 
of  periphery,  913;  blind  spot  of,  916, 
960;  photochemistry  of,  922;  corre- 
sponding or  identical  points,  942,  957 ; 
lines  of  separation,  944 

,  retinal  structures,  fatigue  of,  919; 

retinal  processes,  electric  currents 
developed  by,  928;  muscles,  ocular, 
948 ;  nutrition  of,  969-975 ;  arrangement 
of  blood  vessels,  969;  vaso-motor 
changes  in,  970;  lymphatics  and  lymph- 
spaces,  970;  protective  mechanisms, 
976;  eyelids  and  their  muscles,  976; 
conjunctiva  and  its  glands,  977;  Mei- 
bomian and  lachrymal  glands,  978 ;  eye 
in  old  age,  1153 

Eyeball,  rotation  of,  939,  945;  move- 
ments of,  944;  primary  position  of,  945, 
946;  muscles  of,  948;  simultaneous 
movements,  952 

Faeces,  composition  of,  396 

Fainting,  a  result  of  cardiac  inhibition, 

253,  302 
Fallopian  tube,  1112-1123;    reception  of 

ovum  by  the,  1112 


Falsetto  voice,  1089 

Faradization,  63 

Fat,  its  presence  in  chyle,  402 ;  amount 
absorbed  during  digestion,  413;  mode 
of  absorption,  417;  formation  of,  571, 
606,  628 ;  history  of,  604-609 ;  increase 
of,  in  cell-substance,  604;  disappear- 
ance of,  from  cells,  605;  nature  of,  in 
adipose  tissue,  606 ;  limits  to  construc- 
tion of,  609;  its  potential  energy  as 
food,  633 

Fat-cells,  structure  of,  604 

Fatigue,  its  efi^ect  on  muscular  irrita- 
bility, 122,  129,  259;  sense  of,  its  na- 
ture, 129;  retinal,  negative  images 
produced  by,  935 :  auditory,  1002 

Fats  in  white  corpuscles,  40;  in  blood, 
47 ;  in  nerve  tissue,  105 ;  in  food-stuffs, 
312;  in  chyle,  403;  action  of  gastric 
juice  on,  321,  388;  of  bile,  358;  of  pan- 
creatic juice,  362;  emulsification  of, 
during  digestion,  358,  391,417;  course 
taken  by,  in  digestion,  413, 417  ;  change 
of,  in  the  lacteal  radicle,  418 ;  various, 
melting-points  of,  606 ;  various,  of  milk, 
613;  as  food,  metabolism  lessened  by, 
616,  627 

Fattening,  aids  to,  668 

Feet,  sweating  in  dogs  and  cats  only 
present  in  the,  556 

Fehling's  fluid,  as  test  for  dextrose,  315 

Fenestra  ovalis  and  fenestra  rotunda  of 
ear,  985,  986 

Ferment,  fibrin,  efficient  cause  of  coagu- 
lation, 24;  its  action  on  fibrinogen, 
26 ;  the  amylolytic,  in  saliva,  318 ;  the 
amylolytic,  destroyed  by  gastric  juice, 
321 

Ferments,  organized  and  unorganized, 
(footnote) ,  318 ;  their  presence  in  urine, 
519 

Fever,  metabolism  heightened  by,  649 

Fibres,  muscular,  see  Muscle 

nerve,  see  Nerves 

of  brain,  see  Brain 

Fibrin,  of  the  blood,  15;  its  development 
during  clotting,  16 ;  its  proteid  nature, 
17 ;  structure,  18 ;  causes  of  its  appear- 
ance, 20;  action  of  gastric  juice  on, 
323,  326;  in  clotting  lymph,  400 

Fibrin-ferment,  24,  26 

Fibrinogen,  its  precipitation  from 
plasma,  23 ;  its  conversion  into  fibrin, 
25,  26,  29,  30 

Fick,  spring-manometer  of,  219,  220;  his 
pneumatograph,  430,  431 

Fingers,  clubbed,  in  phthisis,  656 

Flatulence,  390 

Flavours,  sense  of  smell  appealed  to  by, 
1029 :  localization  of  seat  of  perception 
of,  lO.'.S 

Flourens,  '  noeud  vital '  of,  473 


INDEX. 


11C9 


Fluid,  serous,  23 ;  its  identity  with  lymph, 
402;  iu  diet,  602;  amniotic,  1127;  its 
functions,  1127;  composition,  li;Jl 

Fluidity  of  living  blood,  26;  of  blood  in 
the  vessels  after  death,  27 

FcEtus,  nourishment  and  respiration  of, 
1123;  swallowing  movements  executed 
by,  1127,  1130;  transmission  of  food- 
material  to  the,  1127;  growing  differ- 
entiation of  tissue  in,  112'J ;  movements 
of,  1130;  digestive  functions,  1130; 
circulation  in,  1132;  expulsion  of,  1139 

Follicle,  Graafian,  1123 

Food,  amoeboid  absorption  of,  3;  carried 
to  the  tissues  by  the  blood,  8 ;  its  grad- 
ual change  into  living  substance,  40; 
ingestion  of,  by  white  corpuscles,  40, 
44;  its  effect  on  vascular  mechanism, 
300;  effect  of  its  presence  on  gastric 
secretion,  338;  on  pancreatic  secretion, 
306 ;  on  1)ile  secretion,  367 ;  on  stomach 
movements,  383;  on  intestinal  move- 
ments, 386;  as  acted  on  by  saliva,  314; 
by  gastric  juice,  321 ;  by  bile,  357 ;  by 
pancreatic  juice,  362;  by  succus  en- 
tericus,  363;  changes  of,  in  the 
alimentary  canal,  387;  as  income  com- 
pared with  output  of  material,  022; 
potential  energy  supplied  by,  032 

Food-stuffs,  classification  of,  311 ;  changes 
of,  in  the  body,  513;  relative  digesti- 
bility of,  564;  fatty  and  carbohydrate, 
627;  peptones  and  salts,  r)29-030;  vari- 
ous, iu  normal  diet,  658-000 

Foramen  ovale,  course  of  foetal  circula- 
tion through  the,  1132;  gradual  occlu- 
sion of  the,  1135 

Fornix  conjunctiva,  977 

Fovea  centralis,  the,  888;  region  of  dis- 
tinct vision  in,  887 

Freezing,  its  effect  on  muscle,  89 

Frey  and  Ki-ehl's  manometer,  196,  197 

Friction,  peripheral,  as  affecting  circu- 
lation, 163,  166 

Frog,  rheoscopic,  102 ;  capillary  circula- 
tion in,  161 

,   brainless,   phenomena  shewn    by, 

55,  6(),  74,  144,  145 

,  lymph-hearts  of,  405;  winter  stor- 
age of  hepatic  glycogen  in,  5()6 

Fuscin,  its  i^resence  in  the  retina,  922 

Gad,  manometer  of,  190 

Gall-bladder,  changes  in  bile  effected  by 
the,  354;  storage  of  bile  in  the,  3(i7 

Gallstones,  cholesterin,  present  in,  105; 
composition  of,  .3(>4 

Galvanic  battery  described,  'i7 

Ganglia,  spinal,  141 :  of  splanchnic  sys- 
tem, 141 ;  spinal,  of  the  posterior  root, 
678 

Ganglion  stellatum,  253-255 
74 


Gases,   absorption   of,  by  liquids,   447; 
their  presence  in  blood,   447,  504;  in 
urine,  .520;    various,   their   effects  on 
respiration,  494 
Gaskell,  his  method  of  recording  heart- 
beat, 240 
Gastric  juice,    normal    composition    of, 
320 ;  the  amyloly tic  ferment  destroyed 
by,  321 ;  its  action  on  fats,  ib. ;  artifi- 
cial, how  prepared,  321,  322;  its  action 
on    proteids,    323-331 ;    nature  of    its 
action,  328;  secretion  of ,  338 ;  secretion 
of,  inrtuenced  by  absorirtion  of  food, 
340;   formation  of  free  acid  in,  352; 
changes  in  its  character  as  digestion 
proceeds,  388 
Gelatin,  in    food-stuffs,  311;    action  of 
gastric  juice  on,  329 ;  its  effect  as  food, 
029 
Gestation,  human  period  of,  1137 
Giddiness,  a  result  of  disarrangement  of 

coordinating  machinery,  733 
Gland,  salivary,  venous  pulse  in,  236; 
submaxillary,  of  dog,  double  nerve 
supply  of,  207 
Glands,  albuminous,  342;  of  Cowper, 
1115;  gastric,  secretion  from,  inter- 
mittent, 338 ;  of  newt,  340 ;  of  bat,  ib. ; 
changes  in  central  cells  of,  ib. ;  lachry- 
mal, 978  ;  of  Lieberkiihn,  303  ;  lym- 
phatic, multiplication  of  leucocytes  iu, 
42;  mammary,  structure  of,  010;  mam- 
mary, at  birth,  015;  Meibomian,  978; 
of  Moll,  978 ;  storage  of  granular  mat- 
ter in,  ;>48;  oxyntic,  of  frog,  352;  paro- 
tid, double  nerve  supply  to,  338;  paro- 
tid, changes  of,  during  secretion,  342; 
salivary,  venous  jjulse  in,  230 ;  submax- 
illary, of  dog,  double  nerve  supply  of, 
267,  333,  338;  effect  of  stimulation  of 
chorda,  336;  effect  of  cervical  sympa- 
thetic, 337:  cell  changes  in,  344;  ther- 
mal changes  in,  639 
Globin,  the  proteid  constituent  of  haimo- 

globin,  460 
Globulins,  a  group  of  proteids,  19;  their 
changes  to  acid-  and  alkali-albumin, 
88 
Glomeruli,  the,  secretion  by,  535;  special 
substances  excreted  by,  534;  effect  of 
blood-pressure  on  the,  536 ;  complexity 
of  their  action,  538 
Glossopharyngeal  nerve,  335 
Glottis,  the,  1073-1075 :  changes  in,  during 
utterance  of  voice,  1075,  1080 :  narrow- 
ing and  widening  of  the,  1080 
Glycerin,  its  effect  on  the  hepatic  cell, 

578 
Glycin,  a  product  of  metabolism,  539, 586 
Glycocholic  acid,  ."Sti 
Glycogen,  its  jiresence  in  white  corpus- 
cles, 38;  in  muscle  substance,  92,  573; 


1170 


INDEX. 


in  plain  muscle,  131 ;  in  embryonic, 
573  ;  in  hepatic  cells,  566;  in  the  pla- 
centa, 574,  1122,  1129;  in  the  testis, 
1115  ;  in  the  fostus,  1129 

Glycogen,  characters  of,  561;  its  conver- 
sion hy  the  liver  into  sugar,  562  ;  stor- 
age of,  in  the  liver,  565,  570 ;  manufac- 
ture of,  769,  770;  winter  storage  of 
hepatic  of,  in  frog,  566 

Golgi,  organ  of,  lOiM 

Goll,  column  of,  68-1 

Goltz  and  Gaule,  their  maximum  manom- 
eter, 210 

Gout,  accumulation  of  uric  acid  in  the 
blood  in,  596 

Graafian  follicle,  1123 

Granules  in  white  corpuscles,  37,  40 ;  of 
resting  glandular  cells,  343;  of  resting 
albuminous  cells,  ib. ;  of  resting  mu- 
cous cells,  344;  in  leucocytes,  419;  in 
hepatic  cells,  566 

Growth,  human,  curve  of,  1145 

Griitzner's  method  of  preparing  fibrin, 
327 

Guanin,  presence  of  in  urine,  517 

Gudden's  commissure,  785 

Gustatory  sensations,  see  Sensations, 
gustatory 

Gymnema  sylvestre,  gustatory  sensa- 
tions affected  by,  1033 

Hsemacytometer  described,  34 
Haemadromometer  of  Volkmann,  172 
Hjematachometer  of  Vierordt,  173 

of  Chauveau  and  Lortet,  174 

Haematin,  32 ;  its  relations  with  bilirubin, 
34,  584;  oxygen-holding  power  of,  460; 
iron-free,  460, 584 
Hsematoblasts,  development  of,  43 
Hffimatoidin,  585 
Hfematoporphyrin,  585 
Hsemin,  crystals  of,  460 
Hsemoglobin,  32,  450;  an  oxygen-bearer, 
34,    36,    456;    its    proportion    in    red 
corpuscles,    47;     in    red    muscle,    89; 
crystals  of,  451 ;  spectroscopic  features 
of,  451 ;  spectroscopic  features  of,  re- 
duced, 453;  reduced,  change  of  colour 
in,  454 ;  absorption  of  oxygen  by,  455  ; 
its  combination  with  gases  other  than 
oxygen,  457 :  products  of  decomposition 
of,  458;  its  respiratory  functions,  460, 
510;  its  relation  to  bilirubin,  34,  584; 
of  fcEtal  arterial  blood,  1125 
Haemorrhage,  its  effect  on  blood-pressure, 

296 
Haidinger's  brushes,  877 
Hallucinations,    ocular,    937 ;    auditory, 

1020 
Head-voice,  how  produced,  1088 
Hearing,  sensations  of,  796;  mechanisms 
of,  980;  binaural,  1022 


Heart,  visible  movements  in,  181 ; 
changes  in,  during  cardiac  cycle,  182  ; 
auriculo-ventricular  valves,  action  of, 
183;  auricular  systole,  183;  ventricu- 
lar systole,  184;  auriculo-ventricular 
valves,  action  of,  183 ;  semilunar  valves, 
action  of,  184;  change  of  form,  185; 
cardiac  impulse,  187 ;  sounds  of,  188 ; 
pressure  exerted  by  (endocardiac  press- 
ure) ,  191  ;  graphic  record  of,  193-197  ; 
negative  pressure  in,  192,  212 ;  pressure 
in  ventricle,  the  phases,  210;  duration 
of  cardiac  phases,  213,  215  ;  summary 
of  events  in,  215;  work  done  by,  217; 
sequence  of  events  in  beat  of,  241 ; 
power  of  independent  rhythm  in  the 
several  parts,  242;  characters  of  the 
contraction  of  muscular  fibres  of,  243; 
rhythmic  beating  due  to  impulses  pro- 
ceeding from  nerve  cells  of  the  ganglia, 
243 ;  inhibition  of  beat  of  in  frog,  245 ; 
in  mammal,  239;  augmentation  of  beat 
of  in  frog,  246-250;  in  mammal,  250; 
inhibitory  and  augmentor  fibres  in 
frog,  245,  247;  in  mammal,  250-253; 
inhibition,  reflex  of,  249;  centre  of 
inhibition  of  (cardio-inhibitory  centre) , 
250 ;  inhibition  and  augmentation  of 
beat,  nature  of,  256 ;  inhibition  of,  sus- 
pended by  atropin,  257 

Heart-beat,  regulation  of,  238;  intrinsic 
regulation  of,  300,  712;  development  of 
normal,  181 ;  influences  other  than 
nervous  affecting,  260;  relations  of 
with  vaso-motor  system,  306;  normal 
rate  of,  297 ;  slowing  effects  of  venous 
blood  on,  505;  during  asphyxia,  507; 
of  the  babe,  1146;  death  marked  by 
cessation  of,  1159 

Heat,  given  out  by  contracting  muscle, 
95 ;  loss  of  energy  in  the  form  of,  636 ; 
bodily,  measurement  of,  635 ;  sources 
and  distribution  of,  638 ;  modes  of  loss 
of,  640;  regulation  of,  by  variations  in 
loss,  641 ;  production  of,  643 ;  increased 
by  labour,  644;  regulation  of,  by  the 
nervous  system,  645;  increased  pro- 
duction of,  by  injury  to  parts  of  the 
brain,  647;  great,  effects  of,  649 

Heat  and  cold,  sensations  of,  1041-1043; 
separate  terminal  organs  for,  1056- 
1058;  epidermal  seat  of  sensations  of, 
1057 

Helmholtz,  magnetic  interrupter,  04 

Hemianopsia,  789 

Henle,  sphincter  of,  545 

Hepatic  cells,  changes  of  ,566;  glycogen  in, 
567  ;  vein,  temperature  of  blood  in,  640 

Hering,  his  theory  of  colour  vision,  900, 
902  ;  colour  blindness  explained  by, 
909 ;  as  to  simultaneous  and  successive 
contrasts,  935 


INDEX. 


1171 


Hermann  on  muscle  currents,  101 

Hiccough, 511 

Hippuric  acid,  its  presence  in  urine,  517; 
how  formed  in  the  kidney,  53'J 

Hopping,  how  effected,  1105 

Horopter,  the,  957 

Horse,  sweat  of,  551 

Humour,  aqueous,  972;  how  furnished, 
972 ;  vitreous,  971 

Hunger  and  tliirst,  sensations  of,  1048 

HLirthle,  membrane  manometer  of,  194- 
196;  tracings  of  ventricular  and  aortic 
pressure  by  apparatus  of,  20o,  204,  208, 
20'.),  233 ;  differential  manometer  of, 
205 ;  tambour  sphygmoscope  of,  220 

Hydrochloric  acid,  free,  in  gastric  juice, 
322 

Hydrogen,  evolution  of,  in  small  intes- 
tine, 395 

Hyperpncea,  485 

Hj-poxanthin,  i^resence  of,  in  urine,  517 

Illusions,  visual,  9G1 ;  tactile,  1069 

Images,  retinal,  formation  of,  839-845; 
in  relation  to  sensations  excited  by, 
845;  entoptical,  876 

Impulses,  nervous,  54,  106;  electrical 
changes  accompanying,  107 ;  cardiac, 
209 ;  mode  of  recording,  213 ;  afferent 
and  efi'erent,  67G;  their  paths  along 
the  cord,  800,  801, 810;  relays  in  course 
of,  802,  810;  crossing  of,  804;  sensory, 
different  paths  for  different,  805 ;  trans- 
mitted by  grey  matter  and  internuncial 
tracts,  808 ;  ampullar,  732 ;  motor,  effect 
of  efferent  impulses  on  the  coordina- 
tion of,  733,  955;  visual,  781,  834,  908; 
auditory,  how  excited,  980,983;  audi- 
tory, development  of,  980,  986;  voli- 
tional, time  required  for  transmission, 
774  ;  volitional,  course  of,  in  man  along 
the  pyramidal  tract,  775 

Impurities  in  expired  air,  442 

Income  and  output  of  material  in  nutri- 
tion, 622 

Incus,  the,  986 

Indol,  a  product  of  bacterial  action,  361, 
393 

Induction  coil,  construction  of,  59 

Infancy,  characteristics  of,  1148 

Inflammation,  phenomena  of,  290;  oedema 
due  to,  410 

Infusoria,  ciliary  motions  in,  136 

Inhibition,  cardiac,  phenomena  of,  245 
et  supi'H  :  fainting  a  result  of,  253, 
302;  effect  of  atropin  on,  257 

of  secretion  of  saliva,  335;  of  respi- 
ration, 479;  of  reflex  actions,  707 ;  of 
parturition,  1142 

Inhibitory  nerves,  148;  fibres  in  vagus 
of  frog,  248;  in  vagus  of  mammal, 
250;  cardiac  inhibitory  fibres,  contin- 


uous action  of,  253 ;  their  analogy  with 
vaso-dilator  fibres,  269 

Inspiration,  mechanism  of,  433;  move- 
ments of,  434;  laboured,  phenomena 
of,  436 

Intercostal  muscles,  their  work  in  respi- 
ration, 4;i5 

Intermediate  line,  in  muscle  fibre,  83 

Intermittence,  cardiac,  260,  301 

Interruptor,  magnetic,  63 

Intestine,  absorption  of  fats  in  the,  417 ; 
absorption  of  diffusible  substances  and 
water,  420 

,  large,  movements  of,.  381 ;  changes 

of  food  in,  395;  digestion  of  cellulose 
in  the,  396 

,  small,  movements  of,  380;  changes 

of  food  in  the,  390,  391 ;  fermentative 
changes  of  food  in  the,  394 ;  fluidity  of 
food  maintained  in  the,  422 

"  Intrinsic  light  "  of  retina,  904 

Iodine,  coloration  of  starch  by,  316 

Iris,  development  of  the,  837 ;  muscular 
and  vascular  changes  in  the,  857 

Iron,  its  presence  in  hfematin,  460;  in 
hajmoglobin,  450;  in  bile,  355,  585;  in 
the  spleen,  582 

Irradiation,  932 

Irritability,  muscular  and  nervous,  53-80 ; 
their  mutual  independence,  53,  54; 
diminution  and  disappearance  of,  after 
death,  80,  81 ;  as  affected  by  electro- 
tonus,  112;  circumstances  determining, 
125 ;  centrifugal  loss  of,  in  severed 
nerve,  126;  influence  of  temperature 
on,  127;  influence  of  blood-supply  on, 
128 ;  influence  of  functional  activity  on, 
ib. ;  presence  of  oxygen  a  condition  of, 
130 ;  prolonged,  of  heart,  242 

Irritants,  inflammatory  action  of  on 
tissues,  291 

Islets,  extra-vascular,  149 

Jaundice,  how  caused,  370,  587 
Judgments    of   distance    and  size,   how 

formed,  96.^;  of  solidity,  9()6 
Juice,  gastric,  see  Gastric  juice 

,  pancreatic,  see  Pancreatic  juice 

,  intestinal,  see  Succus  eutericus 

Jumping,  how  effected,  1105 

Katabolic  changes  in  living  tissue,  39, 41 ; 

heat  liberated  by,  638 
Katelectronus  defined,  113 
Kathode  or  negative  electrode,  57 
Key,  galvanic,  various  forms  of,  58 
Kidney,  the,  duplexify  of  its  mechanism, 
524;    vaso-motor   mechanisms  of,  525; 
relations,   various,   of    llow    of    blood 
through,  528;    vaso-constrictor  nerves 
of,  530;  effect  of  chemical  changes  in 
the  blood,  531 ;  secretion  by  the  renal 


1172 


INDEX. 


epithelium,  533;  double  vascular  sup- 
ply to,  in  amphibia,  533 ;  work  of  the 
epithelium  of  the  tubules,  538 ;  kidney 
and  skin,  mutual  relations  of  secretory 
activity  of,  541 ;  its  relations  to  water 
absorbed  by  the  alimentary  canal,  541 ; 
influence  of  central  nervous  system  on, 
543;  fcetal,  urea  secreted  by,  1131 

Kilogram-meters,  daily  work  of  heart 
estimated  in,  218;  energy  of  food  and 
body  and  day's  work  estimated  in,  634 

Knee-jerk,  705,  717 

Kreatin,  its  presence  in  the  blood,  47 ; 
chemical  composition  of,  93;  in  un- 
striated  muscle,  131 ;  the  product  of 
muscle  metabolism,  590 

Kreatinin,  its  presence  in  urine,  517 ;  the 
urinary  form  of  kreatin,  591 ;  diffi- 
culties presented  by  its  presence  in 
urine,  591 

Kymograph,  Ludwig's,  for  recording 
blood-pressure,  160 

Labour,  physiological  division  of,  6;  cir- 
cumstances governing  capacity  for, 
510 ;  increased  production  of  heat  from, 
644 

"  Labour,"  the  events  of,  1137 ;  first  stage 
of,  1137;  second  stage  of,  1138;  causes 
determining  its  onset,  1142 

Labyrinth  of  ear,  bony  and  membranous, 
983;  perilymph  cavity  of,  987;  connec- 
tions of  auditory  nerve  with,  983;  the 
cochlea,  982;  vestibular,  parts  of,  1007, 
1008;  probable  functions,  1017;  trans- 
mission of  impulses  through  the,  1007 

Lachrymal  gland,  structure  of,  978 

.Lactalburaiu,  612 

Lactation,  nervous  centre  for,  618 

Lacteal  radicle  of  intestinal  villus,  pas- 
sage of  fat  into,  413,  418 

Lacteals,  the,  absorption  by,  398;  chyle 
contained  by,  in  fasting  animals,  402; 
passage  of  products  of  digestion  into, 
413 

Lactic  acid,  its  presence  in  the  blood,  48 ; 
isomeric  variations  of  (footnote),  90; 
its  effect  on  the  heart,  259;  fermenta- 
tion, 394;  a  product  of  muscular  me- 
tabolism, (J53 

Lactoprotein,  612 

Lactose,  ready  fermentation  of,  613;  its 
formation  in  the  mammary  gland,  617 
"  Laky"  blood,  how  formed,  31 
Laryngeal  nerves,  1081 
Laryngoscope,  larynx  as  seen  by  the,  1070 
Larynx,  the,  its  condition  in  respiration, 
438;  cartilages  of,  1073,   1078;  ventri- 
cles,  uses  of,  1091 ;   muscles  of,  1076, 
1080;    nervous   mechanisms   of,    1082; 
respiratory  movements  of,  1082;  corti- 
cal area  for  movements  of,  1085 


Laughter,  mechanism  of,  512 

Lecithin,  in  stroma  of  red  corpuscles,  32; 
in  white  corpuscles,  38;  in  the  blood, 
48 ;  in  muscle  substance,  92 ;  in  nervous 
tissue,  105;  in  milk,  613;  its  composi- 
tion, 105 

Lens,  the,  development  of,  837;  mech- 
anisms for  changing  curvature  of,  854 ; 
action  of  the  suspensory  ligament  on, 
855 

Leucin,  composition  of,  361 ;  in  intestinal 
contents,  393 ;  a  product  of  nitrogenous 
metabolism,  594;  its  conversion  into 
urea,  595 

Leucocytes,  in  the  lymphatic  system,  42 ; 
their  origin,  43;  their  presence  in  the 
villi,  419;  among  epidemic  cells,  554 

Leucocythjemia,  increase  of  white  cor- 
puscles in,  44 

Levatores  costarum,  their  work  in  respi- 
ration, 436 

Lieberkiihn,  glands  of  succus  entericus 
probably  furnished  by,  363;  cells  of, 
422 

Life,  processes  of,  compared  with  those 
of  death,  1 ;  its  existence  possible  with- 
out organs,  3;  periodic  events  of,  1153; 
factors  of,  1158 

Light,  as  stimulus  to  visual  apparatus, 
858,  878;   "intrinsic,"  of  retina,  904 
changes  in  retina  produced   by,  878 
sensitiveness  of  living  matter  to,  921 
decomposition  of,  891 

Listing,  diagrammatic  eye  of,  842;  his 
law,  946 

Liver,  the,  destruction  of  red  corpuscles 
in,  35;  blood-supply  to,  367;  quality  of 
as  affecting  bile  secretion,  368,  369; 
liver  of  frog,  564;  storage  of  glycogen 
in,  565.;  mammalian,  561-568;  nervous 
control  of  glycogenic  function,  575; 
"acute  yellow  atrophy"  of,  587,  594; 
presence  of  urea  in,  594 ;  conversion 
of  leucin  into  urea  in,  595 ;  heat  set 
free  in,  639;  its  action  on  lactic  acid, 
653;  fcetal,  deposition  of  glycogen  in, 
1129 

Living  substance,  food  and  waste  of,  3 

Locomotor  mechanisms,  1101 

Ludwig,  his  stromuhr,  173;  his  mercurial 
gas-pump,  444 

Lungs,  the,  their  function  chiefly  mechan- 
ical, 424;  entrance  into  and  exit  of  air 
from,  425;  air,  tidal  and  stationary  in, 
425  ;  air,  complementary,  supplement- 
ary and  i-esidual,  426;  results  of  open- 
ing into  pleural  chamber,  42(! :  condi- 
tion of,  before  birth,  427;  elasticity  of, 
pressure  exerted  by,  427 ;  respiratory 
changes  in,  462-466;  effects  of  infla- 
tion and  suction,  482;  first  inflation  of, 
1134 


INDEX. 


1173 


"  Luxus  consumption  "  of  food,  393,  G27 

Lymph,  the,  a  medium  of  exchange  be- 
tween blood  and  tissues,  13,  14,  403; 
salts  present  in,  39;  migration  of  white 
corpuscles  into,  292;  coagnlable,  in  in- 
flammation, lb.;  microscopical  charac- 
ters of,  400;  clotting  of,  400;  chemical 
composition  of,  varying,  401 ;  total 
diurnal  flow,  403;  movements  of,  403; 
its  flow  increased  by  muscular  move- 
ments, 405  ;  transudation  of,  nature  of 
the  process,  406;  its  functions  in  the 
eye,  971 

capillaries,   compared    with    blood 

capillaries,  398 

corpuscles,  400 

spaces,  passage  of  the  white  corpus- 
cles into,  43 

Lymphatic  arrangements  of  brain  and 
cord,  824 

Lj-mphatic  glands,  their  influence  on 
lymph,  401;  lymphatic  system,  398; 
prominence  of,  in  infancy,  1148 

Lymphatics  of  the  eye,  970-975 

Magnetic  interrupter,  C3 

Majendie,  foramen  of,  826 

Making  and  breaking  currents  and 
shocks,  57-65;  contractions  with  the 
constant  current.  111 

Male  breathing,  diaphragmatic  character 
of,  433 

Male  organs  of  reproduction,  1114 

Malleus,  the,  986 

Maltose,  315 

jMammary  gland,  610;  changes  in,  during 
secretion,  GIO;  dormant,  characters  of, 
611;  at  birth,  615;  relations  of,  to  the 
nervous  system,  (il8 

Manometer,  for  measuring  blood-press- 
ure, 156, 157:  maximum  and  minimum, 
210,  211;  endocardiac  pressure  shewn 
by,  191-197;  Gad's  manometer,  196; 
Krehl's,  ib.;  Fick's,  220;  vaso-motor 
actions  observed  by,  277 

Marey's  pneumograph,  430 ;  tambour,  192 

Marrow,  red,  formation  of  red  corpuscles 
in,  35;  yellow,  of  bones,  (505 

Massage,  metabolism  excited  by,  669 

Mastication,  how  effected,  372 

Meatus,  auditory,  external,  983 ;  internal, 
993 

Meconium,  1127;  sources  of,  1131 ;  chemi- 
cal composition,  1131 

Medulla,  loss  of,  in  vaso-constrictor 
fibres,  274;  retention  of  in  vaso-dilator 
fibres,  275 

ol)longata,  cardiac  effect  of  stimula- 
tion of,  249;  centre  for  nerves  of  taste 
in,  277;  for  vaso-motor  imimlsos  in, 
277-2S0;  for  constrictor  impulses  in, 
279-284;  for  secretion  of  saliva,   335; 


for  deglutition,  375;  for  vomiting,  380; 
for  respiration,  473;  effect  on  blood- 
pressure  of  successive  sections  of,  283; 
diabetic  area  of,  575;  see  also  Bulb 

Meibomian  glands,  978 

Melting-point  of  various  fats,  606 

Membrana  i^upillaris,  absorption  of,  be- 
fore birth,  838;  tympani,  983,  989 

Membrane-manometer  of  Hiirthle,  194- 
196,  219 

Me'niere's  disease,  733 

Menstruation,  1111-1113;  causation  of, 
1112 

Mercurial  gas-pump,  Ludwig's,  444,  445; 
Priiiger's,  445;  Alvergniat's,  446 

IMetabolic  processes  of  body,  559 

Metabolism,  defined,  39;  water  of,  41,  42; 
increased  by  exercise,  305:  by  proteid 
food,  617,  626,654;  of  muscle,  products 
of,  591;  of  nervous  tissue,  592;  of 
glands,  593;  proteid,  its  complexity, 
599;  nitrogenous,  625;  products  of, 
653;  of  muscle,  the  chief  source  of 
heat,  639;  conducted  in  the  tissues, 
651 ;  course  of  products  of,  6rj3;  nervous 
control  of,  654 ;  rapidity  of,  in  infancy, 
1147 

Metals,  retention  of,  in  the  liver,  355 

Metameres,  hj-pothetical,  of  spinal  cord, 
139,  140 

Methffimoglobin,  spectrum  of,  4(10 

Micro-organisms,  their  actions  in  diges- 
tion, 394;  in  expired  air,  442 

Micro-unit  of  heat  defined  (footnote),  96 

Micturition,  mechanism  of,  545;  nervous 
mechanism  of,  546;  centre  for,  547; 
voluntary  and  involuntary,  548 

Migration  of  the  white  corpuscles,  43;  in 
inflammation,  292;  aided  by  changes 
in  vascular  walls,  294 

Milk,  action  of  gastric  juice  on,  327,  329; 
of  rennet  on,  330;  double  mode  of 
secretion  of,  616;  nature  of,  612:  con- 
stituents of,  613;  uterine,  1122,  1129 

Millon's  reagent  for  detection  of  protein, 
17 

Mitral  valves,  their  action,  185 

Molecular  basis  of  chyle,  403;  where 
elaborated,  418 

Moll,  glands  of,  978 

Morse  key,  59 

Movements  in  living  bodies,  2;  amoe- 
boid, 3(),  37,  137,  292;  of  body,  how 
accomplished,  51 ;  ciliary,  52, 134  ;  mus- 
cular, lieat  given  out  during,  95,  644; 
cardiac,  visible,  181;  of  alimentary 
canal,  371-387;  gastric.  383;  intestinal, 
ib.  ;  amoeboid,  of  lymph  corpuscles, 
400:  Brownian,  in  chyle  globules,  403; 
muscular,  flow  of  lymph  increased  by, 
404;  respiratory,  428;  bilateral,  760; 
coordinating    machinery    of,    729;    of 


1174 


INDEX. 


cortical  origin,  how  effected,  753;  of 
dog,  740,  772;  of  monkey,  744,  763; 
of  anthropoid  ape,  749 ;  voluntary,  739- 
780 ;  action  of  motor  area  in  effecting, 
769;  as  influenced  by  sensory  impulses, 
772;  skilled,  correlation  of  with  pyra- 
midal tract,  760;  'forced,'  735,  813; 
'forced,'  from  injury  to  optic  lobes  in 
frog,  815;  foetal,  1130;  of  locomotion, 
1101 ;  ocular,  949;  sense  of,  1059 
Mucin,  a  constituent  of  saliva,  313 
Mulberry  gallstones,  364 
Muscse  volitantes,  875 
Muscarin,  its  action  on  cardiac  tissue,  257 
Muscle,  irritability  of,  53  et  supra ;  phe- 
nomena of  contraction  of,  65-134;  te- 
tanic contraction  of,  75-81 ;  gross 
structure  of,  82 ;  wave  of  contraction, 
83;  striated,  86;  striated  under  polar- 
ized light,  ih. ;  chemistry  of,  ih. ;  living 
and  dead,  contrasted,  86,  87;  dead, 
chemistry  of ,  87-95 ;  frozen,  89;  rigid, 
acid  reaction  of,  90 ;  living,  reaction  of, 
91 ;  chemical  changes  due  to  contrac- 
tion, 94 ;  thermal  changes  due  to  con- 
traction, 95 ;  electrical  changes  in,  97 ; 
action  of  the  constant  current  on, 
110-116 ;  work  done  by,  as  influenced  by 
fatigue,  122,  129;  by  load,  123;  by  size 
and  form  of  muscle,  124;  by  tempera- 
ture, 127;  by  blood-supply,  128;  by 
functional  activity,  ib. ;  oxygen  con- 
sumed during  contraction  of,  304 ;  plain, 
structure  of,  131;  chemistry  of,  ib.; 
characters  of  contraction  of,  131,  132; 
spontaneous  contraction  of,  133;  tonic 
contraction  of,  134 ;  nutrition  of,  128 ; 
vascular  changes  in,  271;  changes  due 
to  contraction  of,  304;  vaso-dilator 
fibres  in  nerve  supply  to,  269;  embry- 
onic, glycogen  in,  573 ;  respiration  of, 
469 ;  governance  of  nutrition  of,  714 

Muscles,  skeletal,  result  of  metabolism 
in,  590;  their  proportion  in  body,  619; 
tone  of,  713;  rigidity  of,  717;  their 
mode  of  action,  1101 

Muscle-currents,  98-102;  velocity  of ,  99 ; 
negative  variation  of,  102 

Muscle-curves,  65 ;  analysis  of,  71 ;  vari- 
ations of,  74 ;  tetanic,  75 

Muscle-nerve  preparation,  55-81,  102 ;  as 
a  machine,  119;  muscle  current  shewn 
in,  102,  103 

Muscle-plasma,  89 

Muscle-serum  and  clot,  89 

Muscle-sound,  122,  123 

Musical  sounds,  character  of,  998 

Myocardiogranis,  202 

Myoglobulin,  89 

Myograph,  66;  pendulum,  68 

Myosin  in  dead  muscle,  87,  88 ;  in  white 
corpuscles,  38 


Myosinogen  in  living  muscle,  90 
Myxosdema,  its  connection  with  disease 
of  thyroid,  601 

Nasal  passages,  inspired  air  warmed  in 

the,  438 
Nausea,  sensation  of,  1049 
Negative  pressure  in  heart,  210,  211,  217 
Nerves,    irritability    of,    53    et    supra; 
tested  by  constant  current,  118;   end- 
plates  of,  probable  action  of  urari  on, 
55 ;  connection  of  with  muscular  fibre, 
82 ;  chemistry  of,  105 ;  mixed,  141 ;  in- 
hibitory,   148 ;     vaso-motor,    262-288  ; 
specific  energy  of,  1052 ;  special  sensa- 
tions   not    caused    by   stimulation   of 
trunk,    1053;    abdominal    splanchnic, 
141 ;    vaso-constrictor    fibres    in,    262, 
272,  277,  385  ;  inhibitory  fibres  in,  385 

of  alimentary  canal,  383,  384 

,    brachial    plexus,    constrictor  and 

dilator  fibres  in,  270,  271 

,  cardiac,  253 

,  cervical  sympathetic,  cardiac  aug- 

mentor  fibres,  in  frog,  247-249;  fibres 
of,  to  salivary  glands,  338 ;  vaso-motor. 
fibres  in,  263,  269,  278;  pupil-dilating 
action  of,  862 

,  chorda  tympani,  vaso-motor  fibres 

in,  267,  269;  secretory  fibres  to  sub- 
maxillary gland,  333,  336,  351 ;  its  con- 
nection with  sense  of  taste,  1035 

,  depressor,  vaso-motor  functions  of, 

280,  281 

of  eye-ball,  859 

,  glosso-pharyngeal,  335;  its  connec- 
tion with  sense  of  taste,  1035 

of  larynx,  1081 

,  lingual,  333,  334;  its  connection  with 

sense  of  taste,  1035 

,    optic,   decussation    of,    in    optic 

chiasma,  784 ;  development  of,  835 

,  phrenic,   functions  in    respiration, 

472 

,  sacral,  regulation  of  bladder  action 

by,  546 

,  sciatic,  constrictor  and  dilator  fibres 

in,  269-271 

,  spinal  accessory,  cardio-inhibitory 

fibres  in,  252-255 

,  spinal,  140 ;  anterior  and  posterior 

roots  of,  141,  676;  efferent  and  affer- 
ent fibres  of,  their  separate  paths,  667 

of  stomach,  339;  submaxillary,  333 ; 

thoracic,  279 

,  vagus,    inhibitory  action   of,    148; 

government  of  heart-beat  by,  in  frog, 
249,  258 ;  a  mixed  nerve,  141 ;  of  mam- 
mal, inhibitory  fibres  in,  250;  supply 
to  the  stomach,  339,  383;  to  the  in- 
testines, 383;  cardiac  augmentor  and 
inhibitory  fibres  in,  249,  268,  385  j  in- 


INDEX. 


1175 


fluence  on  respiration  of,  470 ;  on  the 
circulation,  481 
Nerve-endings,  specific  terminal  organs 
of,  for  tastes,  10.'54  ;  for  pressure,  10o7  ; 
cutaneous,  1054;  for  heat  and  cold, 
1054 
Nerve-fibres,  efferent  and  afferent,  141, 
143;  revehent,  142  ;  in  spinal  cord,  143; 
medullated  and  non-niedullated,  147; 
vaso-constrictor  and  vaso-dilator,  209; 
vaso-constrictor,  course  of,  273;  vaso- 
dilator, eourse  of,  27') ;  inhibitory  and 
augniontor,  250 ;  secretory  and  trophic, 
351 ;  nutrition  of,  ()79 
Nervi  erigentes,  vaso-dilator  fibres  of, 
278;  action  of,  on  the  rectum,  385;  ac- 
tion on  penis,  and  roots  of,  1110 
Nervous  system,  central,  centres  for 
automatic  and  reflex  actions  in,  143, 
144;  vaso-motor  functions  of,  277,  282; 
regulation  of  temperature  by  the,  045, 
047  ;  metabolism  governed  by  the,  054 

mechanism,  coordinating,  702,  729, 

737,  950 
Neurin  in  nervous  tissue,  105 
Neuroglia,  of  white  and  grey  matter  of 

cord,  (iyO 
Neurokeratin  in  nerve  medulla,  106 
Newt,  chief  cells  of  gastric  glands  in,  346 
Nitrogen  in  proteids,  17;  in  expired  air, 
441 ;    relations   of,   in  the   blood,  401 ; 
free,  inassimilable  by  living  beings,  023 
Nitrogenous    waste    not    increased    by 
muscle  contraction,  95, 97 ;  equilibrium, 
020 
"  Nttnid  vital "  of  Flourens,  473 
Noises  and  musical  sounds,   998;  char- 
acters of,  1002 
Nostrils,  their  work  in  inspiration,  438 
Notch,  dici'otic,  in  pulse  tracings,  231 
Notes,  hdw  produced  vocally,  1075,  1084 
Nuclcin,  in  Mdiite  corpuscles,  38;  a  modi- 
fied proteid,  41 ;  a  constituent  of  milk, 
()1();  of  semen,  1115 
Nutrition,  statistics  of,  610;  income  and 
output  of  material  in,  022 ;  summary  of 
phenomena  of,  051 ;  of  muscle,  052;  of 
muscle  increased  by  activity,  128;  in- 
fluences   determining,    ()54;     nervous 
control  of,  054,  707  ;  disordered,  phe- 
nomena of,  650;  of  nerve-fibres,  079; 
of  embryo,  1123,  1128 

Odours,  perception  of,  1025 ;  discrimina- 
tion of,  1026 

Qidema,  possible  causes  of,  399,  409;  in- 
flammatory, 410;  of  Brighfs  disease, 
411 

Oesophagus,  movements  of  in  deglutition, 
376 ;  force  of  contraction  in  the,  ib. 

Oil,  clotting  of  blood  prevented  by 
presence  of,  21 


Old  age,  phenomena  of,  1151 ;  degenera- 
tions characteristic  of,  1152 

Olein,  presence  of,  in  blood,  47;  a  con- 
stituent of  animal  fat,  006 

Olfactory  bulb  and  tract,  sensations, 
1025;  judgments,  1027 

Oncograph,  renal,  527 

Oncometer,  renal,  520,  830 

Ophthalmoscope,  principle  of  the,  926 

Optic  chiasma,  784 

lobes,  results  of  removal  in  frog, 

737 

nerve,  its  decussation  in  chiasma, 

785;  an  extension  from  the  brain,  837; 
optic  cup,  836;  fibres,  their  insensibility 
to  light,  917,  1052 

thalami,  results  of  removal  in  frog, 

737 

Optical  systems,  simi^le  and  complex, 
840 

Optogram,  how  obtainable,  924 

Ora  serrata,  838 

Ordeal  by  rice,  its  mode  of  action,  335 

Organs,  definition  of,  8;  terminal,  for 
sensations  of  touch  and  temperature, 
1051 ;  for  sensations  of  pressure,  1054 ; 
for  sensations  of  heat  different  from 
those  for  sensations  for  cold,  1055; 
nature  of,  1057 

Organs  of  reproduction,  female,  1111- 
1113;  male,  1114-1118 

Os  uteri,  expansion  of  during  'labour,' 
1137 

Ossicles,  auditory,  986;  attachments  of, 
986;  conduction  of  -sibrations  through, 
989 

'  Output '  of  blood  by  ventricle,  197 ; 
increased  by  augmentor  action,  256 

Ovum,  escape  of  the,  1111 ;  transference 
to  the  uterus,  1111;  impregnation  of 
the,  1119;  nutrition  of,  in  the  uterus, 
1123 

Oxidation  in  the  tissues,  seat  of,  469 

Oxygen,  its  absorption  by  the  living 
body,  2;  borne  by  the  blood  to  the 
tissues,  13;  in  proteids,  17;  borne  by 
hremoglobin,  32-35,  461 ;  its  entrance 
to  the  blood  by  diffusion,  424:  in  air 
expired  and  inspired,  -140 ;  relative  pro- 
portions of  in  arterial  and  venous  Ijlood, 
443;  varying  amounts  of  in  venous 
blood,  417;  relations  of  in  the  blood, 
447  ;  aljsorption  of,  by  blood  not  ac- 
cording to  'law  of  pressures,'  418; 
its  access  in  the  lung  to  the  corpuscle, 
463;  its  relations  in  laboured  breathing 
and  asphyxia,  4().">;  mode  of  storage  in 
muscle  tissue,  4()8 ;  effect  on  respira- 
tion of  deficiency  of,  487 ;  effect  of 
breathing,  49.", ;  results  of  jiigh  jircssure 
of,  496;  mode  of  measuring  amount 
consumed,  023;  consumption  of,  as  af- 


1176 


INDEX. 


fected  by  temperature,  646 ;  absorption 

of,  in  infancy,  1147 
Oxyhsemoglobin  defined  (footnote) ,  454 ; 

colour  of,  456 
Oxyntic  gland  of  frog,  352 

Pacchionian  glands,  825 

Pain,  sense  of,  1044-1050 ;  localization  of, 
1045 ;  special  nerve-endings  not  needed 
for,  1047 

"  Pains  "  of  labour,  1137 

Pallor  caused  by  emotion,  286 

Palmitin,  present  in  blood,  47;  a  constit- 
uent of  animal  fat,  606 

Palpitation  of  heart,  its  causes,  302 

Pancreas,  histological  changes  during  se- 
cretion, 340;  of  dog,  341 

Pancreatic  juice,  its  action  on  food-stnffs, 
359-362;  on  fats,  362;  on  proteids, 
362 ;  secretion  of,  365 ;  circumstances 
affecting,  366 ;  trypsin  a  constituent  of, 
349 ;  its  composition,  358 

Panniculus  adiposus,  604 

Paraglobulin,  a  constituent  of  blood- 
serum,  19 ;  precipitated  from  plasma, 
23 ;  in  white  corpuscles,  38 

Parapeptone,  326 

Paraphlegia,  reflex  action  in,  706 

Parotid  gland,  nerve  supply  to,  338;  cell- 
changes  in,  342 

Parturition,  1136-1143;  mechanisms  of, 
1137 ;  a  reflex  act,  1140 ;  inhibition  of, 
1142 

Peduncles  of  the  cerebellum,  812 

Pendulum  myograph,  66,  68 

Penis,  erection  of,  1116 ;  nerves  concerned 
in  mechanism  of,  1116;  striated  mus- 
cles assisting,  1116;  nervous  centre 
for,  1117 

Pepsin,  the  ferment  body  of  gastric  juice, 
329;  proteids  converted  into  peptone 
by,  ib.;  secreted  by  the  'chief  gas- 
tric cells,  352;  in  the  foetal  gastric 
membrane,  1130 

Pepsinogen,  an  antecedent  of  pepsin,  350 

Peptone  formed  from  proteids  by  gastric 
juice,  321,  325;  by  pancreatic  juice, 
329 ;  test  for,  325,  326 ;  its  absence  from 
chyle,  416;  its  course  during  absorp- 
tion, 416;  as  food,  629 

Perceptions,  visual,  time  required  for, 
823;  and  judgments,  959;  psychical 
modifications  of,  931 

and  judgments,  auditory,  1006, 1023 ; 

olfactory,  1025  ;  tactile,  1066 

Pericardial  fluid,  its  persistent  fluidity 
in  i^ericardial  bag,  28 

Periodic  events  of  life,  1153 

Peripheral  region,  blood-pressure  in,  161 

resistance,  defined,  163;   its  action 

in  the  circulation,  170;  illustrated 
by   model,    167;    lowered    by    action 


of  depressor  nerve,  281;  affected  by 
vaso-motor  changes,  276;  by  condi- 
tion of  vascular  walls,  294 ;  by  changes 
in  character  of  blood,  ib. 

zone,    in    capillary   contents,   290; 

white  corpuscles  present  in,  291; 
blood  platelets  in,  during  inflamma- 
tion, 292 

Peristaltic  contractions  of  plain  muscle, 
131 

movements  of  alimentary  canal,  371 ; 

excited  by  stimulation  of  vagus,  383; 
influences  bearing  on,  386 ;  of  ureter, 
644;  of  bladder,  547 

Personal  equation  as  affecting  reaction- 
time,  819 

Perspiration,  nature  and  amount  of,  650; 
secretion  of,  555;  regulation  of  tem- 
perature by,  642 

Pfluger's  gas-pump,  445 

Phagocytes,  44 

Phakoscope,  Helmholtz's,  852 

Phantoms,  ocular,  937;  auditory,  1020; 
tactile,  1069 

Phases  of  life,  1144 

Phenol,  a  bacterial  product  in  digestion, 
394 ;  compounds  of,  in  urine,  517 

Phloridzin,  temporary  diabetes  produced 
by,  676 

Phonation,  nervous  mechanism  of,  1083 ; 
centre  for,  1086 

Phosphates  in  muscle  ash,  93;  in  nerve 
ash,  106 ;  in  urine,  518 

Phosphenes,  884 

Phosphorus,  a  constituent  of  nuclein,  38, 
630;  of  serum,  47;  of  lecithin,  106;  of 
nerve  tissue,  106 ;  of  milk,  613 

,  its  importance  in  organisms,  630 

Photochemistry  of  the  retina,  922 

Physiology,  divisions  of,  3 ;  problems  of,  9 

Physiological  unit  defined,  6 

Physostigmin,  its  effect  on  pupil  contrac- 
tion, 866 ;  its  effect  on  accommodation, 
868 

Pigment,  yellow,  of  serum,  47 

epithelium  of  retina,  926 

Pigments,  their  possible  formation  from 
liEemoglobin,  36 ;  of  bile,  256 ;  of  urine, 
619 

Pilocarpin,  its  action  on  the  sweat  glands, 
567 

Pitch  of  sounds,  discrimination  of,  1001 

Pituitary  body,  the,  structure  of,  601 

Placenta,  the,  glycogen  present  in,  574; 
formation  of,  from  the  decidua  sero- 
tina,  1120;  vascular  events  of  the, 
1121 ;  shedding  of  the,  1121 ;  expulsion 
of,  after  parturition,  1139 

Plasmatic  layer  in  capillary  contents,  290 

Plasmine,  properties  of,  23 

Plateau,  systolic,  197 

Platelets,  blood,  44,  292 


INDEX. 


1177 


Plethysmograph,  principles  of  its  action, 
174,  178,  198;  amount  of  blood  in  parts 
determined  by,  270;  for  kidney  meas- 
urements, 525 ;  for  measurement  of 
blood-supply  to  brain,  830 

Pleural  cavity,  result  of  access  of  air  to 
the,  42(j 

Plexus,  brachial,  constrictor  and  dilator 
fibres  in,  271 

Pneumatograpb  of  Fick,  431 

Pneumograph,  Marey's,  430;  tracing  of 
respiratory  movements  by,  432 

Polarizing  current,  irritability  of  nerve 
aifected  by,  112-114 

Posture,  erect,  how  maintained,  1102 

Potassium  salts  in  cell  tissue,  39,  47 ;  in 
muscle  tissue,  90,  91 ;  in  urine,  516 

Predicrotic  wave,  its  causes,  235 

Pregnancy  and  birth,  1119-1143 

Pressure,  arterial,  as  compared  with 
venous,  154,  159;  as  affecting  pulse 
tracings,  223;  heart-beat  in  inverse 
ratio  to,  2(31;  blood,  in  the  small  ves- 
sels, peripheral  region,  IGl,  1G8;  flow 
of  lymph  regulated  by,  403-409;  en- 
docardiac,  191 ;  graphic  records  of,  193- 
199;  negative  during  each  cardiac 
cycle,  192,  193;  how  produced,  201; 
auricular  and  ventricular  compared, 
182,  193 ;  of  salivary  secretion,  337 ; 
of  bile  secretion,  3G9 ;  pulmonary,  427 ; 
thoracic,  431;  thoracic,  negative,  500; 
liartial,  of  gases,  448;  absorption  of 
oxygen,  dependent  on,  4(>2 ;  results 
of,  49(5;  atmospheric,  effect  of  diminu- 
tion of,  494 ;  increase  of,  496 ;  of  car- 
bonic acid  in  pulmonary  alveoli,  466; 
within  the  bladder,  547;  intra-ocular, 
conditions  affecting,  975 ;  sensations 
of,  1037 ;  modified  by  temperature, 
1055 ;  sensibility  of  skin  to  changes  of, 
1039,  1056 

Pressures,  Henry-Dalton  law  of,  449 

Primoi-dial  utricle,  4 

Processus  vocalis,  and  museularis,  1075 

Proteids,  general  composition  of,  17 ; 
changes  in,  produced  by  alcohol,  24; 
in  food-stuffs,  311 ;  action  of  gastric 
juice  on,  321 ;  of  pancreatic  juice  on, 
359;  classification  of,  in  order  of 
solubility,  ;)22 ;  path  taken  by,  during 
digestion,  415;  a  source  of  fat.  (iOS; 
metabolism  of  boily  increased  by,  ()17  ; 
disruption  of,  during  digestion,  626; 
probable  molecular  composition  of, 
361 ;  possible  storage  of,  in  the  body, 
627  ;  "  tissue,"  or  morphotic  and 
"  Hoating  "  or  circulating,  627 

Proteid  material,  a  constituent  of  living 
matter,  41 ;  jiotential  energy  of,  ex- 
pressed in  calories,  633 ;  the  pivot  of 
metabolism,  654 


Protoplasm,  definition  of,  4;  "differen- 
tiated," 4;  undift'erentiated  in  the 
embryo,  37 

Pseudopodia  of  the  white  corpuscles, 
anKJuboid  movements  by  means  of,  137 

Psychical  processes,  analysis  of,  821; 
duration  of,  823;  visual,  complexity 
of,  961 

Ptomaines,  their  bacterial  origin,  394 

Ptyalin,  a  constituent  of  saliva,  318 

Puberty,  phenomena  of,  1150 

Pulse,  the,  153,  219;  methods  of  record- 
ing, 219-223;  artificial,  223,  227;  char- 
acters of,  227;  disappearance  of,  228; 
dicrotism  in,  230-236;  anacrotic,  ib.; 
venous,  236 ;  venous,  respiratory,  237 

Pulse-volume,  200,  217 

Pulse-wave,  changes  of,  in  the  arteries, 
227;  velocity  of  the,  229;  length  of 
the,  ih. 

Pulvinar,  the,  ending  of  part  of  the  optic 
tract  in,  787 

Punctum  lachrymale,  978 

Puncture  of  pleura,  result  of,  426 

Pupil,  the,  see  Eye,  pupil 

Purkinje,  figures  of,  917 

Purple,  visual,  922 ;  bleaching  of  by 
light,  923 

Pus  corpuscles,  their  formation,  43 

Pjdorus,  ejection  of  chyme  through  the, 
389 

Pyramidal  tract  of  cord,  686;  efferent 
nature  of  impulses  of,  753;  not  indis- 
pensable for  voluntary  movements,  772 

Pyramids  of  the  bulb,  decussation  of,  758 

Pyrexia,  causes  of,  648 

Eadial  artery,  tracings  of  the  pulse  in, 
221-223,  226,  231 

Eadical,  lacteal,  contents  of,  418 

Ranke's  diet  table,  633 

Reaction-period,  subdivision  of,  819;  for 
vision,  881 

Rectum,  nervous  control  of  movements 
of,  385 

Recurrent  sensations,  937 

Refiex  actions,  general  features  of,  144- 
146 ;  doubtful  if  carried  out  by  ganglia, 
144;  not  always  proportioned  to 
stimulus,  145;  often  purposive  in  char- 
acter, 146 ;  vaso-motor,  277 

actions  of  the  cord,  698;  features  of 

dependent  on  afferent  impulses,  698; 
nervous  mechanisms  of,  ('>99;  their  re- 
lations to  intelligence,  701 ;  coordin.a- 
tion  of,  702 ;  determined  by  intrinsic 
condition  of  cord,  703;  other  than 
movements,  707  ;  inhibition  of,  705;  in- 
hibitory action  of  the  brain  on,  708; 
time  required  for,  709 

Regeneration  of  organs  in  lower  animals, 
1109 


1178 


INDEX. 


Registers  of  the  voice,  1090 

Relaxation  of  muscular  fibre  an  essential 
part  of  contraction,  72,  21)9 

Rennet,  curdling  action  of,  on  milk,  330 

Rennin,  its  direct  action  on  casein,  331; 
its  formation  in  gastric  cells,  352 

Reproduction,  tissues  and  mechanisms 
of,  1109;  general  features  of,  1129; 
female  organs  of,  1111;  male  organs 
of,  1114 

Respiration,  424-512;  pulmonary,  circu- 
lation aided  by,  171;  its  mechanism, 
425-439;   work  of  the  muscles  of  the 
ribs  in,  435 ;  laboured,  muscles  of,  436 
expiration,  the  expiratory  muscles,  437 
change  of  temperature  of  air  in,  440 
change    of    aqueous  vapour    in,    440 
changes  in  blood  caused  by,  442 ;  chem- 
ical aspects  of,  470 ;  an  involuntary  act, 
472 ;  sequence  of  muscular  contractions 
in,  472 

,  pulmonary,  centre  for,  medullary, 

473 ;  automatic  action  of,  474 ;  influence 
by  afferent  impulses,  476 ;  duplexity  of 
its  action,  479;  effects  of  inflation  and 
suction,  482;  double  action  of  vagus 
on,  483;  nature  of  action,  484;  two 
lateral  halves  of,  484;  influenced  by 
character  of  blood-supply,  485 ;  by  de- 
ficiency of  oxygen,  487, 1134 ;  by  excess 
of  carbonic  acid,  488;  by  other  changes 
in  the  blood,  488 ;  centre  for  apncea, 
phenomena  of,  489 

,   Cheyne-Stokes,    490;    affected    by 

changes  in  atmospheric  pressure,  491, 
494 ;  its  effect  on  arterial  pressure,  497 ; 
artificial,  its  effect  on  the  circulation, 
503 ;  impeded,  its  effect  on  heart-beat, 
507 ;  as  affected  by  muscular  work, 
510;  regulation  of  temperature  by, 
642;  as  affected  by  sleep,  1155 

,  facial  and  laryngeal,  438 ;  cutaneous, 

552;  of  muscle,  469;  of  other  tissues, 
467;  of  the  embryo,  1124;  placental 
compared  with  branchial,  1126 

Respiratory  quotient  in  herbivora  and 
carnivora  compared,  629 

Retina,  see  Eye,  retina 

Rheometer  of  Ludwig,  172 

Rheoscopic  frog,  102;  current  of  action 
shewn  in,  107 

Rhythm,  secondary  respiratory,  490 
Rhythmic  changes  of  calibre  in  artery, 
262 ;  beat  of  heart,  spontaneous  nature 
of,  239;  beat  of  cardiac  substance,  300; 
contractions  of  uterus  during  preg- 
nancy, 1136 
Ribs,  movements  of,  in  respiration,  434, 

435 
Rigor  mortis,  characteristics  of,  86,  87; 
development  of  carbonic  acid  during, 
91-95;  conversion  of  myosinogen  into 


myosin  during,  95;  progressive  order 

of,  128 ;  accession  of  heat  at  onset  of, 

641 
Rima  glottidis,  the,  see  Glottis 
Ritter  Valli  law,  126 
Rods,  of  retina,  922,  926 
Roots  of  spinal  nerve,  141,  677 
Rosenthal's  calorimeter,  635 
Round  ligament  of  uterus,  contractions 

of,  1118 
Roy,    sphygmotonometer    of,    220,  222; 

perfusion  cannula  of,  240 
Roy  and  Adami,  cardiometer  of,  199,  202 
Roy  and  Rollestou,  method  of  recording 

endocardiac  pressure,  193,  194 

Sacculi  of  large  intestine,  peristaltic  con- 
tractions of,  381 

Saline  solution,  normal,  defined  (foot- 
note), 16 

Saliva,  characters  and  properties  of,  313 ; 
its  properties,  314;  its  amylolytic  ac- 
tion, 316,  387;  characters  of  parotid, 
submaxillary,  sublingual,  and  mixed, 
318 ;  amount  daily  secreted,  332 ;  reflex 
secretion  of,  333 ;  centre  for  secretion 
of,  in  medulla  oblongata,  335 ;  of  dog, 
mechanical  use  of,  387;  of  the  babe, 
1146 

Salivary  glands,  venous  pulse  observa- 
ble in,  236 

Salts,  neutral,  needed  for  formation  of 
fibrin,  26 ;  calcium,  clotting  as  affected 
by,  ib. ;  pulsation  of  'washed-out' 
heart  as  affected  by,  259;  in  food- 
stuiTs,  312;  absorption  of,  414;  as 
food,  630;  importance  of,  for  nutri- 
tion of  nervous  system,  630;  essential 
to  life  of  muscle,  652 ;  in  diet,  661 

Santorini,  cartilage  of,  1070,  1078 

Sarcolemma,  structure  of,  86 

Scaleni  muscles,  the,  their  service  in  res- 
piration, 435 

Scheiner's  experiment,  847,  873 

Schlemm,  circular  canal  of,  969 ;  passage 
of  aqueous  humour  by  means  of,  974 

Sclerotic  coat  of  eye,  development  of,  837 

Secretion  of  saliva,  nervous  mechanism 
of,  332 ;  of  gastric  juice,  338 ;  changes  in 
gland  constituting  act  of,  340 ;  changes 
in  albuminous  cells,  342;  changes  in 
mucous  cells,  344;  by  central  cells  of 
stomach,  346;  special  substances  elab- 
orated during,  350 ;  of  pancreatic  juice, 
365 ;  of  bile,  366 ;  of  urine,  524 ;  glome- 
rular and  tubular  in  the  kidney  com- 
pared, 534 ;  glomerular,  its  nature,  535 ; 
of  sweat,  551 ;  mechanism  of,  555 ;  of 
milk,  615 

Secretions,  carbonic  acid  in,  470;  their 
constituents  manufactured  by  glandu- 
lar action,  538 


INDEX. 


1179 


Segmentation  of  the  ovum,  6 

Self-digestion,  352,  353 

Self-iiiduetion,  02 

Semen,  chemical  composition  of,  1115; 
emission  of,  see  Emission  of  semen 

Semicirculai-  canals,  effects  of  injury  to 
the,  730 

Semilunar  valves,  their  action,  184;  di- 
crotic wave  as  affected  by  closure  of, 
233,  234 

Sensations,  special  auditory,  998-1006; 
limits  of,  1000;  fusion  of,  1003 

,  cutaneous,  798-811,  1037-1058;  im- 
portance of  contrast  in,  105G;  of  pres- 
sure, 1037;  localization  of,  1039;  of 
heat  and  cold,  1041;  of  pain,  1044;  of 
touch  and  temperature,  terminal  or- 
gans necessary  for,  1051 ;  of  pressure, 
terminal  organs  for,  10.'j4  ;  of  heat,  ter- 
minal organs  for,  dilfereut  from  those 
of  cold,  1051) ;  connection  of  with  the 
muscular  sense,  10l3(J 

,  olfactory,  1025,  1026 

of  taste,  795,  1029-1036 ;  usually  ac- 

comjianied  by  other  sensations,  1029; 
caused  by  electrical  or  mechanical 
stimuli,  1030;  conditions  of,  1031;  lo- 
calization of,  1032;  distribution  of  ter- 
minal organs  for,  1033 ;  theories  as  to 
mode  of  origin,  1034;  nerves  for,  1035 

,  visual,  probable  progressive  de- 
velopment of,  793;  general  features 
of,  878;  fusion  of,  883,  886;  localiza- 
tion of,  885 ;  of  colour,  891 ;  of  colour, 
due  to  metabolic  changes,  901 ;  psycho- 
logical features  of,  929 ;  their  want  of 
agreement  with  perceptions,  931;  re- 
current, 937 

,  afferent,  as  factors  in  coordination 

of  movement,  733;  crossing  of,  from 
opposite  hemispheres  of  brain,  804;  de- 
velopment of,  along  the  spinal  cord, 
805;  transmission  of,  within  the  brain, 
808;  coordination  of  motor  impulses 
regulated  by,  955 

Sense,  the  muscular,  1059;  of  move- 
ment, of  position,  and  of  effort,  1060; 
afferent  impulses  forming  basis  of,  1061 

Sensibility,  general,  1046;  recurrent,  678 

Serous  cavities,  fluid  of,  402 

fluids,  artificial  clotting  of,  23;  char- 
acters of,  402 

Serum  left  after  clotting  of  fibrin,  15; 
chemical  composition  of,  18-21,  46 

Serum-albumin,  its  characters.  20 ;  action 
of  gastric  juice  and  hydrochloric  acid 
on,  324;  importance  of,  in  nutrition  of 
muscle,  (i52 

Sex,  differences  of,  ll.'O 

Shivering  from  cold,  temperature  raised 
by,  647 

Shock,  induction,  5'.i;  nature  of,  697 


Short-circuiting,  58 

Sight,  see  Vision 

Singing,  power  of,  dependent  on  nervous 
mechanism,  1087 

Sinuses,  venous,  of  brain,  828;  placental, 
1121;  quality  of  blood  in,  1125 

Size,  judgment  of,  !J62 

Skin,  as  regulator  of  heat,  642;  different 
kinds  of  sensations  experienced  through 
the,  1037;  as  field  of  touch,  1039 

Sleep,  phenomena  of,  1154;  afferent  im- 
pulses as  affected  by,  1154;  respiration 
during,  1155;  the  brain  during,  1156 

Smell,  sensations  of,  1025-1028;  cortical 
area  for,  794 

Sobbing  and  sighing  and  sneezing,511,512 

Sodium  chloride,  its  action  on  jjlasma, 
22,  26;  on  mucin,  313;  glycholate  and 
taurocholate,  3"(i;  hydrate,  its  effect 
on  the  heart,  259;  sulphindigotate,  ex- 
cretion of  by  kidney,  534 

Solidity,  judgment  of,  966 

Somatic  nerves,  141 

Sound,  musical,  of  contracting  muscle, 
123,  190;  of  the  heart,  188-191,  213, 
216;  waves  of,  987;  complex,  analysis 
of,  1013;  psychical  nature  of  apprecia- 
tion of,  1016 

Sounds,  musical,  characters  of,  1021 ;  ap- 
preciation of  outwardness  of,  1021; 
judgment  of  direction  of,  1023;  judg- 
ment of  distance  of,  1023 

Spectrum,  limitations  of  visibility  of ,  891 

Speech,  cortical  area  for,  766,  1085;  a 
skilled  movement,  766 ;  movements  of, 
bilateral,  766;  causes  of  various  imper- 
fections of,  769;  special  mechanisms 
of,  1092-1100 ;  sounds  made  use  of  in, 
1092 

Spermatozoa,  movements  of,  1114 ;  action 
of,  in  the  ovum,  1119 

Sphincters  of  stomach,  their  action  dur- 
ing digestion,  377  ;  tone  of  dependent, 
on  cord, 713 

Sphincter  ani,  its  nerve-supply,  381; 
versicje,  545;  iridis,  858 

Sphygmograph,  Dudgeon's,  221 

Sphygraoscoi)e,  220 

Spinal  cord,  .see  Cord,  spinal 

Spirometer,  428 

Splanchnic  nerves,  141,  271;  inhibitor 
and  augmontor  fibres  in,  385;  ganglia, 
141,  142  ;  abdominal  nerve,  266 

Spleen,  the,  possible  formation  of  red 
corpuscles  in,  'Mi;  rhythmical  action  of 
muscle  fibres  in,  171;  its  action  during 
digestion,  3(;8;  movements  of  the,  579; 
chemical  constituents  of,  582  ;  uric  acid 
in  the,  583 

"  Spleen-curve,"  580 

Splecn-pnip,  destruction  of  red  corpus- 
cles in,  35 


1180 


INDEX. 


Spot,  blind,  of  retina,  916 

Spring-manometer,  195 

Stagnation  stage  of  Inflammation,  293 

Stapes,  or  stirrup  bone,  986 

Starch,  action  of  saliva  on,  314;  chemi- 
cal composition  of,  314 ;  action  of  pan- 
creatic juice  on,  314,  315;  'animal,' 
569  ;  its  value  in  diet,  661 

Starvation,  its  effect  in  checking  produc- 
tion of  glycogen,  563,  567  ;  changes  in 
body  during,  620  ;  fall  of  temperature 
attending,  649 

Stearin,  its  presence  in  blood,  47;  a  con- 
stituent of  animal  fat,  606 

Stellate  ganglion,  composite  nature  of, 
255 

Stereoscope,  ocular  movements  affected 
by  the,  953  ;  principle  of  construction, 
966 

Stethometer  of  Burdon-Sanderson,  431 

Stimuli  defined,  53 ;  various  kinds  of, 
56  ;  necessary  characters  of,  121 

Stimulus,  reflex  actions  varied  according 
to  nature  of,  699 

Stolnikoff's  metliod  of  measuring  the 
'  out-put '  of  the  heart,  197 

Stomach,  nervous  supply  to,  339  ;  its  se- 
cretion of  gastric  jviice,  lb.,  340  ;  move- 
ments of,  377 ;  changes  of  food  in  the, 
388 

Storage  of  bile  in  gall-bladder,  367  ;  of 
glycogen  in  the  liver,  565,  571 

Striation  of  muscle  tissue,  86 

Stroma  of  red  corpuscles,  its  composition, 
32  ;  embrj'onic  formation  of,  from  pro- 
toplasm, 35 

Stromuhr  of  Ludwig  described,  173 

Strychnia,  reflex  action  as  affected  by, 
703 

Substance,  living,  compared  with  dead, 
3,  86 ;  metabolic  changes  in,  39-41 ; 
chemical  composition,  41 

Substances,  visual,  hypothetical,  901 

Succus  entericus,  its  nature  and  action, 
363 

Sugar,  its  presence  in  the  blood,  48,  573; 
normally  present  in  blood  and  chyle, 
414 ;  formed  by  saliva  from  starch,  316 ; 
course  taken  by,  during  digestion,  414, 
421;  in  diabetic  urine,  522;  its  con- 
version into  glycogen,  569;  a  product 
of  metabolic  changes,  570 ;  its  value  in 
diet,  661 

Sulcus,  crucial  and  sigmoid,  of  dog's 
brain,  740 

Sulphur  in  proteids,  17,  630;  in  urine, 
518 

Suprarenal  bodies,  the,  601 

Swallowing,  mechanism  of,  373;  its  action 
on  tympanic  air  pressure,  996 

Sweat,  how  secreted,  651,  655;  composi- 
tion of,  652 


Sweat-fibres  of  different  animals,  course 
of,  557 

Sweat-glands,  558;  action  of  pilocarpin 
on,  557 

Sweat-nerves,  558 

Sweating  in  lower  animals,  556 ;  nervous 
mechanism  of,  555 ;  a  reflex  act,  557 

Sympathetic  system,  fibres  to  plain 
muscles  supplied  by,  133;  its  connec- 
tion with  spinal  nerves,  141 

Syntonin,  88 

Systole,  auricular  and  ventricular,  181- 
185  ;  ventricular,  a  simple  contraction, 
190;  and  diastole^  comparative  dura- 
tion of,  212-214;  amount  of  blood 
driven  by  eacli,  153,  217;  work  of 
papillary  muscles  in,  183,  184 

Systolic  plateau,  the,  197,  202,  207,  216 

Tactile  sensations,  1037;  localization  of, 
1039 

Tambour,  Marey's,  192 

Tambour-sphygmoscope  of  Hiirthle,  220 

Tarsus  of  the  eyelids,  976 

Taurocholic  acid,  356 

Tears,  secretion  of,  978 

Tectorial  membrane,  1011 

Teeth,  order  of  their  appearance,  1149 

Tegmentum,  the,  815 

Temperature  of  living  bodies,  2;  as 
affecting  clotting,  20 ;  irritability,  125, 
128;  plain  muscle,  134;  ciliary  action, 
136 ;  vaso-motor  fibres,  271,  288 ;  action 
of  gastric  juice,  328;  action  of  rennet, 
330;  point  of  saturation  of  gas,  440; 
absorption  of  oxygen  by  liquids,  462 ; 
the  cutaneous  vessels,  655;  perspira- 
tion, 560,  665 ;  storage  of  glycogen,  566 ; 
sense  of  taste,  1031 

of  exiDired  air,  440;  regulation  of, 

by  evaporation  from  the  skin,  555;  by 
variations  in  loss  of  heat,  641,  643;  by 
the  nervous  system,  645,  646;  of  cold- 
blooded animals,  641 ;  of  warm-blooded 
animals,  641;  normal,  range  of,  647; 
high,  phenomena  of  death  from,  649; 
low,  effects  of,  650;  its  relation  to 
amount  of  food  needed,  668 ;  of  body, 
maintenance  of,  646 ;  sensations  of, 
1041,  1056;  terminal  organs  for  sensa- 
tions of,  1056 ;  sense  of,  in  parts  other 
than  external  skin,  1043 

Tendon  reflexes,  '  knee-jerk,'  705,  717 

Tenonian  cavity  and  Tenon's  capsule, 
971 

Terminal  organs,  special  sensations  due 
to,  1053;  for  sense  of  touch,  1064;  for 
sense  of  pressure,  1066 ;  for  sense  of  heat 
different  from  those  for  sense  of  cold, 
1056  ;  cutaneous,  their  nature,  1057 

Tetanic  contraction,  its  nature,  55;  due 
to  repetition  of  stimuli,  ib.,  121 


IXDEX. 


1181 


Tetanus,  phenomena  of,  75-81,  111  ;  car- 
bonic acid  evolved  during,  9-t ;  exhaus- 
tion of  irritability  from,  130 

Thermopile,  various  forms  of,  96 

Thermotaxis,  centre  for,  646,  647 

Thirst,  sensation  of,  1048 

Thoracic  duct,  characters  of  lymph  from 
the,  400 

Thorax,  effect  on  blood-tlow  of  pressure 
in  the,  499,  502 

Thrombi,  white,  their  nature,  45 

Thymus  body,  structure  of  the,  602  ;  na- 
ture and  functions,  002 ;  its  size  in  in- 
fancy, 1148 

Thyroid  body,  600  ;  diseases  connected 
with,  601 ;  in  infancy,  1148 

Thyroid-arytenoid  muscles,  1077,  1078 

Thyroid  cartilaiie,  1073 

Tigerstedt,  his  method  of  measuring  car- 
diac output,  198 

Tissue,  connective,  149,  150 

Tissues  not  indispensable  for  life,  3; 
classification  of,  6;  built  up  by  the 
blood,  13;  similarity  of  histological 
elements  of,  39 ;  contractile,  55-138  ; 
nervous,  139-148;  vascular,  149  et 
supra ;  digestive,  311-423 ;  respiratory, 
424-512  ;  relative  proportions  of,  in  the 
body,  619,  620 ;  metabolism  of,  651 ; 
their  death  gradual,  1159 

Tone,  arterial,  264,  2S4;  general,  275- 
283  ;  bulbar  vaso-motor  centre  for, 
280-284;  centre  tor,  in  medulla,  279; 
maintained  by  automatic  action  of 
cord,  713 

of  skeletal  muscles,  713  ;  due  to  cen- 
tral nervous  system,  714 

Tones,  musical,  fundamental  and  par- 
tial, 999 

Tongue,  localization  of  taste  sensations 
in,  1034 

Tortoise,  heart-beat  in,  independent  of 
cardiac  nerves,  243 

Trachea,  effect  on  respiration  of  its  clos- 
lare,  481 

Tract,  optic,  course  of,  785  ;  ascending 
and  descending  antero-lateral,  686 ; 
cerebellar,  689 ;  as  to  functions  of,  807 ; 
median  posterior,  ()84  ;  as  to  functions 
of,  806  ;  i)yramidal,  crossed,  ()86  ;  direct, 
687;  relations  to  volition,  74ii,  769,  773 

Tracts,  afferent,  in  spinal  cord,  SOO ;  in- 
ternuncial,  for  afferent  impulses,  807 

Transudation  into  lymph  spaces,  406  ;  not 
merely  a  filtration,  407  ;  conditions  de- 
termining. 408;  opposite  currents  of, 
through  capillary  walls,  407 

Traube-Hering  curves,  their  origin,  507  ; 
imdulatioiis  in  kidney,  528;  variations 
in  cerebral  blood-pressure,  831,  832 

Tricuspid  valves,  182 

Trypsin,    a   constituent    of    pancreatic 


juice,  349,  360;  in  the  foetal  pancreas, 
1130 

Trypsinogen,  an  antecedent  of  trypsin,  350 

Tube,  Fallopian,  1112 

Tubules,  uriniferous,  epithelium  of  the, 
533;  work  of  the,  538;  special  sub- 
stances excreted  by,  534 

Tuning-fork  for  the  measurement  of  ve- 
locity, 67-71 

Tympanum  of  ear,  983;  conduction  of 
sound  through,  988 ;  structure  and  re- 
lations, 986;  membrane  of,  984; 
muscles  of  the,  993 ;  its  connection  with 
sense  of  outwardness  of  sounds,  1021 

Tyrosin,  a  product  of  pancreatic  diges- 
tion, 3(i0;  chemical  composition,  361 ;  a 
result  of  proteid  decomposition  outside 
the  body,  598 

Umbilical  cord,  1134 

arteries,   1121;    pressure    in,   1124; 

venous  blood  in  the,  1125 

vein,  pressure  in,  1124 

Undulations,  respiratory,  phenomena  of, 
498,  504  ;  luminous,  891 

Unit,  physiological,  defined,  6 

Urari,  the  nature  of  its  action,  54,  83; 
diabetes  in  frogs  produced  by,  576 

Urea,  a  constituent  of  the  blood,  48;  ab- 
sent from  muscle-tissue,  93,  590:  as 
nitrogenous  waste,  93,  513,  589,  593, 
598;  its  relations  to  kreatin,  590,591; 
its  presence  in  the  blood  antecedent  to 
kidney  action,  538;  its  action  on  the 
tubules  of  kidney,  542 ;  brought  to  the 
kidneys  by  the  blood,  589,  599;  its  for- 
mation in  the  liver,  594;  synthesis  of, 
595;  its  relation  with  cyanogen  com- 
pounds, 598;  diminished  excretion  of, 
during  starvation,  621;  excretion  of, 
not  increased  by  exercise,  636;  its  kin- 
ship to  vegetable  alkaloids,  654;  a 
constituent  of  amniotic  fluid,  1131 

Ureter,  peristaltic  contractions  of,  544 

Uric  acid,  596;  chemical  composition  of, 
516;  relations  to  urea,  circumstances 
determining  its  appearance,  59(5;  con- 
stant pressure  of  in  the  spleen,  583 

Urina  hysterica,  543 

Urine,  composition  and  characters  of, 
515 ;  normal  organic  constituents  of, 
51(),  520;  inorganic  salts  of,  517;  aver- 
age composition  of,  520 ;  abnormal 
constituents  of,  520,  .577  ;  secretion  of, 
524;  vaso-motor  mechanisms  for,  525; 
its  relations  to  the  renal  circulation, 
533;  albuminous,  537:  pigments  of,  540; 
discharge  of,  .544  ;  its  secretion  con- 
tinuous, .544  ;  changes  of,  in  the  blad- 
der,549  ;  during  starvation,  621  ;  sugar 
present  in  diabetes,  575  ;  of  children, 
characteristics  of,  1148 


1182 


INDEX. 


Urobilin,  519 

Use,  muscle  substance  increased  by,  128; 
skilled  movements  facilitated  by,  779 

"  Uterine  milk,"  1122,  1129 

Uterus,  the,  reception  of  the  ovum  by, 
1111 ;  changes  in  mucous  membrane 
of,  during  menstruation,  1112 ;  changes 
after  impregnation,  1119;  expansion 
of,  during  pregnancy,  1136 ;  "retrac- 
tion" of,  1137,1139;  rhythmical  con- 
tractions of,  during  pregnancy,  1136; 
during  'labour,'  1137;   nerves  of,  1141 

Utricle,  primordial,  4 ;  of  labyrinth,  1009 

Vagus,  see  Nerves,  vagus 

Valves  of  the  veins,  171 

of  the  heart,  their  action  in  circula- 
tion, 182-185;  sounds  caused  by  their 
closure,  189,  190;  tricuspid,  their 
action,  183;  semilunar,  of  the  pulmon- 
ary artery,  185 ;  their  action,  186 ; 
semilunar,  of  aorta,  189,  207 ;  Eusta- 
chian, 182 

,  ileo-csecal,  mechanism   of,  381;    of 

the  lymphatic  vessels,  401 

Vapour,  aqueous,  in  expired  air,  440 

Vascular  mechanism,  149-307 ;  main  fea- 
tures of  the  apparatus,  150 ;  main  regu- 
lators of,  apparatus,  238,  244,  262 

walls,  their  action  on  the  blood,  27; 

alteration  of,  in  inflammation,  293,  294 

Vas  deferens,  contraction  of  in  emission, 
1117 

Vaso-motor  action,  262-288 ;  arterial  tone 
due  to,  264 ;  effects  of,  275 ;  cutaneous 
and  splanchnic,  compensatory,  305; 
compensatory  in  loss  and  increase  of 
blood,  296 ;  summary  of,  284 ;  its  rhyth- 
mic tendency,  507 ;  regulation  of  tem- 
perature by,  642 

centre,  280-284;  limits  of ,  283 ;  re- 
lations of,  to  other  centres,  284 

fibres,    constrictor,    266-269,    272; 

course  of,  273, 278,  284 ;  loss  of  medulla 
in,  274,  285 ;  tonic  action  of,  275-283 ; 
chief  parts  of  body  supplied  by,  278 ; 
dilator,  269;  course  of,  275;  usually 
employed  in  reflex  action,  278 ;  reten- 
tion of  medulla  in,  285 

functions    of  the   central   nervous 

system,  277;  nerves  of  veins,  288 

Vegetable  cell,  storage  of  metabolic 
products  in,  654 

diet,  results  of,  664;  large  amount 

required,  666 

Veins,  structure  of,  152;  minute,  ib.; 
their  capacity  as  compared  with  ar- 
teries, ib. ;  walls  of,  ib. ;  blood-pressure 
in,  155, 159;  valves  of,  171 ;  vaso-motor 
nerves  of,  288 

Velocity  of  nervous  impulse,  72,  73;  of 
muscular  contraction,  83,  84;  compara- 


tive, of  arterial,  venous,  and  capillary 
circulation,  161-171 ;  of  arterial  current, 
172;  of  flow  in  capillaries,  177;  of  flow 
in  veins,  176 ;  of  blood  current,  229 ;  of 
pulse  wave,  ib. 

Venous  circulation,  aids  to,  171;  pulse, 
230 ;  sinuses  of  brain,  828 

Ventilation,  positive  and  negative  of 
lung,  482 

Ventricle  of  heart  of  frog,  its  action  in 
heart-beat,  240-248;  of  tortoise,  iso- 
lated, spontaneous  heart-beat  of,  243 

Ventricles  of  the  heart,  synchronism  of 
their  action,  182 ;  their  change  of  form 
in  cardiac  cycle,  185;  four  stages  of 
action  of,  212;  tonic  contraction  of,  260 

Vertigo,  causes  of,  733 

Vesicles,  cerebral,  835;  optic,  835;  otic, 
980 

Vesiculse  seminales,  their  action  in  emis- 
sion, 1117 

Vestibule  of  ear,  980;  parts  of,  1008; 
perilymph  cavity  of,  1013 

Vibrations  of  muscle  sound,  122,  123; 
sonorous,  longitudinal  and  transversal, 
990;  of  the  tympanic  membrane,  989; 
through  the  auditory  ossicles,  991; 
through  the  bones  of  the  skull,  993 

of  sound  and  light  compared,  1000 

,  interference  of,  1005 

Vierordt,  his  haBmatachometer,  173 

Vieussens,  annulus  of,  247,  253-256 

Villi,  the,  columnar  epithelium  of,  417; 
pumping  action  of,  419;  of  chorion, 
fcetal,  1121 

Vision,  781;  binocular,  782,  939-958;  its 
action  in  judging  of  distance,  964;  in 
judging  of  solidity,  966;  mechanism  of, 
782;  central  apparatus  for,  788;  imper- 
fections of,  790;  dioptric  mechanisms 
of,  834, 839 ;  astigmatism,  871 ;  spherical 
aberration,  871 ;  entoptic  phenomena, 
874 ;  distinct  limits  of,  887 ;  trichromic 
nature  of,  898,  905;  colour,  Young- 
Helmholtz's  theory  of,  898, 935 ;  Bering's 
theory  of,  900,  935;  field  of,  782,  929, 
940 ;  corresponding  or  identical  points, 
942 ;  struggle  of  the  two  fields  of,  959, 
968 

Visual  areas  in  fovea  centralis,  888 ;  axis, 
939;  centres,  lower,  793;  impulses, 
development  of,  916;  impulses,  origin 
of,  920;  perceptions  and  judgments, 
959 ;  perceptions,  psychical  processes  in, 
961 ;  plane,  940 ;  purple,  922 

sensations,  781-794;  probable  mode 

of  development  of,  793 ;  fusion  of,  886, 
887;  in  relation  to  visual  perceptions, 
929-938 ;  simultaneous,  929 

units,  retinal,  889 

Vitreous  humour,  the,  974 

Vocal  cords,  1071 ;  voice  produced  by  the 


INDEX. 


1183 


vibration  of  the,  1074, 1085 ;  tightening 
and  slackening  of  the,  1081 

Voice,  the,  1070-10!)!;  how  produced, 
1071 ;  fundamental  features  of  the, 
1074;  different  qualities  of,  1085;  chest 
and  head  voices,  1088 ;  registers  of  the, 
1090;  breaking  of  the,  1091 

Volkinanu,  his  hEemadroniumeter,  172 

Voluntary  movements,  their  tetanic  char- 
acter, 122;  nervous  mechanisms  for, 
739,  777 

Vomiting,  mechanism  of,  378 

Vowel  chamber,  1092 

Vowels,  how  formed,  1093 

Walking,  how  effected,  1102 

Walls,  vascular,  their  influence  on  tran- 
sudation, 407 

Warmth,  its  effect  on  skin  action,  541 

Waste  matters,  their  discharge  from  the 
living  body,  2 ;  given  out  by  amoebae,  4 ; 
not  necessarily  useless,  40,  41 ;  nitroge- 
nous, 93 ;  not  increased  by  muscle  con- 
traction, 94,  97,  63Q ;  elimination  of,  513 

Water,  secretion  of,  by  the  glands,  351 ; 
varying  amount  of,  in  li^^ng  tissue, 
409 ;  its  absorption  into  the  portal  sys- 
tem, 414;  intestinal  secretion  of,  422; 
its  discharge  by  the  kidney,  541 ;  by 
the  skin,  555 

Wave-pulse,  dicrotic,  origin  of,  232-23G ; 
predicrotic,  235 ;  anacrotic,  236 

Waves  of  contraction,  muscular,  83;  of 
nerve  and  muscle  impulse,  109;  of 
sound,  987,  999;  of  light,  891 


Web  of  frog,  arterial  changes  visible  in, 
202 

Weber's  law,  880,  1001,  1038 

Weight,  human,  curve  of,  1145 

Whispering,  how  effected,  1095,  1100 

White,  sensations  of,  produced  from  mix- 
ing of  colour  sensations,  897 

Willis,  circle  of,  827 

Winking,  how  effected,  976;  chief  use, 
978,  979 

"  Word-deafness,"  797 

Work,  mechanical,  in  living  body,  2; 
done  by  a  muscle-nerve  preparation, 
119  et  /tzqira ;  amount  of,  done  by  the 
heart,  217 ;  daily,  estimate  of,  634 ;  me- 
chanical source  of  energy  of,  636 ;  pro- 
duction of  heat  increased  by,  644 

Xanthin,    a    constituent  of    urine,  517, 

597 ;  present  in  the  thymus,  602 
Xanthoproteic  test  for  i^rotein,  17,  18 

Yawning,  511 

Yellow  spot,  colour  sensations  as  affected 
by,  913 

Young-Helmboltz,  theory  of  primary 
colour  sensations,  898;  as  applied  to 
colour  blindness,  908,  911 ;  of  simulta- 
neous and  successive  contrasts,  935 

Zinn  zonule,  passage  of   fluid  by  the, 

975 
Zone,  peripheral  of  capillaries,  290;  Lis- 

sauer's,  685,  68() 
Zymogens,  350,  352. 


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WORKS   ON  THE  MEDICAL   SCIENCES 


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for  Consumption  and  Diseases  of  the  Chest,  Brompton,  and  late  President  of 

the  Royal  Meteorological  Society.     8vo,  cloth.     $2.00. 

"  It  is  not  an  exhaustive  work,  it  is  true,  but  it  is  sufficiently  full  for  the  practitioner's  needs, 
accurate,  practical,  and  charmingly  written.  For  those  who  wish  a  small  work  on  the  climatic  treat- 
ment of  pulmonary  diseases,  written  by  a  recognized  authority  on  the  subject,  and  presenting  practi- 
cally and  usefully  most  reliable  information  on  a  very  important  subject,  we  must  heartily  recommend 
Dr.  Williams's  work."  —  N.  Y.  Medical  journal. 

WILLOUGHBY  (E.  F.).— Handbook  of  Public  Health  and  Demography. 

i6mo.     $1.50. 

"  An  admirably  concise  and  lucid  treatment  of  preventive  medicine,  alike  commendable  to  general 
readers,  teachers,  students,  physicians,  and  sanitary  inspectors." —  The  Sanitarian. 

ZIEGLER.  —  A  Text-book  of  Pathological  Anatomy  and  Pathogenesis. 

With  Illustrations.     8vo. 
Part    I.     General  Pathological  Anatomy.     ^3.50. 
Part  II.     Special  Pathological  Anatomy.     Sections  I.-VII.     ^3.50. 
Part  II.     Special  Pathological  Anatomy.     Sections  IX.-XIL    ^3.50. 

ZIEHEN     (Theodore).  —  Introduction    to    Physiological    Psychology. 

Translated  by  C.  C.  Van  Liew  and  Dr.  Otto  Beyer.     With  21  Illustrations. 

^1.50. 

"  The  book  can  be  highly  recommended  to  all  who  wish  a  short  and  clear  outline  of  the  science."  — 
Educational  Review. 

"The  general  reading  of  this  book  would,  we  think,  do  more  for  the  schools  of  America  than  the 
investment  of  the  same  amount  of  time  and  money  in  anything  else."  —  yournal  of  Education. 


MACMILLAN    &    CO., 

66  FIFTH    AVENUE,        -       -       »       NEW   YORK. 


F81 
1895 


